LEVEL Ill STUDY GUIDE EDDY CURRENT METHOD
ASNT
Conl"inuing ducal"ion In Nondes'lruc'live Tes'ling
The American Society for Nondestructive Testing
The Eddy Current Level Ill Study Guide was prepared by:
A. L. Lucero
Publication and review of this Study Guide was under the direction of the Personnel Training and Certification Committee of the American Society for Nondestructive Testing:
Frank Sattler, Chairman Robert Baker Edward Briggs Donald Dodge George Pherigo
Ward D. Rummel Carl Shaw Jack C. Spanner, Vice-Chairman John L. Summers John H. Weiler
Published by the American Society for Nondestructive Testing, Inc., 1711 Arlingate Lane, PO Box 28518, Columbus, OH 43228
Copyright © 1983 by the American Society for Nondestructive Testing, Inc. All rights reserved. Printed in the United States of America.
CONTENTS
Figures/Tables
v
References
vi
Foreword
vii
Preface 1.
viii
Principles of Eddy Current Testing
1
Historical Background 1 Generation of Eddy Currents 2 Field Intensity 3 Current Density 6 Phase/Amplitude and Current/Time Relationships
2.
Test Coil Arrangements
10
Probe Coils 10 Encircling Coils 11 Bobbin Coils 12 Absolute Coils 13 Differential Coils 13 Hybrid Coils 14 Additional Coil Characteristics
3.
14
Test Coil Design
16
Resistance 16 Inductance 16 Inductive Reactance 18 Impedance 19 Q or Figure of Merit 20 Permeability and Shielding Effects Coil Fixtures 21 4.
21
24
Effects of Test Object on Test Coil
Electrical Conductivity Permeability 26 Skin Effect 26 Edge Effect 26 26 End Effect Lift-Off 27 Fill Factor 27 Discontinuities 28 Signal-to-Noise Ratio
8
24
29
iii
5.
Frequency Selection 31 Single Frequency Systems Multifrequency Systems
6.
31
Selection of Test Parameters
31
36
Impedance Testing 41 Phase Analysis Testing 42 Vector Point 42 Ellipse 43 Linear Time Base 44 Modulation Analysis Testing 45 Test Object Handling Equipment
7.
40
Instrument Systems
49 52
Readout Mechanisms
Indicator Lights 52 Audio Alarms 52 Meters 52 Digital Displays 52 CRTs 53 Recorders 54 Computers 55
8.
Applications
57
Flaw Detection 57 Dimensional Measurements 58 Conductivity Measurements 58 Hardness Measurements 59 Alloy Sorting 59
9.
Eddy Current Test Procedures, Standards, and Specifications
ASTM MIL-STD ASME
61
61
62 62 72
ANSWERS to Review Questions
iv
FIGURES/TABLES Figure 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 1.10 2.1 2.2
2.3 2.4 2.5 2.6 3.1
3.2 4.1
4.2 5.1
5.2 5.3 5.4 6.1
6.2 6.3 6.4
6.5 6.6
6.7 6.8 6.9 6.10 6.11 6.12
6.13
Arago's Magnetic Experimentation, 1821 1 Induced Current with Coil and Magnet Induced Current, Electromagnetic Technique 2 Induced Current Relationships 3 Electromagnetic Field Produced by Alternating Current Generation of Eddy Current in a Test Object 4 Induced Current Flow in a Cylindrical Part 4 a. Phasor Diagram of Coil Voltage without Test Object b. Phasor Diagram of Coil Voltage with Test Object Relative Eddy Current Density 6 Eddy Current Phase Angle Radians Lagging 8 Probe Coil 10 Encircling Coil 11 Bobbin Coil 12 Coil Configurations 12 External Reference Differential System Hybrid Coil 14 Multilayer Coil 17 Impedance Diagram
3
5 5
13
19
Measured Conductivity Locus Lift-off Conductivity Relationship
24
27
Effect of Frequency Change a. Primary Impedance Without Secondary Circuit b. Primary Impedance with Secondary Circuit Normalized Impedance Diagram 34 Impedance Variations, f/f 9 = 50 35 Impedance Variations, f/f 9 = 15 36
33 33
Internal Functions of the Electromagnetic Nondestructive Test 40 Four Types of Simple Eddy Current Instruments 41 Vector Point Method 42 Ellipse Method 43 CRT Displays for Dimension and Conductivity 43 Linear Time Base Instrument Diagram 44 Screen Image of a Linear Time Base Instrument with Sinusoidal Signals a. Null Balance Instrument with Amplitude-Phase Detectors 45 b. Typical Response to a Thin Wall Non-ferromagnetic Tube Calibration Standard 46 Instrument Providing Any One of Four Operating Frequencies 46 Multifrequency Instrument Operating at Three Frequencies Simultaneously Commercial Multifrequency Instrument 48 Commercial Microprocessor-Based Instrument 48 Pulsed Waveform Excitation 49
7.4
Discontinuity Response in Thin Wall Non-ferromagnetic Tubing Commercial Strip Chart Recorder 54 Facsimile Recording of a Saw-cut Specimen 55 Computer-controlled Eddy Current System 55
Table 4.1
Electrical Resistivity and Conductivity of Several Common Metals and Alloys
7.1
7.2 7.3
v
44
47
53
25
REFERENCES 1. Recommended Practice No. SNT-TC-1A (1980 Edition), Supplement E. 1980. American Society for Nondestructive Testing, Columbus, Ohio. 2. General Dynamics-CT-6-5 Eddy Current Testing, Programmed Instruction Handbooks, Volume 1, 1967; Volume 2, 1967. General Dynamics Convair Division, San Diego, California. 3. General Dynamics-PI-4-5 Eddy Current Testing, Programmed Instruction Handbooks, Volume 1, 1967; Volume 2, 1967. General Dynamics Convair Division, San Diego, California. 4. H.L. Libby, Introduction to Electromagnetic Nondestructive Test Methods. 1971. John Wiley & Sons, Inc. New York, New York. 5. R.C. McMaster, ed., Nondestructive Testing Handbook. 1959. American Society for Nondestructive Testing, Columbus, Ohio. 6. W.J. McGonnagle, Nondestructive Testing, 2d ed. 1975. Gordon and Breach Publishing Company, New York, New York. 7. R.S. Sharpe, Research Techniques in Nondestructive Testing, Volume 1, 1970. Academic Press, New York, New York. 8. R.S. Sharpe, Research Techniques in Nondestructive Testing, Volume 2, 1973. Academic Press, New York, New York. 9. Metals Handbook, Properties and Selection of Materials, 8th ed. 1961. American Society for Metals, Metals Park, Ohio. 10. J.L. Taylor, ed., Basic Metallurgy for Nondestructive Testing. 1974. British Institute of Nondestructive Testing, Essex, England. 11. Nondestructive Evaluation in the Nuclear Industry (1980). 1981. American Society for Metals, Metals Park, Ohio. 12. Eddy Current Characterization of Materials and Structures-ASTM, STP 722. 1981. American Society for Testing and Materials, Philadelphia, Pennsylvania. 13. Eddy Current Nondestructive Testing-NBS Special Publication 589. 1981. National Bureau of Standards, Washington, D.C. 14. D.J. Hagemaier and A.P. Steinberg, "Low Frequency Eddy Current Inspection of Aircraft Structure." Materials Evaluation, Vol. 40, No. 2, Feb. 1982, pp. 206-210. American Society for Nondestructive Testing, Columbus. Ohio. 15. MIL-STD-1537A (USAF) "Electrical Conductivity Test for Measurement of Heat Treatment of Aluminum Alloys, Eddy Current Method." June 1981. U.S. Department of Defense, Washington, D.C. 16. MIL-STD-271E (SHIPS) "Nondestructive Testing Requirements for Metals." 1973. U.S. Department of Defense, Washington, D.C. 17. ASME Section VI, 1980 edition. American Society of Mechanical Engineers, New York, New York. 18. 1982 Annual Book of ASTM Standards, Part 11 Metallography; Nondestructive Testing. 1982. American Society for Testing and Materials, Philadelphia, Pennsylvania. 19. Metals Handbook, Nondestructive Inspection and Quality Control. 1976. American Society for Metals, Metals Park, Ohio.
vi
FOREWORD
The Personnel Training and Certification Committee of ASNT has prepared a series of Levell! I Study Guides which are intended to present the major areas in each nondestructive testing method. The Levell II candidate should use this Study Guide only as a review, as it does not contain all of the information necessary to pass a typical Level Ill examination. In using this Study Guide, the reader will be given specific references, including page numbers, where detailed information can be obtained. Typical Level Ill questions are available at the end of each section to aid in determining comprehension of the material. A typical use of this Study Guide might include the following sequence: 1. An individual should review the questions at the end of each section in the Study Guide to determine if his or her comprehension of the eddy current method is adequate. The questions will serve as an indicator of the individual's ability to pass a Level Ill examination. 2. If the individual finds questions in a certain section of the Study Guide to be difficult, it is suggested that the individual carefully study the information presented in that section. This review of the information in the Study Guide will serve to refresh one's memory of theory and forgotten facts. 3. If the individual encounters information that is new or not clearly understood, then it is important to note the specific references given throughout the Study Guide and carefully read this information. References are indicated by parentheses and the reference number: (N).
vii
PREFACE
Early experimenters in the field of magnetism and electromagnetism established the basis for the principles of electromagnetic nondestructive examination used today. In 1824, Arago discovered tha·t the vibration of a magnetic needle was rapidly damped when it was placed near a nonmagnetic conducting disk. In 1820, Oersted discovered the magnetic field surrounding a conductor when current was passed through the conductor. In 1820, Ampere discovered that equal currents flowing in opposite directions in adjacent conductors cancelled the magnetic effect. This discovery has led to development of modern coil arrangements and shielding techniques. Faraday discovered the principles of electromagnetic induction in 1831. Maxwell integrated the results of these and other works in a two-volume work published in 1873, and Maxwell's equations are still the basis for investigations of the magnetic and electromagnetic phenomena. The application of these laws and principles has led to the development of an industry whose purpose is to qualitatively and quantitatively investigate the properties and characteristics of conducting materials using nondestructive electromagnetic techniques. As in any industry, controls and guidelines must be established to insure consistent and reproducible products or services. This Study Guide is intended to provide ASNT Level Ill Eddy Current Method candidates with a concise reference with which to prepare for the ASNT Level Ill examination.
viii
1.
PRINCIPLES OF EDDY CURRENT TESTING
HISTORICAL BACKGROUND
Before discussing the principles of eddy current testing, it seems appropriate to discuss briefly facets of magnetism and electromagnetism that serve as the foundation for our study of eddy current testing. In the period from 1775 to 1900, scientific experimenters Coulomb, Ampere, Faraday, Oersted, Arago, Maxwell, and Kelvin investigated and cataloged most of what is known about magnetism and electromagnetism. Arago discovered that the oscillation of a magnet was rapidly damped when a nonmagnetic conducting disk was placed near the magnet (Fig. 1.1). He also observed that by rotating the disk, the magnet was attracted to the disk. In effect, Arago had introduced a varying magnetic field to the disk causing eddy currents to flow in the disk producing a magnetic field by the disk that attracted the magnet. Arago's simple model is a basis for many automobile speedometers used today.
MOTION
MAGNET
CONDUCTING PLATE
Figure 1.1-Arago's Magnetic Experimentation, 1821 (Zetec, Inc.)
Oersted discovered the presence of a magnetic field around a current-carrying conductor, and he observed a magnetic field developed in a perpendicular plane to the direction of current flow in a wire. Ampere observed that equal and opposite currents flowing in adjacent conductors cancelled this magnetic effect. Ampere's observation is used in differential coil applications and to manufacture noninductive, precision resistors. Faraday's first experiments investigated induced currents by the relative motion of magnet and a coil (Fig. 1.2).
,-,,
f'( I
1S1 I I I I
I II II I I I I
+
t
1 I I 1 I I I
~1 'I 'I''
N
(a) Figure 1.2-lnduced Current with Coil and Magnet
(b)
Faraday's major contribution was the discovery of electromagnetic induction. His work can be summarized by the example shown in Figure 1.3. Coil A is connected to a battery through a switch S. A second coil 8 connected to a galvanometer G is nearby. When switch S is closed producing a current in coil A in the direction shown, a momentary current is induced in coil 8 in a direction(- a) opposite to that in A. If Sis now opened, a momentary current will appear in coil 8 having the direction of(- b). In each case, current flows in coil 8 only while the current in coil A is changing.
Figure 1.3-lnduced Current, Electromagnetic Technique
The electromotive force (voltage) induced in coil 8 of Figure 1.3 can be expressed as follows: E
=
Nll
where: E N il
= Average induced voltage = Number of turns of wire in coil 8 = Rate of change of magnetic lines of force affecting coil 8
= 10
8
Maxwell produced a two-volume work "A Treatise on Electricity and Magnetism" first published in 1873. Maxwell not only chronicle·d most of the work done in electricity and magnetism at that time, but he also developed and published a group of relations known as Maxwell's equations for the electromagnetic field. These equations form the base that mathematically describes most of what is known about electromagnetism today (13). In 1849 Lord Kelvin applied Bessel's equation to solve the elements of an electromagnetic field. The principles of eddy current testing depend on the process of electromagnetic induction. This process includes a test coil through which a varying or alternating current is passed. A varying current flowing in a test coil produces a varying electromagnetic field about the coil. This field is known as the primary field.
GENERATION OF EDDY CURRENTS
When an electrically conducting test object is placed in the primary field, an electrical current will be induced in the test object. This current is known as the eddy current. Figure 1.4 is a simple model that illustrates the relationships of primary and induced (eddy) currents. Conductor A represents a portion of a test coil. Conductor 8 represents a portion of a test object. 2
CONDUCTOR 8
CONDUCTOR A
Figure 1.4-lnduced Current Relationships (Zetec, Inc.)
Following Lenz's law and indicating the instantaneous direction of primary current lp, a primary field q,p is developed about Conductor A. When Conductor B is brought into the influence of ci>p, an eddy current IE is induced in Conductor B. This electrical current IE produces an electromagnetic field q,E that opposes the primary electromagnetic field ci>p. The magnitude of q,E is directly proportional to the magnitude of IE. Characteristic changes in Conductor B such as conductivity, permeability, or geometry will cause IE to change. When IE varies, q,E also varies. Variations of q,E are reflected to Conductor A by changes in ci>p. These changes are detected and displayed on some type of readout mechanism that relates these variations to the characteristic that is of interest. FIELD INTENSITY Figure 1.5 presents a schematic view of an excited test coil. The electromagnetic field produced about the unloaded test coil in Figure 1.5 can be described as decreasing in intensity with distance from the coil and also varying across the coil's cross section. The electromagnetic field is most intense near the coil's surface.
GENERATOR
q
)
Figure 1.5-Eiectromagnetic Field Produced by Alternating Current (Zetec, Inc.) 3
The field produced about this coil is directly proportional to the magnitude of applied current, rate of change of current or frequency, and the coil parameters. Coil parameters include inductance, diameter, length, thickness, number of turns of wire, and core material. To better understand the principles under discussion, we must again look at the instantaneous relationships of current and magnetic flux. The exciting current is supplied to the coil by an alternating current generator or oscillator. With a primary current lp flowing through the coil, a primary electromagnetic field
p is produced about the coil. When this excited test coil is placed on a conducting test object, eddy currents IE will be generated in that test object. Figure 1.6 illustrates this concept.
GENERATOR
Figure 1.6-Generation of Eddy Current in a Test Object (Zetec, Inc.)
Note the direction of lp, p, and the resultant eddy current IE. Although Figure 1.6 shows IE by directional arrows on the surface of the test object, IE extends into the test object some distance. Another important observation is that IE is generated in the same plane in which the coil is wound. Figure 1.7 emphasizes this point with a loop coil surrounding a cylindrical test object (4).
Figure 1.7-lnduced Current Flow in a Cylindrical Part (Zetec, Inc.)
4
A more precise method of describing the relationships of magnetic flux, voltage, and current is the phase vector diagram or phasor diagrams (4).
Ep
I
E = Ep
(a)
WITHOUT TEST OBJECT 90 ELECTRICAL DEGREES EXCITATION CURRENT I
~----L
___
L...__ _ __
p
s
PRIMARY MAGNETIC FLUX
SECONDARY MAGNETIC FLUX = 0
Ep~
I
Es~
(b)
E= E T - - t - - -
WITH TEST OBJECT
S
SECONDARY MAGNETIC FLUX
EXCITATION CURRENT I _____...._..__ _ ____,..__ _ _ __
p PRIMARY MAGNETIC FLUX
Figure 1.8-a. Phasor Diagram of Coil Voltage without Test Object b. Phasor Diagram of Coil Voltage with Test Object (Reprinted with permission from Hugo L. Libby, Introduction to Electromagnetic Nondestructive Test Methods, p. 20. Copyright©1971, John Wiley & Sons, Inc.) Figure 1.8 shows the effects of a non-ferromagnetic test object on a test coil. Figure 1.8a shows an encircling coil and the resultant phasor diagram for the unloaded coil. The components of phasor diagram 1.8a are as follows: E Ep I p s
=
Coil Voltage Primary Voltage Excitation Current Primary Magnetic Flux Secondary Magnetic Flux
The current (I) and primary magnetic flux (P) are plotted in phase, and the primary voltage (Ep) is shown separated by 90 electrical degrees. Secondary magnetic flux s is plotted at zero because without a test object no secondary flux exists. Figure 1.8b represents the action of placing a non-ferromagnetic test object into the test coil. The components of phasor diagram 1.8b for a loaded coil are as follows: E ET Ep Es cl>p s T I
= Coil Voltage
Loaded Coil Voltage
= Primary Coil Voltage = = =
Secondary Voltage Primary Magnetic Flux Secondary Magnetic Flux Loaded Coil Magnetic Flux Excitation Current 5
Observing Figure 1.8b we can see by vectorial addition of Ep and Es we arrive at a new coil voltage (ET) for the loaded condition. The primary magnetic flux p and secondary magnetic flux s are also combined by vectorial addition to arrive at a new magnetic flux (T) for the loaded coil. Notice that for the condition of the test object in the test coil, T is not in phase with the excitation current I. Also observe that the included angle between the excitation current and the new coil voltage Ep is no longer 90 electrical degrees. These interactions will be discussed in detail later in this study guide.
CURRENT DENSITY
The distribution of eddy currents in a test object varies exponentially. The current density in the test object is most dense near the test coil. This exponential current density follows the mathematical rules for a natural exponential decay curve (1/ 8 ). Usually a natural exponential curve is illustrated by a graph with the ordinate (Y axis) representing magnitude and the abscissa (X axis) representing time or distance. A common point described on such a graph is the "knee" of the curve. The knee occurs at the 37 percent value on the ordinate axis. This 37 percent point, or knee, is chosen because changes in X axis values produce significant changes in Y axis values from 100 percent to 37 percent, and below 37 percent changes in X axis values produce less significant changes in Y axis values. Applying this logic to eddy current testing, a term is developed to describe the relationship of current density in the test object. Consider the eddy current generated at the surface of the test object nearest the test coil to be 100 percent of the available current, the point in the test object thickness where this current is diminished to 37 percent is known as the standard depth of penetration (4). Figure 1.9 is a relative eddy current density curve for a plane wave of infinite extent with magnetic field parallel to the conducting test object surface.
~
i
1.01
~
I
~
;:)
~
6=1.88
! I
o.37r--------
X~
ONE STANDARD DEPTH OF PENETRATION 0
o 0 w w
oL---~--------~========-1 2 3 4 5
>
~ ..J
w
DEPTH X
a:
DEPTH OF PENETRATION
Figure 1.9-Relative Eddy Current Density (Reprinted [adapted] with permission from Hugo L. Libby, Introduction to Electromagnetic Nondestructive Test Methods, p. 26. Copyright©1971, John Wiley & Sons, Inc.) The current density at any depth can be calculated as follows:
where:
Jx J0 7f
= Current density at depth X = Current density at surface, = 3.1416 = Frequency in hertz
amperes per square meter
Jl
= Magnetic permeability, henries per meter (Him)
X
= Depth from
a
=
surface, meters Electric conductivity, mhos per meter
6
Magnetic permeability
fl
is a combination of terms. For nonmagnetic materials: fl
= 471" • 1Q- 7 H/m
For magnetic materials: fl
= flrflo
where: flr
flo
= Relative = 47r • 10-
7
permeability H/m
The standard depth of penetration can be calculated as follows: 1
o = ._)?Tffla
where:
o = 7T
fl
a
Standard depth of penetration, meters
= 3.1416
= Frequency in hertz = Magnetic permeability, H/m = Electric conductivity, mhos per meter
It should be observed at this point that as frequency, conductivity, or permeability is increased, the penetration of current into the test object will be decreased. We can use the graph in Figure 1.9 (p. 6) to demonstrate many eddy current characteristics. Using an example of a very thick block of stainless steel being interrogated with a surface or probe coil operating at a test frequency of 100 kilohertz (kHz), we can determine the standard depth of penetration and observe current densities at other depths. Stainless steel (300 Series) is non-ferromagnetic. Magnetic permeability (!l) is 4?r • 10-7 H/m and the conductivity is 0.14 • 107 mhos per meter for 300 Series stainless steel. 0 0
1
= ._)?Tffla = --/3.1416.
1
100000. 47r. 10-7
•
0.14. 107
1
b 8
= 743.438 = 0.00135 meters
o = 1.35 mm Using 1.35 mm as depth X from surface a ratio of depth/depth of penetration would be 1. Referring to Figure 1.9, a depth/depth of penetration of 1 indicates a relative eddy current density of 0.37 or 37 percent. What is the relative eddy current density at 3 mm? Depth X equals 3 mm and depth of penetration is 1.35 mm, therefore: 3 1.35
=
2.222
This ratio indicates a relative eddy current density of about 0.1 or 10 percent. With only 10 percent of the available current flowing at a depth of 3 mm, detectability of variables such as conductivity, permeability, and discontinuities would be very difficult to detect. The obvious solution for greater detectability at the 3 mm depth is to lower the test frequency. Frequency selection will be covered in detail later in this text.
7
PHASE/AMPLITUDE AND CURRENT/TIME RELATIONSHIPS
Figure 1.10 reveals another facet of the eddy current. Eddy currents are not generated at the same instant in time throughout the part. Eddy currents require time to penetrate the test part. Phase and time are analogous; i.e., phase is an electrical term used to describe timing relationships of electrical waveforms.
8
5 4 EDDY CURRENT PHASE ANGLE RADIANS LAGGING
3 2 1 0
0
1
2
3
4
5
8
DEPTH X DEPTH OF PENETRATION
Figure 1.10-Eddy Current Phase Angle Radians Lagging (Reprinted [adapted] with permission from Hugo L. Libby, Introduction to Electromagnetic Nondestructive Test Methods, p. 26. Copyright©1971, John Wiley & Sons, Inc.)
Phase is usually expressed in either degrees or radians. There are 21r radians per 360 degrees. Each radian therefore is approximately 57 degrees. Using the surface current phase angle near the test coil as a reference, phase angle current deeper in the test object lags the surface current. The amount of phase lag is determined by 8 = X-,htp.a = Depth/Depth of Penetration, where 8 equals the phase angle lag in radians. Figure 1.10 should be used as a relative indicator of phase lag. The exact phase relationship for a particular system may be different due to other variables, such as coil parameters and excitation methods. The amount of phase lag for a given part thickness is an important factor when considering resolution. Resolution is the ability to separate variables occurring in the test object; for example, distinguishing two discontinuities occurring at different depths in the same test object. As an example, let us establish a standard depth of penetration at 1 mm in a 5 mm thick test object. Refer to Figure 1.10 and observe the phase lag of the current at one standard depth of penetration. Where depth of interest (X) is 1 mm and depth of penetration (o) is 1 mm, the X/o ratio is 1 and the current at depth X lags the surface current by 1 radian. Projecting this examination, let us observe the phase lag for the entire part thickness. The standard depth of pe-Rtrrfation is 1 mm, the part thickness is 5 mm; therefore, the ratio Xlo equals 5. This produces= phase lag of 5 radians or approximately 287 degrees for the part thickness. Having a measurement capability of 1 degree increments, the part thickness could be divided into 287 parts, each part representing 0.017 mm. That would be considered excellent resolution.
a
There is an obvious limitation. Refer to Figure 1.9 and observe the resultant relative current density with an Xlo ratio of 5. The relative current density is near 0. It should become apparent that the frequency can be adjusted to achieve optimum results for a particular variable. These and other variables will be discussed in Section 5 of this study guide. 8
REVIEW QUESTIONS
0.1·1
Generation of eddy currents depends on the principle of: A. wave guide theory. B. electromagnetic induction. C. magneto-restrictive forces. D. all of the above.
0.1-2
A secondary field is generated by the test object and is: A.
B. C. D.
equal and opposite to the primary field. opposite to the primary field, but much smaller. in the same plane as the coil is wound. in phase with the primary field.
0.1-3
When a non-ferromagnetic part is placed in the test coil, the coil's voltage: A. increases. B. remains constant because this is essential. C. decreases. D. shifts 90 degrees in phase.
0.1-4
Refer to Figure 1.8b (p. 5): If ET was produced by the test object being stainless steel, what would the effect be if the test object were copper? A. ET would decrease and be at a different angle. B. ET would increase and be at a different angle. C. Because both materials are non-ferromagnetic, no change occurs. D. None of the above.
0.1-5
Eddy currents generated in a test object flow: A. in the same plane as magnetic flux. B. in the same plane as the coil is wound. C. 90 degrees to the coil winding plane. D. Eddy currents have no predictable direction.
0.1-6
The discovery of electromagnetic induction is credited to: A. Arago. B. Oersted. C. Maxwell. D. Faraday.
0.1-7
A standard depth of penetration is defined as the point in a test object where the relative eddy current density is reduced to: A. 25 percent. B. 37 percent. c. 50 percent. D. 100 percent.
0.1·8
Refer to Figure 1.9 (p. 6). If one standard depth of penetration was established at 1 mm in an object 3 mm thick, what is the relative current density on the far surface? A. 3 B. <0.1 c. 1/3 D. Indeterminate
0.1·9
Refer to Figure 1.10 (p. 8). Using the example in question 1.8, what is the phase difference between the near and far surfaces? A. Far surface leads near surface by sr B. Far surface leads near surface by 171 o C. Far surface lags near surface by 171 o D. Far surface lags near surface by 57 o
0.1·10
Calculate the standard depth of penetration at 10kHz in copper; u A. 0.1 mm B. 0.02 mm C. 0.66 mm D. 66 mm 9
= 5.7 •
107 mhos per meter.
2.
TEST COIL ARRANGEMENTS
Test coils can be categorized into three main mechanical groups: probe coils, bobbin coils, and encircling coils (5).
PROBE COILS
Surface coil, probe coil, flat coil, or pancake coil are all common terms used to describe the same test coil type. Probe coils provide a convenient method of examining the surface of a test object. Figure 2.1 illustrates a typical probe coil used for surface scanning.
Figure 2.1-Probe Coil (Zetec, Inc.)
Probe coils and probe coil forms can be shaped to fit particular geometries to solve complex inspection problems. As an example, probe coils fabricated in a pencil shape (pencil probe) are used to inspect threaded areas of mounting studs and nuts or serrated areas of turbine wheels and turbine blade assemblies. Probe coils may be used where high resolution is required by adding coil shielding (2). When using a high-resolution probe coil, the test object surface must be carefully scanned to assure complete inspection coverage. This careful scanning is very time consuming. For this reason, probe coil inspections of large test objects are usually limited to critical areas. Probe coils are used extensively in aircraft inspection for crack detection near fasteners and fastener holes. In the case of fastener holes (bolt holes, rivet holes), the probe coil is spinning while being withdrawn at a uniform rate. This provides a helical scan of the hole using a "spinning probe" technique.
ENCIRCLING COILS Encircling coil, 00 coil, and teed-through coil are terms commonly used to describe a coil that surrounds the test object. Figure 2.2 illustrates a typical encircling coil.
Figure 2.2-Encircling Coil (Zetec, Inc.)
Encircling coils are primarily used to inspect tubular and bar-shaped products. The tube or bar is fed through the coil (feed-through) at relatively high speed. The cross section of the test object within the test coil is simultaneously interrogated. For this reason, circumferential orientation of discontinuities cannot be determined with an encircling coil (4). The volume of material examined at one time is greater using an encircling coil than a probe coil; therefore, the relative sensitivity is lower for an encircling coil. When using an encircling coil, it is important to keep the test object centered in the coil. If the test object is not centered, a uniform discontinuity response is difficult to obtain. It is common practice to run the calibration standard several times, each time indexing the artificial discontinuities to a new circumferential location in the coil. This procedure is used to insure proper response and proper centering.
11
BOBBIN COILS Bobbin coil, 10 coil, and inside probe are terms that describe coils used to inspect from the inside diameter (10) or bore of a tubular test object. Bobbin coils are inserted and withdrawn from the tube 10 by long, semiflexible shafts or simply blown in with air and retrieved with an attached pull cable. These mechanisms will be described later in the text. Bobbin coil information follows the same basic rules stated for encircling coils. Figure 2.3 illustrates a typical bobbin coil.
Figure 2.3-Bobbin Coil (Zetec, Inc.)
Probe coils, encircling coils, and bobbin coils can be additionally classified (5). These additional classifications are determined by how the coils are electrically connected. The three coil categories are absolute, differential, and hybrid. Figure 2.4 shows various types of absolute and differential coil arrangements.
ABSOLUTE
"'....
UJ ....
0
iii
0 0
DIFFERENTIAL
0
z
z
:::; 0
a:
0 z
UJ
UJ ....
ID
::> 0
0
....UJ
0
z
iii
-{j~
Et
~
J
t-
-i
Iii
.... "' 0 0
UJ ID
0
a: Q.
....UJ
ID
::> 0
-B-~
~
0
Figure 2.4-Coil Configurations (5, p. 38-25)
12
I .:.:::1 -
tL-
- ::::i:=d
~·
8-
d()
ABSOLUTE COILS
An absolute coil makes its measurement without direct reference or comparison to a standard as the measurement is being made (6). Some applications for absolute coil systems would be measurements of conductivity, permeability, dimensions, and hardness.
DIFFERENTIAL COILS
Differential coils consist of two or more coils electrically connected to oppose each other. Differential coils can be categorized into two types. One is the self-comparison differential, and the other is external reference differential. The self-comparison differential coil compares one area of a test object to another area on the same test object. A common use is two coils, connected opposing, so that if both coils are affected by identical test object conditions, the net output is "0" or no signal. The self-comparison arrangement is insensitive to test object variables that occur gradually. Variables such as slowly changing wall thickness, diameter, or conductivity are effectively discriminated against with the self-comparison differential coil. Only when a different condition affects one or the other test coils will an output signal be generated. The coils usually being mechanically and electrically similar allows the arrangement to be very stable during temperature changes. Short discontinuities such as cracks, pits, or other localized discontinuities with abrupt boundaries can be detected readily using the self-comparison differential coil. The external reference differential coil, as the name implies, is when an external reference is used to affect one coil while the other coil is affected by the test object (4). Figure 2.5 illustrates this concept. This system is used to detect differences between a standard object and test objects. It is particularly useful for comparative conductivity, permeability, and dimensional measurements. Obviously in Figure 2.5 it is imperative to normalize the system with one coil affected by the standard object and the other coil affected by an acceptable test object. The external reference differential coil system is sensitive to all measurable differences between the standard object and test object. For this reason it is often necessary to provide additional discrimination to separate and define variables present in the test object.
STANDARD OBJECT
TEST OBJECT
Figure 2.5-External Reference Differential System (Reprinted with permission from Hugo L. Libby, Introduction to Electromagnetic Nondestructive Test Methods, p. 69. Copyright©1971, John Wiley & Sons, Inc.)
13
HYBRID COILS Hybrid coils may or may not be the same size and are not necessarily adjacent to each other (4). Common types of the hybrid coil are Driver/Pickup, Through Transmission, or Primary/Secondary coil assemblies. Figure 2.6 shows a typical hybrid arrangement.
Figure 2.6-Hybrid Coil (Reprinted with perm1ss1on from Hugo L. Libby, Introduction to Electromagnetic Nondestructive Test Methods, p. 198. Copyright©1971, John Wiley & Sons, Inc.) A simple hybrid coil consists of an excitation coil and a sensing coil. In the through transmission coil, the excitation coil is on one side of the test object and the sensing coil is on the other. The voltage developed in the sensing coil is a function of the current magnitude and frequency applied to the excitation coil, coil parameters of the exciting and sensing coils, and test object characteristics. In Figure 2.6 an encircling coil induces circumferential currents in a cylindrical test object, and the disturbances of these currents are detected by a small probe coil.
ADDITIONAL COIL CHARACTERISTICS Coil configuration is but one of many factors to consider when setting up test conditions. Other coil characteristics of importance are mechanical, thermal, and electrical stability; sensitivity; resolution; and dimensions. The geometry of the coil is usually dictated by the geometry of the test object, and often sensitivity and resolution are compromised. The relative importance of test coil characteristics depends upon the nature of the test. A blend of theory and experience usually succeeds in selection of proper coil parameters. Coil design and interactions with test objects will be discussed later in this study guide.
REVIEW QUESTIONS 0.2-1
Differential coils are usually used in: A. bobbin coils. B. probe coils. C. OD coils. D. any of the above.
0.2-2
When using a probe coil to scan a test object, _ _ _ __ A. the object must be dry and polished. B. the object must be scanned carefully to insure inspection coverage. C. the object must be scanned in circular motions at constant speeds. D. the probe must be moving at all times to get a reading.
0.2-3
A "spinning probe" would most likely be a (an): A. bobbin coil. B. ID coil. C. OD coil. D. probe coi I.
14
0.2·4
A "feed-through" coil is: A. a coil with primary/secondary windings connected so that the signal is fed through the primary to the secondary. B. an encircling coil. C. an OD coil. D. both B and C.
0.2·5
When inspecting a tubular product with an encircling coil, which statement is not true? A. OD discontinuities can be found. B. Axial discontinuity locations can be noted. C. Circumferential discontinuity locations can be noted. D. ID discontinuities can be found.
0.2·6
An absolute coil measurement is made _ _ _ __ A. by comparing one spot on the test object to another. B. without reference to or direct comparison with a standard. C. only with probe coils. D. by comparative measurement to a known standard.
0.2-7
When coils in a differential arrangement are affected simultaneously with the same test object variables, the output signal _ _ _ __ A. is directly proportional to the number of variables. B. is "0" or near-"0." C. is indirectly proportional to the number of variables. D. is primarily a function of the exciting current.
0.2·8
Which coil type inherently has better thermal stability? A. Bobbin B. Absolute C. OD D. Differential
0.2·9
A hybrid A. B. C. D.
0.2·10
Proper selection of test coil arrangement is determined by: A. shape of test object. B. resolution required. C. sensitivity required. D. stability. E. all of the above.
coil is composed of two or more coils. The coils _ _ _ __ must be aligned coplanar to the driver axis. may be of widely different dimensions. must be impedance-matched as closely as possible. are very temperature sensitive.
15
3.
TEST COIL DESIGN As discussed earlier, test coil design and selection is a blend of theory and experience. Many factors must be considered. These important factors are determined by the inspection requirement for resolution, sensitivity, impedance, size, stability, and environmental considerations. In order to better understand coil properties and electrical relationships, a short refresher in alternating current theory is necessary. First, we must examine electrical units-for example, current and its representative symbol I. Current not only suggests electron flow but also the amount. The amount of electrons flowing past a point in a circuit in one second is expressed in amperes; 21r • 1018 electrons passing a point in one second is called 1 ampere.
RESISTANCE Resistance is an opposition to the flow of electrons and is measured in ohms. Ohm's law is stated by the equation:
E
=R where: I R E
= Current in amperes = .Resistance in ohms = Electrical potential difference
in volts
The resistance of a coil is determined primarily by the length of wire used to wind the coil; its specific resistance is determined by the type of wire (e.g., copper, silver) and the cross-sectional area of the wire. Resistance
= Specific Resistance
• Length
Area
where: Resistance Specific Resistance Area Length
= = =
Ohms Ohms/Circular mil-foot Circular mils Feet
=
Thus, the resistance of a 10-foot length of 40 gauge copper wire with a specific resistance of 10.4 circular mil-foot at 20 oc would be found as follows:
R
=
10.4 • 10 9.888
= 10.518 Ohms
In an alternating current circuit containing only resistance, the current and voltage are in phase. In phase means the current and voltage reach their minimum and maximum values, respectively, at the same time. The power dissipated in a resistive circuit appears in the form of heat. For example, electric toasters are equipped with resistance wires that become hot when current flows through them, providing a heat source for toasting bread.
INDUCTANCE Heat generation is an undesirable trait for an eddy current coil. If the 10-foot length of wire used in the previous example was wound into the shape of a coil, it would exhibit characteristics of alternating current other than resistance. By forming the wire into the shape of a coil, the coil also would have the property of inductance. The role of inductance is analogous to inertia in mechanics, because inertia is the property of matter that causes a body to oppose any change in its velocity. 16
The unit of inductance is the henry (H). A coil is said to have the property of inductance when a change in current through the coil produces a voltage in the coil. More precisely, a circuit in which an electromotive force of one volt is induced when the current is changing at a rate of one ampere per second will have an inductance of one henry. The inductance of a multilayer air core coil can be expressed by its physical properties, or coil parameters. Coil parameters such as length, diameter, thickness, and number of turns of wire affect the coil's inductance. Figure 3.1 illustrates typical coil dimensions required to calculate coil inductance.
MULTILAYER WOUND COILS
rr~ I
)ooo
I
)ol )oo )oo
I
~
I -jb~ Figure 3.1-Multilayer Coil (Zetec, Inc.)
An approximation of small, multilayer, air core coil inductance is as follows:
L
=
0.8 (rN) 2 6r + 9t + 10b
where:
=
L N
=
=
r I
b
Self-inductance in microhenries(,uH) Total number of turns Mean radius in inches Length of coil in inches Coil depth or thickness in inches
For example, a coil whose dimensions are as follows:
r
= = =
I
b N
0.1 inches 0.1 inches 0.1 inches 100 turns
would have an inductance of:
L
=
0.8 (0.1 • 100) 2 6 • 0.1 + 9 • 0.1 + 10 • 0.1
L
=
0.8(100) 80 = - - = 32 2.5 0.6 + 0.9 + 1
L
=
32 microhenries inductance.
As stated earlier, this inductance is analogous to inertia in mechanical systems in that inductance opposes a change in current as inertia opposes a change in velocity of a body. In alternating current circuits the current is always changing; therefore inductance is always opposing this change. As the current tries to change, the inductance reacts to oppose that change. This reaction is called inductive reactance. 17
INDUCTIVE REACTANCE
The unit of inductive reactance (XL) is in ohms. Because the amount of reactance is a function of the rate of change of current and rate of change can be described as frequency, a formula relating frequency, inductance, and inductive reactance is:
where: XL f
L
=
= =
Inductive reactance in ohms Frequency in hertz Inductance in henries
For example, using the 32 microhenry coil calculated earlier, operating at 100 kilohertz, its inductive reactance would be found as follows: XL L
=
f 27r
=
XL XL
=
= =
Inductive reactance 32 /LH or 0.000032H 100 kilohertz or 100,000 Hz 6.28 6.28 • 100,000 • 0.000032 20.096 ohms
Therefore, this coil would present an opposition of 20.096 ohms to currents with a rate of change of 100 kilohertz due to its reactive component. Unlike a resistive circuit, the current and voltage of an inductive circuit do not reach their minimum and maximum values at the-same time. In a pure inductive circuit the voltage leads the current by 90 electrical degrees. This means that when the voltage reaches a maximum value, the current is at "0." Power is related to current and voltage as follows: P
=
El
where: P E I
= = =
Power in watts Volts Current in amperes
Notice that in a pure inductive circuit, when the voltage is maximum, the current is "0"; therefore, the product El = 0. Inductive reactances consume no alternating power where resistive elements consume power and dissipate power in the form of heat. The opposition to current flow due to the resistive element of the coil and the reactive element of the coil do not occur at the same time; therefore, they cannot be added as scalar quantities. A scalar quantity is one having only magnitude; i.e., it is a quantity fully described by a number, but which does not involve any concept of direction. Gallons in a tank, temperature in a room, miles per hour, for example, are all scalars.
18
IMPEDANCE In order to explain the addition of reactance and resistance with a minimum of mathematical calculations we can again use the vector diagram or phasor diagram to explain this addition (19). A phasor diagram constructed with imaginary units on the ordinate. or (Y) axis and real units on the abscissa or (X) axis is shown in Figure 3.2a.
>a:
<(
z
z 0 <(
::!
REAL
R
(a)
(b)
Figure 3.2-lmpedance Diagram (Zetec, Inc.)
Substituting inductive reactance (XL) and resistance (R) we can find the resultant of the vector addition of XL and R. This resultant vector Z is known as impedance. Impedance is the total opposition to current flow. Further observation of Figure 3.2b reveals XL, R, and Z appear to form the sides of a right triangle. The mathematical solution of right triangles states the square of the hypotenuse is equal to the sum of the squares of the other two sides, or
Substituting Z, X1_, and R, the statement becomes:
further simplified: Z=~X?+FP
Let's try an example. What is the impedance of a coil having an inductance of 100 microhenries and a resistance of 5 ohms and being operated at 200 kilohertz?
First we must convert inductance to inductive reactance and then, by vector addition, combine inductive reactance and resistance to obtain the impedance. XL XL XL
z z
= = = = =
27rfl 6.28 o 200,000 Hz o 0.0001 H 6.28 o 20 = 125.6 ohms .J52+125.6 2 = --)15800.36 125.7 ohms
19
The importance of knowing the impedance of the test coil is more one of instrument consideration than coil design. Maximum transfer of power is accomplished when the driving impedance and load impedance are matched. If, for instance, your eddy current instrument had a driving impedance of 50 ohms, the most efficient test coils would also have impedances of 50 ohms. Other, more common examples of impedance matching are home stereo systems rated at 100 watts per channel into 8 ohms. We can discuss impedance in a more detailed manner by mathematically noting variables using imaginary numbers (4). The indicated square root of a negative number is known as an imaginary number. The imaginary number .J=16 can be written v'(-1)16 or A v'l6 or A •4. The notation A is used extensively and is mathematically noted by i. Since i is also used in electrical terms for current, the i notation for electrical calculations is changed to j. The term j, often called operator j, is equal to the ..;=-1. Instead of noting v'=16 as ../=1•4 we can simply note v'=16 as j4. Since reactance is known as an imaginary component, we can then note impedance Z = R + jXM = IZI.te. where Tan
e
=XL. r:l
The term R + jXM is known as a rectangular notation. As an example, a resistance of 4 ohms in series with an inductive reactance of 3 ohms could be noted as Z = 4 + j3 ohms. The impedance calculation is then:
Z
=
5 ohms
In coil design it is often helpful to know also the included angle between the resistive component and impedance. A convenient method of notation is the polar form where Tan e XL and e is the R included angle between resistance and impedance. In the previous example our impedance magnitude is 5 ohms, but at what angle?
=
A proper form of notation is Z/B.. where complete notation is
z is impedance and /B.. is the included angle. Therefore, the
e
= XL = R
Arc Tan 8
= 36.9
z
= 5~6.9°
Tan
3
4
= 0.750
o
Eddy current coils with included impedance angles of 60 o to 90 o usually make efficient test coils. As the angle between resistance and impedance approaches 0 °, the test coil becomes very inefficient with most of its energy being dissipated as heat.
Q OR FIGURE OF MERIT
The term used to describe coil efficiency is Q or merit of the coil. The higher the Q or merit of a coil, the more efficiently the coil performs as an inductor. The merit of a coil is mathematically stated as: Q
=
XL
R
where:
XL R
= =
Inductive reactance Resistance
For example, a coil having an inductive reactance of 100 ohms and a resistance of 5 ohms would have a Q of 20. 20
PERMEABILITY AND SHIELDING EFFECTS
The addition of permeable core materials in certain coil designs dramatically improves the Q factor. Permeable cores are usually constructed of high permeability "powdered iron." Probe coils, for example, are wound on a form that allows a powdered iron rod or slug to be placed in the center of the coil (4). It is common to increase the coil impedance by a factor of 10 by the addition of core materials. This increase in impedance without additional winding greatly enhances the Q of the coil. Some core materials are cylinder- or cup-shaped. A common term is cup core. The coil is wound and placed in the cup core. In the case of a probe coil in a cup core, not only is the impedance increased, but the benefit of shielding is also gained. Shielding with a cup core prevents the electromagnetic field from spreading at the sides of the coil. This greatly reduces the signals produced by edge effect of adjacent members to the test area, such as fasteners on an aircraft wing. Shielding, while improving resolution, usually sacrifices some amount of penetration into the part. Another method of shielding uses high conductivity material, such as copper or aluminum, to suppress high frequency interference from other sources and also to shape the electromagnetic field of the test coil. A copper cup would restrict the electromagnetic field in much the same manner as the "powdered iron cup core" discussed previously. A disadvantage of high conductivity, low or no permeability shielding is that the coil's impedance is reduced when the shielding material is placed around the test coil. The net effect is, of course, that the coil's Q is less than it was when the coil was surrounded by air. Another coil design used for inspection of ferromagnetic materials uses a saturation approach. A predominant variable that prevents eddy current penetration in ferromagnetic material is called permeability. Permeability effects exhibited by the test object can be reduced by means of magnetic saturation. Saturation coils for steels are usually very large and surround the test object and test coil. A steady state current is applied to the saturation coil. When the steel test object is magnetically saturated it may be inspected in the same manner as a non-ferromagnetic material. In the case of mild steel many thousands of gauss are required to produce saturation. In such other materials as nickel alloys (monel and inconel), the saturation required is much less and can usually be accomplished by incorporating permanent magnets adjacent to the test coil. COIL FIXTURES
Coil fixtures or holders may be as varied as the imagination of the designers and users. After the size, shape, and style have been decided upon, the next consideration should be the test environment. Characteristics of wear, temperature, atmosphere, mechanical stress, and stability must be considered (4). Normally wear can be reduced by selection of wear-resistant plastic compounds, or where severe wear is expected, artificial or genuine jewels may be used. Less expensive and very effective wear materials, such as aluminum oxide or ceramics, are more commonly used. Temperature stability may be accomplished by using coil holder material with poor heat transfer characteristics. Metals have high heat transfer characteristics, and often coils made with metal holders are sensitive to temperature variations caused by human touch. For high temperature applications, materials must be chosen carefully. Most common commercial copper coil wire may be used up to 150-200°C. For temperatures above 200°C, silver or aluminum wire with ceramic or high temperature silicone insulation must be used. Materials must be chemically compatible with the test object. As extreme examples, a polystyrene coil form would not be used to inspect an acetone cooler, or a lead or graphite housing allowed to come in contact with an inconel jet engine tail cone producing service-related stress cracks. Mechanical and electrical stability of the test coil can be enhanced by an application of epoxy resin between each layer of coil winding. This accomplishes many objectives: (1) it seals the coil to exclude moisture; (2) it provides additional electrical insulation; and (3) it provides mechanical stability. 21
Characteristics listed are not in order of importance. The importance of each characteristic is deter· mined by specific test requirements.
REVIEW QUESTIONS 0.3·1
A coil's resistance is determined by: A. wire material. B. wire length. C. wire cross-sectional area. D. all of the above.
0.3·2
Inductance is analogous to: A. force. B. volume. C. inertia. D. velocity.
0.3·3
The unit A. B. C. D.
0.3·4
The inductance of a multilayer air core coil with the dimensions I= 0.2, r= 0.5, b = 0.1, and N = 20, is: A. 1.38 henries. B. 13.8 microhenries. C. 13.8 ohms. D. 1.38 ohms.
0.3·5
The inductive reactance of the coil in 0.3·4, operating at 400 kHz, would be: A. 1380 ohms. B. 5520 ohms. C. 34.66 ohms. D. 3466 ohms.
0.3·6
The impedance of a 100 microhenry coil with a resistance of 20 ohms operating at 100 kHz would be: A. 62.8 ohms. B. 4343.8 ohms. C. 628 ohms. D. 65.9 ohms.
0.3·7
The Q or merit of a coil is the ratio of: A. Z XL B. XL.
of inductance is the: henry. maxwell. ohm. farad.
z
C.
XL,
D.
R R
xL· 0.3·8
The incorporation of magnetic shielding: A. improves resolution. B. decreases field extension. C. increases impedance. D. does all of the above.
0.3·9
The purpose of a steady-state winding incorporated in a test coil is to: A. reduce permeability effects. B. provide magnetic saturation. C. provide a balance source for the sensing coil. D. both A and B. 22
0.3·10
The most important consideration when selecting a test coil is: A. sensitivity. B. resolution. C. stability. D. test requirement and compatibility.
23
4.
EFFECTS OF TEST OBJECT ON TEST COIL
As we have seen, the eddy current technique depends on the generation of induced currents within the test object. Perturbations or disturbances in these small induced currents affect the test coil. The result is variance of test coil impedance due to test object variables. These are called operating variables (19). Some of the operating variables are coil impedance, electrical conductivity, magnetic permeability, skin effect, lift-off, fill factor, end effect, edge effect, and signal-to-noise ratio. Coil impedance was discussed at length in the previous Section. In this Section coil impedance changes will be represented graphically to more effectively explain the interaction of other operating variables.
ELECTRICAL CONDUCTIVITY In electron theory the atom consists of a positive nucleus surrounded by orbiting negative electrons. Materials that allow these electrons to be easily moved out of orbit around the nucleus are classified as conductors. In conductors electrons are moved by applying an outside electrical force. The ease with which the electrons are made to move through the conductor is called conductafce. A unit of conductance is the mho. The mho is the reciprocal of the ohm, or conductance G = - , where G is conductance in mhos and R is resistance in ohms. R In eddy current testing, instead of describing conductance in absolute terms, an arbitrary unit has been assigned. Since the relative conductivity of metals and alloys varies over a wide range, the need for a conductivity benchmark is of prime importance. The International Electrochemical Commission established in 1913 a convenient method of comparing one material to another. The commission established that a specified grade of high purity copper, 1 meter in length and uniform section of 1 mm 2 measuring 0.017241 ohms at 20 oc would be arbitrarily considered 100 percent conductive. The symbol for conductivity is a (sigma) and the unit is % lACS or percent of the International Annealed Conductivity Standard. Table 4.1 lists materials by conductivity and resistivity. A statement can be made about a conductor in terms of conductance or resistance. Note that a good conductor is a poor resistor. Conductance and resistance are direct reciprocals as stated earlier. Conductivity and resistivity, however, have different origins and units; therefore, the conversion is not so direct. As previously discussed, conductivity is expressed on an arbitrary scale in % lACS. Resistivity is expressed in absolute terms of micro ohm-centimeter. To convert to either unit, simply follow the equation: %lACS
. . . ~ 72 ·~ 1
=
Res1st1v1ty m m1cro ohm-em
As the test coil is influenced by different conductivities, its impedance varies inversely to conductivity. A higher conductivity causes the test coil to have a lower impedance value. Figure 4.1 illustrates this concept.
i\- ---------0%
(AIR)
~
"(
CONlUCTI1TY-
'\\
w
(.)
z ~ < w
2%-._
(.)
5%
0::
10%
\
---~ ---\.
100riACi . . . . . RESISTANCE
---+
Figure 4.1-Measured Conductivity Locus (ASM Committee on Eddy Current Inspection, "Eddy-Current Inspection," Metals Handbook, Vol. 11, 8th Ed., Howard E. Boyer, Editor, American Society for Metals, 1976, p. 77.) 24
The coil's inductive reactance is represented by theY axis and coil resistance appears on the X axis. The 0 percent conductivity point, or air point, is when the coil's empty reactance (XLo) is maximum. Figure 4.1 represents a measured conductivity locus (4). Conductivity is influenced by many factors. Table 4.1 (19, p. 206) lists conductivities of materials with different chemical compositions.
Table 4.1-Eiectrical Resistivity and Conductivity of Several Common Metals and Alloys (ASM Committee on Eddy Current Inspection, "Eddy-Current Inspection," Metals Handbook, Vol. 11, 8th Ed., Howard E. Boyer, Editor, American Society for Metals, 1976, p. 79.) Resistivity ohm-em X 10-6
Metal or alloy
1.63 1.72 2.44 2.82
Silver Copper, annealed Gold Aluminum Aluminum alloys: 6061-T6 7075-T6 2024-T4 Magnesium 70-30 brass Phosphor bronzes Monel Zirconium Zircaloy-2 Titanium Ti-6AI-4V alloy 304 stainless steel lnconel 600 Hastelloy X Waspaloy
4.1 5.3 5.2 4.6 6.2 16 48.2 50 72 54.8 172 70 98 115 123
Conductivity
%/ACS 105 100 70 61 42 32 30 37 28 11 3.6 3.4 2.4 3.1 1.0 2.5 1.7 1.5 1.4
Some other factors affecting conductivity are temperature, heat treatment, grain size, hardness, and residual stresses. A change in the temperature of the test object will change the electrical conductivity of that object. In metals, as the temperature is increased, the conductivity is decreased. Carbons and carbon compounds have negative temperature coefficients; therefore, their conductivity increases as temperature is increased. Heat treatment also affects electrical conductivity by redistributing elements in the material. Dependent upon materials and degree of heat treatment, conductivity can either increase or decrease as a result of heat treatment. Stresses in a material due to cold working produces lattice distortion or dislocation (2). This mechanical process changes the grain structure and hardness of the material, changing its electrical conductivity. Hardness in "age hardenable" aluminum alloys changes the electrical conductivity of the alloy. The electrical conductivity decreases as hardness increases. As an example, a Brinell hardness of 60 is represented by a conductivity of 23, and a Brinell hardness of 100 of the same alloy would have a conductivity of 19.
25
PERMEABILITY
Permeability of any material is a measure of the ease with which its atoms can be aligned, or the ease with which it can establish lines of force (2). Materials are rated on a comparative basis. Air is assigned a permeability of 1. A basic determination of permeability, f1- (pronounced "mu"), is:
f1-
Number of Lines Produced with the Material as a Core Lines Produced with Air as a Core
= Number of
Ferromagnetic metals and alloys including nickel, iron, and cobalt tend to concentrate magnetic flux lines (19). Ferromagnetic material or sintered ionic compounds are also useful in concentrating magnetic flux (4). Magnetic permeability is not constant for a given material. The permeability depends more upon the magnetic field acting upon it. As an example, consider a magnetic steel bar placed in an encircling coil. As the coil current is increased, the magnetic field of the coil will increase. The magnetic flux within the steel will increase rapidly at first, and then will tend to level off as the steel approaches magnetic saturation. This phenomenon is called the Barkhausen effect (4). When increases in the magnetizing force produce little or no change on the flux within the steel bar, the bar is magnetically saturated. When ferromagnetic materials are saturated, permeability becomes constant. With magnetic permeability constant, ferromagnetic materials may be inspected using the eddy current method. Without magnetic saturation, ferromagnetic materials exhibit such a wide range of permeability variat_ion that signals produced by discontinuities or conductivity variations are masked by the permeability signal (19). Permeability effects are most predominant at lower frequencies (5). Other magnetic effects include diamagnetic and paramagnetic (4).
SKIN EFFECT
Electromagnetic tests in many applications are most sensitive to test object variables nearest the test coil due to skin effect. Skin effect is a result of mutual interaction of eddy currents, operating frequency, test object conductivity, and permeability. The skin effect, the concentration of eddy currents in the test object nearest the test coil, becomes more evident as test frequency, test object conductivity, and permeability are increased (4). For current density or eddy current distribution in the test object, refer to Figure 1.9 in Section 1.
EDGE EFFECT
The electromagnetic field produced by an excited test coil extends in all directions from the coil. As test object geometrical boundaries are approached by the test coil, they are sensed by the coil prior to the coil's arrival at the boundary. The coil's field precedes the coil by some distance (2) determined by coil parameters, operating frequency, and test object characteristics. As the coil approaches the edge of a test object, eddy currents become distorted by the edge signal. This is known as edge effect. Response to the edges of test objects can be reduced by the incorporation of magnetic shields around the test coil or by reducing the test coil diameter. Edge effect is a term most applicable to the inspection of sheets or plates with a probe coil.
END EFFECT End effect follows the same logic as edge effect. End effect is the signal observed when the end of a product approaches the test coil. Response to end effect can be reduced by coil shielding or reducing coil length in OD encircling or ID bobbin coils. End effect is a term most applicable to the inspection of bar or tubular products. 26
LIFT-OFF
Electromagnetic coupling between test coil and test object is of prime importance when conducting an eddy current examination. The coupling between test coil and test object varies with spacing between the test coil and test object. This spacing is called lift-off(4). The effect on the coil impedance is called lift-off effect.
RESISTANCE - -
Figure 4.2-Lift-off Conductivity Relationship (ASM Committee on Eddy Current Inspection, "Eddy-Current Inspection," Metals Handbook, Vol. 11, 8th Ed., Howard E. Boyer, Editor, American Society for Metals, 1976, p. 79.)
Figure 4.2 shows the relationship between air, conductive materials, and lift-off. The electromagnetic field, as previously discussed, is strongest near the coil and dissipates with distance from the coil. This fact causes a pronounced lift-off effect for small variations in coil-to-object spacing. As an example, a spacing change from contact to 0.001 in. will produce a lift-off effect many times greater than a spacing change of 0.010 in. to 0.011 in. (19). Lift-off e·ffect is generally an undesired effect causing increased noise and reduced coupling resulting in poor measuring ability (12). In some instances, equipment having phase discrimination capability can readily separate lift-off from conductivity or other variables. Lift-off can be used to advantage when measuring nonconductive coatings on conductive bases. A nonconductive coating such as paint or plastic causes a space between the coil and conducting base, allowing lift-off to represent the coating thickness. Lift-off is also useful in profilometry and proximity applications. Lift-off is a term most applicable to testing objects with a surface or probe coil. FILL FACTOR Fill factor is a term used to describe how well a test object will be electromagnetically coupled to a test coil that surrounds or is inserted into the test object. Fill factor then pertains to inspections using bobbin or encircling coils. Like lift-off, electromagnetic coupling between test coil and test object is most efficient when the coil is nearest the surface of the part.
Fill factor can be described as the ratio of test object diameter to coil diameter squared. The diameters squared is a simplified equation resulting in the division of effective coil and part areas. The area of a circle (A) is determined using the equation:
}appears in both numerator and denominator of the fractional equation; therefore~ cancels, leaving the ratio of diameters squared
g 2
2
=
TJ
=
fill factor (5).
Fill factor will always be a number less than 1, and efficient fill factors approach 1. A fill factor of 0.99 is more desirable than a fill factor of 0.75. The effect of fill factor on the test system is that poor fill factors do not allow the coil to be sufficiently loaded by the test object. This is analogous to the effect of drawing a bow only slightly and releasing an arrow. The result is, with the bow slightly drawn and released, little effect is produced to propel the arrow. 27
In electrical terms, we say the coil is loaded by the test object. How much the coil is loaded by the test object due to fill factor can be calculated in relative terms. A test system with constant current capabilities being affected by a conductive nonmagnetic bar placed into an encircling coil can be used to demonstrate this effect. For this example, the system parameters are as follows: (a) (b) (c) (d)
Unloaded coil voltage equals 10 volts. Test object effective permeability (5) equals 0.3. Test coil inside diameter equals 1 inch. Test object outside diameter equals 0.9 inches.
An equation demonstrating coil loading is given by: E
= Eo (1-71 + 1/J.teff)
where: E0 E 11
P.eff
= = =
=
Coil voltage with coil affected by air Coil voltage with coil affected by test object Fi II factor Effective permeability
When the nonmagnetic test object is inserted into the test coil, the coil's voltage will decrease. E
E E E
= = = =
10 (1-0.81 + 0.81 • 0.3) 10 (0.19 + 0.243) 10 (0.433) 4.3 volts
This allows 10- 4.3 or 5.7 volts available to respond to test object changes caused by discontinuities or decreases in effective conductivity of the test object. It is suggested that the reader calculate the resultant loaded voltage developed by a 0.5 inch bar of the same material and observe the relative sensitivity difference.
DISCO NTI N UITI ES Any discontinuity that appreciably changes the normal eddy current flow can be detected. Discontinuities, such as cracks, pits, gouges, vibrational damage, and corrosion, generally cause the effective conductivity of the test object to be reduced. Discontinuities open to the surface are more easily detected than subsurface discontinuities (19). Discontinuities open to the surface can be detected with a wide range of frequencies; subsurface investigations require a more careful frequency selection. Discontinuity detection at depths greater than 0.5 inch in stainless steel is very difficult. This is in part due to the sparse distribution of magnetic flux lines at the low frequency required for such deep penetrations. Figure 1.9 (p. 6) is again useful to illustrate discontinuity response due to current distribution. As an example, consider testing a non-ferromagnetic tube at a frequency that establishes a standard depth of penetration at the midpoint of the tube wall. This condition would allow a relative current density of approximately 20 percent on the far surface of the tube. With this condition, identical near and far surface discontinuities would have greatly different responses. Due to current magnitude alone, the near surface discontinuity response would be nearly 5 times that of the far surface discontinuity.
28
Discontinuity orientation has a dramatic effect on response. As seen earlier, discontinuity response is maximum when eddy currents and discontinuities are at 90°, or perpendicular. Discontinuities parallel to the eddy current flow produce little or no response. The easiest method to insure detectability of discontinuities is to use a reference standard or model that provides a consistent means of adjusting instrumentation (12).
SIGNAL-TO-NOISE RATIO
Signal-to-noise ratio is the ratio of signals of interest to unwanted signals (4). Common noise sources are test object variations of surface roughness, geometry, and homogeneity. Other electrical noises can be due to such external sources as welding machines, electric motors and generators. Mechanical vibrations can increase test system noise by physical movement of test coil or test object. In other words, anything that interferes with a test system's ability to define a measurement is considered noise. Signal-to-noise ratios can be improved by several methods. If a part is dirty or scaly, signal-to-noise ratio can be improverl by cleaning the part. Electrical interference can be shielded or isolated. Phase discrimination and f1irering can improve signal-to-noise ratio. It is common practice in nondestructive testing to require a minimum signal-to-noise ratio of 3 to 1. This means a signal of interest must have a response at least three times that of the noise at that point.
REVIEW QUESTIONS 0.4-1
Materials A. B. C. D.
0.4-2
100% lACS is based on a specified copper bar having a resistance of: A. 0.01 ohms. B. 100 ohms. C. 0.017241 ohms. D. 172.41 ohms.
0.4-3
A resistivity of 13 micro ohm-em is equivalent to a conductivity in % lACS of _ _ _ __ A.
that hold their electrons loosely are classified as: resistors. conductors. semiconductors. insulators.
B.
11.032 0.0625
C.
16.52
D.
13.26
0.4-4
A prime factor affecting conductivity is: A. temperature. B. hardness. C. heat treatment. D. all of the above.
0.4-5
Materials that tend to concentrate magnetic flux lines are _ _ _ __ A. conductive B. permeable C. resistive D. inductive
0.4-6
Diamagnetic materials have _ _ _ __ A. a permeability greater than air B. a permeability less than air C. a permeability greater than ferromagnetic materials D. no permeability 29
0.4·7
When an increase in field intensity produces little or no additional flux in a magnetic test object, the object is considered: A. stabilized. B. balanced. C. saturated. D. at magnetic threshold.
0.4·8
Edge effect can be reduced by: shielding. A. B. selecting a lower frequency. C. using a smaller coil. D. both A and C.
0.4·9
Lift-off signals produced by a 0-10 mil spacing change are approximately _ _ _ _ _ times greater than a 80-90 mil spacing change.
A. B. C. D.
10 2 5 100
0.4·10
Calculate the effect of fill factor when a conducting bar 0.5 inches in diameter with an effective permeability of 0.4 is placed into a 1-inch diameter coil with an unloaded voltage of 10 volts. The loaded voltage is _ _ _ __ A. 2 volts B. 4.6 volts C. 8.5 volts D. 3.2 volts
0.4·11
Laminations are easily detected with the eddy current method. A. True B. False
0.4·12
Temperature changes, vibration, and environmental effects are test coil inputs that generate: A. unwanted signals. B. magnetic fields. C. eddy currents. D. drift.
30
5.
SELECTION OF TEST PARAMETERS
As NOT engineers and technicians, it is our responsibility to industry to provide and perform nondestructive examinations that in some way assure the quality or usefulness of industry products. In order to apply a nondestructive test, it is essential that we understand the parameters affecting the test. Usually, industry establishes a product or component and then seeks a method to inspect it. This practice establishes test object geometry, conductivity, and permeability prior to the application of the eddy current examination. Instrumentation, test coil, and test frequency selection become the tools used to solve the problem of inspection. Test coils were discussed previously, and instrumentation will be discussed later in this text. Test frequencies and their selection will be examined in detail in this Section.
FREQUENCY SELECTION
In Section 1, we observed that eddy currents are exponentially reduced as they penetrate the test object. We also observed a time or phase difference in these currents. The currents near the test coil happen first, or lead the current that is deeper in the object. A high current density allows good detectability, and a wide phase difference between near and far surfaces allows good resolution.
Single Frequency Systems
Unfortunately, if a low frequency is selected to provide good penetration and detectability, the phase difference between near and far surface is reduced. Selection of frequency often becomes a compromise. It is common practice in in-service inspection of thin wall, non-ferromagnetic tubing to establish a standard depth of penetration just past the midpoint of the tube wall (17). This permits about 25 percent of the available eddy current to flow at the outside surface of the tube wall. In addition, this establishes a phase difference of approximately 150 to 170 degrees between the inside and outside surface of the tube wall. The combination of 25 percent outside, or surface current, and 170 degrees included phase angle provides good detectability and resolution for thin wall tube inspection. The depth of penetration formula discussed in Section 1, although correct, has rather cumbersome units. Conductivity is usually expressed in percent of the "International Annealed Copper Standard" (% lACS). Resistivity is usually expressed in terms of micro-ohm-centimeter {J.tOcm) (9). Depths of penetration are normally much less than 0.5 inch. A formula using these units may be more appropriate and easier to use. A depth of penetration formula using resistivity, frequency, and permeability can be expressed as follows:
where: 0
K Q
f It rei
= = = = =
Depth of penetration in inches Constant = 1.98 Resistivity in ~-tOcm Frequency in hertz 1 for non-ferromagnetic materials
31
For non-ferromagnetic materials the term JLrel is ignored. The equation for non-ferromagnetic materials then becomes:
o=K~ As technicians and engineers, our prime variable is frequency. By adjusting frequency we can be selectively responsive to test object variables. Solving the non-ferromagnetic depth of penetration formula for frequency requires a simple algebraic manipulation as follows:
(a) (b) (c)
=K~
0 0
Vf
=
K 02 K2
=
(d)
K2 02
(e)
K2P 02
Q
f
f -Q
= for f =
(1.98)2 e
02
As an example of how this may be used consider inspecting an aluminum plate 0.3 inch thick, fastened to a steel plate at the far surface. Effects of the steel part are undesirable and require discrimination or elimination. The aluminum plate has a resistivity of 5 micro-ohm-em. By establishing a depth of penetration at 0.1 inch, the far surface current will be less than 10 percent of the available current, thus reducing response to the steel part. The frequency required for this can be calculated by applying: f
=
f
=
(1.98)2(5) 0.1 2 1960 hertz
=
19.6 0.01
If detection of the presence of the steel part was required, the depth of penetration could be reestablished at 0.3 inch in the aluminum plate, and a new frequency could be calculated.
f
f
= =
19.6 (0.3) 2
-
19.6 0.09
218 hertz
Another approach to frequency selection uses argument "A" of the Bessel function (5) where argument "A" is equal to unity or 1. A
=
fJLre1ad
2
5066 where: f
= Frequency in hertz
a
=
d JLrel
Conductivity meter/ohm-mm 2 = Diameter of test object, em = Relative permeability
32
A frequency can always be selected to establish factor "A" equal to 1. This frequency is known as the limit frequency and is noted by the term f 9 . By substituting 1 for factor "A" and f for f, the equa9 tion becomes:
=
1
or
Limit frequency (f 9) is then established in terms of conductivity, permeability dimension, and a constant (5066). Since limit frequency is based on these parameters, a method of frequency determination using a test frequency to limit frequency ratio f/f 9 can be accomplished. High f/f 9 ratios are used for near surface tests, and lower f/f 9 ratios are used for subsurface tests. Often results of such tests are represented graphically by diagrams. These diagrams are called impedance diagrams (4). Impedance illustrated by vector diagrams in Section 3 shows inductive reactance represented on the ordinate axis and resistance on the axis of abscissa. The vector sum of the reactive and resistive components is impedance. This impedance is a quantity with magnitude and direction that is directly proportional to frequency. In order to construct a universal impedance diagram valid for all frequencies, the impedance must be normalized (4). Figure 5.1 illustrates a normalization process.
E X
X
......
u
' .... u
~
<( ,_
<(
.... "'
z
z
u
D
1.0
u
......
<(
"'
Rl
c
RES I STANCE - R RES I STANCE - R
(b)
(a)
Figure 5.1-Effect of Frequency Change: (a) Primary Impedance Without Secondary Circuit; (b) Primary Impedance with Secondary Circuit (Reprinted with permission from Hugo L. Libby, Introduction to Electromagnetic Nondestructive Test Methods, p. 37. Copyright©1971, John Wiley & Sons, Inc.)
Figure 5.1 a shows the effect on primary impedance Zp with changes in frequency (w 5.1 a represents primary impedance without a secondary circuit or test object.
= 21rf). Figure
Figure 5.1 b illustrates the effect of frequency on primary impedance with a secondary circuit or test object present. The primary resistance R1 in Figure 5.1a has been subtracted in Figure 5.1b sinceresistance is not affected by frequency. The term wLsG in Figure 5.1 b represents a reference quantity for the secondary impedance. The units are secondary conductance G and wL 5 secondary reactance.
33
Further normalization is accomplished by dividing the reactive and resistive components by the term wl 0 or the primary inductive reactance without a secondary circuit present. Figure 5.2 shows a typical normalized impedance diagram (19).
1.1
0.2
10
-...:;
~ ~ 1.2
09
~06
'~C\ 18\
08
ho,Jw"OI
2~
"'0.7
22 \
:;: :X:
0
SOLID CYLINDRICAL
0
..J
BAR~
1\ce v
\
3
}
06
\
2.6
UJ
u
z
...u
2
>= u
a
I I.O'JwL 0 G
:1 2
:::> Cl
3.4
z
3.6
04
J
3.8/
7
03
0
v ! ,v /
0.2
01
THIN·WALL TUBE
cz
0
25
~ 01
1/. vr
0.2
03
04
j,
1.4
05
0.6
0.7
0.8
NORMALIZED RESISTANCE (R/wlo) OHMS
Figure 5.2-Normalized Impedance Diagram for a long coil encircling a solid cylindrical non-ferromagnetic bar, showing also the locus for a thin-wall tube. (k = electromagnetic-wave propagation constant for a conducting material or ..JWf.La; r = radius of conducting cylinder, meters; w = 21rf; f = frequency; ~ := equivalent of .,Jwf.La for simplified electrical circuits; f.L = magnetic permeability of bar, or = 47r • 10" 7 henries/meter if bar is nonmagnetic; a = electrical conductivity of bar, mhos/meter; 1.0 = coil fill factor.) (ASM Committee on Eddy Current Inspection, "Eddy-Current Inspection," Metals Handbook, Vol. 11, 8th Ed., Howard E. Boyer, Editor, American Society for Metals, 1976, p. 82.)
The terms wl!wlo and R/wlo represent the relative impedance of the test coil as affected by the test object.
34
Signals generated by changes in wl orR caused by test object conditions such as surface and subsurface discontinuities may be noted by L:lwl or L:lR. The L:lwl 0 and L:lR notation indicates a change in the impedance. Figure 5.3 shows the impedance variation in a non-ferromagnetic cylinder caused by surface and subsurface discontinuities (5).
_f =50 9--+-----1
0
0.04
0 INSTITUT DR. FOERSTER
Figure 5.3-lmpedance Variations caused by surface and subsurface cracks in non-ferromagnetic cylinders, at a frequency ratio f/fg = 50 (5, p. 37-21)
Figure 5.3 also illustrates a sensitivity ratio for surface and subsurface discontinuities. Notice with an f/fg ratio of 50, a relatively high frequency, the response to subsurface discontinuities is not very pronounced.
35
Figure 5.4 shows responses to the same discontinuities with an f/f 9 ratio of 15. This lower frequency allows better detection of subsurface discontinuities as shown in Figure 5.4.
0.14 0.12
0.08
0.04 0.02
0 t:.R
0.06(..)Lo INSTITUT DR. FOERSTER
Figure 5.4-lmpedance Variations caused by surface and subsurface cracks in non-ferromagnetic cylinders, at a frequency ratio f/f = 15 (5, p. 37·20) 9
Multifrequency Systems
It becomes obvious that the technician must have a good working knowledge of current density and phase relationships in order to make intelligent frequency choices. The frequency choice discussed to date deals with coil systems driven by only one frequency. Test systems driven by more than one frequency are called multifrequency or multiparameter systems. It is common for a test coil to be driven with three or more frequencies. Although several frequencies may be applied simultaneously or sequentially to a test coil, each of the individual frequencies follows rules established by single frequency methods. Signals generated at the various frequencies are often combined or mixed in electronic circuits that algebraically add or subtract signals to obtain a desired result.
36
One multifrequency approach is to apply a broadband signal, with many frequency components, to the test coil (4). The information transmitted by this signal is proportional to its bandwidth, and the logarithm of 1 plus the signal-to-noise power ratio. This relationship is stated by the equation:
C
= W Log
2
(1
+
~)
where: C W S
N
= =
Rate of information transmitted in bits per second Bandwidth of the signal Signal-to-noise power ratio
This is known as the Shannon-Hartley law.
Another approach to multiparameter methods is to use a multiplexing process (12). The multiplexing process places one frequency at a time on the test coil. This results in zero cross-talk between frequencies and eliminates the need for bandpass filters. The major advantages of a multiplex system are (1) lower cost, (2) greater flexibility in frequency selection, and (3) no cross-talk between frequency channels. If the multiplex switching rate is sufficiently high, both broadband and multiplex systems have essentially the same results. The characterization of eddy current signals by their phase angle and amplitude is a common practice and provides a basis for signal mixing to suppress unwanted signals from test data (12). Two frequencies are required to remove each unwanted variable. Practical multi parameter frequency selection can be demonstrated by the following example: Problem: Eddy current inspection of installed thin-wall non-ferromagnetic heat exchanger tubing. Tubing is structurally supported by ferromagnetic tube supports at several locations. It is desired to remove the tube support response signal from tube wall data. Solution: Apply a multiparameter technique to suppress tube support signal response.
First, a frequency is selected to give optimum phase and amplitude information about the tube wall. We shall call this the prime frequency. At the prime frequency, the response to the tube support and a calibrating through-wall hole are equal in amplitude response. A second frequency called the subtractor frequency is selected on the basis of tube support response. Since the tube support surrounds the OD of the tube, a low frequency is selected. At the subtracter frequency the tube support signal response is approximately 10 times greater than the calibrating through-wall hole. If the mixing unit amplitude adjustments are set so that both prime and subtractor tube support signal amplitudes are equal and phased in a manner to cause signal subtraction, the tube support signals cancel, leaving only slightly attenuated prime data information. For suppression of inside or near surface signals, a higher subtractor frequency would be chosen. A combination of prime, low, and high subtractor frequencies is often used to suppress both near and far surface signals, leaving only data pertaining to the part thickness and its condition. Optimization of frequency then depends on the desired measurement or parameter of interest (11, 12, 4).
37
REVIEW QUESTIONS
0.5·1
What frequency is required to establish one standard depth of penetration of 0.1 inch in Zirconium? A. 19.6 kHz B. 196Hz C. 3.4 kHz D. 340Hz
0.5·2
In order to reduce effects of far surface indications, the test frequency _ _ _ __ must be mixed A. B. must be raised C. must be lowered D. has no effect
0.5·3
The frequency required to establish the Bessel function Argument "A" equal to 1 is called A. B. C.
D.
optimum frequency resonant frequency limit frequency penetration frequency
= 50.6 meter/ohm mm
Q.5·4
Calculate the limit frequency for a copper bar (o correct limit frequency is _ _ _ __ A. 50 kHz B. 50.6 Hz C. 100 Hz D. 100kHz
0.5·5
Using the example in Question 5.4, what is the f/f 9 ratio if the test A. 1.2 B. 120 c. 60 D. 600
0.5·6
In Figure A. B. C.
D.
2
)
1 em in diameter. The
fr~quency
is 60 kHz?
5.1 b the value wl 5 G equaling 1.4 would be indicative of _ _ _ __ a high resistivity material a high conductivity material a low conductivity material a nonconductor
0.5-7
Primary resistance is subtracted from Figure 5.1b because _ _ _ __ A. resistance is always constant B. resistance is not frequency dependent C. resistance does not add to the impedance D. none of the above
0.5-8
The reference quantity is different for solid cylinder and thin-wall tube in Figure 5.2 because A, B. C. D.
0.5·9
the the the the
frequency is different conductivity is different skin effect is no longer negligible thin-wall tube has not been normalized
A 25 percent deep crack open to the near surface gives a response than the same crack 3.3 percent of diameter under the surface (ref. Figure 5.4). A. 10 B. 2.4 C. 1.25 D. 5
38
times greater
0.5·10
When using multifrequency systems, low subtracter frequencies are used to suppress A. B. C. D.
conductivity changes far surface signals near surface signals permeability changes
39
6.
INSTRUMENT SYSTEMS
Most eddy current instrumentation is categorized by its final output or display mode. There are basic requirements common to all types of eddy current instrumentation. Five different elements are usually required to produce a viable eddy current instrument (4). These functions are excitation, modulation, signal preparation, signal analysis, and signal display. An optional sixth component would be test object handling equipment. Figure 6.1 illustrates how these components interrelate.
EXCITATION
MODULATION
TEST OBJECT
SIGNAL PREPARATION
BALANCE OR COMPENSATING NETWORKS FILTERS SIGNAL SHAPING CIRCUITS
DEMODULATION AND ANALYSIS
DISPLAY OR INDICATION
RECORDERS ALARMS RELAYS AUTOMATIC MECHANISMS
T:!~~!~~T ~--------------------------------Figure 6.1-lnternal Functions of the Electromagnetic Nondestructive Test (Reprinted with permission from Hugo L. Libby, Introduction to Electromagnetic Nondestructive Test Methods, p. 60. Copyright © 1971, John Wiley & Sons, Inc.)
The generator provides excitation signals to the test coil. The signal modulation occurs in the electromagnetic field of the test coil assembly. Next, the signal preparation section, usually a balancing network, prepares the signal for demodulation and analysis. In the signal preparation stage, balance networks are used to "null" out steady-value alternating current signals. Amplifiers and filters are
also part of this section to improve signal-to-noise ratio and raise signal levels for the subsequent demodulation and analysis stage. The demodulation and analysis section is made up of detectors, analyzers, discriminators, filters, and sampling circuits. Detectors can be a simple amplitude type or a more sophisticated phase/ amplitude or coherent type. The signal display section is the key link between the test equipment and its intended purpose. The signal can be displayed many different ways. Common displays include cathode ray tube (CRT) oscilloscopes, meters, recorders, visual or audible alarms, computer terminals, and automatic signalling or reject equipment.
40
A series of simple eddy current instruments is shown in Figure 6.2 a, b, c, and d (19).
GROUND (a)
GROUND
GROUND GROUND (b)
(a)
GROUND (c)
GROUND
(b)
GROUND
(c)
GROUND (d)
GROUND
SAMPLE
(d)
Figure 6.2-Four Types of Simple Eddy Current Instruments (ASM Committee on Eddy Current Inspection, "Eddy-Current Inspection," Metals Handbook, Vol 11, 8th Ed., Howard E. Boyer, Editor, American Society for Metals, 1976, p. 86.)
In Figure 6.2a, the voltage across the inspection coil is monitored by an ac voltmeter. This type of instrument could be used to measure large lift-off variations where accuracy was not critical. Figure 6.2b shows an impedance bridge circuit. This instrument consists of an ac exciting source, dropping resistors, and a balancing impedance. Figure 6.2c is similar to Figure 6.2b. In Figure 6.2c a balance coil similar to the inspection coil is used to provide a balanced bridge. Figure 6.2d illustrates a balancing coil affected by a reference sample. This is commonly used in external reference differential coil tests. In all cases, since only the voltage change or magnitude is monitored, these systems can all be grouped as impedance magnitude types (5). Eddy current testing can be divided into three broad groups (2). The groups are impedance testing, phase analysis testing, and modulation analysis testing. Impedance testing is based on gross changes in coil impedance when the coil is placed near the test object. Phase analysis testing is based on phase changes occurring in the test coil and the test object's effect on those phase changes. Modulation analysis testing depends on the test object passing through the test coil's magnetic field at a constant rate. The amount of frequency modulation observed as a discontinuity passes through the test coil's field and is a function of the transit time of the discontinuity through the coil's field. The faster the transit time, the greater the modulation.
IMPEDANCE TESTING With impedance magnitude instrumentation it is often difficult to separate desired responses, such as changes in conductivity or permeability, from dimensional changes. A variation of the impedance magnitude technique is the reactance magnitude instrument (5). In reactance magnitude tests, the test coil is part of the fundamental frequency oscillator circuit. This operates like a tuned circuit where the oscillator frequency is determined by the test coil's inductive reactance. As the test coil is affected by the test object, its inductive reactance changes, which in turn changes the oscillator frequency. The relative frequency variation ~flf is, therefore, an indication of test object condition. Reactance magnitude systems have many of the same limitations as impedance magnitude systems (5).
41
PHASE ANALYSIS TESTING
Phase analysis techniques are divided into many subgroups depending on the type of data display. Some of the various types are vector point, impedance plane, ellipse, and linear time base (2). The vector point circuit and display are illustrated in Figure 6.3. Vector Point
TO HERE
FROM HERE
CONDUCTIVITY HORIZONTAL
"A"
FROM HERE
TO HERE
DIMENSIONAL CHG VERTICAL "B"
STANDARD REFERENCE
PROCESSING CIRCUITS
GENERATOR
o·
TEST ARTICLE'
270. CRT
V,
"' D " = PERMEABILITY
D
=
y
DIMENSION
- - -)1
a = CONDUCTIVITY
/I /
//
I
I
/ VOLTAGE PLANE
42
X V,
0
Figure 6.3-Vector Point Method (2, p. 3·15) Reprinted with permission.
POINT OF LIGHT
The vector point display is a point of light on a CRT. The point is the vector sum of theY and X axis voltages present in the test coil (2). By proper selection of frequency and phase adjustment, voltage V1 could represent dimensional changes and voltage V2 could represent changes in conductivity.
Ellipse The ellipse method is shown in Figure 6.4 .
s. .__
RE-F-ER_E_N_CE_s_T_A_ND_A_R_D_
-~•71.~ PROCESSING CIRCUITS
GENERATOR
~~-TE-STA-RT-ICLE-----~~'l~ PHASE SHIFTER
Figure 6.4-EIIipse Method (2, p. 3-16) Reprinted with permission.
As with the vector point method, the test object and reference standard are used to provide a balanced output. A normal balanced output is a straight horizontal line. Figure 6.5 shows typical ellipse responses (2).
DIMENSION
SMALL CHANGE
LARGE CHANGE
SMALL CHANGE
LARGE CHANGE
SMALL CHANGE
LARGE CHANGE
CONDUCTIVITY
BOTH DIMENSION AND CONDUCTIVITY
Figure 6.5-CRT Displays for Dimension and Conductivity (2, p. 3-17) Reprinted with permission.
43
With the ellipse method the vertical deflection plates of a CRT are energized by an amplified voltage from the secondary test coils (5). The horizontal deflection plates are energized by a voltage that corresponds to the primary magnetizing current. With this arrangement, an ellipse opening occurs when a discontinuity signal is perpendicular to a dimensional variation in the impedance plane. The ellipse method can be used to examine many test object variables such as conductivity, permeability, hardness, dimensions, and discontinuities. When testing ferromagnetic parts with the ellipse and vector point methods, the relative permeability of the part will vary due to the nonlinear magnetization of the magnetizing field. This nonlinear magnetization creates odd harmonic frequencies to appear in the output data (5).
Linear Time Base A test instrument system that is well suited to determine harmonic distortions present in the fundamental frequency uses the linear time base method of analysis (5).
6
7
Figure 6.6-Linear Time Base Instrument Diagram (5, p. 40-29)
The linear time base unit applies a sawtooth shaped voltage to the horizontal deflection plates of a CRT. This provides a linear trace of the CRT beam from left to right across the CRT screen. The linear trace is timed so that it is equal to one cycle of the magnetizing current. This allows one cycle of the magnetizing sine wave voltage to appear on the CRT. Figure 6.7 illustrates a linear time base display. SLIT
Figure 6.7-Screen Image of a Linear Time Base Instrument with Sinusoidal Signals (5, p. 40-31)
44
A slit or narrow vertical scale is provided to measure the amplitude of signals present in the slit (5). The base voltage is normally adjusted to cross the slit at "0" volts, the 180° point on the sine wave. The slit value "M" is used to analyze results. The slit value "M" is described by the equation: M =A sine where: M A e
= Slit value = Amplitude of the measurement in the slit = Angle between base signal and measurement effect
In Figure 6.7, the angle difference A to 8 is approximately 90 degrees. MODULATION ANALYSIS TESTING
Test instruments may also be classified by mode of operation (4). The mode of operation is determined by two functional areas within the instrument type. The first consideration is the method of test coil excitation. The second area is the degree of compensation, or nulling, and the type of detector used. The types of excitation include single frequency or multifrequency sinusoidal, single or repetitive pulse, and swept frequency. Compensation and detection can be accomplished by three modes. The three main input-detector modes are: 1. 2. 3.
null balance with amplitude detector, null balance with amplitude-phase detectors, and selected oft-null balance with amplitude detector.
Mode 1 responds to any signal irrespective of phase angle. Mode 2, using amplitude-phase detectors, can discriminate against signals having a particular phase angle. With this system, the total demodulated signal can be displayed on an X-Y oscilloscope to show amplitude and phase relationships. Figure 6.8a shows a commercial null balance instrument with amplitude phase detectors.
Figure 6.8a-Null Balance Instrument with Amplitude-Phase Detectors (Zetec, Inc.)
45
5 50%
ID
3 75%
OD
Figure 6.8b-Typical Response to a Thin Wall Non-ferromagnetic Tube Calibration Standard (Zetec, Inc.)
Mode 3 is a phase-sensitive system although it has only amplitude detectors. It achieves phase sensitivity by operating at a selected off-balance condition. This off-null signal is very large compared with test object variations. Under this condition, the amplitude detector output varies in accordance with the test object signal variation on the large off-null signal. Two off-null systems are required to present both components of the test coil output signal. Figure 6.9 shows a block diagram of a stepped single frequency phase-amplitude instrument (4).
I
TEST OBJECT
I L...----'1 I
I
I .-----
L~ I,_,
___
__.I
Figure 6.9-lnstrument Providing Any One of Four Operating Frequencies (Reprinted with permission from Hugo L. Libby, Introduction to Electromagnetic Nondestructive Test Methods, p. 65. Copyright © 1971, John Wiley & Sons, Inc.)
46
The circuit in Figure 6.9 is capable of operating at any of the four frequencies. If the four frequencies are over a wide range, several different test coils may be required to use the instrument over the entire range. Most modern single frequency instruments use this principle; however, the four individual generators are usually replaced by one variable frequency generator with a wide operating range. A typical frequency range for such an instrument is 100 Hz to several megahertz. Figure 6.10 shows a block diagram for a multifrequency instrument operating at three frequencies simultaneously.
J_
!GENERATOR
lGENERATOR_r=
I
GENERATORe
----,
!
I
~
TEST OBJECT
COMPENSATION FILTERS AND OR BALANCE f - - AMPLIFIERS CIRCUITS
I
-
MULTIPLE AMPLITUDE· PHASE DETECTORS
-
ANALYZERS
I III MULTIPLE READ-OUTS
Figure 6.10-Multifrequency Instrument Operating at Three Frequencies Simultaneously (Reprinted with permission from Hugo L. Libby, Introduction to Electromagnetic Nondestructive Test Methods, p. 65. Copyright © 1971, John Wiley & Sons, Inc.)
In Figure 6.10, excitation currents at each frequency are impressed on the coil at the same time. Multiple circuits are used throughout the instrument (4). The test coil output carrier frequencies are separated by filters. Multiple dual phase amplitude detectors are used and their outputs summed to provide separation of several test object parameters. A system similar to this is described in "InService Inspection of Steam Generator Tubing Using Multiple Frequency Eddy Current Techniques" (12). Another approach to the multifrequency technique uses a sequential coil drive called multiplexing (12). The frequencies are changed by a step-by-step sequence with such rapidity that the test parameters remain unchanged. The multiplex technique has the advantages of lower cost, continuously variable frequencies, and little or no cross-talk between channels.
47
Figure 6.11 illustrates a commercial multifrequency instrument capable of operating at four different frequencies sequentially. Each of the frequency modules may be adjusted over a wide range of frequencies. In addition, two mixing modules are used to combine output signals of the various channels for suppression of unwanted variables. Results of such suppression are described in "Multi-Frequency Eddy Current Method and the Separation of Test Specimen Variables" (12) .
•• ••
••i••
:
Figure 6.11-Commercial Multifrequency Instrument (Zetec, Inc.) Instruments are being developed that are programmable, computer or microprocessor based. With microprocessor controlled instruments, test setups can be stored in a programmable memory system. This allows complicated, preprogrammed test setups to be recalled and used by semiskilled personnel. Systems are designed with preprograms having supervisory code interlocks that prevent reprogramming by other than authorized personnel. Microprocessor-based instruments can interface with larger computer systems for control and signal analysis purposes. Figure 6.12 shows a single frequency portable microprocessor-based instrument. The CRT display applies the phase analysis technique for signal interpretation.
Figure 6.12-Commercial Microprocessor-Based Instrument (Nortec Corporation) 48
Other instruments being developed will be microprocessor based with the ability to excite several coils at several frequencies. This would allow automatic suppression of unwanted variables and a direct link to larger computers for computer enhancement of test signal information. A test system using pulsed excitation is shown in Figure 6.13 (4).
Figure 6.13-Pulsed Waveform Excitation (Reprinted with permission from Hugo L. Libby, Introduction to Electromagnetic Nondestructive Test Methods, p. 66. Copyright © 1971, John Wiley & Sons, Inc.)
A pulse is applied to the test coil, compensating networks, and analyzers simultaneously. Systems having analyzers with one or two sampling points perform similar to a single frequency tester using sinusoidal excitation. Pulsed eddy current systems (7) having multiple sampling points perform more like the multifrequency tester shown in Figure 6.10. TEST OBJECT HANDLING EQUIPMENT Test object handling equipment is often a necessary component of a test system (4). Bars and tubes can be fed through encircling coils by means of roller fed assemblies. The stock being fed through the coil is usually transported at a constant speed. The transport speed is selected with instrument response and reject system response being of prime importance to the test. Pen marking and automatic sorting devices are common in automated systems. Spinning probes are used where the probe is rotated and the tube or bar is translated. Probe rotational speeds must be compatible with instrument response and translation speeds in order to obtain the desired inspection coverage and results.
Small parts are often gravity fed through coil assemblies. A major problem with small parts ·is loading, inspecting, and unloading. A speed effect (4) occurs when a conducting object is passed through a coil. As the object moves through the coil's magnetic field, an additional induced voltage within the object occurs. This additional induced voltage has the same frequency as the exciting current, and it causes a current flow and associated magnetic fields that produce signals proportional to the speed of the object through the coil. For larger or stationary structures, test probes are scanned over the part surface by manual or remotely operated systems. Scanning considerations are the same as for tube and bar stock: instrument response, marking or reject system response, and desired coverage. In the case of large heat exchangers, a probe positioning device is used to position the test probe over each tube opening to be inspected. Tubes to be inspected are identified by manual templates, or their coordinates are programmed into computer memory. Positive feedback is supplied to computer positioning systems by encoder devices. In manual template systems the tube end is viewed by a video camera. Tube identification and control feedback are supplied to the operator via a video display system. In each system, as the probe guide is positioned correctly, the probe is inserted and withdrawn from the heat exchanger tube bore, and results of the scan are recorded on chart paper and magnetic tape. 49
REVIEW QUESTIONS Q.6·1
Signal preparation is usually accomplished by: A. detectors. B. samplers. C. balance networks. D. discriminators.
0.6·2
Most eddy current instruments have _ _ _ _ _ coil excitation. A. square wave B. triangular wave C. sine wave D. sawtooth wave
0.6·3
When only coil voltage is monitored, the system is considered a(an) _ _ _ _ _ type system. A. impedance magnitude B. phase analysis C. reactance magnitude D. resistance magnitude
0.6-4
It is easy to distinguish dimensional variations from discontinuities in a reactance magnitude system. A. True B. False
Q.6·5
Eddy current systems can be grouped by: A. output characteristics. B. excitation mode. C. phase analysis extent. D. both A and B.
0.6-6
In modulation testing the test object must be _ _ _ __ A. stationary B. moving C. polarized D. saturated
0.6-7
Using the vector point method, undesired responses appear _ _ _ _ _ on the CRT. A. vertical B. horizontal C. at 45 o to horizontal D. random and cannot be set
0.6·8
When ellipse testing a rod, the f/f 9 ratio is lowered from 50 to 5 percent. The response from a 5 percent surface flaw: A. will appear more elliptical. B. will appear less elliptical. C. is unchanged. D. rotates 90° clockwise.
0.6-9
Using the linear time base, harmonics appear: A. as phase shifts of the fundamental waveform. B. as distortions of the fundamental waveform. C. to have no effect on the fundamental waveform. D. as modified sawtooth signals.
0.6-10
Calculate the slit value "M" for a signal phase shift of 45 degrees at 10 divisions vertical amplitude. A. 14
B.
7
D.
0.7 1.4
c.
50
0.6·11
A multifrequency instrument that excites the test coil with several frequencies simultaneously uses the concept. A. multiplex B. time share C. broadband D. synthesized
0.6·12
A multifrequency instrument that excites the test coil with several frequencies sequentially usesthe concept. A. multiplex B. time base C. broadband D. Cartesian
0.6·13
In a pulsed eddy current system using a short duration and a long duration pulse, the short duration pulse is used to reduce _ _ _ __ A. edge effect B. skin effect C. Iift-off effect D. conductivity variations
0.6-14
When selecting feed rates for automatic inspection of tube and bar stock, consideration is given to: A. instrument response. B. automatic sorting response. C. speed effect. D. all of the above.
51
7.
READOUT MECHANISMS
Eddy current test data may be displayed or indicated in a variety of ways. The type of display or readout depends on the test requirements (4). Test records may require archive storage on large inservice components so that corrosion or discontinuity rates of change can be monitored and projected. In some production tests, a simple GO/NO-GO indicator circuit is all that is required. Some common readout mechanisms are indicator lights, audio alarms, meters, digital displays, cathode ray tubes, recorders, and computer printout or displays.
INDICATOR LIGHTS
A simple use of the indicator light is to monitor the eddy current signal amplitude with an amplitude gate circuit. When the signal reaches a preset amplitude limit, the amplitude gate switches a relay that applies power to an indicator light or automatic sorting device. With the amplitude gate circuit, high-low limits could be preset to give GO/NO-GO indications.
AUDIO ALARMS
Audio alarms can be used in much the same manner. Usually the audio alarm indicates only the abnormal condition. Alarm lights and audio alarms are commonly incorporated in eddy current test equipment. The indicator light and audio alarm give only qualitative information about the item, whether a condition is present or not. The degree of condition cannot normally be determined with these devices. Indicator lights and audio alarms are relatively inexpensive and can be interpreted by semiskilled personnel.
METERS
Meters can present quantitative information about a test object. Meters operate on the D'Arsonval galvanometer principle. The principle is based on the action between two magnetic fields. A common meter uses a strong permanent magnet to produce one magnetic field; the other magnetic field is produced by a movable coil wound on a core. The coil and core are suspended on jewelled bearings and attached to a pointer or "needle." The instrument output current is passed through the coil and produces a magnetic field about the coil that reacts to the permanent magnetic field surrounding the assembly. The measuring coil is deflected, moving the meter pointer. The degree of pointer movement can be related to test object variables as presented by the tester output signals.
DIGITAL DISPLAYS
Numerical digital displays or indicators provide the same type of information as analog meter systems. Many eddy current instruments have analog output circuits. Data handling (7) of analog information in digital form requires analog information to be processed through analog-to-digital (A-D) converters. The A-D converter transforms analog voltages to numerical values for display.
52
CRTs Cathode ray tubes (CRTs) play.an important role in the display of eddy current information. Most CRTs are the "electrostatic" type. Three main elements comprise a cathode ray tube: (1) electron gun, (2) deflection plates, and (3) a fluorescent screen. The electron gun generates, focuses, and directs the electron beam toward the face or screen of the CRT. The deflection plates are situated between the electron gun and the screen. They are arranged in two pairs, usually called horizontal and vertical, or X and Y. The plane of one pair is perpendicular to the other pair and therefore considered X and Y. The screen is the imaging portion of the CRT. The screen consists of a coating or coatings that produce photochemical reactions when struck by the electron beam. The photochemical action appears in two stages. Fluorescence occurs as the electron beam strikes the screen. Phosphorescence enables the screen to continue to give off light after the electron beam has been removed or has passed over a section of the screen. All screen materials possess both fluorescence and phosphorescence. Screen materials are referred to as phosphors. The color of fluorescence and phosphorescence may differ as the case for zinc sulfide: the fluorescence is blue-green, and the phosphorescence is yellow-green. Fluorescence may appear blue, white, red, yellow, green, orange, or a combination of colors, depending on the chemical makeup of the screen. The amount of light output from the fluorescent screen depends on the electron beam accelerating potential, screen chemical composition, thickness of screen material, and writing speed of the electron beam. The duration of the photochemical effect is called persistance. Persistance is grouped as to low, medium, or high persistance. To display repetitive signals, a low or medium persistance CRT may be used. To display nonrecurrent or single events, a high persistance CRT should be used. Many modern CRTs have the capability of both low or medium and high persistance. Storage or memory CATs have ·the ability to display nonrecurrent signals. The image from a single event may remain visible on the CRT for many hours, if desired.
Figure 7.1 illustrates a typical eddy current signal response on a storage CRT.
Figure 7.1-Discontinuity Response in Thin-Wall Non-ferromagnetic Tubing (Zetec, Inc.)
The amplitude of the signal in Figure 7.1 is an indicator of the volume of the discontinuity. The phase angle with respect to the X axis represents discontinuity depth and origin, origin indicating whether the discontinuity originated on the inside or outside surface of the tube (13).
53
RECORDERS Recorders are also used to display data and to provide a convenient method of data storage. Recording is accomplished on paper strip charts, facsimile paper, facsimile photosensitive, magnetic tape (AM, FM, or video), or digital memory disks. Strip chart recordings are common in testing tubing or nuclear fuel rods where the discontinuity's location down the length of rod or tube is critical. The strip chart length is indexed to time or distance and pen response indicates normal or abnormal conditions.
~~Nif~i•ti'ty
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:,:··:·:·J; . ~~.\',.. ·· ,_.,:::~:
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.
;
' ..·.·.:····1 .. · · ..
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Figure 7.2-Commercial Strip Chart Recorder (Gould Instruments)
Fascimile recording (12) is a technique of displaying data signals as a raster of lines which have varying levels of blackness which correspond to data-signal voltage changes. Facsimile recording is commonly referred to as C-scan recording. If no data is transmitted to the facsimile recorder, a uniform light or dark (depending on preference) line or series of lines (raster) would be recorded. In the case of light rasters, the incoming data signal would produce areas of different darkness. The darkness would be dependent on the incoming data signal. Facsimile recorders are used in conjunction with scanning mechanisms and scan rates, and locations are synchronized with the facsimile recorder to present an image of the object variances. Figure 7.3 illustrates a typical facsimile recording.
54
•f'"'i
. ;:-07' ·.
',
...,.
Figure 7.3-Facsimile Recording of Saw-cut Specimen (Copyright, American Society for Testing and Materials, 1916 Race Street, Philadelphia, PA. 19103. Reprinted, with permission.)
Another common type recorder is the X-Y recorder. X-Y recorders are usually used to present scanning type data. In X-Y systems, only data signals are printed; no raster is produced in a conventional X-Y recorder system. Magnetic tape recorders, usually frequency-modulated multichannel types, are used to provide a permanent record of test results. In the case of eddy current equipment with X-Y outputs, quadrature information is recorded and played back into analyzers tor post inspection analysis (13). COMPUTERS
Computers may be used to control data acquisition and analysis processes. Data handling techniques (7) take a wide variety of approaches. Dodd and Deeds (12) describe a computer-controlled multifrequency system. Figure 7.4 shows a computer-controlled eddy current system.
Figure 7.4-Computer-controlled Eddy Current System (Oak Ridge National Laboratory, No. 1747-49)
55
REVIEW QUESTIONS Q.7·1
Display requirements are based on: A. test applications. B. records requirement. C. need for automatic control. D. all of the above.
Q.7·2
Amplitude gates provide a method of controlling: A. reject or acceptance limits. B. instrument response. C. display amplitude. D. all of the above.
Q.7·3
Alarms and lights offer only: A. qualitative information. B. quantitative information. C. reject information. D. accept information.
Q.7·4
The galvanometer principle is the basis for _ _ _ __ A. corrosion rates B. metallographic deterioration C. a voltmeter D. light source illumination
Q.7-5
In order for analog information to be presented to a digital computer, it must be processed through _ _ _ _ _ _ __ A. an A-D converter B. a microprocessor C. a phase detector D. an amplitude detector
Q.7-6
In a cathode ray tube, the electron gun: A. directs the beam. B. focuses the beam. C. generates the beam. D. all of the above.
Q.7·7
Photochemical reactions produced by electrons striking a CRT screen cause: A. photosynthesis. B. phosphorescence. C. fluorescence. D. both B and C.
Q.7·8
High persistance CRT screens are normally used for repetitive signal display. A. True B. False
Q.7·9
Length of a strip chart can indicate: A. flaw severity. B. distance or time. C. orthogonality. D. all of the above.
Q.7·10
A series A. B. C. D.
of lines produced in facsimile recording is/are called: grid lines. raster. crosshatch. sweep display.
56
8.
APPLICATIONS
Electromagnetic induction and the eddy current principle can be affected in many different ways. These effects may be grouped by discontinuity detection, measurement of material properties, dimensional measurements, and other special applications (4). With discontinuity, or the flaw detection group, we are concerned with locating cracks, corrosion, erosion, and mechanical damage. The material properties group includes measurements of conductivity, permeability, hardness, alloy sorting or chemical composition, and degree of heat treatment. Dimensional measurements commonly made are thickness, profilometry, spacing or location, and coating or cladding thickness. Special applications include measurements of temperature, flow metering of liquid metals, sonic vibrations, and anisotropic conditions.
FLAW DETECTION
The theoretical response to discontinuities has been discussed in previous Sections of this guide. In this Section, some actual practice examples are given to enhance the understanding of applied theory. A problem common to the chemical and electric power industries is the corrosion of heat exchanger tubing. This tubing is installed in large vessels in a high density array. It is not uncommon for a 4-foot diameter heat exchanger to contain 3000 tubes. This high density and limited access to the inspection areas often preclude the use of other NDE methods. Heat exchanger inspection systems and results are described by Denton (13), Wehrmeister (13), Libby (8), Dodd, Sagar, and Davis (12). In most of these cases, the severity of the discontinuity is determined by analyzing the eddy current signal phase and amplitude. The signal amplitude is an indicator of the discontinuity volume. The phase angle determines the depth of the discontinuity and also the originating surface (ID or OD) of that discontinuity. (See Figure 6.8, pp. 45-46.) Phase angle and amplitude relationships are usually established by using a reference standard with artificial discontinuities of known and documented values. The geometry of real discontinuities may differ from reference standard discontinuities. This difference produces interpretation errors as discussed by Sagar (12). Placement of real discontinuities near tube support members causing a complex coil impedance change is also a source of error. This, of course, is dependent upon the size of the discontinuity and its resultant eddy current signal in relation to the tube support signal. This follows the basic principle of signal-to-noise ratio. The signal-to-noise ratio can be improved at tube-to-tube support intersections by the use of multifrequency techniques (12, 11). In multifrequency applications, an optimum frequency is chosen for response to the tube wall and a lower than optimum frequency is chosen for response to the tube support. The two signals are processed through comparator circuits called mixers where the tube support response is subtracted from the tube wall response signal, leaving only the response to the tube wall discontinuity. Another industry that uses eddy current testing extensively is the aircraft industry. Many eddy current examinations are conducted on gas turbine engines and airframe structures. A common problem with gas turbines is fatigue cracking of the compressor or exhaust turbine blade roots (13).
57
usually these inspections are performed with portable instruments with meter response capability. The meter response is compared to the response of known discontinuities in a reference specimen. A determination is then made of the part's acceptance. The reference specimen and its associated discontinuities are very critical to the success of the test. Often models are constructed with artificial discontinuities that are exact duplicates of the item being inspected. The low frequency eddy current inspection of aircraft structures is explained by D.J. Hagemaier(14). The low frequency (100-1000 Hz) technique is used to locate cracks in thick or multiple layer, bolted or riveted aircraft structures. Again, models are constructed with artificial cracks, and their responses are compared to responses in the actual test object. Pulsed eddy current systems also are used for crack detection in thick structures.
DIMENSIONAL MEASUREMENTS
Dimensional measurements, such as thickness, shape, and position, or proximity of one item to another, are important uses of the eddy current technique. Often materials are clad with other materials to present a resistance to chemical attack or to provide wear resistance. Cladding or plating thickness then becomes an important variable to the serviceability of the unit (6). For nonconductive coatings on conductive bases, the "probe-to-specimen spacing" (6), or lift-off technique can be applied. The case of conductive plating or cladding on conductive bases requires more refinement. The thickness loci respond in a complex manner on the impedance plane (4). The loci for multilayered objects with each layer consisting of a material with a different conductivity follow a spiral pattern. In certain cases, two frequency or multifrequency systems (6) are used to stabilize results or minimize lift-off variations on the thickness measurement. The depth of case hardening can be determined by measuring the nitride case thickness in stainless steel (11). The nitride case thickness produces magnetic permeability variations. The thicker the nitride, the greater the permeability. The coil's inductive reactance increases with a permeability increase. This variable is carefully monitored and correlated to actual metallographic results. Eddy current profilometry is another common way to measure dimensions; for example, the measurement of inside diameters of tubes. using a lift-off technique (11). For this measurement, several small probe coils are mounted radially in a coil form. The coil form is inserted into the tube and each coil's proximity to the tube wall is monitored. The resultant output of each coil can provide information about the concentricity of the tube. An obvious problem encountered with this method is centering of the coil holder assembly. The center of the coil holder must be near the center of the tube. When inspecting for localized dimensional changes, a long coil holder is effective in maintaining proper centering. Another function of the long coil form is to keep the coils from becoming "cocked" or tilted in the tube.
CONDUCTIVITY MEASUREMENTS
Conductivity is an important measured variable. In the aircraft industry, aluminum is used extensively. Aluminum conductivity varies not only with alloy but also with hardness and tensile strength. Eddy current instruments scaled in %lACS are normally used to inspect for conductivity variations. Secondary conductivity standards (12) are commonly used to check instrument calibration. Common secondary conductivity standards range from 8% lACS to approximately 100% lACS. The
58
secondary standards are usually certified accurate within ± 0.35 percent or ± 1 percent of value, whichever is less. Temperature is an important variable when making conductivity measurements. Most instruments and standards are certified at 20°C. Primary conductivity standards are maintained at a constant temperature by oil bath systems. Primary standards are measured by precision Maxwell bridge type instruments. This circuit increases measurement accuracy and minimizes frequency dependence of the measurement (12). HARDNESS MEASUREMENTS Hardness of steel parts is often measured with low frequency comparator bridge instruments (19). The reference and test coil are balanced with sample parts of known hardness. As parts of unknown hardness affect the test coil, the instrument output varies. The amount of output variation depends upon the degree of imbalance created by the unknown test object hardness.
Signal output is then correlated to test object hardness by comparing to known hardness samples of the same geometry. For example, if a cathode ray tube were used to display hardness information, the "balance" hardness could be adjusted to center screen, lower hardness values could appear below center, and higher hardness values could appear above center on the CRT. ALLOY SORTING Alloy sorting can be accomplished in the same comparator bridge manner as hardness. A major consideration in both cases is the selection of correct and accurate reference specimens. Since most eddy current instruments respond to a wide range of variables, the reference SPcecimen parameters must be controlled carefully.
Test object and reference specimens must be the same or very similar in the following characteristics: 1. 2. 3. 4. 5.
geometry, heat treatment, surface finish, residual stresses, and metallurgical structure.
In addition, it is advisable to have more than one reference specimen for backup in case of loss or damage. In the case of steel parts, they should be completely demagnetized to remove the effects of residual magnetism on instrument readings. As in most comparative tests, temperature of specimen and test object should be the same or compensated. Many other measurements can be made using eddy current techniques. The electromagnetic technique produces so much information about a material, its application is only limited by our ability to decipher this information (13). REVIEW QUESTIONS 0.8·1
Conductivity, hardness, and composition are part of the _ _ _ _ _ group. A. defect detection B. material properties C. dimensional D. special
0.8-2
Using an ID coil on tubing and applying the phase/amplitude method of inspection, a signal appearing at 90 o on a CRT would be caused by: A. ID flaw. B. OD flaw. C. dent. D. bulge. 59
0.8·3
Discontinuities in heat exchangers at tube support locations are easier to detect because the support plate concentrates the electromagnetic field at that point. A. True B. False
0.8·4
Using multifrequency techniques on installed heat exchanger tubing, a tube support plate signal can be suppressed by adding a frequency signal to the optimum frequency signal. A. low B. high C. A orB D. none of the above
0.8·5
In the aircraft industry, a common problem in gas turbine engines is: A. corrosion. B. fatigue cracking. C. vibration damage. D. erosion.
0.8·6
Thick or multilayered aircraft structures are normally inspected by: A. low frequency sinusoidal continuous wave instruments. B. high frequency sinusoidal continuous wave instruments. C. pulsed systems. D. A and C.
0.8·7
Response to multilayer varying conductivity structures follow _____ loci. A. orthogonal B. spiral C. linear D. stepped
0.8·8
Nitride case thickness can be monitored in stainless steel cylinders by measuring _ _ _ __ A. conductivity B. dimensions C. permeability D. none of the above
0.8·9
Conductivity is not affected by temperature. A. True B. False
0.8·10
Residual stresses in the test part produce such a small effect that they are usually ignored when selecting reference specimens. A. True B. False
60
9.
EDDY CURRENT TEST PROCEDURES, STANDARDS, AND SPECIFICATIONS Procedures, specifications, and standards are produced to provide a means of controlling product or service quality. Written instructions that guide a company or individual to a desired end result and are acceptable to industry, are the basis of procedures, specifications, and standards. ' Many publications are available to guide or instruct us. Some of the most frequently used references are the American Society for Testing and Materials (ASTM), American Society of Mechanical Engineers (ASME), American National Standards Institute (ANSI), and Military Standards (M I L-STD-XXXX). These publications are laboriously produced by committees made up of scientific and technical people. Usually after a committee produces a draft document, it is submitted to industry and the scientific community for comment and subsequent revision (17). In certain cases, standards combine to assist each other. As an example, ASME Section V Appendix A (17) uses ASTM E268(18) to provide "STANDARD DEFINITIONS OF TERMS RELATING TO ELECTROMAGNETIC TESTING." The military standard, MIL-STD-1537A "Electrical Conductivity Test for Measurement of Heat Treatment of Aluminum Alloys, Eddy Current Method," references ASTM 8193 "Resistivity of Electrical Conductor Materials" and ASTM #18 "Rockwell Hardness and Rockwell Superficial Hardness of Metallic Materials."
ASTM
Standards or Standard Practices, ASTM, usually include in the written instructions headings such as scope, references, method, significance, definitions, apparatus, calibration standards, calibration, and procedure. The Scope heading makes a general statement about the document's applicability and intent. The Applicable Document heading refers to other publications used as references for the standard. Method is usually a summary of how the test method is applied to the test object and what type of measurements can be made. Under the Significance heading is a more detailed discussion of test results and probable causes of indications expected during the examination. The Definition section usually contains definitions of key words or key phrases associated with the inspection. Apparatus describes the general requirements for the inspection system including instrumentation, coils, positioning, and driving mechanisms. Under Calibration Standards the fabrication requirements for artificial discontinuity calibration standards are discussed. Included is usually a discussion of the reference specimen and the geometrical requirements of the artificial discontinuities in it. The Calibration section provides instructions for adjustment and standardization of the apparatus used for the examination. The response to known discontinuities is usually described in this section. Under the Procedure heading, detailed instructions are given to implement the inspection. These instructions usually include acceptance limits and how to handle components that are not acceptable. The ASTM publishes several standards pertaining to the eddy current method. These standards are numbered; for example, E268-81. "E268" refers to the standard, and "81" refers to the year of origin or the year of last revision. Some ASTM standards that pertain to the eddy current method are as follows: E 309-77 E 571-82
Eddy-Current Examination of Steel Tubular Products Using Magnetic Saturation Electromagnetic (Eddy-Current) Examination of Nickel and Nickel Alloy Tubular Products
E 703-79
Electromagnetic (Eddy-Current) Sorting of Nonferrous Metals
E 426-76 (1981) Elecromagnetic (Eddy-Current) Testing of Seamless and Welded Tubular Products, Austenitic Stainless Steel and Similar Alloys E 243·80E 1 Electromagnetic (Eddy-Current) Testing of Seamless Copper and Copper Alloy Tubes E 566-82
Electromagnetic (Eddy-Current) Sorting of Ferrous Metals
61
E 215-67 (1979)e2 Electromagnetic Testing of Seamless Aluminum-Alloy Tube, Standardizing Equipment for E 690-79 In Situ Electromagnetic (Eddy-Current) Examination of Nonmagnetic Heat Exchanger Tubes E 376-69 (1979) Measuring Coating Thickness by Magnetic-Field or Eddy-Current (Electromagnetic) Test Methods E 268-81 Electromagnetic Testing
MIL-STD
The United States Military uses the Military Standard docufTient to control testing and materials. Standard procedures are provided by a series of MIL-STD-XXXXX documents. Special requirements are specified by the Military Specification system. For example, MIL-STD-1537A refers to "Electrical Conductivity Test for Measurement of Heat Treatment of Aluminum Alloys, Eddy Current Method." The "Calibration System Requirements" for MIL-STD-1537A are contained in Military Specification MIL-C-45662. The MIL-STD usually contains several parts and is very descriptive. These parts normally include Scope, Applicable Documents, Definitions, General Requirements, Detail Requirements, and Notes. The Scope contains a general statement of applicability and intent of the Standard. Applicable Documents pertains to other reference or controlling documents such as other MIL-STD, Military Specification, or ASTM publications. The Definition part contains precise definitions of key words and phrases used in the Standard. Under General Requirements, equipment, reference specimen, and personnel requirements are described in sufficient detail to implement the Standard. Included in this part is instrument sensitivity and response, test object variables, reference specimen requirements, and personnel qualification requirements. The Detail Requirements part describes the specific procedure to implement the Standard. The Notes part contains pertinent statements about the process and guidelines for reporting of results.
ASME
In 1911 the American Society of Mechanical Engineers set up a committee to establish rules of safety for design, fabrication, and inspection of boilers and pressure vessels. These rules have become known throughout industry as the ASME Code. The committee designated as ASME Boiler and Pressure Vessel Committee is a very large group of people from industry and the scientific community. The "Committee" has many subcommittees, groups, subgroups, and working groups. Currently, there are approximately 3 committees, 15 subcommittees, 50 subgroups, and 37 working groups. Each subcommittee, subgroup, and working group combines as a unit for a specific area of interest. For example, the Subcommittee on Pressure Vessels (SCVIII) has three working groups and four subgroups reporting to it. The purpose of these groups is to interface with industry to keep pace with changing requirements and needs of industry and public safety. The ASME Boiler and Pressure Vessel Code is divided into eleven sections. ASME Section V is "Nondestructive Examination." Section Vis divided into two subsections, "A" and "B". Subsection "A" deals with "Nondestructive Methods of Examination." Article 8 is "Eddy Current Examination of Tubular Products." Subsection "B" contains "Documents Adopted by Section V." Eddy current standards are described in Article 26. In this case, the ASTM E215 document has been adopted by the ASME and reassigned the designation "SE215." ASME Section V Article 8- Appendix I gives detailed procedure requirements for "Eddy Current Examination Method for Installed Non-ferromagnetic Steam Generator Heat Exchanger Tubing." A procedure designed to meet this requirement can be illustrated by the following example, Document QA 3.
62
Procedure No. QA 3
11-1
EDDY CURRENT INSPECTION OF NONFERROUS TUBING BY SINGLE FREQUENCY TECHNIQUES
A.
PURPOSE
This procedure describes the equipment and methods as well as the personnel qualifications to be utilized for the performance of the eddy current examination of steam generator tubes, and meets the requirements of the N.R.C. Regulatory Guide 1.83, ASME Section XI Appendix IV, and ASME Section V Article 8 of the ASME Boiler and Pressure Vessel Code.
B.
SCOPE
The scope of the examination to be performed is contained in the eddy current inspection program document applicable to the specific plant to be inspected.
C.
PREREQUISITES
1.
Plant Condition
The plant must be shut down with the primary system drained. The steam generators shall be open on the primary side for access to the channel head and the shell cool-down sequence shall be complete. Air movers shall be attached to circulate air through the generator to dry the tube sheet. 2.
Equipment
The examinations shall be performed utilizing an XXXX/XX eddy current instrument with bobbin coil probes designed for testing from the inside of the tubes. The inspection performance shall be monitored by the use of the oscilloscope with a phase sensitive vector display and recorded for later evaluation on both magnetic tape and strip chart. a.
Equipment utilized shall be: i. XXXX/XX eddy current instrument with integral series YYYY two-channel storage display oscilloscope. ii. Differential coil probes. iii. XYZ or YZA two-channel frequency modulated magnetic tape recorder. iv. Brush Mark 220 strip chart recorder. v. Four-channel communications systems. vi. Calibration standard The calibration standard shall be manufactured from a length of tubing of the same size and type of material that is to be examined in the vessel. The standard shall contain 6 intentional defect areas as follows: aa. 100% drill hole (0.052" for 3/4" OD tubing and smaller, and 0.067" for larger tubing). bb. Flat-bottomed drill hole 5/64" diameter X 80% through from the outer tube wall surface. cc. Flat-bottomed drill hole 7/64" diameter X 60% through from the outer tube wall surface. dd. Flat-bottomed drill hole 3/16" X 40% diameter through from the outer tube wall surface. ee. Four flat-bottom holes, 3/16" diameter, spaced 90 degrees apart around the tube circumference, 20% through the tube wall. Each standard shall be identified by a serial number etched on one end and be traceable to the master standard stored at the facility. 63
Procedure No. QA 3
11·2
ft.
b. c. 3.
Circumferential groove 20% deep by 1/16" long by 360 degrees on the inside tube wall surface. gg. Circumferential groove 10% deep by 1/8" long by 360 degrees on the outer tube wall surface. Probe positioning and feeding shall be accomplished remotely for in-service inspection whenever practical. Baseline inspection may be done manually. Communications shall be provided by the use of a Model 1 four-channel mixer system.
Personnel Qualifications
Personnel collecting data in accordance with this procedure shall be qualified to Level I or higher in accordance with Document QA 101. Personnel interpreting data collected in accordance with procedure shall be qualified to Levell lA or higher in accordance with Document QA 101. Prior to receiving a certification, the applicants shall have completed the program recommended by SNT-TC-1A (1968 edition), Supplement E. D.
E.
PRECAUTIONS
1.
All personnel to be engaged in eddy current inspection programs at operating plants shall have received instructions in and understand the radiation protection rules and guidelines in effect on the plant site.
2.
All personnel to be engaged in the test program shall wear protective clothing to the extent of the type defined by the exclusion area work permit.
3.
All personnel entering a radiation work area will have proven their ability to work in a face mask by successfully passing the pulmonary function test during their annual physical.
4.
No entries shall be made into the steam generator channel head without the presence of a qualified health physics technician.
5.
Ensure that nozzle covers (when applicable) are securely in place inside the vessel prior to commencement of the eddy current inspection program.
PERFORMANCE 1.
Preparation
a.
Establish location of control-operation center.
b.
Arrange power distribution box at control-operation center.
c.
Install the four-channel communications system control box at the control-operation center.
d.
Establish communication with one or more headsets at the steam generator.
e.
Install XXXX/XX eddy current test instrument, tape recorder, and Brush recorder at controloperation center.
f.
Connect the horizontal and vertical outputs from the XXXX/XX eddy current test instrument to the H & V input jacks of the FM tape recorder.
g.
Connect the H & V output jacks of the FM tape recorder to the two-channel inputs of the Brush recorder.
h
Connect the microphone to the FM tape recorder. 64
Procedure No. QA 3
2.
11·3
i.
Connect Amphenol plug of the probe to receptacle on XXXX/XX eddy current test instrument. Be sure to use a 100' extension cable in series with the probe.
j.
Po~ition the trace to the center horizontal line of the CRT graticule and sweep the trace honzontally across the screen, utilizing the position knob on the XXXX. NOTE: Trac~ is parallel to the horizontal line on graticule. If necessary, adjust the trace rotat1on knob on the back of the XXXX/XX.
k.
Turn XXXX/XX sensitivity to zero and center oscilloscope spot at the center of the graticule using the position knobs.
I.
Place all front panel switches on the FM tape recorder in the "up" position, start the tape recorder in the record mode. Advance the record knobs until both meters are in the upper position of the red scale. Run at low speed (3-3/4 in/sec).
m.
Depress the microphone key and speak into the microphone. Temporarily, use the communication headset to assure that the voice recorded on the tape is loud and clear.
n.
With the tape recorder operating in the record mode and the oscilloscope spot centered, center the Brush chart pens with both channels set on 200 M.V./div. and in the calibrated position.
0.
With the position controls on the oscilloscope, make sure that the channels are correctly connected and the equipment will swing full scale both directions. One volt deflection of XXXX/XX should give one major division deflection on Brush recorder.
Equipment Calibration a.
Prior to the commencement of the eddy current examination of the steam generator tubes and after the replacement of any component, the equipment shall be calibrated in accordance with the following steps: i. Insert the bobbin coil probe into a section of the calibration standard which is free of defects. ii. Select the desired operation frequency (as per the specific calibration procedure) by operating first the "coarse" and then the "fine" FREQUENCY control knobs on the front panel. iii. Set the balance selector switch to "L" or "C" as required for the specific test frequency/probe combination. iv. Set the sensitivity to zero and center the oscilloscope spot with the position controls. v. Set the sensitivity adjustment knob to 050. vi. Set the R and X balance knobs to 500. vii. Depress the automatic null push button. viii. Release the automatic null button when spot motion ceases or reaches a minimum. This indicates null condition. NOTE: As proper null is achieved, the X and R BALANCE controls will appear to "hunt." This is a normal occurrence. If rotation of the X and R BALANCE controls is not observed, verify that the dial-locking mechanisms are in their fully unlocked positions (full counterclockwise). Manually check freedom of rotation. ix. Set the sensitivity knob as stated in the specific calibration procedures. x. Re-balance with the automatic null. xi. Set the oscilloscope attenuators to 1 volt/division. xii. Rotate the phase control until the probe motion signal is horizontal (as per the specific calibration procedure) and the first lobe of the 100% drill hole goes down first as the probe is withdrawn from the standard. xiii. Position the trace to the center of the CRT display by rotating the position control knobs (inner red knobs on the attenuators).
65
Procedure No. QA 3 3.
11·4
Tube Inspection General
(Refer to Specific Calibration Procedure QA 2)
4.
5.
a.
Eddy current inspection activities shall be performed with equipment sensitivities and speeds set as follows: i. Oscilloscope attenuator settings shall be 1 or 2 volts/division on both the vertical and horizontal amplifiers. ii. The tape recorder shall be run at low speed. iii. The Brush recorder sensitivity shall be set as per the Specific Calibration Procedure. iv. The Brush recorder shall be run at 5 mm/second for all defect inspections and 25 mm/second for all sludge measurement inspections. v. Insert probe completely through the calibration standard. Record pertinent data at beginning on both sides of magnetic tape and corresponding brush paper. Start tape recorder and Brush chart recorder and record calibration standards while withdrawing probe through standard past the defects. The system is now ready for operation.
b.
Visual verification of the identity of the specific tube being inspected shall be performed before and after each fixture change and at the beginning and end of each side of magnetic tape. Verification of the positive identification of tube location shall be recorded on both the magnetic tape and strip charts.
c.
Should the performance of the tube identity verification reveal an error has occurred in the recording of probe location, all tubes examined since the previous verification of location shall be reexamined.
d.
The equipment calibration shall be verified and recorded at the beginning and end of each side of the reel of magnetic tape. The calibration shall be checked frequently by observing that the tube support plates are at the proper phase angle. At a minimum, the calibration will be verified at 4-hour intervals and after any equipment change.
e.
Should the equipment be found to be out of calibration, the equipment will be recalibrated as per Section E-2 of this procedure. The recalibration will be noted on both the magnetic tape and strip chart. The data interpreter will determine if it is necessary to reinspect any of the tubes.
Tube Inspection Manual
a.
The data recording shall be made during probe withdrawal. Withdrawal speed is 14" per second maximum. No minimum speed specification is required, but a good uniform pull of 12" per second is preferred.
b.
Since no inspection is performed during probe insertion, the speed should be as rapid as possible.
c.
The prober shall have continuous communication with the equipment operator and both personnel must concur on the tube number being examined.
d.
Due to radiation exposure or endurance factors, combinations of probe guides and probe pusher/pullers may be used to facilitate the inspection.
Tube Inspection Automatic Remote
NOTE:
a.
Ensure that all probe positioner, probe feeder, and probe and communication connecting cables are clear of access walkways and secured to any available supports.
Install remotely operated probe feeder local to steam generator with "on-off," "forwardreverse" control at control-operation center. 66
Procedure No. QA 3
F.
11·5
b.
Verify correct direction of rotation of probe feeder (make sure slow speed retracts probe).
c.
Check the operation of the remotely operated eddy current positioner and connect the flexible probe conduits to the probe guide tube and the probe pusher.
d.
Install remotely operated probe positioner on tube sheet of the steam generator to provide coverage of the area to be examined.
e.
Connect power supply-control cable assembly to remotely operated probe positioner.
f.
Connect power supply-control cable assembly to the control console at the control-operation center.
g.
Verify the correct operation and control of the remotely operated probe positioner.
h.
Utilizing the probe pusher, feed the probe through the flexible conduit up to the guide tube of the positioner.
i.
Operate the positioner to locate the probe beneath the tube to be examined.
j.
Utilizing the probe pusher, feed the probe into and up the tube to the desired height. Monitor the extent of insertion by reference to impedance signals from tube supports on the oscilloscope screen.
k.
Withdraw the eddy current probe from the tube until the impedance signal on the oscilloscope screen indicates that the probe is clear of the tube sheet. Concurrent with the probe withdrawal, visually monitor the signals displayed on the oscilloscope screen while recording all data on the magnetic tape and strip chart recorder.
I.
Reposition the probe beneath the next tube selected for examination.
m.
Repeat the procedures described in the preceding steps until all the tubes selected for inspection have been examined.
INSPECTION RESULTS AND DOCUMENTATION 1.
Requirements
a.
The data interpreter shall be certified to Level IIA or Ill as per Procedure QA 101.
b.
Data shall be collected with an eddy current test system with a current certification of calibration as per procedure CSP.
c.
Data shall be interpreted with an interpretation system with a current certification of calibratiorr as per procedure CSP.
d.
The data collection station shall be calibrated with an approved standard which is serialized and traceable to master calibration standard.
e.
The identify of the plant site, the steam generator, the date, the test frequency, the calibration standard serial numbers, and reel side numbers shall be recorded at the start of each new reel and side of magnetic tape and its companion roll of chart paper.
f.
The data collection station shall be set up and calibrated as per Procedure QA 3.
67
Procedure No. QA 3 2.
11-6
Performance a.
The i. ii. iii. iv.
data interpreter shall: Determine that all data collected is properly marked. Report tubes whose data is incomplete or uninterpretable. Require a retest at alternate frequency(ies) for any tube(s) that has(have) a signal whose origin is difficult to ascertain. In-service inspections aa. Report all defects > 19% which occur on the outer surface. bb. Report all other indications which appear to be relevant. cc. Identify the approximate axial position of the indication with respect to a known structural member.
v.
b.
Preservice inspections aa. Report all indications observed. Include the approximate axial position of the indication with respect to a known structural member. Interpretation i. All data shall be recorded on form A3 or equivalent. ii. The conversion from signal phase angles to defect depths shall be accomplished via the following: Table Number QA 4.1 QA4.2 QA4.3 iii. iv.
v.
G.
Equivalent graphs, programmed computers, etc., may be used. All strip chart recordings shall be examined in their entirety. Any abnormal signals observed on the strip charts shall be recorded on the data sheets. The magnetic tape recording shall be examined for each tube that is recorded. In some cases, it may be necessary to examine the magnetic tape in its entirety. (This condition would normally require additional data interpretations to meet scheduling requirements.) The strip chart recording is not considered abnormal unless the vertical excursion is more than 3 minor divisions with both channels having significant phasing.
REFERENCES The following documents are required for the performance of eddy current inspection programs utilizing the methods described in this procedure. 1.
Required Documents a.
Eddy current inspection specific calibration procedure document applicable to the plant to be inspected.
b.
Tube sheet maps marked to designate the extent of examination to be performed.
c.
Eddy current inspection program fixtures placement detail document applicable to model of steam generator to be inspected (when applicable).
68
11·7 FORM NUMBER
TITLE: SPECIFIC CALIBRATION PROCEDURE
SITE STEAM GENERATOR#_ _ _ _ TUBE MATERIAL _ _ DIA. _ _ WALL _ _ DATE _________________ HOTLEG __________________ COLDLEG _________________ XXX X/XX 1. Frequency _______________ ± _______________
2. Sensitivity _____________
3. Balance Manual _ _ Auto _ _ L _ _ C _ _
4. Phase _______________ ± --------------PROBE 1. Style Number _______________
2. Length of Shaft _______________ 3. Length of Extension Cable _____________ BRUSH RECORDER MARK 220 1. Vertical Sensitivity ____________
2. Horizontal Sensitivity _____________ YZA 1. Record Meters Vertical Sensitivity ______________ Horizontal Sensitivity _____________ CALIBRATION STANDARD 1. Serial Numbers ______________ 2. Data Conversion Table/Curve # ______________ INSPECTION TECHNIQUE (state probe retraction speed in applicable blank) 1. Hand Probe Local ______________ 2. Hand Probe Remote _______________ 3. Hand Manipulate with Probe Pusher _______________ 4. Remote Control Fixture _______________ 5. Remote Control Fixture with Probe Pusher _____________ YES _ _ NO _ _ 6. Fixture Procedure Document Required?
69
A 2 Rev. 2
REVIEW QUESTIONS
Q.9·1
A precise statement of a set of requirements to be satisfied by a material, product, system, or service is a _ _ _ __ A. standard B. specification C. procedure D. practice
Q.9·2
A statement that comprises one or more terms with explanation is a _ _ _ __ A. practice B. classification C. definition D. proposal
Q.9·3
A general statement of applicability and intent is usually presented in the _ _ _ _ _ of a standard? A. summary B. scope C. significance D. procedure
0.9·4
Military Standards are designated by "MIL·C-(number)." A. True B. False
0.9·5
In the structure of ASME the subcommittee reports to the subgroup. A. True B. False
0.9·6
In example QA 3, personnel interpreting results must be: A. Level I or higher. B. Level II or higher. C. Level IIA or higher. D. Level Ill.
0.9·7
The prime artificial discontinuity used to calibrate the system described in QA 3 is: A. 20% ID B. 50% OD c. 100% D. 50% ID
0.9·8
In QA 3, A. B. C. D.
0.9-9
QA 3 specifies a maximum probe traverse rate of _ _ _ _ _ _ __ 12"/sec A. B. 14"/sec C. 6"/sec D. not specified
0.9·10
The system in QA 3 is calibrated with an approved standard that is traceable to _ _ _ __ A. NBS B. ASME C. a master standard D. ASTM
equipment calibration must be verified at least _ _ _ __ every hour each day every 4 hours every 8 hours
70
0.9-11
In accordance with QA 3, tubes whose data are incomplete or uninterpretable must be A. B. C. D.
0.9·12
Referring A. B. C. D.
reinspected reported reevaluated removed from service to QA 3, QA 4.1 is a _ _ _ __ calibration form data interpretation table data report form certification form
71
ANSWERS TO REVIEW QUESTIONS Numbers i~ parentheses indicate where answers may be checked and verified. For Questions 1-1 through 9-5, numbers in parentheses are keyed to th reference~ on page vi of this Study Guide. For Questions 9-6 through 9-12~ numbers m parentheses refer to the reprint of Procedure No. QA 3 found on ' pages 63-68 of this Study Guide. Q 1-1 Q 1-2 Q 1-3 Q 1-4 Q 1-5 Q 1-6 Q 1-7 Q 1-8 Q 1-9 Q 1-10 Q Q Q Q Q Q Q Q Q Q
Q Q Q Q Q Q Q Q Q Q
Q Q Q Q Q Q Q Q Q Q
3-1 3-2 3-3 3-4 3-5 3-6 3-7 3-8 3-9 3-10
5-1 5-2 5-3 5-4 5-5 5-6 5-7 5-8 5-9 5-10
8-1 8-2 8-3 8-4 8-5 8-6 8-7 8-8 8-9 8-10
B B
c A
B D
B B
c
c
D
c A
(4, p. 19) (4, p. 19) (4, p. 20) (4, p. 20) (4, p. 23) (13, p. 4) (4, p. 25) (4, p. 26) (4, p. 26) (4, p. 45)
Q 2-1 Q 2-2 Q 2-3 Q 2-4 Q 2-5 Q 2-6 Q 2-7 Q 2-8 Q 2-9 Q 2-10
(4, p. 328) (2, p. 36) (4, p. 332)
Q 4-1 Q4-2 Q 4-3 Q 4-4 Q 4-5 Q 4-6 Q 4-7 Q 4-8 Q 4-9 Q 4-10 Q 4-11 Q 4-12
B
c
D
c
D D D
A
B
c c
D
B B
c
B B
B B B A
B D
B
c
B B
(2, p. 38) (19, p. 78) (4, p. 212) (4, p. 195) (4, p. 173) (4, p. 211)
(19, p. 79) (5, p. 37.20) (5, p. 36.13) (5, p. 36.13) (5, p. 36.13) (4, p. 37) (4, p. 37) (19, p. 82) (5, p. 37.20) (12, p. 289)
(4, p. 270) (13, p. 59) (12, p. 282) (12, p. 256) (13, p. 47) (12, p. 129) (4, p. 51) (11, p. 631) (12, p. 121) (19, p. 102)
Q 6-1 Q 6-2 Q 6-3 Q 6-4 Q 6-5 Q 6-6 Q 6-7 Q 6-8 Q 6-9 Q 6-10 Q 6-11 Q 6-12 Q 6-13 Q 6-14 Q 9-1 Q 9-2 Q 9-3 Q 9-4 Q 9-5 Q 9-6 Q 9-7 Q 9-8 Q 9-9 Q 9-10 Q 9-11 Q 9-12
c c A
B D
B A
c B B
c A
c D
B
c
B B B
c c c
B
c
B B
72
(4, p. 60) (4, p. 60) (5, p. 40.1) (5, p. 40.14) (4, p. 64) (2, p. 4-26) (5, p. 40.23) (5, p. 42.4) (5, p. 42.35) (5, p. 40.31) (4, p. 215) (12, p. 219) (7, p. 387) (4, p. 77) (18 Part II, p. iii) (18 Part II, p. iii) (18 Part I~ p. 288) (15, p. 1) (17 Section V, p. X) (QA-3, p. 2) (QA-3, p. 3) (QA-3, p. 4) (QA-3, p. 4) (QA-3, p. 5) (QA-3, p. 6) (QA-3, p. 6)
D
(5, (4, (4, (5, (4, (6, (4, (4, (4, (4,
B D D
c
B B D
B E
p. p. p. p. p. p. p. p. p. p.
38.25) 194) 71) 40.1) 195) 353) 69) 210) 198) 211)
(2, p. 8) (12, p. 95) (9, p. 56) (2, p. 13) (19, p. 78) (4, p. 171) (4, p. 173) (2, p. 26) (6, p. 360) (5, p. 36.17) (19, p, 88) (4, p. 27)
B
c
D D
B B
c
D A
c
B A
Q Q Q Q Q Q Q Q Q Q
7-1 7-2 7-3 7-4 7-5 7-6 7-7 7-8 7-9 7-10
D A A
c
(4, p. 76) (4, p. 76) (7, p. 238)
A D D
B B B
(4, p. 250) (12, p. 450)
