BASIC REFRESHER LEVEL 111
/
NOT MEASUlcERIENT SENSITII'E
25 Januarv 1991 SUPERSEDING MILSI7D-410D 23 JULY 1974
MILITARY STANDARD NONDESTRUClTVi? TESTING PERSONNEL QUALIFICATION AND CERTIFICATION
AMSC N/A
AREA NDTI
DISTRJBUnON STATEMENT A. Approved for public release distribution is unlimited.
.
F O R E W O R D
1. This military standard is approved for use by all Departments and Agencies of the Department of Defense. 2. Beneficial comments (recommendations, additions, deletions) and any pertinent
data which may be of use in improving this document should be addressed to ASDE3ES. Wright-Patterson Air Force Base, Ohio 454334503. by using the self addressed Standardization Document Improvement Proposal @D Form 1426) appearing at the end of this document or by letter. 3. n/m,-STD-410E specifies the qualification and certification requirements for nondestructive testing/nondestructive inspection personnel. Previous revisions of this specification addressed the requirements for personnel using penetrant, magnetic particle, ultrasonic, eddy current and radiographic nondestructive testinglnondestructive inspection methods. This revision adds detailed requirements for acoustic emission and neutron radiographic methods as well as general requirements for any other nondestructive method for determining the acceptability of a product. In addition, this revision upgrades the designation of Level I, eliminates the Level I Special, adds an instructor level of qualification and adds a recertification requirement for Level III.
MILSTD-4IOE
CONTENTS
PAGE
PARAGRAPH 1. 1.1 1.2 1.2.1 1.2.2 1.3 1.4
SCOPE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applicability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Common methods . . . . . . . . . . . . . . . . . . . . . . . . . . Other methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . Levels of qualification . . . . . . . . . . . . . . . . . . . . . . . . Levels of certification . . . . . . . . . . . . . . . . . . . . . . . .
1 1 1 1 1 1 2
APPLTCABLE DOCUMENTS . . . . . . . . . . . . . . . . . . . 2 Non-Government publications . . . . . . . . . . . . . . . . . . . 2 Order of precedence . . . . . . . . . . . . . . . . . . . . . . . . . 2 DEFINITIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Certification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Certifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Closed book examination . . . . . . . . . . . . . . . . . . . . . . 2 Contracting agency . . . . . . . . . . . . . . . . . . . . . . 3 Documented . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Employer ............................... 3 Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Examination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Experience . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 General examination . . . . . . . . . . . . . . . . . . . . . . . . . 3 Indication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Instructor . . . . . . . . . ; . . . . . . . . . . . . . . . . . . . . . . 3 Interpretation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 On-the-job . . training . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Organ~zatlon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Outside agency . . . . . . . . . . . . . . . . . . . . . . . . . 3 Practical examination . . . . . . . . . . . . . . . . . . . . . . . . 4 Prime contractor . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Product form . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Qualification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Specific examination . . . . . . . . . . . . . . . . . . . . . . . . . 4 Technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Test samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Training ......................................... 4 ;
iii
MILSTD-310E CONTENTS PARAGRAPH
PAGE
4. 4.1 4.1.1 4.1.2 4.1.3 4.1.4 4.1.5 4.1.6 4.1.7 4.2 4.3 4.4 4.5
GENERAL F?EQUlREMENTS. . . . . . . . . . . . . . . . . . . . Certification procedure . . . . . . . . . . . . . . . . . . . . . . . . Levels of qualification . . . . . . . . . . . . . . . . . . . . . . . . Perso'nnel . . duties and responsibilities . . . . . . . . . . . . . . . Tralnrng program . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experience requirements . . . . . . . . . . . . . . . . . . . . . . . . Examination practices . . . . . . . . . . . . . . . . . . . . . . . . . . Records and documentation administrative practices ..... Recertification requirements . . . . . . . . . . . . . . . . . . . . . Personnel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Compliance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Outside agency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5. 5.1 5.1.1 5.1.2 5.1.3 5.1.4 5.1.5 5.2 5.2.1 5.2.2 5.2.3 5.2.4 5.3 5.3.1 5.3.2 5.4 5.4.1 5.4.2 5.4.3 5.4.4 5.4.4.1 5.4.4.2 5.4.4.3 5.4.5 5.4.6 5.4.7 5.5 5.6 5.6.1
DETAILED REQtJIREMENTS . . . . . . . . . . . . . . . . . . . . . 6 Levels of qualification . . . . . . . . . . . . . . . . . . . . . . . . . 6 Trainee . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Level I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Level I1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Jnstructor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Level . .ID . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Training . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Specialist personnel . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Exams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Minimum required training hours . . . . . . . . . . . . . . . . . 7 Previous training . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Experience . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Previous experience . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Equivalent . . experience . . . . . . . . . . . . . . . . . . . . . . . . 10 Exam~natlons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Physical . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Specific . . . . . . . : . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Practical . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Level l . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Level II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 LevelIII . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Administxation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Grading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Re-examination . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Designation of instructors . . . . . . . . . . . . . . . . . . . . . 12 Certification . . . . . . . . . . . . . . : . . . . . . . . . . . . . . . . 13 Records . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5 5 5 5 5 5 5 5 5 5 5 6 6
MILSTD-410E
CONTENTS PARAGRAPH
Loss of certification . . . . . . . . . . . . . . . . . . . . . . . . . . Reinstatement of certification . . . . . . . . . . . . . . . . . . . . Recertification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13 13 14
NOTES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Level1 Special ..................................... Intended Use ....................................... Subject tenn (key word) listing . . . . . . . . . . . . . . . . Changes from previous issue . . . . . . . . . . . . . . . . . . . . .
14 14 14 14 15
1. SCOPE 1.1 Pumose. This standard establishes the minimum requirements for the qualification and certification for personnel involved in the application of nondestructive inspection WI) or nondestructive testing (NDT) personnel. These requirements include training, experience and examination. 1.2 ~oplicability.This standard applies to personnel using NDI or NDT methods to accept materials, products, subsystems, components or systems for the Government, prime contractors or subcontractors. It also applies to those individuals directly responsible for the technical adequacy of the NDI and NDT methods used as well as those providing the technical training or supervision for NDI or NDT personnel. This standard is not intended to apply to individuals with administrative authority only over the above identified personnel or to research personnel developing technology for use by qualified and certified NDI or NDT personnel. 1.2.1 Common methods. This standard contains detailed requirements for the applicable training, experience, and examination for the following methods: Liquid penetrant Magnetic particle Mdy current Ultrasonic Radiography Acoustic emission Neutron radiography
(pr)
0
m) m
(RT) (AE) (NRT)
1.2.2 Other methods. This standard may apply to other NDI or NDT methods such as leak testing. thermography, holography, computed tomography. or any other method that can determine the acceptability or suitability for intended service of a material, part, component, subsystem, or.system without impairment of the intended function. The requirements for personnel training, experience, and examination for these other methods shall be as established by the contracting agency and shall be in accordance with the guidelines established for the methods listed in 1.2.1. 1.3 Levels of are: Trainee Level I Level I1 hstructor Level III
The levels of qualification established by this standard
1.4 Levels of certification. The levels requiring certification in accordance with this standard are: Level I Level I1 Level Lll 2. APPUCABLE DOCUMENTS 2.1 Non-Government ~ublications.The following documents form a part of this document to the extent specified herein. Unless otherwise specified, the issues of the documents which are DoD adopted are those listed in the issue of-the DODISS cited in the solicitation. Unless otherwise specified, the issues of documents not listed in the DODISS are the issues of the documents cited in the solicitation (see 6.2). AMERICAN SOCIETY FOR NONDBTRUCTIVE TESTING ASNT-CP-189 - ASNT Standard for Qualification and Certification of Nondestructive Testing Personnel AShT Recommended Practice No. ShT-TC-1A Certification in Nondestructive Testing
- Personnel Qualification
and
(Applications for copies should be addressed to the American Society for Nondestructive Testing, 1711 Arlingate Plaza, Columbus OH 43228-0518.) 2.2 Order of ~recedence.3n . the event of a conflict between the text of this document and the references cited herein, the text of this document takes precedence. Nothing in this document, however, supersedes applicable laws and regulations unless a specific exemption has been obtained.
3.1 Activiq. One of the organizational elements of an agency of the Department of Defense.
3.2 Certification. A written statement by an employer that an individual has met the applicable requirements of this standard. 3.3 Certifier. A designated representative of the employer with the responsibility and authority to document that an individual meets the applicable'requirements of this standard. 3.4 Closed book examination. An examination administered without access to reference material except that provided with or in the examination. Questions utilizing such reference material shall require understanding of the information contained therein rather than mere location.
3.5 Contracting agency. A government activity, prime contractor or subcontractor procuring the product requiring testing or the nondestructive testing services. 3.6 Documented. The condition of being in written form. 3.7 Emplover. The government activity, prime contractor, subcontractor, or outside agency employing individuals performing NDI or NDT. 3.8 Evaluation. The determination of the significance of relevant indications. 3.9 Examination. A formal, controlled, documented interrogation conducted in
accordance with a procedure. 3.10 Ex~erience.Actual performance or observation conducted during work time resulting in the acquisition of knowledge and skill. This does not include classroom or laboratory training but does include on-the-job training. 3.1 1 ~ e n e r a examination. l A written examination addressing the basic principles of the applicable NDI or NDT method. 3.12 Indication. The response, or evidence of a response, occurring during a nondestructive inspection or test. 3.13 Instructor. An individual qualified and designated, LAW this standard, to provide classroom or laboratory training for NDTMDI personnel and to administer and grade qualification examinations. 3.14 Interoretation. The determination of whether indications are relevant or nonrelevant. 3.15 Method. One of the disciplines of nondestructive inspection or testing (e.e. .radiography) within which different techniques exist. 3.16 Qn-the-iob train in^. Training. during work time, in learning insuumentation set up, equipment operation, recognition of indications, and interpretation under the technical guidance of a designated Level I[ or Level DI individual. 3.17 Organization. The entity, Government or private, having the responsibility of complying with this standard. 3.18 Qutside aeency. The organization under contract for NDI or NDT services which may include the training and examination of personnel to the requirements of this standard. Consultants and self employed individuals are included in this definition
3.19 Practical examination. The examination used to demonstrate an individual's ability in conducting the NDI o r NDT methods that will be performed for the employer. Questions and answers need not be written, but observations and results must be documented. 3.20 Prime contractor. The organization having responsibility to the government for a system, component, or materials. 3.21 Procedure. A detailed, written instruction for conducting NDI or NDT or certifying personnel. All procedures shall be approved by a Level ID. 3.22 Product form. Materials, parts, or components having similar NDI or NDT characteristics. Examples of individual product forms are: castings. extrusions, plate, aeldments, pyrotechnics, bonded assemblies, composite materials, and printed circuit boards. 3.23 Oualification. The skills, training, knowledge and experience required for personnel to properly perform to a particular Level. 3.24 Soecific examination. The written examination to determine an individual's understanding of procedures, codes, standards, and specifications for a given method used by the employer. 3.25 Techniaue. A category within a method, for example: ultrasonic immersion testing or fluorescent dye penetrant inspection. 3.26 Test samples. Parts containing known defects and used in the practical examination to demonstrate the candidate's proficiency in using a particular method. Test samples will not be production parts unless the Level ID has previously investigated the parts and documented all abnormal or out of specification conditions within the samples. Alternatively, test samples can refer to images of actual hardware, i.e.. radiographs, when the candidate's required proficiency is in the interpretation of the image rather than the generation of the image. 3.27 Training. An organized and documented program of activities designed to impart the knowledge a n d skills t o b e qualified to this standard. This program may be a mix of classroom, laboratory, programmed self-teaching and on-the-job training as approved by the appropriate Level III.
4 . GENERAL REQUIREMENTS
4.1 Certification ~rocedure.All organizations involved in any aspect of NDI or NDT
shall develop and maintain a procedure for the qualification and certification of their NDI or NDT personnel. This procedure shall be in accordance with the requirements of this standard. The procedure shall be available for review by the organization's customers. The procedure, as a minimum, shall include: 4 . l . i Levels of aualification. This shall include identification of the levels of qualification covered by the procedure. The organization may add any additional levels that are appropriate; however, in no manner can the organization eliminate or reduce minimum requirements of this standard in its qualification and certification procedure. 4.1.2 Personnel duties and responsibilities. This shall include the identification of the duties and responsibilities for the different levels of qualification. 4.1.3 Trainine Droeram. This shall include-outlines of the instruction provided by the organization as well a s sources of outside training utilized by the organization. 4.1.4 Exoerience reauirements. This shall include the techniques within the method and the minimum amount of time for each technique. 4.1.5 Examination oractices. This shall include the designation of the individuals or organizations that will perform the examinations as well as the number of questions. and the specific types of physical tests to be used. 4.1.6 Records and documentation administrative oractices. This shall include the description of the details to be recorded for each certified individual and identification of the individuals responsible for developing, administering, and maintaining the employer's certification program. 4.1.7 Recertification requirements. This shall include the employer's requirements for recertification of personnel. It shall also include the requirements for the loss and r&nstatement of certification. 4.2 Personnel. Personnel (Government. prime contractor, subcontractor, outside agency, etc.) performing, specifying, reviewing, monitoring, supervising, or evaluating . NDI or NDT functions for the purpose of accepting items for the Government shall be qualified to the appropriate requirements of this standard. Personnel performing specialized NDI or NDT, such a s ultrasonic thickness gauging or e l e d & l conductivity tests, with equipment designed for and limited to such usage and that produces clearly recognizable output for both acceptable and unacceptable conditions, do not require qualification to this standard. 4.3 Methods. For the common methods listed in paragraph 1.2.1 of this standard, the requirements for training, experience and examination are detailed in section 5 of this standard. These requirements, as well as those requirements contained in the two publications referenced in paragraph 2.1, shall serve as guidelines for those methods not listed in paragraph 1.2.1.
4.4 Com~liance.Prime contractors shall be responsible for compliance to this standard by their subcontractors. Those organizations utilizing outside sources for training or examination of their personnel shall be responsible for assuring that the appropriate requirements of this standard are met. The employer'is solely responsible for the certification of its employees and cannot certify for another employer. Individuals cannot certify themselves.
4.5 Outside agency. An employer may utilize an outside agency to develop a certification program, train and examine NDI or NDT personnel and perform any other Level LU function. An outside agency cannot certify personnel. The employer shall document the suitability of any outside source selected to perform any function to meet the requirements of this standard. This documentation shall be sufficient to justify that the outside agency is capable of performing the required Level III functions. 5. DETAILED R E Q W S 5.1 Levels of qualification. There shall be five levels of personnel qualification. 5.1.1 Trainee. A trainee is an individual who is participating in a training program for an NDI or NDT method and is not certified. Trainees shall obtain work experience only under the direct supervision of a Level Il, Level III or Instructor in the same method. Trainees shall not independently conduct tests, make accept or reject decisions, or perform any other NDI or NDT functions. 5.1.2 Level I.Level I is the first certifiable qualification level. The Level 1 certification shall be for a specific technique in a given method. The Level I individual shall have the skills and howledge to perform specific tests, specific calibrations, and, with prior written approval of the appropriate Level III individual, specific interpretations and evaluations for acceptance or rejection, and document the results in accordance with specific procedures. The individual shall be knowledgeable of any necessary preparation of parts before or after inspection. The individual shall be able to follo\r procedures in the techniques for which certified and shall receive the necessaiy guidance or supervision from an Level II or Level E l individual. 5.1.3 Level II. Level I1 individuals shall have the skills and knowledge to set up and calibrate equipment, conduct tests, and t o interpret, evaluate, and document results in accordance with procedures approved by the appropriate k v e l LU. The individual shall be thoroughly familiar with the scope and limitations of the method in which he is certified and shall be capable of directing the work of trainees and Level I personnel. The individual shall be able to organize and document NDI or NDT results. The individual shall be familiar with the codes, standards, and other contractual documents that control the method as utilized by the employer.
.,
5.1.4 In~rrucror. Lnstructors shall have the skills and kn~wledgeto plan, organize, and present classroom, laboratory, or on-the-job training programs of instruction, in accordance with approved course outlines, in the method for which appointed. The individual shall be familiar with the codes, standards, and other contractual documents that control the method , a s utilized by the employer. C/O*
5.1.5 Level
5I'4LC
JD. Level III individuals shall have the skills and knowledge to interpret
codes, standards. and other contractual documents that control the method as utilized by the employer; select the method and technique for a specific inspection; and prepare and verify the adequacy of procedures. Only individuals certified to Level I II shall have the authority to approve procedures for technical adequacy in the method to which they are certified. The individual shall also'have general knowledge of all other NDI or NDT methods utilized by the employer. The individual shall be capable of conducting or directing the training and examination of personnel in the method certified. The individual shall nor conduct NDI or NDT for the acceptance of parts unless the demonstration of proficiency in this capability was included in the practical examination upon which, in part, the certification is based. 5.2 Training. Candidates for certification as Level I or Level II shall complete sufficient organized training to become familiar with the principles and practices of the applicable test method and techniques. The training shall be conducted in accordance with a detailed course outline approved by a Level El. The training shall cover basic principles, products, equipment, operating procedures and techniques, and the applicable specifications, codes and instructions used by the employer. The supplements to SNT-TC-IA may be used to develop the training outlines. Subjects not covered in the instruction shall not appear on the training outline. The training outlines shall include the list of references from which the training material is derived.
5.2.1 S~ecialist~ersonnel.The training shall be presented by an Irstructor or a Level IU with the exception that specialist personnel not qualified to this standard may be used to provide instruction on highly specialized topics. Selection of such pers~nnel must be approved by the Level ID. 5.2.2 Exams. An individual must pass-a final exam in order to receive credit for a block of training hours. Such examinations given in conjunction with training shall not be used to satisfy any of the qualification examination requirements of section 5.4.
5.2.3 Minimum required trainine hours. The minimum training hours for Levels I and
In are given in table I for a variety of NDIMDT methods. The minimum training hours for those methods not covered by table I shall b e as determined by the Level III and agreed upon by the facility's customer. There are no additional training requirements to transition from Level II to Level ID nor can an individual have sufficient training to allow certification to Level IU without prior certification as a Level 11 or performance equivalent to a Level II.
RIILST?)-4 IOE
TABLE I. h4WTMU4 TRAJNING HOURS. LEVELS I AND II
CONDITION ~
O
D
[I1
I21
I31
Penetrant
8
8
16
Magnetic particle
12
8
20
Eddy current
12
40
52
Ultrasonics
40
40
80
Radiography
40
40
80
Acoustic Emission
40
40
80
Neutron radiography
28
40
68
[I ] k v e l I [2] Level 11, with prior Level I Certification 131 Level 11, no prior Level I Certification
5.2.4 Previous training. Training obtained from a prior employer must be documented and verified by the previous employer in order to be accepted by the current employer. For personnel credited with training from a prior employer or those not certified within 6 months of their training, refresher training must be provided. The refresher training shall cover the following subjects with the depth of coverage of each subject determined by the Level III responsible for the employer's certification program:
Standardization and calibration Operation of applicable test or inspection equipment Specific test o r inspection procedures Interpretation and evaluation of test or inspection results Safety Applicable codes, standards and specifications
5.3 Ex~erience.Candidates for certification at Levels 1. II or IU shall have sufficient
practical experience to assure that they are capable of performing the duties of the level for which certification is sought. The minimum requirements for Levels I, II and Dl are given in Table II. TABLE II. h4NMUvl EXPERIENCE REQUIREh4ENTS ,
CONDITION
PI
PI
[I1
[21
Penetrant
130 hrs
270 hrs
400 hrs
4 yrs
2 yrs
Magnetic particle
130 hrs
400 hrs
530 hrs
4 yrs
2 yrs
1 Yr
Eddy current
130 hrs
1200 hrs
1330 hrs
4 yrs
2 yrs
1 yr
Ultrasonics
400 hrs
1200 hrs
1600 hrs
4 yrs
2 yrs
1 yi
Radiography
400 hrs
1200 hrs
1600 hrs
4 yrs
2 yrs
1 Yr
Acoustic Emission
400 hrs
1200 hrs
1600 hrs
4 yrs
2 yrs
1 Yr
Neutron radiography 800 hrs
2400 hrs
3200 hrs
4 yrs
2 yrs
1 yr
METHOD
[51
,
[61
[I] Trainee experience for Level I. Experience in method must be at least half this tine. [2] Level I experience for Le\.el II. Experience in method must be at least half this time. [3] Trainee experience for direct certification to Level II. . Evperience in method must be a t least half this time. [4] Level Il experience required for Level
III with no college degree.
[5] Level II experience required for Level degree.
III with technical associate
[6] Level II or equivalent work experience required for Level E l with technical bachelors degree. Equivalency of the work experience shall be determined and documented by the Level III responsible for the employer's certification program.
5.3.1 Previous ex~eriencc.A candidate's experience with a previous employer may be accepted by the current employer only if that experience is documented and verified by the former employer. 5.3.2 Eauivalent ex~erience.For personnel certified under previous revisions of this document or other qualification/certification programs, the equivalency of their previous experience to the requirements of table TI will be determined and documented by the Level III.
5.4 Examinations. The examinations to verify the physical and technical qualifications of candidate personnel shall consist of a physical examination. a general examination, a specific examination, and a practical examination. The requirements for the physical examinations; the questions utilized for the general and specific examinations and the checklist for the practical examination shall be available for review by the facility's customers. If the actual test questions given during certification examinations are not kept in each certified individual's records, then the listing of questions from which examinations are derived shall be available for review by the facility's customers. The questions shall be made available to certification candidates only during administration of the examinations. 5.4.1 Phvsical. The physical examination shall assure that the applicants near vision and color perception meet the following requirements. Near vision tests shall be administered annually and color perception tests shall be administered prior to certification or recertification. These tests shall be administered by an individual approved by the Level III responsible for the maintenance of the certification program or by the outside agency utilized for the examination of personnel:
Near vision - Jaeger #I test chart at not less than 12 inches, or equivalent with one eye, either natural or corrected. Color oerception - Distinguish and differentiate between the colors used in the methad for which certification is sought. 5.4.2 General. The general examination for all levels shall be a closed book
examination consisting of questions that cover the cross-section of the applicable method at the appropriate level. The questions, answers, and references in the appliixble SNT-TC-IA supplement and other publications may be used to develop the general examination. A minimum of 40 questions shall be used for the general examination at each level. For Level IlI. the general examination questions will address the general knowledge of other methods as well as the method for which certification is sought. Possession of a current ASNT NDT Level III certificate by the candidate shall be satisfactory evidence that the general examination requirement is satisfied.
5.4.3 Suecific. The specific examination for all levels shall be a closed book examination and shall cover the specifications, codes, equipment, operating procedures, and test techniques the candidate may use in the performance of his duties. A minimum of 30 questions shall be used for the specific examination at each level. 5.4.4 Practical. The practical examination shall consist of a demonstration of
proficiency by the candidate in performing tasks that are typical of those to be accomplished in the performance of his duties. Test samples used in the examination may be actual hardware, if the candidate is required to demonstrate proficiency in the application of the process as well as interpretation of results, or may be images, such as radiographs, if the candidate is only required to interpret the results and not perform the process of generating the image. Written checklists covering the topics detailed below shall be developed by the Level IJIto assure adequate coverage and to assist in the administration and grading of the examination. 5.4.4.1 b v e l I.The candidate shall demonstrate proficiency by using the appropriate
method to examine at least one test sample for each technique to b e used and document the results. The test samples shall be representative of the products to be encountered by the candidate in the performance of his duties. The checklist shall address proficiency in the use of the procedures and equipment or materials, adherence to procedural details and the documentation of the results. If the Level I candidate is to accept products, then the checklist shall also include proficiency in the interpretation and evaluation of indications. 5.4.4.2 Level Il. The candidate shall demonstrate proficiency by using the appropriate
method to examine a t least one test sample for each technique. The candidate shall interpret, evaluate and document the results of the examination of the test samples. At least two test samples shall be evaluated for each method. The test samples shall be representative of the products to be encountered by the candidate in the performance of his duties. The checklist shall include proficiency in the use of the procedures and equipment or materials. adherence to procedural details, and the accuracy and completeness of interpretations and evaluations of indications. 5.4.4.3 Level. III. The candidate shall demonstrate proficiency by preparing an
NDIMDT procedure appropriate to his employer's requirements. When the candidate's duties will include inspection or evaluation of products, then proficiency in performance of such tasks shall be demonstrated also. The checklist shall address the practical and technical adequacy of the procedures prepared by the candidate, and when applicable, the adequacy of the interpretation and evaluation of indications. In the event that the candidate has already developed satisfactory procedures, then it is not necessary to develop another one for the practical examination. The results of the practical examination shall be documented. Procedures developed for a previous employer can be used to satisfy this requirement if their adequacy can be verified and documented.
5.4.5 Administration. A Level HI, knowledgeable and familiar with the specifications,
standards, codes, techniques and products associated with the employer, and certified Level III in the method for which the examinations are given, shall be responsible for the administration of all qualification examinations. The administration and grading of those examinations using multiple choice or truelfalse type questions can be delegated by the.level III. If an outside agency is used to provide this function, then the employer shall assure that the individual who performs the administration of the examinations is fully qualified. In no case can an examination be administered by one's self or by a subordinate. 5.4.6 Grading. The candidate for certification must achieve a minimum grade of 70%
on the general and specific qualification examinations. The candidate must detect all discontinuities or conditions specified by the Level HI during the practical examination and achieve a minimum score of 70% on the remainder of the practical examination. The candidate must have an average score of no less than 80% in order to be eligible for certification. All examination scores shall be of equal weight in determining the average score. 5.4.7 Re-examination. Candidates failing any examination (general, specific or
practical) shall receive additional training or wait at least 30 days before attempting re-examination. The additional training shall be documented and shall address those areas found deficient in the candidate's skills or knowledge. The re-examination shall not utilize the same questions or specimens that were used in the initial examination.
5.5 Designation of Instructors. Instructors shall be designated by the Level JJ3 responsible for the employer's certification .program and shall meet a least one of the . following criteria: a. Be certified to Level
in the method for which they will be designated Instructors
b. Possess the equivalent of a B.S. in engineering, physical science or technology and have adequate knowledge in the method for which they will be designated Instructors. c. Possess an associate's degree in physic21 science or technology and have a minimum of 5 years experience, or equivalent, as a Level II in the method for which they will be designated ~nstructors. d. Possess a minimum of 10 years experience as a Level 11, or equivalent, in the method for which they wili be designated Instructors.
5.6 Certification. Personnel who have demonstrated that they possess the appropriate
qualifications shall be certified by their employer in accordance with the employer's certification procedure. Certification is not required for personnel who are trainees or those who are designated as Instructors. 5.6.1 Records. The employer shall maintain certification records for personnel for as long a s their certification is in effect. Such records shall be available for audit by the facility's customers. The records shall include, as a minimum:
a. Name of the individual certified. b. Level, method, and techniques for which individual is certified, c. Results of all qualification examinations, including the separate test scores, that the individual has taken. d. Date and expiration of current certification(s). e. History of all previous NDTMDI certifications with current employer. f. Training history which identifies source and dates of training, course hours and
-grades (if given after training), and instructor's name.
g. Experience history, both with current and previous employers, sufficient to justify satisfaction of experience requirements for certification. h. Results of physical examinations. i. Extent and documentation of formal education. 5.6.2 Loss of certification. Certification may expire, be suspended or be revoked. Certification shall expire when employment is terminated or when the cenification interval has lapsed with no recertification attempted. Certification shall be suspended when the periodic physical examination is overdue, the individual does not perform in the method certified for at least 12 consecutive months, or the individual's performance is found to be deficient in any manner. Certification shall be revoked when the individual does not perform in the method certified for at least 24 consecutive months or the individuals conduct is found to be unethical or incompetent. 5.6.3 Reinstatement of certification. Certifications which have been suspended may be reinstated when the cause for suspension has been corrected and the correction verified by the employer. Certifications that have expired or been revoked may not be reinstated except by recertification.
5.6.4 Recertification. Level I and I1 personnel shall be recertified ar intervals not to exceed three years. Level lIl personnel shall be recertified at intervals not to exceed 5 years. The physical and practical examinations, equivalent to those required for initial certification, shall be given prior to recertification. The extent to which the individual's knowledge of the general and specific examination subject areas is examined shall be determined by the Level III responsible for the employer's certification program and shall be documented in the individuals certification records. 6. NOTES (This section contains information of a general or explanatory nature that may be helpful, but is not mandatory.) 6.1 Level I Soecial. The Leve! I designation in this revision is equivalenL to the Level I Special designation of MIL-STD-410D. The MIL-STD-4IOD Level I Special was limited to the ultrasonic and eddy current methods. Experience has shown that the Level I Special designation is an effective way of designating the entry level certification for nondestructive inspection and that it should be allowed a for all methods; thus the change was made in this revision. Because of the increased responsibilities assigned to the Level I, minimum required classroom training hours are no? specified (see table 1).
ur
6.2 Intended use. When invoked in a Request for Proposal (RFP),lnvitation for Bid of other similar document, the contracting agency should request that a copy of the offeror's existing qualification/certification procedure for NDI o r NDT personnel be included with the technical proposal. If the offeror has no existing procedure or if the existing procedure does not comply with this standard, then the contracting agency should request that the offeror's approach for establishing a procedure that complies with this standard b e included in the technical proposal. In addition, if the contacting agency intends that personnel using methods other than those listed in paragraph 1.2.1 be qualified and certified to this standard, then details on the offeror's approach to conducting such an effort should be requested as part of the technical proposal.
m),
6.3 Subiect term (key word) listing. Acoustic emission Certification Eddy current Liquid penetrant Magnetic particle Neutron radiography Nondestructive testing Qualification Radiography Ultrasonic
6.4 Chanees from orevious issue. Marginal notations are not used in this revision to identify changes with respect to the previous issue due to the extensiveness of the changes.
Custodians: Army.- MR Navy - AS Air Force - 11 Reviewer Activities: Army - AR
Preparing Activity: Air Force
- I1
(Project No. NDTI-0176)
INTERNATIONAL STANDARD First edition
1992-05-15
Non-destructive testing - Qualification and certification of personnel
Reierencc number
is0 9712:1992(E)
INTERNATIONAL STANDARD
I S 0 97121992(E)
Non-destructive testing personnel
- Qualification and certification of
.
;
For the purposes of this lnternational Standard, the following definitions apply:
This lnternational Standard establishes a system for the qualification and certification, by a cenlral independent body, of personnel to perform industrial nondestructive testing (NDT) using any of the following methods:
3.1 authorization: Permission to work. issued by the employer or responsible agency and based on the individual's suitability for a specific job. In addition to the certification. amongst othels the jobspecific knowledge. skill and physical ability could be assessed.
a) eddy-current testing; b) liquid-penetrant testing;
d) radiographic testing;
3.2 qualilication: A demonstration of the knowledge. skill. training and experience required to property perform NDT tasks.
e) ultrasonic testing.
3.3
c) magnetic testing;
certification: The orocedures. leading to a written testimony of the 4alification of an individual's competence in an NDT
The system described in this lnternational Standard may also apply to visual inspection. leak testing. neutron radiography, acoustic emission and other NOT methods where national certification pmgrammes exist.
L,;Cp
3.4
certificate: Written testimony of qualification.
J\ 3.5 naUonal ceNfying body: The agency that administers procedures for certification of NOT personnel in accordance with the requirements of this lnternational Standard.
,,,,/ \*
&' r2
Abbreviations qualifying body: A competent organizalion. independent of the employer or responsible agency, authorized by the national certifying body to prepare and administer examinations to qualify NOT personnel.
3.6
The following abbreviations shall be used to identify the five NDT methods covered by this International Standard: English Eddy current
f 3
CF
Liquid penetrant Magnetic Radiography
PT MT RT
RS MG RI
Ultrasonic Nondestructive testing
French Couranls de Foucault
Ressuage Magnetoscopie Rayonnements Ionisants UT US Ultrasons NOT END Essais non destructifs
candldate: The individual seeking certilication under the qualification and certification scheme.
3.7
employer or responslble agency: The organization lor which the candidate works on a regular basis. 3.8
NOTE 1
Candidates may be self-employed.
basic education: The minimum formal education required for qualification.
3.9
I1 may be used to determine duralion and level training and experience required prior to
b) The practical test lor levels 1 and 2 is to verify
'TE 2
3.10 NDT training: A process o l instruction in theory and practice in the NDT methods in which certification is being sought, which may take the form of training courses to an approved syllabus in addition to periods of practical work under qualified supervision but shall not include the use of specimens used in practical examinations.
ability to set Up and Operate test equipment, and perform the necessary settings to yield satislaclory test results. specific examination: The specific examination C-d" 4 includes both a written and a practical part for levels
3.11 experience: The period during which the candidale performed the specific NDT method as his main activity under qualified supervision. inciuding personal application ofthe NDT method to materials, parts or structures but not including tests performed during training courses. 3.12 NDT method: The application of a physical principle in nondestructive testing (for example: ultrasonic testing). 3.13 NDT technique: A specific way of utilizing an NDT method (for example: immersion ultrasonlc testing). 3.14 NDT procedure: An orderly sequence of ~ l e s which describe in detailed terms where, how and in .fhich sequence an NDT method should be applied ,o a product. 3.15 NDT inshuctions: A written document detailing the.precise steps to be lollowed in testing in accordance with an NDT procedure.
3.19
Iand 2. and only two written parts for level 3.
a) The written test is concerned with components, systems, equipment, operating procedures and test techniques commonly used in a particular industry or industrial sector. It involves the demonstration of knowledge related to the product being tested and covers the applicable specifications, codes and acceptance criteria. For level 3 only, this examination includes the writing of one or more satisfactory procedures. b) The practical test involves, for levels1 and 2. the demonstration of familiarity with and the ability to operate the necessary test equipment on prescribed mmponents and the ability to record and analyse the resultant informalion to the degree required.
&
3.20 lob-specific examination: Any additional examcnation concerned with the application of an NDT methog to a specialized product not commonly involved in a particular industrial sector. This examination. which supplements this International Standard. is carried out following written guidelines with results recorded to meet quality-assurance or customer-audit requirements.
-
This examination is outside the scope of lhis International Standard. NOTE3
3.16 industrial sector. A particular area in industry or technology where specialized NOT practices are utilized requiring specific skill. knowledge, equipment or training to achieve satisfactory performance. An industrial sector may be interpreted to mean a product (welds, castings, elc.) or an Industry (aerospace, steel, etc.). qualiIica~lonexamlnauon: An examination administered by the national certifying body or by an authorized qualifying body, which shall include a general examination and a specific examination for each level of competence.
3.17
3.18 general examinauon: The general examination includes both a written and a Practical part for levels 1 and 2. and only a written part for level 3.
3.21 bainee: An individual who works under the supervision of certified personnel but who does not conduct any tests independently. does not interpret test results and does not write reports on test results. This individual may be'regislered as being in the process of gaining appropriate experience to establish eligibility for qualilication lo level 1 or for direct access to level 2.
4 Levels of c o m p e t e n c e 4.1
Classification
An individual certified in accordance with this national Standard shall be classified in one of three levels depending upon the Individual's respective a) The written test Is mncerned with the principles whereas one who has not yet of Of the method and' at least lorlevel attained certllicallon may be registered as a trainee, level 3, covers basic knowledge of other NDT melhods. of materials and pmcesses. and of 4.2 NDT level 1 discontinuities arising through the use of various materials, manufacturing processes or service An individual certified to NDT level 1 is qualified to conditions. For level 3, the requirements for carry out NDT operalions in accordance with written certification of NDT personnel are also Included.
.
instructions and under the supervision of level 2 or level 3 personnel. The individual shall be able to set up the equipment. carry out the tests, record the resulls obtained, classify the results in accordance with written criteria, and report the results. He shall not be responsible for the choice of the test method or technique to be used. nor for the assessment of test results.
4.3
NDT level 2
An individual certified to NDT level 2 is qualified to perform and direct nondestroctive testing in accordance with established or remgnized techniques. The individual shall be competent to choose the test techniques to be used; to set up and calibrate equipment; to interpret and evaluate results i n accordance with applicable codes, standards and specifications; to carry out all duties for which a level 1 lndlvldual is qualifed and to check that they are property executed; to develop NDT procedures adapted to problems which are the subject of an NDT specification; and to prepare written instructions and organlze and report the results of nondestructive tests. The individual shall also be familiar with the scope and limitations of the method for which helshe is qualifed, and be able to exercise assigned responsibility for on-the-job tralnlng and guidance of trainees and NDT level Ipersonnel.
4.4
NDT level 3
An individual certified to NDT level 3 shall be capable of assuming full responsibility for a test facility and stall; establishing techniques and procedures; interpreting codes, standards. specifications and procedures; and designating the particular test methods, techniques and procedures to be used. The individual shall have the competence to interpret and evaluate results in anordance with existing codes. standards and specifications: have a sulficient practical background in applicable materials. labrication and product technology to select methods and establish techniques and to assist in establishing acceptance criteria where none are olherwise available: have general familiarity with other NDT methods; and have the ability to train level I and level 2 personnel.
5 5.1
G e n e r a l principles of certification Administration
The certification activity that includes ail procedures adopted to demonstrate the qualification of an individual to carry out tasks in a specific NDT method and leads to a written testimony of hislher competence shall be adminlstered in each country by the national certifying body, with the assistance. where necessary, of duly authorized qualifying bodles.
0) 5.2
(/
National certifying body
The national certifying body shall be a non-profit organization which has no direct involvement in training of NOT personnel and which is recognized by the I S 0 member body of the country Concerned. 5.2.f
Composition
The national certifying body shall be Supported by an adminislrative committee, which shall i n c l d e eminent representatives of NDT societies, cummittees, users, suppliers. government departments and other interested parties as appropriate. The NCB shall establish. In writing. the number of members of this committee, their qualifications (including education. training and experience), the means and extent of documentation of their qualifications. and their tenure. 5.22
Responsibilities
-
The national certifying body a) shall initiate, maintain and promote the national certification scheme as specified in this International Standard; b) shall administer the procedures and operations for certification in accordance with national documents meeting the minimum requirements of this International Standard, and a stringent code of ethics. including sanctions, which shall apply to committee members and certificate holders; c) may delegate, under its direct responsibility. the detailed administration of the certification pmcedure to other organizations which will act as qualifying bodies and which could represent industrial sectors: d) shall take the ultimate responsibility for the certilication scheme, including technical and adminislrative requirements; e) 'shall approve, either directly o r through a qualifying body, properly stafied and equipped examination centres which it shall monitor on a periodic basis and f)
shall keep ail appropriate records and issue, or delegate the issuing of, written testimonies. o r responslble agency --a-
The employer or responsible agency shall introduce the candidate to the national certifying body and document the validity of the personal information provided, including the declaration of educatlon.
:ng and experience needed to eslablish the el,w,lity o f (he candidate, but shall not be directly iniolved in the certification procedure itself. The employer or responsible agency shall be fully responsible for all t11at concerns the authorization to operate and the validity of the results of NDT operations.
proved by the national certifying body in that method. Table 1 and annex 8 are provided for guidance; however national certifying bodies shall lake into consideration education. certification in other methods, training facilities and other factors. Table 1 - Minimum duration of trainina Training hours
If the individual is self-employed, or introduces himself* he shall assume all responsibilites deskribed for the employer or responsible agency.
5.4
Examination c e n t r e s
Examination centres established by the national certifying body o r through authorized qualifying bodies shall, as a minimum requirement.
NDT method
1 1
Eddy-current testing Liquid-penetrant testing
1
1 1
40
l6
Radiographic testing Ultrasonic tesliog
I
use only specimens prepared or approved by the national certifying body for the practical examinations conducted at that centre.
1
80
(
40
I
Magnetic testing
a) have adequate qualified stan; premises and equipment t o ensure satisfactory qualification . . indusexaminations for the levels, methods and trial sectors concerned; b) use only those documents and .examination questionnaires established or approved by the national certifying body;
+._
.
40
80
40
80
1 Training hours include both praclical and lheoretical lrainlng courses.
I
2 Direct access to level 2 implies the total of the. hours shown lor levels 1 and 2
Whenmore than one authorized examination centre exists, each shall have specimens containing comparable defects. Under no circumstances shall examination specimens be used for training purposes.
6.3.2
6 6.1
Taking into account the scientific and technical potential of candidates for level 3 certification. it i s considered that preparation for qualification could be done in dilierent ways: by taking training courses. attending conrerences o r seminan such as organized by industrial or independent associations, and studying books. periodicals and other specialized printed matter. No training hours have therefore been specified in table 1, although references cited in annex B do suggest course content and duration.
General
Candidates shall have a combination o f education. training and experience adequate to ensure that they have the polential t o understand the principles and procedures o f the applicable NDT method.
6.2
Level 3
Eligibility for examination
Education
fvldence of education may be required to establish the eligibility o f a candidate.
6.4
6.3 T r a i n i n g
6.4.1
j.3.1 Levels 1 and 2 To be eligible to apply for certification in any NDT method. the candidate shall provide evidence of successful completion of a training programme ap-
.
Experience Levels 1 and 2
To be eligible lor certification. the candidale shall have the minimum experience Indicated i n table2 for the method in which helshe i s seeking certiSication. .
Table 2
- Minimum experience requirements
Table 3
- Minimum experience requirements lor level 3 Degree
NDT method
Experience (months)
four-year accredited science or engineering mllege or university programme
I
Successful mmpletion of at leas1 Wo years of engineering or science study at an accredited college. university or technical school
I
NOTES 1 Work experience in months Is based on a nomlnal 40 hlweek (175 h/month). When an lndivldual Is workIng more lhan 40 hlweek. helshe may be credited wilh experience based on the total hours. but helshe shall be required lo produce evidence of this experience.
I I
Direct access lo level 3 by a now ceriilied -&rator wilh experience euuivalent to level
2 For level 2 certification, lhe.intent of lhls International Standard is lhat work experience consists of time accrued as a level I . If the individual Is being qualified directly to level 2, wilh no time at level 1. the experience shall consist of the sum of the periods r e quired for level 1 and level 2
I
I
Graduate of a four-year accredited science or engineering college or university . pro. gramme Suacessful wmpletion of at least two years of engineering or xience study at an amedited college. university or technical school
3 Credit for work experience may be gained simultaneously in two or more of the NDT melhods covered by this International Standard, with the reduction i n total required experience as follows:
l
I I
a) two testing methods time by 25 %;
c) four or more testing methods - reduction of total required time by 50 %.
6.4.2
I No degree
- reduction of total required
h) three lesting methods -reduction of total required lime by 33 %:
The candidate shall be required to show Illat. for each of the testing methods for which helshe seeks certification, helshe has at least half of the lime required in labie2.
48
No degree
I
I I
Level 3
Level 3 responsibilities require knowledge beyond the technical scope of any specific NDT method. This broad knowledge may be acquired through a variety of combinations of education, training and experience. Table3 details rnlnimum experience related t o formal education. All candidates for level 3 certification in any NDT method shall have successfully completed the practical examination for level 2 i n that method.
I
NOTE - 11 the college or university degree is issued In non. deslludive testing. lhe experience required lor a m s to level 3 may be reduced by 50 %.
6.5. V l s l o n requirements
/
~
-
-
-
-
-
-
-
-
The candidate shall provide documented evidence o f satisfactory vision, in accordance with the following requirements: a) distant vision shall equal Snellen fraction 20130 o r better In at least one eye, either unmrrecled o r corrected;
b) near vislon shall permit reading a minimum o f Jaeger number 2, or equivalent type and size letters, at not less thanJ&cm on a standard Jaeger test chart for near vision, in at least one eye. corrected or uncorrected; c) colour vision shall be sumcient that the candidate can distinguish and dillerentiate contrast
between the colours used in the NDT method concerned.
b) the date of certification;
c) the date upon which certilication expires;
7 7.1
Examinations Examination content
The qualification examination shall consist of a general and a specific examination and normally m v e r a given NDT method as it is applied in one or more specific industrial sectors. For level Iand level 2. each of these two examinations shall include both a written and a practical test. For level 3, however, besides the written general examination. the specific examination shall consist of two written tests t o b e respectively designated 'specific (seclor)" and "specific (procedure)'. No level 3 practical test as such is required.
In the general examination, the candidate shall demonstrate sullicient proficiency in performing the NDT method. In the specific examination, he shall demonstrate his ability t o use the same NDT method In the industrial sector concerned.
-2
Administration o f examinations
All examinations shall b e conducted in examination centres established o r approved by the national certifying body. Detailed procedures for the s t ~ c ture, monitoring and grading of examinations by the nalional certifying body are contained in annex A.
Criteria applicable to re-examinalion with respect to (a) partial o r complete failure o f examination and (b) extension of certificalion t o other methods or sectors are described i n annex A: subclause A.l.5 refers to levels 1 and 2. and A.2.4 t o level 3.
8.1
Administration
Based o n the results of the qualification examinalions, the national certifying body, directly or through its authorized qualifying bodies, shall announce the certification. and issue cerlificates and/or corresponding wallet cards.
8.2
d) the level o f certification; e) the NDT method; I) the industrial sector(s) concerned: g) a unlque identification number; h) the siqnature of the individual certified: i) a photograph o f the individual certified and
j) the cold seal of the national certifying body o r the approved qualifying body cancelling the pholograph to avoid falsification. NOTE 4 By issuing the certilicale and/or the mrra sponding wallet card, the national certifying body or the qualifying body attests lo the qualification of the individual but does not give any authority lo operate. There may be a special space on both lhe certilicale and lhe wallet card for ihe signature of the employer or responsible agency authorizing the holder of the cwtificale lo operale and taking responsibilily for leal results. This authorization also serves as testimony of aclivity of the certified lndividual.
9 9.1
Validity and renewal Validity
The period of validity shall not exceed a maximum o f live years from the date of certification indicated on the certificate and/or wallet card. Certification shall be invalid a) i f the lndlvidual changes from one industrial sector to another. i n which case he/she shall successfully mmplete supplementary examinations lor the new industrial sector; b) at the option of tile national certifying body afler reviewing evidence of unethical behaviour; c) i f the individual becomes physically incapable of perlormlng hls/her dutles. based upon the visual examination taken at least every second year under the responsibilily of his employer o r responsible agency.
Certiflcates a n d wallet c a r d s
9.2 Zertificates and corresponding wallet cards shall bear: a) the name of the individual certified;
Renewal
ARer the first period of validity. certification may be renewed by the national certifying body, directly or through an authorized qualilying body, for a new
period of similar duration, provided the individual meets the following criteria:
national certifying body will have the option of replacing this simplified examination by an alternative. structured credit system under its control).
a) helshe provides evidence at least every second
year of satisfactory visual examination and b) heishe provides evidence of continued satisfactory work activity without significant interruption.
if the individual fails to achieve a grade of 80 % or better in the simplified examination, helshe shall apply for new certification.
NOTE 5 A significant interruption means an absence or a change of activity which prevents h e certifiedindividual
10
from practising the duties corresponding to his/her level in the method and the industrial sector(s) for which helshe is certified, for one or several periods for a total time exceeding one year.
The national certifying body or its authorized quatiwing bodies shall keep
If the criteria for renewal are not met. the individual shall apply for recertification.
9.3
Recertification
Upon completion of each second period of validity, or at least every ten years. certification shall be renewed by the national certifying body. directly o r through an authorized qualifying body, for a similar period, provided the individual meets the two criteria for renewal and successfully completes a slmplified examination to assess hislher current knowledge. This simplified examination shall consist of:
Files
a) an updated list of all Individuals certified. classified a m r d i n g to level. test melhod and industrial sector; b) an individual file for each lndividual certified and for each individual whose certification has been withdrawn, containing I)application forms.
2) examination documents, including questionnaires, answers. descriptions of specimens, records, results of tests. written procedures and/or techniques, and grade sheets, 3) renewal documents, including evidence of
physical condition and continuous activity.
a) Level I and level 2: a practical examination or. ganized in accordance with a simplified procedure:
4) reasons for any withdrawal of certification and details of any further penalty inflicted.
b) Level 3: a written examination which includes 20 questions on the application of the test method in the industrial sector concerned and 5 questions on this International Standard (the
Individual liles shall be kept under suitable conditions of safely and discretion for a period at least equal to the total of the initial period of validity plus the renewal period.
Annex A (normative)
Administration of examinations A.l
Examinations for l e v e l 1 and l e v e l 2
A.l.l
Table A.l
- Required number of questions General examination
Qualification examination
Number of questions The qualification examination administered under this International Standard shall include a general examination and a specific examination for each level of competence. Each examination shall mnsist of a written parl and a pradlcal part The pradlcal parl shall be of sullicient duration, complexity and smpe to verify adequately the candidate's ability to apply the NDT method to real test situations.
A.1.2
NDT method
Examination content
1.21 General exarnlna~on
of
basic-knowledge
questions valid at the date of examination. The candidate shall be required, as a minimum, to give anto the fixed number Of multiple-choice questions shown in tableA.1. The practical test in the general examinalion is to verify the candidate's ability to make the required settings and operate the test equipment properly in order to obtain satisfactory results and correctly interpret these results. The candidate shall therefore be required to demonstrate this ability. with mmments; using the means of verification>vailable for each test method. such as calibration blocks, Image-quality, indicators and magnetic-field lndifalOE.
For the radiographic test method, there shall be an additional examination on radiation safety.
"OTE 6
Eddycurrent testing
30
30
Liquidpenelrant testing
30
30
Magnetic testing
30
30
Radiographic testing
40
40
Ultrasonic testing
40
40
Examinations on the radiographic test method .ay indude either X- or garnrna-radiation, or both, depending upon the procedure of the national certirying MY.
Specific examlnation
In .the specilic examlnation, the candidate shall demonstrate his ability to use the relevant test method in the industrial sector concerned.
The written test in the general examination shall include only questions selected from the national cer-
collection
Level 2
A.1.22
In the genera! examlnation, the candidate shall demonstrate proficiency in performing the relevant NDT method.
body's
Level 1
'
The written test in the specific examination shall include only questions selected from the national certifying body's current mllection related to all industrial sectors or from the mllection of specific questions maintained by an authorized qualifying body related to the industrial sector concerned. During the specific examination. the candidate shall to a fixed number of be required to give questions. as defined in tableA.2. including multiple-choice questions. calculations. written procedures and questions on codes, standards and specilications. ~ h , practical test inthe specific examination is to verify the ability to perlorm testing of prescribed components relating to the industrial sector concerned, and to record and analyse the resultant information to the degree required, accurding to specific testing instructions or specifications. and to the NDT level being sought. The specimens used for the practical test shall be Selected from a ml~ection of representative specimens chosen b~the national certifying body or by its authorized qualifying body.
For level 2. the candidate shall be required to demonstrate the ability to prepare written instructions for level 1 personnel.
cedure which includes at least ten check points. 7his procedure shall be developed by the national certi. lying body o r an authorized qualifying body.
If the practical test in the specific examination covers two o r more industrial sectors, the number of specimens to be tested shall be increased proportionally to examine the candidate's competence in each of the industrial sectors concerned.
A candidate for a practical examination may use his own apparatus. The examiner shall investigate the reliability o f the test apparatus made available to the candidate. and unreliable apparatus shall be re_ placed. as well as any apparatus that may be rendered unserviceable during the course of the examination. Any item of apparatus brought by a candidate that i s unreliable o r rendered unserviceable during the examination shall be replaced by the candidate himself.
Table A.2
- Required number of questions Specific examlnation Number of questions
NDT method Level 1
Level 2
Eddy-current testing
15
15
Liquid-penetrant testing
20
15
Magnetic testing
20
15
Radiographic testing
20
20
Ultrasonic testing
20
20
A.1.4
The general examination shall be graded separately from the specific examination so that the candidate may be examined later for certification in another branch of industry without having t o take the general examination again; thus a certified operator changing from one industrial sector t o another keeps the benefit of the general examination valid for all industrial sectors. To be certified. the candidate shall obtain a grade of at least 70 % in each of tfi&ur tests I examination ana a cumnosite arade of at least
-
If the written part o f the specific examination covers two or more industrial sectors. the number of questions shall be increased proportionately t o reasonably cover each o f the industrial sectors, and evaluated accordingly.
A.1.3
The composite grade for.the respeclive level shall be determined by adding the weighted marks obtained from multiplying each of the four test marks by a weighting factor t o be selected from tableA.3. The total of the selected weighting factors shall equal 1.00.
Conduct of examinations
All examinations shall be conducted in examination centres approved and monitored by the national certifying body, either directly or through an author-' ized qualifying body. At the examination. the candidate shall have in his possession a valid proof o f identification and a n official notification o f the examination, which shall be shown to the examiner o r invigilator on request. Any candidate who, during the course of the examination, does not abide by the examination rules o r who perpetrates, o r is an accessory to, fraudulent conduct shall be excluded from further participation. The written and practical tests shall be conducted and supervised by an examiner chosen among NDT level 3 personnel and designated by the natlonal certifying body, either directly o r through an authorlzed qualifying body. The examiner may be assisted by one o r more invigilators placed under his responsibility. The examiner shall mark the written tests completed by the candidate; he shall judge and mark the results of the practical tests in accordance with a oro-
Grading
/(
Table A.3
/c,
- Weighting factors for gradlng - Levels 1 and 2
I Level
I
Weighting factor General
Speciflc
Written
Practical
Written
Practical
1
0.2 to 0.4
0.2 to 0.4
0.2 to 0.4
0.2 to 0.4
2
0.2 to 0.4
0.2 lo 0,4
0.2 to 0.4
0.2 to 0.4
1
A candidate failing for reasons of unethical behaviour shall wait at least 12 months before reapplying. A candidate who fails to obtain the pass grade for the whole examination may take one. and only one. retest in a maximum o f two parts. provided the minimum percentage (70%) was obtained i n each
-t and that retesting takes place within 12 months o f the failed examination.
A candidate for re-examination shall apply for and take the examination in accordance with the procedure established far new candidales. A certified operator wanting to extend certification in a given NOT method to new industrial sectors keeps the benefit of the general examination and shall be required to take only the related specific examination.
A2 A.21
E x a m i n a t i o n s f o r level 3
A.2.1.2
Specilic examlnation
The specific examination shall include two parts, to be marked separately. The first part is designated "specific (sector)" and the second "specific (procedure) ". The specific (sector) test shall include 20 questions on the application of the NDT method in each industrial sector concerned. The necessary questions shall be chosen from a list maintained by the national certifying body o r by an authorized qualifying body. The specific (procedure) test shall require the drafling of one or more satisfactory NOT procedures.
Examination content
The qualification examination for level 3 candidates shall consist only of a written examination. normally covering a specified test method applied in one or more industrial sectors. This examination shall cover a) basic knowledge relating to the test method applied for and to materials, processes and dis2 general-examination continuities; level questions relating to at least two other test methods covered by this International Standard and selected by the candidates themselves; and requirements for the certification of NDT personnel; b) specific knowledge relating to the application of the NOT method i n which the candidate is being examined in the industrial sector concerned. including the applicable codes. standards and specifications, plus knowledge of the product being tested. If the candidate is not certified to NOT level 2 at the time of application, then helshe shall also successfully complete the level 2 practical examination in the relevant NOT method.
The general examination shall include only l~iultiple-choice questions, selected from the national certifying body's collection of basicknowledge questions valid at the date of the examination. The number of questions shall be as follows: a) 30 questions on the main test method and materials, processes and discontinuities: b) 10 level 2 questions on each of at least two additional test methods; c) not less than 5 questions o n the personnelcertification scheme.
A.2.2
Conduct of examinations
All examinations shall be conducted in examination centres established or approved by the national certifying body. and shall be monitored by the national certifying body, directly or through an authorized qualifying body. At the examination, the candidate shall have i n his possession valid proof of identification and an official notilication of the examination, which shall be shown to the examiners o n request. Any candidale who. during the course of the exa'mination, does not abide by the examination rules o r who perpetrates, or is an accessory to, fraudulent condud, shall be excluded from further participation. Examinations shall be conducted and supervised by at least two examiners chosen among level 3 operators and designated by the national certifying body. directly or through an authorized qualifying body. Each examiner shall correct and grade separately the dillerent parts of the examination i n accordance with procedures established by the national certifying body. During a meeting, each o f the examiners shall present and explain his grades, and a n average grade shall be calculated for each part o f the examination.
The written general examination shall be graded separately so that the candidate may be examined later for certification in another branch o f industry without having to repeat the general examination. To be certified, the candidate shall obtain a grade of at least 70 % in each part of the examination and a composite grade o f at least 80 %. The composite grade for the respective level shall be determined by adding the weighted marks obtained from multiplying the test marks in each part
..
of the examination by a weighting factor to be selected from tableA.4. The total of the selected weighting factors shall equal 1.00. Table A.4
- Weighting factors for grading Level 3
minimum percentage (70 %) was obtained in each part and that retesting takes place within 12 months of the first failure. In the case of a second failure to obtain the pass grade, the candidate shall be re. examined in all three Parts. A candidate for re-examination shall apply for and take the examination in accordance with the procedure applicable to new candidates.
A certified operator changing from one industrial sector to another, but who keeps using the same NDT method, retains the benefit of the general examination and shall be required to take only the two specific (sector and procedure) examinations concerning the new industrial sector. A candidate failing for reasons of unethical behaviour shall wait at least 12 months before reapplying. A candidate who fails to obtain the pass grade for the whole examination may take one, and only one, retest in a maximum of two parts, provided the
A special procedure may be apptied in the case of a candidate taking examinations for certification in several testing methods within a period of one year. to avoid the duolication of level 2 ouestions relatino to the additiorial test methods & well as thos; questions relating to codes or standards and the certification scheme.
Annex B (informative) Technical knowledge of NDT personnel 6.1
General
This annex provides a bibliography of international publications detailing course content. The minimum hours of training recommended to confirm eligibility for examination are detailed in the main text of this International Standard.
8.2
[2] The cdrnpiete Recommendations o n international harmonization offraining qualification and cerfilication or nondestructive testing personnel. Prepared by the lnternational Cornrnittee on Non-Destructive Testing. adopted November 1985. Available from tlie Foundation lor the Qualification o f NDT Personnel. P.O. Zoetermeer. The Box 190. 27M1- AD
.'
References
C13 Technical Document IAEA-TECDOC-407 (1987). Training guidelines in nondestructive testing techniques, International Atomic Energy Agency, WagramrnerstraCe 5. P.O. Box 100. A-1400 Vienna. Austria.
[3] ASNT recommended praclice SNTITC-IA 1988 Edition. Tables I-A l o I-H (recommended training courses). Published by the American Society for Non-destructive Testing. 1711 Arlingate Lane, P.O. Box 28518. Columbus. Ohio 43228-0518. USA.
GENERAL DESCRIPTION
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LIQUID PENETRANT TESTING A.5
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Liquid Penetrant testing is a quick and reliable nondestructive test method used for detecting various types of discontinuities which are opened to the surface of a material or part. During normal operation, critical components of aircraft engines, airframes, missiles, space vehicles, nuclear reactors, and other modern machinery, are often subjected to extreme loads and vibrations. In time, these extreme loads and vibrations may cause a component to develop an intemption in its normal physical structure or configuration. This is called a DISCONTINUITY.. Although the discontinuity may not affect the usefulness of a part when it occurs, or even alter the parts appearance to the naked eye (since the discontinuity may be minute) repeated stresses or overloading may eventually cause that part to fail. It can be seen therefore, that detection of small discontinuities before they progress into a DEFECT, which is detrimental to part serviceability, is of vital importance to prevent loss of equipment and personnel. Failure of the part may cause one of the following: $-~)s&~2 P-0 1 ._Maior Repair: "Down %me" for major repair caused by part failure is expensive inc_;u) &&& ,A , (2 terms %st time. 2. Lpss of Eclui~meG:Total loss of equipment due to part failure is expensive in terms of lost time and equipment. \o\\,2 . 7 b ' -' 3. Loss of Personnel: Total loss of the equipment may result in the loss of operating .$, @-' personnel. -3 t
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PENETRANT INSPECTION CAPABILITIES Penetrant inspection can detect open to the surface discontinuities, such as: Ip +6+.r.~ 23 f ~ r a c k s Laps . yoPorosity (hole through a wall) --:I' Leaks ,*'<& Seams Pits + ,') , "
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bD
~ndercut.~
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Note: This is only a partial listing. A listing of all discontinuities caused by metal and non-metallic material preparation, material forming, and material processing would be too unwieldy for this study guide. Penetrant inspection can be used w$h reliable accuracy on the following nonabsorbent materials: 1. 2. 3. 4. 5. 6.
Aluminum Magnesium Brass Copper Titanium Bronze
7. 8.
9. 10. 11. 12.
Cast lmn Stainless Steel Non-Magnetic Alloys Ceramics .. Hard Rubber Plastic
Caution: As some plastics, rubber, and synthetic products may be affected by oil, tests should be made before penetrant inspecting such materials to avoid damaging the part under test.
\ \
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BASIC PRINCIPLES OF PENETRANT INSPECTION
xv*\
The basic principle of penetrant inspection is capillary action. Capillary action is the action by which the surface of a liquid, where it is in contact with a solid, is elevated or depressa The materials, processes, and procedures used in liquid penetrant testing are all designed to facilitate capillarity and to make the results of such action visible and capable of interpretation. The forces of capillarity, or capillary action, may be obsewed when a plastic straw is inserted into a glass of water. When the straw is inserted, the water molecules enter the straw and begin to attract other nearby molecules, pulling them up the straw by cohesion. This process continues as the water rises higher and higher. The water continues to rise until the pull of the surface tension is equalized. Cohesive forces prevent the water from falling back down the straw. Capillary action as applied in n O n d e S t ~ ~ ttesting i ~ e is somewhat more complex, since various surface conditions hindering or assisting the action are encountered. Liquid penetrants in nondestructive testing have low tension and high capillarity. Capillary action is illustrated in Figure 1-1.
$6 WATER LEVEL IN STRAW WATER LEV€!.IN GLASS
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Figure 1-1 CAPILIARY ACTION
The basic objective of liquid penetrant inspection is to increase the visible contrast between the discontinuity and its background. This is done by treating the whole object with an appropriag searching liquid of high mobility and penetrating power (which enters the surface opening of the discontinuity), and then encouraging the liquid to emerge from the discontinuity to reveal the flaw pattern to the inspecting personnel under daylight conditions (visible dye penetrants) or, when exposed to black light (fluorescent penetrants).
.-
There are several methods by which the basic principles of penetrant inspection can be administered. -In each method, however, there are certain general procedures which must be followed. GENERAL PROCEDURES FOR PENETRANT INSPECTION The following are general procedures for penetrant inspection:
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1. Selection of the Aoorooriate lnsoection Process: ,,
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The appropriate inspection process shall be determined by the testing facilities available, the type and amount of parts to be tested, and the results anticipate desired. 2. Pre-Testing: If the material to be tested could be affected by oil, sulphur or c tests shall be performed to ensure that the parts are not damaged, when placed .L-8 under penetrant inspection method test. 3. Pre-Cleaning: The part to be inspected shall be pre-cleaned in order to remove any contaminating material. A .< , CAUTION: Inadequate pre-cleaning is the source of most of the false indications encountered. 24 4. Pre-Drying: Parts which have been precleaned shall be dried to remove all traces of j ,yJ2L.A 2 . p cleaning material. 5. Penetrant Application: Penetrant shall be applied to a part under test in a manner c&'d \ /.appropriate to the type of part or facilities available. Sufficient dwell time shall be allowed for optimum penetration. Figure 1-2. \ fee +
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Fguro 1-2 PENETRANT APPLICATION AND DWELL TIME
J
2
6. Penetrant Removal: Penetrant shall be removed from the surface of the part under test
in the manner dictated by the type of penetrant used. Figure 1-3.
Fgure 13
REMOVAL OF EXCESS SURFACE PENElRANT
7. Developer Application: Developer shall be applied to the part under test as appropriate to the process being used and the configuration of the part under test. Sufficient dwell time shall be allowed for optimum results. Figure 1-4. 3
figure 1-4 DEVELOPER APPLICATION
,
i I
8. Inspection Interpretation: The part shall be inspected and the discontinuity interpreted
and evaluated to the applicable acceptance standard. Figure 1-5.
Fgure 1-5 lNSPECTlON AND INTERPRETATION OF INDICATIONS
t 9. Post-Cleaning: The developer shall be removed after inspection interpretation and prior to returning the part to service.
PENETRANT SELECTION FACTORS The proper selection of a penetrant to be used for penetrant inspection is dependent on many factors such as penetrabilu visibility, particular type of discontinuity sought, configurationof part, surface conditions, facilities and equipment available, etc. Selection of the proper penetrant, therefore, should be based on penetrant sensitivity. PENETRANT SENSITIVIPI: Penetrant Sensitivity is herein defined as the ability of the penetrant, along with compatible family items in its group, to effectively find discontinuities of the type sought under the. penetrant inspection circumstances involved. Using this definition, the penetrant most adaptable to the majority of penetrant inspection conditions that will exist, is the proper penetrant. COMPATIBILITY: Penetrant materials supplied by qualified producers are not compatible or interchangeable for the purposes of penetrant inspection. Use only one manufactureCs group of materials in an inspection line or portable inspection operation. This is known as a farnilygroup, and intermixing of families is not permitted unless the "mixed family" has been previously qualified. + PENETRANT MATERIALS Penetrants: Penetrants are classified by Method and Type as follows: Method A Fluorescent dye c+&+$\a Method B Visible dye TYPe 1 Water-washable Post emulsifiable, lipophilic, or , , Type 2 Post emulsifiable, hydrophilic Type 3 Solvent removable Emulsifiers are classified as either: Emulsifier: An emulsifier that is water-soluble Hydrophilic An emulsifier that is oil-soluble and not water-soluble Lipophilic Solvent removers are classified as follows: Solvent Remover: Halogenated Non-halogenated / Developers: Developers are classified by form as follows: Dry powder I ;./ Water soluble Water suspendible Nonaqueous Specific application (i.e.Plastic film) All penetrant materials are supplied in either bulk form or in small pressurized canisters.
, '
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F
SELECTION OF LIQUID PENETRANTTEST METHOD When a specific liquid penetrant test method is not specified by the contract, the selection of a suitable penetrant inspection process is made by the Level Ill who makes this decision based on seven basic factors. 1. Requirements previously established by component drawings applicable documents on material or Darts to be laced under examination. 2. Type and siie of disc&tinuity to bGetected. 3. Suriace c ~ n d i t i gof i part to be examined. - - jw;bc4, 'mih) 4. Configuration of part to be examined. . 5. The number of parts lo be examined. -+ i 6. Facilities and equipment available. 7. Effect of the penetrant chemicals on material being examined.
f %.iY.3 i
PT Ill BASIC
-
TABLE 1 ASME CODE CLASSIFICATION OF LIQUID PENETRANT METHODS AND TYPES METHOD A -FLUORESCENT PENETRANTS Type 1 Water Washable Penetrant (Procedure A-1) Dry, Wet, or Nonaqueous Developer Type 2 Post-emulsifiable Penetrant (Procedure A-2) Lipophilic or Hydrophilic Emu!sifier Dry, Wet, or Nonaqueous Developer Type 3 Solvent Removable Penetrant (Procedure A-3) Solvent Rernover/Cleaner Dry. Wet, or Nonaqueous Developer METHOD 8--VISIBLE PENETRANTS Type 1 Water Washable Penetrant (Procedure B-1) Wet or Nonaqueous Developer Type 2 Post-emulsifiable Penetrant (Procedure B-2) Lipophilic or Hydrophilic Emulsifier Wet or Nonaqueous Developer Type 3 Solvent Removable Penetrant (Procedure 8-3) Solvent RemoverICleaner Wet or Nonaqueous Developer
PT Ill BASIC
TABLE 1.a MIL STD 6866 CLASSIFICATIONOF LIQUID PENETRANT METHODS AND TYPES
TYPE Type I Type II Type Ill
Fluorescent Dye Visible Dye Dual mode (visible and fluorescent dye)
METHOD Method A Method B Method C Method D
Water-washable Post emulsifiable, lipophilic Solvent removable Post emulsifiable, hydorphilic
SENSITIVITY Level 1 Level 2 Level 3 Level 4
Low Medium High Ultrahigh
DEVELOPERS Form a Form b Form c
Form d Form e
Dry powder Water soluble Water suspendable Nonaquesous Specific application
SOLVENT REMOVERS Class (1) Hologenated Non-halogenated Class (2) Class (3) Specific application
METHOD A TYPE 1 INSPECTION PROCESS The Method AType 1 Penetrant Inspection process uses a water-washable fluorescent penetrant and a dry, wet, or non-aqueous wet developer. The penetrant has self-emulsifying properties to make it water removable. Method A Type 1 Process is generally used when: 1. Examining large volume of parts. 2. Discontinuities are not wider than their depth. . 3. Surfaces are very rough (i.e., sand castings, rough weldments). 4. Examining large areas. 5. Examiningthreads and keyways. 6. The lowest fluorescent penetrant sensitivity is sufficient to detect the discontinuities inherent to the part. 7. Removal of excess penetrant may be difficult due to rough surfaces. 8. Sulphonates in emulsifying agents will not affect nickel bearing TABLE 2 ADVANTAGES AND DISADVANTAGES OF SPECTION PROCESS
--
... ..~.. ... ..
1. The use of fluorescence ensures good
visibility of flaw indications. , n\
2. Process is not reliable in finding
2. Process can'be cofisidered as a one-step
d-i
i.'process and, therefore, fast and economical.
scratches and shallow discontinuities.
->-,.
3. Process &n be used for detecting a wide
3. Penetrant can be affected by acids
-
ard chrom!es.
-.of. discontinuities. range
4. Penetrant used can be easily washed off with
water. 5. Process is easily adaptable to a large volume
of small parts.
4. Process is not reliable on anodized
1
surfaces.
1
5. Process is susceptible to over-
washing.
6. Process is excellent for rough surfaces,
keyways, and threads. 7. Process is relatively inexpensive.
6. Water contamination may destroy \\
'
\
usefulness of penetrant.
17. Not good forwide shallow 1
PT Ill BASIC
discontinuities (width greater than
METHOD A TYPE 2 INSPECTION PROCESS The Method A Type 2 Penetrant Inspection process uses a post-emulsifiable fluorescent penetrant, a lipophilic emulsifier, and a dry, wet, or non-aqueous wet developer. The materials used in this process are very similar to that described for Method A Type 1 process, except that these penetrants are not selfemulsifiable. A lipophilic or hydrophilic emulsifier is used to make the penetrant water washable. METHOD A TYPE 2 INSPECTION PROCESSES ARE GENERALLY USED WHEN: 1. Examining large volume of parts. 2.
1-
A higher sensitivity than Method A. Type 1 is required or 'desired.
3.
y7 1
penetrants. 4.
5.
?j ,I/
The part is contaminated with acid or other chemicals that will harm'water-washable
Discontinuities are wider than their depth. Variable, but controlled, sensitivities are necessary so that nondetrimental discontinuities can be disregardedwhile harmful or detrimental discontinuities are
6.
detected. Examining parts which may have discontinuities contaminated with in-sewice soils.
7.
Examining for stress, cracks or intergranular corrosion.
8.
Examining for grinding cracks.
9.
High visibility is required.
-
TABLE 3 ADVANTAGES AND DISADVANTAGES OF METHOD A TYPE 2 INSPECTION PROCESS,/"
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ADVANTAGES 1. Fluorescence used in this p w s s is more
brilliant, thus ensuring greater visibility of flaw indications. 2. High sensitivity for very fine discontinuities.
3. Good on wide shallow discontinuities.
(width greater than depth)
I
DISADVANTAGES-1. Process is a two-step process, and
therefore requires more time. 2. Additional equipment is required for
application of the emulsifier. 3. Not as good on parts with complex
shapes (i.e. threads) as Type 1.
4. Process good for high volume production.
4. Additional material increases cost.
5. Process normally not affected by acids.
5. Emulsifier dwell time very critical.
6. Process not as susceptible to over-washing.
PT Ill BASIC
METHOD A TYPE 3 INSPECTION PROCESS The Method A Type 3 Penetrant lnspection process uses a solvent-removable fluorescent penetrant, a penetrant remover (solvent) and non-aqueous developer. The penetrant is not water-washable but is removed instead with the solvent remover. Method A Type 3 lnspection Process is generally used when: 1.
Spot examination is required.
2.
Water-rinsing method is not feasible because of part size, weight, surface condition, no water available, no heat for drying, or field use. TABLE 4 ADVANTAGES AND DISADVANTAGES OF METHOD A TYPE 3 INSPECTION PROCESS ADVANTAGES
DISADVANTAGES
1. Process can be used for spot inspection
1. Use of solvent to remove penetrant
prohibits inspecting large areas.
on large parts. 2. Process can be used when water-rinsing
methods are not feasible.
+-
2. Sensitivity can be reduced by the
application of excessive amounts of remover.
METHOD B TYPE 1 INSPECTION PROCESS Method B Type 1 Penetrant lnspection process uses a water-washable visible dye penetrant and wet or non-aqueous developer. The penetrant has self-emulsifyingproperties to make it water removable and is of a brilliant red color. Method B Type 1 Process is generally used when: 1.
The lowest sensitivity is sufficient to detect the discontinuities inherent to the part.
2.
Examining large volume of parts.
3.
Discontinuities are not wider than their depth.
4.
Surfaces are very rough (i.e., sand castings, rough weldments, pitted areas).
5.
Examining large areas.
6.
Examining threads and keyways.
7.
Removal of excess penetrant may be difficult due to rough surfaces.
PT Ill BASIC
TABLE 5. ADVANTAGES AND DISADVANTAGES OF METHOD B TYPE 1 PROCESS ADVANTAGES
DISADVANTAGES
1. No blacklight or darkened area required.
1. Process is not reliable in finding scratches.
2. Process can be considered as a one-step
2. Process is less sensitivity for fine
discontinuities.
process and, therefore, fast and economical. 3. Process can be used for detecting a wide
3. Penetrant can be affected by acids and
range of discontinuities. 4. Penetrant used can be easily washed off
with water.
ch-omtes. 4. Process is not reliable on anodized surfaces.
5. Process is susceptible to over-washing.
5. Process is easily adaptable to a large
6. Water contamination may destroy usefulness
of penetrant.
volume of small parts. 6. Process is excellent for rough surfaces,
7. Not good for wide shallow discontinuities
keyways, and threads.
(widlh greater than depth).
7. Process is relatively inexpensive. METHOD B, TYPE 2 INSPECTION PROCESS Method B, Type 2 Penetrant lnspection process uses a post-emulsifiablevisible dye penetrant, an
.
emulsifier, and a dry, wet or non-aqueous developer. The materials used in this process are very similar to that described for Method A, Type II process, however, the eenetrants are not self-emulsifiable. An emulsifier is applied over the penetrant to make it water washable. Method 8, Type 2 lnspection process is generally used when: 1.
Examining large volume of parts.
2.
A higher sensitivity than Method B, Type 1 is required or desired.
3.
The part is contaminated with acid or other chemicals that will harm waterwashable penetrants.
4.
Discontinuities are wider than their depth.
5.
Examining parts which may have discontinuities that are contaminated with inservice soils.
6.
Examining finished suiiaces and other general purpose examinations.
TABLE 6 ADVANTAGES AND DISADVANTAGES OF METHOD B TYPE 2 INSPECTION PROCESS ADVANTAGES
DISADVANTAGES
1. No blacklight or darkened area required.
1. Process is a two-step process, and
2. High sensitivity for fine discontinuities.
therefore requires more time.
3. Good on wide shallow discontinuities.
(width greater than depth) 4.
Process good for high volume production.
5. Process normally not affected by acids. 6. Process not as susceptible to overwashing.
2. Additional equipment is required for
application oi the emulsifier. 3. Not as good on parts with complex
shapes (i.e. threads) as Type 1. 4. Additional material increases cost.
5. Emulsifier dwell time very critical.
METHOD B. TYPE 3 INSPECTION PROCESS The Method B, Type 3 Penetrant lnspection Process uses a solvent-removable visible dye penetrant, a
- -
penetrant remover (solvent) and a dry, wet or non-aqueous developer. The penetrant is not waterwashable but is removed instead with the penetrant remover. Method 8, Type 3 lnspection Process is generally used when: 1.
Spot examination is required.
2.
Water-rinsing is not feasible because of part size, weight, surface condition, no water available, or remote location. TABLE 7 ADVANTAGES AND DISADVANTAGES OF METHOD B TYPE 3 INSPECTION PROCESS ADVANTAGES
1. Process can be used for spot inspection
on large parts. 2. Process can be used when water-rinsing
methods are not feasible. 3. No blacklight or darkened area required. 4. Process is highly portable.
DISADVANTAGES 1. Use of solvent to remove penetrant
prohibits inspecting large areas. 2. Sensitivity can be reduced by the
application of excessive amounts of remover. 3. Visibility of indications is limited.
As shown in the previous paragraphs, the test method is dependent upon the materials used. It should be obvious that in order to achieve the desired results.the proper selection and use of materials is of vital importance , and mandatory that the written procedure be followed to the letter.
:
Figure 1 TYPICAL PENETRANT INSPECTION EQUIPMENT
INSPECTION BOOT11
,
ULTRA VIOLET LIGHTS
N0TE:WHEN THE EQUIPMENT IS USED FOR A TYPE 2 INSPECTION PROCESS, THE EXTRA TANK (SHOWN BY THE DASHED LINES) WlLL BE USED FOR M E PENETRANT. IN M I S EVENT, THE TANK IDENTIFIED ABOVE AS THE PENETRANT TANK WlLL BE USED FOR M E EMULSIFIER. WHEN THIS EQUIPMENT IS USED FOR THE TYPE 1. PROCESS, THE ADDITIONAL TANK IS NOT REQUIRED.
PT Ill BASIC
14
7
PENETRANT INSPECTION KITS. Penetrant inspection is practical for field use, because these materials are supplied in the form of portable kits. Both Fluorescent and Visible Dye Penetrant inspection kits are available, but it is essenlial that only the complete family of penetrant inspection materials be employed for these field kit inspection operations. PORTABLE VISIBLE DYE PENETRANT KITS. Portable Visible Dye Penetrant Kits are available for field inspection. A typical Visible Dye Penetrant Kit is illustrated in Figure 2. A VlSlBLE DYE PENETRANT KIT usually contains: 1.
Spray cans of cleaning or removal fluid
2.
Spray cans of visible dye penetrant.
3.
Spray cans of nonaqueous developer.
4.
Wiping cloths and brushes.
BRUSH A N D WIPES
PENETRANT
CLEAN
Figure 2 Portable Visible Dye Penetrant Kit
PORTABLE FLUORESCENT DYE PENETRANT KITS. Portable Fluorescent Dye Penetrant Kits are available for field inspection. A typical Fluorescent Dye Penetrant Inspection Kit is illustrated in Figure 3. A FLUORESCENT DYE PENETRANT KIT usually contains: 1.
A portable black light and transformer.
2.
Spray cans of cleaning or removal fluid.
3.
Spray cans of fluorescent dye penetrant.
4.
Spray cans of nonaqueous developer.
5.
Wiping cloths and brushes.
PENETRANT
DEVELOPEfi
\
Figure 3 Portable Fluorescent Dye Penetrant Kit
PT Ill BASIC
PORTABLE BLACK L I G H T
In summary, let's consider the advantages and limitations of the liquid penetrant test method ADVANTAGES OF PENETRANT TESTING Materials are relatively inexpensive Some methods are relatively fast Sensitive: can detect discontinuities .001" or greater. Versatile: can be used on any non-porous, non-absorbent material. LIMITATIONS OF PENETRANTTESTING Some methods are time consuming and therefore expensive. Can only detect discontinuities open to the surface. /
Surface of part should be 60 to 125 degrees F. Cannot be used on very rough surfaces..-----+ Procedure can be messy. May require good ventilation. No easy pemlanent record.
LEARNING MODULE 4 INTERPRETATION AND EVALUATION OF INDICATIONS This learning module describes the interpretation and evaluation phases of NDT, discontinuity characteristics. and the classifications of indications and discontinuities. THE INSPECTOWEXAMINER' Since correct evaluation of a discontinuity depends on accurate interpretation the inspector is the key in the inspection process. The success and reliability of any NDT depends upon the thoroughness with which the inspector conducts the examination from the initial step all the way through to the final interpretationof the indications. The inspector must carefully follow the procedure, search out indications and then decide the seriousness of discontinuities found to determine the disposition of parts according to the severity of the flaw indications. Remember poor processing can be worse than no inspection, because, if improper processing yields no indications for the inspector to interpret the part would be considered acceptable whether it is or not. In some cases, the inspector may perform only the inspection phase of the process. At other times, the inspector may perform all phases of the process. In either case, the success and reliability of the inspection depends on the thoroughness of the inspector, and proper
f-
processing of the part. The" inspector* as used in this learning module is referred to as the "examine? in the ASME Code. PERSONNEL QUALIFICATION The personnel performing the liquid penetrant test must be qualified and certified in accordance with S M TC-?A. A review of the company's "Written Practice" would be necessary to determine the specific requiiements for qualificationto any level of competency as recommended by SNT-TGIA
- . . ..
TERMINOLOGY
. .
Quite often inspectors will confuse the various terms used and will use them incorredly. Therefore, it is important that the inspector have a clear understanding of the terms relating to liquid penetrant testing. INDICATION: a response, or evidence of a response, that requires interpretationto determine its significance. DISCONTINUITY: a broad term relating to a condition that is foreign to the normal structure of a material. A discontinuity may or may not be detrimental to the intended service life of a part and must therefore be evaluated. HelMer Associates, lnc PTMcd4 O 1989
DEFECT: a term applied to a discontinuity which may be detrimental to the intended service life of a part, and exceeds the limits of the applicable acceptance criteria. INTERPRETATION: the action performed by the inspector in determining the cause of an indication. EVALUATION: the action performed by the inspector in comparing the magnitude and severity of an indication to a predetermined acceptance criteria in order to determine acceptance or rejection of the part. RECOGNrrlON OF TYPES OF INDICATIONS it must be recognized that all indications are not caused by discontinuities. Some indications are the result of faulty processing of the part, while other indications are the result of part design. The penetrant inspector must be able to recognize the various indications that might appear. Penetrant indicationswill fall into one of three categories:
.
1.
False Indications
2.
Nonrelevant Indications
3.
True or Valid Indications
Usually there are specik differences between all three and a well-trained inspector should be able to determine into which of the three categories an indication is to be classified. Qualified inspectors, using acceptable procedures and codes, can usually determine the cause and category of the penetrant indication. FALSE INDICATIONS
In nondestrudiwe testing, an indication that may be interpreted erroneously as a discontinuity is considered a false indication. In all NDT disciplines, false indications can become major pmblems in the the NDT .- . . .inspection process. Usually a thogugh knowledge of the manufacturing processes ,involved, . process, and previous experience of the inspector is necessary to readily and accurately classiiy a false indication. The most common causes of false indications are the improper or inadequate precleaning of the part, and the improper or inadequate removal of the excess surface penetrant. If all the surface penetrant is not completely removed in the removal process, the remaining penetrant may produce false indications. This is true for both the fluorescent and visible penetrant methods. The use of the black light during the removal of fluorescent penetrants is very helpful in determining that adequate removal has been achieved.
Hellier Associares, Inc PTMcd4 O 1989
A properly cleaned part would show only a very faint, or no pink background if visible penetrants were
used, or only very faint, or no areas of background fluorescence when fluorescent penetrants are used. False indications due to incomplete washing are usually easy to identify, since the penetrant will be in broad areas rather than in the sharp patterns found in the true indications. The danger of poorly cleaned parts, which produce the false indications, lies in the fact that there may be actual discontinuities in the improperly cleaned areas which would be masked by the false indications. If false indications interfere with interpretation of true indications found on the parts complete reprocessing of the parts would be required. NON-RELEVANT INDICATIONS Non-relevant indications are true indications produced by uncontrolledtest conditions. However, the conditions causing them are present by design or accident, or other features of the part having no relation to the damaging flaws being sought. The term signifies that such an indication has no relation to discontinuities that might constitute defects. NON-RELEVANT INDICATIONS DUET0 FILLETS, THREADS, AND KEYWAYS: Sharpfillets, threads, and keyways will often retain penetrant at their base and produce indications despite a good removal
f'-
technique. This is particularly tiue when post emulsified penetrants are employed. Because heat-treating or fatigue cracks often do m
r at such locations it is essential that the inspector check these locations
very carefully. NON-RELEVANT INDIGATTONS DUE TO PRESS-FIT: Anotherwndition which may create nokrelevant indications is when parts are press-fitted into each other. if a wheel is press-fitted onto a shaft, penetrant will show an indicationat the fit line. This is perfectly normal since the two parts are not welded together. The only problem with such indications is that penetrant from the press fit may bleed out and mask a true . . dis~ontinuity.
CWUliUOW: Where penetrant bleed out may mask discontinuities on press-fit parts, the time between application of developer and inspection should be held to a minimum to prevent excessive bleed out.
Hellier Associates, Inc. PTMod 4 Q 1989
TRUE INDICATIONS The last classification of indications is the group of which we are most interested and is called the true indication which is caused by a discontinuity. True indications can be further classified into four major groups. They are: inherent, primary processing, secondary processing, and service discontinuities. These are covered in detail in another module. Three basic questions must be answered to facilitate proper interpretation of the flaw indications: 1.
What type of discontinuity would cause the indications?
2.
What is the extent of this discontinuity?
3.
What effect will this discontinuity have on the anticipated service of the part?
NOTE: The answers to the first two questions are the prime responsibility of the inspector. The answer to the third question, unless specific acceptance criteria are specified, usually requires special assistance. SPECIFIC TYPES OF DlSCONTlNUrflES Generally speaking, we can divide discontinuities into five basic types. These are:
f
1
Fine, Tight Surface Cracks. Such cracks may be shallow or deep, but their most signifmnt characteristics is their very small and tigM surface opening. Deep cracks of this type, once well penetrated, may provide a reservoir of penetrant, and
2.
therefore, may be easier to show than shallow cracks. Broad, Open Surface Discontinuities. Discontinuitiesof this type may be shallow or relatively deep. Their significant characteristic is their width which tends to permit penetrants to be removed from the discontinuity, especially when water spray removal techniques are employed. Care must be taken to ensure this does not occur.
3.
Porosity. Generally speaking, porosity is a discontinuity having a cavity below the surface which is connected to the surface by a very small channel. Porosity is typically found in castings and welds and is sometimes referredto as gas holes.
4.
Shrinkage: Micro or sponge shrinkage in castings which is opened to the surface by machining and etching may be hard to differentiate from cracks. Much care must be used in evaluating this type of indication.
5.
Leaks or Through Cracks. Discontinuities of this type are cracks or openings which pass from one surface to another.
Hellier Associates, lnc. PTMcd4 63 1989
FLAW INDICATION CATEGORIES
There are five basic types of indications which may be seen by the inspector. These indication types caused by the discontinuities listed in the above paragraph are as follows: 1.
Continuous linear indications
2.
Intermittent linear indications
3.
Rounded indications
4.
Small dot indications
5.
Diffuse or weak indications
It is possible to examine an indication of a discontinuity and determine its cause as well as its extent. such an appraisal can be made if something is known about the manufacturing processes or the operational use to which the part has been subjected. The extent of the indications, or acurmulation of penetrant, will show the extent of the discontinuity. The vividness of the visible dye penetrant on the contrasting white developer or the brilliance of the fluorescent dye penetrant will give some indication of the discontinuity's depth. Deekdiscontinuities will hold penetrant and therefore, will be broader and more brilliant. Very fine discontinufies can hold only small amounts of penetrant and will therefore appear as fine lines. In many instances, more accurate flaw evaluation may be obtained by removing the indications and reapplying nonaqueous wet developer so that the rate and amount of penetrant bleed out can be closely
f!
observed to facilitate the interpretation of the flaw discontinuity. CON'NUOUS LINEAR INDICATIONS Cracks, cold shuts, and forging laps usually show as a continuous line indication. A crack will appear as a sharp or faint-jagged line, straight line or intermittent line, while cold shuts will usually appear as smooth, straight, narrow lines. Scratches and die marks will also appear as straight lines, but the bottom of the .
,di.s.continuityis usually visible. \
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CONTINUOUS LINEAR INDICATIONS \
Hellier Associates. Inc P T M w ' 4 @ 1989
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INTERMITENT LINEAR INDICATIONS The same discontinuities that appear as straight lines may also appear as linear intermittent indications. This condition is caused by the discontinuity being pattially closed at the surface due to metal working such as machining forging, extruding, peening, grinding, etc. As an illustration, grinding cracks are caused by local overheating of the surface being ground, but these cracks may be partially closed by the plastic flow of the metal over the crack caused by the high shear forces produced on the surface of the metal. Grinding cracks can show as a craze pattern made up of a network of very fine cracks. f
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INTERMITTENT LINEAR INDICATIONS
ROUNDED INDICATIONS Rounded indications generally indicate porosity caused by gas holes or pin holes or a generally porous material depending on the extent of the indication. Deep crater cracks in welds frequently show up as rounded indications, since there is a large amount of dye penetrant entrapped. The indications may appear rounded because of the volume of penetrant entrapped, ailhough the actual defects may be irregular in outline. \
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Hellier Associales, Inc. PTMcd4 O 1989
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SMALL DOT INDICATIONS Discontinuities of this nature result from a porous condition of the material. Such indications may denote small pin holes, excessively coarse grains in a casting, or may be caused by micro-shrinkage or certain cast alloys. A series of aligned dots might result from a very tight crack. NOTE: Internlittent dot indications, or even a generally heavy background may also result fmm surface corrosion pining, general intergranular surface corrosion or even an excessively mugh surface. This type of indication may obscure indications from genuine cracks.
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DIFFUSE OR WE3K INDICATIONS This condition may be caused by a porous surface, insufftuent cleaning, incomplete removal of dye penetrant or excess developer. Weak indications extending over a wide area should be viewed with suspicion. When this condition is encountered, the part should be completely reprocessed.
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DIFFUSE OR WEAK IHDICWTIOMS
Hsllisr Associalss. Inc. PTMod4 @ 1989
1
FATIGUE OR SERVICE CRACKS Fatigue cracks or sharp shallow cracks developed while the part is in service are extremely dangerous and represent an eventual part failure. Care must be taken to detect these discontinuities.
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FATIGUE OR SERVICE CRACKS \
Hellier Associates. Inc. PTMod4 8 1989
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LEARNING MODULE 1 MAGNETIC PARTICLE TESTING PRlNClPLES OF MAGNETISM
In order to understand how and why a magnetic particle test works it is necessary to understand the principles of magnetism.
HORSESHOE MAGNET
The most familiar type of magnet is the horsehoe magnet shown
in figure 1-1. It will attract magnetic materials to its ends where a leakage field occurs. These ends are commonly called "north" and "south" poles, indicated by N and S on the diagram. There will be no attraction except at these poles. Magnetic flux lines, or lines of force flow from the north to the south pole
.
as long as they are external to the magnet. Since these lines of force always form a complete circuit, they also pass through the iron or steel of which the magnet is made. Note thatwithin the magnet the lines are
Figure 1-1. Horseshoe h f q n e t
Ifthe ends of the horseshoe magnet are bent so that they are close together, as shown in figure 1-2, the
ends will sti!l attract magnetic materials. However, ifthe ends of the magnet zre benl closer together, and the two poles completely fused or welded into a ring as shown in figure 1-3, the magnet will no longer attract or hold magnetic materials because there is no longer a leakage field. The magnetic field remains as shown by the arrows, but without poles there is no attraction. Such a piece is said to have a circular field, or to be circularly magnetized, because the magnetic lines of force are circular.
Fiwre 1-2. H o m s h a e hiagnel w i t h Polci CloseTocclher
MT MOD 1
Any crack in the fused magnet or cicularly magnetized part which crosses the magnetic flux lines will immediately create noflh and south poles on either side of the crack. (see figure 1-4). This will lorce some 01 the rnagnetic flux (lines ol force) out of the metal path and is referred to as lluxleakage. Magnetic
materials or particles will be attracted by the pole created by the crack, forming an indication of the discontinuity in the metal part. This is the principle whereby rnagnetic particle indications are formed by means of circular magnetization.
Figure 1-4. Cnck in Fused Horjeshoe Magnet
BAR MAGNET
If a horseshoe magnet is straightened, a bar magnet is created a s shown in figure
1-5. The bar magnet has poles at either end and magnetic lines of force flowing through the length of it.
Magnetic particles will be attracted only to the poles. Such a piece is said to have a longitudinal field, or to be longitudinally magnetized.
Figure 1-5. f f o n e s h o e Magnet Straightened t o Form
831Magnet
MT M O D 1
- -
A slot or discontinuity in the bar magnet which crosses the magnetic flux lines will create north and south
poles on either side ol the discontinuity (see tigure 1-6). These poles will attract magnetic parlicles. In a similar manner, if the discontinuity is a crack even though it is very fine, it will still create magnetic poles as indicated in figure 1-7. These poles will also attract magnetic particles. The strength of these poles wiil be a function of the number of flux lines, the depth of the crack and the width of the air gap at the surface. The greater the pole strength, the greater the leakage field. The strength of this leakage field determines the number of magnetic parlicles which will be gathered to form indications: strong indications at strong fields, or large discontinuities, and weak indications at weak fields of small discontinuities.
MAGNETIC PARTICLES
-7
Figure 1-6. $lot in Bar Magnet Attracting Magnetic Particles
TMAGNETIC
I
PARTICLES
\
CRACK
Figure 1-7. Crack in Bar Magnet AtLncting M q n e l i c Partides
I
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MAGNETIC FLUX CHARACTERISTICS
Magnelic lines of.lorce (flux lines) may be described by several characteristics,
A. They are closed loops.
B. They can be distorted (like a rubber band). C. They return upon themselves. 'D. They never cross.
E. They seek the path of least resistance.
F. They are most densely concentrated at the poles oi the magnet. G. They flow from north to south outside the magnet, and from south to north within the magnel.
CLASSIFICATION OF MATERIALS All materials read to a magnetic field in one of three ways. They are. therefore, classilied as diamagnetic,
. paramagnetic, or ferromagnetic. When made into a rod, a diamagnetic material is repelled by a magnetic field and will align itself at right angles to the field. When a paramagnetic or a lerromagnetic material is made into a rod, it will be attracted by a magnetic field and will align itself parallel to the field.
1.
Diamagnet~cmmaleria!s have permeabilities slightly less than unity. Bismuth has the iowest
permeability known (.9998).Other diama~neticmaterials are phosphorus, antimony, flint glass, and mercuky. Such materials are usually consideredto be nonmagnetic.
2.
Paramagnetic materials have permeabilities greater than unity. Those whose permeablities are
only slightly grealer than unity such as platimum (1.00002), are called paramagnetic and are usually considered to be nonmagnetic.
3.
Ferromagnetic materials have permeabililies great than unity and are usually Considered to be
magnetic. Ferromagnetic materials are iron, nickel, cobalt, and many alloys such as permalloy, alnico, permivar, elc. Usually materials wiih permeabilities of 1.1000 or greater aree referred lo as lerromagnelic.
MAGNETIC PROPERTIES OF MATERIALS
Low Carbon Content Sleel Vs. High Carbon Content steel
Low
Large grain with a very simple structure
High
Smaller grain size; the structure is more complex for added strenth
Example of each before a magnetizing force is applied:
Low Carbon
Figure 1-8 ATOM ARRANGEMENT Hiah Carbon
Figure 1-9 When a magnetizing lorce is applied to low carbon content steel, the aloms align easily.
F i g u r e 1-10
More magnetizing force is required to align the atoms of high carbon steel into magnetic domains. As illustrated in figure 1- 9, the atom directions are more disarranged than low carbon content in figurel-8
I
magnetizing force
F i g u r e 1-11
When the magnetizing force is removed from low carbon content steel, most of the atoms return to their normal orientation (figure 1-8), leaving little magnetism. High carbon content steel is different. Because it is much harder to align the atoms; when the magnetizing force is removed many atoms will stay aligned and the material will retain a greater amount of magnetism as shown in figure 1-12.
Figure 1 - 1 2
You will notice thal a malerial of high reluctance has:
1. Low permeability
..
.
2. High retentivity
3. High coercive force
4. High residual magnetism
And material with low reluctance is easy lo magnethe. It has:
1. High permeability
2. Low retentivity /-
3. Low residual magnetism
4. Low coercive force
PROPERTY
LOW CARBON
HIGH CARBON
Permeability
High
Low
Reluctance
Low
High
Relentivily
Low
High
Residual Field
Low
High
Coercive Force
Low
High
MAGNETIC PARTICLE TESTING MAGNETIC FIELDS
INDUCED MAGNETIC FIELDS Magnetism may be induced into a material by placing the material in an already existing magnetic field. This can be illustrated by making a screwdriver magnetic by mbbing it against a permanent magnet. An easier method is through the use of electrical current. Ifa wire is wrapped around a screwdriver and electric current passed through the wire, the screwdriver becomes magnetized. PERMANENT MAGNETS Permanent magnets are sometimes used to induce magnetic fields within a test specimen. The use of permanent magnets for magnetization has many limitations and they are, therefore, only used when these limitations do not interfere or prevent the formation of adequate leakage fields at the site of a discontinuity. ELECTRIC CURRENTS Electric currents can be used to create or induce magnetic fields in ferromagnetic materials. Magnetic lines of force are always at right angles (900) to the direction of the magnetizing current flow. Therefore, the direction of the magnetic field can be altered, and is controlled by the direction of the magnetizing current. It is important to know how to use electric currents to induce the magnetic lines of force so that they intercept and are, as near as possible, at right angles to the discontinuity. Eiiher circular or longitudinal magnetic fields can easily be created in a test specimen. The strength of the magnetic field can be varied, and through the use of several types of current, variations in field strength and distribution can be accomplished. There are basically two types of electric current used as a magnetzing force. These are alternating current (AC), direct current (DC). Alternating Current or AC is current that reverses its direction of flow at regular intervals. Such current is frequently referred to as AC. Direct Current or DC, as the name implies, refers to an electric current flowing continually in one direction through a conductor. Such current is frequently referred to as DC. AC VS DC The magnetic fields created by alternating current and by direct current differ in many respects. The most important difference in magnetic particle testing is that the magnetic field created by alternating current is confined near the surface of the part referred to as skin effect, while the magnetic field created by direct current penetrates below the surface of the part.
MOD 2
Although different types of magnetizing current can be used in magnetic particle inspection only one type is generally best suited for each type of inspection to be performed. Alternating current (AC) is used for the detection of surface discontinuities only, due to the skin affect. Direct current (DC) or Halfwave direct current (HWDC) is used for detection of either surface or subsurface discontinuities. Regardless of the type of current used for magnetization, the magnetic field created in the test part will be either a circular field or a longitudinal field. CIRCULAR MAGNETIZATION. Circular magnetizationderives its name from the fact that a circular magnetic fiekl atways surrounds a conductor such as a wire or a bar carrying an electric current (see figure
. 2-1). The direction of the magnetic lines of force (magnetic field) is always at right angles to the direction of the magnetizingcurrent. An easy way to remember the direction of magnetic lines of force around a conductor is to imagine that you are grasping the conductor with your hand so that the extended thumb points parallel to the electric current flow. The fingers then point in the direction of the magnetic lines of force. Conversely, if the fingers point in the direction of current flow, the extended thumb points in the
f .
direction of the magnetic lines of force.
Magnetic Field Surrounding an Electrical Conductor
F i g u r e 2-
1
Since a magnetic part is in eHect a large conductor, electric current passing through this part creates a magnetic field in the same manner as with a small conductor (see figure 2-2). The magnetic lines of force are at right angles to the direction of the current as before. This type of mangetization is called circular mangetization because the lines of force, which represent the direction of the magnetic field, are circular within the part. The strength of the magnetic field is dependent upon the current passing through the conductor.
Magnetic Field in Part Used as a Conductor
F i g u r e 2-
2
CIRCULAR MAGNETIZATION WITH INSPECTION EQUIPMENT. To create or induce a circularfield in a part with stationary magnetic particle inspection equipment, the part is clamped betweenthe contact plates and current is passed through the part as indicated in figure 2-3. This sets up acircuiar magnetic field in the part which creates poles on either side of any cradc or discontinuity which wns parallel to the length of the part. The poles will attract magnetic particles, forming an indication of the discontinuity. T CONTACT PLATE
.
CONTACT P L A T E7
Cteating n Circular Magnetic Field in a Part
On parts that are hollow or tubelike, the inside surfaces are as important to inspect as the outside. When such parts are circularly magnetized by passing the magnetizing current through the part, the magnetic field on the inside surface is negligible. Since there is a magnetic field surrounding the conductor of an electric current it is possible to induce a satisfactory magnetic field by placing the part on a copper bar or other conductor. This situation is illustrated in figures 2-4 and 2-5. Passing current through the bar induces a magnetic field on both the inside and outside surfaces.
CRACKS O.D. OR 1.0. MAGNETIZING CURRENT Figure 2- 4
Circular Magnetization of a Cyclinder Using a Central Conductor
Figure 2- 5
Circular Magnetization of Ring-Type Parts Using a Central Conductor
LONGITUDINAL MAGNETIZATION. Electric current can also be used to create a longitudinal magnetic field in a piece of magnetic material. The nature and direction of this field is the result of the field around the conductor which forms the turns of the coil. Application of the rule of the thumb to the conductor at any point in the coil illustrated in figure 2-6 will show that the field within the coil is lengthwise as indicated. F A G N E T I C FIELD
Figure 2- 6
WIRE COIL
7
Magnetic Lines o f Force i n a Coil
When a part made of magnetic material is placed inside a coil as shown in figure 2-7, the magnetic lines of force created by the magnetizing current concentrate themselves in the part and induce a longitudinal mangetic field. Inspection of a cylindrical part with longitudinal magnetizationis shown in figure2-18. If there is a transverse discontinuity in the part, such as that in the illustration, small magnetic poles are formed on either side of the crack. These poles will attract magnetic particles, forming an indication of the discontinuity. Compare figure 2-8 with figure 2-3 and note that in both cases a magnetic field has been induced in the part which is at right angles to the defect. This is the most desirable condition for reliable inspection. The strength of the magnetic field within a coil is dependent upon the current flowing through the coil, the number of turns in the coil, and the diameter of the coil.
,-WIRE
LM,*GNETIZING Figure 2-17
COIL
CURRENT
Longiludinal Magnetic Field in a Fart Placed in a Coil
1
Figure 2- 8
.
MAGNETIZING CURRENT 7
Longitudinal Magnetic Field Shows Transverse Crack
INDUCED CURRENT MAGNETIZING. When a direct current in a circuit is instantly cut off, the field surrounding the conductor collapses, or falls rapidly to zero. The rapid change of field tends to generate a voltage (and current) which is opposite in direction to that which had been established in the circuit. When ferromagnetic material is under the influence of such a collapsingfield, the effect is greatly increased.
I
Under certain conditions the rapid collapse of the field can generate very high currents inside ferromagnetic material, and the phenomenon can be made useful in some magnetizing problems. An extremely useful application of a collapsing field method fo magnetization has been developed for the magnetizing of ring-shaped parts such as bearing races, without the need to make direct contact with the surface of the part. Regardless of the type of magnetizing current employed, whether DC, AC or halfwave, the induced current method is usually faster and more satisfactory than the contact method. Only one operation is required and the possibility of damaging the part due to arcing is completely eliminated since no external contacts are made on the part.
MAGNETIC PARTICLE TESTING INSPECTION METHODS CURRENTIPARTICLEAPPLICATION. Two methods of processing are used in magnetic particle inspection. The Continuous Method and the Residual Method. Which of the two methods to use in a given case depends upon the magnetic retentivity of the part being inspected and the desired sensitivity of the inspection to be made. The continuous method must be used on parts having low retentivity. Highly retentive parts may be inspected using the residual method. For a given magnetizing current or applied magnetizing field the continuous method offers the greatest sensitivity for revealing discontinuities. CONTINUOUS METHOD. This method implies that the magnetizing force is acting while the magnetic particles are applied. When the current is on, maximum flux density will be created in the part for the magnetizing force being employed. In some cases, usually when AC or half-wave DC is being used as the magnetizing current, the current is actually left on, sometimes for minutes at a time, while the mangetic particles are applied. This is more often needed in dry method applications than in the wet. Leaving the current on for long periods of time is not practical in most instances, nor is it necessary when using the wet method. The heavy current required for proper magnetization can cause overheating of parts and contact /
burning or damage to the equipment if allowed to flow for any appreciable length of time. In practice, the magnetizing current is normally on foronly afractiin of a second at a time. All that is required is that a sufficient number of magnetic particles are in the zone and free to move while the magnetiiing current flows. The bath ingredients are so selected and formulated that the particles can and do move through the film of liquid on the surface of the part and form strong, readable indications. The viscosity of the bath and the bath concentration are important, since anything that tends to reduce the number of available particles or to slow their movement tends to reduce the build-up of indications. RESIDUAL MRF1OD. The residual method is a method of inspection in which magnetic particles are applied to parts after the parts have been magnetized. The residual method is used only when parts are magnetized with DC and the parts have sufficient retentivity to form adequate magnetic particle indications at discontinuities.
Usually the use of the residual method is limited to the search for discontinuities which
are open to the surface, such as cracks. WET VS DRY M E M O D The magnetic particles may be applied to the surface of the test part in the form of a wet suspension, in which the magnetic particles are held in suspension in a liquid vehicle, which is flowed over the test part, or in the form of dry powder which is dusted over the test part. The particular method to be used would be determined by the test conditions, or dictated by specification. Each method has distinct advantages and limitations.
t
WET METHOD ADVANTAGES AND LIMITATIONS. As is true of every process, the wet method has both good points as well as less favorable characterisitics. The more important good points of the wet method, which constilute the reason for its extensive use, as well as the less attractive characteristics are tabulated as follows:
a
It is the most sensitive method for very fine surface cracks.
b.
It is the most sensitive method for very shallow surface cracks.
c.
It quickly and thoroughly covers all surfaces of irregularly-shaped parts, large or small, with
d.
magnetic particles. It is the fastest and most thorough method for testing large numbers of small parts.
e. f.
The magnetic particles have excellent mobiliy in liquid suspension. It is easy to measure and control the concentration of particles in the bath, which makes for uniformity and accurate reproducibility of resuls.
9. h.
It is easy to recover and reuse the bath. It is well adapted to the short, timed shot technique of magnetization for the continuous method.
i.
It is readily adaptable to automatic unit operation.
j.
It is not usually capable of finding defects lying wholly below the surface if more than a few
k.
thousandthsof an inch deep. It is messy to work with, especially when used for the expendable technique, and in field testing.
I.
A recirculatingsystem is required to keep the particles in suspension.
rn
it sometimes presents a post-inspection cleaning problem to remove magnetic particles linging to the surface
Fluorescent magnetic particles used in suspension in liquids have the same unfavorable characteristics
-
which go with the usual wet visible method techniques. There is the additional requirement for a source of black-light, and an inspection area from which the white light can be excluded. Experience has shown that
these added special requirements are more than justified by the gains in reliability and sensitivity.
GENERAL. The dry powder method is primarily used for the inspection of welds and castings where the
detection of defects lying at or very close to the surface is considered important. The particles used in the dry method are provided in the form of a powder. They are available in red, black, yellow and gray colors. The magnetic properties, particle size and shape, and coating method are similar in all colors making the particles equally efficient. The choice of powder is then determined primarily by which powder will give the best contrast and visibility on the parts being inspected and the degree of sensitivity desired.
ADVANTAGES AND LIMITATIONS. The dry powder method has good points and less favorable characteristics. These advantages and disadvantages which may influence its use for a specific application are summarized in the following list: Excellent for locating defects wholly below the surface and deeper than a few thousandths of an inch. Easy to use for large objects with portable equipment. Easy to use for field inspection with portable equipment. Good mobility when used with alternating current (AC) or half-wave direct current (HWDC). Not as messy as the wet method. Equipment may be less expensive. Not as sensitive as the wet method for very fine and shallow cracks. Not easy to cover all surfaces properly, especially of irregularly-shaped or large parts. Slower than the wet method for large numbers of small parts. Not readily usable for the short, timed shot technique of the continuous method. Difficult to adapt to a mechanized test system.
MAGNETIC PARTICLE TESTING EQUIPMENT
GENERAL. Considerations involved in the selection of magnetic particle inspection equipment include the type of magnetizing current and the location and nature of inspection. Magnetic particle inspection equipment serves two basic purposes, which dictate requirements for the size, shape and functions. These two purposes are to provide convenient means for accomplishing proper magnetization and to make possible, rapid inspection of parts, with assurance that the inspection results will be reliable and reproducible. STATIONARY EQUIPMENT. A typical stationary horizontal wet magnetic particle inspection unit of intermediate size is shown below. The unit has two contact heads for either direct contact or central conductor, circular magnetization using a copper rod between the heads or a cable connected to a contact block between the heads. Units contain a coil used for longitudinal magnetization. The coil and one contact head are movable on rails. The other contact head is iiied; the contact plate on it, being air cylinder operated, provides a means for clamping the part. The unit has a self-contained power supply with all the necessary electrical controls. Magnetiuing currents are usually three phase full-wave DC or AC depending upon usage requirements. The units are made in several different sizesto accomodate different length parts and with various maximum output currents. A full length tank with pump, agitation and circulating system for wet inspection media is located beneath the head and coil mounting rails. A hand hose with nozzle is provided for applying the bath. On special units automatic bath application facilities are provided.
Typical Wet Horizontal Magnetic Particle Test Unit
This unit is used tor the wet method with either the visible or the fluorescent magnetic particles. The unit is equipped with a black light seen mounted on the back rail, and a hood and wrtains which may be drawn to exclude white light when the fluorescent particles are used in the wet suspension. Direct current up to 6,000 amperes, derived from full wave rectified three phase AC, is delivered to the adjustable contact heads, for circular magnetization. A built-in coil is provided for longitudinal magnetization. This unit is equipped with the infinitely variable current control by means of a saturable core reactor, and also with the self-regulating current control. A great number of variations of these typical magnetizing units is available. These variations are in size, in current output and kinds of current, in the methods of current control, and in numerous types of fittings to expedite magnetization of odd-shaped parts. In addition there are many accessories, such as contact pads, automatic bath applicators, contact clamps, leech contacts, steady-rest for heavy shafts, prod contacts. special shaped coils, powder guns, etc. MOBILE EQUIPMENT A versatile mobile inspection unit is shown below. These units are available in several sizes ranging from 2000 to 6000 amperes of AC and HWDC oulputs. The units have remote control current out-put, ONIOFF and MAGlDEMAG controls which permit one-man operation at the site of the inspection. The units are used with either rigid or cable wrapped coils for longitudinal magnetization and demagnetization. Cables connected to a part or passing through it are used for circular magnetization or demagnetization. Mobile units can be easily moved to any inspection site where suitable line input voltages and current capacity are available.
Typical AClHWDC Mobile Magnetic Particle Test Unit
MOI7 7
\'
,q'
PORTABLE EQUIPMENT
A small portable unit which can be handcarried is shown below. These
units have both AC and HWDC outputs and must be used with a portable coil or cable wrapped coils to Ion gitudinally magnetize, or with prods or clamps for circular magnetization. The units usually have a remote ONlOFF control permitting a one-man operation for many applications. They can be used wherever an adequate 115 volt AC power source is available.
MAGNETIC YOKES
A
Magnetic yokes are small and easily portable. They are very easy to use and are
adequate when testing small castings or machined parts for surface cracks and for weld inspection. They induce a strong magnetic field into that portion of a part that lies between the poles or legs of the yoke. The induced field flows from one leg of the yoke to the other in an orientation as shown below and yokes and probes are available with either fixed or articulated legs, also shown below. Yokes are available for operation from a 115 volt, 60 hertz AC outlet, and some are equipped with a rectifier so HWDC may be used. A permanent magnet yoke is also available, permitting inspections to be performed without the use of electric current.
INTERPRETATION AND EVALUATION OF INDICATIONS DEFINITIONS in order to properly and accurately intrepret and evaluate magnetic particle indications the magnetic particle inspector should understand certain definitions which are used in connection with this inspection method. Since these terms are used frequently in this learning module, the inspector must fully understand the meaning of each of the following. INDICATION. in magnetic particle inspection an indication is an accumulationof magnetic particles being held by a magnetic leakage field to the surface of a part. The indication may be caused by a discontinuity (an actual void or break in the metal) or it may be caused by some other condition that produces a leakage field. DISCONTINUIN. A discontinuity is an interruption in the normal physical structure or configuration of a part. These discontinuities may be cracks, laps in the metal, folds, seams, inclusions, porosity, and similar conditions. A discontinuity may be very fine or it may be quite large; it will generally be a definite separation or void in the metal. DEFECT. A defect is a discontinuity which exceeds the limits of the acceptance criteria and, therefore, interferes with the usefulness of a part. BASIC STEPS OF INSPECTION. Magnetic particle inspection can be divided into these three basic steps: a. Producing an indications on a part. b. Interpreting the indication c. Evaluating the indication. PRODUCING AN INDICATION. In order to produce a proper indication on a part it is necessary to magnetize the part using the proper magnetizing force necessaryto produce the desired magnetic flux oriented in the proper direction (i.e. circular or longitudinal). INTERPRETING THE INDICATION. After the indication is created, il is necessary to interpret that indication. Interpretation is the deciding of what caused that indication, what magnetic disturbance has attracted the particles in the particular pattern found on the part. If the operator knows something about metal processing, it is possible to determine from the appearance and location of an indication the cause of the indication.
i
NON-RELEVANT INDICATIONS NATURE AND TYPE. It is possible to magnetize parts of certain shapes in such a way that magnetic leakage fields are created even though there is no discontinuity in the metal at the point. Such indications are sometimes called erroneous indictions or false indications. They should be called "non-relevant indications" since they are actually caused by distortion of the magnetic field. They are real indications but since there is no interruption in the metal they do not affect the usefulnessof the part. It is important that the operator know how and why these non-relevant indications are formed and where to look for them on the parts being inspected. EXAMPLES OF NON-RELEVANT INDICATIONS MAGNETIC WRITING. This is a condition caused by a piece of steel wbbing against another piece of steel which has been magnetized. Since either or both pieces contains some residual magnetism the rubbing or touching creates magnetic poles at the points of contact. These local magnetic poles are usually in the form of a line or scrawl and for this reason the effect is referred to as magnetic writing. COLD WORKING. Cold working consists of changing the size or shape of a metal part without raising its temperature before working. When a bent nail is straightened by a carpenterwith a hammer the nail is being cold worked. Cold working usually causes a change in the permeability of the metal where the change in size or shape occurs. The boundary of the area of changed permeability may attract magnetic particles when the part is magnetized. HARD OR SOFT SPOTS. If there are areas of the part which have a different degree of hardness than the remainder of the part these areas will usually have a different pemteabirQ. When a part w l h such areas of different permeability is inspected with magnetic particle inspection, the boundaries of the areas may create local leakage fields and altract magnetic particles to form indications. BOUNDARIES OF HEATTREATED SECTIONS. Heat treating a part mnsists of heating it to a high temperalure and then cooling it under controlled conditions. The cooling may be relativity rapid or it may be done quite slowly, depending upon the characteristics of the metal which are desired. It is possible to increase or decrease the hardness or the grain size of the metal by varying the temperature and the rate of cooling. On a cold chisel the point is hardened to cut better and to hold an edge. The head of the chisel, which is the end struck by the hammer, is kept softer than the cutting edge solhat il won't shatter and break. The edge of the hardened zone frequently creates a leakage field when the chisel is inspected with magnetic particle inspection.
MOD 2
..
ABRUPT CHANGES OF SECTION. Where there are abrupt changes in section thickness of a magnetized part, the magnetic field may be said to expand from the smaller section to the larger. Frequently thiscreates local poles due to magnetic field leakage or distortion. These leakage fields will attract magnetic particles thereby creating an indication. The non-relevant indication will usually be "fuzzy" like an indication which is produced by a discontinuity beneath the surface. INTERPRETATION AND ELIMINATION OF NON-RELEVANT INDICATIONS, INTERPRETATION. It may at first appear to the operator that some types of non-relevant indications discussed and illustrated in the preceeding material would be difficult to recognize and interpret. For example, the non-relevant indications shown in figures 9-5 and 9-6 may look like indications of subsurface discontinuities. However, there are several characteristics of non-relevant indications which will enable the operator to recognize them in the example cited and under most other conditions. These characterisitics of non-relevant indications are: a.
On all similar parts, given the same magnetizing technique, the indications will occur in the same location and will have identical patterns. This condition is not usually encountered when dealing with real subsurface defects.
b.
The indications are usually uniform in direction and size.
c.
The indications are usually "fuzzy" ratherthan sharp and well defined.
d.
Non-relevant indications can always be related to some feature of construction or cross section which accounts for the leakage field creating the indication.
ELIMINATION OF NON-RELEVANT INDICATIONS. Although non-relevant indications can be recognized in most cases, they do tend to increase the inspection time, and under certain condiiions may mask or cover up indications of actual discontinuities. Therefore it is desirable to eliminate them whenever possible. In most cases non-relevant indications occur when the magnetizing current is higher than necessary for a given part. consequently, these indications will disappear if the part is demagnetized and reinspected using a sufficiently low magnetizingcurrent. TRUE OR VALID INDICATIONS. If the indication is caused by a discontinuity it is termed a true indication or a valid indication. If the indication is caused by a discontinuity at the surface of the part the particles are usually tightly held to
the surface by a realtively strong magnetic leakage field. The line of particles is sharper and well defined and there is a noticeable "build-up" of the particles. This build-up consists of a slight mound or pile of
MOD 2
15
particles which on deep surface cracks is sometimes high enough above the surface of the part to cast a shadow. If such an indication is wiped o f f the discontinuity can usually be seen. Ifthe indication is caused by a discontinuity below the surface it will be a broad fuzzy looking accumulation
of particles rather than being sharp and well defined. The particles in such an indication are less tightly held to the surface because the leakage field is weaker. EVALUATING THE INDICATION. After the indication has been formed and has been interpreted, it must be evaluated. It is necessary for the operator to decide whether that indication in that particular location on that particular part will affect the usefulness of the part. Evaluation is the determination of whether the part can be used in spite of the indication, whether the cause of the indication can be removed without affecting the strength of the part, or whether th epart must be scrapped. As a guide, the following basic considerations may be used in conjunction with the operatoh knowledge and experience to help in the evaluation of indications. a.
A discontinuity of any kind lying at the surface is more likely to be harmful than a discontinuity of the same size and shape which lies below the surface
b.
Any discontinuity having a principal dimension or a principal plane which lies at right angles or at a considerable angle to the direction of principal stress, whether the discon tinuity is surface or subsurface is more likely to be harmfulthan a discontinuity of the same size, location and shape lying parallel to the stress.
c.
Any discontinuity which occurs in an area of high stress must be more carefully con sidered than a discontinuity of the same size and shape in an area where the stress is low.
d.
Discontinuities which are sharp, such as grinding cracks or fatigue cracks, are severe stress-raisers and are more harmful in any location than rounded discontinuities such as scratches.
e.
Any discontinuity which occurs in a location close to a keyway or fillet must be considered to be more harmful than a discontinuity of the same size and shape which occurs away form such a location.
LEARNING MODULE 9 MAGNETIC PARTICLE TESTING INTERPRETATION AND EVALUATION In order to properly and accurately interpret and evaluate magnetic particle indications the magnetic particle inspector must understand certain definitions which are used in connection with this inspection method. Since these terms are used frequently in this learning module, the inspector must fully understand the meaning of each of the following. INDICATION. In magnetic particle inspection an indication is an accumulationof magnetic particles being held by a magnetic leakage field to the surface of a part. The indication may be caused by a discontinuity or it may be caused by some other condition that produces a leakage field. DISCONTINUITY. A discontinuity is an interruption in the normal physical structure or configuration of a part. These discontinuities may be cracks, laps in the metal, folds, seams, inclusions, porosity, and similar conditions. A discontinuity may be very fine or it may be quite large; it will generally be a definite separation or void in the metal. The word "Discontinuity covers the condition before it is determined whether it is a defect or not. The cause of magnetic particle indications is usually a discontinuity - whether physical or magnetic. And if we exclude those discontinuities that are present by design and consider only those present in the metal by accident or as the result of some manufacturing process, these may still not make the part defective in the sense that t s service performance will be affected unfavorably. we come, therefore, to the conclusion that a discontinuity is not necessarily a defect. It is a defect only when it will interfere with the performance of the part or material in its intended service. So we should be careful to refer to a discontinuity as a defect only when it makes the specific part in which ... . it . occurs unsuitable for the purpose for which it was designed and manufactured.
DEFECT. A defect is a discontinuity.whichinterferes with the usefulness of a part.
P Magnetic particle inspection can be divided into these three basic steps: a. Producing an indications on a part. b. Interpreting the indication. c. Evaluating the indication.
Hellier Associates, Inc. MTMcdS 0 1989
PRODUCING AN INDICATION In order to produce a proper indication on a part it is necessary to have some knowledge of the principles of magnetism, the materials used in inspection, and the technique employed. Since these subjects have been covered in previous learning modules observance of the procedural steps outlined should insure that a proper indication is produced. INTERPRETINGM E INDICATION Aiter the indication is created, it is necessary to interpret that indication. Interpretation is the deciding of what caused that indication, what magnetic disturbance has attracted the particles in the particular pattern found on the part. If the operator knows something about metal processing, it is possible to determine from the appearance and location of an indication the cause of the indication. NON-RELEVANT INDICATIONS NATURE AND TYPE It is possible to magnetize parts of certain shapes in such a way that magnetic leakage fields are created even though there is no discontinuity in the metal at the point. Such indications are sometimes called erroneous indications or false indications. They should be called "non-relevant indications" since they are actually caused by distortion of the magnetic field. They are real indications but since there is no interruption in the metal they do not affect the usefulness of the part. It is important that the operator know how and why these non-relevant indications are formed and where to look for them on the parts being inspected. NOTFFThe use of fluorescent magnetic particles on parts with non-relevant indications is recommended since they emphasize the contrast between the particle build-up at a relevant discontinuity and that due to the non-relevant field. ..
.-...
Non-relevant indications are divided into the following five classes depending upon their cause: a. Magnetic writing. b. Cold working. c. Hard or soft spots.
d. Boundaries of heat treated sections. e. Abrupt changes of section.
Hellier Associales. Inc. MTMod 9 C3 1989
MAGNmC WRITING This is a condition caused by a piece of steel Nbbing against another piece of steel which has been g touching magnetized. Since either or both pieces contains some residual magnetism the ~ b b i n or creates magnelic poles at the points of contact. These local magnetic poles are usually in the form of a line or scrawl and for this reason the effect is referred to as magnelic writing. In figure 9-1 the part in the top view is magnetized wilh a circular field. If another part made of magnetic material is ~ b b e against d or. comes into contact with the magnetized part, as in the second view, a weak field will be induced into the smaller part. Afler the smaller part has been removed the circular field in the original part will be altered or distorted to some extent as shown in the bottom view. Since there is no force to change the direction of the altered field, there will be some leakage at the point of distortionwhich will attract magnetic particles.
FEURE 9-1 CREATION OF MAGNETIC WAITING
Hellier Associates. Inc. MTMod9 G3 1989
COLD WORKING. Cold working consists of changing the size or shape of a metal part without raising its temperature before working. When a bent nail is straightened by a carpenterwith a hammer the nail is being cold worked. Cold working usually causes a change in the permeability of the metal where the change in size or shape occurs. The boundary of the area of changed permeability may attract magnetic particles when the part is magnetized. HARD OR SOFT SPOTS If there are areas of the part which have a different degree of hardness than the remainder of the part
these areas will usually have a different permeability. When a part with such areas of different permeability is inspected with magnetic particle inspection, the boundaries of the areas may create local leakage fields and attract magnetic particles to form indications. BOUNDARIES OF HEATTREATED SECTIONS Heat treating a part consists of heating it to a high temperature and then cooling it under controlled conditions. The cooling may be relativity rapid or it may be done quite slowly, depending upon the characteristics of the metal which are desired. It is possible to increase or decrease the hardness or the grain size of the metal by varying the temperature and the rate of cooling. On a cold chisel the point is hardened to cut better and to hold an edge. The head of the chisel, which is the end struck by the hammer, is kept softer than the cutting edge so that it won't shatter and break. The edge of the hardened zone frequently creates a leakage field when the chisel is inspected with magnetic particle inspection. ABRUPT CHANGES OF SECTION Where there are abrupt changes in section thickness of a magnetized part, the magnetic field may be said to expand from the smaller section to the larger. Frequentlythis creates local poles due to magnetic field leakage or distortion. If a part as shown in figure 4 2 is magnetized in a coil, poles are set up at each end and some leakage occurs at A and B. also, the change of section at C is quite abrupt and there may be a leakage across this angle as shown. These leakage fields will attract magnetic particles thereby creating an .-indication. The indications formed at A and B are usually very easily interpreted; that at C may be more difficult to recognize as being non-relevant. If the indication is continuous around the shaft it should be suspected as being caused by the shape of the part ratherthan by a discontinuity. The non-relevant indication at C will usually be "fuzzy" like an indication which is produced by a discontinuity beneath the surface. If there is a crack ordiscontinuity in that area it will usually produce an indication which is sharper and it probably will not run completely around the part.
Hellier Associates, Inc. MTMod 9 01989
FKGURE 9-2
LOCAL POLES CREATED BY PART CONFIGURATION
On parts with keyways a circular magnetic fieki can also set up non-relevant indications as in figure 9-3. Particle accumulations may occur at A where there are leakage fields. A keyway on the inside of a hollow shaft may also create indications on the outside as indicated at area B in figure 9-4. Here the magnetic field is forced out of the part by the thinner section at the keyway.
Figure 9-3 Concentration of Field in a Keyway
Figure 9 4 Exlernal Leakage Field Created by an Internal Keyway
Hellier Associates, Inc. MTMod 9 0 1989
The gear and spline shown in figure 9-5 were magnetized circularly by passing current through a central conductor. The reduced cross section created by the spline ways constricts the magnetic lines of force and some of them break the surface on the outside diameter. Particles gather where the magnetic lines of force break through the surface thereby creating indications.
Fgure 4 5 Gear and Shaft Showing Non-relevant lndicalions Due to Internal Splines
Figure 9-6 shows a non-relevant indication on the under side of a bolt head. The indication here is caused by&e slot in the head.
Figure 9-6 Non-relevant indications under head, created by slot on top of head
Hellier Associates, Inc. MTMod 9 @ 1989
INTERPRETATION AND ELIMINATION OF NON-RELEVANT INDICATIONS. INTERPRETATION It may at first appear that some types of non-relevant indications discussed and illustrated in the preceeding material would be difficult to recognize and interpret. For example, the non-relevant indications shown in figures 9-5 and 9-6 may look like indications of subsurface discontinuities. However, there are several characteristics of non-relevant indications which will enable the operator to recognize them in the example cited and under most other condiiions. These characteristics of non-relevant indications are:
a
On all similar parts, given the same rnagnefiing technique, the indicationswill occur in the same location and will have identical patterns.
b.
The indications are usually uniform in direction and size.
c.
The indications are usually "fuzzy" rather than sharp and well defined.
d.
Non-relevant indications can always be related to some feature of condruction or cross section which accounts for the leakage field creating the indication.
ELIMINATIONOF NON-RELEVANT INDICATIONS Although non-relevant indications can be recognized in most cases, they do tend to increase the inspection time, and under certain conditions may mask or cover up indications of actual discontinuities. It is-therefore,desirable to eliminate them whenever possible. In most cases non-relevant indications occur when the magnetizing current is higher than necessary for a given part. consequently, these indications will disappear if the part is demagnetized and reinspected using a sufficiently low magnetizing current. Under most conditions the value of magnetizing current which is low enough to eliminate non-relevant indications will still be sufficient to produce indications at actual discontinuities. This will be true where the non-relevant indication is magnetic writing, and for . sewml other types, but may not hold where there are abrupt changes of section. It is therefore desirable
to determine whether the non-relevant indication was caused by an abrupt change of section before reinspecting. The proper procedure is to demagnetize and reinspect using a lower value of magnetizing current, repeating the operation with still lower current if necessary until the non-relevant indications disappear. Care must be taken not to reduce the current below the value required to produce indications of all actual discontinuities. Where there are abrupt changes of section two inspections may be required: one at a fairly low amperage to inspect only the areas at the change in section, the other at a higher current value to inspect the remainder of the part. Hellier Associates, lnc. MTModS 01989
TRUE OR VALID INDICATIONS If the indication is caused by a discontinuity it is termed a true orvalid indication. Ifthe indication is caused by a discontinuity at the surface of the part the particles are usually tightly held to the surface by a relatively strong magnetic leakage field. The line of particles is sharper and well defined and there is a noticeable "build-up" of the particles. This build-up consists of a slight mound or pile of particles which on deep surface cracks is sometimes high enough above the surface of the part to cast a shadow. If such an indication is wiped off the discontinuity can usually be seen. If the indication is caused by a discontinuity below the surface it will be a broad fuzzy looking accumulation of particles rather than being sharp and well defined. The particles in such an indication are less tightly held to the surface because the leakage field is weaker. The difference in appearance between indications of surface and subsurface discontinuities is clearly shown in figures 9-7 and 9-8. Notice the sharpness and definition of the line of magnetic particles in figure 9-7. The pattern in figure 9-8 is much broader than that in figure 9-7 and is quite typical of the indications formed over subsurface discontinuities.
Figure 9-7 indication of surface discontinuity
Helliar Associates, lnc. MTMcd9 0 1989
Figure 9-8 Indicationofsubsurface dismntinuS
8
EVALUATING THE INDICATION Lastly, after the indication has been formed and has been interpreted, it must be evaluated. It is necessary for the operator to decide whether that indication in that particular location on that particular part will affect the usefulness of the part. Evaluation is the determination of whether the part can be used in spite of the indication, whether the cause of the indication can be removed without affecting the strength of the part, or whether the part must be scrapped. As a guide, the following basic considerations may be used in conjunction with the operator's knowledge and experience to help in the evaluation of indications. a.
A discontinuity of any kind lying at the surface is more likely to be harmful than a discontinuity of the same size and shape which lies below the surface.
b.
..
Any discontinuity having a principal dimension or a principal plane which lies at right angles or at a considerable angle to the direction of principal stress, whether the discontinuity is surface or sub-surface is more likely to be harmfulthan a discontinuity of the same size, location and shape lying parallel to the stress.
c.
Any discontinuity which occurs in an area of high stress must be more carefully considered than a discontinuity of the same sue and shape in an area where the stress is low.
d.
Discontinuitieswhich are sharp, such as grinding cracks or fatigue cracks, are severe stress-raisers and are more harmful in any location than munded discontinuities such as scratches.
e.
Any discontinuity which occurs in a location close to a keyway or fillet must be considered to be more harmful than a discontinuity of the same size and shape which occurs away form such a location.
Hellier Associates, Inc. MTModQ @3 1989
R T LESSON 100 INTROI?UCTION T O RADIOGRAPHY . . Radiography is an important part of the inspection and development process within industry. It is used to check structural materials, castings and weld integrity in the construction of . . buildings, power stations, pressure vessels, pipelines, bridges and oil drilling platforms. It is also used in the routine inspection of materials and component parts for the airrrafrlaerospace, automotive, and shipbuildingindusnies. . Radiography is recognized by various organizations thfoughout the world as a reliable nondestructive inspection technique for revealing hidden defects that might lead to failure in se~ce.
...<
ADVANTAGES OF RADIOGRAPHY .
, -
. .
-
Radiographic inspection is superior to other methods in a number of applications:
I)
It is a nondestructive test method
2)
Reveals the internal condition of the materid.
3)
Applicable to mast materials.
4)
Discloses fabrication and assembly errors.
5)
Reveals structural discontinuities.
6)
Provides a permanent visual representation of the object
L ?a
c f l 4
LIMITATIONS OF RAIIIOGRAPW RadiographiciDspection has s e v d inhumtlimhtions: 1)
Two-sided acccxibility of the spechen is required.
2)
Specha sizt and coafigun;ltionmay limit the exteat to which a specimen may be radiographed
3)
Radiography wiU not ddece all discontinuities.
4)
T i invoIved and equipmeat costs makeradiography apemiye.
5)
Presents a potential safety hazard.
IQ erod)
Radiopphy uses X or
&&on
A radiograph records the radiation that has passed through a component so thatflaws can be derectcd A comwnent of uniform seaion without flaws or defects allows the radiafion to pass through the film and produce a uniform image.
to produce an image on a f i l m
A defect in a component such as a bIow hole is deteaed by producing a darker image on the film.
MAKING A RADIOGRAPH
Beam of radiation
Film in a caswttc
. .
I
~ hcomponent t to be tested orinpxted is placed betweenaliadiation s o m and a speiAIY prepared film Precautions are taken to wure that unauthorized persons are kept away from the area to.preventU n I l M a r y exposnre to radiation.
When the equipment is operated some radiation penetrates the component and is recorded on the film.After cxposnre the film is p e in a darkmom m M o p the image.
RT LESSON 101
-.
.,-
INTRODU&'~ON TO IONIZING RADIATION T H E STRUCTURE OF MATTER
AU matter whether solid, liquid or gas consists of elements, or combiinations of elements. .
:.'
'
An element is a substance which cannot be broken down into simpler substances by . chemical m a s . Two or more dements can combine chemically to form compounds as follows: combine chemically to form the solid
1)
at room temperature, sodium and sodium chloride (NaCI).
2)
!hydrogen and oxygen combine to form water (HzO).
3)
carbon and hydrogen combine toform the gas methane (m).
There are 92 mmally d
g dements. If an element is qeatedly divided a stage wilI b e d e d where it can no longer be subdivided and still possess its chemical form. These individual particles of matter, whose existence was suggested by the Greeks, are called 'atoms'.
THE STRUCTURE OF THE ATOM The atdm is the basic building block of all matter. The atom.is the d e s t particle that possesses all the chamcteristics of an element
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planets orii~ting-the sun.
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~ ;.'''.~ Atd.l&-of&atomisthe-'Z'.Y ~ ~ ; ~ ~ h : ' . bulk or 'nude& which is positively . . , .,.. .,.: . charged Whirling around .. . . - ........... '. - n u c l e u s i. nthep ~ ~ ... i m ~ ~ . . . . . . , .. . . . . . . . . . . . . . . ::" .: . . 4 d & & & * . * & are & & ~ Y Y . "-;i>:... ... . .. . . . ..., -. . --...?,. .. .. ... . , ;:jr;.; 'charged !'> -.:...;; . ': .: , .. . . . ->c. :. .... . . . .. .. ' .
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Prolon +vc
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charge
charge
. The nucleus itselfis made up of two types of paaicles of appro-Y equal mass: 'protons' which are positively charged and 'neutrons' which cany no charge.
EIelectrons are 1840 timeslighter than protons or neutrons, and have negligible. . mass.
?he total negative charge of all h e elelectrons orbiting the nucleus balances the positive m e o f theprotonsin the nucleus. The atom as a whole therefore has no e l d c a l charge.
EICC~O& the ~udens in c ~ defined y paths or shells, Each shellcontainsa catainnumbex of elechons. The oqta shall has fcwaclccwns t&anit can @kcIt'is pnpand to khan:'.the . 'vacantplaceswitfi c l c m u ~from s other atoms. This aIIows one atamto combine ch+caIIy with anotha: . . '
Thechemicalpro~ofatomsarc~odbythenumbaofelectronsindx:outn:shelL A . t m of a particuk clementhave a fixed andequal nambcrofelectrons and protons Mdanormal ciratmma The numbm of protons in the n u d w of aparthhclanent is known as tfie atomic number of the clrmcnt
...
MASS NUMBER
("A" number)
_ -,.
? h e m s of an atom is fix& by the number of protons and neutrons in the nucleus. L .
2 Neutrons
13 Protons 14 Neufrons
00 00
Hydrogen
Helium
1proton + 0 neutrons Mass number = 1
2 p ~ t o nt s 2 neutrons Mass number = 4
13protons t 14neutrons
Mass number =27
Theatomic n u m b i s the number of protons, and is a ~ c m i s t iofc thc atom of a p m m i d~nent, for example:
Hydrogen Helium AlmDinium
ISOTOPES The atoms of each elemerit kiitain aJe6nite number of protons but may have a different number of neutrons. The& atoms are called 'isotopes' and are given an identifying number related to the weight of the nucIeus. Such atoms have the same chemical properties, but have different weights and radiation properties. The three different isotopes of hydrogen are an example. These isotopes are chemically identical but are written:
.:
...
Other well known isotopes used for indastrial radiography are: 1)
Cobalt 60
Nore: the fop number represents he total number of prorons and neutrons in the nudm Ehe alorm-c weight. The bottom number representr the as~micnrunber.
-
3)
Iridium 192
1P2Ir 77
.Th& arc isot~pcswfiich;givt off some form of ionizing radiatioa In the process they may themscva be mnsfhmd into otfierelements by lmingpartides.fromthtnuclcus. 1sornpcs chat have a @eat& number of neutrons than pmmns in tfie n&cIeusan said to be 'unsta&'. Unstable isotopes try to s t a b i i themselves spontaneously by a nnmba.of diffwprocesses:
-
1)
by nlcasingneutrons .thatis ejectingneutrons frmnthe nucleos.
2)
by s p W g neutrons into anew proton plos an electron, which fIies off at hi& speed.
The isotopes which arc usefnlforradiography give offgmmaradiation as a aof some and affects of these spontaneouschanges. Gammamdiation is vay penphotographic fiIa - hence its uxfdness.
i
TYPES OF RADIATION
- . Dlning the radioactive decay process, caused by the splitting of the neutrons, radiation is released in three different forms: - '
1)
2) 3)
alphapdcles (a) betaparticles (p) gamma rays (Y)
Alpha particles - are 2 neutrons and 2 protons (helium nucleus) bound together to behave as one fundamental particle. Alpha particles are emitted from heavy nuclei containing a large number of nucleons (neutrons and protons) such as Americium 241, an anificial elment Beta particles - arc high speed elecEons which are emitted from the nucleus. Beta particles are emitted dlning the decay of Iridium 192 and Cobalt 60.
-
Gmitna rays are electromagneticradiations (as are radio waves and Light waves) that are emitted from the nucleus. After the emisdon of alpha and beta particles, the nucleus can re-adjustits ehergy still fkther by the emission of gamma rays. This emission does not ftnther change the element These gamtna rays are used for radiography.
Apart fromxntudy oarnringradioisotopes, it is also possible to produceradioactivity in n d y stable elements by the use of a nuclear reactor,or a high energy particle amelemlor. This is done by introducingenergy into the stable nucleus in the form of an energetic paaide such as a neutron. The nuc1ev.s then loses this excess energy by giving off radiation in the form of gamma rays Radioisotopes are producbi in nuclear reactors by twomethods:
1)
They can be sepaiatedout of fission hgments, ifenemfedwhen afnel element like uranium 235 is used E m m ~ l of e ~comma nidioiitopes this manner - rxoducedin . are cesium. 137, strontium90 and krypton 85.
2)
'~$ble~ents&bcmaderadi~by~adngthrmina~e~d~onina ndearreactor, shieldtd by specially designed aaxssholes. Nentrons originating f h m the mxta are used to irradiate thcsestable clancats. Examples of mdioisotopes prodnced by this m&od am cobalt60 from cobaIt59, iridium 192 fromiridium 191,'and thulium 170f r o m t h h 169.
All these radioisotopes may be used for i n d d radiography.
.
RT LESSON 102
'RADIATION SAFETY DANGERS OF IONIZLNG RADIATION It is vital thatpeople who use and operate X-ray and gamma-ray equipment obsave the proper safety standards. Radiation may damage your health and shorten your Iife. Your . safety is of utmost importance.
-
-
Ionidngradjationsarc pa&cularly dangerous becanstthey arc invisible and cannot be detected by the human senses. They can canse injury to human tissues and organs, for example those that prodnce red blood corpuscles in bone marrow.
&tcn&e dosesof ionizing mdiations can m n ~localized e damage such as radio&m&is or gangrene. They may also cause wad h& disordas, such as Ieukaunia, {cancer of
ffie bl&)
which may eventualIylead to death.
;
?he darnaee that on be done to vow
health by &nidng radiations m$ also affect future generations.
Some effects ofradiation accumulate with time. Each radiadon dose received adds to those already gained,
However. X-raysarid gamma rays used for industrialpurposes cannot make a room or an object or the air radioactivt. When m s u r e is over the radiomhed o b i a i s hamless and can be approached and handled dsafety. A m e y met& n k bc t b d after each expasme to ensure that the sourceis safe to approach.
...
CONTROL OF RADIATION EXPOSURE
Employers and employees are rcqkedto do aIl t h i s rmsonably praaicablc m restrict the extent to which people an:exposed to ionizing radiations. ?he unit of mdiation dosc is the REM Forpractical~nrposcs when measming X and gamma &on the rem can bc considered to be apvahu to the RAD or the Rocntgeo
j-j \
-:
MAXIMUM PERMISSIBLE DOSES ?he statutory regulations'sI;ecify' h
MEASURING RADIATION
~adiationdoseratr:ismwsm?edwidi.'. .: aradiationmetcrorm o n k S o q ' ...
. . .. .. .
typesuseGdga:m~tnbcstodetcct gamma or X-rayi o n i d o n and m battayoperatcd. 'Ihtrcadontscalc i s i n r ~ ~ t g u i s p e r h o n r, .a n d ~ t g e 'n s .. :.. . . per hour. . '
-
Thtaccnmnlatedwllo'lebody dose of ionizingradiation I X W ~ V Cby ~ rnonimnd pmsoPQM)nnel must not ex& 125 remiper calendar quarter or 5remiper year.
-
SAFETY EQUDPMENT AND REQUIREMENTS .
. - _ -._
PERSONAL PROTECTION
To ensure that you are efEectivelyproteaed from ionidngradiaton ahd that the ma%imum dose rates are not wrceedwi, statutory @tiom appIy to aU industrial radiographic options. These legularions are cantakedinthe Ccde of Federal Regulations and State Regulations.
RADIATION DOSE RATE METER (SURVEY METER)
This is the most important item of eqnipmcnt far yoar safety and protection. .Thesnrvcy nebx is a dclicatc inshmnmt usually calibrated in milliroentnens .. - per - hour (mdhr). ~ndicationofthe dose ratcisdirect-it is used to: 1)
. Check theposition at which bmim should be set up.
A m e y meter shouId always be available to each nvliogaphy team.
The survey meter s h o d be a p p r o p for ~ the type of radiation in use. Where necessary scale conversion data s h o d be available. Survey mters should be tested by a qualifiedperson before use. niey must be calibrated at 90 day intends and afta: dl repairs. Rtconls of calibrations must be kept by your employer.
AUDIBLE ALARM D O S l M E T m
Theseinstrwnentsindicatethcprcse~xof~onbyanaudibIehignaL w a n d e r and Wter thanthc smvey m e and an designed to be canid on your paw& They
. gi~cwamiogofhighdoselatc~~mustbenvirdrcdondrtrSngchewhokpaiodof
ps@leep~sure.l k y m p a r t i c n l a r y v a h a b I e w h e n u S i n g X - ~ ~ y m ~ ~ o n eqmp~becaasefheZrgiwaaimmtdiaawamiog. kisdtotesttheuuits~~y
tbensmtthatthtyanmgoodworIdngorder,andthatthe~~~icatestht~~~of . ionizingradiatioa
FILM BADGES AND THERMO-LUMINESCENT DOSIMETERS
Regulations require you to wear a H.mbadge so that thc amount of radiation you are exposed to is documented Thc f%n badge consists of a photomphic film in a special holder, which you should attach to your trousers belt or to the outside of your normal clothing.
FROM
R(CWB€TAWINWW ~OPEM QIP
mhLORPLLSIXCLSE OWVESLSBETA6HIEU)l RU8
Atthccndofthcase~thcfilmkpnxxssedandassesscdtod~ethcamo~of radiation received. The film badge pmvidcr a -record of your dose. Your p a x d radiation dose noprdis kzpt by yonr cmploya. Yon may ask to see it at any misonable time. ~ ~ ~ 6 ~ n a , ) m a y b e n s e d ~ o f a f i l m b a a g e t o m a & your personal dose. A TLZ) is a phosphor in a solid cyrstd shactnre that, when cxpostdto lonizlngradiationsbncs~~ergy. W h W h e n h e a t c d t h c ~ i s n l ~ i n & e f o r m o-f. Wwhich is p e o n a l to tkc exposing radiation. ..
,--.%
Always wear your film badge or TLD on the outside of your normal clothing, at the fmnt of your body. - . .- ^
2
1
.
If you remove your coat or coverall when wotking,.make sare that you W e r the film - badge to either your shirt or tronsem.
.
Dating "offwork" paiods keep your film badge away fium high tcmperahncs, such as hot pipes andradiators. Protea it frompssibIe ch* attadc,and do not keep it near luminous articles sach as alarm docks,watches, compasses,and mdhtion sources.
:p-
'00
wear y o u r f i badge dming6e whole w0rki.a~M o dincIuding preparation 6tmnspt, d g up and storing equipmat
If yo0 notice any defects in the film badge holder, partiaxlady if any of themctaIinserts aremissing, orif you 10%or damage it, inform your snpavisor at once.
R+hnn yoor film badge pmmpfly at thetimespccifie& Alwayscnsore thatyou have a new badge to wear before giving up the old one.
POCKET DOSIMETER
C
A dinxX nadingpocbdosimctarteadingform0 to at lease200 mR must be wom in addition to a film badge to detamine your acaunnlateddose. It is not an acceptable : substimtc for a f ilmbadge, but is an optionalcheck. Thc major a d m g e of &edimx nading pocloct dosimeter is thatit gives an imnmbfcindicationof your wgosmc dose.
ALARMJNG DOSIMETER
an audible alarm whenever the dose rate equals or
J
.-
An darning dosimeter pmvi&g ex& 500 mR&rmWhr must be worn in addition the the film badge and direct reading pocket dosimeter.
SAEETY WITH RADIOGRAPHIC EQUIPMENT
Allequipment must be maintained in a good, cIean, safe, working order.
It must be
'[
chccked b e f q and afteruse by the raaiographer at each site. A record of these checks shouId be kept showing details of any defects, and the action taken to remedy them. In the case of ga& expos& devices, a &ey meter must be used during the exinination. This will aIso confirm that the survey meter is wod6.u~.Ifthe survey meter pives - no reading, check with another survey &w.Report a~unwual readi;lgs.
Esniprmnt must have a k of pmmting useby d & people. X-ray equipmenti s usually fittedwith akcy switch on the corn1 panel or box. The key s h d be in the custody of themiiograph'?~IImnst .only be used when the equipment is in m o v e d aftetuse.
Gamma containes are fitted withdtherabyoraspecial typc of lock. You mnst cnsmethat it is kept locked at dlti- cxocpt during exgosure. The key should be Inthe custody of the mdiographer.
0'
GENERAL PROTECTION Everybody in the area w h & - i o ~ ~ is ~ being o n usedmust be protected fiom radiation. In a prmauent instalIation shielding is provided by an enclosure of thick walls. On sites where it is not reasonably practicable to provide walled enclosures alternative protection must be ananged
Distance is an effeclive protection from radiation. ?he greater the distance from the source, the lower the radiation level will be. For example, at Mice the distance fiom the source, the radiation level wilI be a quarter of its original level. Thisfollows the inverse square law. . -
BARRIERS . -
-.
TOmake sure that otherpeopl'oii the site are adequately protected you must set up a suitabIy marked area, to keep out a l l except authorized persons
If for good reason this barrier is not set up at the 2 mWhr dose rate boundary it must be so indicated and explained on the daily repod -
:
RADIOGRAPHIC PERSONNEL - . _ - --
People working wirh ionizing radiations an:categorized amrdhg to heir degree of and involvement There are three categories: Radiation Safetv Officer, Radiographer, - . Radiographefs Assistant
-
-
-
The Radiation Safety Officer is usually a supervisor appointed by the Licensee who . has the knowledge of, responsibility for, and authoriey to enfom appropriate radiation protection rules, standanls, and practices on behalf of the liceme. - The Radioeraoher is an individual who perfom or who, in attendance at the site where the &tion exposure device or sealed source is being used, personally supervises radiographic operations and who is responsible to the licensee for assuring compliance with. the re-dati~niand conditions of the license.
-
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The Radiographer's Assistant & an individual who, under the personal supervision of a radiographer, rises radiographic wgosme devices, sealed sources or related handling tools, or radiation survey instruments in radiography. STORING AND TRANSPORTING SOURCES When a sourceis not in usc or in transit.you must cnsunthat it is kept in a secure storaEe a m . Ihestorage area m& be ~ d et r h c ~ ~ ofoan nddperson, who keeps a
record of thc utilization of somces, and who has castody of the keys. Waming notices mnst be fixed to the ootdde of the storage area. Tne notices mast include the internationally agreed symbol for ionizing radiation.
L.
EAUTIDN -
W D E[AT110N *
AREA n
..
TRANSPORTING A SOURCE
1)
Enson'thatit is id:a locked, shielded containez Check thatit is suitably shielded by usbga m e y meter.
2)
Enson thax you know its type, andactivity.
3)
Enson that yon an wearing a film badge cn-'ED, a d i m readingdosimeterand an alarming dosimeter.
4) . Follow tlteFeda& State a n d l o w l r e ~ o o s .
Note: you should chcck that every sealed source containeryou receive k marked with a proper labelb h @ t n g the cu~otdarts, anda "RodiwaivekiA.iataial"label.
.-
LESSON 103
l.,-RT A .
X-RAY EQUIPMENT THE NATURE O F X-RAYS
X-rays
Light rays
X-raysare elecfmrnagneticradiarions and have rhe same nature as'radiowaves,light and ultra-vi01e light They mvel at the same speed as light and obey most of the same laws. They differin that their wavdength is much shorter than Iigkrays.
Ilis this cbaracttaistic thaf forms the basis of the a b i i of X-lays to penetrate solid matexiak.
PRODUCTION
X-RAYS
l-7 Whm electrons travelling at bigh speed collide wit&rnattcrin any form, heat and X-rays are produced. To do this the following are nccdcd:
I) 2) 3)
a source of ibe elections.
a rneans of acoclera(ing them to high s p e d a m&od of stoppingthua
This is done within the modem X-ray tube - a glass or ceramic tube or envelope in which a vacuum has been produced- Two e l d e s are placed inside the tube: an anode or positive electrode and a cathode or negative electrode. These are connected to e l e a r i d circuits with a Iow voltage and a u m ~flow t on the cathode side and a high voltage induced into the anode by a transformer. Electrons are produced by: Heating the &ent of the cathode with an decwic current libxxtes the electrons. Increasing the cunrent raises the temperam of the £ilamentand hence increases the number of electrans liberated. Electrons
Cathode
2)
&ode
Applying a high voltage across the anode and cathode from the cathode towards a target faceon the anode.
a beam of electrons
Electrons
Cathode
3)
h d e
The electrons arc stopped by ailowingthrm m hit the target hon the an& Whcn the cleumns are brought to an abmpt halt by the target Eacc a BnalI amomof their energy (about1percent)isnalizedasX~ays.Thercmainiog99percentisdissipatedasheat
i
CONTROL PANEL
All the controls necessary for the 6puadon of the X-ray tube head are collected together in a control box or in an operating paneL
?he diagram shows an example of a portable conlroIarrangement that 'Alows the mdiogmpher to carry it to a convenient, safe opemting position It is made of sheet stwi, has a lid and wxyhg handle and is weatherproof. It includes the following f e a m . .
-.
I
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RcmovabIe safety Ley vhi& bm&s the p o w happy to give the ~ o g r a p h s control against accidentalradiation. X-rays on button X-rays off button. Delay light CabIe connector to power supply. CaJdeconnector for anxiIiary powcr to warning system. CabIc conneaor to mbe head MiEmpaage control to alta the quantifyof radiatian.
MiIIiammctcrtokdimemk~ Kilovoltage control to aZtapeeetratingpowaof racktion. Rilovoltagc metawhi& may also indicate the line voItagc T iwith automatic wtposm\: met
,
Note: on some wmolpanels theremay be other wmning lighrs:
-
Red light tube head on orX-rays on. Green light - tube head off orX-rays ofi
. Some ako have ma~mnhcwarning of ' may be &le
Hellier, Inc.
or light wmnings.
q
o about ~ to begin', or 'exposmeon'.
Thuc
(---I .-.
TUBE HEAD
,. -
L
The x-ray tube is enclosed in a metal cannister, connected to gcounci, and f i e d with an
insulating liquid (oil) or gas. It also houses a aansformer, supplying high and low voltage, and a cooling system (as a lot of heat is generated). The tube head provides shielding against unwanted radiation. .X-rays emerge from a window made from a material which allows the passage of most of. the radiation Some windows on low kiIovoltage units are made of BeryLIium which has a low rate of absorption of X-rays. - A red light may be incorporated in the head which flashes a warning when X-rays are W i g generated
&ss-section of a tube head I
...
o$
'
VYl
The X-ray tube is made of a toughened glass such as pVrex. It is shielded to restrict the escape of radiation and contains an anode and a cathode seaIed in a vacuum The calho& is connected to the ncguivcpole of the high voltage circuit At the end of the tube there is a fiIament madcof tungsten as it ha5.a high melting point ~ hnumber c of electrons c&tted depends on the Wqemimereached by the filament on the cathode, when it is heated by the eI&c ammt Varying the cumnt varies the
tcmpaahne and So in tmn controk the emission of elabnns.
The m&is co~ccted to the positivepole of thc voltage Wt, ?he anode usually consists of a solid block of coppawith its end cnt away to form an an& of about 709 Thisprovides a focal spot of &dent size, and spreads the heat so that the target does not mcIt Thetarget is made of tungsten set into tfic faceof the anode. As much heat is . generated at thc anode, a large area of impact is desirable. U~lliorIn-
The elecrron beam of negativeIy charged electronsi s accelerated towards the anode by appIying a very high voItagc to the cathode. This voltage is rimmedin kilovolts 0. The tube c m t from the cathode to the anode is low and is measured in x d J i a n ~
(dl. The impact ofdeamns on the target faceof the anode generafesxgys. Theinteaityo£ the X-rays emitted by the Eube are in proportion to the tube current
.
Note: only thew&X-ray from rhe uugetface.
b m i% shown. X-rays are hawever ememrned in all directions
A bmm of radiation can be produced either laterally, axblly or obliquely panoramic depending upon the shape of the target face.
APPLICATION OF VARIOUS TYPES OF X-RAY EQUWMENT X-ray equipment commercially a v W l e for i n d u s t r i a l m d i ~ ~ ~work h i c is class3ied according to maximumkilovoltage. The choice of equipment depends upon the type of work to be undertaken There are other types of X-ray equipment but they are not suitable for work on site locations.
Approximate thickness of steel which it is practical to examine using X-rays Self-rectified wuivment Ak&num (kV)
kilovoltage
Hellier, Inc. RT Lesson 103
routine work inches
maximum thickness inches
onstant potential equipment mutine work inches
r~aximum
thickness inches
RT LESSON 104 GAMMA RAY-SOURCES AND EQUIPMENT ADVANTAGES OF GAMMA-RAY EQUIPMENT
ww
1)
-
Portability gammaradiography is paaiahdy Snitable for use on site locations, becansc it is portable and requires no power supply or cooling system.
3)
--
-
2)
Accessibility - gamma-ray source containas are genaallysmallandcanbe takEninto places which are lnacccssibleto X-ray @p-t
SmaIf source-to-film distance a gamma-ray somce is suitable where a small sourceto-filmdistance is necessary, such as when radiographing weMs on small diameterpipes.
Hellier, - IIC.
~n
-
High penetrating power some gamma-ray sources have a very high energy (penetrating power) which makes it possible to reduce the time of l e exposure, and obtain satisfactory radiographs of verymetal components.
-
I
Capital outlay Iow ove& cost compared to X-ray equipment 6)
-
Scatter less scatter compared to X-rays.
DISADVANTAGES OF GAMMA-RAY EQUIPMENT
1)
Gamma radiation cannot~bkwitchedoff. Thereforeradiographers need to be protectad at all times from these penctratingrays by c o d y designed equipment and procedures.
2)
The quality of the radiograph cannot be readily controlled as it can with X-rays. The gamma-ray wavelength caonot be altexed using the same isotop~
3)
Gamma rays give a higher energy r a w o n than X-rays, with less contrasting images. This makes the radiographs more difficult to interpret .
4)
The activity of some radioactive isotopes with a shoa haJf-life decreases quickly in a short time. It is therefore necessary to periodically replace thc source.
5)
Precautions are essential when storing or tranrpo&g the s o w and container.
-
INSPECTING A LARGE COMPONENT ON S r r E
EXPOSURE DEVICES The gamma-ray source is eontainedinside a radiation shield known as an exposure device. Each type of radiogmphic - - exposure device has a source holder and is fiped with an arrangement for exposing the source when required. Exposure devices used for site indusb3 radiography fall ioto two geneml categories: shutter tweand umiection me. There are manv variations of each. Radiomnhm must .. ensure &I they &-familiar&h any special f&es of h e equipment to be-usad Employers are responsible for providing traiaing in the use of this equipment - -
SHUTTER TYPE DEVICES W~tha shutter type device the sealed source is exposed for radiography without the source leaving the W + o fthe container. ?his is done by either swingingaside, or rotating part of the shielding.
7
1)
Front shutter
The radiation beam is exposed by raising theh n t shutter. These containers are mggcd,rcliab1e and s u i t a b 1 e f o r r m o s t ~ otedniqoes, n exceptpipewelds.
2)
Rotating shutter In this type the s o w is exposod by rotating the shntttr by hand, or by ranotc cable o p d o a When s o m of high adivity are used the shutter should be o p t e d by remote w n t d
Care must be taken to ensure that the source is exposed away frorn the radiographer.
Rotating shutter type devices are us& for radiography of pipe and other applications qujring a directiondl exposure.
PROJECTION TYPE DEVICE (CABLE OPERATED) The s o w is moved along a guide tube to an external working position by means of a cable. The cable is driven forward bv the radiomphu. using a hand&& wind-out gear. At the end of the exposure the &ble is retfaced to &the source to its shielded position. . -
Control Cable /
I
I
Projector
Exnose 8----
Lbl Source i n Transit
I
I
(c) Source at Radiographic Site
Scurce
I
SAFETY REQUIREMENTS FOR USERS OF RADIOACTIVE SOURCES To protect radiographers, 0the.r workers on the site and the public, there an:a number of regulatory agency reqkments that must be compIied with by anyone usingradioactive sounzs. In addition, each lecensee must prepare an Operating andEmergency Prowdim Manual which contains deiailed instruction for the operation of the equipment, safety p&m, anddetailed instructions in the event an equipment malfunction, an accident or other unusual incident occurs.
1)
Radiographas most be given proper safety inshuctions on thc dangersofionizing radiation,and the use ofequipment.
2)
ThqymnstwmrafilmbadgeorTIl)torccordthe~tion&s~dved, Rtcords ofpefional dose must be mainrainedby the anpIoyer.
3)
73ey mnst war a direct rcadingpockct dosimeter to snpp1&ent the film badge or 'ILD for personal monitodng.
-2 C
Establisl> ~ e s t r i c t e dArea
5)
6)
?hey must take reasonable precautions to minimize exposure to radiation by establishing the unrestricted area around the workplace on the site. Limiting the useful b&m to the minimum sizepracticable will aIso minimize exposme and reduce the size of the radiation area. Any additional shielding that can be utilized wiU also minim& exposure.
They must use a qxkymeter to checkthe level of radiation at regular intervals, and at source contajnm after every exposure.
7) - - Thcy must ensm~ that gamma radiation cxpmx~ devices arc kept locked at all times except when acmally bdng used for radiographic q o m m . 8)
Hellier, Inc.
They must ensm~ that -radiation somces am stored scantly in a dcsignatcd, locked storage area when not bdng used on ajob site
RT LESSON 105
RADIOGRAPHI~FILM$ AND INTENSIFYING SCREENS RADIOGRAPHIC FILMS
Most Nms used for radiography have emulsion on both sides, however, films with emulsion on only one side are availble. Single-sided &requires much longer exposure times than doublesided films. .
<
:
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An x-ray film is made up of seven layers:
1.
The super-marum which is a thin layex of clear hardened gelatin which protects the underlying emukion from damage during normal handling. (a)
2.
The &ion which is sensitive to X-rays, b r a y s , light, heat, pressure and some chemicals. The mulsion consistsof a large nnmbw of minute grains of silver *bromide (silver halide) embedded in a supportingmedium of gelatin. When radiation strikes the emulsion a change takes place kihe physical stTuctme of the grains. effed:is called ' which is invisible.untilthe filmis chemically p~~ @)
3.
A substratumwhich wnsistr;of a miof and a b i g m a w It ensures that the thin emulsion Layer adheres £irmly to the base dtuing the stages of processing. lhisis particularly important in high tempaatrne automaticpmming techniques, and processing under tropical conditions.,..(c)
4.
?he base which is a ce11ulose tciacate or polyster such as 'Esta~'which forms a tough, transparent, but flexible base. (d)
&\"@ protective coating (a)
#
Protective coating (a) .. ..-..-
, -53
THELATENTIMAGE .
-
The word 'latent' means hidden and it is used to indicate the invisiblephysical change that takes place when the grains of silver bromide suspended in the gelatin are affected by light, X-rays, gamma rays or other radiations. This small physical change is then exploited by development, the farmaton of tiny grains of black, metallic silver, to produce a visible image. . .
It is these grains,suspended in the
gelatin on both sides of the pliable base, that form the image that is visible after the frlm is processed
.
RcMive speed
-
Relative
expo-
d-- -gcain
(mediumhigh speed)
Note: thesefigures rd2.r to a rypicul range of-
.
The nature of the emnIsion, and the onxxssin~of the film. moduces a 'imkiness' in the imam= " which is the random clnmphg of G s i l v a ~&IS. The &a t.t&i&b.d @VQbromide the less graininess t h a t will be in the image. The grain s i z is dated m the smsitivirv ofthe film to radiati0~& d y , fine grain is associatedw& slow speed films. and g & wibhip$ -films. ~
6 i
FILM CASSETTES Fdm cassettes can be flexible, &-h*d or rigid The flexible cassette is made fmrn strong, vinyl and is used extensively for site radiography because it can be readily adapted to various shapes and sections, such as pipework and circumference welds. There are two designs: 1)
-
Double envelope cassettes These have an inner and outer envelope. The outer envelope is closer to the film size than is possible with a rigid cassette. This makes accuratepositioning of the film for exposure easier.
2)
Single envelope cassettes ?hex have a nylon press . . down fastener which gives good light-tight sealing. It also enables the-cassetteto be opened or closed at a touch.
3)
Semi-rigid cassettes Theseconsistof a cardboardfront and back hinged together with flaps on the inside that are folded ovetthe filmto pmduce aligkt-tigfitmvclope. Although not as flexible as the previously desuibed cassettes they can be formed around a large -ex pipe or casting.
4)
Rigid cassettes T~.CFC con& of a thin albminiuinot had plastic front andbackwiih a felt prcssmr:pad attachedta thehideof th'ebackto'keepthcfiImandscrceztmdkces ininbmtecontact Thcsc a m used when farming tfie film.mundthe part is not nqoinxl.
Hellier, Inc. RTLesson IOS
F.,.3
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INTENSIFYING SCREENS The d e w of photographic effect of X-rays and gamma rays depends on the amount of radiation enerm that is absorbed by the sensitized C O ~ M ~of: the f i This is about 1 per cent for radiation and of m7&um penebrating power. The remaining % per cent of radiation pass&through the fh is not used. To overcome this. the film mav be sandwiched between two intensWnt! - - meens. . Under the action of X-rays and gamma these screens either emit electrons (lead screens) or fluoresce (fluorescent screens) which results in an e m photographic effect upon the film emulsion layers. Intimate contact between the f h and the screens is necessary to obtain sharp images.
rays
.
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There are three main types of screen in general use: lead foil, salt or fluorescent and fluorometallic. LEAD FOE SCREENS These are used extensively for industrial radiography. The intensifying effect is caused by the liberation of electrons fium the lead foil under the excitation of radhion. These electrons strike the film creating and intensification of the photographic action in the emulsion of the film. This intensifying actions results in a reduction in exposure t h up to 75 per cent ?he lead screens also absorb the low energy smaer &tion resnlting in improved contfdst
Lead screens art made np from thin sheets of lead foil, which is d~ uniform in stcuc& and stu& on t6 a thinbase, such as stiff paper or card.
Flaws on scrtens,.such as scratches or mcks in the d a c e of the metaL are visiile on the radiographic image. T h d o r e damaged m n s shonId not b c ' d
Narmally two lead screeos are nsed. The thickness of the front screen mustbe matched to the M e s s of the radiation used Thisis to allow the primary radiation m pass throur31, while stouuing as much as ~ossib1eof the secondanr radiation k . which h&i a Ionm&veheth The front screen is usually 0.005" thick, and the rear screen about 0.10" thick. It is however possibIe to use two screens of the same thickness.
Lead i u t a s i f y k g scnens arc not particulariy effective w i t .x-ray equipment below about 120 kV.
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FLUORESCENT SCREENS (OR SALT SCREENS) . . These consist of a thin flexible. base coated with a fluorescent layer made up from fine crystals of a suitable metallic salt usually calcium tungstate. Two main types are used in industrial radiography:
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1)
high definition (fine grain) screens, made of small salt c~ystals
2)
screens giving Figh intensification (rapid or high speed screens) made of larger salt crystaIs.
Exposun to X-rayscauses the salt crystals to glow with a blue light 'ilk light a6E& the fihandproducest%emainpaiiof thelatent film image.
Theradiographic film isplaced bctwccn two screens coated with thescsalts,sothatthesaltcoatingis in contaa with the film
Salt screens rcducccxposnn time and allow a lower kilovoltage to be used. However, definition is affeded by salt screens. dcpendingonthcrjzc6f thesaltaykals. Faster smms givc the worst &ect
Thtscnensandthcfilmarcthen p W in a mml or plastic c8ssette.or film holdex. so that thcv arc in
Salt screens shouId be examined frequently to ensorc tfiat they are &txh mdust and dia. They can be cleaned with a sIightly soapy sponge, or wad of cotton wool, applied gently until all traces of dirt have been removed. At no time shonld thc sponge or cotton wool be wet enougfi to allow b p s of water to fall on the scrceps. over once with a iwistcneddmp cloth wad. Dry with a clean so& cloth f k e from loose fibres.
Note: salr screensare rarely used with gmnma radiation
Hellier, Inc. RT
T~.v.vnn7nF
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FLUOROMETALLIC SCREENS
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These meens are a combination of &&I& screen and the salr screen giving the elecmn emission effect and the fluorescent effect They consist of pairs of s m m s made up of flexible or mrd support thin lead foil layer of fine grain fluorescent salt They are normally used with fine grain, high conblast direct type fikn giving an i n t d c a t i o n which can reduce the exposure by a s much as nine times, yet without losing too much sensitivityof flaw detection They are made in different grades to suit different X-ray and gamma-ray energies. Their use is largely confined to routine inspection when speed of exposure is essential but when . ordinary salt ScTeens would give too great a loss in critical inspection.
THE INTENSIFICATION FACTOR This is express as the ratio of the exposure without using screens to that of using screens as follows: Intensification factor =
JZmosurewithout screens Exposure with srreens
It varieswith the kilovoltage andfhe circuitry of the X-ray set b e i g used.
The graph shows the intemiEcatibn factor with the kilovoItageused for salt and lead screens.
..
With salt screens the maximum &&is o b ~ e atdabout 200 kilovolts. With I& screans the inleosification dfect is only obtained above 120kilovolts.
~ellier,Inc.
R T T ~ r m n1 n F
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RT LESSON 106 PRODUCING THE RADIOGRAPHIC IMAGE
IMAGE FORMATION A radiopph is a shadow picture of a component which has been placed between an X-ray tube,or a gamma-ray source and the film. The appearance of the shadow picture produced is influenced by the relafive positions to each other of the items in the diagram
I
It is imp0rc1.1~ that i n d m radiographm are f a d i a r with the geometricalprinciples of image formation. Because X-rays and gammarayshavdin straight lines like rays of E a t , the shadow or image foxmation they produce is easia to wcplain in tams of light as shown in the diagrams.
If a beam of light firoma fh&@t shines through a hole in a card onto an object placed between the card and a screen,then aprimary shadow image of the o b j a wilI be formed on the s a e u ~ This primary shadow is r c f d to as the umbra.
T3e sharpness of the shadow image of the object depends on the items in the diagram. .
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Ethcseitems or factors are not correctthe wnbra will bG surroundedby a secondary shadow refened to as the penambra, The width ofthc nmXnalW o w is n : f d to as g e o d e ~ ~ t ~ h a r p(Ug. m s If the width of thc pea& shadow docr not exceed 0.020.. the image will. appear sharp to the unaided eyc In practice, since the source always has sorne dimension there will always be sorne peII~bra1 shadow.
PRODUCING TEEE SHARPEST POSSIBLE IMAGE The foIIowing conditionswilI implove tttc sharpness of the shadow image:
.
1)
Use the smaUat possible focalpoint or source
2)
Ensnre thc d
3)
~nsartthe&possib~e~Ct~~somccandthe~(~)
e s t possible distance bctwm the oobject and the screen (fSlm).
FACTORS THAT AFFECT IMAGE FORMATION AND PENUMBRA .- *. . SOURCE SIZE If the size. of the sourceor focal spot is inaeased from a small source to a larger source,the resulting shadow image of the object wiU be less sharp, (ie. the penumbra increases).
SOURCE-TO-FILM DISTANCE The distance betweenthe sornrr.ar focal pint and the film is known as the s ~ m t o - f i l m distancc Kthe s o m is moved farther away from the film the amoont of shadow ovcdap - or puuunbra is nduoed. t h d y pmkcing a sl'larper image. When the sourceis close to the screen or 6lm it produces alarge penumbral shadow, resulting in an unsharp image.
When the source is moved Wier away from the smm or film it produces a nantowcr penumbral shadow, thatby . reducing the geomefric unsharpness.
OBJECT-TO-FILM DISTANCE (OFD) .
.
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<
1.
This is always calculated as the distance berwmthe s o w side of the object aridthe film It is essential that the image of any discontinuitiespresent in the objectshould be as sharp as possible. A space b e ~ e the n objea and the film.should be avoided as this has an adverse effect upon the sharpness of the irnage.
When the object is movcd awayhmthcscnxnorf3m it gives an increastd shadow owdap and an nnsharp h k g e
When the obj& is moved close tothesmmorortilmit gives a reduced shadow overlap and a sharper image.
2)
To prevent distortion the f h or screen should be at right*. angles to the source of radiation
3) d.
The plane of the object and the fih or screen should be paallel to give a sharp image and reduce distortion.
CALCULATING GEOMETRIC. UNSHARPMESS (Ug)
A sharp image has a small width of penumbra (I0.020")med to as geometric unsharpness (Ugj. This is obtained uskig the d e s t avaiIabIe somoe, the longest pracfical source-to-film distance with the object placed in mntaa with the film hoIder.
The size ofthe peuumbm (Ug)can be CaICnlatedfromthediagram using the following formula: u g = fxd D
Ug = h'ze of source3 x (obicb-to-filrn distance) source-to-obj& distance
CALCULATING TRE EVLZWIMUR.P FOCUS-TO-FILM DISTANCE The minimum sourceto filmdisthce that would prodnce an image with a Ug of 0.020"can be dcuIated using the following formnla:
Sotmeto-fiImdismnce = focal mot size x obiect-to-fiIm distance i objea-to-fitn distance (tninimm) Imximmugpermittcd
GAMMA RAY EXPOSURE FACTORS - _ ..
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TYPE OF SOURCE The rype of source used, ie. Iridium 192 or Cobalt 60, wiU determine the energy (penetrating power) of the radiation. Since S e r g y o f a m of source remains constant the only way- . To change the energy used is to change &e ty@of so&.
ACTIVITY OF SOURCE
.
.
The activity of the source in Curies govems the inteusity of the r a w o n emitted and it cannot be varied. 'Ihe intensity of ~adiatiouis proportional to the d e strengthof a source and will affect the exposure time required to produce aradiograph oE a given dMsity. That is, radiographs of equal density would be produced if the s a m specimen is radiographed with a 50 Curie source with an exposure of two minutes and with a 100 Curie source with an exposun: of one minute. It is esse;ltial that the s o w decay chart be availabIe as the exposure rime must be adjusted as the activity of the source'decreases thpugh decay.
OTHER FACTORS C o n s i d d o n of other factors such as source-to-film distance, film-type,intensyfing screens, and processing would be the same as4&scu.sed with an x-ray exposure.
DETERMINING RADIOGRAPHIC EXPOSURES
X-KAY EXPOSURE CHARTS Methods of detumining c o w radiographic exposure are by: I.
Rcfaence to acposmc charts these provide exposure conditions r t q thichcss of matcdat ~ hcxposm t is usnaIly cxpnssed in tams of u
M for a given
n
d
s or
An cxponnc chaa is a graphplotted on semi-log graph paper. Tht exposurerequired to
achieve a fixed density isrdated to the ma& thlckmss. An exposurcchact is dcvdoped for a particolarX-rayma&ine using a fixedset of conditions such as matuial type, film type,SFD, iil&,.pmcessing, processing a n d d t i n g dens%%y.If any of these factors changethe c q c m b chartis no longer valid and some compensation must be made.
2.
Rcfuwicc to pnxious cxposmcmrds.. theseprovide infounation to pmducc coxrect cxwmw, but the data may not always bc awBabIc or sai3icicnt for the uarti&
3.
Trialandemx:althougIrthis~odkoftcnnse&,itisnsnalIyvaywastefuIandcostly .. both in teams of time and tilm. This method of demmmng an exposure is not recommended except in u n d W.
USE OF AN X-RAY EXPOSURE CHART To detemine the proper exposure,enter the chart at the base for the thichess of the specimen and move vertically to intersect the desired Kv. Move horizontally to the left h r n this intersection point and read the required exposure in mAS or mAM X-ray Exposure Chart- 160 KvP Unii
USE O F A GAMMA RAY EXPOSURE CHART
. - .% ;
These cham are used although sbnie&hii't&s to x-ray exposure charts are different in that the exposure time must be determined using the formula shown on the chart Iridium 192 f3pasure Chart
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IRIDIUM 192 EXWSURE FACTORS FOR
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1
T -TIME (MINUTES1 FOR OENSINZO' EF EXPOSURE FACTOR 0 -SOURCE=CO-FILM DISTANCE (FEETI S -SOURCE STRENGTH (CURIES1
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INCHES OF STEEL
t
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RT LESSON 107
After exposing the film to radiation rhe hlm must be processed (developed) to make the latent image visible. This is carried out in a darkroom under subdued light (safelight) of a color and intensity that will not affect the film.
High quality results depend upon deanIiness, the quality and concentration of the processing solutions and the c o w combination of temperatme, time and agitation. There are two main methods of pmxssing, manual and automatic, which incolporate the essential steps oE /lAex& wrc 4 b . +,!$I ;ki. development -5'r'q s i l v r ~J t ; l - ' + @ ~ g @ washing (Jdrying
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MANUAL PROCESSING The filmsare suspendedvertically in the tanks on suitable hangers or clips so that s e v d films can be processed together. The operator agitates the films and transfers thuniium one tank to another.
PREPARING
FILM FOR PROCESSING
Check that the developer solntiou'is nady and at the right
n o d y 6S°F (2O0C).
Check that the darkroomis seam, the white light off and a n d m safety Light on.
UNLOADING THE CASSETTES - ., FLEXIBLE CASSETTES
.
Undo the £41 of the cassette carefully. W~thdrawthe film.and screensfrom the cassette, slowly, to
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RIGID CASSETTES
Place the cassette on a bench with the backside up. 0p.u and careNly pick up the 61mby its edges. T d e r the film into avatical position with a flowing motion, avoiding bending it.
surface.
Radiographic films are sensitiveto pressme, acashg, kinking and friction. Friction may produce an clectdcal dkharge known as 'static' which causes marldngs on the film.
Aaach the film to the hanger, asming that the clips hold it m t l y and GY on the hanger.
PROCESSING THE FILM Processing is canied out in deep tanks containing ch& solutions. The tanks are immersed in a jach m help corn1the tcflperatmc of thc solution.
-.
Whcn the film is pnxxssed,it is i m m d in each of the tanksfor artcommended period of time. A timer.isused to control the time.
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Film processing comprises five stages which are numbered in the iUustradon. .
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1)
Immerse the f h fully in each tank in sequence for the time periodrecoq~n~ended by the manufacturers of the p&sing chemicals.
',
I
3)
Agitate the fiIm up and dawn for about ten seconds when it is first immersed and then for about ten seconds eveq minute during the developing time.
4)
When d e u i n gfilm from tank to tank, drainback smpIns so1ntion off filmtostopcaqylngovoIi@from one tank to another. Use running water in the washing tank If a static
2)
Tap the hanger on the tank after immsing the film to free any air bubbles which may be attached to it
5)
'Ihn&erthcfilmfromthe~~ tankand place in a drying cabiiet for approximately.twenty minutes, or . nnriIit is dry. Do not placc wet films over or near films already drying. Do not place films too close together as hey may touch and stick togethir as they dry.
wattrtankisustd, agitate thefilm when washing and change the wam frequentIy.
AUTOMATIC PROCES
This allows radiographic films to be processed and dried automatically, without constant operator attention It is quicker than manud pnx~ssingand can be kept working for 24 hours a day if require&. .
-.
The filmis fedin through a sIot and fecd tray fium the darkroom side of the walL
The processcdradiograph is delivered so thatit can b checked as it comes out of the machine.
OncetheiiImhas beenfedinto.tht processor, it is tcansprted at constant speed through developer, fuccr, wash and drying sections by three racks of mllers immersed in deep tanks.
The controlpanel has warning lights to indicateconditions inside the processor.
PREPARING FlL?dS FOR READING u;arnincthcfiImfor~gfanlls(~)~andchtckthe5density. P b it m apmtcctiw avelope,madcthe unnlope with thenfqlpce nmbmofthe f3.m and .
g-*
presenttheiihtothe5nreadm.
EXAMINING T m FILM FOR PROCESSING FAULTS (ARTIFACTS) It is important that radiographs b&kc of in the arm ofintarst as they may bt cause for rejection of the radiograph. It is important not only to rtcognizc film aaifacs,but to also undastand their cause and how to remedy them. . '
UNSUTrABLE SMRAGE OR CARLESS HANDLING OFFKMS ATTIME OF EXPOSURE
CAUSE
AR'I?FACI'
'Pre-exposure or 'doubteexposure' to X-rays or gamma rays giving either overall fogging of film or showing other inexplicable patterns (Emrn intervening objects). Image present on both sides of f i
.
L,/
REMEDY -
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Insufiicient protection of film fmm radiation in storage or in transport
Make sure that frlm is stored or transported under radiation proof conditions
Elmi left in the vicinity of tube or somce while making exposures
Keep films away fiom tube or source during exposure
Back scatter
Use adequate
backing sheet during exposures
G M d mottle and greater
1) Film has been stored for too
&>,&FA
1) Do not over-stock
-$43 fim t r y t o m film a t h i .about thteemonths
2)Storeinacooldry P9 C
Note: thisfbr@ab is rare $hameacalEy sakipackage is unbroken. Wavy mottlc with a 'watay'
-~*-gt
conditions
Ston in dry place and avoid constant dampness in dark room comparatively rarefault with modem packaging
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MIS-HANDLING THEF E M AND FAULTY DARKROOM EQUIPMENT
0-
ARTIFACT
CAUSE
REMEDY
Overall veiling or fogging. Image may be discernible on one side only (usually by physical test)
Excessive exposure to safelight, or faulty safelighting
Test safelights before use; follow maker's instructions particularly with regard to wattage of lamps and handling distance
Patches of heavy density or streaks of density near edges of Wm
Improperly closed cassettes or film holders .
charge cause&& p f i g film out of package
Darkdots with lines radiating fiam them Image on bob sides of61m
W~thdrawfilm slowly
from packet
tfao
. ,
E'"
Check that cassettes. and fiIm holders are closed before exposure
1
@'
-fl Pressure on or buckling of fiImswhile loadinginto cassettes or holde?;~
Handle carefully. Avoid buckling or bending hlm
As above but dark areas Cyi'mb W r K
As above but caused after
D& cr&ceat Lhumb nail'
Heavy prcssmc or kink marks
Avoid 'czimpii or kinking fiIm while hmdhg. This fault is more likely with large filmsor whith long lengths of film
Pressun of heavy specimen on film; more likely when usiagenvelope wrapped film or ffcxibk holdax May be due to ovUti@t binding of film on
Forheavy objects use
expoSnE
marks, often s m u n d e d by
lighter areas
Light or dark marks corresponding to contour ofspedmen
weId surface
As above. This is a fairly rare fault
rigidcassettes preferably, or pIace
themcarefully on envelapcpackedfilm, Avoid overtightening securingstraps
.-*:x,
f
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- . . -. MIS-HANDLING 'JXEFILM AND FAULTY DARKROOM EQUIPMENT - CONTINUED ARTIFACT
CAUSE
REMEDY
Small sharp spots of reduced density. Images may be on both sides of film but do not coincide
Dust trapped between intensifying screens and film
CIean screens and avoid dust in darkroom as much as possible. This fault is commonplace but not usually confusing
Light or discololned streaks along film, usually on one side only
Inseaion of Om for processing into a channel hanger which is contamhtd with fixer
Wash and dry all hangers thoroughly before use.
F ^
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,.
rDark rounded spots or smears on one side of film
Gght spots as h
e
Light or dark spots but not as obvious as above two
PeriodicalIy thoroughly clean all hangers and clips
Developer splashedprior to processin& Bad darkroom iayont a d gross ~ e s s n e s s
Keep loading bench dry. Do mt splash
Irisplashes
As above
Warmsplashes Iffilmis developedimmktely these may not show. If left for some time they may bc d&
As above
distrlcalsh a b o r n
.
ARTIFACT
CAUSE
REMEDY
Light fogging - excessive
Attempted impdon in front of safelight during development Incorrect safelighting
Use timeand'
-
I""-veil
temperature method of development Do not ' h e l o p by inrpecrion' --
'
Solarisation: partjalor complete rev& to a positive instead of a negative image
Exposure during development I) Unsafe lighting
2 9
d).A
.
s ' YVF
I) Check safelamps
to:
Uneven development Patchy. streaky and mottled @ns. k g e s on both sides
" e/\
--
3'
4-
2)
Makesure white Iight is off
2) White light
This fault is rare
1) Lack ofagitation
1) Asitate adequateIy as recommended
2) Overshortdevelopment in wann soIutions
2) Give coma time md for development
3) Attanpfingto over-sure conpeaate by under development
-
3) 'Developmeat inspCCtion1is bad by pmctice. Conect expm.timc and always qroixss stan*
-P Flow marks, bromide sfmamers, dark areas
below light areas and vice versa
Lack of agitationcausing nnevcn development, dae to rclease of by-products in developmentprocess
properly. ie,
10-15seconds on hrst immersion and 5-10 seconds in each subsequent minute
Undeveloped, unfixed or unwashed area at top edge offilm
Hellier. Inc.
Failrne to maintain solution levels
RcpI&tanksas WP&~
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ARTIFACTS OCCURRU\TG DURZNG PROCESSING - CONTlMJED
-
FAULT
Reticulation: fine network of lines on surface of film
Dichrornic fog. p W h colour when viewed by transmitted light, greenish coIour when viewed by reflected light NL
-CAUSE
llFMEDY
Gmss temperature diaFerence bween various processing solutions
Mainrain solutions at recommended temperatures. This . fault is rare with most modern Nms
1) Contamina~onof developer by fixer or vice versa
1) Discard
rinse or stop 2) ~nade~uatk bath
2) Make sure rinse
%. .
contaminated developer water is i s g , or renew stop bath more often
3) Makeacid fixer bath -
-
ARTIFACTS OCCURRING DURING WASFENG AND DRYING
Small blisters oruinkled spots on film; areas of emulsion missing
-
REMEDY
Excessive washing, usually by allowingfilm to remain in static water at higher than
Wash in cold running water for not more than half an hour (ten minutes is usually adequate)
Drying rm& usually light spots with slightly darkex edges, or can be streaks. Visible on one side only
1) Failure to use wetting-agent rinse after washing
1) Use wetting-agent bath to promote ~ v wa g
2) Poor drying discipline
2) Do notput wet films above dry or nearly dry films in drying cabinet
Density change due to uneven W g
Avoid placiog films too close tog&er in hot air c a b i i In extremccasesfib may stick together
CAUSE
AR.TFACI'
--
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/ Patches of density change usndlIy darkerand in central area of film
( 2 2
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Hellier, Inc. RT
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60
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4m
Designation: E 748 - 90
Standard Practices for Thermal Neutron Radiography of Materials' Thir rlmbrd is isucd ""dcr ,hc Crcd dciignxion E 7": Ihc nurnkr irnrn~diatclylollau.inc thc drrbgn3lion indicalrr ihc ? u i a i original adoption or. in ihc nw ofrcririan. thc )czr uilrsl rcvirion. A nurnkr in prcnthitiJlcatr\ ille ?car liilall rc$npra\al. A rumrwnpl cprilon (,) indintcs zn Ldir~rirldangc rincc ~ h cIan rclirion or rapprov3l.
I. Scope
I. I Purpose-A practice lo be employed for the radiographic examination of materials and components with thermal neutrons is outlined herein. It is intended as a guide for the production of neutron radiographs that possess consistent quality chancteristics, as well as aiding the user to consider the applicability of thermal neutron radiology (radiology, radiographic, and related terms are defined in Terminology E 1316). Stitemens concerning preferred pnct i e are provided without a discussion of the technical background far the preference. The nffenary technial background can be found in Refs (1-24).~ I 2 Limirafiom-Acceptance standards have not been established for any material or production process. Adherence to the practice .will, however, produce reproducible rsults that could serve as standards. Neutron radiography, whether performed by means of a racror, an acceferator, subcritical assembly, or radioactive saurc+ will be consistent in sensitivity and resolution only if the consistency of all details of the technique, such as neutron saurce, collimation. gmmefq film, ctc., is maintained through the practices. These practices are limited to the use of photographic or radiographic film in combination with conversion screens for image remrding; other imaging systems arc available. Emphasis is placed on the use of nuclear reactor neutron SQUTCIS
L .3 Inierprelafion and Acceprance Standnrds-Interprewlion and acceptance standards are not covered by these practices. Designation of accept-reject standards is iecognirred to be within the cognizance of product specifications. 1.4 Safay Prarfices-General problems of personnel proteaibn against neutron and anociated radiation peculiar to the neutron radiologic prare discussed in 15.1. For further information on this important as* of neutron radiology, refer to current documents of the National Committee on Radiation Protection and Measuremen& the Federal Register, the U.S. Nuclear Regulatory Commission. the U.S. Department of Energy, the National Bureau of ' Standards, and to applicible state and l a d codes. 1.5 Orher Aspecls offhe Neutron Radiographic PracessFor many important aspects of neutron radiography such as technique, files, viewing of radiographs. storage of radio-
'
Thcrr: pndm arc undcr ihc juridictian of ASTM Curnrnittcc El7 on N a n d m c u ' ~Tcning m d arc lhc dirm mpocuibiliry of Submrnmitrcc E07.05 an Nmlmn Padiopphy. Cumnt dilian rppmvn( k c . 28. 1994. Publirhcd Fcbrunp 1991. Orifjnally publhhd z E 748 80. L a picviour cdilion E 748 89. Thc boldfxr nurnkn in mrrn,hicicr 10 ihc list of mkrcncn 31 ihc cnd
-
-
graphs, film processing. and record keeping, refer to Guide .. E 94. (See Section 2.) 1.6 Perso~i~~cl.lor T/~c,r~iial Neuron Radiographic Inspec. rion-Training and certification of personnel to perform thermal neutron radiographic examinations is imponant to a successful neutron radiologic operation. For additional information refer to American Society for Nondeitrucrive Teiting Recommended Practice No. SNT-TC-IA. 1.7 The agency performing the testing or examination shall meet the requirements of Practice E 543. 1.8 This sra~rdarddocs nor purpon ro address all offhe safery problcnrs associared wirlr irs use. Ir is [he responribiliry of rhe user qfrlris standard ro esrablislr appropriate safay and hedrh praciices and defermine rhc applicabiliry of reooulafory limirationr prior ro use.. (For more specific safety precautionary information see , I .4.) 2. Referenced Documents
2.1 A S T M Srandards: E 94 Guide for Radiographic Tening' E 543 Practice for Evaluating Agencies that Perform Nondertructive Testing3 E 545 Method for Lktermining' Image Quality in Direct Thermal Neutron Radiographic Examination" E 1316 Terminology for Nondeitructive Examinations" 2 2 ASNT Srandard: SNT-TC-IA Recommended Practice for Personnel Qualification and Cerlificationi 3. Signifiance and Use 3.1 This practice includes typ& of materials to be examined, neutron radiographic examination techniques, neutron production and collimation methods, radiographic film, and convener meen selection. Within the p m m state of the neutron radiologic a this practice is genelally applicable to specific material combinations, processes, and techniques.
4. Neutron Radiography 4.1 The hfe~lrod-Neutron radiography is basically similar to X radiography in that both techniques employ radiation beam intensity modulation by an object to image macroscopic object derails. X rays or gamma rays are replaced by neutrons as the penetrating radiation in a through-transmission examination. Since the absorption characterinin of matter for X rays and neutrons differ 'Annual Bad < ! l ' . t T . WSmndord~.Vol 01.03. 'Anil;lblc from lhc Arncriun k c t y lor Nondcnmnivr Tci~ing. 171 1 Adinslc I-lnr P.O. b x 18518. Calurnhur. 01-153223-11518.
'-
Fast Neutron Source
I Maderalor
Anerlure
i
I
Gamma
I
cay
Fillel FIG.
1
' --i
ot ~ , ~ ~ D, , t ~ ~
@vevg
Obiecl
Neutron Beam
Typical Neutron Radiography Facility with Divergent Callirnator
dradcally, the two techniques in general serve lo c O m ~ l e ment one another. 4.2 Faciliries-The basic neutron radiography facility consists o i a source of f a neutrons, a moderator, a gamma filter, a collimator, an object. a conve~ionscreen. a film image recorder or other imaging system. a cassene, and adequate biological shielding and interlock systems. A schematic diagram of a representative neutron radiography facility is illustrated in Fig 1. 4.3 Thermalimion-The p r o c w of slowing down neutrans by permining the neutrons to come to 'Iherma1 equilibrium with their surroundings.
5. Neutron Sources 5.1 General-The thermal neutron beam may be obdried from a nuclear reactor, a subxitical asembly, a radioactive neutron source, or an accelerator. Neutron radiography has been achieved successfully with a l l four sources. In all cases the initial neutrons generated possess high energies and must be reduced in energy (moderated) to be useful for thermal neutron radiography. This may be achieved by surrounding the source with light materials such as water, oil, plastic, paracf~n,beryllium, or graphite. The preferred moderator will be dependent on the constraints dictated by the energy of the primary neutrons, which will in turn be dictated by neutron beam parameters such as thermal neutron yield requirements, cadmium ratio, and beam gamma ray contamination. The characteristics of a particular system for a given application are left for the seller and the buyer of the service to decide. This is an easier task in the erase of neutron radiography than that of X radiography. Characteristics and capabilities of each trpe of source are referenced in the References section. A comparison of s o u r m is shown in Table 1. 5.2 Nuclear Reaclors-Nuclear reactors are the prererred thermal neutron source in general, since high neutron fluxes are available and exposures can be made in a relati\'ely short TABLE 1 Cornoarisan of Type 01 Sam%
Typcwl RadagrJphlc Flux, n/a7iz.s
Nudear readw S~bcntml assrmMy Aoceleratm
lO5lto l V lo' to 1@ l@ to l@ lo' to l~l'
Radiwatw
tinic span. 'IFiic higll nculroti intcl>silymakcs it possible provide a liglllly collimated k x n > : ~hcrcforc.Iri~h-resolutio,, radiographs w ~ bc i produced. 5.3 Subcri~icol ~s.~e~?tb/j-Asubcritical assembly is achieved by the addition of suficienl fissionable material surrounding a moderated source oT neutrons, usually a radioisotope source. ~ l t h o ~ gtlie h total thermal neulron yield is smaller than that o f a nuclear reactor, such a systenl olTen the attractions of adequate image quality in a r-nable exposure tinie. relative eare oT licensing. adequate neutron yield Tor most industrial applications, and the possibility of tfanspomblc operation. 5.4 Acccl~~ralor So~rrces-Acceleraton used for thermal neutron radiography have generally been of the low-voltage type which utilize the 'H(d,n)'He reaction, high-energy X-ray machines in which the (x,n) reaction is applied and Van de GraalT accelerators which employ the 'Be(d,n)I0~ reaction. I n all cases, the mrgeLs are surrounded by a moderator lo reduce the neutrons to thermal energies. The total neutron yields of such can be in theorder of 1 0 1 2 . ~ . ~ -the ~ ; thermal neutron flu of such sources before collimation can be in the order of 109n.cm-2.s-', for example, the yield from a Van de Graaff a d e m o r . 5.5 ]soropic Sortrcr3-Many isotopic sources have been employed Tor neutron radiologic applications. Those that have been most widely utilized are outlined in Table 2. ~ ~ dsources i ~ the ~ best~ posibjty ~ i for pomble ~ ~ operation. However, becauie of the relatively low neutron yield. the exposure times are usually long for a given imaze quality. The isotopic soura: Z52Cf offers a number of advantages for thermal neutron radiology, namely, low neutron energy and small physical size, both ofwhich lead to eficient neutron moderation. and the posibility for high total neutron yields. 6 . imaging Methods and Conversion Screens 6.1 General-Neutrons are nonionizing particulate radiation that have little dirm effea on radiographic film. T o obtain a neutron radiographic image on film, a canversion screen is normally employed, upon neutron capture, screens emit prompt and delayed decay products in the form of nuclear radiation or light. In ail cases the screen should be placed in intimate contact with the radiographic film in order to obtain sharp images. 6.2 Direcr h4e1110d-In the direa method, a film is placed on the source side of !he conversion screen (front film) and exposed to the neutron beam together with the conversion screen. Electron emission upon neutron capture is the mechanism by which the film is exposed. The screen is generally one of the following types: ( I ) a free-standing gadolinium metal screen accessible to film on both sides; ( 2 ) a sapphirei-oated, vapordeposited gadolinium screen on a substrate such as aluminum; or (3) a light-emitting fluorescent screen such as gadolinium oxysulfide or 6LiF/ZnS.
Thermal Neutron Sources Radlagiaphc Resdutlar
UWIJCLemlrZ
excclbt
S
&turn
s W opaabm. px!amy dfollt ar-dlopenthn st* openm w l t y pos~lble
@
paoc lo m e d ~ n
W
opaaucn ml pcrtable
TABLE 2
SOV~CC
~
TYW
d
i
~Sources ~ ~ t Employed i ~ e lor Thermal Neulron Radiography
1i.n)
60 days
"OPo-ae
lo.")
2"Am-Be
(=."I
138 days 458 yea's 163 days 2.65 years
251c(
11l.n)
SPonlanMuS lissl~n
mese comments canpare w r c e s m
Commmls'
Iiail.Llle
"'Sb-Qe
219~m.212 Cm.Be
A
~
s h m hait.LIe and high ~~bachground.low rieulian energy is advanlaqe lor modcrallan. high yield source shed hattJde. ww 7-background long hall4fe. easily shieldca ,-bachgrouna shEn hallJ8le. high nculron yvcld long hallJile. hjgh neulrw yield. nmall slle and low energy oncr advanIa~e5in moderalsan
lne table
Exposure of an additional film (without object) is often useful to resolve anifacts that may app&r in radiographs. Such anifam could result from screen marks, excess pressure, light leaks, developmen\ or nonuniform film. In the case of light-emitting conversion screens, it is recommended that the spectral response of the light emission be matched as closely as possible to that of the film used for optimum results. The direct method should be employed whenever high-resolution radiographs are required, and high beam contamination of low-energy gamma rays or highly radioactive o b j w do not preclude its use. 6.3 Indirecf Merhod-This method makes use of conversion screens that can be made temporarily radioactive by neutron capture. The conversion screen is exposed alone to the neutron-imaging beam; the film is not present. Candidate conversion materials include rhodium, gold, indium, and dysprosium. Indium and dysprosium are recommended with dysprosium yielding the greater speed and emitting less energetic gamma radiation. It is recommended that the conversion xreens be activated in the neutron beam for a maximum of three half-lives. Further neutron irradiation will result in a negligible amount of additional induced activity. ffier irradiation, the conversion sueens should be placed in intimate contad with a radiographic film in a vacuum cassette, or other light-tight assembly in which good contact can be maintained between the radiographic film and radioactive screen. X-ray intensification screens may be used to increase the speed of the autoradiographic p r o m if desired. For the indirect type of exposure, the material from which the cassette is fabricated is immaterial as there are no neutrons to be scatIered in the exposure process. In this case, as in the activation process, there is litde to be gained for conversion screen-film exposures extending beyond three half-lives It is recommended that this method be employed whenever the neutron beam is highly contaminated with gamma rays, which in turn cause fdm fogging and reduced contran sensitivity, or when highly radioactive objects are to be radiographed. In shorf this method is beam gammainsensitive. 6.4 Orher Imaging Sysrems-The scope of these practices is limited to fdm imaging (see 1.3). However, other imaging systems such as track-etch or real-time are available. 7. Neutron Collimators 7.1 General-Neutron sources for thermal neutron radiology generally involve a sizeable moderator region in which the neutron motion is highly multidirectional. Collimators are required to produce a beam and thereby produce adequate image raolution capability in a neutron radiology facility. It should be noted that in the definitions of collimator parameters, it is assumed that the object under
Fast Neutron Source
ol Diameler 0
/
Moderator
Film
I
Diverging
Gamma Ray
Neutron
Filler
FIG. 2
Object
Beam
Pinhole Catlimatar
examination is placed as close to the imaging system as possible to decrease both magnification and image unsharpnes due to the finite neutron source s'm.Several types of collimators are available. These include the widely used divergent type, multichannel, pinhole, and slraight collimators. The image spatial resolution properties of the beams are generally set in part by the diameter or longest dimension of the collimator entrance port (D) and the distance behyeen that apetture and the imaging system (L). An exception is the multichannel collimator in which D is the diameter of a channel and L is the length of the collimator. It should be noted that the detection system used in conjunction with a multichannel collimator will register the collimator pattern. Registry can be eliminated by empirically adjusting the distance between the collimator and the imaging system until the pattern disappears. Ratios of LID as low as 10 are not unusual for low neutron yield sources, while higher resolution capability systems ohen will display LID values of several hundred or more. The actual spatial resolution or image unsharpness in a particular radiologic examination will depend, of course, on factors additional to the beam characterinics. Thex include the object size, the geometry of the system. and scatter conditions. The size of the X-radiologic source. F. would be replaced by the size of the eReaive thermal neutron radiologic source (D) in the calculation of geometric unsharpness. The geometrical aspects ofthe problem are discussed in Guide E 94. 7.2 Divcrgcnr Collimaror-The divergent collimator is a tapered reentrant porl into the point of highest thermal neutron flux in the moderator. The walls of the collimator are lined with a thermal neutron absorbing material to permit only unscattered neutrons from the source to reach the object and the image plane. This type of collimator is preferred when larger objects will be radiographed in a single exposure. It is recommended that the divergent collimator be lined with a neutron absorber which produces neutron
capture decay products that \\.ill ,I<,( result in background fogging ofthe film. sucl~a s " ~ cjrt~onalc. i A typical divergent collimating SYS1Cm is illustrated in t l ~ eschematic diagram of Fig. I . 7.3 A~rillicl~onnelCaflinraror-The multichannel collimator is an array of tubular collimaton stacked within a larger coliima1or envelope. It is recommended as a means of achieving a high degree of collimation within a shon collimation length. When this type of collimator is employed, a suitable collimator to detector distance should be maintained to avoid regin? of the collimator pattern on the radiologic image. 7.4 Slraiglrr Colli~naror-A straight-tub? reentrant port can also be used instead ofthe tapered assembly described in 7.2. Although such collimaton were widely used in early neutron radiologic work. the need to examine larger objects and to achieve higher resolution has fostered the use of divergent collimators. 7.5 Pinhole CoIIirriaror-Higher resolution can be obtained with a straight collimator when it is employed in conjunction with a pinhole iris. The pinhole is generally fabricated from a neutron-opaque material such as Cd, Gd, or 'OB. The rmlution attainable will be dependent on the pinhole diameter D. A schematic diagram ofthis system is illustrated in Fig. 2. 8. Beam Filters 8.1 Thermal Neurron Radiograpl~y--In general, filters may not be neassary. 11 may be desirable to employ Pb or Bi filters in the neutron beam to remove beam gamma-ray contamination. Whenever Bi gamma-ray filters are employed in a high neutron flux environment, the filter should be encased in a sealed aluminum can to contain alpha particle contamination due to the 2'0Po produced by the neutron capture reaction in '@Bi. Gamma rays can cause Glm fogging and reduced contrast sensitivity. In particular, xinlillator converter screens exhibit sensitivity to beam gamma-ray contamination. This effect can be minimized by careful selection ofthe screcnffilm combination.
9. Masking 9.1 General-In general, masking is not ofien used in thermal neutron radiology. Where it is desirable to reduce scatter or to reduce unusual contrasts, the choice of masking materials should be made carefully. Materials that scatter readily, such as those containing hydrogen or materials that emit radiation that may be readily detected, for example, as indium, dysprosium, or cadmium, should be avoided or used with exceptional care. Lithium-containing materials may be useful for masking purposes. Background fogging may result from the 470 keV gamma ray from boron. 10. Effect or Materials Surrounding Objwl and Cassette 10.1 Backscarrer-As in the case of X radiography, effects of back-scattered radiation. for example, from walls, etc., can be reduced by masking the radiation beam to the smallest practical exposure area. Effects of backscatter can be determined by placing a neutron-absorbing marker of a material such as gadolinium and a gamma-absorbing marker of a material such as lead on the back of the exposure cassette. IT problems with backscatter are shown, one should minimize
in tl~ec r l , ~ ~ u ar cx a illatcri3ls ~1131sca11~rO r enlit radiatio,, 3s discussed it1 Scctiot~9. nacksr~itcrcan minimized I,,. placing 3 neutron absorbcr sucll as g~doliniumbehind 11,; cassette. I I. Cassettes 1 1 . 1 ,\forcriol oJ COI~X~I-rmio11-The casseue frame and
back may be fabricated of aluminum or magnesium as employed in standard X-ray film cassettes. Aluminum or magnesium entrance window X-ray cassettes can be used directly for neutron radiography. Special vacuum cassettes designed specificall? for neutron radiography are preferred'io conventional X-ray cassettes. Plastic window X-ray cassettes should not be used. The plastic entrance face may be replaced with thin, 0.010 to 0.062-in. thick 11OO reactor grade, or 6061T6 aluminum, or magnesium lo eliminate image resolution degradation. The use ofhydrog-*nous materials in the construction of a cassette can lead to image degradation and the use of these materials should be considered carefully. 1 2 a c u i Cassctrres-Whenetfer possible, vacuum cassettes should be employed to hold the converter foil or scintillator screen in intimate contact with the film both in the direct and indirect exposure methods. Cassettes of the type that maintain vacuum during the exposure or that must be pumped continuously during the exposure are equally applicable. Vacuum norage minimizes atmospheric corrosion of dysprosium conveners and subnanually increases their useful life. 12. Thermal Neutron Radiographic Image Quality 12.1 irnage Quali~yIndicaors-Image quality indicator; for thermal neutron radiography are described in Method E 545. The devices and methods d a a i b e d therein permit: (I) the measurement of beam composition, including relative thermal neutron to higher energy neutron composition and relative gamma-ray content; and (2) devices for indicating the sensitivity of detail visible on the neutron radiograph. 13. Contrast Agents 13.1 I~~rproved Conrrasr--Contrast agents are useful in thermal neutron radiology for demonstrating improved contrasto f a tagged material or component. For thermal neutron radiography even simple liquids such as water or oil can serve as efkctive contrast agents. Additional useful marker materials can be chosen from neutron-at~enuatingmaterials such as boron. cadmium. and gadolinium. Of coune, the deleterious effect of the contrast agent employed upon the test object should be considered. 14. Types of Materials To Be Examined with Thermal
Neutron Radiography 14.1 General-This section provides a categorization of applications according to the characteristics of the object being examined. The following paragraphs provide a general list of four separate categories for which thermal neutron radiographic examination is particularly useful. Additional details concerning neutron attenuation are discussed in Appendix XI. 14.2 Derccrio~r a[ Similar Densir)! hfarerials-Thermal neutron ndiognphy can oNer advantages in c a w of objects
of similar-density materials. that can represent prohlems for
X-radiography. Some brazing mntcrials, such as cadmium snd silver for example, are rcadily shown by thermal neutron adiography. Contrast agents can help show materials such as ceramic residues in investmentian turbine blades. Inspection of castings for voids or uniformity and of cladding materials can often be accomplished with thermal neutron radiography. Material migration in solid-stale electronic components, electrolfle migration in batteria, difusion between light and heavy water, and movemenr of moisture through concrete are examples in which thermal neutron radiography has proved useful. 14.3 The Detection of LowDensity Conipoiienls and Materials in High-Densiry Con!ainmenrs-This recommended category includes the examination of metal-jacketed explosive devices, location and measurement of hydrogen in cladding materials and weldmenrs, and of moisture in assemblies, location of fluids and lubricants in metal containment systems, examination of adhesive bonds in metal parts including honeycomb, locadon of liquid metals in metal par&, location of corrosion products in aluminum airframe components, examination of boron-filament composites, studies of fluid migration in sealed metal systems, and the determination of poison distribution in nuclear reactor fuel rods or control plates. 14.4 The Examination of Higl11v Radioadive ObjeclsThe technique of indirect neutron imaging is insensitive to gamma radiation in the imaging beam or from a radioadive object that could produce fogging of the film with the resulting loss in contrast sensitivity. This category of r F m mended examinations includes the inspeaion of irradiated reactor fuel capsules and plates for cracking and swelling, the determination of highly enriched nuclear fuel distribution in assemblies, and the inspeaion of weld and braze joints in irradiated subassemblies. 14.5 D~FerentiafionBerween Isoropes of the Same Elemenl-Neutron anenuation is a function of the particular isotope rather than the element involved. There are certain isotopes that have either very high or very low anenuation and, therefore, are subject to detection by thermal neutron radiology. For example, lI3Cd is the only isotope of cadmium with a high thermal neutron attenuation. Also, one can differentiate baween isotopes such as 'H and 'H or 13'U and ='U.
~ t ior Objects ~ ~ t and i 13sposurc ~ ~ i\lalerials 15.1 Obj~,c[s-Cenain objccts pl3ctd i n the neutron beam may be activated, depending upon the incident neutron energy, intensity and exposurc time, and the material activation cross section and half-life. Therefore, objects under examination may become radioactive. In extreme cases this could produce lilm fogging, thereby reducing contrast. Safety is a nrong consideration; radiation monitoring of objects should be performed after each exposure. Objects that exhibit a radiation level too high for handling should be set aside to allow the radiation to decay to acceptable levelk In practice, since neutron exposure times are normally short, a short decay period will usually be satisfactory. 15.2 Casselles-Radiographic cassettes containing materials such as aluminum and steel can become activated, padcularly on multiple exposures. Monitoring of radiation to determine safe handling levels can alleviate safety problems and minimize film fogging. Activated cassettes, screens, and objects should be kept away from unexposed film. Converted X-radiography cvsettes are virtually worthless for high-resolution industrial neutron radiography. Vacuum cassettes should be employed whenever poaible to maintain the film and convener foil in intimate contact during the exposure. This holds for both the direct and indirect methods. 15.3 Coiisersion Screens-Conversion screens used for direct exposure methods are usually chosen for low activation properties. Conversion meen materials such as gadolinium, boron, or lithium seldom cause problems. However, conversion screens for the indirect exposure method are chosen for high-activation potential. Therefore, exposed and activated screens such as indium, dysprosium, rhodium, or gold should be handled with care. Screens should be handled with gloves or tongs and should be moved in a shield. High-radiation exposures to the fingers are a potential hazard. A cassette will shield much of the beti radiation emitted by the commonly used indirect exposure converter screens. Conversion screens should normally be allowed at leas a three half-life decay period before reuse to prevent double exposures.
15. ~
16. Keywords
16.1 neutron attenuation; neutron collimator; neutron radiography; neutron sources
XI. .4TTENUATION O F NEUTRONS R\' MATTER X I . I A major advantage oi using neutrons for radiography is that radiologic observation of certain material cambinations is easily accomplished \\*ith slow neutrons where. because OFauenuation difkrences. problems will arise with X rays. For example, the high attenuation of slow neutrons by elements such as hydrogen, lithium, boron, cadmium. Thermal Neutron Linear Anenuation Coeniuenls Using Average Scanering and Thermal Absorption Cross Sections lor the Naturally Occuning ElementsA
T A B L E X1.l
0 0 5 5 Sedion (barns)'
Eiwoenl ~ l o m i cNO.
symba(
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
H He
20
Ca
21 ' 22 23 24 25 26 27 28 29 30 31 32
Sc Ti V
33 34
Ar,
35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55
8,
Li Be
a C N
0 F Nc Na
Mg Al Si
P S Cl A K
Cr Mn Fe
M Nt
Cu Zn Ga Ge
se Kr
Rb Sr Y
Zr ~b
MO Tc Ru ~h PO Aq
Cd I" Sn Sb
Te I Xe cs
scanenng 38.0 0.8 1.4 6.14 3.6 4.75 10.6 3.76 4 .o 2.42 32 3.42 1.49 2.2 .5.0 0.98 16.0 0.6 1.5 32 24.0 4.0 4.93 3.8 2.1 10.9 6.7 17.3 7.9 42 6.5 7.5 7 10.0 6.1 7.5 6.2 10.1 7.60 6.40 5.0 5.8 5.0 7.5 5.0 5.1 6.2 5.7 2.2 4.0 4.2 5.5 3.6 4.30 7.0
~bwxplian 0.332 0.0 70.7 0.0092 759 0.W3 1.85 0.00 0.010 0.04 0.530 0.053 0230 0.16 0.18 0.52 33.2 0.678 2.10 0.44 26.5 6.1 5.04 3.1 13.3 2.55 37.2 4.43 3.8 1.1 2.9 2.3 4.3 11.7 6.8 25 0.37 1.21 1.28 0.185 1.15 2.7 22.0 2.56 150 6.9 63.6 2450 193.5 0.625 5.4 4.7 6.2 24.5 29.0
lhw Anenualii hff-'.on-'
9"
9" 335 0.76 99.4 0.535
9as 5 a 5 s 0.095 0.150 0.104 0.122 0.183 0.052 5 s
9az 0.047 0.0849 1.69 0.n 0.702 056 122 1.14 4.W 1.99 0.99 0.35 0.48 0.44 0.52 0.797 027
9" 0.071 0203 0.330 0279 0.341 0.54 demilymknawn 0.723 11.3 0.77 4.10 113.4 7.50 0.171 '0.36 0.30 0.23 gas 0.306
Etnnen~
56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73
Ba La
74 75 76 77 78 79 80 81 82 83 84
W
86 87 88 89 90 91 92 93 94
Conlinucd
Crassna'%
(barnslo
Scatler,n9
No,
85
T A B L E X1.1
Nd Rn
8.1 9.3 4.7 3.3 160 ...
Srn Eu
8.0
cc Pr
...
Gd
...
Tb Oy
20.2 100 9.4 11.0 12.2 25 8 8 6.2
Ho Er Tm Yb
Lu HI Ta
Re
0s Ir Pl Au Hg Tl Pb
Bi PO At Rn Fr Ra AC TI, Pa
5 11.4 15.2 ( a h ) 14 112 9.3 20 9.8 11.4 9
... ... ... ... ... ...
12.7
...
U
9.0
Np Pu
...
...
1.2 9.0 0.63 11.5 50.5 60 58W 4600 49 WO 25.5 930 66.5 162 103 38.6 77 102 21 18.5 88 15.3 426 10.0 98.8 375 3.4 0.170 0.033
...
...
... ...
130 510 7.40 15W ( f a ) 7.68 9 W (fks'osion) 160 (nrsion)
Linear Anenualcon Coellicien,, ern-, 0.143 0.49 0.154 0.412 1.68 density unxnown 179 95.3 1497 1.446 36.1 2.43 5.68 3.83 1.50 2.85 5.3 1.5 1.49 6.60 2.17 30.9 1.40 6.39 16.1 0.46 0381 026
..
...
density u n b w n g= densityunknawn 1.69 density unkrour. 0.60 W.4 0.788 densify unkwr* 7.96
a Updaled I r a n prevaus edilion v i l h data pimarily fcan BNL 325.3rd ed..\'oJ 1.1973. OAIl cross.seclbn v a l m arc mas1 p r h k values.
and several rare earths means that these materials can readil! be shadowed with neutrons even when they are combined ir a n assembly with some high atomic weight material such a: steel, lead, bismuth, o r depleted uranium. Although thl heavy material would make X radiography difficult, neutrol radiography should yield a succasful inspection. Further, th diKerences in slou, neutron attenuation ohen found betwec: neighboring materials in the periodic table offer a n a d \ w tage for neutron radiologic discrimination between material that have similar X-ray artenuation characteristics. X1.2 This advantage is illustrated in Fig. X l . l in whit the mass attenuation coefficients pJp are plotted as function of atomic number o f the attenuating element fi both X rays (about 120 kVp energy) and slow neutron There are many apparent attenuation differences. T h e ficient plp is normally used in attenuation calculations in (1 exponential relationship
0 FIG. XI.?
10
20
30
40
50
60
70
ATOMK
100
ABSORPTIO.
NUMBER
Calculated Themal Nwlron and 100 & 500 KEV X-Ray Linear AHenuation Coefficients as a Fundion of Atomic Number
/,lo = p-~w~e~ox
.=
90
ATOMIC NUMBER Approximate Mass Anmuation Coeffiaents as a Function of Atomic Number
CALCULATIONS UTILIZED AVERAGE SCATTERING AND 2200 m l s
FIG. X1.2
80
(1)
ere: ratio of emergent radiation intensity to the intensity incident on a material, = linear attenuation coefiicient, = density, and = thickness.
X1.3 For neutrons, it is more convenient to have the relationship between attenuation coellicient and cross seclion, as rollOws: 11=
+
Po, = 40,
0,)
where: P = number of nuclei per cm3 o i attenuating material,
(2)
I.,
a,""
-1,.
-
01NI.'10".1WI
*.".I_ m,."l*F,,,,.WUI.U."rEIn .,'1II1*",ru.c+tr.ic.u. c0,,w,,0 UYO( ,*IU",.OII,LO*I*,rn'", r.. ' C . C l U 1 . 3 . *I1CILi(
@ '. ' "
".,lxtl"C.
.",U'"*
FIG. X1.3
..Y,m*UOnUI..",$.
<,I
I,,"
.#.n"rn.'"olwi
*.".L
U u U l l .
"Uannr.l~l*;i
,mu,-
Half-Value Layers of Selecled Materials fat Thermal Neutrons
Courtesy ol Aeroresl Operations. Inc.
a, = total
cross section (cm'), equal to the sum ofabsorption and scattering moss sections (a, = a%),and p = the linear attenuation coefficient (cm-I). A tabular listing of linear attenuation coefficients is shown in Table X1.I and a comparative plat is given in Fig Xl.2. The data presented in fig. XI .3 give half-value-layer thicknesses for thermal neutrons for many materials.
XI .4 In radiologic situations, radiation that is transmitted through the object being examined is recorded so that those areas in which radiation has been removed, either by absorption or by scattering, may be observed. Equations (I ) and (2) are valuable in assessing the relative change in transmitred radiation intensity for several materials and thicknesses within an object of interest.
X2. CALCULATION O F T H E LINEAR ATTENUATION C0EI:FICIENT O F A COMPOUND X2.1 Ifthe material under examination contains only one element, then the linear attenuation coefticient is as follo\vs: fl = P -
A'o A
where: p = linear auenuation coefficient, cm-I, p = material density, grn.cm-', N = Avogadro's number = 6.023 x 10'' a1oms.g-mol-'. a = total cross section, cm', and A = gram atomic weight of material. X2.2 If on the other hand, the material under cxarnination contains se\ceral elements, or is in the form o i a compound, then the linear attenuation coefficient is as iollows:
where:
)1
= linear attenuation coefficient of the compound, cm-'.
= compound density, g.cm-', A' = Avogadro's number = 6.023 X LO2' atoms.g-mol-'. A4 = g n m molecular weight of the compound, u , = number of absorbin2 .atoms of ith kind uer compound molecule. and a , = total cross section of the ith atom, cm'. X2.3 As an esample. consider the calculation of the linear attenuation coeflicicnt. p. for the compound polyethylene Cliz: p
where: p = 0.9 l g.crn-'. N = 6.023 x 10" atonis,g-mol-I, A4 = 14.0268 g.
Lecture Guide: UT Basic Principles INTRODUCTION Ultrasonic testing
- introduces high frequency sound waves into test object - to obtain information about object - measures two quantities
- time for sound to travel - amplitude of received signal Primary Applications
- Thickness measurement
- Discontinuity detection - Material properties Ultrasonic Signal Terminology
-
Indication: displayed s i p d
- Reflector: source of an echo - Discontinuity: interruption of the test materid - Defect: unacceptable discontinuity
Advantages and Limitations
Advantages
- deep penetration - portable equipment
- pulse echo testing requires access to only one side of test object - accurate for thickness measurement and discontinuity location - permits volumehic examination - suitable for go / no-go testing: audio and visual alarms - no known hazards
Limitations
- test object must be able to conduct sound - liquid couplant is required
- need a nained operator - dead zone: discontinuities just beneath surface may not be detectable
SOUND
Sound is the passage of mechanical energy, in the form of vibrations, through a medium The medium provides two properties required for vibration to occur
- Mass:
matter that the energy can move
- Elasticity: restoring force
.
Sound can propagate in all three states of matter
- solids - liquids
- gases
.
Ability to propagate depends upon:
- type of sound wave - material composition
- sonic wavelength
GENERATION OF SOUND
. . .
Transducer (often called a search unit or probe)
-
A device which converts energy from one form to another
An ultrasonic transducer is the link between the instrument and the test object Operates on piezoelecnic principle
- piezoelecDic crystals develop a voltage when subjected to mechanical
.
pressure (i.e., when deformed)
piezoelectric process will operate in reverse
- piezoelectric crystals change shape (and be caused to vibrate) when a voltage is applied to them
.
Transducer assembly (contact type)
- crystal element: thickness determines frequency of vibration - elemcdes: establish electrical contact with the crystal - frontal member
- Contact transducers: wear plate provides protective contact surface - damping block: controls crystal ringing; absorbs rear sound waves
Damping Block
!;: >;
i/i
8
8
i $
*
ii
I
I Electrodes
Wear Plate
.
.
Types of Transducers
- straight beam: introduces sound perpendicular to the test surfaces
- single crystal: for testing thicker materials
- dual crystal: for testing thinner materials - especially thickness gauging of corroded and eroded materials
- delay line: high resolution for near surface flaw detection, plus thickness gauging on thin materials -paintbrush: long, rectangular active area, usually made from a "mosaic" of crystals, for rapid scanning of large surfaces
- angle beam: introduces sound at an angle to the test surface
- immersion: for use in a liquid environment - focused: concave surface
The Test Sequence
- instrument's timebase (sweep generator) initiates time/distance display - insmment's pulser emits initial pulse
- to activate transducer, sending sound into test object - initial pulse appears on display - sound travels through test object - sound reflects from material boundaries and discontinuities
- these reflections strike transducer, are converted into electrical signals and displayed
.
TimelDistance Relationships
- sound travels at different speeds in different materials -
speed of sound is constant in a given material
- ~hereforcwe can measure distance by measuring sound travel time roo i 90
-
............3.................................. i
..............>.............:..............: ............. :..............:..............:..............:............. .............:..............:..............i:..............:..............: ' Ti. ......"!..............................:........ : ............. ...........:.... i i -' i
. . .L
70
3".
. . . . . ............. . . . .'.............. . . . . '. 5.. ..........:............ . . :............ :.............. : ........................:..............6............!..............j 1.............i..............1........... i 40 .... ..".'"............:..............:i..............:..-.........i.............i'."........ ........:.............. ...........: 1............'..............'..............!.............. -I :. ...........:................... :............"! 20 i... ..........i..............'..............:.............. ...... .............! ............:............ ..... :..............: j I/..'..........! .............:..............:...-.........:..............' .......... ..............'.............. "
.L
f.
i"""
:
.
. . i
.L
i
IIIII~IIII~IIII~IIII
0
.
1
2
3
4
5
I I ~ I ~ ~ I I I ~ I I I I ~ I I I I ~
6
7
8
9
1
0
Delay a n d range controls
- provide time base adjustment
- DELAY control horizontally shifts reflections without altering space between them
- RANGE control expands or contracts space between reflections
Echo Amplitude/Signal Height Relationship
Echo amplitude determines height of the echo signal on the display
Gain control provides amplitude adjustment
Wavelength and Its Elements
Properties of sound waves include:
- velocity - frequency
- wavelength Velocity is defined as the speed of sound
- i.e., distance mveled per unit time - velocity depends on: - density and elasticity of test material
- wave mode (shear, longitudinal, surface, etc.) - material temperature
Frequency is the rate of vibration
- i.e., the number of complete waves that pass a given point in one second - a wave is generated from one full cycle of transducer vibration - frequency depends on the number of cycles per second - frequency units - Hertz (Hz): cycles per second
- Kilohertz (KHz): thousands of cycles per second - Megahertz (MHz):millions of cycles per second Importmt frequency ranges
- audible (human hearing) range: 20 to 20,WO Hertz
- ultrasound: above 20,000 Hertz - commercial testing range: lOOICHz to 25 MHz+ a sound wave is sonic vibration in motion
- apuise or wave wain is a series of sound waves
- defrned as the distance from one point on a wave train to the next identical point
- also defined as the distance sound travels within the duration of one complete cycle
Wavelength (mm) = Velocitv (kmlsecl Frequency (MHz)
IV.
REFLECTION PRINCIPLES
Sound reflects when it strikes an acoustic interface
An echo is a reflection from an acoustic interface
An acoustic interface is the boundary between two materials of different acoustic impedance
.
Acoustic impehnce is the opposition that a material offers to the passage of sound
.
Acoustic impedance =Velocity x Density (2 =V x r)
s
. .
The greater the acoustic impedance difference, the greater the percentage of reflection Echo performance also affected by size, shape, orientation, and texture and thickness of reflector Sound can be absorbed and scattered as it mvels b u g h a given material
- because the material's structure may include grain boundaries, porosity, or impurities
V.
MAJOR TEST VARIABLES
Basic Test Method
Coupling Technique
Wave Travel Mode
.
Sound Travel Geometry
Data Presentation Method
Basic Test Method
Thru-transmissiontechnique
- sound is transmitted in one direction thru object
- Received at the other end of the object - Test sample compared with reference sample - Reduced amplitude indicates interruption of sound travel -Display shows amplitude of received signal
- Requires fucturing of transducers - Requires access to both sides of test object - Does not provide individual echo signals for each reflector
Pulse-cclm technique
Test object information provided by reflected sound energy
- Individual echo signal for each reflector - Displayed Information: echoes reflected from acoustic interfaces
.
Resonance tests were used for thickness measurements
- Resonance occurs when material thickness equals 112 of wavelength - has been replaced by pulse-echo method
.
Coupling
.
Liquid couplant is needed to exclude air and act as medium for hansmitting ultrasound into test material because:
- high reflectivity due to impedance mismatch at air interfaces - wavelength is too short in air at the high frequencies used for testing
.
Couplant considerations: -Wetting ability
- Viscosity - Should not damage test material - Ease of removal
.
Typical couplants: -water
- oil - cellulose and water mixture
- grease
Contact testing technique: couplant is applied to test surface
- Advantages of contact testing
- portability -allows the transducer to be moved by hand over complex part geometries
- requires a lower initial investment in equipment
SEARCH
IBI
COUPLANT
TEST
.
Immersion testing technique: transducer and test object are immersed in water
- the water usually contains additives (wetiing agent, anti-fungicide, etc.)
- Advantages of immersion
- uniform coupling
- high speed testing
- recording of test results
- virtually immune to transducer wear caused by abrasion - allows use of higher frequency transducers
- abiity to angulate transducer
- ability to use focused transducers
-
.
precise control over transducer movement
Surface Condition
-
smooth surface is preferred
- rough entry surface scatters the sound, reducing test sensitivity
.
Wave Motion
Sound waves travel through a material by displacing tiny particles (molecules) in the material
Various wave modes
- longitudinal, shear, surface, plate
.
Wave rZ1odesare defined by particle movement in relation to direction of navel
Longitudinal waves
- also known as compressional waves
- particle motion parallel to wave travel
- alternating zones of compression (high particle density) and rarefaction (low particle density)
- travel in solids, liquids, and gases
- highest velocity wave mode
Transverse waves
- also known as shear waves
- particle motion perpendicular to wave travel
- alternating zones of peaks (upward particle displacement) and troughs (downward particle displacement)
- travel in solids only
- approximately h q the velocity of longitudinal waves
.
Rayleigh waves
- also known as surface waves
- travel across material surface
-velocity is 90 percent of shear waves
-penetrates to approximately one wave length
.
Plate waves
- propagation occurs only in thin sheet materials
- when material thickness is less than three wavelengths - two modes; symmetrical and asyinmebical
1-25
Sound Travel Geometry
.
Maximum sound reflection is obtained when sound beam is perpendicular to reflecting surface
- discontinuity pardel to the sound enhy surface: straight beam transducer
- discontinuity obliquely oriented to the test surface:
n
/ 1
angle beam transducer
Display presentation techniques
- A-scan - horizontal scale: distance I time
-
vertical scale: echo amplitude I transducer output voltage
- B-scan; side view of test object: profile of interfaces reflecting sound beam
- C-scan: plan(top) view through test object
VI.
TEST INSTRUMENTS
.
Introduction
- Ultrasonic test instruments are comparirors
- Therefore ultrasonic instruments must be calibrated prior to use
Ultrasonic Instrument Functions
.
Clock Circuit (Timer, Synchronizer)
- Clock initiates the chain of events that results in one complete cycle of an ultrasonic test
- Clock sends mgger signal, at regular intervals, to sweep generator and pulser
- Trigger signal is repeated at a given frequency, called pulse repetition rate
-When repetition rate is too fast, wraparound (display of echoes from previous test cycles) occurs
Display:
Conventional Cathode Ray Tube
- Provides a visual display of test signals
- Contains an electron gun which generates a narrow beam of elections directed toward front of tube
Sweep generator
- generates a display of sound travel time on the horizontal scale - for distance readout
- RANGE control adjusts horizontal scale for desiml distance range
- scale will be valid for a given sound velocity only
- horizontal display is adapted for different material velocities using a M A T E W VE%C€lTY contml
.
Pulser
- Emits electrical signal which activates transducer - Called initial pulse or main bang
- Duration of transducer ringing determines the length of the dead zone - Dead zone is the depth range in test material from which no indications can be displayed
- DAMPING and/or PULSE ENERGY
adjust initial pulse
Receiver
- Receiver circuit processes and amplifies signals enroute to CRT -Processing is provided by detector and filter subcircuits
- Filtering is a cosmetic change to the signal that removes test information - Videoflters smooth out pulse cycle information - Frequency
filter selects of either narrow band or broad band display .
- Narrow band display provides an improved signal to noise ratio .
- improves test sensitivity - Broad band display is for high resolution testing
,
.
Amplifier
- A subcircuit in the receiver circuit - Multiples the voltages of signals - Controlled by GAIN control
- Gain controls are calibrated in decibels (dB) -Decibel values are logarithmic
- To estimate discontinuity size - determine difference in echo amplitude between discontinuity signal and reference signal, with use of a calibrated gain control -REJECT control adjusts the amplifier's input sensitivity
- prevents the display of undesired low amplitude signals - for example: grass or hash (metal noise signals such as echoes from material grain boundaries or inherent fine porosity)
VIII. REFLECTORS IN THE SOUND BEAM
.
Sound Beam Geometry
- The sound beam consists of a near field (Fresnel zone) and a far field (Fmunhofer zone)
- The end of the near field (and the beginnjmg of the far field) is called the
- Point sources: Sound originates on the crystal surface as a number of individual point sources radiating spherical waves
- As the waves progress outward from the transducer, they interfere with each other
- the interference in the near field causes varying wave amplitudes - therefore, it is difficult to estimate reflector size in the near field
- at the yo point, waves combine into a single spherical wave front
- far field: predictable decrease in sound pressure as distance from the transducer increases
- therefore, reflector size can be estimated in the far field
Laws of Distance
- Infinite reflectors -- intercepts the entire sound bearn
- Small reflectors - intercept only a portion of the sound bearn
Material Loss Attenuation
- amplitude losses caused by the structure of the test material - scattering of sound by coarse grain structure or fine porosity
- conversion of sound into heat by absorption
- Distance Am~litudeCorrection IDAC) curve techniaue: a curve showing amplitude versus distance for a given reflector is manually or electronically plotted on the CRT screen
- Electronic distance am~litudecompensation techniaue: the test i n s m e n t varies gain as a function of distance so that a given reflector exhibits the same displayed amplitude at all distances
- Test block technique: reflectors in test objects are compared to machined reflectors in standardized test blocks
-Test block (ASTh4 Block) specifications
- Area Amplitude Blocks Set (Alcoa A) - 8 blocks - 314 deep flat-bottomed hole in each block - labeled #1- #8 for 64th~of an inch hole diameter - used to check test system linearity
- Distance Amplitude Blocks Set (Alcoa B) - 19 blocks - 314 deep flat-bottomed hole in each block - lengths vary to provide metal paths of 1/16" - 5-314" from test surface to hole interface - used to evaluate discontinuities, set sensitivity, set DAC - Basic Blocks Set - 10 blocks, each 2" in diameter - combination of portions of area amplitude and distance amplitude block sets
X
.
TEST PERFORMANCE VARIABLES
-
a Penerratiorl: the ability to pass through a material interface of a given size (e.g., grain boundaries and inherent porosity). Penetration improves by decreasing test frequency.
-
b. sensitivity: the ability of the test system to display small reflectors, to display a given size reflector of a given distance along the sound beam axis. Sensitivity depends primarily on five factors: (1)
Beamspread: As beamspread is decreased, more sound pressure per unit area strikes a reflector, thus increasing echo amplitude. Beamspread is decreased by increasing transducer area and/or increasing frequency.
(2)
Near Field Len&: As near field length varies, the position
of a reflector relative to the yo point likewise varies. Sensitivity is optimized when the reflector is positioned near the beginning of the far field. (3)
Freauencv Bandwidth: As bandwidth is decreased, sensitivity increases. Bandwidth i s decreased by decreasing transducer damping.
(4)
Transducer Crvstal Material: Piezoelectric crystal materials vary in their efficiency as both transmitters and receivers of sound.
(5)
Test Svstem Simal to Noise Ratio: Signal/Noise Ratio depends on a number of factors such as penetration and test instrument design.
-
c. Resolun'on: the abiity of the test system to individually display reflectors located at slightly different depths along the sound beam. Resolution depends primarily on Frequency Bandwidth. As bandwidth is increased resolution increases.
XI.
ANGLE BEAM THEORY
Straight beam transducers are only effective for detecting flaws parallel to the test surface
.
Angle beams are required for detecting flaws obliquely oriented to the test surface Angle beams are produced in the test material using the principle of refraction Refraction is the bending of a sound beam when it passes through an interface between two materials of different velocity
In contact angle beam testing, the transducer crystal element is mounted on an angle wedge to produce refraction
.
In immersion angle beam testing, the transducer is "an,dated" to produce refraction
Angle Beam Transducer Assembly
1-44
Sound beam approaching interface is called incident beam
.
Sound beam is reflected at the interface
Angle of reflection equals the angle of incidence
Mode Conversion
- occurs when a sound beam is incident to an interface at an angle other than 90 degrees - A pomon of incident beam's energy converts at the interface to a h a m of the opposite wave mode
- reflects at an angle other than the angle of incidence
:Mode Converted
.
Refraction
- When a sound beam passes at an angle other than perpendicular to the interface, between two materials of different acoustic velocity, a change in beam direction called refraction occurs
:Mode Converted
Shear Beam
.
Snell's Law defines the relationships between incident and refracted sound beams: Sjn (Incident) Sin (Refracted)
Velocity (Incident) Velocity (Refracted)
.
Critical Angles
- Thefirst critical angle is the incident a n p l ~that causes the refracted longitudinal wave to be refracted 90 degrees
: Mode Converted : Beam
- The second critical angle is the incident angle that causes the refracted shear wave to be refracted 90 degrees
t
I Mode Converted
: Surface Waves
- A surface wave starts to develop at the second critical angle
Lecture Guide: ET Basic Principles
Overview Summarv of the Eddy Current Test Process
-
An alternating current generator applies an alternating voltage to a coil, causing ac current to flow through the coil.
-
The current flow in the coil causes an alternating magnetic field to develop around the coil.
-
When the coil is brought near to an electrically conductive test object, the alternating magnetic field develops circulating electrical currents in that object.
-
The current flow provides test information that can be displayed and interpreted.
Copyright 1993 Hcllicr bsociates, Inc.
=
Maior aoulication areas -
In-service inspection of tubing at nuclear and fossil fuel power utilities, at petrochemical plants, on nuclear submarines and in air conditioning systems
-
Inspection of aircraft structures and engines
-
Production testing of tubing, pipe, wire, rod, and bar stock Eddy current applications result from sensitivity to several variables: Conductivity variations Presence of surface and subsurface discontinuities distance) Spacing between test coil and test material (18-08 Material thickness Thickness of plating or cladding on a base metal Spacing between conductive layers Permeability variations
Copyright 1993 HeUier Associntes, Lnc.
Advantages and Limitations
The advantases are:
1.
Sensitive to numerous material variables
2.
Much of the equipment is portable, lightweight, and battery powered.
3.
The method is virtually nondestructive -
4.
Test results are usually instantaneous
5.
No couplants, powders, or other physical substances are link applied to test material; aaugnetic field is the only&t4y.eencoiLaud_t.esf.naterial
Exception: computer analysis of recorded multi-channel test data
Ideal for "go/no-go" testing -
Audible and visual alarms available for high speed testing
-
Alarms triggered by threshold gates or box gates
6.
No known safety hazards
7.
Material preparation is usually unnecessary; cleanup is not required
Copyright 1993 Hellicr Associates. Inc.
The limitations are: 1.
Sensitive to numerous material variables
2.
Test material must be electrically . conductive -,*-
/
But it is possible to measure thickness of nonconductive coatings on conducting materials
1
' d)(i f l
4 -
,,
3.
Eddy currents normally cannot penetrate ferromagnetic materials Consequently, testing on ferromagnetics is limited to surface defects only
-
unless material has been magnetically saturated using direct current field coils
Magnetic saturation limited to certain test geometries only
4.
Likely demagnetization after testing is completed.
Limited penetration even on nonferromagnetic materials
5.
Penetration limited to fractions of an inch in most materials.
Requires a trained, skilled, experienced operator
Copyright 1993 Hellicr Associntcs. Inc.
=
-
Magnetism A magnet's force field can be visualized as a number of closed loops -
The magnetic loops are called lines of force orflux lines
-
the lines of force flow from the north to the south pole around the outside of a magnet; and from the south to the north pole within the magnet
Copyright 1993 Hcllicr Ass?ciatrs. Inc.
-
Field intensity depends on flux density
-
Flux density is the number of flux lines per unit area perpendicular to direction of flow
-
Gauss is the unit of measure for flux density
-
One gauss is one line of force per square centimeter
-
Flux density is greatest within the core of a magnet and at the poles
-
Flux..density.decreases with distance from the magnet according to the inverse square law i.e. flux density is inversely proportional to the square of the distance from the poles of the magnet
Flux Field
Copyright 1993 Hellier Associates. Inc.
*
Electromagnetic Principles The Induction Process 1.
An alternating current generator applies alternating voltage to a coil circuit. A portion of this voltage, (VR), is applied across the resistance of the coil wire.
2.
(VR), causes a current (Ip) to flow through the coil.
Copyright 1993 Hellicr Associates. Inc.
3.
Electromagnetism occurs. -
3.
The alternating current flowing through the coil causes an alternating magnetic field (CDp) to develop around the coil.
Self induction occurs.
-
The coil's alternating magnetic field induces a back voltage (VL) into the coil.
-
According to Faraday's Law, the quantity of induced voltage is proportional to the rate of flux variation.
-
Since the flux is varying the most through 00,1800, and 3600; and varying the least through 900 and 1800, the back voltage is 900 out of phase with the coil current and flux.
Copyright 1993 Hellier Associates. Inc.
4.
Inductive Reactance occurs.
-
Since the back voltage is 900 out of phase with the coil current, it will oppose changes in the coil current.
-
Since amplitude change is the very nature of alternating current flow, opposition to change in AC is effectively opposition to flow of AC.
Copyright 1993 Hellicr Associntcs. Inc.
5.
If a secondary circuit is placed in proximity to the primary, a voltage will be induced into it, current will flow through it, and an aItemating magnetic field will develop around it.
-
Lenz's Law will take effect: the direction of current flow in the secondary will be opposite in direction to current flow in the primary.
-
In addition, the polarity of the secondary flux will be opposite to the polarity of the primary f l u .
' Copyright 1993 Hellicr Assacintcs. Inc.
6.
Due to Lenz's Law, the secondary flux will be opposite in polarity to the primary flux.
-
the secondary flux will therefore cancel some of the primary flux.
-
this reduces the amplitude of peak primary flux
-
which reduces the rate of variation of primary flux
-
less variation of primary flux results in reduced back voltage
-
which results in a reduction of inductive reactance
-
when the coil is moved toward a more conductive portion of the test material test coil inductive reactance decreases
Copyright 1993 Hcllicr Associates. Inc
Summarv of Induction Process Terminology
.
Electromagnetism
-
electric current flowing thr~ugh-g~~~&~u_ctor causes a magnetic f a d to develop around that conductor, .-perpendicular to i F -_-_A
*
-
A more concentrated magnetic field can be obtained by winding the conductor into a coil
-
Rux density decreases with distance from a magnet according to the inverse square law
Electromagnetic induction (Faraday's Law)
D
Self Induction
-
.
relative motion between a magnetic field and conductor causes an electrical current flow in that conductor
Relative motion between an AC magnetic field and the conductor developing that field induces a voltage into that conductor
Back Voltage (Back EMF)
-
The voltage induced as the result of self induction
-
Because it is induced 90 degrees out of phase with the coil current, back voltage will oppose changes in the coil current
Copyright 1993 Hellier Associntcs. Inc.
.
Inductive Reactance
-
The opposition to change in alternating current flow caused by back voltage
-
Since amplitude change is the very nature of alternating current flow, opposition to change in AC is effectively opposition to flow of AC
-
Inductive reactance depends on coil design and test frequency
-
When more flux lines cut across more coil turns per unit time, inductive reactance increases
-
Hence, the formula:
X ~ = 2 n f L
-
Lenz's Law the direction of an induced current will be such that its own magnetic field will o d the induced current
Copyright 1993 Hcllicr Associates. Inc.
Eddy Current Test Process
-
Circulating electrical currents induced in an isolated, electrically conductive object by an alternating magnetic field
-
In contrast to electricity conducted along the length of a wire, the electricity generated by the test coil's lines of force has a circular eddy-like pattern
.
Seauence of Events During an Eddv Current Test The test instrument and coil assembly function together, so that: 1.
The test instrument's AC generator applies alternating voltage to the test coil, causing an alternating current to flow through the coil
The frequency of the eddy currents alternating in the test material depends on the test instrument's ac frequency generator
Copyright 1993 Hellier Associates. Inc.
;
2.
The current in the coil develops a magnetic field around the coil (the primary flux)
-
the primary flux induces a back voltage into the coil, causing inductive reactance
Copyright 1993 Hcllicr Associntcs, Inc.
3.
The primary flux also induces a voltage into the test material, causing eddy currents to circulate
Copyighr 1993 Hcllicr Associntes. Inc.
5.
The eddy currents generate a magnetic field of their own (called the secondary flux)
- which reacts with the primary field that the coil is generating
-
test material conditions (defects, conductivity changes, thickness changes) affect the flow of eddy currents
-
changes in the flow of eddy currents cause changes in the secondary field
-
changes in the secondary field cause changes in the impedance of the coil
4.
Changes in the impedance of the coil cause a change in the indication on the display (test output).
Copyright 1993 Hcllier Associates. Inc.
Eddy Current Characteristics *
Flow Patterns
-
They flow in closed loops
-
They flow in concentric circular paths parallel to the turns of the coil perpendicular to the coil's flux
-
orientation of eddy current flow in the test material therefore depends upon the orientation of coil flux to the test material which, in turn, depends on the orientation of the turns of the coil to the test material orientation of the coil's turns and, thus, eddy current distribution are determined by the coil's configuration
Copyright 1993 Hcllicr Associates. Inc.
-
Eddy current flow is least disturbed by discontinuities oriented parallel to their flow paths
-
Most disturbed by discontinuities oriented perpendicular to their flow paths
-
In their attempt to flow in unbroken loops, eddy currents follow the path of least resistance around nonconducting obstacles
Copyiight 1993 Hellier Associates. Inc.
-
eddy currents behave like compressible fluids the flow paths are circular as long as the eddy currents are undisturbed by nonconducting material boundaries and discontinuities the flow paths will distort and compress to accommodate intrusion of theiqflow
-
The direction of travel continually alternates between clockwise and counter-clockwise movement
Copyright 1993 Hellicr Associatcs. Inc.
Skin Effect -
-.
-
Eddy currents are subject to skin effect current density is maximum at the material surface and decreases rapidly (exponentially) with depth standard depth of penetration (6) is the material depth at which current density decreases to 36.8% of surface current density
skin depth refers to the layer of material thickness extending from the surface to the standard depth of penetration the skin depth formula applies to thick materials only (t > 56)
*
-
Phase Lag Eddy currents experience a linear phase lag with depth as depth increases, eddy current activity is progressively delayed phase lag in the test material proceeds at the rate of one radian (57.3") per standard depth of penetration
Test Output
-
During an eddy current test, a primary circuit (the test coil) induces eddy currents into a secondary circuit (the test material)
-
The test material behaves the same as a single turn secondary coil
-
Variations in the test material change the test coil's inductive reactance and effective resistance, producing indications on the instrument display
Copyright 1993 Hcllier Associntcs. Inc.
21
-
Note the use of the term effective resistance the resistance of the coil's wire does not change however, the eddy currents circulating in the test material cause friction and dissipate a part of their energy as heat thus the secondary acts as a load on the primary, causing a resistance change on the display
Copyright 1993 Hellicr Associ;ltes. Inc.
*
Impedance
-
When resistance and inductive reactance are combined they produce a quantity called impedance
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Impedance amplitude is the magnitude of the vector "* .,-sum of inductive .reactance andFsl3Eice
impedance amplitude is the coil's total opposition to current flow as inductive reactance and/or resistance increases, impedance amplitude increases
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Impedance phase angle is the proportional relationship between inductive reactance a n d resistance
as inductive reactance increases relative to resistance, impedance phase angle increases as resistance increases relative to inductive reactance, impedance phase angle decreases
Copyright 1993 Hcllicr Associntcr. Inc.
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With most eddy current instruments, the coil assembly is connected to the instrument via a bridge circuit
at the start of the test, the instrument operator balances the bridge to provide a reference signal during testing, the display provides a readout of bridge imbalance caused by interaction of the coil with the test material -
When an instrument is balanced during test setup, it is balanced for impedance values at a particular point on the impedance plane
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the balance point serves as a display reference during testing
Copyright 1993 Hellicr Associates. hc.
Impedance Plane Display -
The impedance plane is a graphic plot of values present in the test coil
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The total voltage affecting coil current consists of two components voltage across the coil's resistance induced back voltage that causes inductive reactance.
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The voltage across the coil's resistance is in phase with the current
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The induced back voltage is 90 degrees out of phase with the current
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A graph of these two voltages would therefore place them on axes that are 90 degrees opposed
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Likewise, a plot of the impedance components associated with these voltages, inductive reactance and resistance, would require axes that are 90 degrees opposed -
Resistance values are shown on the X axis
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Inductive reactance values on the Y axis
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Such a plot is called an impedance plane and is used for displaying eddy current test data.
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Impedance plane display instruments present both impedance amplitude and phase angle simultaneously on a CRT screen
Copyright 1993 Hellicr Associalcs. Inc
Signal Analysis
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Test information on an impedance plane instrument is interpreted by observing the movement of the display dot on a cathode ray tube screen while the test coil interacts with the test material
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Each type of condition that an eddy current test can detect is characterized by a certain pattern of display dot movement
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Test variables are arranged along curves or "loci" on the impedance plane
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Generally, there are separate curves for each variable
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Distribution of information on the impedance plane can be altered by changing test frequency
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Redistribution of information on the impedance plane by adjustment of frequency is a key technique in optimizing test performance
Copyright 1993 Hellicr Associalcs. Inc.
Lift-Off Curves
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The zero conductivity point, also called the coil in air or empty coil point is typically located at a position of low resistance, but high inductive reactance
Resi s t a n c e
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This is the impedance point for a coil whose flux is not near any conductive material
Copyright 1993 Hcilier Associatrr. Inc.
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As a coil is moved toward a conductor, secondary flux changes the coil's impedance and the display dot moves
Res i s t a n c e
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The position where movement terminates depends on the conductivity of the test material
Copyright 1993 Hellier Asxrciatcs. Inc.
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The more conductive the test material, the greater the cancellation of primary flux
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Thus, the greater the drop in inductive reactance, the further downward the display dot moves
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In addition, since the coil and test material are mutually coupled, the test material acts as a load on the coil and the effective resistance of the coil changes
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The movement of the display dot is therefore a combination of variations in both inductive reactance and effective resistance.
I Resistance
Copyright 1993 Hcllicr Associates. Inc.
Conductivity Curve
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The conductivity curve originates at the zero conductivity point and terminates at the infinite conductivity point
counterclockwise extreme represents zero conductivity clockwise extreme represents infinite conductivity sometimes called the comma curve because of its shape
Copyright 1993 HcUier Associntes. Inc.
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Different positions along this curve represent nonferromagnetic materials of different conductivities whose thicknesses are infinite relative to electromagnetic penetration i.e., the flux lines entering the material, as well as the eddy currents that they generate are not touching the bottom surface of the material
d U
Rir Point
C
t i v 8
R 8
a C
t a
n
C
Resistance
Capyight 1993 Hcllicr Associatcs. Inc.
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As frequency is increased, the impedance points for the various conductivities move clockwise along the curve
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Thus, as frequency is increased, the lower conductivity materials spread apart along the curve while the higher conductivity materials become compressed at the bottom end of the curve
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Higher frequencies provide greater separation fdr conductivity tests on lower conductivity materials
Copyright 1993 HeUicr Associntes. Inc.
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As test frequency is decreased, the impedance points for the various conductivities move counter-clockwise along the curve
I
$ U
R i r Point
C
t I
v
e
R e a C
t a n C
Resistance
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And, as frequency is decreased, the higher conductivity materials spread apart while the lower conductivity materials become compressed at the top end of the curve
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Lower frequencies provide greater separation for conductivity tests on higher conductivity materials
Copyright 1993 Hellicr Associntes. Inc.
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Frequency adjustment also helps separate the lift-off and conductivity variables At low frequencies, lift-off curves for low conductivity materials are almost parallel to the conductivity curve As frequency is increased, the operating point moves clockwise along the coaductivity curve, increasing the angle between the lift-off curve and conductivity curve
Maximum separation is achieved at the so-called "knee" of the conductivity curve, where the lift-off curve approaches it almost perpendicularly
Coil Diameter
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Increases in coil diameter move the display dot clockwise on the conductivity curve
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Decreases in coil diameter move the display dot counter-clockwise on the conductivity curve
Copyright 1993 Hcllier Associarcs. Inc.
Thickness Curves -
As stated above, the conductivity curve consists of impedance points for materials whose thicknesses are infinite, relative to electromagnetic penetration
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At lesser thicknesses, eddy current flow in the material becomes restricted and the impedance point spirals away from the conductivity curve
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As thickness.approaches.zero, the impedance point approaches the zero conductivity point
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One standard depth of penetration is approximately located on thickness curves at a point slightly to the right of initial intersection with the conductivity curve
Copyright 1993 Hellier Associates. Inc.
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Frequency adjustment is again available to optimize performance
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As frequency is decreased, material penetration increases, but thickness resolution on thinner materials decreases
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As frequency is increased, material penetration decreases, but thickness resolution on thinner materials increases
I
t
L"r=
4
. - .,
Resistance
Copyright 1993 Hcllier Associntcs. Inc.
Discontinuitv Signal Displav
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A discontinuity causes an interruption of current flow
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The magnitude of an eddy current discontinuity signal depends on the quantity of interrupted current flow
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Eddy current density decreases exponentially with depth
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Discontinuity volume, shape, and position all affect signal magnitude
The depth of the disturbance, however, causes a linear phase lag of the signal
Copydghl 1993 Hellier Associntcs. Inc.
Test Variables a
Test Performance Parameters
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Eddy current test performance is generally defined by the following criteria: Sensitivitv: The minimum size of discontinuity that can be displayed from a given material depth Penetration: The maximum depth from which a useful signal can be displayed for a particular application Resolution: The degree to which separation between signals can be displayed
Copyright 1993 Hellicr Associnm, Inc.
Control of Test Performance
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Test performance is primarily influenced by conductivity, permeability, frequency, and coil design
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In that only test frequency and coiI design are selectable, these two are the primary controls over test performance
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Conductivitv: The greater the conductivity of the test material, the greater the sensitivity to surface discontinuities, but the less the penetration of eddy currents into the material Explanation: As the coil's flux field expands, voltage is induced first on the surface and then at increasing depths in the test material
In high conductivity materials, a considerable eddy current flow and thus a strong secondary flux field is developed at the surface This results in a substantial cancellation of primary flux Because the primary flux has been greatly weakened, less primary flux is available to develop eddy currents at greater depth
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Permeability: As material permeability increases, signals resulting from permeability variations increasingly mask eddy current signal variations this effect becomes more pronounced with increased depth permeability thus limits effective penetration of eddy currents
Copyright 1993 Hcllicr Associates. Inc.
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Freauencv: As test frequency is increased, sensitivity to surface discontinuities is increased, permitting increasingly smaller surface discontinuities to be detected as frequency is decreased, material penetration is increased the test frequency for obtaining standard depth penetration in.a given material can be estimated from a Penetration Chart because of the number of variables affecting eddy current behavior, standard depth should only be used as a starting point the optimum frequency is best determined by experimentation
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Coil Desi~n: Penetration and sensitivity are affected by coil geometry penetration: larger coils produce flux fields that extend further in both the lateral and depth dimensions. Rule of thumb: eddy current penetration is limited to coil diameter sensitivity: since a small surface defect would cause a proportionally greater disturbance in the field of a smaller coil, smaller coils are preferred for detection and localization of small surface defects Rule of thumb: coil diameter should not exceed the length of he discontinuity that is to be detected
Copyright 1993 Hcllicr Associarcs, Inc.
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TEST MATERIAL VARIABLES
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Response of the test system to the test material can be classified according to three types of test material variable: J I. Conductivity
2. Geometry J
3. Permeability ! ,
Copyright 1993 Hellier Associates. Inc.
Conductivitv
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Conductivity is the ease with which electrons pass ihrough a given material Conductivity depends on relative ability of a material's atoms to allow electron flow Each metal is assigned a conductivity value on a scale called the International Annealed Copper Standard (IACS) G . . : :,.:.>,,,zd..
:,-: .::;.<
; ~L-
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According to the IACS, conductivity values - are rated in percent, with the conductivity of pure copper being 100%
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Factors causing conductivity variations include: 1.
Variations in chemical comoosition: The various metallic elements and alloys can be sorted as long as none of the materials has overlapping conductivities
2.
Mechanical processing: Cold working affects lattice structure, causing minor conductivity changes
3.
Thermal processing: Heat treatment causes hardness changes that are detectable as conductivity variations
4.
Unrelieved residual stresses cause unpredictable conductivity variations. Thus. it is a undesirable variable
- 5. 6.
Variations in thickness of plating or cladding are a combination of both conductivity and geometric variables Material temperature: As material temperature increases, conductivity decreases an undesirable variable -
Variations in temperature can be caused by environment, materials processing, and eddy currents themselves
Copyright 1993 Hcllicr Associates. Inc
Geometry
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Geometric variables result from restriction of eddy current flow due to differences in distance or size:
1.
Material thickness can be measured because changes in thickness affect eddy current flow.in the.test material As the material becomes thinner, eddy current flow becomes restricted Eddy current density is greatest at the material surface and decreases exponentially with depth (skin effect) Eddy current sensitivity to thickness variations also decreases with depth Recall that at standard depth of penetration eddy current density decreases to 36.8% of surface density Optimum performance is obtained up to this depth
2.
Material discontinuities cause indications to the extent that discontinuity dimensions and depth disturb eddy current flow discontinuities whose major dimensions are perpendicular to eddy current flow paths and which are located near the test surface will provide the strongest indications, since eddy currents attain peak amplitude progressively later with depth
Copyright 1993 Hellier Aswcintcs. Inc.
3.
Material boundaries. Restriction of current flow called "edge effect" occurs when an eddy current surface coil approaches the end of a plate
a current flow restriction called "end effect" occurs when an encircling or internal coil approaches the end of a tube or pipe Both effects produce strong signals The effects are intensified by the wider eddy current fields developed by large diameter coils and lower test frequencies. Smaller diameter coils reduces edge effect; use of shielded coils virtually eliminates it When a surface coil is drawn perpendicularly toward a material edge, an edge effect signal increases in amplitude If ~e field simultaneously intercepts a discontinuity during this approach, the two conditions will produce a combined signal, rather than separate edge and discontinuity signals
Copyright 1993 Hellier Associates. Inc.
Thus the discontinuity may not be detected The problem can be eliminated by scanning the coil parallel to the material edge at a constant distance from the edge; this technique maintains edge effect at a constant value. Interception of a discontinuity will then cause a signal change. Simple fixtures to accomplish this can be easily fabricated
Copyrighl 1993 Hellier Associates. Inc.
4.
Coil couvling. When distance between the test coil and test material varies, the intensity of the flux field induced in the test material likewise varies The spacing between a surface coil and the test material is called "lift-off' The spacing between either an internal coil or encircling coil and concentrically positioned test material is caIIed "fill factor" Coupling effectiveness between inner diameter probes and the inner wall of the tube is calculated as fill factor Lift-off is useful for measuring the thickness of paint or other nonconductive coatings on the surface of a metal Lift-off can also be used to measure the thickness of nonconductive materials, as long as such materials are placed on a conductive surface Fill factor deflections can indicate material variations such as wall thickness changes or ovality conditions Fill factor is calculated from the following formula: rC ~ 5 ; e s~c..y-o...
7!
Copyright 1993 Hcllier Associatcs. Inc.
fiw
/'
b
~4~ii
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Permeability
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Permeability is the measure of a material's ability to be magnetized, that is, a material's ability to concentrate magnetic flux
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Permeability is quantitatively expressed as the ratio of flux density to magnetizing force
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A hysteresis loop is a plot of a material's flux density variations as magnetizing force is varied
C o m p l e t e d H y s t e r e s i s Loop
Copyiight 1993 Hellier Associates. Inc.
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Saturation occurs at that point on the loop where further increases in magnetizing force do not cause significant increases in flux density
Copyright 1993 Hcllier Associates. Inc.
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Residual magnetism is the amount of flux density remaining in the material after the magnetizing force has been reduced to zero
B Residual Magnetism
Saturation
H
Copyright 1993 Hcllier Associates. Inc.
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Ferromagnetic metals, including iron, carbon steels, 400 series stainless steel, nickel, and cobalt, have high permeability
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The alternating magnetic field of an eddy current coil becomes highly concentrated in such materials and overpowers the eddy current response, causing the test system to display permeability, rather than conductivity, variations
Copyright 1993 Hcllicr Associntcs, Inc.
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TEST EOUIPMENT
Instrument Overview
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All eddy current instruments require at least three circuit elements: AC generator, coil, and processing/display circuitry
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During testing, the instrument should be checked at regular intervals against the reference standard to ensure that it is operating properly and is still set up correctly for the test being performed
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If a variation in instrument performance or setup is discovered, all material tested since the last verification of proper performance and setup should be retested.
Single Frequency Instruments
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The AC generator of a single frequency instrument drives the test coil with only one frequency. Basic Control Functions
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Frequency: Adjusts the frequency at which the ac generator drives the test coil
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Gain (Sensitivity, dB): Adjusts amplification of the bridge output signal for display
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Phase Rotation: Rotates the direction of dot deflection
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Balance (Null, Zero): Adjusts impedance to be identical on both sides of the bridge
Copyright 1993 Hcllicr Associntcs. Inc.
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The AC generator@) of a multi-frequency instrument drives the test coil with two or more frequencies.
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Multi-frequency instruments offer potential for substantial enhancement of performance. Use of more than one test frequency has two advantages:
1.
Use of -multiple frequencies aLIows more than one frequency-dependent performance variable to be optimized simultaneously For example: during in-service tube inspection using internal coils, a higher frequency provides sensitivity to inner diameter discontinuities, with a lower frequency for sensitivity to outer diameter discontinuities
2.
Test signals generated by the various frequencies can be "mixed" to prevent display of undesirable signals Suppression of signals from steel supports during inspection of nonferromagnetic tubes is an example Each e.dditiona1 frequency enables the mixing out of an additional variable
Copyright 1993 Hcllier Azsociatcs. Inc.
Coils
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Two classifications of eddy current test coils
I.
Basic configuration: determines how the coil physically "fits" the test object
2.
Absolute vs differential operation: determines how the coil assembly is wired to the instrument's circuitry, which determines the material conditions to which the system is sensitive
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Coil design, as well as magnitude and frequency of the applied field developed by the coil. current, all affect the electro~~lagnetic
Copyright 1993 Hcllicr Associates. Inc.
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..
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Configurations
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Surface Coils are built into probe type housings for scanning material surfaces
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In addition, the coil can be wound around a ferromagnetic core for even more field strength
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Wide surface coils permit rapid scanning and deeper penetration, but cannot pinpoint the-location of small discontinuities They are useful for conductivity testing because they tend to average out localized conductivity variations along material surfaces
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Narrow coils are preferred for detecting and pinpointing the location of small surface discontinuities Because of their smaller diameter electromagnetic fields, narrow coils are less susceptible to edge effect
Copyright 1993 Hcllicr Assxiafcs. Inc.
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Encirclin~Coils completely surround the test material
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normally used for production testing of rods, wire, bar stock, pipes and tubing
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Material tested with encircling coils should be centered in the coils by means of guides, so that the entire circumference will be tested with equal sensitivity
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Because of "center effect", eddy currents oppose and therefore cancel themselves at the center of solid cylindrical materials tested with encircling coils Thus, discontinuities located at the center of rods and bar stock cannot be detected with encircling coils
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Encircling coils inspect the entire circumference of the test object, but cannot pinpoint the exact location of a discontinuity along the circumference
Coppight 1993 Hcilier Associntcs. Inc.
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"Spinning coils", which are actually surface coils that revolve around cylindrical test material, are employed when identification of circumferential location is required in encircling coil applications
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Since spinning coils couple to only a limited segment of test material circumference, they are not subject to center effect
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However, spinning coils inspect with a spiral pattern, so their material coverage depends on coil rotation speed versus material transport speed
Copyright 1993 Hcllicr Associates. Inc
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Internal Coils pass through the cores of pipes and tubes, and are normally employed for in-service inspection Like encircling coils, standard bobbin-wound internal coils inspect the entire circumference of the test object at one time but cannot pinpoint the exact location of a discontinuity along. the circumference
Copyright 1993 Hellier Associates. Inc.
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Absolute, differential, and external reference modes can be used with any of the three basic coil configurations: surface, encircling, and internal coils
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With most eddy current instruments, the coil assembly is connected, to the instrument via a bridge circuit
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The bridge must be balanced by connection of matching impedance values to each side of the bridge
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The display,circuit is connected across the bridge to provide an indication whenever there is an impedance variation between the two sides of the bridge
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Absolute coil configurations place a single coil on the test material and employ a balance load, remote from the test material, to balance the bridge
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Absolute coils detect any'condition which affects eddy current flow
Copyright 1993 Hellier A s s x i a m . Inc.
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Differential self-comuarison configurations use a matched pair of coils to perform a comparison. both coils are coupled to the test material, with one portion of the test material being compared to another Conditions sensed by both coils cancel and are not detected Conditions sensed by only one coil are detected Differential coil signals are difficult to interpret The displayed signal represents the difference between two coil's impedances, rather than the impedance of a single coil's interaction with the test material
Copyright 1993 Hcllicr Associates. Inc.
Other Coil Setups
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External Reference employs one coil coupled to the test material, with the other coil coupled to a reference standard provides an indication whenever the test material differs from the standards
Copyright 1993 Hcllier Associotcs. Inc.
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Transmit-Receive configurations use one coil assembly to induce eddy currents into the test material and a second coil assembly to sense the secondary field
\Reflection coils employs two coils on the same side of the test object
c Display
Through transmission coils position transmitting and receiving coils on opposing sides of the test object.
p'.
a,"
\,\-
Copyright 1993 Hcllicr Associates. Inc
! I
LEAK TESTING Defiaai.tion - NDT method used: 1) for the detection and location of leaks and 2) for the measurement of fluid leakage in either pressurized or evacuated systems or components Leak - the physical hole that exists not the quantity of fluid passing through the hole
Leaks can be due to cracks, crevice, fissure, hole or passageway, that contrary to what is intended admits water, air or other fluid or lets fluids escape Leakage - the flow of fluid through a leak without regard to the physical size of the hole through which flow occurs
Leak rate - amount of fluid passi-ngthrough the leak per unit of timeof t h e under a given set of conditions (expressed as units of quantity or mass per unit of time)
Minimum detectable leak - smallest hole or discrete passage that can be detected Minimum detectable leak rate - smallest detectable fluid-flow rate
Leaks can have influence on the safety or performance of a system Leak Testing performed for: 1) prevent material loss which can interfere with system operation 2) prevent environmental contamination . hazards or nuisances caused by accidental leakages 3) to detect unreliable components and those whose leakage rates exceed acceptance criteria Sensitivity - how small a physical leak can be detected Part surface must be clean of any contaminents that could interfere with the test and be dry
Types of leaks: 1) real leaks - localized leak such as a hole 2) virtual leak -gradual desorption of gases from surfaces or escape of gases from nearly sealed components within a vacuum system Mean Free Path - the average distance that a molecule travels between successive collisions with the other molecules in the gas phase
Types of flow in leaks: 1) permeation - passage of a fluid into, throughand out of a solid barrier having no holes large enough to perinit more than a small fiaction of the total leakage to pass through any one hole 2) molecular flow (< 10-6 atm cm3Isec) when mean fiee path of the gas is greater than the largest cross-sectional dimension of the leak 3) transitional flow (10-4 to 10-6 atm cm31sec) - when mean free path of the gas is approximately equal to cross-sectional dimension of the leak
4) viscous flow - when mean fiee path is smaller than the cross-sectional dimension of the leak (consists of laminar and turbulent flow) 5) laminar flow (10-2 to 10-6 atm cm3isec) where velocity distribution of the fluid in the passage or orifice is parabolic; particles follow straight lines 6) turbulent flow (>lo-2 atm cm31sec) particles follow very erratic paths 7) choked flow - if upstream pressure is held constant and downstream pressure is gradually lowered, the velocity of the fluid through the passage will increase until it reaches the speed of sound
Four (4) primary leak testing methods Bubble Testing (BT) Halogen Diode Leak Testing (ELIIT) Pressure Change Measurement Test (PCMT) Mass Spectrometer Leak Test (MSLT)
Sensitivih ranges of the leak testing methods Sensitivity range in cm3/second METHOD PRESSURE VACUUM Bubble test - liquid film 10-1 to 10-5 10-1 to 10-5. Bubble test - immersion 1 to 10-6 Pressure - increase 1 to 10-4 Pressure - decreaselflow 1 to 10-3
1 to 10-4
Halogen (heated anode)
10-1 to 10-6
10-1 to 10-5
Mass spectrometer
10-3 to 10-5
10-3 to 10-10
A gas pressure differential is first established across a pressure boundary, therefore preventing the test liquid fkom entering or clogging the leaks Gas leakage through pressure boundary is then detected by the formation and observation of bubbles in the dectection liquid at the exit points of leakage
Provides immediate indications of the existence and location of large leaks Three classifications of bubble testing 1) Liquid immersion technique pressurized test object or system is submerged in test liquid e bubbles form at exit point of gas leakage and rise to the surface of the test liquid
2) Liquid film application technique thin layer of test liquid is flowed over the low pressure surface of test object bubbles form at exit point of gas leakage 3) Foam application technique used for detection of large leaks test liquid is applied as thick suds or foam rapid escape of gas tiom large leaks 'blows a hole' through foam blanket indicating leak
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Advantages relatively simple, rapid and inexpensive fairly sensitive technique location of exit points of leaks very accurate in immersion technique, entire pressurized component can be inspected simultaneously on exposed surfaces visible to the examiner large .leaks can be detected first and sealed or repaired, then smaller leaks can be detected with more refined testing apparatus required level of operator training and skill is minimal Disadvantages contamination of test spechan surfaces improper temperature of part surface contamination or foaming test liquids improper viscosity of test liquids excessive vacuum over surface of test liquid
*
low surface tension of test liquids leading to clogging of leaks prior use of cleaning liquids that might clog leaks air in test liquids or outgassing from test surfaces causing bubble formations OGEN DIODE E E K TESTmG
In a halogen leak detector, minute quantities of halogen vapor enter a detector cell and are ionized catalytically on a heated platinum anode Ions are collected on a cathode electrode which has a negative potential A current proportional to the rate of ion formation flows in an external circuit to produce an indication on a meter
Rate at which ions are formed is proportional to the halogen concentration in the gas which passes into the detector cell
A unique feature of the dector cell is that the ionization process can take place at atmospheric pressure Ionization process is specific to halogen vapors produced by halides Halides are produced by elements containing halogens such as chlorine, iodine, bromine, fluorine and astatine Most common tracer gas used in this method are those containing chlorine such R-12 and R-22
R- 12 R-22
Dichlorodifluoromethane CC12F2 Monochlorodifluoromethane CHClF2
Since R-12 liquifies at 70 psig and R-22 liquifies at 122 psig at 70°F, systems tested at room temperature can not have 100% tracer gas pressure greater than these pressures
Pressurized air is added to tracer gas when testing at higher pressures or to minimize the quantity of tracer gas used due to cost Dilution with air without increasing the pressure will reduce the testing sensitivity
Test instrumentation will also respond to solid particles of iodides, chlorides, bromides and fluorides These may be found in cigarette smoke, solder fluxes, cleaning compounds and aerosol propellants Items such as rubber and plastic tubing should be avoided since halogen gases are absorbed by these and could interfere with test readings
Test sensitivity can be effected by background contamination caused by a large leak masking a signal from a small leak nearby
Five classifications of halogen diode leak testing 1) Direct halogen leak testing with no significant halogen contamination in the atmosphere with a standard halogen detector halogen pressurized component is sniffed locally with probe 2) Direct halogen leak testing with significant halogen contamination in the atmosphere with a proportional detector halogen pressurized component is sniffed locally with probe 3) Shroud Test air is passed over a halogen pressurized component which is contained in a close fitting container or shroud and the discharged air is sampled by the halogen detector useful for components with maximum cross sectional diameters of 2" 4) Air curtain shroud a coniponent previously subjected to bombing (pressurized with a
halogen gas) is placed in an open top shroud and the lower end of the shroud is sampled by the halogen leak detector useful for high production testing of small items such as transistors 5) Accumulation test a halogen gas blanket is between an outer shroud and the component exterior surface and the internal atmosphere of the component is sarnpled by the halogen detector useful for components up to several cubic meters in volume
In pressure change measurement testing, leakage rates are determined by quantitative measurement of pressure changes or flow rates of air or pressurizing gases, without requiring use of tracer gases.
Typical gases - atmospheric air and i~itrogen
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FIG. 2 FIG. 1
Vacuum
Box
Vacuum Chamber Technique
polyethylene or structural plastic, the test fluid must not promote environmental stress cracking (E.S.C). 7.7 Ifthe test fluid is to be used on oxygen systems it m u a meet the requirements of MIL-L-25567D.
8. Immersion Technique 8.1 Applicafion-This technique is applicable to test specimens whose physical size allows immersion in a container of fluid when the test specimen can be sealed prior to the test. 8.2 Techniquesjbr Creasing Pressure D~rerenfialr 8.2.1 Pressurizcllion of Terr SpecimenSeal components and apply an elevated pressure, or if accessible, increase the internal pressure for test purposes. 8.2.2 Elevased-Temperasure Tesf Fluid-Heat the test fluid to a temperature not exceeding the maximum rated temperature of the test specimen. This will cause expansion of the gas inside the test specimen, creating a pressure differential. This technique is usually limited to use on very small parts. 8.2.3 Vacuum Technique-Immerse the test specimen in the test fluid and then place the test fluid container in the vacuum chamber. Reduce the pressure in the chamber to a point that does not allow the test fluid to boil, thus creating a presure differential. This technique is normally used on very small parts. 8.3 Test Fluids Used in Immersiorl Technique-The following test fluids mav be used. orovided thev are not d e m m e n ~ lo l the component being'tested: 8.3.1 Wafer-Should be treated with a wetting agent up to % by volume to reduce surface tension and promote bubble gowth. 8.3.2 Mefhyl Alcohol (Teclinical Grade). Undilufed-Not suitable for the heated-bath technique or the vacuum technique. 8.3.3 Ethylene Glj~col(Technical Grade), Undiluted. 8.3.4 Mineral Oil-Degreasing of the test specimens may be necessary. This is the most suitable fluid for the vacuum technique. 8.3.5 Fl~ioracarbo~is or Glj~ccrfn-Ruorocarbons are nor
recommended Tor stainless steel nuclear applications. . 8.4 Proced~rres: 8.4.1 Pressurized Tesr Specilnetz: 8.4.1.1 Specimens Sealed at Elevored Pressures-Place the test specimen or area being tested in the selected test fluid and observe for a minimum period of 2 min. Interpret as leakage a stream ofbubbles originating from a single point or two or more bubbles that grow and then release from a single ~inl. 8.4.1.2 Very S m l l Specimens Sealed as Ambienr or Reduced Pressures-Place the test specimen in a pressure chamber and expose to an elevated pressure. The actual pressure is dependent on the specimens. Place the specimen in the selected test fluid within 2 min after removal from the pressure chamber and observe for a minimum period of 2 min. Interpret as leakage a stream of bubbles originating from a single point. 8.4.2 Elevrued Temperasure Test Fluid-Place the test specimen in the test fluid which is stabilized and maintained at an elevated temperature at a temperature dependent on the specimen. Observe for a stream of bubbles originating from a single point or two or more bubbles that grow and then release from a single point Interpret either as indicating leakage. The time of observation shall be dependent o n the internal volume of the specimen and the case materials of the enclosure. Dwell time must be sufIicient to allow a pressure increase to a pressure dependent on the specimen. 8.4.3 ifaantum Teclmique-Place the test specimen in a container of the selected test fluid and place the container in a vacuum chamber with viewing porn. Reduce the pressure in the vacuum chamber and observe for a stream of bubbles originating from a single point or two or more bubbles that grow and then release from a single point. The amount of vacuum used will be dependent on the test fluid and should be the maximum obtainable without the test 'fluid boiling. This technique is also applicable to unsealed components or specimen sections by use ofthe apparatus s11own in Fig. I.
9. Liquid Application Tccl~nique 9.1 Applicarion-This technique is applicable to any test
specimen On which a pressurc din.ercnti3i can he c r a t e d across the area to be examined. An example oftl>istechnique is the application ofleak-test solutions to pressurized gas-line joints. It is most useful on piping synems, p r w u r e vessels. tanks, spheres, pumps, or other large - a .~.n a r a t u son which the immersion techniques are impractical. 9.2 Locotion of B~tbblcT m Fl~tid-Apply the test liquid to the low-pressure side ofthe area to be examined and then examine the area for bubbles in the fluid. Take care in applying ihe fluid to prevent formation ofbubbles. Flow the slution on the test area. Joints must be completely coated. The pressure dinerential should be created before the fluid is applied, to prevent clogging of small leaks. 9.3 Tj'pe o/Bubble Tesi Fluid-A solution of commercial leak-testing fluids may be used. The use of soap buds or household detergents and water is not considered a satisfactory leak-test fluid fur a bubble test, because of lack of sensitivity due to masking by foam. The fluid should be capable of being applied free- of bubbles so that a bubble appears only at a leak. The fluid selected should not bubble except i n response to leakage. 9.4 Vaoium Technique-Place a vacuum box (see Fig. 2)
over the bubble lest nuid. In testing equipment, such as storage tank floors and roofs, place the vacuum box over a section of the weld seam and evacuate to 3 psi (20.68 kPa) (or what the applicable nandard requires) and hold for a minimum time Of 15 s. 10. Precision and Bias 10.1 A c c t r r a q ~ T h emethods are not intended to measure leakage rates but lo locate leaks on a go, no-go basis. Their accuracy for locating leaks of lo-' atm.cm3/s (1. x Pa.m3/s) and larger is t5 %. Accuracy for locailng smaller leaks depends upon the skill of the operator. 10.2 Rcpcorobili1.1~-On a go, no-go basis, duplicate testr; by the same operator should not vary by more than 2 5 % for leaks of I x lo-' atm-cm3/s (I X loes Pa.rn3/s), 10.3 Rcprod~tcibiliry--On a go. no-go basis, duplicate tests by other trained operaton should no1 vary by more than 10 % for leaks of I x lo-'' atm.cm3/s (I X lo-') Pa.m'/s 2nd larppr 11. Keywords
11.1 bubble leak resting; film solution leak test; immersion leak test; leak testing; vacuum box leak testing
T w Amer~canSmrly tor Tesl,ng an0 IAalerfiJlrIJhcr noposn.on nupenmg 11s *Jlldfly ot a n / p l e n l ngatr S e n e a .o cannecr.on wnh any .lcm mnl.oncd .n lhsr rieoaard Urcn 01 Inn rtandara ale erpesrty aavaw lnal dererm rwl on 01 the talo!ly 01 an, such me rrsX 01 mlr~ngemenlor sucn rqhlr. ore cnllretf I e s r own reSpn%b Icy plmt n(mlz.
and m M be reviewed evwy five years snd Thk slandard ir subjen to revision a1 any time by the respansible lechnical ilmt rsvirw: eilhvreapprovedor wllhdrawn. Ywrmments are invilw'eilherlor rwtsion 01lhk standardor far edd8b.d standards and should be addressed to ASTM Headquarlers. Ywr mmmenls wiN receive carefulamideration a1 a meeting ot the responrlblc t&nical cammhee. which yw may attend. 11 yw 1-1 lhal yaur ramments have nol received e fair hearing you lhwld make y w r vkw Wown to the ASTt.4 Cammatee M Slandards. 1916 Race St.. Philadelphia, PA 19103.
d STb
Designation: E 427
- 94
Standard Practice for Testing for Leaks Using the Halogen Leak Detector (Alkali-Ion Diode)' Thjr standard ir i\>urd undcr Grcd dniUnrtalrn I: 417, 8Iv znuinl*.i tn~nlnlurrl?I;tlltl~(ttl~ Ibr ~I~-.~~nrlt~tul ~~JIC.IIC, CIIC ,,I o: caw .n icrtsinn. ,hc :cny orbs rc.>sitin. A nunthrr in p.rcnll,ru-. ~ndlmbn! l a !.mr of la<$rr.ppia\a~. A Jupcncripl cp8lon 1.) indirdlrs 2 0 rdltondl chlngr llncc ihc 1a1I rvvlutln
I. Scope 1.1 This practice covers procedures for testing and locating the sources of gas leaking af the rate of I x lo-' Standard cm3/s ( I x lo-' Pa m3/s). The tcsf may he conducted on any device or component across which a pressure differential of halogen tracer gas may be created.
and on which the effluent side of the area to be leak tested is accessible for probing with the halogen leak detector. 1.2 Five methods are described: 1.2.1 A4ethod A-Direct probing with no significant halogen contamination in the atmosphere. 1.2.2 Atefhod B-Direct probing with significant halogen contamination in the atmosphere. 1.2.3 Method C-Shroud test. 1.2.4 Method D-Air-cunain shroud test. 1.2.5 Melhod E-Accumulation test. 1.3 The vaIus stated in inch-pound units are to be regarded as the standard. The metric equivalents of inchpound units may be approximate. 1.4 This standard does nor prtrpon lo address the safii!? concerns f i j a , associared lvifh hs use. If is the responsibility of rhe user of rhis standard tu estubiish appropriae sajely a n d health practices and defermine !he opp/icabi(ilj?of reerqIdoqr limilaionr prior to itse. 2. Referenced Documents 2.1 ASTM Sfandard: E 1316 Terminology for Nondestructi\*eExaminations' 2 2 Orher Documents: ASNT "Leak Testing Handbook" Volume One of :Nondestructive Testing Handbook"' SNT-TC-IA Recbmrnended Practice for Penonnel Quaiification and Certification in Nondest~ctiveT a i n g 3 ANSIIASNT CP-189 ASNT Standard Tor Qualification and Certification ofNondestructi\.e Testing Personnel' 3. Terminology 3.1 Definitions-For definitions of terms used in this standard, see Terminology E 1316, Section E. 'Thir pmcricc k undcr lhc jurisdiction 01 ASTM Commiltcc E-7 an N o n d a ~ c t i v ~ T a u ' n g a nislfrcdimt d rrspanribilil?ol'Submmmiller £07.08 on h k T a i n g Mcrhod. C u m n l edition appro& March IS. 1994. Published May 1994. Origirwlly PUblkhcd ;rs E427-71. hp,nviour edition E 4 2 7 -911. Annirol B w k oJ.4ST.li Slondurds. Val 03.03. Available from Arncrion Saciny far Nondcnmciiw Tcrtinr. 171 1 ArIin~x18e f'hZ3. P.0. Box 28518. Columbus. OH 43228451X.
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4. Summary or l'racticc 4.1 Section 1.6 of NASA's Loukugc T~~srinl: Hundhook' will be of value lo somc users in determining which leak test method lo use. Section I I of the .4SAfT T<,.~iili,r: Ifand/~o~ik
may also be of value. 4.2 Thex methods require halogen leak detection equipmen1 with a full-scale readout ofat least 3 x lo-? Std cm3/s (3 x 10-"' Pa m3/s) on the most sensitive range, a maximum I min drift of 0 and sensitivity drift o i f 1 5 percent of full scale on this range. and f 5 percent or less on others (see 8.1.5). 4.3 Method A (Fig I ) is the simplest test. requiring only that a halogen tracer-gg pressure be created across the area to be tested. and the xarching of the atmospheric side of the area with ihe detector probe. This method detects leakage and locales its source or sources, when used in a tesi area free from significant halogen contamination in the atmosphere (see 7.1). Experience has shown that leakdetedon down to 1 x Sld cm3/s (1 X. lo-' Pa m3/s) in factory environments will usually be satisfaclory if reasonable precautions are taken against releasing halogens in the building If a ten booth is constructed so as to be purged with clean outdoor air, this level may be reduced to 1 x lo-' Std cm3/s (1 x Pa m3/s). Testing down to I x Std crn3/s (I x IO-"' Pa m3/s) will require additional halogen removal. This can be accomplished by paning the ten-booth purge air through a bed of activated charcoal. 4.4 Method B (Fig 2) is essentially the same as Method A, except that the amount of air drawn by the probe from the test area is reduced, and the required sample flow is made up with pure ( b l is, zero-halogen) air. This reduced sample intake has the disadvanmge of reducing the vacuum-cleaner efiect of the larger flow and thus requires closer and more careful probing However, the tolerance to atmospheric halogen can be increased up to 100 times. Also, large leaks beyond the range ofMethod A can be accurately located (but not measured) by Method B. 4.5 Method C (Fig. 3A and B) is suited for leak testing items which have an approximate cross-section dimension of 2 in. (50 mm), but may be as long as 30 fi (10 m). In this method, air, either atmospheric or purified, is passed over the halogen-pressurized which is inside a close-fining container. The discharge air from the container is sampled by the halogen detector, and any additional halogen content indicated. The shroud principle may be applied in a manner
:,
as sirnplc 3s Fig. 3l3, wllcrci,l l l i ~ ~ .orc 13pC I \ 3ppliCti around a flanged joint to bc tested. or as complete as in Fig 3A. T h e latter provides isolation of the detector rrom atmospheric halogens, a pure-air reference supply, and a convenient calibration means. This enables detection of
Hologcn Leok 0r:cctor
t4clnod C
Hologcn
FIG. 1 Halogen Leak Detedor. Method A Propo
Hologcn Leak Oeleclor
-.
--b
2 .
RG. 2 Pmpwtioning Pmbe, Halogen Leak Detector, Method 6 Air(S0-100 psiql
Ic;~hs:IS small 3s I x 10-' Std crn3/s ( I x lo-' Pa mr/s). 4.6 MeLhod D (Fig. 4) is userul for high-produclion testing orsmall items such as transistor; which have been previously subjected to a halogen gas prcsure above atmospheric (bombed), or testing the sealed-offend o f a till tube, and the like. In this method, the end or the shroud is always open, and the detector always draws a sample from the lower end. Atmospheric halogens arc prevented from entering by a laminar-flow pure-air curtain. When any leaking object is insened below the flow division level, the leakage is t!lcn picked up by rhe detector. This method is useful for deteciing leaks down to 1 X 1 OT7 Std cmr/s (1 X 1 0-a Pa m3/s) in size. 4.7 Method E (Fig. 5) is similar to Method C (Fig. 3A), except it provides for testing p a r 6 up to several cubic meters in volume. This is accomplished by allowing the leakage to accumulate in the chamber for a fixed period, while keeping it well mixed with a fan, and then testing the internal atmosphere for an increase in halogen content. The practical sensitivity attainable with this method depends primarily on two things. First, o n the volume between iheshroud and the object; and second, o n the amount of halogen outgassing produced by the object. Thus, a part containing rubber, plastin, blind cavities o r threads cannot be tested with the sensitivity obtainable with a smooth metallic pan. The sensitivity of the test and net volume of the system are related as follows: A, = LFIV where: A, = the rate of halogen increase in the volume, Std cm3/s2, L = the leak rate into the volume, Std.cm3/s. F = the flow rate in the detector probe, Sld m 3 / s , and V = the net volume o f the system, em3 For practical operating considerations, the minimum value of A, that should be used is about 2 x lo-" Std crn3/js2 (2 x 10-12 Pa m3/s). (This will give a detector readout of 100 x
Shroud Leak Test
Method C
'90-
Close-filling Cover
0
Pressurizing Cann (If Requiredl
tMinirnum FIG. 3 A
Shroud Leak Test. Method C
Plug Valve
I Clearance
Sample Shioud L e o k ierf ,,,elhad
O p e n l o g In Tape
L
appropriate ior NDI' I 11 qualilication according to Rccommcndcd I ' r a ~ ~ i c c No. SNT-TC-IA or the American Society for Nondestructive Testing or ANSIIASNT Standard CP-189.
c
Tape O v e r G a p B e l w e e o TWO ~ l a n g e s
a
6. Significance and Use 6.1 Halogen leak testing can be used to indicate the
presence, loca~ionand magnitude of leaks in a closed vesscl. This test method is normally used Tor production examination. Its use with halogenated refrigerant gases has been declining because orconcerns about the efict of these gases on the ozone layer.
Pipe Flong~
FIG. 38
Simple Shroud Leak Test, Methad C
3.5 A l m . r r l r t r o..,rr scrceo
FIG. 4
Air-Curlain-Shroud Leak Tesf Method 0
Accumutotian
Leak Test
DEVICE
kyessurizing
Method
E
1-
'
Connection
FIG. 5 Accumulation Leak Tesf Method E
Std cm3/s ( 1 x 10-lo Pa m3/s) after a 50-s or I x accumulation period.) Thus, (based on F = Std cm3/s) a 5 X Std cmvs leak ( 5 x lo-" Pa m3/s) may be detected in a system of 10' cm3 net volume, or a 5 x 10-5-Std cml/s or Pa m3/s) leak in a 10'-cml system. Where variables, time, volume, and leak rate permit, values of readout should be set in the lo-' or 10-8-Std cm3/s range for less critical operation. Methods C, D, and E are well adapted for automation of valving and material handling.
5. Personnel Qualification 5.1 It is recommended that personnel performing leak testing attend a dedicated training coune on the subject and pass a written e\amination. The training course should be
7. Interferences 7.1 .4r1i1o.~plreric Hulu,q~~tis--When direct probing (Methods A and B) is used to locate leaks, the leak detector probe is drawing in air from the atmosphere. If the atmosphere is contaminated with halogen to a degree that produces a noticeable indication on the detector, the detection of halogen from leaks becomes much more difficult. Significant atmospheric conramination with halogen' is defined as the level where the detector response, when the probe is moved from zero-halogen air to test-area atmosphere, exceeds that expected from the s m a l l a leak to be detected. For reliable testing, atmospheric halogen must be kept well below this level. 7.2 Halogens Ourgassed jrom Absorben1 MaterialsWhen leak testing is done in enclosures which prevent atmospheric conramination from interfering with the test (Methods A, B, and C), halogen absorbxi in various nonmetallic materials (such as rubber or plastics) may be released in the enclosure. If the amount released starts to approach the amount from the leak in the same period of time, then a reliable leak test becomes more difficult. The amount of such materials in the enclosure, o r their exposure to halogen must then be reduced to obtain a meaningful test 7.3 Pressurizing wiih Tesr Gas-In order to evaluate leakage accurately, the test gas in all paris of the device must contain subnantially the same amount of tracer gas. When the device contains air prior to the introduction oftest gas, or when an inen gas and a tracer gas are added sepmely, this may not be true. Devices in which the effective diameter and length are not greatly different (such as tanks) may be tested satisfactorily by simply adding tracer gas. However, when long or restricted systems are to be tested, more uniform tracer distribution will be obtained by lint evacuating to a few tom, and then filling with the test gas. The latter must be premixed if not 100 percent tracer. 8. Apparatus 8.1 Hologerl Leak Defector-To perform leak tests as specified in this standard, the leak detector should meet the following minimum requirements 8.1.1 Scnsor-Alkali-ion diode or electron capture. 8.1.2 Rcadoltr-Panel instrument or digital readout. 8.1.3 Ra~lge(Linear)-l x 104 to I x Std cml/s ( 1 x lo-' to I X lO-'"a m3/s) full scale. 8.1.4 Response Tinie-3 s or less. 8.1.5 S~obilifyof Zero and Sensirivirp-A maximum variation of 21 5 percent of full scale on most sensitive nnge while probe is in pure air; a maximum variation of 2 5
;,
pcrcent O r rull sc3le on otl,cr ranges, I;,~period ol. I riiiri 8.1.6 Co~rtrolr. . 8.1.6.1 Range-Preferably i n s o l e steps or about 3 times or 10 times. 8.1.6.2 Zero-Automatic zeroing option is desirable. 8.2 Halogen Leak Standard-To perform leak tests as specified in this standard, the leak standard should meet the following minimum requirements: 8.2.1 Ranges-I0 x lo-'' to 10 x 1 0 ' ' ~Std cm3/s (lo-' to lo-" Pa m3/s) full scale. 8.2.2 Adjusrabiliry-Adjustable lwk standards are a convenience, but are not mandatory.. 8.2.3 Accurac)i-225 percent of full-scale value or better. 8.2.4 Temperarure Coeflcienr-Shall be srated by manufacturer. 8.3 Orher Appamlus-Fixtures or other equipment specific to one test method are listed under that method. ~
9. Material 9.1 Tesr G a r 9.1.1 Tesr-Gas Requtreme~irs-To be satisfactory, the test gas should be nontoxic, nonflammable, not detrimental to common materials, inexpensive, and have a response factor of one. R-12 (dichlorodifluoromethane, CCI,F,) and R-22 (monochlordfluoromethane, CHClF,) have these characteristics. R-12 is commonly used unless the higher pressure of the more expensive R-22 is needed (130 psig versus 70 psig at 70 F). If the test specification allows leakage of f X loe5 Std cm3/s (I x lo4 Pa m3/s) o r more, or iflarge vessels are to be tested,oonsideratidn shouId be given to diluting the tracer gas with nonhalogen gas such as dry air or nitrogen. This will avoid operating in the nonlinear portion of the sensor output, or in the case of large venels, save tracer-gas expense. However, the balogen content of the qxdication leak should remain compatible with the expeded level of atmospheric halogen and the test method as outlined in Seaion 4. NOTE I-Whm a vcacl is not evacuated prior to adding t a t gsr, the Lana is automatially d~lutedby I atm orair. 9.1.2 Producing Premixed Tesr Gas-If the volume of the device or the quantity to be tested is small, premixed gases can be conveniently obtained in cylinders. The user can also mix gases by batch in the same way. Continuous mixing using calibrated orifices is another simple and convenient method when the test pressure does not exceed 50 percent of the tracer gas pressure available (Note 2). Another method is to pars the nonhalogen gas through the liquid tracer. This produces test gas containing the maximum amount of tracer gas. NOTE2: Gution-Thc liquid tnccr gar supply should not be h a t e d abve ambient tcmpenture. 9.2 Pure Air, Air J?om Which Halogens Have Been Removed la a Level oJless Than 1 ppb (or Ofher Suirable A'onhalogen Gas. Such as Nirrogen). 9.2.1 Requiremenrs: 9.2.1.1 Less than I ppb of halogen. 9.2.1.2 Less than 10 ppm of gases reactive with oxygen, .uch as petroleum-base solvent vapors. 9.2.1.3 Dew point IB'F (IO'C) or more below ambient temperature, and
9.2. I .4 SII;III ilc rcasonabl? frcc rrom rust. din, oil, etc. 9.2.2 I'rod~rc~ioitoj 1'11l.r' .-lil: 0). Otln,). Gus-Air or gas ol. suitable purity, may be produced by first passing it through a
conventional filterdrier activated charcoal.
(ir
necessary.) and then through
10. Olibrntion
10.1 The leak detectors used in making leak tests by these methods are not calibrated in the sense that they are taken to thestandards laboratory, calibrated, and then returned to the job. Rather, the leak detector is used as a comparaior between a leak standard (set to the specified leak size) which is pan of the instrumentation, and the unknown leak. I-iowever, the sensitivity of the leak detector is checked and adjusted on the job so that a l a k of specified size will give a readily observable, but not off-scale reading. More specific details are given in Section I I under the test method being used. To verify detection. reference to the leak Standard should be made before and after a prolonged test. When rapid repetitive testing of many items is required, refer to the leak standard alien enough to assure that desired test sensitivity is maintained. 11. Procedure
I I. I General Cansiderarions: I 1.1.1 Tesr Speci/icarions-Use a testing specification that includes the following: 11.1.1.1 The gas pressure on the high side ofthe device to be tested; also on the low side if it need differ from atmospheric. 11.1.1.2 The test gas composition, if there is need to specify i t 11.1.1.3 The maximum allowable ieak rate in standard cubic centimeters per second. 11.1.1.4 Whether the leak rate is for each leak or for total leakage of the device, and 11.1.1.5 if an 'each leak" spedfication, whether or not areas other than seams, joints, and fittings need to be tested. 11.1.2 Safe1.v Factor-Where feasible, ascertain that a reasonable safety factor has been allowed between the actual operational requirements of the device, and the maximum specified for testing Experience indicates that a factor of at least 10 should be used when possible. For example, if a maximum total leak rate for satisfactory operation of a Pa m3/s), the test device is 5 x 10" Std cm3/s (5 x requirement should be 5 x Std cm3/s (I x Pa mvs) or less. 11.1.3 Tesl Pressure-Test the device at or above its operating pressure and with the pressure drop in the normal direction, where practical. Take precautions so that the device will not fail during pressurization, or that the operator is protected from the consequences of a failure. 11.1.4 Disposition or Recovery of Tesr Gas-Do not dump test gas into the test area if further testing is planned. Either vent it outdoors or recover for reuse if the volume to be used makes this wonhwhile. 1 1.1.5 Derrimenral Effecrs o j R-12 and R-22 Tracer Gases-These gases are quite inert, and seldom cause any problem with most materials, patticularly when used in gaseous form for leak testing and then removed. Test gas should not be left in the device unless it h d q and sealed, as
most halogens in the presence 01.moisture acccleratc corrosion over a period of time. When tiicrc is a question as to the compatibility of the tracer with a particular malerial, an authority on the latter should be consulted. This is panicularly true when the material may be subject to chloride stress corrosion under conditions of use. I 1.1.6 Correlario~io j TCSI-GosL<,akagc ~eirhOrlrcr Gascs or Liqlrids a1 Di//i'~-cvir O[~e,zlri~~g I ~ r r s s ~ ~ r r s - 4 i v the en normal variation in leak geometry. accurate correlation is an impossibility. However, if a safety factor of ten or more is allowed (see 1 1.1.2) adequate correlation for gas leakage within these limits can usuall!. be obtained by assuming V ~ K O U Sflow and using the following relation: Q2 = Q,(N/KI)[P?2- /',')/(PI1 - P3')1 where: Q, = test leakage. Q, = operational leakage. = viscosity of test gas (Note 4). I\'Z = viscosity of operational gas (Note 4). N, p,, P I = absolute pressures on high and low sides at test, and P,, P3 = absolute pressures on high and low sides in operation. Experience has shown that, at the same pressures, gar leaks Std cm3/s ( 1 x will not show visible smaller than I x leakage of a liquid, such as water, that evaporates fairly rapidly. For slowly evaporating liquids such as lubricating oil, the gas leak should be another order of magnitude smaller, 1 x lod Std cm3/s.* NOTE3-Viwosity difiercnm between gasa is a rcladvely minor cITcci and u n bc igno~dif daircd. 11.2 Method A (See 2.3 and Fig. I): 11.2.1 Appararw 11.2.1 .I Test specification. 11.2.1.2 Halogen leak detector; standard probe type. 11.2.1.3 Halogen leak standard, upper 9/10 of scale to include halogen content of maximum leak in accordance with the specification, with response factor correction. 11.2.1.4 Test gas, at or above specification pressure. 11.2.1.5 Pressure gages, valves and piping for introducing test gas, and if required, vacuum pump for evacuating device. 11.2.1.6 Pure-air supply, if not part of halogen leak detector. 11.2.1.7 Test booth or other atmospheric contamination control, if shown to be necessary by 11.2.2. 11.2.2 Procedure: 11.2.2.1 Set the halogen leak standard at the maximum halogen content of the specification leak. Exanrple: if the maximum leak rate is 1 x lom4Std cm3/s (I x lo-' Pa m3/s) and the test gas is I percent R-12 in air. set the standard at I X lo4 x .O1 = I x lo4 Std crn3/s (I X lo-' Pa m3/s). 11.2.2.2 Stan the pure-air supply and adjust to flow in excess of that of the leakdetector probe, couple the probe loosely to the supply, so that air is not forced into the detector. 'Sanlclcr. U. I.. and blollcr. T . W.. "Fluid Flaw Convcrrian in k 3 L s and Gpillrria." I'acorrn Sy!ttpri,,n! Tronronionr. 1956. p. 29. Alu, Gcncnl E l c n k Ca. Kcpan R56GL261.
11.2.2.3 Svan tile detector. warm up and adjust in accorinstructions Tor detection of dance with the nru~~uCdcturci's leaks of size of I 1.2.2.1, using the "Manual Zero" mode. 11.2.2.4 Remove the probe from the pure-air supply to the test area, and note the reading, and also minimum and maximum readings for a period or I min. 11.2.2.5 Rezero the instrument. place the probe on the leak standard, and note the reading. Nore 4-ll neccrwv to obtain a rwmnahlc inslrumcnt denemion in 11.2.2.4 and lI.Z.Z.5. relurn the p r a k lo lhc pure-air supply. adjun i l ~ c -nnge' control and rczcro il ncccswry. 11.2.2.6 IT 11.2.2.4 is larger than 11.2.2.5. or if the I-min variation is more than 30 percent of 11.2.2.5. lake steps to reduce the atmospheric halogen content of the test area before proceeding with the leak test. 11.2.2.7 If the "automatic zero" mode is to be used. increase the sensitivity by a factor of three. 11.2.2.8 Evacuate (if required) and apply test gas to the device at the specified pressure. 11.2.2.9 Probe areas suspected of leaking. Hold the probe on or not more than 0.2 in. (5 mm) from the surface of the device, and move not faster than I .O i n . 1 ~(30 mm/s). If leaks are located which cause a "reject" indication when the probe is held 0.2 in. ( 5 mm) from the apparent leak source, repair all such leaks before making final acceptance test. If a marginal indication is observed while detecting in uautomatic zero" mode, reduce the sensitivity by a factor of rhree, switch to the 'manual zero" mode and compare the leak reading on the leak standard and on the device. 11.2.2.10 Maintain an orderly procedure in probing the required areas, preferably identifying them as tested, and plainly indicating points of leakage. 11.2.2.1 1 At the completion of the tesf evacuate or purge, or both, the test gas from the device. 11.2.2.12 Write the test report, or athenvise indicate test results as required. 1 1.3 Merhod B (See 4.4 and Fig. 2): 11.3.1 Apparafw-Same as for Mehod A (see 11.2) except 11.2.1.2, halogen leak detector to be proportioning probe type. 11.3.2 Procedure-Same as for Method A except as follows: 11.3.2.1 Use a self-contained pure-air supply. Activate by closing the probe tip valve tightly, which sends 100 percent pure air to the sensor. 11.3.2.2 in 11.2.2.4. open the probe value wide (about two turns), which sends 100 percent atmospheric sample to the sensor. 11.3.2.3 If the conditions of 11.2.2.6 are met, proceed with the test. If not. partially close the probe valve until they are. However. do not reduce the valve ouenine below the ~ o i n at t which the resuonse to the leak standard is reduced 30 percent. 11.4 hferliod C (See 4.5 and Fig. 3): 1 1.4.1 Apparalw: 11.4.1.1 Test specification. 11.4.1.2 Purge the sample detect and calibrate unit (PSDC), Fig. 3A, plus the shroud to fit the device under test (the upper 9/10 of halogen leak standard scale shall include halogen content of maximum leak in accordance with the speciliotion, with response factor correction).
1 1.4.1.3 Test gas, at or above specification pressure if the device is not already pressurized. I 1.4.2 Procedure: 11.4.2.1 Set the halogen leak standard at the maximum halogen content of the specification leak (see 11.2.2.1). 11.4.2.2 Adjust the air pressure. air flows (except purge valve V1) and valves V4 and V7 as indicated in the diagram for this method. (The addition of flowmeters and pressure gages at appropriate places in the circuit to facilitate these adjustments is recommended.) 11.4.2.3 Start the detector, warm up and adjust in accordance with the manufacturer's instruction for detection of leaks of size 1 1.4.1.1, using the "manual zero" mode. 11.4.2.4 Place a device not containing halogen (dummy) in the shroud and open valve V2 for as long as is required to purge the shroud of a~mospherichalogens. 11.4.2.5 Turn valve V7 to "calibrate" and valve V4 to the "sample" position, note detector indication, adjust the sensitivity if required, and return the valves to the original ("standby") positions. Remove the dummy device of 11.4.2.4. 11.4.2.6 I m r t the device to be tested inside the shroud and connect the evacuate or pressurize line, or both. if device is not already pressurized with tracer gas. 11.4.2.7 Open valve V2 for as long as is required to purge the shroud of atmospheric halogens. 11.4.2.8 .Turn valve V4 to the "sample" position. 11.4.2.9 If the device is already pressurized, read the leakage, if any, on the detector. 11.4.2.10 If the device is not pressuriml, check the leak detector for indication of incomplete purging, then pressurize and read the leakage, if any. An indication of the leak detector greater than that obtained during calibration 11.4.2.4 shows leakage greater than allowed by the tion. 1 1.4.2.1 1 If the device has been pressurized with halogen tracer for the leak test only, exhaust the test gas outside the test area, or r m v e r for reuse. 11.4.2.12 Remove the device from the shroud and write the test report, or othenvise indicate the results of test as required. 1 1.5 Method D (See 4.6 and Fig. 4): 1 1.5.1 A p p a r a w 1 1.5.1.1 Test specification. 11.5.1.2 PSDC unit (fig. 3A) plus shroud as in Fig. 4 to fit device (the upper 9/10 of the halogen leak standard scale shall include halogen content of maximum leak in accordance with the specification, with response factor correction). 1 1.5.2 Procedure: 11.5.2.1 Set the halogen leak standard at the maximum halogen content of the specification leak (see 11.2.2.1). 11.5.2.2 Adiust the air Dressure and flows as indicated in the diagram fo; this metho>. Valve V2 is open, and valve V4 is set at the "sample" position continuously. 11.5.2.3 Stan the detector, warm up, and adjust in accordance with the manufacturer's instruction for detection of leaks of size 11.5.1.1, using the "manual zero" mode. 11.5.2.4 Place a device not containing halogen (dummy) in the shroud. Turn valve V7 to the "calibrate" position, note detector indlcatlon, adjust the sensitivity if required and ,
return the valve to the original (standby) position. Remo\sc. the dummy device. 11.5.2.5 Insert the device to be leak-tested (and which has previously been "bombed" or which is pressurized with halogen tracer) in the shroud. NOTE5-Any pan or the device rhnr is below lh~.purge air omning.
lo
be luk-lesled must be
11.5.2.6 Read the leakage, if any. An indication on the leak detector greater than that obwined during calibration (see 11.5.2.4) shows leakage greater than that allowed by'the specification. 11.5.2.7 Remove the device and record the test results as desired. 11.5.2.8 If a large leak is detected, the clean-up of the shroud and sensor can be expedited by turning valve V7 to "standby" for a few seconds. This will purge shroud, lines and sensor with pure air. 11.6 Method E (See 4.7 and Fig. 5): 1 1.6.1 Apparatus: 1 1.6.1.1 Test specification. 11.6.1.2 PSDC unit (Fig. 3A) plus shroud as in Fig. 5 (the upper 9/10 of halogen leak standard scale shall include halogen content of maximum leak per specification, wirh response factor correction). 11.6.1.3 Test gas, at or above specification pressure, if the device is not already pressurized. 11.6.2 Procedure: 11.62.1 Set the halogen leak standard at maximum halogen content of the specification leak (see 11.2.2.1). 11.6.2.2 Adjust the air pressure, air flows (except purge valve V 3 as indicated on the diagram for this method. 11.6.2.3 Start the detector, warm up, and adjust in accordance with the manufacturer's instructions for detecting leaks of size of 1 1.6.1.1, using- the "manual zero" mode. 11.6.2.4 Place a device not containing hatogen (dummy) under the shroud. 11.6.2.5 Open valve V2 for as long as is required to purge the shroud of atmospheric halogen. 11.6.2.6 Turn valve V7 to the "calibrate" position, allow an appropriate ammulation period (with fan running), turn valve V4 to the "sample" position, and note detector indication. If necessary adjust the sensitivity and repeat 11.6.2.5 and 11.6.2.6. Remove the dummy device. 11.6.2.7 Insert the device to be tested inside the shroud and connect the evacuate or pressurize line, or both, if device is not already pressurized with tracer gas. 11.6.2.8 Open valve V2 for as long as is required to purge the shroud of atmospheric halogens. 11.6.2.9 Turn valve V4 to the "sample" position. 11.6.2.10 If the device is already pressurized, note whether the detector reading increases (in the allotted accumulation period) beyond that obtained during calibration (see 11.6.2.6). If so, reject the device. 11.6.2.1 1 If the device is not pressurized, check the leak detector for indication of incomplete purging, then pressurize and proceed as in 11.6.2.10. 11.6.2.12 Alternatively, sampling for leakage (V4) may be delayed until the end of the accumulation period. However, if this is done, time is lost and the sensor will be subjected to
the lest repon (Fig. 6). or otherwise indicate the results or the test as required.
a more concentrated halogen sample, ifthc device has a large leak. 11.6.2.13 If the device has been pressurized with halogen tncer for leak tesl only, exhaust rhe test gas outside the r a t area, or recover for reuse. 11.6.2.14 Remove the device from the shroud and write
12. Keyords
12.1 rreon leak testing; halogen leak testing; heated anode halogen detection; leak testing
HALOGENLfiAKTESTREPORT Tester Tesl witnessed by TeslW per ASTM Sld. Oevicelested No. accepled Mar. kakuge, acn?(lledpa. x 10 Sld, cm31s TOM -a eachwlleakage Device evacualed belae chaiging II evawaled, pessuie T Test pessure psg Tesl gas: I _ Tram: - g a s Atmospheric h&gm equivaienl x 10 Leak Denectw SeMl No. Leak Standard S e a No.
-
-
FIG. 6
..
-
Melhcd pieces NO. rejected -
Dale al Test
NO.
-
-
-
Sample Test Report Form
J n e h r . w n Soc,c!y i m Tallog and Malcrrsls lanes ncposllmrespMmng Ih€v a l d i f 01 any wren1 r ~ g n :arrcncdm cmnmlao any ncm mcnlsonw in rncs s t m a d U s m a1 l h n slamiad z e w a d y Z O d r s M lnal aclcrm n.?l,oo 01 ine ,~l,dby 01 on, swn palcnl r,ghlr and the r d r ~ol mlr ngcmcnl d s w h ngnlr am eNlrely tiwr a n rcspwibla, ant,
This slmdard is subjecl lo revision e( any lime by lhe mpmsible lachnicalmmhee wd mosl be reviewed every live y e u s and Nnol rwised, &her reappoved or wahdrawn Your m m m m s ere imiled &her lor revision 01l h k srandard or lor sddifiwldmdards &shooM be addrerred lo ASTM Headquarrers. Your mmmems will receive carefulm i d e r a l i o n a1 a meeling o( Lhe rerpwr;lble IRhnM emmillee, which you may Mend. H you feel the1 yaur m m M s have md received a lab hearing you should make your view known lo
451b
Designation: E 499
- 94
Standard Test Methods for
Leaks Using the Mass Spectrometer Leak Detector in the Detector Probe Mode',2 Thtl rlandard is irrucd undcr tllc fixed dcsignaion E199: lhc nunihcr imrnedhicly iollorlnp ihc di-rienat~anindsr3lcr the ?car a1 aripnrl adopsion or. in ihc caw orrcuirion. llic ?.car o i l a n revision A numbcr in w r c n l h c a l indlcaicl lhc ?car arlarl rc~pprourl.A lurnwnpt imilao (.) indicxca an cdisonal r h a n y sinrc lhc Ian rcvirion or rapliro\.al
I . Scope 1.1 These test methods cover procedures for testing and louting the sources of gas leaking at the rate of I x standard cm3/s) or greater. The test may P a m3/s (1 % be conducted on any device or component across which a pressure differential of helium or other suitable tracer gas may be created, and on which the emuent side ofthe leak to be tested is accwible for probing with the mass spectrometer sampling probe. 1.2 Two test methods are described: 1.2.1 Tesr Merhod A-Direct probing, and 1.22 Tesr Merhod B-Accumulation. 1.3 This standard does nor purport ro address ihe sa/cry concerns, $any, associared,n~irhirs tise. I r is [he responsibility o/ rhe user ofrhis standard ro esrablish uppropriare saJery and health pracrices and derermine rhe applicabiliry of reguiafory limirarionsprior lo use.
2. Referenced Documents 2.1 ASTM Srandard: E 1316 Terminology for Nondestructive Examinations3 2.2 Other Documens: SNT-TC-1A Recommended Practice for Personnel Qualification and Certification in Nondestructive Testing4 ANSIjASNT CP-189 ASNT Standard for Qualification and Certification of Nondestructive Testing Personnel4 3. Terminology 3.1 Dejinilions-For definitions of terms used in this standard, see Terminology E 1316, Section E.
4. Suuunaty of Test Methods 4.1 W o n 1.8 of the Leakage Testing will be ofvalue to some uses in determining which leak test method to use. 4.2 These test methods require a leak detector with a Pa. m3/s ( i x 1 OM' full-scale readout of at leas I x
andb book'
'nae
id mclhodr arc u n d a the j u r l d i d a n of ASTM Cammilicc E-7 on Nanduwaitivc: Tcriinc and are ihc d i r m raoonsibililv of Subcommiticc E07.08 on Lnk T d n g . C u m n l cdilian aoorortd March 15. 1994. Pubiirhd May . 1994. Orieinallv . P U M Wa E 499 -71. ~ uprrviour t &lion ~ 4 9 -91. 9 '(Almmpheric prmurc cxenwl. p-urc abavc =~rnarphcricinlemall. This dnument (he D c t m o i Pmbc Modedcraibcd i n Guidc E 412. ' A n n u l sod. 01.4n Sondordr. .v VOI 03.01. ' A ~ h b k i r o mAmninnSadcly l a r N o n d a r u a i v c T m ~ i n g .1711 Arlingrlc P k 4 P.O. Box 28518. Calumbu. OH 432286518. 'Ulrr. 1. Willism. -Leakage T a i n g H a n d b m k " prrpand ror Liquid Prapul1 Scctian. la Prooulrion Labaniom. Natioml Acronaudcs and S D ~ CAdminC -Inllon. Pmdcna. CA. Conlnn NAS 7.196. Junc 1961
standard cm3/s) on the mosi sensitive range, a maximum I-min drin of zero and sensitivity of f S % of full scale on this range, and 2 2 % or less on others (see 7.1). The above sensitivities are those obtained by probing an actual standard leak in atmosphere with the detector, or sampling, probe, and nor the sensidvity of the detector to a standard leak attached directly to the vacuum system. 4.3 Test hlerhod A. Direcr Probing (see Fig. I), is the simplest test, and may be used in pans ofany size, requiring only that a tracer gas pressure be created across the area to be tested, and the searching of the atmospheric side of the area be with the detector probe. This test method detects leakage and ils source or sources. Experience has shown that leak tesing down to 1 x lo-' Pa.m3/s ( I x 10" standard cm3/s) in factory environmenls will usually be satisfactory if reasonable precautions against releasing gas like the tracer gas in the ten area are observed, and the effects of other interferences (Section 6) are considered. 4.4 Tesr Merhod 8. Accumulaion Testing (see Fig 2), provides for the tesiing of parts up to several cubic metres in volume as in fig. 2(a) or in portions of larger devices as in Fig 2(b). This is accomplished by allowing the leakage to aocumulate in the chamber for a f i e d period, while keeping it well mixed with a fan, and then testing the internal atmosphere for an increase in tracer gas content with the detector probe. The practical sensitivity attainable with this method depends primarily on two things &f on the volume between the chamber and the objed; and second, on the amount of outgassing of trdcer gas produced by the object Thus, a pan having considerable exposed subber, plastic, blind cavities or threads cannot be tested with the sensitivity of a smooth metallic part. The time in which a leak can be detected is directly proportional to the leak rate and inversely proporiional to the volume between the chamber and the pan. In theory, extremely small leaks can be detected by this test method; however, the time required and the effecrs of other interferences limit the practical sensitivity of this test method to about I x Pa.m3/s ( I x standard cm31s) for small pans.
5. Personnel Qualification 5.1 It is recommended that personnel performing leak testing a dedicated uaining course on the subject and pass a written examination. The training courje should be appropriate for NDT level I1 qualification according to Recommended Practice No. SNT-TC-1A of the American Society NondeStrucrive Or Standard CP-189
4m E 499 6. Significance and Use 6.1 Test Method A is frequently to test large systems and complex piping installations that can be tilled with a trace gas. Helium is nomally used. ~h~ test method is used to locate leaks but cannot be used to quantify except for approximation. Care must be taken to provide sufficient ventilation to prevent increasing the helium background at the test site. Resulu are limited by the helium background and the percentage of the leaking trace gas wptured by the probe. 6.2 Test Method B is used to increase the concentration of trace gas coming through the leak by capturing it within a n enclosure until the signal above the helium background can be detected. By introducing a calibrated leak into the same volume for a recorded time interval, leak rates can be measured.
7. Interferences 7.1 Almospheric Heliltm-The
atmosphere contains about
five pans per million (ppm) ofhelium, which is being contin. U O U S ~ Y drawn in by the detector probe. This background must be "zeroed out" before leak testing using helium an proceed. Successful leak testing is contingent on the ability of the detector to discriminate betwen normal atmospheric helium, which is very c o n m n S and a n increase in helium d u e '0 a leak. Ifthe normally stable atmospheric helium level is increased by release of helium in the test area, the referes and leak testing more diflicuit. ence level b e ~ ~ m unsmble, 7.2 Heliuni Ourgassed fiom Absorben1 Malerials--.~~. lium absorbed in various nonmetailic materials (such as mbber or plastics) may be relased during the test. If the rate and magnitude of the amount released approaches the amount released from the leak, the reliability of the test is decreased. T h e amount of such materials or their exposure to helium must then be reduced to obtain a meaningful test. 7.3 Pressurizing wilh Tesf Gas-in order to evaluate leakage accurately, the test gas in all pans of the device must contain substantially the same amount of tracer gas. When the device contains air prior to the introduction of test gas, or Elecrricrl Pawcr
Trap
Rovgh Pump
Pump
Leak. Note That Probe O o a Not Pick Up All of
FIG. 1
Method A ,n<
Rough Pump
the Lcakrge
I
Helium
I
9"
I
I
Probe
I-
Dctecror
Prerruri2ini) Connection
a ] Accumulation Leak Test, Complete Device in Chamber
bl Accumulation Leak Test. Flexible Shroud over a Small Portion of Device FIG. 2 Method B
when an inert gas and a tracer gas are added separately, this may not be true. D e v i w in which the effective diameter and length are not greatly different (such as tanks) may be tested satisfactorily by simply adding tracer gas. However, when long or restricted systems are to be tested, more uniform tracer distribution will be obtained by fim evacuating to less than 100 Pa (a few tom), and then filling with the test gas. The latter must be premixed if not 100 % tracer. 7.4 Dirt and Liquih-As the orifice in the detector probe is very small, the pa- being tested should be clean and dry to avoid plugging. Reference should be frequently made to a standard leak to ascertain that this has not happened. 8. Apparatus 8.1 Helium Leak Defecfor, equipped with atmospheric detector probe. To perform tests as specified in this standard, the detector should be adjusted for testing with helium and should have the following minimum features: 8.1.1 Sensor Mass Analyzer. 8.1.2 Readour, analog or digital. 8.1.3 Range (linear)-A signal equivalent to 1 x lo-' Pa.m3/s (1 x standard cm3/s) or larger must be detectable. 8.1.4 Response lime, 3 s or less.
8.1.5 Sfabilify of Zero and Sensifivily-A maximum variation of &5 % of full scale on the most sensitive range while the probe is active; a maximum variation of 2 2 % of full scale on other ranges for a period of 1 min. NOTE I-Variations may he a funmion of cnvironmenwl interfcrcnm rather than equipment limiulions. 8.1.6 Conrrols: 8.1.6.1 Range, preferable in scale steps of 3x and lox. 8.1.6.2 Zero, having sufficient range to null out atmospheric helium. Automatic null to zero is preferred. 8.2 Heliltnz Leak Sfandard-To oerfonn leak tests as specified in this standard, the leak standard should meet the following minimum requirements: 8.2.1 Ranges-1 x to Pa.m3 (lo-' to lo-% standard cm3/s) full scale calibrated for discharge to atmosphere. 8.2.2 Adjusfabilify-Adjustable leak standards are a convenience but are not mandatory. 8.2.3 Accuracy, &25 % of full-scale value or better. 8.2.4 Temperalure Coeficienf, shall be stated by manufacturer. 8.3 Helium Leak Sfandard, as in 8.2 but with ranges of iOmRor Pa.m"s (lo-' or lo-% standard cm31s). 8.4 01Aer Apparalus-Fixtures or other equipment spe-
increasing the prL.rsurc
u.!iil
: I ~ , , I Ilrss , c ~r.,l,cns,ve
&35. such 2s
air.
11.2.2.1 1 At cornplelton of thc test evacuate or purge tesl gas from the device. if required. 11.2.2.12 Write a test repon or otherwise indicate test results as required. NOTE 5-lr neceswry l o obwin a rwronablc inslmmcnl deflecrion. adjusl range, rczero if nccesww, and reapply s;lmpling probe lo leak slandard. 1 1.3 Tessr A4erhod B (refer to 4.4 and Fig. 2): 11.3.1 ilppara1li.r-Same as for Test Method A. except that equipment for enclosing all or part of the item to be tested is required as shown in Fig. 2. 1 I .3.2 Procedure: I 1.3.2.1 Ser-up-Same as I 1.2.2.1 through 1 1.2.2.7, Test Method A, except that somewhat larger variations in atmospheric helium can be tolerated due to the isolation of the pan during test. 11.3.2.2 Sensitivirj~Setring-In general, it will be advantageous to use the maximum stable sensitivity setting on the leak detector, in order to reduce the accumulation time to a minimum. 11.3.2.3 Insert the pan to be tested (unpressurized), the leak standard ( 1 1.2.1.3), and the detector probe in the Fig. 2 enclosure. 11.3.2.4 Note the rate of increau: of detector indication. 11.3.2.5 Remove the leak standard, pressurize the pan with test gas, and again note rate of rise. if any. If 11.3.2.5 exceeds 1 1.3.2.4, reject part. 11.3.2.6 Remove the part from the enclosure and purge out any accumulated helium. 11.3.2.7 Evacuate or purge test gas from the pan, if required. 11.3.2.8 Write a test report or otherwise indicate test results as required.
11.2 Tesr Mmhod A (refer to 4.3 and Fig. I): I 1 .2. 1 Apparart~s: I 1.2.1. I Tesr SpeciJkorioi~. 11.2.1.2 Nclitrm Leok Derecror. with atmospheric detector, sampling probe. 1 1.2.1.3 Heliu~vLeok Slandard, discharge to atmosphere. Size equal lo helium content of maximum leak rate per specification. 1 1.2.1.4 Heliuti~Leok Srandard. discharge to vacuum. Pa-m3/s ( I Size: anywhere between I x lo-' and I x x and 1 x lo-' standard cml/s), unless otherwise specified by maker of leak detector. 11.2.1.5 Tesf Gas, at or above specification pressure. 1 1.2.1.6 Pressrrre Gages. I/alves, and Piping, for introducing test gas, and if required, vacuum pump for evacuating device. 11.2.1.7 Liquid Nirrogoi. if required. 1 1.2.2 Procedure: 11.2.2.1 Set helium leak standard at maximum helium content of specification leakage. Example: Maximum leak rav: I X lo-' Pa.m'/s ( I x lo-' standard cm3/s). Tat gas I lo helium in air, w1 the standard a1 1 x 10-'XU.01 or I X 10-'Pa.m3/s(I x 104cm'h). 11.2.2.2 Start detector, warm up, fill trap with liquid nitrogen if required, and adjun in accordance with manufacturer's instructions, using leak standard 11.2.1.4 attached to vacuum system. 11.2.2.3 Attach atmospheric detector probe to detector sample port in place of leak standard and open valve of detector probe, if adjustable type is being used, to maximum leak rate under which detector will operate properiy. 11.2.2.4 Rezero detector to compensate for atmospheric 12. Precision and Bias helium. 12.1 Precision 11.2.2.5 With orifice of leak standard (1 12.1.3) in a statement on precision is 12.1.1 Tesr Melhod A-No horizontal position, hold the tip of the detector probe made. direaly in line with and 1.5 +. 0.5 mm (0.06 k 0.02 in.) away 12.1.2 T a r Method B-Replicate tests by the same operfrom the end of the orifice, and observe reading (Note 5). ator with the same equipment should not be considered 112.2.6 Remove probe from standard leak and note suspect if the results agree within j125 %. Replicate tests minimum and maximum readings due to atmospheric from a second facility should not be considered suspect if the helium variations or other instabilities. results agree within +SO %. 11.2.2.7 If 11.2.2.6 is larger than 30 % of 11.2.2.5, take 12.2 Biax steps to reduce the helium added to the atmosphere, or to 12.2. t . Test Merhod A-Due to the nature of the test no eliminate other causes of inskbility. If this cannot be done, statement of bias is possible. Calibration standards are used testing at this level of sensitivity may not be practical. only to ensure that the leak detector is funciioning properly. 11.2.2.8 Evacuate (if required) and apply test gas to device No leak measurement is intended. at specified pressure. 12.2.2 Test Method B-Bias of leak rates between lo-' 11.2.2.9 Probe Areas Stcrpecred of haking-Probe shall and Pa.ml/s (lo-'' to 10-I standard cm3/s) are typically be held on or not more than I mm (0.04 in.) from the surrace +25 %. of the device, and moved not faster than 20 mm/s (0.8 in.1~). 13. Keywords If leaks are located which cause a "reject" indication they 13.1 bell jar leak test; bomb mass spectrometer leak test; must be repaired before making final acceptance test. helium leak test; helium leak testing; leak testing; mass 11.2.2.10 Mainrain an orderly procedure in probing the spectrometer iwk testing: sealed object mass spectrometer q u i r e d areas, preferably identifying them as rested, and leak test plainly indicating points of leakage.
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Designation:
E 1 6 0 3 - 94
Standard Test Methods for Leakage Measurement Using the Mass Spectrometer Leak Detector or Residual Gas Analyzer in the Hood Mode'
1. Scope
1.1 These test methods cover procedures for testing the sources o l gas leaking at the rate of 4.4 x lO-I4 molesjs ( I x standardim3/s at O'C) or greater. These test methods may be conducted on any object that can be evacuated and to the other side of which helium or other tracer gas may be applied. The object must be structurally capable of being tor). evacuated to pressures of 0.1 Pa (approximately 1.2 Three test methods are described; 1.2.1 Test Merhod A-For the object under test capable of being evacuated, but having no inherent pumping capability. 1.2.2 Tesl Merhod B-For the object under test with integral pumping capability. 1.2.3 Tesr Merhod C-For the object under test as in Ten Method B, in which the vacuum pumps of the object under test replace those normally used in the leak detectar (LD). 1.3 The values mted in SI units are to be regarded as the standard. The values given in parentheses are for information only. 1.4 This standard doer nor purporr 10 addrers ail of rhe safely wncerns, $ any, arsociafed u~ifh IS use. [I! is rlre responsibility of the user of fhis srandard io er~ablishappropriare s&y and health pradices and determine the applicabiliry ofregulafary limi[afions prior to use
2. Referenced Documents 2.1 ASTM Srandard:
E 1316 Terminology for Nondestructive ExaminationZ 2.2 Ofher Documenls= SNT-TC-IA Recommended Practice for Personnel Qualification and Certification in Nondestructive Testing3 ANSIIASNT CP-189 ASNT Standard for Qualification and Certification of Nondestructive Testing Personnel3 3. Terminology 3.1 Dejinirions-For definitions of terms used in these test methods, see Terminology E 1316.
4. Summary of Test Methods 4.1 These test methods require a helium LD that can provide a syslem sensitivity of 10 % or less of the intended leakage rate to be measured. 'Thcw
Cumnt miition rpprovcd Mar& 15. 1994. Publishmi May
1994.
4.2 7'c~rA,i~ri~od A-This test method is used to helium leak test objects that are capable of being evacuated to a reasonable test pressure by the LD pumps during an acceptable length of lime (Fig. 1). miis requires that the object be clean and dry. Auxiliary vacuum pumps having greater capacity than those in the LD may be used in conjunction with them. The leak test sensitivity will be reduced under these conditions. 4.3 Tesr Merhod B-This test method is used to leak test equipmenl that can provide its own vacuum (that is, equipment that has a built-in pumping system) at least to a level of a few hundred pascals (a few torr) or lower. Refer to Fig. 2. 4.4 Tesr Merhod C-When a vacuum system is capable of producing internal pressures of less than 2 x 105 Pa (2 x 104 ton) in the presence of leaks,these leaks may be located and evaluated by the use of either a residual gas analyzer (RGA) or by using the specVometer tube and controls from a conventional MSLD, provided that the leakage is within the sensitivity range of the RGA or MSLD under the conditions existing in the vacuum system. Refer to Fig. 3.
5. Personnel QualiIiuGon 5.1 It is recommended that personnel performing leak testing attend a dedicated training m u m on the subject and pass a written examination. The training course should be appropriate for NDT Level I1 qualification in accordance with Recommended Practice SNT-TC-1A or ANSIIASNT Standard CP-189. 6. Significance nnd Use 6.1 Tesl Merhod A-This test method is the most frequently used in leak testing components. Testing of components is correlated to a standard leak, and the actual leak rate is measured. Acceptance is based on the maximum system allowable leakage. For most produetion needs, acceptance is based on acceptance of pans leaking less than an eslabliihed leakage rate, which will ensure safe performance over the
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FIG. 1
Test Meihad A
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FIG. 3 Test Method C
projected U e of the component. Care must be exercised to ensure that large systems are calibrated with the standard leak located at a representative place on the test volume. As the volume tends to be large (>I m3) and there are often low conductance paths involved, a check of the response time as well as system sensitivity should be made. 6.2 Tesc Method B-This test method is used for testing vacuum systems either as a step in the final test of a new system or as a maintenance practice on equipment used for manufacturing, environmental test, or conditioning paN. As with Test Method A. the reswnse time and a system sensitivity check may be requireb for large volumes. 6.3 Test Method C-This test method is to be used only when there is no convenient method of connecting the LD to the outlet of the high-vacuum pump. I f a helium LD is used and the high-vacuum pump is an ion pump or cryopump, leak testing is best accomplished during the roughing cycle, as these pumps leave a relatively high percentage of helium i n the high-vacuum chamber. This will limit the maximum sensitivity that can be obtained.
hours lo build u p llie partial pressure of helium in [llc volumc belu,ecn the two leaks so that enough helium enlers the vacuum syslem lo be detected by the LD. This type or leak occurs frequently under the following conditions: 7.1.1 Double-welded joints and lap welds, 7.1.2 Double O-rings. 7.1.3 Threaded joints. 7.1.4 Ferrule and flange-ppe tubing liltings. 7.1.5 Casling will1 internal voids. 7.1.6 flal polymer gaskers. and 7.1.7 Unvented O-ring grooves. 7.2 In general. the solution is proper design lo elimina~e there conditions; however, when double seals must be used, an access pon belween them should be provided for attachmen1 to the LD. Leaks may then be located from each side of the seal. The access port can be sealed or pumped continuously after repair by a holding pump (large vacuum system). 7.3 Temporarily plugged leaks often occur because of poor manufacturing techniques. Water, cleaning solvent, plating, flux. grase. paint, etc. are common problems. These problems can be eliminated to a large extent by proper preparation of the p a m before leak testing. Proper degreasing. vacuum baking, and testing before plating or painting are desirable. 7.4 The time constant for evacuation and for the rise of the helium signal is invenely proportional to the pumping speed and directly proportional to the volume being evacuated. Low-condumnce tubing, or any other flow impedance, can reduce the pumping speed of the system very significantly, thus extending the system response time constant. If such an impedance connects two volumes under test, a LD connection to each volume should be provided. 7.5 When unusually long pumping times are necesr;uy, aU of the connections not being tested should be protected from continuous exposure to the helium. This will reduce undesired high-helium background levels due to permeation of helium through the O-rings. This can be effected by donbleseals (with evacuation of the space between), or sometimes by more informal shielding approaches. 7T.57 MEIHOD A-HELIUM
LEAK TESl7NG OF COMWNENIS/SYSTEMS USING THE LD
8. Apparatus 8.1 Leak Derecror, having a minimum deteciable lcak rate as required by the test sensitivity. 8.2 A t ~ ~ i l i a Pumps, r!~ capable of evacuating the object to be tested lo a low enough pressure that the LD may be connected. 8.3 Suirable Connecror and Valves, to connect to the LD test port. Compression fitting and metal tubing should be used in oreference to a vacuum h o w 8.4 ~iandardLcaks of Borh Capsrile o p e (Containing 11s Own Ifeliutn Stipply) and Capillary Type, an actual leak that is used to simulate the reaction of the test system to a helium leak. The leak rate of the standard lcak used for the system calibration shall be equal to or lrss then one half of the acceptance level (maximum permissible leakage rate). Temperature correction of the permeation capsule-type swndard ~
7. interferences 7.1 Series leaks with an unpumped volume between them present a difficult if not impossible problem in helium leak testing. Although the &atrace gas enten the first leak readily enough since the pressure difference of helium across the first leak is approximately one atmosphere. it may take many
Calibration Setup w i t h a
capillary
CL
9.2 Adjust the LD readout to correspond to the temperd. lure-corrected standard leal, value in accordance with tile manufacturerz' instructions. NOTE I-Valve closurn mnv be accom~lishcdautomalimllv n,, ~, s a m c LDr ~ n some d C O U ~ I C ~ O U - I Y htS1.D~ W ( = q u i r t conlinucd UU. the rouy>xlng purnc, dunng terl~np, Refcr lo lhc m ~ n u l ~ c l u r c rown,,,,,! 'r
manual. 9.3 Disconnect tile capsule standard leak from the LD and connect the test system to the LD.
Calibration S e t u p
with a
Capsule CL
FIG. 4 Calibration Setups
leaks should be performed when the ambient temperature has a difference of 3°C (5°F) from the calibration temperature of the standard leak. The leakage rate error may become significant (>I2 %) without temperature correction. 8.5 Vacuum Gage, to read the pressure before-the LD is connected. 8.6 Heliwn Tank and Replalor, with attached helium probe hose and jet. 8.7 Test Component/Sysrem Enclosure (Hood)-Either a rigid structure or heavy plastic cover to contain and surround the test pan totally in helium tracer gas.
9. Instrument Cdibration 9.1 Attach the capsule leak to the LD and tune the LD to achieve the desired sensitivity scale in accordance with the manufacturer's instructions. Allow sufficient time for the flow rate from the capsule leak to equilibrate. The permeation-type capsule leak should be stored with the shutoff valve (if present) open, and the leak should be allowed to equilibrate to ambient temperature for several hours. Capillary-type capsule leaks should be stored with the shutoff valve closed to prevent unwanted decay of the reservoir pressure.
10. System Calibration and Test Procedure 10.1 For small-volume tests (a few litres and less) or when the standard leak cannot be attached directly to the test component, the instrument calibration shall be used for the system calibration. The correction factor (CF) used to multiply the instrument calibration value for the system leak rate is one. 10.2 For large-volume systems, attach one of the slandard leaks to the test system at a location that provides the lowest conductance path to the LD. NOTE 2-11 using a capsule l u k . open the calibtated I& (CL)and pump isolation wlva. and clow the ulibntion vslvc. Turn on the CL vacuum pump. Refer to Fig. 4. 10.3 Evacuate the device to be tested until near equilibrium pressure is reactikd on the rough vacuum gage. Open the valve to the LD and check the background helium concentration. When the helium background is equal to or less than one half the acceptance level (maximum permissible leakage rate), close the valve(s).to the roughing pumps. 10.4 System Calibraion or Procedure Quolifiiccion: 10.4.1 Record the helium background level. 10.4.2 Open the valve of the system standard leak (calibration valve) attached to the test component/system (Fig. 4). NOTE3-Iiusing a clpillary leak, apply helium of one atmosphere to the smndard l u k For the clpsule standard Ids, dmc the pump isolation valvc immcdiatdy prior to opening the calibration valvc. 10.4.3 Graph the LD response as a function of time until a steady-state condition is reached. Refer to Fig 5. 10.4.4 Close the standard leak valve, and reduce the helium background of the test componentlsystem to the same level as that obtained before system calibration. It may be necessary to open roughing pump valves and use the roughing pumps to expedite the reduction of the helium background. 10.4.5 Calculate the LD C F for adjusting the instrument calibration reading to a system calibration reading. For tests on large-volume systems, the amplitude response of a leak in the system is less than the amplitude response from the instrument calibration standard leak 10.4.5.1 This CF should be calculated at either the time gt which a steady-state response (SS) is reached or at the time at which the LD response is 63 % of the change. This shall be the minimum test period. The formula for the C F at this test time is as follows:
where: CL,. = temperature-corrected standard leak rate,
.-
Steady S t a t e value
---
MSLD
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t
Background level
'c
Sr T e s t Time
v
z
= system time constant
x = o
v
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r;
volume of test system Pumping speed of eystem
r = 6 3 s of amplitude change cauned by C L 5r = 99.9% of amplitude change (Steady State condition)
FIG. 5 System Time Constanl
LR = indicated LD reading (0.63 SS or SS) at the end of the test period (T or 57 respectively), and BR = background reading (initial reading). 10.5 Set the LD on the appropriate range. 10.6 Close the valves to the roughing pump(s) if they were opened to expedite the reduction of the helium background. 10.7 Fd the test wmponent/system enclosure with helium or place the test patt in the enclosure. Large, enclosures should be purged sumciently to remove the trapped air. For any concentration other than 100 % helium atmosphere, the system aoxptance level should be adjusted for the reduced sensitivity. 10.8 Keep the test wrnponent/system in the test enclosure for the test period established in accordance with 10.4.5 and record the LD reading at the end of the period. N m 4--The system time raponst may be longer than tile innrumen1 rapomtime.
10.9 Calculate the system leakage by multiplying the LD reading by the C F to obtain the corrected system leakage. For tests in which a system calibration was not performed (that is, test volumes less than a few litres), use a C F of one. 10.10 Write a test report. or otherwise indicate the test results as required. TEXT MEITiOD B-HELIUM LEAK TESTING OF VACUUM E O U l P M E M AND SYSTEMS THAT HAVE INIECRAL PUMPING SYSTEMS OF THEIR OWN
11. Apparatus 11.1 Helium LD-Same
tion. All connections should have as high a conductance as is practical. 12.2 Attach the standard leak to the vacuum chamber of the object to be tested and as far as practical from the inlet to the pumping system. Refer to Fig. 4. 12.3 Operate the equipment until equilibrium vacuum is reached in the vacuum chamber. 12.4 Slowly open the inlet valve to the LD. Do not allow the LD pressure to exceed the manufacturer's recommendations. 12.5 if the inlet valve can be opened fully without exceeding the safe LD operating pressure, close the equipment roughing pump valve slowly. If this valve can be closed completely. the maximum sensitivity of the test will be achieved. 13. instrument Glibration 13.1 See Section 9. 14. System Calibration and Test Procedure 14.1 See Section 10. TEST hlETl1OD C-USE 01:RCA OR OF HELIUM MSLD SPECTRORlETEl TUBE AND CONIROL IN LEAK l T S n S C (NO VACUUM SYSTEM IN THE LD)
15. Apparatus 15.1 RGA or lCfSLD and Cottlrols. tunable to the tnce gas.
apparatus as Section 8.
12. Preparation of Apparatus 12.1 Connect the inlet valve of the LD of the foreline of the object to be tested. If possible, insert a valve in the foreline between the mechanical pump and the LD connec-
15.2 Standard Leak, of approximately the size of the minimum leak to be located. 15.3 Slrirable Filling and Isolaling Valves, for attachment to the hi&-vacuum chamber. 15.4 Liquid ~ i t r o g e nCold Traps, to be used if the system conta~nscondensable vapors harmful to the RGA or the MSLD
16. Preparation of Apparatus 16.1 Attach the RGA or the MSLD tube to the highvacuum section of the test object to he tested. The conneclion should be located near the pumped end of the system and attached with as short and as lame a diameter tube as practical. Maximum test sensitivity & obtained when the high-vacuum pumps are throttled, by means of the highvacuum valve. so as to maintain as hi& a oressure in the ~n'imlationvalve volume under'test as is safe for the may be used between the detector and the system to allow servicing the detector without loss of vacuum in the system and to protect the detector from contamination when nor in use. When a liquid nitrogen trap and isolating valve are both being used, the cold trap should be located between the test object and the isolating valve. 16.2 Attach a standard capillary or permeation leak to the system as far away from the pumps as possible, using the lowest conductance path. A small high-vacuum valve should be used between the standard leak and the system, and a dust cap should be provided for the capillary standard leak if it is
LL
to he left in place. Refer to Fig. 4 for the calibration setun
17. Instrument Calibration 17.1 See section 9. 18. System Calibration and Test Rocedure 18,1 seesection 19. Precision and Bias 19.1 Precision-The precision of these test methods..will vary with each instrument and the sensitivity level of the leak test. 19.2 Biu-The bias of the leak t a t will be equal to that ofthe standard leak used for the system calibration when test conditions are the same as the system operating conditions.
20. Keywords 20.1 helium leak test; helium mass spectrometer leak test; hood leak test; leak testing; mass spectrometer leak test
Ths nK1ric.w W r y lor Testing and Malerials lakes m p a r h ~ q € c I i n gthe validi7y dany w e n 1 righls asserted in mnnedion wifh any i(Mt memiaoed in lhb standad. US- d l h b slandard are eqx€dy advised lhal delenniMiion ol the validify d any such wen(righls, and ihe rirk d i n f r i m olslhh rigMs, are €direly their avn respnsibil#y. Thb slandard is sobjeQ 10 revision al any W-9 by ihe respririMe l & n b l m m i n e e andmusl be revwew live yesrs and fndrrnired, ei7herresppro"edorwilhd-. Y o u r c ~ m w m a r e i n u ~ e d ~ I ~ ~ r ~ and shooM ba a d d w lo ASTM Hea+&es. Ywr cMmwm wiilreccire M u 1 m M e ( a l i o n a1 a meeling d lhe r s s p ~ ~ i b l e l&nW m m e e . Which you may anand. H you lee1 lhal your mmmnls have nd mceh'ed a fair hearing you shwld make your views kMwn lo Uw ASTM Commnec an Si&s. 1916 Race Si. Philad#@ia. PA 19103.
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THERMOGRAPHY Introduction
.
. . .
all methods in which heat sensing devices are used to measure temperature variations in components, structures, systems or physical processes used for detection of subsurface flaws or voids, provided depth of flaw is not large compared to its diameter can inspect complex shapes or assemblies of similar or dissimilar materials need only one side accessibility
Basic Principles of Thermography
.
thermography uses non-contact infrared scanning equipment to detect invisible infrared radiation (heat) and converts this energy -- to visible light, - or an electrical simal to be displayed:
-
visible
ELECTROMAGNETIC SPECTRUM
. . .
involves the measurement or mapping of surface temperatures when heat flows from, to or through a test object thennograph - map of isotherms or contours of equal temperatures, over a test surface examples of detectable changes - heat leaking out of a component causes a hot spot on the part surface or unbonded area on a component which so uniformly heated will produce a hot spot since the heat does not flow to the substructure compared to the normal area the larger the imperfection and the closer it is to the surface, the greater the temperature Merentid
Heat Transfer Mechanisms
. .
heat flows from hot to cold within an object by conduction and between an object and its surroundings by conduction, convection and radiation electromagnetic radiation is emitted from a heated body when electrons within the body change to a lower energy state. Both the intensity and the wavelength of the radiation depend on the temperature of the surface atoms or molecules
Material Heat Transfer Characteristics
.
Material characteristics that affect conduction and convection 1) Specific heat (c) -
. .
the amount of heat a mass of material will absorb for a given temperature interval. the mass per unit volume of the material 2) Density (p) 3) Thermal conductivity Q - the amount of heat that flows in a given direction when there is a temperature difference across the material in that direction 4) Thermal diffusivity (a)- the speed at which the heat flows away from a region of higher temperature to the surrounding material 5) Convection heat transfer coefficient @) - a measure of how efficiency heat is exchanged . between a surface and a flowing - gas - or liquid a measure of the heat energy ( local thermal agitation) 6) Temperature Q contained at each point in the test object Important material characteristic in radiation heat transfer is the emissivity (E) of a test surface Emissivity indicates the efficiency of a surface as a radiator (or absorber) of electromagnetic radiation
e
blackbodies, the most efficient radiators or absorbers of electromagnetic radiation, have an emissivity of 1.0, all other bodies have an emissivity less than 1.0
o
emissivity is a function of several variables such as color and surface roughness
e
variations in emissivity change the power of the radiation emitted at given temperature and thus affect infr-dlwl.temperature measurements
Surface Preparation e
surface condition can affect test results i.e. roughness, cleanliness, foreign materials, uniformity and condition of paint or other surface coatings
Establishing Heat Plow
. . .
test piece to be inspected by thermography are considered to be either active or passive passive - test pieces are artificially heated or cooled during the inspection to obtain a thermal profile active - test pieces that use the heating or cooling effects inherent in normal service durini inspection
Inspection Equipment
. .
Temperature sensors used in thermal inspection can be separated into two categories: noncontact temperature sensors (used for thermography) and contact temperatures sensors Noncontact sensors depend on the thermally generated electromagnetic radiation from the surface of the test object This energy is typically in the infrared region TYPES OF NONCONTACT SENSORS A.
Infrared imaging svstems 1) hand held scanners - respond to wavelengths (h)of 8-12 pm emitted by objects at or near room temperature but have poor imaging qualities and are not suitable for accurate measurement of local temperature differences 2) high resolution infrared imaging systems - these systems use either pyroelectric vidicon cameras with image processing circuitry or cryogenically cooled mechanical scanners to provide good image resolution (150 pixels, or picture elements per scan line)
. . .
Temperature sensitivity to 0.1" C (0.2" E) [some claim down to 0.001" C (0.002 " F)] Response time < 0.1 second to detect transient temperature changes or differentials Systems use either a gray scale or a color scale which are correlated to temperature ranges to depict the temperature distribution within the image
3) the& wave interferometer systems - use modulated laser excitation with rapid phase and amplitude sensing that can be scanned across a surface to produce an image
.
system senses the interaction between the thermal waves of the laser and the thermal variations from coating defects and thickness variations
B. Radiometers and pyrometers Devices for measuring radiation, or spot or line temperatures, without the spatial resolution needed for an imaging system usually have slow response time, so they are good for monitoring constant or slow varying temperatures Pyrometers - used as noncontacting thermometers for temperatures of 0" - 3000" C ( 32" - 5400" F)
. . .
Both radiometers and pyrometers are low cost devices that can be, used for long term monitoring of processes Contact Temperature Sensors - include material coating an thermoelectric devices advanta~es- usually low in cost disadvantages - provides qualitative temperature measurements which can show small changes in temperatures and coatings can change the thermal characteristics of the past surface
TYPES OF CONTACT SENSORS A. Cholesteric liquid crystals
.
.
greaselike substances that can be blended to compounds hive color transition ranges at temperatures from -20 to 250 " C (-5 to 480" F) Compounds can have a color response for a particular temperature range and differentials of 1' - 50" C (2' - 90" F)
B. Thermally quenched phosphors
. .
. .
organic compound that emit visible light wwhh excited by ultraviolet light. brightness of phosphorus inversely proportional with temperature over a range from room temperature to - 400' C (750" F) some can change as much as 25% / "C or 14% 1 "F other coatings - heat sensitive paints, therrnochromic compounds, heat sensitive papers, meltable frosts and waxlike substances can indicate surface temperatures.
C. Thermoelectric devices Thermocouples -
consist of a pair ofjunctions of two different metals. As the temperature of one of the junctions is raised, a voltage relative to the other (reference)junction is produced that is proportional to the temperature difference between the two junctions.
Thermopiles -
are multiple themocouples used electrically in series to increase the output-voltage. They have greater output (which results in greater sensitivity) but have a slower response time due to the increased mass.
Thermistors -
are electrical semiconductors that use changes in electrical resistance to measure temperature
Acoustic emission
Reprinted from ResearchlDwelooment. May 1971, Volume 22, Number 5. oases 20-24
Acoustic emission -. Does metal 'shriek' when it's under stress or strain? Indeed it does. . . and instruments and methods have been developed to 'listen in' on materials and predict failures before they occur
President, ~ u n e G nR e e a r d ~ o r ~ a r a t i b n
and A. S. Tetelrnan
Schwl of Engineering. UCLA Acoustic emission &ling offen a new method for performing nondestructive tating of materials, manufacturing processes and suuctural components. When a material is strained beyond its elastic limit, it emits a characteristic noise signal thal is called ucomic c m b sion. T h e total amount of acoustic emission increases until the material fractures. Detedion of acoustic emission signals allows an engineer o r scientist to predict when a material is about to fail and gives him
-
Digital printer bo \
or computer cL 2
Reamplifier Transducer Alarm
AJ Fig. 1. Simplified block diagram of acoustic emission system. Sensing transducer in contact with structure being investigated converts low-level stress waves to electrical signals that are amplified, filtered and processed in varietv of wan.
the opportunity to prevent (fie failure in such wses. The family physician employs one form of acoustic emission Lcsting when he listens to the human heartbeat with a stethoscope. From the pulse rate and amplitude of the e m i M sound, h e determines whether there are any defects in the heart However, the sound emitted by a deforming metal o r nonmetallic structure is much more difFicult to detect Sensitive pieuxlearic transducers must be u t i i i i to hear the key events o f deformation and fracture and convert these p u b s to cleclmnic signals. Filters are required to screen out unwarranted background o r extraneous noise. T h e electronic signals need to he amplified, pro& and presented to the user in a simple display. Finally, the scientist o r engineer must have some understanding of the "software" of this technique if he is lo use it efficiently. A great deal of r and d effort has been expended on acoustic emission testing during the last several years. An acoustic emission working group is in existence and, in c a p e r a t i o n with ASTM, is sponsoring a twoday technical conference on emission tesiing in Florida in Decemher 1971. Apart from the original efforts of Kaiser in Germany. almost all of the work has been performed in the U.S. Acoustic emission techniques haqe been fouhd to be one of the most informative methods of determining material behavior and stNcturd performance. T h e techniques have been used for nondesmctive inspection of ordnance and pressure vessels, for determining the efficiency of welding and adhesive bonding processes: and for understand-
ing [he microscopic proccsscs o i iatiguc, slress corrosion cracking and composite failure. Malcrials such as steel, titanium, aluminum. concrete, woad aod fiber reinforced resins have been investigated. How It Works Acoustic emissions are the impulsively generated small amplitude elastic stress wavcs created by deformations in a material. T h e rapid release of kinetic energy from the deformation mechanism propagates elaslic waves from the source, and these arc detected as small displacements on the surface of the specimen. The emissions indicate the onset and continuation o f deformation and may be used to locate the source of deformation through Lriaugulation techniques. A particular feaNre which makes acoustic emission analysis a most useful tool for the study of the behavior o f materials is that the pattern of emission u determined by the lime distribution of the impulsive deformations that occur within the material. Coosequently, the study of local defects can be carried out without prior knowledge of their location, o r even existence. In addition. emission data dscribe the volumetric deformation p r o c m not adquately available from surface phenomena (such as strain), thus permining a mom comprehensive insight into the deformation p r o e s e s (such as plastic flow, fracture and phase transformations) that occur. The application o f acoustic emission technology involves a f f i n g the senson to the article under i n v s ligation; the detected emissions are then amplified, selectively filtered. and conditioned, and then counted either on a periodic basis, as a rate of emission, o r as a cumulative total. Typically. inflection points in the data curves obtained through either counting method are used to determim such items as the onset o f plasticity a n d l o r crack growth, continuatioo of slow o r stable crack gmwth. and .the transition to unstable crack growth. Emission signals are frquently also recorded on magnetic tape for post-test analysis. Figure 1 is a simplified block diagram showing the detector in cootad with a strudurc. T h e sensing transducer is normally constructed from a piaoelectric crystal that converts low level s t r w waves in the structure to electrical signals that are amplified, band pars filtered and processed in a variety of ways. T h e signals are usually transient in nature and tend to ring the detection transducer a t resonana. This rrcults in an electrical signal that is a damped sinusoid with a carrier frequency strongly dependent on the traosducer characteristics. In many cases the signals are counted with a digital counter. This count is converted to a dc voltage and displayed on an x-y recorder. The digital counting technique has other advantages in the event one wishes to process the data with a digital computer. The use of several acoustic emission channels on a large structure can be used lo triangulate to a source and thereby locate a flawed area. This is accomplished in much the same manner as louting sources o f earthquakes. .+Applications
Strain
Fig. 2. Acoustic emission rate data observed fmm metal specimen pulled in tension. Note that emission rate is maximum near yield strength and decreases in workhardening range.
Renure Fig. 3. Typical summation of acoustic emission curves o h tained from identical pressure vessels with different initial flaw sizes. Slopes increase rapidly prior to failure. Data can be used to predict failures before they occur.
k e e p growing
Acoustic emission testing techniques are rapidly being used in many a r e a . Metallurgisls and materials engineers are finding useful information concerning
Fig. 4. Summation acoustic emission as function of time for three different heat treated specimens of an aluminum alloy under load i n 3 per cent salt solution.
crack area (square inches\
~ i g . 5 . Summation of awustic emission signals a s function of area of hydmgen-induced cracking for several values of stress intensity factor K.
the deformation mechanisms operating in materials. These cover the gamut from glass and ceramics, through conventional metals, into the more modern composites. Figure 2 shows the typical awustic emission response normally observed from a metal tensile specim e n T h e eminion rate is maximum near the yield strength of the material and dsreases in the workhardening range The type of activity observed from an nnftawed specimen of this kind is related to microscopic dislocation pmcascs and requirs a high sensitivity instrumentation system to be detected. T h e signal levels can vary by orders of magnitude depending on such factors as the crystallme sWclure of the materials, yield strength and past history. Flaw de(ectioa The introduction of a Baw into a material significantly changes the awustic c m k i o n pattern in comparison to the unflawed specimen. T h e data in Fig. 2 are primarily due to a uniform, homogeneous yielding that occurs in the gage section; in this situation the emission incrcves to a maximum. and decreases in the work-hardening region. When a flaw is introduced, l o c a l i i yielding will occur in the vicinity of the Baw even though the gross stress in the specimen is weU below the yield stress. If the Baw is large enough to caw failurc below g e n e d yield, a continuous build up of crnission will occur until the specimen fails. Any anomaly that will c m t e l o c a l i i yielding will result in awustic emission. Figure 3 shows typical data from p r m r c vascls containing flaws o f sufficient sizc to cause failure to c o x r below general yield. Note the large incrwwr in the Slopes of the summation of acoustic emission-prcnure curves prior to failure. Over 100 pressure vessels of different materials have been monitored over lhe past 8 years. This rapid
increase in cn~issionprior to failure was observed in all cases for failures bclow general yield. Many limes the beginning of this rapid build up in activity occurs at approximately 70-80 per cent of [he failure pressure. This allows failure to be anticipated in snme cases. Thin-wall vessels constructed from tough materials. and containing small Raws often show a peak in the acoustic emission data prior lo failure. similar to the unflawed tensile specimen. It is then marc difficult to make failurc predictions based on the slope of the emission data. However, predictions can sometimes be' made by periodically holding the vessel at constant pressure on the increasing pressure cycle. Almost all malerials 'containing flaws exhibit a creep effect at some percentage of the critical stress where failure occurs. This results in continuing acoustic emission during t h a e constant-pressure hold periods. Laboxtory tests on fracture specimens can determine at what percentage of the critical stress intensity factor this creep effect ocmm and the pressure vessel test can be used to estimate wheiher the stress intensity at the largest flaw is above o r below this value. In many materials the stress intensity factor must be 89 lo 90 per cent of the critical value Kcbefore tnc c m p effect is observed. Predicting suxrptibility to strm cormSon cracking and hydmgen embritUemenL Many materials exhibit susceptibility for subcridcal u a c k gmwth when cxposed to a combim~ionof certain environments, high s@cscs. c, and pre-existing Ram of length a. This phenomenon is known a s sires w m i o n cracking. Acoustic emission techniques can easily detect stcorrosion cracking long before any visible evidence of attack is p r e x n t To demonsbate ihu effect, three compact tension fradure specimens of an alu- minum alloy, containing sharp machined notches were loaded to the same value of Ncss intensity factor, K = (-)k, and subjected to a salt water solution. Acoustic emision tranducers were attached, and the notched region of the specimens were placcd in a 3 per a n t salt solution. The spacimem had becn heat treated prior (D the test to such an extent that one was s w r p t i b i e to strsr c o r n i o n cracking and the other two were n o t The summation of acoustic erninion counts p r s e n t t o m each specimen was then plotkd as a function o f time for a n 8 h o u r period. The d t i n g curves are shown in Fig. 4. Note that specimen A shows aaivity after only 20 minufm in thesolutionand goes to 10,000 counu in less than an hour. Specimens B and C were not expected to besusceptible to theenvironment. As expected; C did not show any activity over the 8hour period. In subsequent tesesIs,C was held for as long as 86 hours with n o activity occurring. Spetimen B did showsome susceptibility to stress corrosion. a[though in a much less dramatic manner Lhan A. Following ~e 8-hour test the spccirncns were removed and examined a t 8 X magnification. No visible difierenas were observed b e t w m the specimens. In subsequent 86-hour t a u . A began to show a small crack in the vicinity o f the notch tip. Most p r s e n t . methods of determining stress corrosion suweptibiity involves loading many specimqns simultaneously and waiting for failures to occur. Thus. w n k s and months are required to obtain the needed data. Acoustic . emission techniques can shorten the time considerably and arc idcally suilcd for determining whether or not
a particular cnvironmunt is hostilc to a given m~lcrial.
The role or dissolved hydrogen in promoling (r;lclure or high strength stecl components has been the object o f numerous investications. The acoustic emission resulting from the initiation of microcracks and crack propagation can be easily recorded, and a quantitative relationship has been established between the acoustic emission data and the amount of crack area generated. Figure 5 shows the rclationshi~between the summation of acoustic emissions present as a function of the amount of hydropen-induced crack extension in a cathodically chirgedTspecimen of 4340 steel. Note that the number of counts present for a given amount o f crack area swept out is strongly dependent on the stress intensity factor K present at the crack tip. Acoustic emission testing can thus be used to continuously monitor slow crack growth in cadmium plated steel fasteners, and predict when a bolt is approaching failure (when K -tKJ. Mevuring coating thickness. There are many coating processes for protecting materials from erosion o r corrosion under dilierent environmental conditions. Anodizing is used by the aluminum industry. Thermal oxidation can occur when materials such as titanium alloys are subjected to high temperatures in the presence of oxygen. Both of these processes result in the formation of brittle coat on the surface of the material. When the materials are deformed, the resulting microcracks generated in this coating give rise to acoustic emission signals that can he easily recorded. A technique has been developed for measuring anod ' i coating thickness. It involves recording the total number of counts for a given pressure from clamped diaphragm specimens of thin aluminum with varying anodized coating thickness. Experiments have been performed recently on diaphragm specimens of 6 A I 4 V titanium alloy subjectcd to IS00 F in air, for different lengths of time. The diaphragm specimens were clamped at the edges and subject2 to ;n increasing hydro&tic pnsrure>n one side, while acoustic emissions were recorded from the opposite s i d e The summation of acoustic emission counts were recorded up to 3M)O psi on each specim e n The total number of counts observed from each was plotted as a function of the time of exposure of each diaphragm to the 1500 F environment These data. are presented in Fig. 6. Note that the longer exposure results in a higher number of wunts to the maximum prusure. Since the oxide coating thickness is proportional to some function of the time of exposure, a test of this type can be used to determine the average wating thickness. The 3000 psi was not sufficient lo plastically deform the diaphragm to such an extent that a noticeable dimpling occurred. It was determined that a few hundred psi was sufficient to distinguish t h e difference between specimens; thus, a device can be envisioned that would pressurize a given area on a sheet specimen and provide a measure of the wating thickness in a nondes&ctive manner. Delcctinc - hich - temuenture failure. Many measurement techniques such as strain gaging, holography and eddy currents, require access to the surface o f the material. at o r adjacent to the area to be measured. Thus, certain limitations are present when high temperature closed environmental conditions are present. On the other hand, acoustic emission signals generated in materials will propagate for large distands
0
0.25 0.5 0.75 1.0 1.25 1.50 1.75 2.0 2.25 Hours
Fig. 6. Sqmmation acoustic emission as function of time of exposure at 1500 F for 6A1-4V titanium diaphragms pressurized to 3000 psi.
with little attenuation in most engineering materials; and therefore the transducers can be located at some distance from the activity without a loss of data. One example of the use of this "wave guide" principle is shown in Fig. 7. A Rene' 41 tensile specimen containing a transverse weld was tested at 1400 F i n a gleeble machine. T h e determination of the on& of microcracking was of ~ r i r n a r yinterest. The soecimen was heated ti 1403 F during ;he first three mihutes. a t which time load was applied until a preset stress was obtained. At this point constant displacement was maintained on the specimen. The signals from the transducer (mounted on the water-uxrled grips external to the hot environment) were accumulated and plotted as a function of time. The acoustic emission data (Fig. 7) shows considerable activity during Load application; the emission quiets down a s the displacement is held constant. After approximately 30 seconds at constant displacemenl, crack initiation begins and continues. amlcrating until complete failure occurs. The emission data provided detailed insight into a time dependent phenomenon, controlled by applied stress, temperature and material composition. The acquisition of these data was conveniently accomplished even under fairly severe elearical and thermal noise conditions.
Future Applications All materials and structures contain defects of one sort or another. 'Generally, these defects wuse no reduction in the strength of a part. However, if the defects reach a certain range o f size, they become dangerous and can cause a substantial reduction in strength. AET offers the possibility of detecting these cracks before they reach this critical size range. TO understand how this is accomplished, it is necessary to digress a bit and consider the mechanics of crack growth. Briefly, a crack of length o residing in
10
Crack ini!ia$ion
conaant dirplocemcnt
Holding
8
.. . . .. . .:...+:;,.<;'";,.i' . : . , .
7 ;,
.
,
, ... ..-.. ... .. . . -
.,-. .
."
5.:
and failure ,
a i n =85 db ,,~B W = 1W-300 KHz
'-
x'
'
4 ' :
Differential transducer
3 2 ,
, ,,
.. 1
2
3
4
5
.... ... .. . 6
7
6
Minuter
Fig. 7. Summation acoustic emission as function of time for welded Rene' 41 tensile specimen. Specimen was heated to 1400 F in a gleeble machine loaded to a constant displacement and held until failure occurred.
- ...
a n elastic volume under a tensile stress s is described by a stress-intensity-factor K. K= (1) K gives a measure of the local strain energy concentration, G. at the crack tip. For example. K = (EG)H, where E is the elastic modulus. Unstable (rapid, final) crack propagation occurs when G reaches a critical value, G,, such that K reaches a critical value K, = (EG,)" (2) From Eq. 1. we see that the fracture s ~ e s or s is rhen given by eF = Kc/ (a~,)" (3) where a, is the critical flaw size. Kc is called the fracture roughness. This key equation indicates that in brittle materials (K, low), small flaws will become critiul a t a particular slress level, whereas a, will be large in tough materials (Kc high). Flaws that are smaller than critical size when introduced into the suucture (by poor welding) can sfill grow out to critical size under random loads (fatigue) o r in reactive environments (stress corrosion cracking). Rare of slow crack growth also depends on K. da/dr = f(K) = ffd;io)*) (4) It is necessary to detect a crack when its K I K , ratio is low, if the stress is to be removed and brittle fracture is to be prevented. consis& of a series of disSlow crack crete movemenls within the structure. Each movement rapidly removes the strain energy G stored near the crack tip. A portion of this released energy is spent by the increased crack-surface area as surface energy, while another portion is released as elastic waves in the form o f acoustic emissions. The results of rising load tests have shown that the total number of acoustic emission signals, ZN, can be directly related lo the applied stress-intensity-factor K, through a relationship o f the form I N = AK(5) where nr is a constant for a material and ~hickness. Eq. 5 suggests thal acoustic enlission could be
applicd to thc dctcctlun ol cracks and their suhcritical growth by continuous niunitoring oi a structure. However, i n practical usage, cxccssive bockground noise during service. such as accun in aircraft nuclear power generating facilities, eliminates this procedure i n many cases. As a n allernative to contin. uous monitoring, a procedure lhat lakes advantage of the irreversibility of acoustic emission is possible. For example, i f a cracked structure is loaded to a particular value of K and (hen unloaded, emission will not occur during reloading until this previous value o f K is exceeded. I t is therefore possible to take advan: tage of this irreversible nature to determine whether or not a crack has grown during service loading, by periodically overstressing the structure to a stress level higher than the service stress and simultaneously monitoring for acoustic emission. If Raws have grown since the previous overstress, then the applied stress intensity factor during the new overstress application will have increased. and emission will be observed. Alternatively, if no Raw extensions had occurred, the applied stress-intensity-factor would remain as before and no new plastic deformation, and hence no emission would occur. 1t is entirely f e G b l e that this technique could be used to periodically overstress selected structural components to determine if fatigue cracks are growing in critical areas. This concept of using.acoustic emission and a scheduled applicalion of stress to estimate service life may be applied to the case of turbine discr. Many of the discs that are retired after a specified life could undoubtedly experience further safe usage, provided nondestructive testing techniques couid reliably predict that any parricular disc would be safe for a specified period of additional service life. During each cycle of loading. combinations of stress, time. and temperature produce some creep and fatigue damage which may, in turn, be accelerated by metallurgical changes that occur during service. From Eq. 5 , we see that a certain number of acoustic emission signals, TN, (or also a certain acoustic emission rate, N) indicates that a crack is approaching K c and that the disc should be removed from service. If the acoustic emission number is below XN, then it is p w i b l e to guarantee that K is below a particular value and that the disc has a certain guaranteed lifetime remaining to it, depending on the exact form of equation (4). The problem of estimating !he residual life of a turbine disc is but one of a number of problems that might be solved by AET. Consider weld cracking as another example. Many failures of welded high strength steel p a m resulting from the growth of cracks are .difficult to detect by conventional NDI methods. However, it should be possible to detect the formation of a weld crack from the sound emitted during the growth of the crack. Continuous AET monitoring during welding should reduce the incidence of undetected weld cracks. Bridges, dams and aircraft are all struc~uralsystems in which failure occurs by slow crack extension prior to failure. If this crack growth can be monitored. either periodically o r continuously, it should be possible t? determine when K approaches K,, and hence when'structural failure is impending. The one major problem thal remains involves the screening out of extraneous noise. but in many instances this is nonexistent or can be ovcrcome with existing technique$. 0
121ACOUSTlC EMISSION TESTING
PART 1
INTRODUCTION TO ACOUSTIC EMISSION . TECHNOLOGY
The Acoustic Emission Phenomenon
" Acoustic emission is _ & e - e b t i w 1-
) J?,'
I, ,
,
'
.,
?
?!. J
.
'I 41
\
i!
that is spotane-
\ ously releazed by materials when they u n e o r m a t i o n .
In the early 1960s.a new nondestructie testing technology was born when it was r e c o g n i d that discontinuities in pressure versels amusmonitoring their acoustic emission .-- - signals. AIthou tic emission is7he most wbel usedte;;iiiTior this p enomenon, it has also been d i edY stress waoe emission, stress mw, microseirm, microsdmiic adimty and mdc noise. Formally defined acoustic emission is "the ckrs of phenomena where transient elastic w a w are generged by the rapid release of energy f m m I& sourcer within a material, o r the transient elastic wavs so generated.": This is a debition embracing bath the process of wave generation and the wave itself.
t'
Source Mechanisms Sources of acoustic emission include many different mechanisms of deformation and fracture Earthquakes and rockbursts in mines are the largest naturally occurringemir sion sources. Sources that haw been i d e n a d in metals include crack gmvth, moving dislocltions, slip, hvinning, grain boundary sliding and the fracture and demhesion of inclusions. In composite materials, sources indude mahir cracking and the debonding and fracture of fibers. These mechanisms typify the classical response of matelials to applied load. Other mechanisms fall \\
Acoustic Emission Nondestructive .. Testing Acoustic emission examination is a rapidly maturing nondestructive testing method with demonstraid capabilities for monitoring structural integrity, deteain l e a k and incipient failures in mechanical equipment, an for characterinng materials behavior. T h e first documented application of acoustic emission to an engineerin s t t u b was published in= and all of the a ~ i l a b 1 e . i n 8 dapplication exp5rience has been accumulated in the comparatively short time since then.
5
di5 h ! i k&\a
Comparison with Other Techniques
h&ec+ecS LJ Amustic emission differs from most other nondem~ctive 4 methods in tw significant respects. Fi.the ene that is detected is released from within the test obi& S e r tfian being supplied by the nondestructiw met!&, as in ultrasonics o r radiography. Second, the acoustic emission method is capable of detecting the d y ~ m i pnxerses c associated with the d dation of stmctud integrity. Crack fj and plastic eformahon an: major s o u r n of acouz- 4 h m m i o n . Latent drswnhnuities that enkrge under load G E F a z i v e sources of acoustic emission by virlue of their size. location or orientation are also t h e most likely to be significant in t e q of structural integrity. Usually, certain areas within a structural system will de\dop local instabilities long before the structure fails. These instabilities result in minute dynamic movements such as plastic defonnation, slip o r crdck initiation and propagation. Although the s t r e s ~ e s 7 i T : a ~ a l I , ~ % a ~well b e below the elastic design limit, the regtoion near a crack tip may undergo plastic delbniGtGn as ;I resr~ltof high l w d l stresses. In this situntion, t l ~ epropag;tting disco~ltinuitl/acts as a source of stress waws ind l ~ c o ~ n eans active acoustic emission Sotlice. +cornustic elnissiuo csaivlit~ation is no~tdirectiond. Most acoustic emission sources appear to functio~ias point source emitters that r.idi;ttc wauefmnts. Often. a sei~sorlwittrd int of anacoustic etniszion source c;111drtect tlle resulting acoustic emission.
&
-
$ " .
.-
-
'.FUNDAMENTALS O F ACOUSTIC EMISSION
This is in contrast lo other i~>cthods or tnu~idest~uctiw testing, which depend on prior knowledge of tlie probable location and orientation or a discclntinuit). in order to direct :I beam o l energv through the structure on a path tliet will intersect tine area or interest. Advantages
of Acoustic Emission Tesu
Tlie acoustic emission ~iiethodoKers the followi~igit(lvai~tages over other nondestructive testing methods:
1. Acoustic emission is a dynamic inspection mew
in that it provides a response to discontinui~gmwthunder an imposed structural stress; static discontinuities will not Renerate amustic emission sippals. 2. ~mustkemissiona n detect and ev&ate the signil,. u n c e ofdiscontinuities throughout an entire structure during a single test. 3. Since only limited access is required, discontinuities may be detected that are inaccessible to the more tnditional nondestructive methods. 4. Vessels and other pressure systems can often be requali6ed during an in-5e~c-cinspection that r e q u i r e little o r no downtime , 5. ?he acoustic emission method may be used to p s n t c c a t a s t q h i c failure of systems with unlolown discuntinuities. and to limit the maximum pressure during containment system tests.
'
/.y57?
TESTING113
rlniissiu~itests. lbhle 1 gives all overview of tlie ,"anner by \\*hicllv~riotts11i:tterial properties and testing conditions in,Ihtencr acm~sticel~iissionresponse arnplitnides.The Llctors ~liotlldgenerzilly be runsidered as indicative. rather than ils nbsolttte.
Application of Acoustic Emission Tests A classilication of the runctionnl categories of amustic emission applications is given belo\": 1. mechanical pmperty testing and characterization;
2 ~reseg.9 pmol tezting; 3. in-%,* (requalifiution) testing; 4. qn-linh~onitoEng; 5. in-process weld .,. monitoring; -, , 6. m!l%id?ignature anabis; Zleak detection and-l--%o<-and -& 8. geological a=.
-
By definition, on-line monitoring may be continuous o r intermittent, and may involve the entire structure o r a limited mne only. Although leak detection and amustic signature analysis do not involve acoustic emission in the shictest sense of the tenn. amustic emission twhniques and equipment are used for these applications.
'4y'<~? Amustic emission is a wave ohenomenon and acoustic .
y
C
I
emission teaine user the amibutes of particular wdw to help character& the material in which the waw are trawl&eSing. \ F z e n z l a n d _ - m p l i t u d @ are -pIez.of f6rm paramete? that are +ady monitored in acoustic
Structures and Materials A wide variety of structures and materials (metals, nonmetals and various combinations of these) can be monitored
WBLE 1. Factors that affect t h e relatfve a m p l i t u d e of acoustic emlsslon response Factors That Tend t o Increase Acoustlc
Factors That Tend t o Dmease Acoustic
Emlnlon R e y x x u e Amplitude
Emblcn Reponre Amplitude
High mength High main rate LOW temperature AZ!imtrow Nonhomogeneity Thitk smiom Brittle failure [cleavage) Material containing dixonunuities Manemitic phaw rransformations Crack propagation Casl materials Large grain size Mechanically induced
LOW mength
FROM IPANNER
LOW srrain rate
High temperature tsotropy Homogeoeity Thin senions Ductile failure (shear) Material withwt dixoncjnuiries Diffusion-controlledphase transformations Plastic deformation Wrwghr materials Small grain size Thermally induced twinning
fiCOUmCEMLWON rECHNlOUESANDAPPUUnON~.R E P R I M E 0 WIIH PERMISSION.
14lACOUSTIC EMlSSION TESTING
11y acoustic emission t e c l i ~ ~ i ~during u e s the application of a11 external stress ( l o a d ) . l l ~ pri~naryacoustic e emission mechanism varies with different materials and should be cliar~ct e d M o r e applying amustic emission techniques to a new type of material. Once tlie cliaracteristic amustic emission response has been defined, acoustic emission tests can be used to evaluate the structural integrity of a mmponent.
Testing of Composites Amustic emission nionitoring of fiber reinforced c n m p s ite materials has proven quite effecthe when compared wvitll other nondestructive testing methods. H-r. attenuation of the amustie emission signals in fiber reinforced materials presents unique problems. ENediw acoustic emission monitoring of fiber reinforced components requires much doser sensor spacings than would be the case with a metal mmponent of similar size and configuration. With the proper number and location of sensors, monitoring of composite structures has p m n higbly effective for detecting and locating areas of fiber b k g e , d e l a m i n a t i o ~and other types of s h u d degradation.
Acoustic Emission Testing EquipmentEquip~nent for processing acoustic emission siglals is available in a varie? of fonns ranging from small prtaI,le iastru~iie~~ts to large multichannel systems. Conipo~ie~its m~iiniouto all systems are sensors, preamplifiers, filters and amplifiers to make the s i ~ i a measurable. l Methods used Tor mwuretnent. display an2 storage my more widely according to tlie demands of the appliation. Figure 1 shows a block diagram of a generic four-channel acoustic emission system.
Acoustic Emission Sensors
When an amustic emission wavefront impinges on the surface of a test object, w r y minute mduements of the surface molecules m u r . A sensois function is to detect this mechanical m m m e n t and conwrt it into a specific, usable electric signal. The sensors used for acoustic emission testing often resemble an ultrasonicsearch unit in configurationand generally utilize a piemelectric tnnzducer as the electromechanical conversion device h e sensors may be resonant Pressure System Tern o r broadband. The main considerations in sensor selection are (1) operating frequency; (2)sensitivity; and (3) environpressure are sd using h tacit or mental and physical characteristics. For high tern &re other pressure t e s t The level of stress s o d d normally b e tm. W ' W i d e s may b e ud to isolate the sensor E r n the held &Iw the r e l d -. B ~ i ~ lg & a~ b e inenvimnment Tbis be i ~ ~sherses ~ ~ ~ is a convenient ~ l alternative to the use of d u c e d to beamed high temperalure sensors. Waveguides haw also been used generated in rotary shafts. h e r m a l ztresses may b e created to ~ m d i t i o the n acoustic emission signat as an interpreeither be unil d Y .Tension and bending messes tation aib lateral o r +C to best simulate service induced stresses. Issues such as wave type and directionality are difficult to handle in this technology, sin= the naturally occumng amustic emission mntains a complex mixture of we modes.
t-
Successful Applications
Examples of proven app~iutions for the amustic emission method include those listed belmv.
1. Periodic o r continuous monitoring of pressure vessels and other pressure mntainment SFtems to detect and locate active dismntinuities. 2. Detection of incipient fatigue failures in aerospace and other eneineerine structures. ,. 3.' Monitorin materials behavior tests to characterize various fai ure meclianisms. 4. Monitoring fusion or resistance weldmen6 during welding o r during the cooling period. 5. Monitoring acoustic emission response during stress corrosion crackine.. and ii\drocen embrittlement suscept~bilitytests.
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~. .. Selection P~~E!~!/fiers and Frequency. The preamplifier must be located J o s e to the sensor. Of-
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ten it is actually in+rated into tbe sensor housing. T h e preamplifier provides required filtering, gain (most cornmonly 3 dB) and cable drive ca&-&ty. F F i l t e ~ gin tbe preamplifier (together with sensor selection) is the primary meam of defining the monitoring frequency for the amustie emission test. This may be suoolemented by additional filat the n'ainframeChoosing the ~nonitoringfrequency is an operator function, since the acoustic emission wurce is esseniially wide band. Reported frequencies range from audible clicks and rnwxks un . 7 . .= to 50 MH-I . Although noTal\vaF fully appreciated by operators, the obsened frequr~icvspectrum of amustic emission sienals is c e dthe resonance and tnnsmission significantly ~ n f l t ~ r ~ i bv
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FUNDAMENTALS OF ACOUSTIC EMISSION TESTING/ 15
FIGURE
1.
YMOFS
Schematic diagram of a basic four-channel acousric emission testing system PROIMRlFlEG wlrn FILIEG
M E N AMPIIFIEG W~TH FIUERI
MEAIuREMENI
C\RCUIT~
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SrORhGE
YREEN
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MIA BUFFEFS
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MlCllOCOMRmR
V ORRIJOR
characteristics of both the specimen (geometry as well as acoustic properties) and the sensor. In p h c e , the h r uency limit is governed by backgmund noise; it is unuto go below 10 kHz e~ceptin microseismic work f i e upper frequency limit is governed by wave attenuation that restricts the uzehrl detection range; it is unusual to go above 1 M H z The single mcst common frequency range for acoustic emission testing is 100 to 300 kHz
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System Mainframe The first elements in the mainframe are the main amplifiers and thmholds, which are adjusted to determine the test sensitiviv. Main amplifier gains in the range of 20 to 60 dB are most commonly used. Thereafter, the available processin depends on the size and cost of the G e m . In a small portafle instrument. acoustic emission events or threshold crossings may simply be counted and the count then converted to an analo voltage for plotting on a chart recorder. In more a d v a n 2 hardware systems, pnnisions may be made for energy or amplitude measurement, spatial filtering, time gating and automatic alarms.
Acounic Emission System Accessories Accessory items often used in acoustic emission w r k include oscilloscopes, transient recorders and s+wm a d ? zrs, magnetic tape recorders, rms voltmeters, special calibration instmnients, and devices for simulating acoustic
emisdon. A widely accepted rimuktor is the Hsn-Xielsen source. a modirted s-d n d &at prwide5 a remarkably reproducible simulatdPamustic emission signal when the lead is broken against the test structure.
Microcornputen in Acoustic Emission Test Systems Signal Processing and Displays Nearly all modem acuusiic emission Nstems use micmmmputen in various configurations. as determined by the l system size and performance requirements. In e ~ i c a implementations, each acoustic emission signal is meamred by hardware circuits and the measured parameters are passed through the central microcomputer to a disk fie of signal descriptions. The customary signal description includes the threshold crossing counts, amplitude, duration, rise time and often the energy of the signal, along with its time of occurrence and the values of slowly changing variables such a? load and background noise l e d . During or after data recurding, the mtem exiracts.data for graphic displays and hardcopy report. Common displays include history plots of acoustic emission versus time or load, distribution functions, cmqlots of one signal descriptor against another and source location plots. Installed systems of this t y p range in size from 4 to 128 channels.
IbIACOUSTIC EMISSION
,
TESTING
Some ;illo!.s :i~tdt~i;lte!i:~l> itt:ir in)l cslli1)it ; I I I ~t~ieasur&le Kaiser effect el ;ill. h4icrocomputer based systems are i~sually\*en \rr;atile. Becmise or the Kaiser d k c t , e;lcl~acoustic signal 1n;iy allowing data filtering (to remove noise) and exte~lsivepostonly occur once so 11131 itisliectinns ]lave a nmv-or-never test display capability (to analyze and interpret results). Tllis qnalily. In this respect. iicot~sticemission is at a disversatility is a great advantage in new or difficult applica;idvant~ge\when compared to tecl~niquesthat can 1% aptions, but it places high demands on the knwletlge auld ~ operaton or with differplied again and again. I I differer~t technical training of the opentor. Other kinds o f ~ ~ u i p ~ n e ~ ic tt ~ tinstn~me~lts. wvilhot~t ;iffecting the stnrcture or the have been dewloped for routine industrial appliritio~~ it1 the discontinuity. hands of less highly tnined penonnel. The outcome in practical terms is that acoustic ernission Examples are the systems used for bucket truck testing must be used at carefully planned times: during p m f tests. (providing preprugrammed data repoits in a m d a n c e with before plant shutdowns or during critical moments of conASTM recommended practices) and systems for resistance tinuous operation. This seeming restriction sometimes weld p r o c w control (these are inserted into the current h o m e s the biggest advantage of the i&ustic emission control system and terminate the welding process automattechniques. By using acoustic emission during service, proically as m n as ~ u l s i o nis detected). duction can continue unintermpted. Eqienzive and time Acoustic emission equipment was among the first nondeconsuming processes sucli as tlie erection of scaffolding and structive testing equipment to make use ofcomputers in the extensive surface preparation can be completely awided. late 19605.Performane. in terms of acquisition speed and real-time a n a l e capability, has been much aided by advances in miemcomputer technology. Trends apected in the future include advanced kin& of wdveform analysis. Acoustic Emission Test Sensitivity more standardized data interpretation p d u r e s and more . dedicated industrial products. Althou h the acoustic emission method is quite sensitiw, cornpare lw.t h other nondestructiw methods such as ultrasonic testing o r radiographic testing, the sensitivity decreases with i n d g distances behveen the amustic emirCharacteristics of Acoustic don source and the sensors. T h e same Cadors that affect the Emission Techniques propagation of ultrasonic waves a h affect the propagation of the acoustic (stress) waves used in acoustic emission The acoustic emission test is a passive method that monitechniques. tors the dynamic redishibution of stresses within a material. Wave mode conversions at the surfaces of the test object o r component. ThereTo% acoustic emission monitorin is and other acoustic interfaces. combined with the faa that only effeaive while the material o r structure is subject$ to direrent wave modes propa ate a t diilerent velodties. are an introduced stress. Examples of these strenes include factors that complicate anafysis of acoustic emission repressure testing of vessels o r piping, and tension loading o r sponse signals and produce uncertainties in d m l a t i n g bend loading of stmctunl components. acoustic ernission source locations with hianguiation o r other source locating techniques. O p e r a t o r Training a n d System Uses
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Irreversibility a n d t h e Kaiser Effect
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-Background Noise a n d Material Properties An important feature aflecting acoustic emission applicaIn principle, overall acoustic emission system sensitivily , ; tions is the generally irreversible response from ~iiostmetals. in practice, it is often found that once a given load has depends on the sensors as tvell as the cl~aracteristicsof the .. 4 k e n appliedand the acoustic emission fmnl a w r ~ i m o d a t - specific instrumentation system. In practice, houwer, the ing that stress has ceased, additional acoustic emissio~iwill se~isitivityof the acoustic emission metl~odis often primarily not OM^ until that s t i b s level iraceeded. ewn if t l ~ eb a d limited by ambient background noise considentions for is mn~pletelyremoved and then reapplied. This oftell useful engineering materials with good acoustic transmission cl~aracteristia. - $ (and sometimes troublesome) behavior has been nnn~edthe cv I Kniscr@kt in h s ofthe researcher who first reported it. \\'hen monitoring structures made of materials that ex? T h e degree to which' th-mt is present varies Ilil>ithigh acoustic attenuation (due to scattering or absotp,/ behwen metals and may even disappear completely after tion), tlie acoustic properties of the material usually limit , , s w r d hours (or days) for alloys that esliibit apprecktble the t ~ l t i ~ ~ ltest a t e sensitivily and will certainly impose !imits ;.; .'d ; tempemlure annealilig (remveYl cliar;lrlc~~istics. 0x1 the maximum sensor spacings that can lx used.
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FUNDAMENTALS OF
Effecuof System Sensors Sensor coul~lilig:rtrd reproducilrilit).ciiiol rcsputi\c arc ilnportant factors that nust l>econsi
Interpretation of Test Data Proper interpretation of the acoustic emission response obtained during monitoring of presmrized systems and other structures +ly requires considerable technical nence with the acoustic emission b&en the acoustic emiaion system opentos, the data inte retation personnel and those conhulling the pprocess o?-ing the shucture Since most computeri& multichannel acoustic emission +ems handle m i p n s e d a in~ a pseudo batch p d u r e , an intrinsic d a d time m r s durine the b t a transfer o m ess. This is usually not a problem b$ can &onally -it in anal* ermn when the quantity of acoustic emission signals is Nffiaent to mrload the data handling capabilities of the acoustic emission system.
Compensating for Background Noise When acoustic emission monitoring is used during hydrostatic testing of a vessel or other pressure sytem, the acoustic emission system will often prwide the first indieation of leakage Pump noise and other vibrations. or leakage in the pressurizing y t e m , can also generate background noise that limits the aerall systeni sensitivity and hamlxn acr.rtrate interpretation. Special precautions and fhturing may be necessary to reduce such background noise to tolenble levels. Acoustic emission monitoring of production processes in a manufacturing environment inwl\.es special problems related to the high ambient noise levels (lmtl~electrical and acoustical).
ACOUST~CEMISSION TESTING! 1 7
i'r~-,enIive llle;~s~~r<~s CII:II, ti<~c~~ss:tt~ to ])rovidc sr!r[icivti[ c:lect~ic:~l or ;icotbstioil ir'<~l:tti<)~i to ;tcl;ic?.ceSfecti\c a a i ~ t s t i ~ etnission irio~iitorinfi. \';iriot~sprwtrcl~trrsI r ; ~ \ r licc-1111sed to re(liicc lire cfr~cts of l>;tckgro~~nd iroise sottrces. In~.lrxledalrlong tlrcse are ineclr;i~ricidand amr~sticisol;~tiul~; elect~icalisulatio~~; electronic filtering within the acoustic eliiission system: nilxlificatio~rsto the iiiecl~i~~riwl or li!dr:rrtlic 1o;rditrg process; special sensor cunfi~uratiotisto co~itlolelectronic gates for noise blocking; and statistically l~.uedelectronic currnterIneasures includi~rgautorurrelatic~nilrd cross mrrelatioti.
The Kaiser Effect Josef Kaiser is credit& as the founder of niodem acoustic emission tecl~nolo~y and it was his pioneeringwrk in Germany in the 1950s that triggered a connected, continuous flow of sul)seyoent de\rrloprnent. He made two major discaveries. The first w w the near uni\eIsality of the acoustic emission phenomenon. He observed emission in all the materials he studied. The second was the e K i that bears his name in translation of llis own words: 'Tests on various mate": ah (metals. woods or mineral materials) have shown that larv level emissions begin even at the lawst strw levels (less than 1 MPa or 100 psi). They are detectable all the way tl~mughto the failure load. but only ifthe material has exper i e d no previous loading. This phenomenon lends a spedal significance to acoustic emission by the measurement of emission dusion can be dram about the loading experienced prior to tile test by the material under investigation. In this. the magnitude and duntion of die earlier loading and the time between the eariier loading and the test loading are of no imp~rtance."~ Thu &ect has attracted the attention ofacousticemission workers ever since. In fa<+.all the yean of acoustic elnission reseadl have yielded no other generalization ofcomparable power. As time went 11y. lmth practical applications and controwrsial ex&-ptions to the rules were identified. The Dunegan
Corollary
The first nwjor application of the Kaiser efiect w.tl ;I test stntegv for diagnosing dainage in pressure vessels and other engineering stnlctttres."lre s t n t e v included a clarification of the I>elrn\iorexpected o f i ~pressure wssel subjected to a series of lo;idi,rgs (to a prwfpressure w i t 1 1 inten-ning periods at a Iwer working pressure). Should the \essel sufier no damage during a particular working period, tlie Kaiser erect dictates that no elaissioo will be obsenjed during tlie si~bsequent loading. I n
ISIACOUSTIC EMISSION TESTING
the event of discontinuiy gr~nvilidunlig a uwrking peliid. subsequent proof loading twuld s~~bject the inaterial at tlie discontinuity to higher stresses tli;io I d o r e and tile discu~itinuity would emit. Emission during the proof loading is therefore a measure of damage eq~eriencedduring the preceding working period. This socalled Dunegao wrollonj Ixrame a standard diagnostic approach in practical field testing. Field operators learned to pay particular attention to emission behueen the w o h g p m r e and the pnmf pressure. and thereby made many effeaive diagnoses. A superficial &ew of the Kaiser eR& might lead t; the concluhon that practical application of amustic emission technioua reauires a series of ever increving loadings. ~cnwwr,'effecti$ engineering diagnoses can be made by. repeated applications of the same proof -. prersure
that colitest. Btlt actuiilly. K~iscr'sidc:~;il)plies 1110ref~x~idamentally to stress in a 111:iterili. hiolcriols entit a,ily itnrlcl: rrr~pircrdenle(lssfrer.7 is the root principle to wnsider. Evaluated point-hyimi~it tllro~tglithe threedi~nensionalstress field witliin the structure. this principle h;u wider truth than the statemelit that structures enlit only under unprecedented load. Pmided that tlie microstnictr~rehas not been altered between loadings. the Kaiser principle 1119 men have the universal \.alidi? that the Kaiser eKwt evidently lacks. at l w t for acthe deformation and discontinuity growth. In composite materials. an important acoustic emission mechanism is friction b e b e e n free surfaces in damaeed rrgions. Frictional acoustic emission is also p b s e d fGm fatigue cracks in metals. Such source mechanisms contnvene both the Kaiser eKect and the Kaiser princi~le,but they can be important for p r a c t i d detection of damage and discontinuitia.
The Felicity Effect T h e second major application of the Kaiser effect arose from the study of rases where it did not occur. Specifically in fiber reinforced components, emission is often obs e n d at loads lower ihylthe previous maximum, especially when the material is in poor condition o r close to failure. l%s breakdown of the Kaiser effect war; s u d u l l y used to predict failure loads in composite pressure vessels4 and bucket huck booms.s W was inhod'& to ddescribethe The was debreakdownof the Kaiser effect and thefeIi* vised as the d t e d quantitative measure The felicity ratio has p d to be a valuable diagnostic too1 in one of the emission a plicationr;, the most succeszful or of fib*^ w s e l s and fionge d . 6 1' &ct- the Kaiser effect maybe as a caseof the effect (a felidty ratio 1). The of cases where the Kaiser eKect breaks down was at firstquite confusingand contmrsial but evensome Furtherjnsighls emerged. The Kaiser mo* no'iceabl~h situations where time dependent nisrns conmil the deformation. The rheological flow or rekxation of the matrix in highly stressed mmposites is a "lour the prwiprime -pie. 'Iow of the matrix ous maximum can transfer stress to the fibers, causing them to break and emit. Other cases where the Kaiser ellkct will fail are corrosion processes and hydrogen embrittlement, which are also time dependent.
3
The Kaiser Principle Further insight can he gained b!. considering load on a structure versus stress in the material. I n pnctical situaHans, test specimens or eiigineerina structures e\-pcrience loading and most discussiotis o f t h e kiiser erfect come from
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Overview of Acoustic Emission Methodology . ?hir Nonderrrudice Testing Handbmk volume contains detailed descriptions of acoustic eminion sou-, a rich topic that involves the sciences of material, deformation fracture Another topic appwing in this ,,lume is the subject ofwax: propagation, the pmcerr shaper the signal and brings the information from source to sensor. Attenuation d the wave d d e n n i n u .i d e t d h f i i t y and therefore be considered when placing sensors; howledge of the wave> velocity is a h needed for precise source lacation. These are uncontrolled factors that must be avessed for
,a
tested, Measurement and ana+& of the acoufic signals is another major component of tile tecllno\ogy -red this Acoustic emission from defornation may be so rare that a single detectedevent is enough to wanant rejeaion of the object under test. Or. they may be so frequent that the acorntic signal is continuous. Compoundin interpretation difliculties are amplitudes of the reive% signals that range mer five orders of magnitude. T h e time diflerenes used to l o u t e acoustic emission sources range from less than a microsffond to liundreds of milliseconds. In addition to handling all of these variables. an acoustic emission system should allow any of several techniques for reducing background noise and spurious sigoals that orten interfere acoustic emission measurements. Tile acoustic emission t e c l w o l ~cwmprises ~ a of
ranee
FUNDAMENTALS O F ACOUSTIC EMISSION TESTING1 19
is still i r l tile ~ ! : i rst:i$c;. \~ Advx!tvc~\t ~ % \ l l ~ i l fc>? ] ~ l~\iscoi~ti. ~~s powerflll t ~ h n i q r t e s fur e ~ p i o i l i $tile ~ ~11:lturai ;tcoustic .. tittity c h ; ~ r ; ~ c t e r i z by ~ l i~ ~~ ~: ~I l\ I ~ :111:1ixsis ~ I ~ I I ;lre'\vy prh~alis. emission process and for piit~inj; l)ri~ctical\,due Fro111t l l r l er rl will ~ sig. available information. These teclltli~~ues include ~~lell~cnls iog. 1)ut it retll:tit>s I 0 1%. l ~ ~ ~ l ~ ~ r\ ~l ~l l lic~t ~~thy t~ific:ts~tly ~llfvct1 1 1 X ~ J~ ~)r:tctic:~l i ~ c ~ ~ ~eiiIissi1111 t s t i c t s t i t ~ gis lor cllaracterizing tile aruustic e1nissi1111 ~ W I I i)artio~lar I 111:tterials a t ~ dprocesses; methods fur eli~llinntiojinoise; I i ~ r p < , r k w ~ ~ ~ e ~ l . Tile t e c l ~ t ~ o 1:tclis l o ~ ~~ni\.ers:~l rvz~t~ersarks lor tile de. checking wave propagation properties of engioeeri~lgstntcscripti1111of ~oaterkllet~~issi\ities and the interpretakion or tures and applying the results to test deign; lor loadi~lgtlvzt structural test data. T l ~ e r ris a w ~ ~ s t a need a t to improve inwill optimilz the acoustic e~nissio~i data ftnm a structure strumentation p r f o n l ~ a n c c111d noise rejection t e c l ~ n i ~ u e s without causing appreciable damage; for louting acuustic 3s acoustic enlission is pressed illlo service ill tougller entiemission sources, either mugllly or precisely; methods of mnments and more demanding applintions. 6 d e accepdata analysis and presentation; and rnet11od.s for acceptance. tance is continuing b11t slmvly. Perl~apsmost ofall, there is a rejection or further inspection of the test stmdure. major need to useft11 inlonnation in assimilable The field olacoustic emission testing is still growing n g form to the many nonspecialists who 11ak a use for acoustic orously and presents many challenges. Signifiunt wearc11 emission testing but find the subject difTicult to ap roach uestions are still unanswered. The mathematid theory of This wlurne or the Notldcstntdiu: Tiiity: IfondbmEis one %e acoustic emission source llas been developed beginning way of satisfying tliat in~l)ortantneed. in the inid 1970s and tile pra~iicalapplication olthir; theor?l
VISUAL TESTING 1
INTRODUCTION
The oldest and most commonly used NDE method is Visual Testing (VT). It may also be the least understood and least effectively used of all methods. . There is a difference between just looking at an object and really seeing it through a trained eye. VT may be defined as "an examination of an object using the naked eye, alone or in conjunction with various magnifying devices, without changing, altering, or destroying the object being examined." In VT the most important tools are the ones you were born with, your eyes. Visual acuity is of prime importance to the visual examiner. According to recent stati~tics, at least fifty percent of the American population over twenty years of age are required to wear some type of corrective lenses. However, in the early stages of eyesight failure, either many persons are not aware that they need corrective lenses or they just do not wear them. As with any sensitive tool, the most important tools in visual examination must b e checked for accuracy at regular-intervals to ensure that they remain accurate and sensitive. Most standards require that visual examiners have annual eye examinations to check: Near vision acuity, Far vision acuity, and Color perception. Although the eyes are the most important tool, in many situations they are not sensitive enough, not accurate enough, or cannot get to the area to be examined. In those cases, the use of optical aids is necessary in order to complete the visual examination. 2 BASIC PROCEDURE
VT is the observation, either directly or indirectly, o f ' a specimen by an . . . . . examiner .in -.such 'a .fashio.&' as 30 determine :the .piesence or,absknce of. swiqde:'. . . disr5ohti"uities br irregularities: VT should .be the' first NDE &thod td' be applied to a specimen. Other NDE methods may or may not be required after VT. The procedure is usually quite simple: '
1.
2. 3.
Prepare Surface Assure adequate illumination Observe
Visual Testing is composed of the following six basic elements that interact with each other, with each affecting the end results: 1. 2. 3. 4.
5. 6.
2.1
The examiner The test object Illumination of the test object Optical aids Mechanical aids (measuring devices) Recording method Examiner
The visual examiner must be competent. Many specifications and codes require that visual examiners be qualified through formalized training programs and on-the-job experience, and certified to ensure their competency. SNT-TG1A (1988 edition).'has included VT as a recognized NDT method and recommends that an indiiidual with a high school education have a minimum of 24 hours formal training and at least 3 months on-the-job experience in order to qualify as a Level 11. 2
Jest Obiect
The test object's size, shape, and surface condition are important in determining what optical aids and mechanical tools need to be used to complete the examination, and what illumination will be required. Some of the test object factors to be considered include the following:
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Size of the test object; Configuration of the test object; Accessibility of areas; .. . . .. : Dire~tioh. . d f . view: :. ... . . Surface reflectivity; and Discontinuity type, size, and shape. ',:
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Illumination
2.3
lllurnination of the examination surface is extremely important for the effectiveness of any VT. lllurnination is usually measured in footcandles. A footcandle is the amount of light given off by a candle at a distance of one foot from the eye. A standard 100 watt incandescent bul6 provides about five footcandles at a distance of five feet. Some specifications establish minimum light intensity for VT while others only specify "adequate illumination." Adequate illumination levels for different types of examinations are referenced in some standards and specifications.
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An example of one means of establishing illumination levels is"the requirement of being able to resolve a 1/32-inch black line on an 18 percent neutral gray card when held within 24 inches of the eye, at an angle of not less than 30 degrees to the surface of the card. Figure 1 illustrates this method. .2.4
.
Qotical Aids
Optical aids that may be used in visual examination include the following: Mirrors Magnifiers Borescopes Fiberscope Mirrors provide the examiners with the ability to look 2.4.1 Mirrors. inside castings, pipes, threaded and bored holes, and around corners. The mirrors most commonly used in VT include the dental mirror and the pivoting end mirror.
2.4.2 fvlaanifiers. Magnifiers are used as an aid in almost every type .of . . VT: to b'rjng - out:sinalt details .and .for close 'ex.amiriati.on of discontinu[ties. :The.. magnifiers most commonly used in visual examinations include t i & fbllowing:
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Single lens magnifier, Headband magnifier, Pocket comparator, and Eye loupe magnifier. The single lens magnifier is normally used when 1 . 5 X to 10X .' magnification is needed. It usually is a single bi-convex lens 1 to 3 inches in diameter mounted in a holder. The headband magnifier (which is a pair of lenses in a frame attached to a headband) is normally used for fine detailed examination of small objects because it leaves the hands free for manipulating the object. The pocket comparator measuring magnifier is a hand-held, double- lens magnifier that may be from 7X to 20X, with any one of several available scales or reticles, that allows measurements in inches, millimeters, angles, and circle diameters. The eye loupe magnifier, similar to a pocket comparator magnifier without the measuring reticule, is usually attached to a headband or a clip that attaches to regular eyeglasses. The loupe, with a magnifying lens from 5X to 30X, is normally used when extremely fine detail is required for examination of small items. Borescopes and F i b e r s c o ~ e ~Borescopes . and f i b e r k p e are used 2.4.3 for examining pipes and tubes, deep holes, long bores, pipe bends, and other internal surfaces that cannot be viewed by direct viewing because of inaccessibility. Borescopes come in many sizes from tiny needle-like instruments to large instruments 6 inches in diameter and 100 feet long. Most borescopes are equipped with a light source near the tip to illuminate the area being examined, as illustrated in figure 2A. The fiberscope is a flexible instrument used when access to the surface to be examined is such that the examination instrument must go around corners and curves. The fiberscope is made up of a bundle. of numerous very fine glass fibers that transmit light... .Fiberscopes. provide. a .light 'source through the tip tb illuminate the area of interest (also 'i~lustratkdin figure 26).
Mechanical Aids
2.5
To early man, measurements were related to different body part sizes, even though they were not standard. For example, a cubit was the length of a man's arm from the tip of the elbow to the tip of the middle finger. Depending upon the length of his arm, Noah's ark, which was 300 cubits long, could have' been from about 400 feet to about 500 feet long. Figure 3 illustrates this variation. Since Noah's time, man has been improving the science of measurement (metrology). Today many different measuring instruments are used by the VT examiner, some of which are very simple devices such as the &inch scale while others ar.e more complex precision measuring devices. The following are .., examples of measuring devices used by the VT examiner: Steel rules Vernier calipers Dial indicating calipers Micrometers (OD, ID, and depth) Dial indicators Combination squares Thread pitch gages. Thickness gages Levels Weld gages (Fillet, Palmgren, Hi-Lo) Steel rules are available in a wide variety of Steel Rules. sizes and graduations to suit specific needs. The most popular is the 6-inch and inch,. and 0.01 and 0.1 inch, as shown in rule with gradation of figure 4-4. Steel tapes are available in lengths up to at least 100 feet and are reasonably accurate for non-precision measurements of long parts.
2.5.1
.2.5.2 Vernier Caiioers. Vernier calipers are more precise measuring devices than rules because they allow measurements to the thousandth of an inch. Vernier calipers .are available i n standard lengths from 6 inches to 48 inches (see figure 5).
'
..
2.5.3 Dial lndicatina Calioers. Dial indicating calipers are very similar to the vernier calipers, have gradations on the bar and a dial indicator is used 6) to indicate the precision rather than the vernier plate (see figure measurement (thousandth of an inch). 2.5.4 Micrometers. Micrometers allow the examiner to obtain measurements within 0.0005 inch with an accuracy of 0.0001 inch. Micrometers are available in a variety of types and sizes, to enable the examiner to make OD or length measurements, ID measurements. or depth measurements. Figure 7 illustrates a standard micrometer.
2.5.5 Dial Indicators. Dial indicators are the most commonly used measuring devices for VT examinations. The dial indicator is an instrument consisting of graduated dial, an indication hand, a contact point attached to a spindle, and an amplifying mechanism. The dials, which are graduated"to indicate at least 0.001 inch, are generally used with a base stand having an adjustable arm or a magnetic base stand with an adjustable post and arm (see figure 8). 2.5.6 Combination S a w . The combination square set consists of a blade (a 12-inch steel rule), and ttiree interchangeable heads: a square head, a center head, and a protractor head. W h e n equipped with the square head, the tool can be used as a depth gage, a height gage, or a scribing gage, and also for checking if surfaces are plumb andlor square. When equipped with the center head, it is useful i n locating the center of round stock; when equipped with the protractor head, it becomes a bevel protractor and permits measurement of angles. Figure 9 illustrates a combination square set. B r e a d Pitch Gaaez. Thread pitch gages are used to determine the 2.5.7 number of threads per inch and the thread pitch on screws, bolts, nuts, pipe, and other threaded parts (see figure 10). The teeth on the various leaves of the thread pitch gage, which correspond to the standard thread forms, are used like a profile gage. Thickness Gases. Thickness gages such as bevel protractors are used for gaging clearance between objects such as bearing clearance, gear play, pipe-pipe flange clearance, or gaging narrow slots. Commonly called feeler gages, they are available in sets that contain leaves ranging in thickness from 0.0015 to 0.200 inch. 2.5.8 .
Levels. Levels are tools designed for use in determining whether a plane or surface is truly horizontal or vertical. Some levels are calibrated to indicate the angle on inclination in degrees in relation to a horizontal or 11). vertical surface (see figure 2.5.9
Weld Gaoes. Weld gages come in a variety of designs either for 2.5.10 general purpose or for specific detail gaging. Some of the weld gages can be used to make quantitative measurements while others are used for go-no-go judgment only. Figure 12 illustrates a Palmgren weld gage. Figure 13 shows how a fillet weld gage is used to measure a convex weld. A fillet weld gage can also be used to measure the size of fillet welds and concave conditions (see .. figure 14). The Hi-lo welding gage (figure 15A) can provide measurements of internal alignment on the inside after fit-up, pipe wall thickness after alignment, length between scribe lines, root opening, 37112' bevel, fillet weld leg size, and reinforcement on butt welds. The Deerman Hi-lo'gage (figure 158) has functions similar to the Hi-lo welding gage but it is mdie applicable to small diameter pipe. This gage can provide measurement of inside diameter mismatch after fit-up, root opening, undercut and pit depth, weld reinforcement height and outside diameter offset.
The information gathered from the VT examination may be recorded either as a hard copy or by the subjective method. . The hard copy method produces a visual record by means of a photograph, videotape, or movie film. This method permits comparison of the present condition to a set of standards or to previously recorded conditions to determine what, if any, changes have taken place. Eye fatigue is minimized and corrections .. for differences in individuals' visual acuity can easily be accomplished. The hard copy provides more objective data and therefore a higher degree of accuracy.
The subjective method is used when the visual examiner makes an immediate decision based solely on what he or she sees and an interpretation of what is seen. Although this is the most commonly used method of data recording, it makes standardization difficult as it relies heavily on the visual examiner's memory. visual acuity, and competence. Therefore, the degree of accuracy is less than when data is recorded on hard copy.
3 CONCLUSION
In summary, VT is the oldest, the first recorded, and the most commonly used NDE method. It requires a high degree of training and skill on the part of the visual examiner and should always precede any other NDE method to be applied.
-
Simplicity Speed Low cost (usually) Extensive training usually not necessary Minimal equipment needed Can be performed while specimen is in use
3.2 Limitations Only surface conditions can be detected or measured Poor or variable resolution of eye Fatigue Distractions Some equipment is expensive
,
oper Viewing Ang
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Basic Test for Adequate Illumination
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BORESCOPE
RUBBEREYECUP
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STEEL SHEATH DIRECTION OF VIEW FIELDS OF VIEW FORE-OBLIQUE
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3.
Early Modes of Measurement
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INSIDE MEASL'=EMEN
MEASURING LENGTH OF SHOULDER CW TURNED RlER LOCK
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VERNIER R A T E ADJUSTAELE JAW
OWSIDE READING 1.4323.
Figure
5.
Vernier Caliper
INSIDE MEASURING CONTACTS A
DEPTH ROD c.
ADJUSTING
BEZEL CLAMP
EMXIS W E G . BAR GAAWATIONS .1W: DWINWCATOA .MI' GWUXIAnON. .lW PWREVOCmON
METRIC:W E 1 3 mm BAR GAAWATIONS 2 mm, WAClNMCATOA 0.02 M M GRADUATION. 2 mm W E PER REVOLUTION
MEASURING CONTACTS
OUTSIDE MEASUREMENT
Figure
INSIDE MEASUREMENT
6.
DEPTH MUSUREMEN1
dial Indicating Calipers
Outside Micrometer
Figure
7.
Micrometer
Figure
8.
Dial Indicator
LOCATINGCENTER . OF ROUND WORKPIECE
PROTRACTOR HEAD
CHECKING OUTSIDE SOUARENF
Figure
9.
combination Square Set
GAGING SINGLE PITCH OCTERIOR THREAD
GAGING INTERNALTHREAD
Figure
10. Thread Pitch Gages
.
.
.
. .
Figure
11.
Levels
To determine the size of the convex fillet weld
To check the permissible tolerance of convexity
Figure
To determine the size of a concave fillet weld
To check the permissible tolerance of reinforcement
12. Palrngren Weld Gage
Figure
13.
Measuring Convex Fillet Weld Size
Figure
14.
Measuring Concave Fillet Weld Size
1
118" Mismatch
WELD HEIGHT GAGE
PIT DEPTH GAGE
OUTSIDE HI-LO GAGE
Internal Misalignn Fit-Up or Alignment
A.
B. Deerman Hi-lo Gage
Hi-lo Welding Gage
Figure
15. Weld Gages
C H A P T E R 7: C O M P A R I S O N A N D S E L E C T I O N O F N D T P R O C E S S E S TABLE OF CONTENTS Paragraph
Page
..................... ............. ..........
GENERAL METHOD IDENTIFICATION NDT DISCONTINUITY SELECTION DISCONTINUITY CATEGORIES DISCONTINUITY CHARACTERISTICS AND METALLURGICAL ANALYSIS NDT METHODS APPLICATION AND LIMITATIONS BURST COLD SHUTS FILLET CRACKS (BOLTS) GRINDING CRACKS CONVOLUTION CRACKS HEAT-AFFECTED ZONE CRACKING HEAT-TREAT CRACKS SURFACE SHRINK CRACKS THREADCRACKS TUBING CRACKS HYDROGENFLAKE HYDROGEN EMBRITTLEMENT INCLUSIONS INCLUSIONS LACK OF PENETRATION LAMINATIONS LAPS AND SEAMS LAPS AND SEAMS MICROSHRINKAGE GAS POROSITY UNFUSED POROSITY STRESS CORROSION HYDRAULIC TUBING MANDREL DRAG SEMICONDUCTORS HOT TEARS INTERGRANULAR CORROSION
............ ............ ... ...................... ................... .............. ................ .............. ......... ............... ............. ................. .................. ................ ............ .................... ......................
.............. ................... ................. ................. ................. .................. ................ ................ ............... ..................
.................. .................... ...........
LIST OF FIGURES Page
Figure 7-1 7-2 7-3 7-4 7-5 7-6 7-7 7-8 7-9 7-10 7-11 7-12 7-13 7-14 7-15 7-16 7-17 7-18 7-19 7-20 7-21 7-22 7-23 7-24 7-25 7-26 7-27 7-28 7-29 7-30 7-31 7-32
Liquid Penetrant Test . . . . . . . . . . . . . Magnetic Particle Test Ultrasonic Test Eddy Current Test Radiographic Test Burst Discontinuities Cold Shut Discontinuities Fillet Crack Discontinuity Grinding Crack Discontinuity Convolution Crack Discontinuities Heat-Affected Zone Cracking Discontinuity Heat-Treat Crack Discontinuities Surface Shrink Crack Discontinuities Thread Crack Discontinuities Tubing Crack Discontinuity Hydrogen Flake Discontinuity Hydrogen Embrittlement Discontinuity Weldment Inclusion Discontinuities Wrought Inclusion Discontinuities Lack of Penetration Discontinuities Lamination Discontinuities Lap and Seam Discontinuities in Rolled Threads Lap and Seam Discontinuities in Wrought Material Microshrinkage Discontinuity Gas Porosity Discontinuity Unfused Porosity Discontinuity Stress Corrosion Discontinuity Hydraulic Tubing Discontinuities Mandrel Drag Discontinuities Semiconductor Discontinuities Hot Tear Discontinuities Intergranular Corrosion Discontinuity
...
............... ................... ................. .................. ................ ............... .............. ............. ........... ...... ........... ......... ............. ..............
............. ......... .......... ........... .......... .............. ..... .... ............. .............. ............ ............ ........... ............. ............ ............... .........
7-3 7-4 7-4 7-4
I
I .a
7-5
7-9 7-11 7-13 7-15 7-18 7-20 7-22 7-25 7-27 7-29 7-32 7-34 7-36 7-38 1-40 7-43 7-45 7-47 7-49 7-52 7-54 7-56 7-57 7-59 7-61 7-64 7-66
I
I
I
1
CHAPTER 7: COMPARISON A N D SELECTION O F N D T PROCESSES 700
GENERAL
mjs chapter summarizes the characteristics of various types of disconti-
*uities, and lists the NDT methods that may be employed to detect each type of discontinuity. m e relationship between the various NDT methods and their capabilities and limitations when applied to the detection of a specific discontinuity is shown. Such variables as type of discontinuity (inherent, process, or service), manufacturing processes (heat treating, machining, welding, grinding, or plating), and limitations (metallurgical, structural, or processing) also he@ in determining the sequence of testing and the ultimate selection of one test method over another. 701
METHOD IDENTIFICATION
Figures 7-1 through 7-5 illustrate five NDT methods. Each illustration shows the three elements involved in all five tests, the different methods in each test category, and tasks that may be accomplished .with a specific method. ELEMENT
PROCEDURE
TASK -
Figure 7-1. Liquid Penetrant Test 702
NDT DISCONTINUITY SELECTION
The discontinuities that are discussed in paragraphs 706 through 732 are only some of the many hundreds that are associated with the various products of today's industry.' During the selection of discontinuities for inclusion in this chapter, only those discontinuities which would not be radically changed under different conditions of design, configuration, standards, and environment were chosen.
'
ELEMENT
r
TASK 1
DRY VISIBLE TESTING
PERSONNEL
=-t EIY
AND NEAR-SURFACE DISCONTINUITIES
TECHNIQUES
WET VISIBLE TESTING
EQUIPMENT
WET FLUORESCENT TESTING
Figure 7-2. ELEMENT
-
PROCEDURE
I
Magnetic Particle Test
PROCEDURE
PERSONNEL DETERMINE
THRU TRANSMISSION
1
SPECIALIZED APPLICATIONS
I
Figure 7-3. Ultrasonic Test ELEMENT
PROCEDURE
PERSONNEL
a El-' TECHNIQUES
MANUAL COATING AND PLATING
EQUIPMENT
Figure 7-4.
Eddy Current Test
TASK
ELEMENT
PROCEDURE
TASK
DETECT DISCONTINUITIES TECHNIQUES
X-RAY TESTINGFILM
DETERMINE BOND EQUIPMENT
GAMMA RAY FILM TESTING
SPECIALIZED APPLICATIONS
Figure 7-5. Radiographic Test 703
DISCONTINUITY CATEGORIES
Each of &e specific discontinuities are divided into three general categories: inherent, processing, and service. Each of these categories is further classified as to whether the discontinuity is associated with ferrous or nonferrous materials, the specific material configuration, and the manufacturing processes if applicable. 1.
Inherent Discontinuities
Inherent discontinuities h e those discontinuities that are related to the solidification of the molten metal There are two types.
2.
a.
Wrought. Inherent wrought discontinuities cover those discontinuities which are related to the melting and original solidification of the metal or ingot.
b.
Cast. Inherent cast discontinuities are those discontinuities which are related to the melting, casting, and solidification of the cast article. It includes those discontinuities that would be inherent to manufacturing variables such as inadequate feeding, gating, excessively high pouring temperature, entrapped gases, handling, and stacking.
Processing Discontinuities
F'rocessing discontinuities are those discontinuities that are elated to the various manufacturing processes such as machining, forming, extruding, rolling, welding, heat treating, and plating.
3.
Service Discontinuities
Service discontinuities cover those discontinuities that are related to the various service conditions such as stress corrosion, fatigue, and wear. 704
DISCONTINUITY ANALYSLS
CHARACTERISTICS AND
METALLURGICAL
"Discontinuity characteristics," as used in this chapter, encompasses an analysis of specific discontinuities and references actual photos that illustrate examples of the discontinuity. The discussions cover:
705
1.
a.
Origin and location of discontinuity (surface, near surface, or internal).
b.
Orientation (parallel or normal to the grain).
c.
Shape (flat, irregularly shaped, or spiral).
d.
Photo (micrograph discontinuity).
e.
Metallurgical analysis (how the discontinuity is produced and a t w h a t stage of manufacture).
andlor
typical overall view
of
the
NDT METHODS APPLICATION AND LIMITATIONS General
The technological accomplishments in the field of nondestructive testing have brought test reliability and reproducibility to a point where the design engineer may now seiectively zone the qecific article. Zoning is based upon the structural application of the end product and takes into consideration the environment as well as the loading characteristics of the article. Such an evaluation in no way reduces the end reliability of the product, but evaluation does reduce needless rejection of material that otherwise would have been acceptable.
Just as the structural application within the article varies, the allowable discontinuity size will vary depending on the configuration and method of manufacture. For example, a die forging that ;has large masses of material and extremely thin web sections WOUIC not require the same level of acceptance over the entire forging. 'Re forging can be zoned for rigid
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The nondestructive testing specialist must also select the method which will satisfy the design objective of the specific article and not assume that all NDT methods can produce the same reliability for the same type of discontinuity. 2.
Selection of the NDT Method
In selecting the NDT method for the evaluation of a specific discontinuity keep in mind that NDT methods may supplement each other and that several NDT methods may be capable of performing the same task. The selection of one method over another is based upon such variables as: a.
Type and origin of discontinuity
b.
Material manufacturing processes
c.
Accessibility of article
d.
Level of acceptability desired
e.
Equipment available
f.
Cost.
A planned analysis of the task must be made for each article requiring NDT testing. The NDT methods G t e d for each discontinuity in paragraphs 706 through 732 are in order of preference for that particular discontinuity. However, when reviewing the discussions, it should be kept in mind that rapidly developing new techniques in the NDT field may alter the order of test preference. 3.
Limitations
The limitations applicable to the various NDT methods will.vary with the applicable standard, the material, and the service environment. Limitations not only affect the NDT method but, in many cases, &o affect the structural reliability of the test article. For these reasons, limitations that are listed for one discontinuity may also be applicable to other discontinuities under slightly different conditions of material or environment. In addition, the many combinations of environment, location, material, and test capability do not permit mentioning all limitations that may be associated with the problems of locating a specific discontinuity. The intent of this chapter is fulfilled if you are made aware of the many factors that influence the selection of a valid NDT method. 7-7
706
BURST
1.
Categorx. Processing
2.
Material. Ferrous and Nonferrous Wrought Material
3.
Discontinuity Characteristics
Surface or internal. Straight or irregular cavities varying in size from wide open t o very tight. Usually parallel with the grain. Found in wrought material that required forging, rolling, or extruding. (See Figure 7-6.) 4.
5.
Metallurgical Analysis a.
Forging bursts are surface or internal ruptures caused by processing at too low a temperature, excessive working, or metal movement during forging, rolling, or extruding operation.
b.
A burst does not have a spongy appearance and, therefore, is distinguishable from a pipe, even when it occurs a t the center.
c.
Bursts are often large and are very seldom healed during subsequent working.
NDT Methods Application and Limitations a.
b.
Ultrasonic Testing Method. (1)
Normally used for the detection of internal bursts.
.(2)
Bursts are definite breaks in the material and resemble a crack, producing a very sharp reflection on the scope.
(3)
Ultrasonic testing is capable of detecting varying degrees of burst, a condition not detectable by other NDT methods.
(4)
Nicks, gouges, raised areas, tool tears, foreign material, or gas bubblei on the article may produce adverse ultrasonic test results.
Eddy Current Testing Method. Not normally used. Testing is restricted to wire, rod, and other articles under 0.250 inch (6.35 mm) diameter.
A FORGING EXTERNAL BURST
B. BOLT INTERNAL BURST
..,,. . '.. . ,: ..
.
C. ROLLED BAR INTERNAL BURST
D. FORGED BAR INTERNAL BURST
Figure 7-6.
Burst Discontinuities
c.
Magnetic Particle Testing Method.
(1)
Usually used on wrought ferromagnetic material in which the burst is open to the surface or has been exposed t o the surface.
(2)
Results are evaluation.
limited
to surface and near
surface
d.
LiquidtPenetrant Testing Method. Not normally used. When fluorescent penetrant is to be applied to an article previously dye penetrant tested, all traces of dye penetrant should first be removed by prolonged cleaning in applicable solvent.
e.
Radiographic Testing Method. Not normally used. Such variables as the direction of the burst, close interfaces, wrought material, discontinuity size, and material thickness restrict the capability of radiography.
1.
Category. Inherent
2.
Material. Ferrous and Nonferrous Cast Material
3.
Discontinuity Characteristics
Surface and subsurface. Generally appear as smooth indentations on the cast surface resembling a forging lap. (See Figure 7-7.) 4.
Metallurgical Analysis
Cold shuts are produced during casting of molten metal. They may result from splashing, surging, interrupted pouring, or the meeting of two streams of metal coming from different directions. Cold shuts are also caused by the solidification of one surface before other metal flows over it, the presence of interposing surface films on cold, sluggish metal, or any factor that prevents fusion where two surfaces meet. Cold shuts are more prevalent in castings formed in a mold having several sprues or gates.
A SURFACE COLDSHUT
8. lNTERNAL COLD SHUT
C. SURFACE COLD SHUT MICROGRAPH
Figure 7-7. Cold 'shut Discontinuities 5.
NDT Methods Application and Limitations a.
Liquid Penetrant Testing Method. (1)
Normally used to evaluate surface cold shuts in both ferrous and nonferrous materials.
(2)
Indications appear as a smooth, regular, continuous or intermittent line.
(3)
Liquid penetrants used to test nickel base alloys, certain stainless steels, and titanium should not exceed 1% sulfur or chlorine.
(4)
Certain castings may have surfaces that are blind and from which removal of excess penetrant may be difficult.
(5)
b.
e.
The geometric configuration (recesses, orifices, and flanges) of a casting may permit buildup of wet developer thereby masking any detection of a discontinuity.
Magnetic Particle Testing Method. (1)
Normally used for the evaluation of ferromagnetic materials.
2
The metallurgical nature of 431 corrosion-resistant steel is such that, in some cases, magnetic particle testing indications are obtained which do not result from a crack or other harmful discontinuities. These indications arise from a duplex structure within the material, wherein one portion exhibits strong magnetic retentivity and the other does not.
Radiographic Testing Method. (1)
Cold shuts are normally detectable by radiography while testing for other casting discontinuities.
(2)
Cold shuts appear as a distinct dark line, or band, of variable length and width, and definite smooth outline.
(3)
h e casting configuration may have inaccessible a r e a . that can only be tested by radiography.
708
d.
Ultrasonic Testing Method. Not recommended. Cast structure and article configuration do not, as a general rule, lend themselves to ultrasonic testing.
e.
Eddy Current Testing Method. Not recommended. Article configuration and inherent material variables restrict the use of this method.
FILLET CRACKS (BOLTS)
1.
Category. Service
2.
Material. Ferrous and Nonferrous Wrought Material
3.
Discontinuity Characteristics
Surface. Located a t the junction of the fillet with the shank of the bolt and progressing inward. (See Figure 7-8.)
A FILLET FATIGUE FAILURE
8. F R A C N R E AREA OF (A) SHOWING
TANGENCY POINT OF FAILURE
C. CROSSSECTIONAL AREA OF FATIGUE CRACK I N FILLET SHOWING TANGENCY POINT IN RADIUS
I
Figure 7-8. Fillet Crack Discontinuity 4.
Metallurgical Analysis
Fillet cracks occur where a marked change in diameter occurs, such a s a t the head-toshank junction where stress risers are created. During the service rife of a bolt repeated loading takes place whereby the tensiIe Ioad fluctuates in magnitude due to the operation of the mechanism. These tensile loads can cause fatigue failure starting a t the point where the stress risers occur. Fatigue failure, which is surface phenomenon, starts a t the surface and propagates inward. 5.
I
I
NDT Methods Application and Limitations
a.
1
Ultrasonic Testing Method. (I)
Used extensively for service associated discontinuities of this type. 7-13
b.
c.
d.
e.
-
*.
(2)
A wide selection of transducers and equipment enable onthespot evaluation for fillet crack.
(3)
Since fillet cracks are a definite break in the material, the scope pattern will be a very sharp reflection. (Propagation can be monitored by using ultrasonics.)
(4)
Ultrasonic equipment has extreme sensitivity, and established standards should be used to give reproducible and ' reliable results.
1
Liquid Penetrant Testing Method. (1)
Normally used troubleshooting.
during
inservice
overhaul
or
(2)
May be used for both ferromagnetic and nonferromagnetic bolts, although usually confined to the nonferromagnetic.
(3)
Fillet cracks appear as sharp, clear indications.
(4)
Structural damage may result f k m exposure of highstrength steels t o paint strippers, alkaline mating removers, deoxidizer solutions, etc.
(5)
Entrapment of penetrant under fasteners, in holes, under splices, and in similar areas may cause corrosion due t o the penetrant's affinity for moisture.
II
Magnetic Particle Testing Method. (1)
Only used on ferromagnetic bolts.
(2)
Fillet cracks appear as sharp clear indications with a heavy buildup.
(3)
Sharp fillet areas may produce nonrelevant magnetic indications.
(4)
17.7 pH steel is only slightly magnetic in the annealed condition, but becomes strongly magnetic after heat treatment, when it may be magnetic particle tested.
Eddy Current Testing Method. Not normaIIy used for detection of fillet cracks. Other NDT methods are more compatible to the detection of this type of discontinuity. Radiographic Testing Method. Not normally used for detection of fillet cracks. Surface discontinuities of this type would be difficult to evaluate due to size of crack in relation to the thickness of material.
!
i !
I
709
GRINDING CRACKS
1.
Category. Processing
2.
Material. Ferrous and Nonferrous
3.
. Discontinuity Characteristics
surface.' Very shallow and sharp a t the root. Similar t o heat-treat crack and usually, but not always, occur in groups. Grinding cracks generaus occur a t right angles to the direction of grinding. They are found in highlj heat-treated articles, chrome plated, case hardened, and ceramic materials that are subjected to grinding operations. (See Figure 7-9.)
A TYPICAL CHECKED GRINDING CRACK PATTERN
0. GRINDING CRACK PATTERN NORMAL TO GRINDING
C. MICROGRAPH OF GRINDING CRACK
I I
!
Figure 7-9. Grinding Crack Discontinuity
4.
Metallurgical Analysis
Grinding of hardened surfaces frequently introduces cracks. These thermal cracks are caused by local overheating of the surface being ground. The overheating is usually caused by lack of, or poor, coolant; a dull, or improperly ground, wheel; too rapid feed; or too heavy cut. 5.
NDT Methods Application and Limitations a.
b.
Liquid Penetrant Testing Method.
(1)
Normally used on both ferrous and nonferrous materials for the detection of grinding cracks.
(2)
Liquid penetrant indication will appear as irregular, checked, or scattered pattern of fine lines.
(3)
Grinding cracks are the most difficult discontinuity to indicate and require the longest penetration time.
4
Articles that have been deareased may still have solvent entrapped in the discontinuity and should be allowed sufficient time for evaporation prior t o the application of the penetrant.
Magnetic Particle Testing Method.
(1)
Restricted to ferromagnetic materials.
(2)
Grinding cracks generally occur at right angles t o grinding direction, although in extreme cases a complete network of cracks may appear, in which case they may be parallel to the magnetic field.
(3)
Magnetic sensitivity decreases as the size of grinding crack decreases.
c.
Eddy Current Testing Method. Not normally used for detection of grinding cracks. Eddy current equipment has the capability and can be developed for a specific nonferrous application.
d.
Ultrasonic Testing Method. Not normally used for detection of grinding cracks. Other forms of NDT are more economical, faster, and better adapted to this type of discontinuity than ultrasonics.
! i
I i
e.
710
Radiographic Testing Method. Not recommended for detection of grinding cracks. Grinding cracks are too tight and small. Other NDT methods are more suitable for detection of grinding cracks.
CONVOLUTION CRACKS
1.
Category. Processing
2.
Material. Nonferrous
3.
Discontinuity characteristics
Surface. Range in size from microfractures t o open fissures. Situated on the periphery of the convolutions and extend longitudinally in direction of rolling. (See Figure 7-10.) 4.
Metallurgical Analysis
The rough "orange peel" effect of convolution cracks is the result of either a forming operation that stretches the material or from chemical attack such as pickling treatment. The roughened surface contains small pits that form stress risers. S-dbsequent service application (vibration and flexing) may introduce stresses that a c t on these pits and form fatigue cracks as shown in Figure 7-10.
5.
NDT Methods Application and Limitations a.
Radiographic Testing Method. (1)
Used extensively for this type of failure.
(2)
The configuration of the article and the location of the discontinuity limits detection almost exclusively to radiography.
(3)
Orientation of convolutions t o X-ray source is very critical since those discontinuities that are not normal to X-ray may not register on the film due to the small change in density.
(4)
Liquid penetrant and magnetic paGticle testing may supplement but not replace radiographic and ultrasonic testing.
C.
A WPlCALCONVOLUTlON DUCTING
8. CROSSSECTION OF CRACKED CONVOLUTION
HIGHER MAGNIFICATION OF CRACK SHOWING ORANGE PEEL
D. MICROGRAPH OF CONVOLUTION WITH PARTIAL CRACKING O N SIDES
Figure 7-10. Convolution Crack Discontinuities (5)
b.
The type of marking material (e.g., grease pencil on titaniux) used to identify the area of discontinuities may affect the structure of the article.
Ultrasonic Testing Method. Not normally used for the detection of convolution cracks. The configuration of the article (doublewalled convolutions) and the prescence of internal micro fractures are all factors that restrict the use of ultrasonics.
711
c.
Eddy Current Testing Method. Not normally used for 5+ detection of convolution cracks. As in the case of u k E ~ c i c testing, the configuration does not lend itself to this m e t M ?f testing.
d.
Liquid Penetrant Testing Method. Not recommended for t* detection of convolution cracks. Although the discontinuitis are surface, they are internal and are superimposed over exterior shell which creates a serious problem of entrapment.
e.
Magnetic Testing Method. nonferrous.
Not
. applicable.
Materid
HEAT-AFFECTED ZONE CRACKING
1.
Category. Processing (Weldments)
2.
Material. Ferrous and Nonferrous
3.
Discontinuity Characteristics
Surface. Often quite deep and very tight. Usually run parallel with the weld in the heat-affected zone of the weldment. (See Figure 7-11.) 4.
hietallurgical Analysis
Hot cracking of heat-affected zones of weldments increases in severity with increasing carbon content. Steels that contain more than 0.30% Carbon are prone to this type of failure and require preheating prior to welding. 5.
NDT Methods Application and Limitations a.
Magnetic Particle Testing Method. (1)
Normally used for ferromagnetic weldments.
(2)
Prod burns are very detrimental, especially on highly heat-treated articles. Burns may contribute to structural failure of article.
(3)
Demagnetization of highly heat-treated articles can be very difficult due to metallurgical structure.
A MICROGRAPH OF WELD AND HEAT.AFFECTED ZONE SHOWING CRACK. NOTE COLD LAP MASKING THE ENTRANCE OF THE CRACK
8. MICROGRAPH OF CRACK SHOWN IN (A)
Figure 7-11. Heat-Affected Zone Cracking Discontinuity
b.
(1)
Normally used for nonferrous weldments.
(2)
Material that has had its surface obliterated, blurred, or blended due to manufacturing processes should not be penetrant tested until the smeared surface has been removed.
(3)
Liquid penetrant testing after the application of certain types of chemical film coatings may be invalid due to the covering or filling of the discontinuities.
c.
Radiographic Testing Method. Not normally used for the detection of heat-affected zone cracking. Discontinuity orientation and surface origin make other NDT methods more suitable.
d.
Ultrasonic Testing Method.
i
e.
712
Liquid Penetrant Testing Method.
(1)
Used where specialized applications have been developed.
(2)
Rigid standards and procedures are required t o develop valid tests.
(3)
The configuraticn of the surface roughness (i.e., sharp versus rounded root radii and the slope condition) are major factors in deflecting the sound beam.
Eddy Current Testing Method. Although not normally used for the detection of heat-affected zone cracking, eddy current testing equipment has the capability of detecting nonferrous surface discontinuities.
HEAT-TREAT CRACKS
1.
Category. Processing
2.
Material
3.
Discontinuity Characteristics
. Ferrous and Nonferrous Wrought and Cast Material
Surface. Usually deep and forked. Seldom follow a definite pattern and can be in any direction on the part. Originate in areas with rapid change of material thickness, sharp machining marks, fillets, nicks, and discontinuities that have been exposed to the surface of the material. (See Figure 7-12.) 7-21
A FILLET AND MATERIAL THICKNESS CRACKS C'OP CENTER) RELIEF RADIUS CRACKING (LOWER LEFT)
B. HEAT-TREAT CRACK DUE TO SHARP MACHINING MARKS
Figure 7-12. Heat-Treat Crack Discontinuities 4.
Metallurgical Analysis
During the heating and cooling process, localized stresses may be set Up by unequal heating or cooling, restricted movement of the article, or unequal :rosssectional thickness. These stresses may exceed the tensile strength of the material causing it to rupture. Where built-in stress risers occur (keyways or grooves) additional cracks may develop.
5.
NDT Methods AppLication and Limitations a.
b.
Magnetic Particle Testing Method. (1)
For ferromagnetic materials, heat-treat cracks are normally detected by magnetic particle testing.
(2)
Indications normally appear as straight, forked, or curved indications.
(3)
Likely points of origin are areas that would develop stress risers, such as keyways, fillets, or areas with rapid changes in material thickness.
(4)
Metallurgical structure of age-hardenable and heattreatable stainless steels (17.4, 17.7, and 431) may produce nonrelevant indications.
Liquid Penetrant Testing Method.
(1)
Liquid penetrant testing is the recommended method for nonferrous materials.
(2)
Likely points of origin for heat-treat cracks are the same as those listed for magnetic particle testing.
(3)
Materials or articles that will eventually be used in LOX qystems must be tested with LOX compatible penetrants.
c.
Eddy Current Testing Method. Although not normally used for the detection of heat-treat cracks, eddy current testing equipment has the capability of detecting nonferrous surface . discontinuities.
d.
Ultrasonic Testing Method. Not normally used for detection of heat-treat cracks. If used, the scope pattern wiU show a definite indication of a discontinuity. Recommended wave mode would be surface.
e.
Radiographic Testing Method. Not normally used for detection of heat-treat cracks. Surface discontinuities are more easily detected by other NDT methods designed for surface application.
713
SURFACE SHRINK CRACKS
1.
Category. Processing (Welding)
2.
Material. Ferrous and Nonferrous
3.
Discontinuity Characteristics
Surface. Situated on the face of the weld, fusion zone, and base metal. Range in size from very small, tight, and shallow, to open and deep. Cracks may run parallel or transverse to the direction of welding. (See Figure 7-13.) 4.
Metallurgical Analysis
Surface shrink cracks are generally the result of improper heat application, either in heating or welding of the article. Heating or cooling in a Iocalized area may set up stresses that exceed the tensile strength of the material causing the material t o crack. Restriction of the movement (contraction or expansion) of the material during heating, cooling, or welding may also set up excessive stresses. 5.
NDT Methods Application and Limitations
'a.
Liquid Penetrant Testing Method. (1)
Surface shrink cracks in nonferrous materials are normally detected by use of liquid penetrants.
(2)
Liquid penetrant equipment is easily portable and can be used during in-process control for both ferrous and nonferrous weldments.
(3)
Assemblies that are joined by bolting, riveting, intermittent welding, or press fittings will retain the penetrant, which will seep out after developing and mask the adjoining surfaces.
(4)
When articles are dried in a hot air dryer or by similar means, excessive drying temperature should be avoided t o prevent evaporation of penetrant.
A TRANSVERSE CRACKS I N HEAT-AFFECTED ZONE
8. TYPICAL STAR-SHAPED CRATER CRACK
C. SHRINKAGE CRACK A T WELD TERMINAL
Figure 7-13. Surface Shrink Crack Discontinuities b.
Magnetic Particle Testing Method.
(1)
Ferromagnetic weldments are normally tested by magnetic particle method.
(2)
Surface discontinuities, that are parallel to the magnetic field will not produce indications since they do not interrupt or distort the magnetic field.
(3)
c.
Areas such as grease fittings, bearing races, or other similar items that might be damaged or clogged by the bath or by the particles should be masked before testing.
Eddy Current Testing Method. (1)
Normally confined to nonferrous welded pipe and tubing.
(2)..
A probe or encircling coil could be used where article configuration permits.
d.
Radiographic Testing Method. Not normally used for the detection of surface discontinuities. During the radiographic testing of weldments for other types of discontinuities, surface indications may be detected.
e.
Ultrasonic Testing Method. Not normally used for detection of surface shrink cracks. Other forms of NDT (liquid penetrant and magnetic particle) give better results, are more economical, and are faster.
I I
I
714
THREAD CRACKS
1.
Category. Service
2.
Material. Ferrous and Nonferrous Wrought Material
3.
Discontinuity Characteristics
! 11 8
Surface. Cracks are transverse to.the grain (transgranular) starting at the root of the thread. (See Figure 7-14.) 4.
Metallurgical Analysis
Fatigue failures of this type are not uncommon. High cyclic stresses resulting from vibration and/or flexing act on the stress risers created by the thread roots to produce cracks. Fatigue cracks may start as fine submicroscopic discontinuities or cracks and propagate in the direction of applied stresses.
A. COMPLETE THREAD ROOT FAILURE
!
C. MICROGRAPH OF (Al SHOWING CRACK AT BASE O F ROOT
'0. TYPICALTHREAD ROOT FAILURE
0. MICROGRAPH OF IBI SHOWING TRANS-
GRANULAR CRACK ATTHREAD ROOT
!
Figure 7-14. Thread Crack Discontinuities 5.
NDT Methods Application and Limitations a.
Liquid penetrant Testing Method. (1)
Fluorescent penetrant nonfluorescent.
is
recommended
over
(2)
Low surface tension solvents such as gasoline and kerosene are not recommended cleaners.
(3)
When applying liquid penetrant to components within an assembly or structure, the adjacent areas should be effectively masked to prevent overspraying.
b.
Magnetic Particle Testing Method. (1)
Normally used to detect cracks a t the threads on ferromagnetic materials.
(2)
Nonrelevant magnetic indications may result from the thread configuration.
(3)
Cleaning titanium and 440C stainless in halogeneated hydrocarbons may result in structural damage to the material.
'
c.
Eddy Current Testing Method. Not normally used for detecting thread cracks. The article configuration would require specialized equipment if adaptable.
d.
Ultrasonic Testing Method. Not recommended for detecting thread cracks. Thread configuration does not lend itself t o ultrasonic testing.
e.
Radiographic Testing Method. Not recommended for detecting thread cracks. Surface discontinuities are best screened by NDT method designed for the specific condition. Fatigue cracks of this type are very tight and surface connected. Detection by radiography would be extremely difficult.
715
TUBING CRACKS
1.
Category. Inherent
2.
Material. Nonferrous
3.
Discontinuity Characteristics
Tubing cracks formed on the inner surface (I.D.), parallel to direction of grain flow. (See Figure 7-15.) 4.
Metallurgical Analysis
Tubing I.D. cracks may be attributed to one or a combination of the following:
A TYPICAL CRACK ON INSIDE OF TUBING SHOWING COLD LAP
B. ANOTHER PORTION O F SAME CRACK SHOWING CLEAN FRACTURE
2.
C. MICROGRAPH OF (Bl
Figure 7-15. Tubing Crack Discontinuity a.
Improper cold reduction of the tube during fabrication.
b.
Foreign material may have been embedded on the inner surface of the tubes causing embrittlement and cracking when the cold worked material was heated during the annealing operation.
c.
Insufficient heating rate to the annealing temperature with possible cracking occurring in the 1200-1400°F (645-760°C) range.
5.
NDT Methods Application and Limitations a.
b.
Eddy Current Testing Method.
(1)
Normally used for detection of this type of discontinuity.
(2)
Tube diameters below 1 inch (2.54 cm) and wall thicknesses less than 0.150 inch (3.8 mm) are well within equipment capability.
(3)
Testing of ferromagnetic material may be difficult.
Ultrasonic Testing Method. (1)
Normally used on tubing.
(2)
A wide variety of equipment and transducers are available for screening tubing for internal discontinuities of this type.
(3)
Ultrasonic limitationri.
(4)
Certain ultrasonic contact couplants may have high sulfur content, which will have an adverse effect on high-nickel alloys.
transducers
have
varying
temperature
c.
Radiographic Testing Method. Not normally used for detecting tubing cracks. Discontinuity orientation and thickness of material govern the radiographic sensitivity. Other forms of NDT (eddy current and ultrasonic) are more economical, faster, and more reliable.
d.
Liquid Penetrant Testing Method. Not recommended for detecting tubing cracks. Internal discontinuity would be difficult t o process and interpret.
e.
Magnetic Particle Testing Method. Not applicable. Material is nonferrous under normal conditions.
16
HYDROGEN FLAKE
1.
Category. Processing
2.
Material. Ferrous
3.
Discontinuity Characteristics
lternal fissures in a fractured surface, flakes appear as bright silvery reas. On an etched surface they appear as short discontinuities. oinetimes known as chrome checks and hairline cracks when revealed by ~achining. Flakes are extremely thin and generally align parallel with the rain. They are usually found in heavy steel forgings, billets, and bars. h e Figure 7-16.) 4.
Metallurgical Analysis
lakes are internal fissures attributed to stresses produced by localized :ansformation and decreased solubility of hydrogen during cooling after ot working. Usually found only in heavy alloy steel forgings. 5.
NDT Methods Application and Limitations a.
b.
Ultrasonic Testing Method.
(1)
Used extensively for the detection of hydrogen flake.
(2)
Material in the wrought condition can be screened successfully using either the immersion or the contact method. The surface condition will determine 'the method most suited.
(3)
On the A-scan presentation, hydrogen flake will appear as hash on the screen or as loss of back reflection.
(4)
All foreign materials (loose scale, dirt, oil, grease) should be removed prioi to any testing. Surface irregularities such as ~ c k s gouges, , tool marks, and scarfing may cause loss of back reflection.
Magnetic Particle Testing Method. (1)
Normally used on finished machined articles.
(2)
Flakes appear as short discontinuities and resemble chrome checks or hairline cracks. 7-31
A 4340CMS HAND FORGING R E J E T E D FOR HYDROGEN FLAKE
8. CROSSSECTION OF IA) SHOWING FLAKECONDITION I N CENTER O F MATERIAL
Figure 7-16.
Hydrogen Flake Discontinuity
(3)
Machined surfaces with deep tool marks may obliterate the detection of the flake.
(4)
Where the general direction of a discontinuity is questionable, it may be necessary to magnetize in two or more directions.
I
'
c.
Liquid Penetrant Testing Method. Not normally used for detecting flakes. Discontinuities are very small and tight and would be difficult to detect by liquid penetrants.
d.
Eddy Current Testing Method. Not recommended for detecting flakes. The metallurgical structure of ferrous materials limits their adaptability t o the use of eddy current.
e.
Radiographic Testing Method. Not recomniended for detecting flakes. The size of the discontinuity and its location and orientation with respect to the material surface restricts the application of radiography.
HYDRQGEN EMBRITTLEMENT Category. Processing and Service Material. Ferrous Discontinuity Characteristics :ace. Small, nondimensional (interface) with no orientation or direction. nd in highly heat-treated material that was subjected to pickling and/or ing or in material exposed t o free hydrogen. (See Figure 7-17.) Metallurgical Analysis .ations such a s electroplating or pickling and cleaning prior t o electrong generate hydrogen at the surface of the material. This hydrogen trates the surface of the material creating immediate or delayed ittlement and cracking. NDT Methods Application and Limitations a.
Magnetic Particles Testing Method.
(1)
Magnetic indications appear as a fractured pattern.
(2)
Hydrogen embrittlement cracks are randomly oriented and may be aligned with the magnetic field.
(3)
Magnetic particle testing should be accomplished before and after plating.
A. DETAILED CRACK PATTERN OF HYDROGEN EMBRIVLEMENT
B. HYDROGEN
EMBRtlTLEMENT UNDER CHROME PLATE
Figure 7-17.
C.
HYDROGEN E M B R l l T L E M E N T PROPAGATED M R O U G H CHROME PLATE
Hydrogen Embrittlement Discontinuity
(4)
Care should be taken so as not to produce nonrelevant indications or cause damage to the article by overheating.
(5)
301 corrosion resistant steel is nonmagnetic in the annealed condition, but becomes magnetic with cold working.
718
b.
Liquid Penetrant Testing Method. Not normally used for detecting hydrogen embrittlement. Discontinuitites on the surface are extremely tight, small, and difficult to detect. Subsequent plating deposit may mask the discontinuity.
c.
Ultrasonic Testing Method. Not normally used for detecting hydrogen embrittlement. Article configurations and size db not, in general, lend themselves to this method of testing. Equipment has capability of detecting hydrogen embrittlement. Recommend surface wave technique.
d.
Eddy Current Testing Method. Not.recommended for detecting hydrogen embrittlement. Many variables inherent in the specific material may produce conflicting patterns.
e.
Radiographic Testing Method. Not recommended for detecting hydrogen embrittlement. The sensitivity required to detect hydrogen embrittlement is in most cases in excess of radiographic capabilities.
INCLUSIONS
1.
Categorg. Processing (Weldments)
2.
Material. Ferrous and Nonferrous Welded Material
3.
Discontinuity Characteristics
Surface and subsurface. Inclusions may be any shape. They may be metallic or nonmetallic and may appear singly or be linearly distributed or scattered throughout the weldment. (See Figure 7-18.) 4. r
I I 1
iI
Metallurgical Analysis
Metallic inclusions are generally particles of metals of different density as compared to the density of the weld or base metal. Nonmetallic inclusions are oxides, sulphides, slag, or other nonmetallic foreign material entrapped in the weld or trapped between the weld metal and the base metal.
A METALLIC INCLUSIONS
B. INCLUSIONSTRAPPED I N WELD
C. CROSSSECTION OF WELD SHOWING INTERNAL INCLUSIONS
Figure 7-18. Weldment Inclusion Discontinuities 5.
NDT Methods Application and Limitations a.
Radiographic Testing Method. (1)
This NDT metbod is universally used.
(2)
Metallic inclusions appear on the radiograph a s sharply defined, round, erratically shaped, or elongated white spots and may be isolated or in small linear or scattered groups.
(3)
Nonmetallic inclusions will appear on the radiograph as shadows of round globules or elongated or irregularly shaped contours occurring singly, linearly, or scattered throughout the weldment. They will generally appear in
the fusion zone or a t the root of the weld. Less absorbent material is indicated by a greater film density and more absorbent materials by a lighter film density. (4)
I
b.
I
c.
Foreign material such as loose scales, splatter, or flux may invalidate test results.
Eddy Current Testing Method. (1)
Normally confined to thin wall, welded tubing.
(2)
Established standards are required if valid results are to be obtained.
Magnetic Particle Testing Method. (1)
Normally not used for detecting inclusions in weldments.
(2)
Confined t o machined weldments where the discontinuities are surface or near surface.
(3)
The indications would appear jagged, irregularly shaped, individually or clustered, and would not be too pronounced.
(4)
Discontinuities may go undetected when improper contact
exists between the magnetic particles and the surface of the article. d.
Ultrasonic Testing Method. Not normally used for detecting inclusions. Specific applications of design or of article configuration, however, may require ultrasonic testing.
e.
Liquid Penetrant Testing Method. Not applicable. are normally not open fissures.
(
119
1
1.
Category. Processing
2.
Material. Ferrous and Nonferrous Wrought Waterial
3.
Discontinuity Characteristics
1:
.' <. ! I
Inclusions
INCLUSIONS
Subsurface (original bar) or surface (after machining). There are two types: one is nonmetallic with long straight lines parallel to flow lines and quite 7-37
tightly adherent. Often short and likely to occur in groups. The other type is nonplastic, appearing as a comparatively large mass not parallel t o flow lines. Found in forged, extmded, and rolled material. (See Figure 7-19.)
A TYPICAL INCLUSION PATTERN ON MACHINED SURFACES
C. MICROGRAPH OF TYPICAL
B. STEEL FORGING SHOWING NUMEROUS
INCLUSIONS
INCLUSION
Figure 7-19. Wrought Inclusion 4.
is continuities
Metallurgical Analysis
Nonmetallic inclusions (stringers) are caused by the existence of slag or oxides in the billet or ingot. Nonplastic inclusions are caused by particles remaining in the solid state during billet melting. Certain types of steels are more prone to inclusions than others.
5.
NDT Methods Applications and Limitations
a.
b.
c.
d.
Ultrasonic Testing Method. (1)
Normally used to evaluate inclusions in wrought material.
(2)
Inclusions will appear as definite interfaces within the metal. Small, clustered condition or conditions on 'different planes cause a loss in back reflection. Numerous small, scattered conditions cause excessive "noise."
(3)
Inclusion orientation in relationship to ultrasonic beam is Critical.
(4)
The direction of the ultrasonic beam should be perpendicular to the direction of the grain flow whenever possible.
Eddy Current Testing Method.
(1)
Normally used for thin wall tubing and small diameter rods.
(2)
Eddy current testing of ferromagnetic materials can be difficult.
Magnetic Particle Testing Method. (1)
Normally used on machined surface.
(2)
Inclusions will appear as a straight intermittent or as a continuous indication. They may be individual or clustered.
(3)
The magnetizing technique should besuch that a surface or near surface inclusion can be satisfactorily detected when its axis is in any direction.
(4)
A knowledge of the grain flow of the material is critical since inclusions will be parallel t o that direction.
Liquid Penetrant Testing Method. Not normally used for detecting inclusions in wrought material. Inclusions are generally not openings in the material surface.
'
e.
720
Radiographic Testing Method. Not recommended. NDT methods designed for surface testing are more suitable for detecting surface inclusions.
LACK OF PENETRATION
1.
Category. Processing
2.
~ a t e r i * Ferrous and Nonferrous Weldments
3.
Discontinuity Characteristics
Internal or external. Generally irregular and filamentary occurring a t the root and running parallel with the weld. (See Figure 7-20.)
k INADEQUATE ROOT PENETRATION
8. INADEQUATE
ROOT PENETRATION OF BUTT W E L D E O T U B E
C. INADEOUATE FILLET WELD PENETRATION KNOWN AS BRIDGING
Figure 7-20. Lack of Penetration Discontinuities
4.
Metallurgical Analysis
Caused by root face of joint not reaching fusion temperature tjefore weld metal was deposited. Also caused by fast welding rate, too large a welding rod, or too cold a bead. 5.
l
I
NDT Methods Application and Limitations a.
b.
I
I
I
I
c.
d.
Radiographic Testing Method. (1)
Used extensively on a wide variety of welded articles to determine the lack of penetration.
(2)
Lack of penetration will appear on the radiograph as an elongated, dark area of varying length and width. Lack of penetration may be continuous or intermittent and may appear in the center of the weld at the junction of multipass bends.
(3)
Lack of penetration orientation in relationship to the radiographic source is critical.
(4)
Sensitivity levels govern the capab&ty t o detect small or tight discontinuities.
Ultrasonic Testing Method.
(1)
Commonly used for specific applications.
(2)
Weldments make ultrasonic testing difficult.
(3)
Lack of penetration will appear on the scope as a definite break or discontinuity resembling a crack and will give a very sharp reflection.
Eddy Current Testing Method. (1)
Normally used to determine lack of penetration in nonferrous welded pipe and tubing.
(2)
Eddy current testing can be used where other nonferrous articles can meet the configuration requirement of the equipment.
Magnetic Particle Testing Method. (1)
Normally used where backside of weld is visible.
(2)
e.
721
Lack of penetration appears as an irregular indication of varying width.
Liquid Penetrant Testing Method.
(1)
Normally used where backside of weld is visible.
(2)
Lack of penetration appears as an irregular indication of varying width.
(3)
Residue left by the penetrant and the developer could contaminate any rewelding operation.
LAMINATIONS
1.
Catezory. Inherent
2.
Material. Ferrous and Nonferrous Wrought Material
3.
Discontinuity Characteristics
Surface and internal. Flat, extremely thin, generally aligned parallel t o the work surface of the material. May contain a thin film of oxide between the (See surfaces. Found in forged, extruded, and roIled material. Figure 7-21.) 4.
;vietallurgical Analysis
Laminations are separations or weaknesses generally aligned parallel t o the work surface of the material. They may be the result of pipe, blister, seam, inclusions, o r segregations elongated and made directional by working. Laminations are flattened impurities that are extremely thin.
5.
I
NDT Methods Application and Limitations a.
Ultrasonic Testing Method.
(1)
For heavier gauge material the geometry and orientation of lamination (normal to the beam) makes their detection limited to ultrasonic testing.
(2)
Numerous wave modes may be used depending upon the material thickness or method selected for testing. Automatic and manual contact or immersion methods are adaptable.
k LAMINATION IN 0.25 IN. 1635mml PLATE
I
C.
LAMINATION IN PLATE SHOWING SURFACE ORIENTATION
6. LAMINATION IN TITANIUM S H E R
0. LAMINATION IN 1 IN. (25.4mm) BAR SHOWING
SURFACE ORIENTATION
Figure 7-21. Lamination Discontinuities
I
I
(3)
Laminations appear as a definite interface with a loss of back reflection.
(4)
Through transmission and reflection techniques are applicable for very thin sections.
! b.
Magnetic Particle Testing Method. (1)
Articles fabricated from ferromagnetic materials are normally tested for lamination by magnetic particle testing methods. 7-43
c.
(2)
Magnetic indication will appear as a straight, intermittent indication.
(3)
Magnetic particle testing is not capable of determining the overall size or depth of the lamination.
Liquid Penetrant Testing Method.
(1)
722 1.
2. 3.
. Normally used on nonferrous materials.
(2)
Machining, honing, lapping, or blasting may smear surface of material and thereby close or mask surface lamination.
(3)
Acid and alkalines seriously limit the effectiveness of liquid penetrant testing. Thorough cleaning of the surface is essential.
d.
Eddy Current Testing Method. Not normally used to detect laminations. If used, the method must be confined to thin sheet stock.
e.
Radiographic Testing Method. Not recommended for detecting laminations. Laminations have very small thickness changes in the direction of the X-ray beam, thereby making radiographic detection almost impossible.
LAPS AND SEAMS Category. Processing . Material. Ferrous and Nonferrous Rolled Threads
Discontinuity Characteristics
Surface. Wavy lines, often quite deep and sometimes very tight, appearing
as hairline cracks. Found in rolled threads in the minor pitch, and major diameter of the thread, and in direction of rolling. (See Figure 7-22.) 4.
Metallurgical Analysis
During the rolling operation, faulty or oversized dies, or an overfill of material may cause material to be foIded over and flattened into the qurface of the thread but not fused.
A TYPICAL AREAS OF FAILURE LAPS AND SEAMS
8. FAILURE OCCURRING AT ROOT O F THREAD
C. AREAS WHERE LAPS A N D SEAMS USUALLY OCCUR
Figure 7-22. Lap and Seam Discontinuities in Rolled Threads 5.
NDT Methods Application and Limitations
a.
Liquid Penetrant Testing Method.
(1)
Compatibility with both ferrous and nonferrous materials makes fluorescent liquid penetrant the first choice.
b.
723
(2)
Liquid penetrant indications will be circumferential, slightly curved, intermittent or continuous indications. Laps and seams may occur individually or in clusters.
(3)
Foreign material may not only interfere with the penetration of the penetrant into the discontinuity but may cause an accumulation of penetrant in a nondefective area.
(4)
Surface of threads may be smeared due to rolling operation, thereby sealing off laps and seams.
(5)
Fluorescent and dye penetrants are not compatible. Dye penetrants tend to kill the fluorescent qualities in fluorescent penetrants.
Magnetic Particle Testing Method. (1)
Magnetic particle indications of laps and seams generally appear the same as liquid penetrant indications.
(2)
Nonrekevant threads.
(3)
Questionable magnetic particle indications can be verified by liquid penetrant testing.
magnetic
indications may
result
from
c.
Eddy Current Testing Method. Not normally used for detecting laps and seams. Article configuration is the restricting factor.
d.
Ultrasonic Testing Method. Not recommended for detecting laps and seams. Thread configurations restrict ultrasonic capability.
e.
Radiographic Testing Method. Not recommended for detecting laps and seams. Size and orientation of discontinuities restricts the capability of radiographic testing.
LAPS AND SEAMS
1.
Category. Processing
2.
Material. Ferrous and Nonferrous Wrought Material
3.
Discontinuity Characteristics a.
Lap Surface. Wavy lines -usually not very pronounced or tightly adherent since they usually enter the surface a t a small
!
i
angle. Laps may have surface openings smeared closed. Found in wrought forgings, plate, tubing, bar, and rod. (See Figure 7-23.)
A TYPICAL FORGING LAP
6. MICROGRAPH OF A LAP
Figure 7-23. Lap and *am Discontinuities in Wrought Material b.
4.
Seam Surface. Lengthy, often quite deep and sometimes very tight; usually occur in parallel fissures with the grain; and, a t times, spiral when associated with roUed rod and tubing.
Metallurgical Analysis
Seams originate from blowholes, cracks, splits, and tears introduced in earlier processing and elongated in the direction of rolling or forging. The distance between adjacent innerfaces of the discontinuity is very small. Laps are similar to seams and may result from improper rolling, forging, or sizing operations. During the processing of the material, corners may be folded over or an overfill may exist during sizing that results in material being flattened, but not fused into the surface. Laps may'occur on any part of the article. 5.
NDT Methods Application and Limitations a.
Magnetic Particle Testing Method.
(1)
Magnetic particle testing is recommended for ferromagnetic material. 7-47
(2)
Surface and nearsurface laps and seams may be detected by this method.
(3)
Laps and seams may appear as straight, spiral, or slightly curved indications. They may be individual or clustered and continuous or intermittent.
(4)
Magnetic buildup a t laps and seams is very small. Therefore a magnetizing current greater than that used for the detection of cracks is necesssry.
,
(5)
b.
c.
d.
e.
Correct magnetizing technique should be used when examining for forging laps since the discontinuity may lie in a plane nearly parallel to the surface.
Liquid Penetrant Testing Method.
(1)
Liquid penetrant testing is. recommended for nonferrous material.
(2)
L q s and seams may be very tight and difficult to detect especially by liquid penetrant.
(3)
Liquid penetrant testing of laps and seams can be improved slightly by heating the article before applying the penetrant.
Ultrasonic Testing Method. 1
Normally used machining.
to
test wrought
material prior
to
(2)
Surface wave technique permits accurate evaluation of the depth, length, and size of laps and seams.
(3)
Ultrasonic indications of laps and seams will appear as definite inner faces within the metal.
Eddy Current Testing Method. (1)
Normally used for the evaluation of laps and seams in tubing and pipe.
(2)
Other articles can be screened by eddy current where article configuration and size permit.
Radiographic Testing Method. Not recommended for detecting laps and seams in wrought material.
724
MICROSHRINKAGE
1.
Category. Processing
2.
Material. Magnesium Casting
3.
Discontinuity Characteristics
Internal. Small filamentary voids in the grain boundaries appear a s concentrated porosity in cross section. (See Figure 7-24.)
A CRACKED MAGNESIUM HOUSING
8. CLOSE-UP VIEW OF (A)
C. MICROGRAPH OF CRACKED AREA
Figure 7-24. Microshrinkage Discontinuity 7-49
4.
Metallurgical Analysis
Shrinkage occurs while the metal is in a plastic or semimolten state. If sufficient molten metal cannot flow into different areas as it cools, the shrinkage w U leave a void. The void is identified by its appearance and by the time in the plastic range it occurs. Microshrinkage is caused by the withdrawal of thelow melting point constituent from the grain boundaries. 5.
NDT Methods Application and Limitations a.
b.
Radiographic Testing Method.
(1)
Radiography is universally used t o determine the acceptance level of microshrinkage.
(2)
Microshrinkage will appear on the radiograph as an elongated swirl resembling feathery streaks or as dark irregular patches that are indicative of cavities in the grain boundaries.
Liquid Penetrant Testing Method. (1)
Normally used on finished machined surfaces.
(2)
Microshrinkage is not normally open t o the surface. These conditions will, therefore, be detected in machined areas.
(3)
The appearance of the indication depends on the plane through which the microshrinkage has been cut. The appearance varies from a continuous hairline to a massive porous indication.
(4)
Penetrant may a c t as a contaminant by saturating the microporous casting affecting its ability to accept a surface treatment.
(5)
Serious structural or dimensional damage to the article can result from the improper use of acids or alkalies. They should never be used unless approval is obtained.
c.
Eddy Current Testing Method. Not recommended for detecting microshrinkage. Article configuration and type of discontinuity do not Iend themselves t o eddy current testing.
d.
Ultrasonic Testing Method. Not recommended for detecting microshrinkage. Cast structure and article configuration are restricting factors.
e. 725
Magnetic Particle Testing Method. Not applicable. Material is nonferrous.
GAS POROSITY
1.
Category. Processing
2.
Material. Ferrous and Nonferrous Weldments
3.
Discontinuity Characteristics
Surface or subsurface. Rounded or elongated, teardrop shaped, with or without a sharp discontinuity a t the point. Scattered uniformly throughout the weld or isolated in small groups. May also be concentrated a t the root or toe. (See Figure 7-25.) 4.
Metallurgical Analysis
Porosity in welds is caused by gas entrapment in the molten metal, too much moisture on the base or filler metal, or improper cleaning or preheating. 5.
NDT Methods Application and Limitations a.
b.
Radiography Testing Method.
(1)
Radiography is the most universally used NDT method for the detection of gas porosity in weldments.
(2)
m e radiograhic image of a "round" porosity will appear as oval shaped spots with smooth edges, while "elongated" porosity will appear as oval shaped spots with the major axis sometimes several times longer than the minor axis.
(3)
Foreign material such as loose scale, flux, or splatter will affect validity of test results.
Ultrasonic Testing Method. (I)
Ultrasonic testing equipment is highly sensitive, capable of detecting microseparations. Established standards should be used if valid test results are to be obtained.
(2)
Surface finish and grain size will affect the validity of the test results.
A TYPICAL SURFACE P O R O S I N
B. CROSSSECTION OF (A) SHOWING
EXTENT OF P O R O S I N
C. MICROGRAPH O F CROSSSECTlON SHOWING TYPICAL S H R I N K A G E POROSITY
Figure 7-25. Gas Porosity Discontinuity C.
d.
Eddy Current Testing Method. (1)
Normally confined t o thin-wall welded pipe and tube.
(2)
Penetration restricts testing t o a depth of more than oneq u a r t e r inch.
Liquid Penetrant Testing Method. (1)
Normally confined to inprocess control of ferrous and nonferrous weldments.
e.
(2)
Liquid penetrant testing, like magnetic particle, restricted to surface evaluation.
is
(3)
Extreme caution must be exercised to prevent any cleaning material, magnetic (iron oxide), and liquid penetrant materials from becoming entrapped and contaminating the rewelding operation.
Magnetic Particle Testing Method. Not normally used to detect gas porosity. Only surface porosity would be evident. Near surface porosity would not be clearly defined since indications are neither strong nor pronounced.
726
UNFUSED POROSITY
1.
Category. Processing
2.
Material. Aluminum
3.
Discontinuity Characteristics
Internal. Wafer-thin fissures aligned parallel with the grain flow. Found in wrought aluminum that has been rolled, forged, or extruded. (See Figure 7-26.) 4.
Metallurgical Analysis
Unfused porosity is attributed to porosity in the cast ingot. During the rolling, forging, or extruding operations it is flattened. into wafer-thin shape. J f the internal surface of these discontinuities is oxidized or is composed of a foreign material, they will not fuse during the subsequent processing, which results in an extremely thin interface or void.
5.
NDT Methods Application and Limitations a.
Ultrasonic Testing Method.
(1)
Used extensively for the detection of unfused porosity.
(2)
Raw materials may be tested in the "as received" configuration.
(3)
Ultrasonic testing fixes the location of the void in all three directions.
A. FRACTURED SPECIMEN SHOWING
UNFUSED POROSIW
8. UNFUSED POROSITY EQUIVALENT TO 1/64 IN. 10.40 mm). 3/64 IN. 11.17 mm) 5/64 IN. 11.98 mm) AND 8/64 IN. (3.18 mml lleftto ri&tl
C. WPICALUNFUSED POROSITY
Figure 7-26. Unfused Porosity Discontinuity
b.
(4)
Where the general direction of the discontinuity is unknown, it may be necesary to test from several directions.
(5)
Method of manufacture and subsequent article configuration will determine the orientation of the unfused porosity to the material surface.
Liquid Penetrant Testing Method. (1)
Normally used on nonferrous, machined articles.
(2)
Unfused porosity will appear as a straight line of varying lengths running parallel with the grain. Liquid penetrant testing & restricted to surface evaluation.
(3)
Surface preparations such as vapor blasting, honing, grinding, or sanding may obliterate possible indications by
masking the surface discontinuities, thereby restricting the reliability of liquid penetrant testing. (4)
727
Excessive agitation of penetrant materials may produce foaming.
c.
Eddy Current Testing Method. Not normally used for detecting unfused porosity.
d.
Radiographic Testing Method. Not normally used for detecting unfused porosity. Wafer-thin discontinuities are difficult to detect by a method that measures density or that requires that the discontinuity be perpendicular to the X-ray beam.
e.
Magnetic Particle Testing Method. Not applicable. Material is nonferrous.
STRESS CORROSION
1.
Categorx. Service
2.
Material. Ferrous and Nonferrous
3.
Discontinuity Characteristics
Surface. Range from shallow to very deep, and usually follow the grain flow of the material; however, transverse cracks are also possible. (See Figure 7-27.)
4.
Metallurgical Analysis
Three factors are necessary for the phenomenon of stress corrosion to occur: 1) a sustained static tensile stress, 2) the presence of a corrosive environment, and 3) the use of a material that is susceptible to this type of failure. Stress corrosion is much more likely to occur a t high levels of stress than at low levels of stress. The type of stresses include residual (internal) as well a s those from external (applied) loading.
5.
NDT Methods Application and Limitations a.
Liquid Penetrant Testing Method. (1)
Liquid penetrant is normally used for the detection of stress corrosion.
Figure 7-27. Stress Corrosion Discontinuity (2)
In the preparation, application, and final cleaning of articles, extreme care must be exercised t o prevent overspraying and Contamination of the surrounding ar.ticles.
(3)
Chemical cleaning immediately before the application of liquid penetrant may seriously affect the test results if the solvents are not given time t o evaporate.
(4)
Service articles may contain moisture within the discontinuity which will dilute, contaminate, and invalidate results if the moisture is not removed.
b.
Eddy Current Testing Method. Not normally used to detect stress corrosion. Eddy current equipment is capable of resolving stress corrosion where article configuration is compatible with equipment limitations.
c.
Ultrasonic Testing Method. Not normally used to detect stress corrosion. Discontinuities are perpendicular to surface of material and require surface technique.
728
/I
I
d.
Magnetic Particle Testing Method. Not normally used to detect stress corrosion. Configuration of article and usual nonferromagnetic condition exclude magnetic particle testing.
e.
Radiographic Testing Method. Not normally used t o detect stress corrosion. Surface indications are best detected by NDT method designed for such application. However, radiography can and has shown stress corrosion with the use of the proper technique.
,
'
HYDRAULIC TUBING
1.
Category. Processing and Service
2.
Material. Aluminum 6061-T6
3.
Discontinuity Characteristics
Surface and internal. Range in size from short to long, shallow to very tight and deep. Usually they will be found in the direction of the grain flow with the exception of stress corrosion, which has no direction. (See Figure 7-28.)
A INTERGRANULAR CORROSION
B. LAP IN OUTER SURFACE OF TUBING
C . EMBEDDED FOREIGN MATERIAL
D. TWIN LAPS IN OUTER SURFACE O F TUBING
Figure 7-28. Hydraulic Tubing Discontinuities
7-57
4.
Metallurgical Analysis
Hydraulic tubing discontinuities are usually one of the following:
5.
a,
Foreign material coming in contact with the tube material and being embedded into the surface of the tube.
b.
Laps which are the result of material being foIded over and not fused. .
e.
Seams which originate from blowholes, cracks, splits and tears introduced in the earlier processing, and then are elongated during rolling.
d.
Intergranular corrosion which is due to the presence of a corrosive environment.
NDT Methods Application and Limitations a.
Eddy Current Testing Method.
(1)
Universally used for testing of nonferrous tubing.
(2)
Heavier-walled tubing, 0.25 in. (6.3 mm) and over, may not be successfully tested due to the penetration ability of the equipment.
(3)
The specific nature of various discontinuities may not be clearly defined.
(4)
Test results will not be valid unless controlled by known standards.
(5)
Testing of ferromagnetic material may be difficult.
(6)
All material should be free of any foreign material that would invalidate the test results.
b.
Liquid Penetrant Testing Method. Not normally used for detecting tubing discontinuities. Eddy current is more economical, faster, and, with established standards, is more reliable.
c.
Ultrasonic Testing Method. Not normally used for detecting tubing discontinuities. Eddy current is recommended over ultrasonic testing since it is faster and more economical for this range of surface discontinuity and nonferrous material.
d.
Radiographic Testing Method. Not normally used for detecting tubing discontinuities. The size and type of discontinuity and
the configuration of the article limit the use of radiography for screening of material for this group of discontinuities. e.
729
Magnetic Particle Testing Method. Not applicable. Material is nonferrous.
MANDREL DRAG
1.
Category. Processing
2.
Material. Nonferrous Thick-Wall Seamless Tubing
3.
Discontinuity Characteristics
Internal surface of thick-wall tubing. Range from shallow even gouges to ragged tears. Often a slug of the material will be embedded within the gouged area. (See Figure 7-29.)
C. ANOTHER TYPE OF EMBEDDED SLUG
D. GOUGE ON INNER SURFACE OF PIPE
Figure 7-29. Mandrel Drag Discontinuities 7-59
During the manufacture of thick-wall seamless tubing, the billet is ruptured as it passes through the offset rolls. As the piercing mandrel follows this fracture, a portion of the material may break loose and be forced over the mandrel. As it does, the surface of the tubing may be scored or have the slug embedded into the wall. Certain types of material are more prone t o this type of failure than others.
5.
NDT Methods Application and Limitations a.
b.
Eddy Current Testing Method. (1)
Normally used for the testing of thin-wall pipe or tube.
(2)
Eddy current testing may be confined to nonferrous materials.
(3)
Discontinuities are qualitative, not quantative indications.
(4)
Several factors simultaneously affect output indications.
Ultrasonic Testing Method. (1)
Normally used for the screening of thick-wall pipe or tube for mandrel drag.
(2)
Can be used to test both ferrous and nonferrous pipe or tube.
(3)
May be used in support of production line since it is adaptable for automatic instrumentation.
(4)
Configuration of mandrel drag or tear will produce very sharp and noticeable indications on the scope.
c.
Radiographic Testing Method. Not normally used although it has been instrumental in the detection of mandrel drag during examination of adjacent welds. Complete coverage requires several exposures around the circumference of the tube. This method is not designed for production support since it is very slow and costly for large volumes of pipe or tube. Radiograph will disclose only two dimensions and not the third.
d.
Liquid Penetrant Testing Method. Not recommended for detecting mandrel drag since discontinuity is internal and would not be detectable.
I
I
e.
730
Magnetic Particle Testing Method. Not recommended for detecting mandrel drag. Discontinuities are not close enough to the surface to be detectable by magnetic particles. Most mandrel drag will occur in seamless stainless steel.
SEMICONDUCTORS
1.
Category. Processing and Service
2.
Material. Hardware
3.
Discontinuity Characteristics
Internal. Appear in many sizes and shapes and various degrees of density. They may be misformed, misaligned, damaged, or may have broken internal hardware. Found in transistors, diodes, resistors, and capacitors. (See Figure 7-30.)
1
k STRANDS BROKEN IN HEATER BLANKET
C. BROKEN ELECTRICAL CABLE
8.
FINE CRACK IN PLASTIC CASING MATERIAL
0. FOREIGN MATERIAL WITHIN SEMICONDUCTOR
Figure 7-30. Semiconductor Discontinuities
4.
Metallurgical Analysis I
Semiconductor discontinuities such as loose wire, weld splash, flakes, solder balls, loose leads, inadequate clearance between internal elements and case, and inclusions or voids in seals or around lead connections are the product of processing errors.
5.
NDT Methods Application and Limitations a.
b.
Radiographic Testing Method.
(1)
Universally used as the NDT method for the detection of discontinuities in semiconductors.
(2)
The configuration and internal structure of the various semiconductors limit the NDT method to radiography.
(3)
Semiconductors that have copper heat sinks may require more than one technique due t o the density of the copper.
(4)
Internal wires in semiconductors are very f i e and may be constructed from materials of different density such as copper, silver, gold and aluminum. If the latter is used with the others, special techniques may be needed t o resolve test reliability.
(5)
Microparticles may require the highest sensitivity to resolve.
(6)
The complexity of the internal structure of semiconductors may require additional views to exclude the possibility of non-detection of discontinuities due to masking by hardware.
(7)
Positive positioning of each semiconductor will prevent invalid interpretation.
(8)
Source angle should give minimum distortion.
(9)
Preliminary examination of semiconductors may be accomplished using a vidicon system that would allow visual observation during 360 degree rotation of the article.
Eddy Current Testing Method. Not recommended for detecting semiconductor discontinuities. Nature of discontinuity and method of construction of the article do not lend themselves to this form of NDT.
731
c.
Magnetic Particle Testing Method. Not recommended for detecting semiconductor discontinuities.
d.
Liquid Penetrant Testing Method. Not recommended for detecting semiconductor discontinuities.
e.
Ultrasonic Testing Method. Not recommended for detecting semiconductor discontinuities.
HOTTEARS
1.
Categorx. Inherent
2.
Material. Ferrous Castings
3.
Discontinuity Characteristics
Internal or near surface. Appear as ragged Line of variable width and numerous branches. Occur singly or in groups. (See Figure 7-31.) 4.
Metallurgical Analysis
Hot cracks (tears) are caused by nonuniform cooling resulting in stresses which rupture the surface of the metal while its temperature is still in the brittle range. Tears may originate where stresses are set up by the more rapid cooling of .thin sections that adjoin heavier masses of metal, which are slower to cool. 5.
NDT Methods Application and Limitations a.
b.
Radiographic Testing Method. (1)
Radiographic testing is the first choice since the material is cast structure and the discontinuities may be internal and surface.
(2)
Orientation of the hot tear in relation to the source may influence the test results.
(3)
The sensitivity level may not be sufficient to detect fine surface hot tears.
Magnetic Particle Testing Method. (1)
Hot tears that are exposed to the surface can be screened with magnetic particle method.
A. TYPICAL HOTTEARS IN CASTING
8. HOTTEARS IN FILLET OF CASTING
C. CLOSEUP OF HOTTEARS I N IAI
D. CLOSE-UP OF HOTTEARS I N IB)
Figure 7-31. Hot Tear Discontinuities
c.
(2)
Article configuration and metallurgical composition may make demagnetization difficult.
(3)
Although magnetic particle testing can detect near surface hot tears, radiography should be used for final analysis.
(4)
Foreign material not removed prior to testing will cause an invaliu test.
Liquid Penetrant Testing Method. (1)
Liquid penetrant testing is recommended for nonferrous cast material.
(2)
Method is confined to surface evaluation.
732
'
(3)
The use of penetrants on castings may act as a contaminant by saturating the porous structure and thereby affect the ability to apply surface finish.
(4)
Repeatability of indications may be poor.
d.
Ultrasonic Testing Method. Not recommended for detecting hot tears. Discontinuities of this type when associated with cast. structure do not lend themselves to ultrasonic testing.
e.
Eddy Current Testing Method. Not recommended for detecting hot tears. Metallurgical structure along with the complex configurations do not lend themselves to eddy current testing.
INTERGRANULAR CORROSION
1.
Category. Service
2.
Material. Nonferrous
3.
Discontinuity Characteristics
Surface or internal. A series of small micro-openings with no definite pattern. May appear singly or in groups. The insidious nature of intergranular corrosion results from the fact that very little corrosion or corrosion product is visible on the surface. Integranular corrosion may extend in any direction following the grain boundaries of the material. (See Figure 7-32.) 4.
Metallurgical Analysis
Two factors that contribute to intergranular corrosion are:
5.
a.
Metallurgical structure of the material that is prone to intergranular corrosion such as unstabilized 300 series stainless steel.
b.
Improper stress relieving or heat treat may create the susceptibility to intergranular corrosion. Either of these conditions colipled with a corrosive atmosphere will result in intergranular attack.
NDT Methods Application and Limitations a.
Liquid Penetrant Testing Method. (1)
Liquid Penetrant testing is the first choice due to the size and location of this type of discontinuity. 7-65
A. MICROGRAPH OF INTERGRANULAR CORROSION SHOWING LlFFlNG OF
SURFACE FROM SUBSURFACE CORROSION
OF I N T E R G R A N U L A R CORROSION. ONLY MINOR EVIDENCE O F CORROSION IS E V I O E N T FROM SURFACE
8. MICROGRAPH SHOWING N A T U R E
Figure 7-32. Intergranular Corrosion Discontinuity
b.
(2)
Chemical cleaning operations immediately before the application of Liquid penetrant may contaminate the article and seriously affect test results.
(3)
Cleaning with solvents may release accelerate intergranular corrosion.
(4)
Trapped penetrant solution may present a cleaning or removal problem.
chlorine
and
Radiographic Testing Method.
(1)
Intergranular corrosion in the more advanced stages has been detected with radiography.
c.
(2)
Sensitivity levels may prevent the detection of fine intergranular corrosion.
(3)
Radiography may not indicate the surface on which the intergranular corrosion occurs.
Eddy Current Testing Method.
(1)
Eddy current can be intergranular corrosion.
used
for
the
screening of'
(2)
Tube or pipe lend themselves readily to this method of NDT testing.
(3)
Metallurgical structure of the material may seriously affect the output indications.
d.
Ultrasonic Testing Method. Not normally used although the equipment has the capability to detect intergranular corrosion.
e.
Magnetic Particle Testing Method. Not recommended for detecting intergranular corrosion. Type of discontinuity and material restrict the use of magnetic particles.
