ULTRASONIC TESTING TRAINING HANDBOOK
Abstract This is the official training handbook of my course cour se “Ultrasonic Level 1 training” presented online. It covers all the training outlines with the maximum information that the students need to understand the course and to be well prepared for the official UT-L1 exam. Find my UT-L1 course for only $5 here: https://www.udemy.com/ultrasonictesting/?couponCode=UTReaders
El mustapha BEN BIHI (VVI UT/TOFD L2 No.054 & ASNT L2: UT)
[email protected]
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Want to learn more? I hope you enjoy this book! If you’d like more information about this book,
its author, please surf to www.udemy.com/u/benbihielmustapha www.udemy.com/u/benbihielmustapha
Dedications This handbook is dedicated to the pioneer in the field of nondestuctive testing who made me intersted in this field: Dr. Othmane BOUKSOUR Without the contributions of these “giant,” the world would not be the same for me.
Ownership and copyright This handbook is the result of the author’s personal efforts. It must not be published or copied. Any reproduction in part or as a whole without the written permission of the author is prohibited.
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Table of Contents Introduction Historical perspective of the use of ultrasound
.........................................................2
Comparison between the commonly used NDT methods .....................................4
Fundamental Properties of Ultrasound Definition of ultrasonic vibrations
..................................................................................10
Relationship of velocity, frenquency and wavelenght ..........................................12 Diffraction Theory
..................................................................................................................13
Mode of particle vibration Ultrasonic uses
..................................................................................................15
..........................................................................................................................18
Generation of Ultrasonic Waves Piezoelectric crystal types and characteristics ..........................................................21 Search unit construction .....................................................................................................25 Search unit characteristics: resolution sensitivity ..................................................35 Search unit types
....................................................................................................................37
Contact transducers ....................................................................................................................37 Immersion transducers ...............................................................................................................38 Twin crystal transducers ............................................................................................................39 Delay line transducers .................................................................................................................40 Angle beam transducers .............................................................................................................41 Normal incidence shear wave transducers ................................................................................42 Paint brush transducers...............................................................................................................42
Test material characterestics Velocity
.......................................................................................................................................43
Sound attenuation
.................................................................................................................46
Acoustic impedance ................................................................................................................48
Sound beam characterestics Intensity variations
.................................................................................................................50
Dead zone, near zone, far zone
.........................................................................................50
Dead zone ......................................................................................................................................50 Near zone ......................................................................................................................................51 Far zone .........................................................................................................................................52
Probe diameter and frequency effects
........................................................................53
Angular incidence Reflection ....................................................................................................................................56 Refraction
..................................................................................................................................57
Mode conversion
....................................................................................................................58
Snell’s law ....................................................................................................................................60
Critical angles
.........................................................................................................................63
Ultrasonic inspection techniques Through transmission ..........................................................................................................65 Pulse echo ....................................................................................................................................76 Angle beam
.................................................................................................................................84
Contact testing
.......................................................................................................................88
General knowledges .....................................................................................................................88 Pulse echo method .....................................................................................................................107 Pitch and catch method .............................................................................................................113
Immersion testing
................................................................................................................117
Ultrasonic testing equipement Basic pulse echo instrument ............................................................................................121 Control functions and uses ..............................................................................................124 Information displays
............................................................................................................129
Computer enhanced displays ..........................................................................................134
Ultrasonic reference blocks Calibration methods
..........................................................................................................136
Introduction to the common standards .....................................................................137 Area amplitude block ...........................................................................................................138 Distance amplitude block ..................................................................................................139 The international institute of welding block ...........................................................140 Minuature angle beam block ...........................................................................................143
Uses of artificial reflectors compared ..........................................................................144 Comparison of echo amplitude ................................................................................................144 Distance amplitude curve .........................................................................................................147
Inspected part variations Effect of surface roughness ..............................................................................................150 Effect of surface coatings Surface curvature Grain size
................................................................................................156
................................................................................................................157
...................................................................................................................................160
Discontinuity variations Sizing methods
........................................................................................................................164
Scanning methods ......................................................................................................................164 Evaluation of small discontinuities: the DGS methods ...........................................................167
Shape
...........................................................................................................................................173
Distance from the entry surface
....................................................................................179
Appendix 1: Locating a discontinuity ...........................................186 Appendix 2: Vocabulary ..................................................................................201 Appendix 3: Acoustic characteristics of materials ......208 References ............................................................................................................................212
1. Introduction Non-destructive Testing
The field of Nondestructive Testing (NDT) is a very broad, that plays a critical role in assuring that structural components and systems perform their function in a reliable and cost effective fashion. NDT technicians and engineers define and implement tests that locate and characterize material conditions and flaws that might otherwise cause serious accidents such as, planes to crash, reactors to fail, trains to derail, pipelines to burst, and a variety of troubling events. These tests are performed in a manner that does not affect the future usefulness of the object or material. In other words, NDT allows parts and materials to be inspected and evaluated without damaging them. Because it allows inspection without interfering with a product's final use, NDT provides an excellent balance between quality control and cost-effectiveness.
Nondestructive Evaluation
Nondestructive Evaluation (NDE) is a term that is often used interchangeably with NDT. However, technically, NDE is used to describe measurements that are more quantitative in nature. For example, a NDE method would not only locate a defect, but it would also be used to measure something about that defect such as its size, shape, and orientation. NDE may be used to determine material
1
properties such as fracture toughness, ductility, conductivity and other physical characteristics.
Uses of NDE
· Flaw Detection and Evaluation · Leak Detection, Location Determination · Dimensional Measurements · Structure and Microstructure Characterization · Estimation of Mechanical and Physical Properties · Stress (Strain) and Dynamic Response Measurements · Material Sorting and Chemical Composition Determination 1.1. Historical perspective of
the use of ultrasound
Paul Langevin (Mason) first propagated ultrasonic waves in 1917 in seawater and has been credited as the father of ultrasonics. For a complete history of the early work on acoustics please refer to R.Bruce and Lindsey. The piezoelectric effect which was very critical in the development of ultrasonic method was found in 1880 by Curies. They discovered that alternating voltage applied to piezoelectric crystal produces sound waves in the surrounding medium. The Other effect, namely magnetostricitive effect is also used for generating ultrasonic waves. J.P.Joules whose results are given in a paper dated 1847 discovered magnetostrictive effect.
2
The first use of sound waves in water was for the detection of submarines and was developed during World War l. As electromagnetic waves cannot penetrate water, use of acoustic waves to penetrate seawater, found importance and Prof. Paul Langevin produced the first underwater transducer.
World War II saw the development of many new piezoelectric crystals. The first ultrasonic flaw detection system for locating flaws in materials and for measuring the thickness of material was developed between 1939 and1945. Ultrasonic waves have also been applied to a large number physical phenomena investigations. Some of the studies done are oscillatory magneto acoustic phenomenon in metals, giant quantum oscillations, effect of dislocations and impurities on sound wave propagation, Fermi surfaces
of
metals
and
other
phenomena.
The present day ultrasonic transducers are limited in frequency to 100 GHz (1011 Hz).
HIGH INTENSITY ULTRASONIC (US) WAVES
As opposed to low amplitude devices described earlier, high intensity sound waves have also been applied in areas like ultrasonic cleaning, to produce fine grain size in metals and in producing biological changes. High amplitude devices have also been used to study internal friction, acoustic emission and fatigue.
3
In the recent past, surface wave devices have evoked some interest. They are used as delay lines, complex coded waveforms and wide band signals for missile guidance and airport flight control. Bombs in Pacific Ocean were probably the starting point for the ultrasonic technique. The famous Krautkramer brothers of Germany had in the meantime perfected the technique to such an extent that reliable ultrasonic testing method was available. Technique they employed was based on valves, which was subsequently
changed
transistors
and
ultimately
to
use
of
microprocessors. Most of the present day efforts in improving the technique are centered on employing the microprocessor. Design of Superior probes, and application of the technique to tough ceramics.
1.2. Comparison between the commonly used NDT methods Below you will find a general outline of the capabilities of common nondestructive testing methods. This is intended as brief summary of each discipline's capabilities.
Excellent
Fair
Poor
Defect type
Eddy Current
Magnetic Particle
Cracks/chips/porosity/voids Missed processes/operations Material property Structurally significant
Product lot variations
4
Ultrasonic
Radiography
Resonant Acoustic
Defect Location Surface (external)
Eddy Current
Magnetic Particle
Ultrasonic
Radiography
Resonant Acoustic
Internal Brazing/bonding/welding Speed/Training/Cost
Part troughput Training requirements Overall inspection costs Automation Capacity
Quantitative results Automation requirements
NA
Automation cost
NA
Figure 1: comparison between different NDT methods
Advantages and disadvantages of the NDT methods RESONANT INSPECTION
Detects resonant frequency shifts resulting from changes in mass, stiffness or damping of a part. Defects such as cracks, voids, chips, brazing problems, nodularity, porosity, variations in hardness, missed manufacturing
processes
and
delamination
can
be
detected.
Applications include powder metal parts, ductile iron parts, brazed assemblies, forgings, stampings and ceramic parts. Advantages
Disadvantages
Whole part test for internal and Not diagnostic - does not indicate external flaws
where flaw is, just there is one
5
Fast as 1 part per second
Materials that resonate only - metal, composites and ceramic parts
No part preparation required
Large parts (> 60lbs) difficult to test
Objective pass/fail result
Significant lot to lot variations can mask defect detection
No consumables expenses Easily automated Permanent record capability Easily finds first n number of natural frequencies for NVH applications Best
for
high
volume
quality
inspection Designed to be on the plant floor
ULTRASONICS
Measures thickness, velocity or detects internal defects and variations, such as cracks, lack of fusion, delaminations and lack of bond. Applications include wrought metals, welds, brazed joints, adhesive or bonded joints, non-metallic materials and in-service parts. Advantages
Disadvantages
Most sensitive to cracks
Couplant required
Immediate results
Complex, or small parts may be difficult to check
Automation possible
Reference standards required
Permanent record capability
Trained
operators
inspections Portable
Special probes
6
for
manual
High penetration capability
Surface condition
RADIOGRAPHY
Measures or detects, internal defects and variations, porosity, inclusions, cracks, lack of fusion, corrosion, geometry variation, density changes, misassembled and misaligned parts. Advantages
Disadvantages
Permanent records
Radiation hazard
Portable
Expensive
Geometry variation does not affect Trained operators needed direction of radiation beam Linear defect may be missed Depth of defect not indicated Access needed to at least two sides of the part
EDDY CURRENT
Measures or detects surface and subsurface cracks and seams, alloy content, heat treatment variations, wall and coating thickness, crack depth, conductivity and permeability. Advantages
Disadvantages
High speed
Conductive material only
Low cost
Shallow depth of penetration
Permanent record capability
7
No couplant required No probe contact required
LIQUID PENETRANT
Measures or detects defects open to the surface of parts such as cracks, porosity, seams, laps and through wall leaks. Advantages
Disadvantages
Low cost
Defect must be open to the surface
Portable
Parts must be cleaned before and after testing
Indications may be further examined
Surface films, such as coatings, scale, and smeared metal may visually mask defects
MAGNETIC PARTICLE
Measures or detects surface and qualified subsurface defects, cracks, seams, porosity, inclusions, and very sensitive for locating small tight cracks.
Advantages
Disadvantages
Low cost
Ferromagnetic materials only
Portable
Alignment of magnetic field is critical
8
Subsurface defects
Demagnetization required after the test Surface coatings can mask defects Pre and post cleaning necessary Messy
INFRARED THERMOGRAPHY
Measures or detects, hot spots, heat transfer, temperature ranges, and temperature monitoring and electrical assemblies. Advantages
Disadvantages
Permanent record or thermal picture
Expensive
Remote sensing
Reference standards required
Portable
Poor resolution on thick sections
9
2. Fundamental Properties of Ultrasound 2.1.
Definition of ultrasonic vibrations
The definition of ultrasonic, also known as ultrasound, is sound waves that have a higher frequency than the human ear can hear. An example of ultrasonic is an ultrasound image of an unborn baby An example of ultrasonic is a dog whistle that cannot be heard by humans. An example of ultrasonic is the detection of ships and objects underwater by bouncing a high frequency sound wave off their hulls, a process discovered by Paul Langevin. Sound is made when something vibrates. You can twang a ruler on a table or flick a stretched elastic band to verify this. The stretched surface of the rubber band or the ruler vibrates and sets up a series of vibrations, sound waves, in the air. As the surface of the band or ruler pushes into the air, the air molecules are forced together and a region of high pressure forms: compression. As the surface moves back, the air molecules move apart, forming a low pressure area or rarefaction. As the surface vibrates, alternate compressions and rarefactions set up in the air and travel out from the surface to form a sound wave. The air molecules don’t move with the wave they vibrate to and fro
in time with the vibrating surface. If we plot the displacement of the particle against time it will produce a sine wave as shown below.
10
Figure 2: A Wave Illustration
Mechanical vibrations for nondestructive tests can be generated by electromechanical transducers -devices with the ability to transform electrical into mechanical and vice versa. For ultrasonic testing at frequencies above 200 kHz, piezoelectric transducers are generally used
(they
can
be
used
also
at
lower
frequencies).
Such materials generate electric charges when mechanically stressed and conversely become stressed when electrically excited. Piezoelectric elements mounted for ultrasonic testing are commonly identified as transducers, search units, probes, transmitters (or receivers) or crystals. Sound is created when something vibrates. It is a stress wave of mechanical energy. The piezo-electric effect changes mechanical energy into electrical energy. It is reversible, so electrical energy - a voltage - can be changed into mechanical energy or sound, which is
11
the reverse piezo-electric effect. The first people to observe the piezo-electric effect were the Curie brothers who observed it in quartz crystals.
2.2.
Relationship of frequency velocity and wavelength
We are all familiar with waves in one form or another. We shall define a wave as a disturbance or vibration propagating through space. In general, waves can be classified as being either transverse or longitudinal. In a longitudinal wave, the medium vibrates in a direction parallel to the direction the wave is traveling. Sound is an example of a longitudinal wave. In a transverse wave, the medium vibrates in a direction perpendicular to the direction the wave is traveling. Figure 3 shows a wave with its corresponding wavelength, amplitude and crest.
Figure 3
The wavelength (λ) is the distance of one complete “wave”. The
amplitude of a wave is the maximum displacement of any part of the wave from its equilibrium position. The time it takes for a wave to travel a distance of one wavelength is called the period (T). The frequency (f) is the number of oscillations or cycles that occur during
12
a given period of time. Frequency is usually measured in cycles per second, or Hertz. The period (T) is usually measured in seconds. The frequency (f) and the period (T) are related by the following expression: Period (T) = 1/ Frequency (f) One of the properties of a wave is that the velocity of a wave is related with its frequency and wavelength by the following equation.
Velocity of a wave (v) = frequency (f) × wavelength ( λ ) A wave traveling along a plucked string is transverse in nature. Note that as the wave travels to the right, the medium (the string) is actually only moving up and down.
2.3.
Diffraction theory
Diffraction of Light at Ultrasound Waves
In 1932 Debye and Sears discovered in the USA and Lucas and Biquard discovered in France that transparent media diffract light when an ultra- sound wave is sent through them. This effect is a consequence of a periodical variation of the refractive index, which in turn is a consequence of a local periodical pressure change caused by the ultrasound wave.
13
Figure 4: Sound diffraction
A thin slit, lit up by the lamp La serves as a source of light. The lens L1 is placed in the distance of its focal length from the gap and thus produces a broad beam of parallel light. The light then penetrates a transparent medium (gas, liquid or solid) in which an ultrasonic wave transducer Q, located perpendicularly to the direction of the incidence light beam, produces elastic waves. For experiments with liquids or gases a container with plane parallel glass walls is required. The second lens L2 projects a real image S0 of the gap S on a screen. If the ultrasound wave is excited, several orders of the spectrum of the lamp La can be seen on both sides of S0. By introducing alter F into the path of the light beam we produce monochromatic light and obtain only one interference strip of each order near S 0. To understand this phenomenon we must assume that the local periodical pressure changes of the elastic wave create local changes in the refraction index of the medium. The surfaces of equal phase S(x; y; z) of the light wave are then no longer plains (S(x; y; z) = k0x for light
14
propagation in the x-direction.) but they become a sine function with the same period as the ultrasonic wave. This arises from the fact that in an area with a higher refractive index n the light wave travel slower than in an area with a lower refractive index (c = c 0n ).
2.4. Modes of particle vibration All material substances are comprised of atoms, which may be forced into vibrational motion about their equilibrium positions. Many different patterns of vibrational motion exist at the atomic level; however, most are irrelevant to acoustics and ultrasonic testing. Acoustics is focused on particles that contain many atoms that move in harmony to produce a mechanical wave. When a material is not stressed in tension or compression beyond its elastic limit, its individual particles perform elastic oscillations. When the particles of a medium are displaced from their equilibrium positions, internal restoration forces arise. These elastic restoring forces between particles, combined with inertia of the particles, lead to the oscillatory motions of the medium. In solids, sound waves can propagate in four principal modes that are based on the way the particles oscillate. Sound can propagate as longitudinal waves, shear waves, surface waves, and in thin materials as plate waves. Longitudinal and shear waves are the two modes of propagation most widely used in ultrasonic testing. The particle
15
movement responsible for the propagation of longitudinal and shear waves is illustrated in the figure.
Figure 5: Longitudinal and shear wave illustration
In longitudinal waves, the oscillations occur in the longitudinal direction or the direction of wave propagation. Since compression and expansion forces are active in these waves, they are also called pressure or compression waves. They are also sometimes called density waves because material density fluctuates as the wave moves. Compression waves can be generated in gases, liquids, as well as solids because the energy travels through the atomic structure by a series of compressions and expansion movements.
16
Figure 6: Longitudinal wave representation
In the transverse or shear waves, particles oscillate at a right angle or transverse to the direction of propagation. Shear waves require an acoustically solid material for effective propagation, and therefore, are not effectively propagated in materials such as liquids or gasses. Shear waves are relatively weak when compared to longitudinal waves. In fact, shear waves are usually generated in materials using some of the energy from longitudinal waves.
Figure 7: Shear wave representation
17
2.5.
Ultrasound uses
Ultrasonic testing (UT) has been practiced for many decades. Initial rapid developments in instrumentation spurred by the technological advances from the 1950's continue today. Through the 1980's and continuing through the present, computers have provided technicians with smaller and more rugged instruments with greater capabilities. Thickness gauging is an example application where instruments have been refined make data collection easier and better.
Built-in data
logging capabilities allow thousands of measurements to be recorded and eliminate the need for a "scribe." Some instruments have the capability to capture waveforms as well as thickness readings. The waveform option allows an operator to view or review the A-scan signal of thickness measurement long after the completion of an inspection. Also, some instruments are capable of modifying the measurement based on the surface conditions of the material. For example, the signal from a pitted or eroded inner surface of a pipe would be treated differently than a smooth surface. This has led to more accurate and repeatable field measurements. Many ultrasonic flaw detectors have a trigonometric function that allows for fast and accurate location determination of flaws when performing shear wave inspections. Cathode ray tubes, for the most part, have been replaced with LED or LCD screens. These screens, in most cases, are extremely easy to view in a wide range of ambient lighting.
Bright or low light working conditions encountered by
18
technicians have little effect on the technician's ability to view the screen. Screens can be adjusted for brightness, contrast, and on some instruments even the color of the screen and signal can be selected. Transducers can be programmed with predetermined instrument settings. The operator only has to connect the transducer and the instrument will set variables such as frequency and probe drive. Along with computers, motion control and robotics have contributed to the advancement of ultrasonic inspections. Early on, the advantage of a stationary platform was recognized and used in industry. Computers can be programmed to inspect large, complex shaped components, with one or multiple transducers collecting information. Automated systems typically consisted of an immersion tank, scanning system, and recording system for a printout of the scan. The immersion tank can be replaced with a squitter systems, which allows the sound to be transmitted through a water column. The resultant C-scan provides a plan or top view of the component. Scanning of components is considerably faster than contact hand scanning, the coupling is much more consistent. The scan information is collected by a computer for evaluation, transmission to a customer, and archiving. Today, quantitative theories have been developed to describe the interaction of the interrogating fields with flaws. Models incorporating the results have been integrated with solid model descriptions of real part geometries to simulate practical inspections. Related tools allow NDE to be considered during the design process on an equal footing with
other
failure-related
engineering
19
disciplines.
Quantitative
descriptions of NDE performance, such as the probability of detection (POD), have become an integral part of statistical risk assessment. Measurement procedures initially developed for metals have been extended to engineered materials such as composites, where anisotropy and inhomogeneity have become important issues. The rapid advances in digitization and computing capabilities have totally changed the faces of many instruments and the type of algorithms that are used in processing the resulting data. High-resolution imaging systems and multiple measurement modalities for characterizing a flaw have emerged. Interest is increasing not only in detecting, characterizing, and sizing defects, but also in characterizing the materials. Goals range from the determination of fundamental microstructural characteristics such as grain size, porosity, and texture (preferred grain orientation), to material properties related to such failure mechanisms as fatigue, creep, and fracture toughness. As technology continues to advance, applications of ultrasound also advance. The high-resolution imaging systems in the laboratory today will be tools of the technician tomorrow. Looking to the future, those in the field of NDE see an exciting new set of opportunities. The defense and nuclear power industries have played a major role in the emergence of NDE. Increasing global competition has led to dramatic changes in product development and business cycles. At the same time, aging infrastructure, from roads to buildings and aircraft, present a new set of measurement and monitoring challenges for engineers as well as technicians.
20
Among the new applications of NDE spawned by these changes is the increased emphasis on the use of NDE to improve the productivity of manufacturing processes. Quantitative nondestructive evaluation (QNDE) both increases the amount of information about failure modes and the speed with which information can be obtained and facilitates the development of in-line measurements for process control. The phrase, "you cannot inspect in quality, you must build it in," exemplifies the industry's focus on avoiding the formation of flaws. Nevertheless, manufacturing flaws will never be completely eliminated and material damage will continue to occur in-service so continual development of flaw detection and characterization techniques is necessary. Advanced simulation tools that are designed for inspectability and their integration into quantitative strategies for life management will contribute to increase the number and types of engineering applications of NDE. With growth in engineering applications for NDE, there will be a need to expand the knowledge base of technicians performing the evaluations. Advanced simulation tools used in the design for inspectability may be used to provide technical students with a greater understanding of sound behavior in materials. UTSIM, developed at Iowa State University, provides a glimpse into what may be used in the technical classroom as an interactive laboratory tool. As globalization continues, companies will seek to develop, with ever increasing frequency, uniform international practices. In the area of
21
NDE, this trend will drive the emphasis on standards, enhanced educational offerings, and simulations that can be communicated electronically. The coming years will be exciting as NDE will continue to emerge as a full-fledged engineering discipline.
22
3. Generation of Ultrasonic Waves 3.1.
Piezoelectric crystal types and characteristics
An Introduction to Piezoelectric Transducer Crystal Piezoelectric Materials and their Properties
Certain single crystal materials exhibit the following phenomenon: when the crystal is mechanically strained, or when the crystal is deformed by the application of an external stress, electric charges appear on certain of the crystal surfaces; and when the direction of the strain reverses, the polarity of the electric charge is reversed. This is called the direct piezoelectric effect, and the crystals that exhibit it are classed as piezoelectric crystals.
Figure 8. The direct piezoelectric effect.
Conversely, when a piezoelectric crystal is placed in an electric field, or when charges are applied by external means to its faces, the crystal
23
exhibits strain, i.e. the dimensions of the crystal change. When the direction of the applied electric field is reversed, the direction of the resulting strain is reversed. This is called the converse piezoelectric effect.
Figure 9: The converse piezoelectric effect.
Many of today's applications of piezoelectricity use polycrystalline ceramics instead of natural piezoelectric crystals. Piezoelectric ceramics are more versatile in that their physical, chemical, and piezoelectric characteristics can be tailored to specific applications. Piezo-ceramic materials can be manufactured in almost any shape or size, and the mechanical and electrical axes of the material can be oriented in relation to the shape of the material. These axes are set during poling (the process that induces piezoelectric properties in the material). The orientation of the DC poling field determines the orientation of the mechanical and electrical axes.
24
The direction in which tension or compression develops polarization parallel to the strain is called the piezoelectric axis. In quartz, this axis is known as the "X-axis", and in poled ceramic materials such as PZT the piezoelectric axis is referred to as the "Zaxis". From different combinations of the direction of the applied field and orientation of the crystal it is possible to produce various stresses and strains in the crystal. For example, an electric field applied perpendicular to the piezoelectric axis will produce elongation along the axis as shown in Figure 9. If, however, the electric field is applied parallel to the piezoelectric axis, a shear motion is induced. This type motion is shown in Figure 10.
Figure 10: Shear motion generation.
3.2. Search unit construction Transducer Design Requirements
In the design of practical transducers for various applications, additional requirements include mechanical (contact area, wear resistance, waterproofing and connectors), electrical (voltages, wave shapes, capacitance and grounding) and acoustic (noise level, cross
25
coupling, damping and face plates). Construction of three types of transducer assemblies is shown in Fig. 11. Figure 11a shows a straight beam contact transducer. These use thin wear plates to prevent crystal breakage and to protect the front electrode, which provides internal grounding. Facings of ceramic, metal, plastic and rubber have been used. Applications include tests of rough surfaces and electrical nonconductors. Angle beam transducers direct the ultrasonic beam away from normal incidence toward selected areas within a test object using a wedge between the crystal and the test object (see Fig. 11b). These wedges or shoes are usually made from plastic materials.
26
Figure 11: Typical ultrasonic testing transducers: (a) straight beam contact unit; (b) angle beam transducer; (c) immersion transducer
An ultrasonic search unit consists of an appropriate housing with electrical
connector,
backing
material
for
dampening,
active
piezoelectric material, lens for acoustic focusing, or time delay material if required. In addition, a thin coupling layer, protective layer, or wear plate may be used ahead of the active element. The effectiveness of the search unit for a particular application depends on Q, bandwidth, frequency, sensitivity, acoustic impedance, and
27
resolving power. The Q of a transducer is given by the following equation: =
Fr F2−F1
Where F2 - F1 = bandwidth Fr = resonant frequency of the element F2 = frequency above Fr where amplitude = 0.707 Fr amplitude F1 = frequency below Fr where amplitude = 0.707 Fr amplitude Note: In most nondestructive testing applications, the Q of the search unit will vary from 1 to 10. Sensitivity is the ability of the search unit to detect reflections or echoes from small defects or flaws. Search unit sensitivity is directly proportional to the product of its efficiency as a transmitter and its efficiency as a receiver. The acoustic impedance of a transducer is the product of its density and the velocity of sound within it. Its resolving power includes the ability to separate reflections from two closely spaced flaws or reflectors. For example, in contact testing, the transducer must have good resolution to separate the front surface pulse, initial pulse, or “main bang” from the
near-surface defect reflections. The ability of a transducer to resolve or detect near-surface defects is related to its pulse width or length. The transducer must stop vibrating or “ringing” after it is shocked in order to resolve or “see” small near -surface flaws. Resolving power is also
related to crystal dampening and bandwidth. Ideally, we would like to
28
reduce crystal ringing or bandwidth without adversely affecting sensitivity.In practice, compromises must be made to achieve test goals. For most search units, the product of bandwidth and sensitivity is a constant. As bandwidth increases, sensitivity decreases; as bandwidth decreases, sensitivity increases. By knowing the sensitivity bandwidth product and bandwidth of the transducer, its sensitivity can be calculated. Piezoelectric elements are usually coated with gold or silver on their front and back surfaces to form electrodes. Figure 12 shows a few of the available electrode configurations for flat crystals and some typical transducers. Small wire leads are then attached to the electrode surfaces by welding, soldering, or cementing them in place with conductive epoxy. Good electrical insulation must be used in search unit construction because high-voltage pulses (100 – 2000 VDC) are applied to the electrodes for 10ms or less. The high voltage pulses are transmitted from the pulse circuit of the electronics unit to the search unit through a coaxial cable to an ultrahigh frequency (UHF) or baby N connector (BNC)-type coaxial connector on the search unit.
29
Figure 12 (a, b) Crystal electrode arrangements and (c) factory assembled transducers. Electrode diagrams courtesy of Valpey-Fisher Corp.; transducer photo courtesy of Krautkramer Branson, Inc., manufacturer of Ultrasonic Nondestructive Testing Equipment.
30
The backing material of the search unit serves two purposes. First, it controls crystal dampening and therefore bandwidth, and second, it attenuates the energy on the back side of the crystal so that unwanted reflections will not be received from the back side of the transducer. Transducers can be air backed or backed by fibrous or cellular plastic material that effectively attenuates sound on the back side of the crystal while reducing its undampened bandwidth from as much as 20,000 to a range of 1 to 10, depending on the application. Some manufacturers embed a hardened surface ring and coat the front sides of their transducers with an aluminum oxide coating to reduce transducer wear. In contact testing, plastic wedges are used to position crystals at an angle for shear wave testing. Delay tips are also attached to contact-type transducers for testing thin materials. While increasing the versatility of the transducer, the Lucite wedges and shoes decrease transducer sensitivity. Lucite shoes are contoured wedges that are shaped to the contour of the test surface. In high-speed production lines, the Lucite shoes may actually ride on the test surface, become worn, and have to be periodically replaced. Acoustic lenses can also be attached to the front surface of the transducer to act as a lens for focusing the sound beam. As the radius of curvature of a curved lens increases, the focal length of the lens increases. Ideally, the acoustic impedance of the focusing lens is between that of the transducer and material under test. Transducers can be spherically or “spot” focused and cylindrically focused. Spherically
31
(spot or point) focused transducers are used when improved resolution to small flaws is required for the test material. Cylindrically focused transducers are typically used for pipe and tubing inspections. Acoustic lenses with cylindrical curvatures focus sound energy into cylindrical surfaces normally or at right angles. Focusing lenses effectively shorten the Fresnel zone by shifting the transition distance N
toward
the
transducer.
Backing
material
variations,
lens
misalignment, or lens porosity can result in the propagation of nonsymmetrical beams when using focused transducers. Focusing can increase the echo amplitude from small flaws near the focal point. This technique is used to obtain better near-surface resolution without increasing transducer frequency. The disadvantage is that the sensitivity to defects in the far field is greatly decreased. Cylindrical focusing is used to shape the sonic wave front to conform to part geometry. This produces a clean front surface reflection and better resolution of near-surface flaws in pipe and tubing. This type of focusing is also called “contoured focusing.” The effects of crystal
diameter, focal distance, and water path length are given in Table1. a
(Test conditions ) Crystal
Focus b
Beam
Beam
Actual
Beam FD- Beam
diameter
FD-water diameter
water
Aluminum FD-Stell
(in.)
(in.)
(in.)
path (in.)
(in.)
(in.)
1.50
S
4.2
0.13
1.0
0.75
0.82
1.50
M
6.5
0.23
1.2
1.25
1.35
1.50
L
9.2
0.37
1.8
1.75
1.87
0.75
S
2.5
0.05
1.5
0.25
0.25
32
0.75
M
4.0
0.09
1.9
0.50
0.53
0.75
L
5.2
0.15
1.0
1.00
1.07
0.50
S
1.7
0.04
0.9
0.18
0.19
0.50
M
2.7
0.06
1.1
0.38
0.41
0.50
L
3.7
0.12
1.9
0.44
0.46
0.37
S
1.3
0.03
0.8
0.13
0.13
0.37
M
2.2
0.05
1.2
0.25
0.25
0.37
L
3.0
0.08
1.4
0.38
0.41
0.25
S
0.8
0.02
0.4
0.09
0.10
0.25
M
1.5
0.05
0.8
0.16
0.18
0.25
L
2.2
0.07
0.9
0.31
0.33
0.19
S
0.6
0.02
0.4
0.05
0.05
0.19
M
1.0
0.04
0.6
0.10
0.10
Table 1: Focused search unit characteristics
a
(1) Water path may vary ±25% without adversely affecting test sensitivity. (2) Data applicable
to frequencies of 10MHz or above. b
S = short, M = medium, L = long.
To calculate the focal distance (FD) under other conditions, the following equation may be used: FD (water) = metal path (water equivalent) +water path Where metal path (water equivalent) = metal path \ (velocity of sound in metal/velocity of sound in water) water path = distance from the transducer face to the part surface.
The relationship between crystal frequency and thickness is given in Table 2.
33
Crystal Frequency (MHz)
Crystal Thickness (in.)
0.20
0.500
0.50
0.200
1.00
0.100
2.25
0.50
5.00
0.20
10.00
0.10
15.00
0.007
20.00
0.006
25.00
0.005
Table 2: Crystal Frequency versus Thickness (*) (*) Crystal diameters vary widely, typically in the range 0.5 – 1.5 in. for 1 MHz to 0.125 – 0.375 in. for 25MHz.
Some special-purpose transducers are mosaic and paintbrush transducers. A multiple crystal transducer, whose crystals operate in the same plane and in phase with each other, is known as a crystal mosaic. A paintbrush transducer is a large-area, rectangular-shaped transducer with uniformintensity beam pattern. Since both mosaic and paintbrush transducers are used for rapid scanning of relatively large areas, search units with smaller crystals are frequently used to accurately locate and further evaluate discontinuities found with these transducers. Today’s modern search units are quite rugged and designed to survi ve
heavy use. However, some common sense should be used with regard to the care and storage of search units. They should not be dropped or
34
mechanically deformed; their front surfaces should be protected to prevent scratches and minimize wear. Units should be stored at normal operating temperatures and not subjected to excessively harsh* chemical environments. Tank-type immersion units should be immersed or loaded before they are pulsed, and the transmitter should be turned off before they are removed from the immersion tank. 3.3. Search unit characteristics: resolution; sensitivity The I.I.W. VI block is used to determine the resolution of a flaw detector using a normal beam probe. This block has three target reflectors at ranges of 85 mm, 91 mm and 100 mm. The probe is placed on the block as shown in Figure 13a and echoes from the three reflectors are obtained. The separation of the echoes from each other indicates the degree of resolution of the flaw detector for that particular probe. Figure 13b shows the degree of resolution for flaw detectors using two different normal beam probes.
Figure13: Estimating the resolving power of normal beam probe
35
Another block (described in B.S. 3923: Part 3: 1972) used for the determination of resolution of flaw detectors using either normal beam or angle beam probes is shown in Figure 5.26. With this block the resolution is determined by the minimum distance apart that flaws can be indicated clearly and separately. In use the probe is placed on the center line of the block over the change in radius from one step to the next. Its position is adjusted so that echoes from the two radii are of the same height and approximately 1/2 full screen height. The steps are said to be resolved when their echoes are clearly separated at half maximum echo height or less.
Figure 14: Test block for measuring probe resolution.
36
3.4. Search unit types Ultrasonic transducers are manufactured for a variety of applications and can be custom fabricated when necessary. Careful attention must be paid to selecting the proper transducer for the application. A previous section on Acoustic Wavelength and Defect Detection gave a brief overview of factors that affect defect detectability. From this material, we know that it is important to choose transducers that have the desired frequency, bandwidth, and focusing to optimize inspection capability. Most often the transducer is chosen either to enhance the sensitivity or resolution of the system. Transducers are classified into groups according to the application.
3.4.1. Contact transducers
Contact transducers are used for direct contact inspections, and are generally hand manipulated. They have elements protected in a rugged casing to withstand sliding contact with a variety of materials. These transducers have an ergonomic design so that they are easy to grip and move along a surface. They often have replaceable wear plates to lengthen their useful life. Coupling materials of water, grease, oils, or commercial materials are used to remove the air gap between the transducer and the component being inspected.
Figure 15: A simple transducer
37
Contact transducers are available in a variety of configurations to improve their usefulness for a variety of applications. The flat contact transducer shown above is used in normal beam inspections of relatively flat surfaces, and where near surface resolution is not critical. If the surface is curved, a shoe that matches the curvature of the part may need to be added to the face of the transducer. If near surface resolution is important or if an angle beam inspection is needed, one of the special contact transducers described below might be used.
3.4.2. I mmersion transducers
Immersion transducers do not contact the component. These transducers are designed to operate in a liquid environment and all connections are watertight. Immersion transducers usually have an impedance matching layer that helps to get more sound energy into the water and, in turn, into the component being inspected. Immersion transducers can be purchased with a planer, cylindrically focused or spherically focused lens. A focused transducer can improve the sensitivity and axial resolution by concentrating the sound energy to a smaller area. Immersion transducers are typically used inside a water tank or as part of a squirter or bubbler system in scanning applications.
38
Figure 16: Immersion probes
3.4.3. Twin crystal probes
Also called Dual element transducers contain two independently operated elements in a single housing. One of the elements transmits and the other receives the ultrasonic signal. Active elements can be chosen for their sending and receiving capabilities to provide a transducer with a cleaner signal, and transducers for special applications, such as the inspection of course grained material. Dual element
transducers
are
especially
well
suited
for
making
measurements in applications where reflectors are very near the transducer since this design eliminates the ring down effect that singleelement transducers experience (when single-element transducers are operating in pulse echo mode, the element cannot start receiving reflected signals until the element has stopped ringing from its transmit function). Dual element transducers are very useful when making thickness measurements of thin materials and when inspecting for near
39
surface defects. The two elements are angled towards each other to create a crossed-beam sound path in the test material.
Figure 17: Twin crystal probe
3.4.4. Delay line transducers
Delay line transducers provide versatility with a variety of replaceable options. Removable delay line, surface conforming membrane, and protective wear cap options can make a single transducer effective for a wide range of applications. As the name implies, the primary function of a delay line transducer is to introduce a time delay between the generation of the sound wave and the arrival of any reflected waves. This allows the transducer to complete its "sending" function before it starts its "listening" function so that near surface resolution is improved. They are designed for use in applications such as high precision
40
thickness gauging of thin materials and delamination checks in composite materials. They are also useful in high-temperature measurement applications since the delay line provides some insulation to the piezoelectric element from the heat.
Figure 18: Delay line probe
3.4.5. Angle beam transducers
Angle beam transducers and wedges are typically used to introduce a refracted shear wave into the test material. Transducers can be purchased in a variety of fixed angles or in adjustable versions where the user determines the angles of incidence and refraction. In the fixed angle versions, the angle of refraction that is marked on the transducer is only accurate for a particular material, which is usually steel. The angled sound path allows the sound beam to be reflected from the backwall to improve detectability of flaws in and around welded areas. They are also used to generate surface waves for use in detecting defects on the surface of a component.
41
Figure 18: Angle beam probe 3.4.6. Normal i ncidence shear wave transducers
Normal incidence shear wave transducers are unique because they allow the introduction of shear waves directly into a test piece without the use of an angle beam wedge. Careful design has enabled manufacturing of transducers with minimal longitudinal wave contamination. The ratio of the longitudinal to shear wave components is generally below -30dB. 3.4.7. Paint brush transducers
Paint brush transducers are used to scan wide areas. These long and narrow transducers are made up of an array of small crystals that are carefully matched to minimize variations in performance and maintain uniform sensitivity over the entire area of the transducer. Paint brush transducers make it possible to scan a larger area more rapidly for discontinuities. Smaller and more sensitive transducers are often then required to further define the details of a discontinuity.
42
4. Test Material Characteristics 4.1.
Velocity
The speed of sound is the distance traveled per unit of time by a sound wave propagating through an elastic medium. In dry air at 20 °C (68 °F), the speed of sound is 343.2 meters per second (1,126 ft/s). This is 1,236 kilometers per hour (667 kn; 768 mph), or about a kilometer in three seconds or a mile in about five seconds. In fluid dynamics, the speed of sound in a fluid medium (gas or liquid) is used as a relative measure of speed itself. The speed of an object divided by the speed of sound in the fluid is called the Mach number. Objects moving at speeds greater than Mach1 are traveling at supersonic speeds. The speed of sound in an ideal gas is independent of frequency, but does vary slightly with frequency in a real gas. It is proportional to the square root
of
the
absolute
temperature,
but
is
independent
of pressure or density for a given ideal gas. Sound speed in air varies slightly with pressure only because air is not quite an ideal gas. Although (in the case of gases only) the speed of sound is expressed in terms of a ratio of both density and pressure, these quantities cancel in ideal gases at any given temperature, composition, and heat capacity. This leads to a velocity formula for ideal gases which includes only the latter independent variables. In common everyday speech, speed of sound refers to the speed of sound waves in air. However, the speed of sound varies from substance to substance. Sound travels faster in liquids and non-porous solids than
43
it does in air. It travels about 4.3 times as fast in water (1,484 m/s), and nearly 15 times as fast in iron (5,120 m/s), as in air at 20 degrees Celsius. Sound waves in solids are composed of compression waves (just as in gases and liquids), but there is also a different type of sound wave called a shear wave, which occurs only in solids. These different types of waves in solids usually travel at different speeds, as exhibited in seismology. The speed of a compression sound wave in solids is determined by the medium's compressibility, shear modulus and density. The speed of shear waves is determined only by the solid material's shear modulus and density. Dependence on the propreties of the medium
The speed of sound is variable and depends on the properties of the substance through which the wave is travelling. In solids, the speed of transverse (or shear) waves depends on the shear deformation under shear stress (called the shear modulus), and the density of the medium. Longitudinal (or compression) waves in solids depend on the same two factors with the addition of a dependence on compressibility. In fluids, only the medium's compressibility and density are the important factors, since fluids do not tolerate shear stresses. In heterogeneous fluids, such as a liquid filled with gas bubbles, the density of the liquid and the compressibility of the gas affect the speed of sound in an additive manner, as demonstrated in the hot chocolate effect.
44
In gases, adiabatic compressibility is directly related to pressure through the heat capacity ratio (adiabatic index), and pressure and density are inversely related at a given temperature and composition, thus making only the latter independent properties (temperature, molecular composition, and heat capacity ratio) important. In low molecular weight gases such as helium, sound propagates faster compared to heavier gases such as xenon (for monatomic gases the speed of sound is about 75% of the mean speed that molecules move in the gas). For a given ideal gas the sound speed depends only on its temperature. At a constant temperature, the ideal gas pressure has no effect on the speed of sound, because pressure and density (also proportional to pressure) have equal but opposite effects on the speed of sound, and the two contributions cancel out exactly. In a similar way, compression waves in solids depend both on compressibility and density — just as in liquids — but in gases the density contributes to the compressibility in such a way that some part of each attribute factors out, leaving only a dependence on temperature, molecular weight, and heat capacity ratio (see derivations below). Thus, for a single given gas (where molecular weight does not change) and over a small temperature range (where heat capacity is relatively constant), the speed of sound becomes dependent on only the temperature of the gas. In non-ideal gases, such as a van der Waals gas, the proportionality is not exact, and there is a slight dependence of sound velocity on the gas pressure.
45
Humidity has a small but measurable effect on sound speed (causing it to
increase
by
about
0.1%-0.6%),
because oxygen and nitrogen molecules of the air are replaced by lighter molecules of water. This is a simple mixing effect. 4.2. Sound attenuation When sound travels through a medium, its intensity diminishes with distance. In idealized materials, sound pressure (signal amplitude) is only reduced by the spreading of the wave. Natural materials, however, all produce an effect which further weakens the sound. This further weakening results from scattering and absorption. Scattering is the reflection of the sound in directions other than its original direction of propagation. Absorption is the conversion of the sound energy to other forms of energy. The combined effect of scattering and absorption is called attenuation. Ultrasonic attenuation is the decay rate of the wave as it propagates through material.
Figure 19: Attenuation diagram
Attenuation of sound within a material itself is often not of intrinsic interest. However, natural properties and loading conditions can be related to attenuation. Attenuation often serves as a measurement tool that leads to the formation of theories to explain physical or chemical phenomenon that decreases the ultrasonic intensity.
46
The amplitude change of a decaying plane wave can be expressed as:
In this expression A0 is the unattenuated amplitude of the propagating wave at some location. The amplitude A is the reduced amplitude after the wave has traveled a distance z from that initial location. The quantity
is the attenuation coefficient of the wave traveling in the z-
direction. The dimensions of
are nepers/length, where a neper is a
dimensionless quantity. The term e is the exponential (or Napier's constant) which is equal to approximately 2.71828. The units of the attenuation value in Nepers per meter (Np/m) can be converted to decibels/length by dividing by 0.1151. Decibels is a more common unit when relating the amplitudes of two signals. Attenuation is generally proportional to the square of sound frequency. Quoted values of attenuation are often given for a single frequency, or an attenuation value averaged over many frequencies may be given. Also, the actual value of the attenuation coefficient for a given material is highly dependent on the way in which the material was manufactured. Thus, quoted values of attenuation only give a rough indication of the attenuation and should not be automatically trusted. Generally, a reliable value of attenuation can only be obtained by determining the attenuation experimentally for the particular material being used. Attenuation can be determined by evaluating the multiple back wall reflections seen in a typical A-scan display like the one shown in the
47
image at the top of the page. The number of decibels between two adjacent signals is measured and this value is divided by the time interval between them. This calculation produces an attenuation coefficient in decibels per unit time. This value can be converted to nepers/length by the following equation.
Where v is the velocity of sound in meters per second and Ut is in decibels per second.
4.3. Acoustic impedance
The acoustic impedance Z is the ratio of sound pressure (measured in Pa) to volume flow (measured in cubic meters per second). Let's consider a duct or pipe with cross sectional area A. If the wavelength of sound is large compared to the lateral dimensions of the duct, a plane wave will propagate along the duct. Let's suppose for the moment that there are no reflections coming back from the other end of the duct. In that rather special case, we have a one dimensional (plane) wave travelling to the right, with pressure p and particle velocity u in phase, as we saw in the Sound wave equation.
48
The specific acoustic impedance z is the ratio of sound pressure to particle velocity, and z = ρ*v, where ρ is the density and v the speed of sound. So for our duct with cross sectional area A, provided that the wave is strictly one dimensional and travelling in one direction, the acoustic volume flow is just U = Au For this very special case, we define the characteristic acoustic impedance Z0, where Z0 = p/U = p/Au = z/A So Z0 = ρv/A
49
5. Sound Beam Characteristics 5.1.
I ntensity variations
It is convenient to define the beam 'edge' as the point, across the beam, where the intensity of sound has fallen to one half, or sometimes one tenth of the intensity at the center of the beam. Whenever possible we use the Far Field in ultrasonic testing, the near field usually being accommodated within the Perspex shoe of the probe.
Figure 20: Beam intensity variation 5.2.
Dead zone; Near zone; F ar zone 5.2.1.
Dead zone
The Dead Zone is a zone where it is not possible to detect defects. Due to imperfect damping of the crystals some waves will interfere with the returning waves. This problem can be overcome by using twin crystals, one transmitting, and one receiving. The higher the probe frequency the shorter the Dead Zone.
50
Figure 20: Dead zone representation
5.2.2. Near zone
The Near Field is an area of 'turbulence' and varying sound intensity. Due to the effect of interference in the near field the signal height from the same size of defect may increase when it is positioned further away from the crystal. Similarly, small defects may be completely overlooked.
51
5.2.3. F ar zone
In the Far Field the beam diverges and the signal height from the same size of defect decreases in relation to the distance in accordance with the inverse square law.
Figure 21: The ultrasonic beam profile
It can be seen from the formula, (Figure 21) that by increasing the probe diameter or increasing the frequency (shorter wavelength), the solid angle of the beam will decrease.
52
5.3.
Probe diameter and frequency effect
Most transducers use a piezoelectric element. When piezoelectric ceramics were introduced, they soon became the dominant material for transducers due to their properties and their ease of manufacture into a variety of shapes and sizes. The first piezo-ceramic in general use was barium titanate and that was followed during the 1960s by lead zirconate titanate (Pb(Zr,Ti)O3, PZT) compositions, which is now the most commonly employed ceramic for making transducers. For ferroelectric materials the piezoelectric effect takes place only below the Curie temperature, for barium titanate it is 110 °C, for PZT 320 °C. To compare different piezoelectric materials, it is popular to compare everything to quartz. A basic conflict is given through the fact that for the maximisation of the transmission other crystal properties are relevant than for the maximisation of the receiving properties. Sometimes it can be of use to choose one material for the transmitter and another material for the receiver. The choice of the transducer material has also to include a consideration of the load (immersion medium, wedge or delay line material). The damping is done by a backing, usually a highly attenuating dense material that is used to control the vibration of the transducer by absorbing the energy radiation from the back face of the active element. When the acoustic impedance of the backing matches the acoustic impedance of the active element, the result will be a heavily damped
53
transducer that displays good range resolution. If there is a mismatch in acoustic impedance more sound energy will be reflected forward into the test material. The frequency f depends on the thickness d of the transducer crystal and the sound velocity v of the transducer material. Transducers with higher frequency are more complicated to produce and they are therefore more expensive. Transducers can be used as transmitter or as sensor alternatively or as both of them intermittently. Probes with only one transducer are also called transceivers.
Figure 22: Various piezoelectric crystals
The basic purpose of the transducer wear plate is to protect the transducer element from the testing environment. For immersion, angle beam and delay line transducers the wear plate has the additional purpose of an acoustic transformer between the high acoustic impedance of the active element and the water, the wedge or the delay line all of which are of lower acoustic impedance. This is accomplished by selecting a matching layer that is ¼ wavelengths thick and of the desired acoustic impedance. The choice of the wear surface thickness
54
is based upon the idea of superposition that allows waves generated by the active element to be in phase with the wave reverberating in the matching layer. Not properly designed wear plates result in disruptions in the wave front. Piezo-composite materials usually have a structure called 1-3, where the piezoelectric rods are embedded in a polymer matrix by a dice-andfill technique. The ceramic and the resin are chosen according to the characteristics required for the composite material. The geometry of the microstructure itself can be adapted. One of the characteristics of a 1-3 structure is that the percentage of the ceramic can be varied by modifying the size of the rods and their spacing. The designation simply codes the type of composite: piezoelectric material hits the surface in one direction; the resin does so in three. The height of the ceramic rods – long compared to their lateral dimensions – favor their vibration according to the thickness mode to the detriment of the radial mode. This results in improved electroacoustic efficiency that gives the sensor a high level of sensitivity and a high signal/noise ratio. In addition, the natural damping of composite materials allows a relative bandwidth of 60% to 90% to be obtained while retaining a very good level of sensitivity. The optimum size of the rods, not only their length, depends on frequency. The mechanical properties of the polymers are used to enable the piezocomposite materials to be shaped for focused transducers or such that are adapted to the surface.
55
6. Angular Incidence 6.1.
Reflection
If an acoustic wave meets an interface of two materials with different impedances (impedance mismatch) a part of the energy is reflected while the other part is transmitted. For perpendicular incidence the reflection coefficient R and transmission coefficient T in terms of pressure are defined as = =
Zt − Zi Zt + Zi 2∗Zt Zt+Zi
Zi and Zt are the acoustic impedances for the incident and the transmitting material, respectively. Clearly, the transmission coefficient is always positive, the reflection coefficient, however, can be positive or negative. A change of sign corresponds to a phase change of the reflected wave.
Figure 23: transmission and reflection coefficients variations
56
6.2.
Refraction
Refraction is a change of beam direction at a boundary two media in which ultrasound travels at different velocities. It is caused by a change of wavelength as the ultrasound crosses from the first medium to the second while the frequency remains unchanged. We recall that: Velocity= frequency x wavelength Therefore. When velocity changes but frequency remains the same, the wavelength must undergo change.
Figure 24: Phenomenon of refraction
57
The refraction occurs when the angle of incidence at the boundary is zero. In the case of normal incidence, part of the beam energy is reflected directly backwards, and the remaining energy is transmitted into the second medium without directional change. At any other angle of incidence, the transmitted beam is deviated from the original direction of the incident beam, either towards or away from the normal, depending on the relative velocities of ultrasound in the two media.
6.3. Mode conversion When an ultrasonic wave obliquely impinges on an interface between two media as shown in Fig.3, several things happen depending on the incident angle of the wave as well as the material properties of the two media. One of important things is refraction in which a transmitted wave has a different angle from the incident. The refraction is basically caused by the velocity difference on either side of the interface. The refraction angle can be calculated from Snell’s law if the velocities of the two media and the incidence angle are known. Another important phenomenon is mode conversion that is a generation of one type of wave from another type in refraction as shown in Fig. 3. For example, a longitudinal wave incident on an interface between liquid and solid is transmitted partially as a refracted longitudinal wave and partially as a mode converted shear wave in the solid. Mode conversion can also take place on reflection if the liquid shown in Fig. 3 is a solid. It is noted that any types of waves can be converted to
58
another type, e.g. from a shear wave to a longitudinal wave, and from a longitudinal wave to a surface wave. The angles of reflection and/or refraction by mode conversion can be calculated from Snell’s law.
Figure 26 shows a simulation result for refraction and mode conversion, calculated by a finite difference method. We can see that an incident plane wave (longitudinal wave) of 10º in water is refracted at the refraction angle of 43º in steel and simultaneously converted to shear wave at refraction angle of 22º.
Figure 25: Schematics of reflection, refraction and mode conversion at an oblique interface.
Figure 26: Simulation result for refraction and mode conversion.
59
6.4. Snell's Law Snell's law (also known as the Snell – Descartes law and the law of refraction) is a formula used to describe the relationship between the angles of incidence and refraction, when referring to light or other waves passing through a boundary between two different isotropic media, such as water, glass and air. In optics, the law is used in ray tracing to compute the angles of incidence or refraction, and in experimental optics to find the refractive index of a material. The law is also satisfied in metamaterials, which allow light to be bent "backward" at a negative angle of refraction with a negative refractive index. Although named after Dutch astronomer Willebrord Snellius (1580 – 1626), the law was first accurately described by the scientist Ibn Sahlat the Baghdad court in 984. In the manuscript On Burning Mirrors and Lenses, Sahl used the law to derive lens shapes that focus light with no geometric aberrations. Snell's law states that the ratio of the sines of the angles of incidence and refraction is equivalent to the ratio of phase velocities in the two media, or equivalent to the reciprocal of the ratio of the indices of refraction:
With each as the angle measured from the normal of the boundary, as the velocity of light in the respective medium (SI units are meters per
60
second, or m/s) and
as the refractive index (which is unitless) of the
respective medium. The law follows from Fermat's principle of least time, which in turn follows from the propagation of light as waves. Snell's law is used to determine the direction of light rays through refractive media with varying indices of refraction. The indices of refraction of the media, labeled ,
and so on, are used to represent
the factor by which a light ray's speed decreases when traveling through a refractive medium, such as glass or water, as opposed to its velocity in a vacuum. As light passes the border between media, depending upon the relative refractive indices of the two media, the light will either be refracted to a lesser angle, or a greater one. These angles are measured with respect to the normal line, represented perpendicular to the boundary. In the case of light traveling from air into water, light would be refracted towards the normal line, because the light is slowed down in water; light traveling from water to air would refract away from the normal line. Refraction
between
two
surfaces
is
also
referred
to
as reversible because if all conditions were identical, the angles would be the same for light propagating in the opposite direction. Snell's law is generally true only for isotropic or specular media (such as glass). In anisotropic media such as some crystals, birefringence may split the refracted ray into two rays, the ordinary or o-ray which follows Snell's law, and the other extraordinary or e-ray which may not be co planar with the incident ray.
61
When the light or other wave involved is monochromatic, that is, of a single frequency, Snell's law can also be expressed in terms of a ratio of wavelengths in the two media, λ 1and λ 2:
Example 1: Suppose you wish to calculate the refracted angle within a material when you know the incident angle (20°), incident material velocity (2330 m/s) and refracted material velocity (5960 m/s): A1 = 20° V1 = 2330 m/s A2 = we don’t know! V2 = 5960 m/s Substituting these figures into the equation above gives us:
We cross multiply the fractions to give us
62
6.5.
Critical angles
When the ultrasonic wave passes from one medium (material) to another it changes speed. This is because the speed of a wave is determined by the medium through which it is passing. When the wave speeds up as it passes from one material to another, the angle of refraction is bigger than the angle of incidence. For example, this happens when the wave passes from water to aluminum or from glass to water.
Figure 27: A wave incident on a water-air interface.
Angle of incidence is the angle between an incident wave and the normal. Angle of refraction is the angle between a refracted wave and the normal.
63
First Critical Angle
Before the angle of incidence reaches the first critical angle, both longitudinal and shear waves exist in the part being inspected. The first critical angle is said to have been reached when the longitudinal wave no longer exists within the part, that is, when the longitudinal wave is refracted to greater or equal than 90°, leaving only a shear wave remaining in the part.
Second Critical Angle
The second critical angle occurs when the angle of incidence is at such an angle that the remain shear wave within the part is refracted out of the part. At this angle, when the refracted shear wave is at 90° a surface wave is created on the part surface. Beam angles should always be plotted using the appropriate industry standard, however, knowing the effect of velocity and angle on refraction will always benefit an NDT technician when working with angle inspection or the immersion technique.
64
7. Ultrasonic Inspection Techniques 7.1.
Through transmission
Through-transmission ultrasonic testing (UT) is used for detection, verification, sizing, and growth rate monitoring of cracks in piping, vessels, cylindrical shapes, and sometimes noncylindrical shapes. Through-transmission UT is a two transducer technique in a pitch-catch arrangement. While there are many types of UT techniques, because of the wide variety of component shapes, sizes, and orientations it is sometimes valuable to have an alternative technique for verification, such as through-transmission. Through-transmission UT is most widely known as a method of inspection in automated immersion testing for detection of laminars in steel or disbanding in composite materials where two opposite and parallel surfaces can be used for scanning — Fig. 27. The through-transmission UT discussed in this article is done manually from external cylindrical surfaces such as the outside of a pipe, vessel, valve, or casting. There are many possible arrangements for sending and receiving search units. One typical arrangement uses two transducers radially oriented to each other on a heavy-wall pipe — Fig. 28. In automated immersion testing, the ultrasonic instruments have settings for pulse-echo and dual just as conventional flaw detectors do. The immersion instruments also have a setting for through-
65
transmission. Conventional flaw detectors usually don’t have a setting
for through-transmission, but the dual setting can be used. The only difference is that when the dual setting is used on a conventional flaw detector for through-transmission testing, the sound path distance indicated by the instrument must be doubled — Fig.29. It is often valuable to have an additional technique for verifying the suspected length or depth of a crack. Three examples are described here.
Figure 27 — Examples of ultrasonic through-transmission testing:
(A) Thick steel plate is shown with no laminations present to block the sound traveling from the sender to the receiver, resulting in an
66
indication brought to 100% full screen height (FSH); (B) the same sample plate, but there is a large midwall lamination present that blocks all of the sound from being transmitted, resulting in no echo on the screen; (C) the transducer pair is centered over the edge of the large lamination and half of the original sound is transmitted, resulting in a 50% FSH indication.
Figure 28: through-transmission Setup
(A) Example of a simple setup for through-transmission testing using angled shear waves on part of a pipe with no interference. All of the sound is transmitted, resulting in a 100%
67
FSH indication; (B) a different location where there is a deep inside connected axial crack that blocks all of the sound; (C) the angle beam transducers are centered at the edge of the crack and half the sound is transmitted, resulting in a 50% FSH indication.
Figure 30: When a conventional UT flaw detector is used for through-
transmission testing, and there is no “throughtransmission” option in the instrument setup menu, “dual” can be used instead. If dual is used,
the measured soundpath will be half of the true soundpath distance because the instrument is calculating a “round-trip” distance rather than
the actual one-way trip. (A) Dual element staight beam transducer; (B) through transmission travel time is the same even though the material is twice as thick.
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Example 1: Radial cracks in a small-diameter branch bore hole in a heavy-walled pipe. A heavy-walled, high-energy pipe in a fossil-fuel power plant had a small-diameter branch connection that leaked and was repeatedly seal welded in a series of temporary repairs. The leaks were at the outside edges of the fillet weld joining the branch and the main pipe — Fig. 31. After the branch was cut off flush to the main pipe surface, the source of the leak was found to be a series of radial cracks around the bore hole that extended down to the inside of the main high-energy pipe — Fig. 32. Engineers planning repairs were especially interested in knowing the lengths of the cracks inside the main pipe at the upstream and downstream orientations of the bore hole — the 12 o’clock and 6 o’clock orientations. This would be an important factor in planning for
removal of all the cracks by boring a new, larger-diameter drilled hole centered on the old bore hole. The lengths of the cracks inside the main pipe at the upstream and downstream orientations were estimated with conventional shear wave and then verified with through-transmission UT. The probe separation distance and the search unit angles chosen were based on the size and schedule of the main pipe. A full-size sketch was made, and distances and angles were measured directly from the sketch — Fig. 33.
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Figure 31 — The high-energy branch connection shown here had a
recurring series of leaks. A number of temporary repairs had been made, which included the addition of a fitting to make the seal welded fillet weld a larger diameter in an attempt to seal off the leak. As the cracks grew, new leaks appeared.
Figure 32 — After removal of the branch connection shown in Fig. 4, the
outside surface of the pipe shows a series of radial cracks that continue down the bore hole all the way to the inside of the main high-energy
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pipe. The maximum length of the radial cracks at the upstream and downstream orientations (12 o’clock and 6 o’clock) was 0.85 in. long
at the outside surface. Through-transmission UT was used to verify the crack’s axial length at the inside surface.
Figure 33 — The sending and receiving transducers are separated to a
position where the sound path travels close to the inside of the main pipe wall as shown in Section A-A’. The transducers are then moved in tandem together up the pipe until half the sound is blocked by the crack from the bore hole as shown in Section B-B’.
Example 2: Heater channel head outlet penetration crack. A vertically oriented heater had inlet and outlet penetrations in the heater channel head — Fig. 34. The outlet nozzle had a crack where the outlet penetration meets the inside of the channel head. When there was access to the inside of the
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channel head, the crack lengths could be measured at the inside surfaces — Fig. Fig. 35. It was difficult to ultrasonically measure the subsurface depth of cracking from the inside because all the surfaces were concave. It was also difficult to ultrasonically measure the crack depth from the outside of the heater with conventional techniques because of the curved surfaces, the heavy-walled thickness of the components, and the fillet weld around the outside of the outlet nozzle. A full-size sketch was made of the cross section at the penetration. Scan lines were established for through-transmission testing at a series of probe separations to measure the location of the edge of the crack at various depths. These measurements were precisely measured and documented. During later outages, there was no scheduled access to the inside of the heater to look for and measure crack growth. Throughtransmission UT was repeated from the outside and the measurements did not change, indicating that the crack had not grown — Fig. Fig. 37.
Figure 34: vertically oriented heater heater outlet pipe.
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Figure 35 — View of red dry powder magnetic particle indications of the
crack during access to the inside of the heater channel head. A view of the crack looking down the outlet; B — view of the crack inside the channel head.
Figure 37 — Four pairs of scan lines were set up on the outside of the
channel head for four pairs of through-transmission scans. The elevations of each scan where the transmission dropped from 100% to 50% FSH was recorded as the boundary edge of the crack. Example 3: Mud drum penetration repair weld crack sizing.
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A large power boiler had cracks around arou nd the outside edges of most of the large-diameter penetrations on the inside of the mud drums. Many ultrasonic techniques were used and cracks were estimated to be shallow, probably less than 0.25 in. deep. Some of the cracks were at old repair welds that visually looked severe, as if they could be quite deep. The cracks were wide open and had longitudinal and transverse orientations — Fig. Fig. 38. It was suggested that it would be reassuring if another technique could be used to verify the shallowness shallown ess of those cracks. Through-transmission was used as an additional technique for verification. Full-size sketches were developed for the section at the worst-looking cracks — Fig. Fig. 39. A pitch-catch through-transmission setup was chosen using a 0-deg straight beam search unit as the sender from the inside of the mud drum’s penetrating pipe, the crossover pipe. The other search unit was
a 45-deg L-wave used on the outside surface of the mud drum head as the receiver. Scan patterns were drawn on the inside and outside of the mud drum — Fig. Fig. 40. The location of the edge of the cracks was verified with throughtransmission testing. The crack edge was located as in the previous examples where the amplitude of the transmitted signal was 50% of the sound transmitted in an unflawed area. Through-transmission testing verified that the cracks at the 3 o’clock orientation were close to 0.5 in. deep.
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Figure 38: Cracks at the weld toe of a large penetration into a mud drum
of a power boiler.
Figure 39: Mud drum setup utilized a 0-deg straight beam probe from
inside the crossover pipe, and a 45-deg L-wave transducer from outside the mud drum head.
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Figure 40: (A) Scan lines drawn on the outside of the mud drum; (B)
scan lines drawn on the inside of the mud drum crossover pipe. 7.2.
Pulse echo
Contact Scanning Using Compression Waves. Compression wave pulse-echo techniques usually employ, either a single or dual crystal transducer
directing
ultrasonic
energy
perpendicular
or
near
perpendicular to the scanning surface. These techniques are often known as “straight beam testing” techniques. There ar e some special
techniques using compression waves at steeper angles used, for instance, in detecting cracks in ferrous materials under stainless steel cladding and for time of flight diffraction (ToFD) testing, but these will be covered separately. In the standard compression wave techniques, reflections from the back wall and discontinuities are used to assess the suitability of a component for service. In order to obtain a reflection, it is necessary for the reflector to be orientated so that part of its surface is parallel to the scanning surface, in other words, normal to the beam. Laminar discontinuities and volumetric discontinuities like gas pores and nonmetallic inclusions
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are all suitably orientated. Discontinuities that are angled to the scanning surface may either not reflect at all or may reflect the sound away from the transducer. Figure 41a shows a single crystal compression wave transducer set up for thickness measurement of a metal part, and Figure 41b shows the corresponding ultrasonic A-scan trace in which the time base has been calibrated for 25 mm full scale. The initial pulse appears at zero on the left of the trace and the back reflection signal appears three-quarters along the time base, indicating a sample thickness of 18.75 mm. Thickness gauging is one of the simplest examples of compression wave testing. Notice that the initial pulse occupies almost a quarter of the time base so that 6 mm of metal path are obscured. This obstructed area is known as the “dead zone.”
Figure 42a shows a dual element transducer set up for thickness measurement on a sample that is 4 mm thick. Figure 42b shows the trace for this sample with the time base again calibrated for 25 mm. Notice that the selection of “dual” operation of the flaw detector isolates the
transmitter from the receiver circuit, so there is no initial pulse and, therefore, no dead zone. The first back reflection signal (also called “back wall echo” or “BWE”) shows at 4 mm on the time base. Notice also that multiples of
the BWE appear at 8, 12, 16, 20, and 24 mm on the time base.
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One way in which the reading accuracy can be improved is to take a reading from a multiple and divide the result by the number of passes corresponding to that multiple. Take, for example, the reading at 24 mm (which is the sixth signal). Divide 24 by 6 and the answer is 4 mm. However, suppose the actual thickness was 4.15 mm. It would be difficult to read that accurately on the first back reflection signal, but the sixth signal would have been judged at 24.9. This number divided by 6 equals 4.15 mm. In practice, this sort of accuracy could only be expected on samples with very smooth scanning and back wall surfaces. For thickness measurement in the field on corroded surfaces, errors of up to ±0.5 mm are typical. Figure 42c illustrates an actual thickness gage being used to read the thickness of a plate that is corroded. Figure 43a shows a single element transducer set up to detect laminations in steel plate 20 mm thick. The lamination is smaller than the beam. Notice in Figure 43b that the position of the lamination echo occurs on the screen at 11 mm below the scanning surface and the back reflection echo indicates a 20 mm thickness. The back reflection echo is reduced in amplitude because part of the beam is reflected by the lamination. If the lamination had been bigger than the beam, there would be no back reflection echo. If, on the other hand, the lamination had been smaller, the signal from the lamination would have been smaller in amplitude and the back reflection echo bigger. It might have been necessary to increase the equipment gain to see the lamination at all. In the extreme case, the lamination might be so small that it could
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not be detected at the test frequency or gain used. Therefore, the detection of suitably orientated discontinuities is dependent on the size of the discontinuity, the test frequency, and the gain used. Higher frequencies can detect smaller reflectors due to their shorter wavelength. It must be remembered that attenuation of the ultrasonic beam also has an effect on detection. As the energy penetrates deeper into the material, it weakens. Eventually, the beam is too weak to allow small echoes to get back to the receiver. The higher the test frequency, the greater the attenuation and the less penetration that can be achieved. The material and its grain structure also affect attenuation. The practitioner must balance the conflicting requirements of discontinuity size to be detected, material properties, and ultrasonic beam properties in the choice of transducer and test frequency. Figures 44a and b show two discontinuities unfavorably oriented to the sound beam. The inclined discontinuity in Figure 7-65a is reflecting the energy away from the transducer, but also obscuring the back wall. The result would be no signals visible on the display, but there would be a reduction in the back reflection. In Figure 44b, the vertical crack allows the sound to pass on either side without reflecting but would give a normal back wall echo. The possible orientation of the discontinuity must also be considered in devising a test procedure. Finally, a test technique for the detection of laminations in thin plate is illustrated in
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Figures 45a, b, and c. The technique is called the “multiple echo” technique for reasons that are obvious from Figures 7-66b and c. The time base has been calibrated for 50 mm for a sample 3 mm thick. With the transducer in position 1 (sound material), the multiple echo pattern stretches to 30 mm (14 signals) as shown in Figure 7-66b. With the transducer in position 2, over the lamination, the multiple echo pattern only stretches to 15 mm, as can be noted in Figure 7-66c. This is because the echoes are closer together; so close, in fact, that they interfere with each other, leaving no clear time base in between echoes. Contact Scanning Using Angle Beam Shear Waves. If the possible orientation of any discontinuity is considered to be unfavorable to a beam perpendicular to the scanning surface, it will be necessary to tilt the beam to an appropriate angle to ensure that the beam strikes the discontinuity as near perpendicular as possible. For small angles (up to about 10° in the test material), compression waves may be used. However, for larger angles, mode conversion to shear wave energy makes the use of compression waves alone impossible. It therefore becomes necessary to increase the incident angle beyond the first critical angle, leaving only a shear wave in the part. The lowest practical angle for testing with a shear wave alone is about 35° refracted shear wave angle. This does not mean that testing at angles between 10° and 35° is impossible. However, if an angle in this range needs to be used, the practitioner must consider carefully the geometry of the part. The next decision is whether to use the compression wave or the simultaneous shear wave,
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depending on what happens to the unwanted mode. Regular off the shelf transducers are either straight beam compression wave or shear wave angle transducers of 35° to 70°. The common or “preferred” angles available in ultraso nics for shear
wave testing are 45°, 60°, and 70°, although other angles can be made to order. The angles marked on a shear wave transducer are for steel, unless followed by an identifying letter for other materials. For instance, “45°Al” would denote a 45° shear wave angle transducer
for aluminum.
Figure 41: Thickness measurement
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Figure 42a and 42b: Thickness measurement
Figure 42c: Thickness gauge on corroded plate
82
Figure 43: Perpendicular reflector
Figure 44: Adverse reflector
Figure 45: Lamination detection
83
7.3. Angle beam The angle beam technique is used to transmit ultrasonic waves in to a test specimen at a predetermined angle to the test surface. According to the angle selected, the wave modes produced in the test specimen may be mixed longitudinal and transverse, transverse only, or surface wave modes. Usually, transverse wave probes are used in angle beam testing. Transverse waves at various angles of refraction between 35° and 80° are used to locate defects whose orientation is not suitable for detection by normal beam techniques.
Calculation of Various Distances for Angle Beam Probes Half Skip and Full Skip Distances and Beam Path Lengths
Figure 46 defines the half-skip-distance (BSD), fullskip distance (FSD), half-skip-beam-path-length (HSBPL) and full-skip-beam-path-length (FSBPL) for an angle beam probe of refraction angle.
Figure 46: BSD FSD
Distance AB = Half - Skip - Distance (HSD)
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Distance AC = Full - Skip - Distance (FSD) Distance AD = Half - Skip - Beam - Path - Length (HSBPL) Distance AD + DC = Full-Skip-Bearn - Path - Length (FSBPL) The relations used to calculate HSD, FSD, HSBPL and FSBPL for a specimen of thickness t, are given below: HSD = t * tanɵ FSD = 2 * t * tanɵ HSBPL = t/cosɵ FSBPL = 2*t/cosɵ If the actual probe angle is exactly equal to the nominal probe angle then these distances can be calculated by the following formula: Distance required = F * t
Where F is the appropriate factor from Table 3. F probe angle factor
35o
45o
60o
70o
80o
HSD factor
0.7
1.0
1.73
2.75
5.67
FSD factor
1.4
2.0
3.45
5.49
11.34
HSBPL factor
1.22
1.41
2.0
2.92
5.76
FSBPL factor
2.44
2.83
4.0
5.85
11.52
Table 3: Probe factor for different angles Calculation of Maximum Penetration Thickness for Thick Wall Pipes
The normal range of transverse wave angle beam probes (45 , 60 & 70), when used on thick wall pipe may not penetrate to the bore of the pipe,
85
but cut across to the outside surface again, as shown in Figure 47 and miss the defect.
Figure 47
For a given probe angle, the maximum wall thickness of a pipe that allows the center of the beam to reach the bore of the pipe can be calculated from the following formula: =
∗ (1 − ɵ) 2
Where: T: maximum wall thickness. D: Outer diameter (OD) of the pipe Ɵ: probe angle
The previous Equation can be rewritten to determine the best angle for a given wall thickness as: ɵ = Sin-1 (1 - (2t/d))
For convenience the equation above can be simplified for standard angle probes as
86
t=d*f Where f is the probe factor given in Table 4. Probe angle (ɵ)
35o
45o
60o
70o
80o
Probe factor (f)
0.213
0.146
0.067
0.030
0.0076
Table 4: Probe factor
Table 5 gives values of maximum wall thickness for various pipe sizes and probe angles. 60o
Probe angle Pipe O.D
35
45o
o
Maximum thickness
4” (100 mm)
21.3 mm
14.6 mm
6.7 mm
6” (150 mm)
31.95 mm
21.9 mm
10.05 mm
8” (200 mm)
42.6 mm
29.2 mm
13.4 mm
10” (250 mm)
53.25 mm
36.5 mm
16.75 mm
12” (300 mm)
63.9 mm
43.8 mm
20.1 mm
14” (350 mm)
74.55 mm
51.1 mm
23.45 mm
16” (400 mm)
85.2 mm
58.4 mm
26.8 mm
18” (450 mm)
95.85 mm
65.7 mm
30.15 mm
20” (500 mm)
106.5 mm
73.0 mm
33.5 mm
Table 4: Maximum wall thickness
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wall
7.4.
Contact testing 7.4.1. K nowledge 7.4.1.1.
Normal Beam I nspection
Pulse-echo ultrasonic measurements can determine the location of a discontinuity in a part or structure by accurately measuring the time required for a short ultrasonic pulse generated by a transducer to travel through a thickness of material, reflect from the back or the surface of a discontinuity, and be returned to the transducer. In most applications, this time interval is a few microseconds or less. The two-way transit time measured is divided by two to account for the down-and-back travel path and multiplied by the velocity of sound in the test material. The result is expressed in the well-known relationship: v ∗ t 2∗d = = 2 t
Where d is the distance from the surface to the discontinuity in the test piece, v is the velocity of sound waves in the material, and t is the measured round-trip transit time.
88
Figure 48
Figure 49
89
Figure 50
Precision ultrasonic thickness gages usually operate at frequencies between 500 kHz and 100 MHz, by means of piezoelectric transducers that generate bursts of sound waves when excited by electrical pulses. A wide variety of transducers with various acoustic characteristics have been developed to meet the needs of industrial applications. Typically, 1. Lower frequencies are used to optimize penetration when measuring thick, highly attenuating or highly scattering materials, 2. While higher frequencies will be recommended to optimize resolution in thinner, non-attenuating, non-scattering materials. In thickness gauging, ultrasonic techniques permit quick and reliable measurement of thickness without requiring access to both sides of a part. Accuracy's as high as ±1 micron or ±0.0001 inch can be achieved in some applications. It is possible to measure most engineering materials ultrasonically, including metals, plastic, ceramics, composites, epoxies, and glass as well as liquid levels and the thickness of certain biological
90
specimens. On-line or in-process measurement of extruded plastics or rolled metal often is possible, as is measurements of single layers or coatings in multilayer materials. Modern handheld gages are simple to use and very reliable. 7.4.1.2.
Angle Beams
Angle Beam Transducers and wedges are typically used to introduce a refracted shear wave into the test material. An angled sound path allows the sound beam to come in from the side, thereby improving detectability of flaws in and around welded areas.
Figure 51
Ɵ = Angle of reflection, T=Material thickness, S= Sound path,
Surface distance = SinƟ * S, Depth= CosƟ * S
91
Figure 52
Angle Beam Transducers and wedges are typically used to introduce a refracted shear wave into the test material. The geometry of the sample below allows the sound beam to be reflected from the back wall to improve detectability of flaws in and around welded areas.
Figure 53
Ɵ = Angle of reflection, T=Material thickness, S= Sound path,
Skip = 2(T * TanƟ), Leg = T/CosƟ, V Path = 2 * Leg
92
Figure 54
Figure 55
Why angle beam assemblies are used
Cracks or other discontinuities perpendicular to the surface of a test piece, or tilted with respect to that surface, are usually invisible with
93
straight beam test techniques because of their orientation with respect to the sound beam. Perpendicular cracks do not reflect any significant amount of sound energy from a straight beam because the beam is looking at a thin edge that is much smaller than the wavelength, and tilted cracks may not reflect any energy back in the direction of the transducer. This situation can occur in many types of welds, in structural metal parts, and in many other critical components. An angle beam assembly directs sound energy into the test piece at a selected angle. A perpendicular crack will reflect angled sound energy along a path that is commonly referred to as a corner trap, as seen in the illustration below.
Figure 56
The angled sound beam is highly sensitive to cracks perpendicular to the far surface of the test piece (first leg test) or, after bouncing off the far side, to cracks perpendicular to the coupling surface (second leg test). A variety of specific beam angles and probe positions are used to accommodate different part geometries and flaw types. In the case of angled discontinuities, a properly selected angle beam assembly can direct sound at a favorable angle for reflection back to the transducer.
94
Figure 57
There are two advantages to designing common angle beams to take advantage of this mode conversion phenomenon: (1) First, energy transfer is more efficient at the incident angles that generate shear waves in steel and similar materials. (2) Second, minimum flaw size resolution is improved through the use of shear waves, since at a given frequency, the wavelength of a shear wave is approximately 60% the wavelength of a comparable longitudinal wave, and minimum flaw size resolution increases as the wavelength of a sound beam gets smaller.
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Flaw location and echo display
Figure 58
Figure 59
96
Figure 60
Figure 61
97
Figure 62
Figure 63
98
Figure 64
Figure 65
99
Figure 66
Figure 67
100
Figure 68
Figure 69
101
Selecting the right angle beam assembly
The parameters that affect angle beam performance include not only the (1) Beam angle generated by the wedge, but also (2) transducer frequency and (3) element size. The optimum beam angle will generally be governed by the geometry of the test piece and the orientation of the discontinuities that the test is intended to find. Transducer frequency affects penetration and flaw resolution: 1. As frequency increases, the distance the sound wave will travel in a given material decreases, but resolution of small discontinuities improves. 2. As frequency decreases, the distance the sound wave will travel increases but the minimum detectable flaw size will become larger. 3. Similarly, larger element sizes may decrease inspection time by increasing coverage area, but the reflected echo amplitude from small discontinuities will decrease. Smaller element sizes will increase reflection amplitude from small discontinuities, but the inspection may take longer because the smaller beam covers less area. These conflicting factors must be balanced in any given application, based on specific test requirements.
102
Figure 70
The IIW recommends the use of a contoured wedge whenever the gap between the wedge and the test surface exceeds 0.5 mm (approximately 0.020 in.). Under this guideline, a contoured wedge should be used whenever part radius is less than the square of a wedge dimension (length or width) divided by four:
Figure 71
Where R = radius of test surface
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W = width of wedge if testing in axial orientation, length of wedge if testing in circumferential orientation. Of course switching to a small wedge, if possible within the parameters of inspection requirements, will improve coupling on curved surfaces. As a practical matter, contouring should be considered whenever signal strength diminishes or couplant noise increases to a point where the reliability of an inspection is impaired.
Focused dual element angle beams
The vast majority of angle beam assemblies use single element, unfocused transducers. However, in some tests involving highly attenuating or scattering materials such as coarse grain cast stainless steel, focused dual element angle beams are useful. Because they have separate transmitting and receiving elements, dual element transducers can typically be driven at higher excitation energies without noise problems associated with ringdown or wedge noise. Focusing permits a higher concentration of sound energy at a selected depth within the test piece, increasing sensitivity to discontinuities in that region.
104
Figure 72
High temperature wedges
Standard angle beam assemblies are designed for use at normal environmental temperatures only. For situations where metal must be inspeced at elevated temperature, special high temperature wedges are available. Some of these wedges will tolerate brief contact with surfaces as hot as 480° C or 900° F. However, it is important to note that high temperature wedges require special attention with regard to the sound path they generate. With any high temperature wedge, sound velocity in the wedge material will decrease as it heats up, and thus the refracted angle in metals will increase as the wedge heats up. If this is of concern in a given test, refracted angle should be verified at actual operating temperature. As a practical matter, thermal variations during testing will often make precise determination of the actual refracted angle difficult. Surfaces as hot as 480°C / 900°F
105
7.4.1.3.
Automated Scanning
Ultrasonic scanning systems are used for automated data acquisition and imaging. They typically integrate a ultrasonic instrumentation, a scanning bridge, and computer controls. The signal strength and/or the time-of-flight of the signal is measured for every point in the scan plan. The value of the data is plotted using colors or shades of gray to produce detailed images of the surface or internal features of a component. Systems are usually capable of displaying the data in A-, B- and C-scan modes simultaneously. With any Ultrasonic scanning system there are two factors to consider: ■ How to generate and receive the ultrasound. ■ How to scan the transducer(s) with respect to the part being inspected.
7.4.1.4.
Precision Velocity Measurements
Changes in ultrasonic wave propagation speed, along with energy losses, from interactions with a materials microstructures are often used to nondestructively gain information about a material's properties. Measurements of sound velocity and ultrasonic wave attenuation can be related to the elastic properties that can be used to characterize the texture of polycrystalline metals. These measurements enable industry to replace destructive microscopic inspections with nondestructive methods.
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Of interest in velocity measurements are longitudinal wave, which propagate in gases, liquids, and solids. In solids, also of interest are transverse (shear) waves. The longitudinal velocity is independent of sample geometry when the dimensions at right angles to the beam are large compared to the beam area and wavelength. The transverse velocity is affected little by the physical dimensions of the sample. 7.4.1.5.
Pulse-E cho and Pulse-E cho-Overlap Methods
Rough ultrasonic velocity measurements are as simple as measuring the time it takes for a pulse of ultrasound to travel from one transducer to another (pitch-catch) or return to the same transducer (pulse-echo). Another method is to compare the phase of the detected sound wave with a reference signal: slight changes in the transducer separation are seen as slight phase changes, from which the sound velocity can be calculated. These methods are suitable for estimating acoustic velocity to about 1 part in 100. Standard practice for measuring velocity in materials is detailed in ASTM E494. 7.4.2. Pulse E cho Method
Pulse Echo Method: Sound pressure on axis (schematic) for the incident wave (top) and the wave reflected from a reflector in form a circular disc (bottom).
107
Figure 73
Figure 74
108
Pulse Echo Method- Schematic screen pictures obtained by the pulseecho method. (a) Small flaw in sound beam; (b) two small flaws in sound beam; (c) large flaw in sound beam, smaller second flaw and back wall masked; (d) large, obliquely orientated flaw, back wall masked; (e) small flaw but no back wall echo because the axis of the beam is not incident at right angles on back wall; (f) strong attenuation of sound beam due to scattering, no echo from flaw or back wall, only "grass"
109
Figure 75
Pulse Echo Method- Multiple echoes in a plate. (a) schematic; (b) actual screen picture without time or thickness scale; steel plate 50 mm thick, frequency 4 MHz
Figure 76
110
Figure 77: Amplitude loss: Inverse Square Law
Figure 78: Influence of Shadow on axial defects defects
111
Figure79: Influence of reflector orientation orientation on signal
Figure80: Influence of reflector size on signal
112
Figure 81: Basic pulse echo testing presentation
Figure 81: Basic pulse echo testing presentation presentation
7.4.3. Pitch-catch methods
Advantage: Sensitive to near surface defect Capable of penetrating thicker material due to pitch-catch mode.
113
Disadvantage: It measures only sound energy loss at the receiver, without giving details information of location. 7.4.3.1.
Pi tch-Catch M ethods- Through Tr ansmission
Through transmission testing uses two search units; one unit is used as a transmitter and the other unit is used as a receiver, as shown in Figure below. With this technique, the ultrasonic beam passes through the test piece or is attenuated by one or more discontinuities. Total or partial attenuation of the signal is possible depending on the severity of the discontinuity. Both transducers must be properly coupled with a liquid coupling agent to obtain reliable results. As with other techniques using two search units, greater efficiency may be obtained by using a ceramic element in the transmitting search unit and a lithium sulfate element in the receiving unit.
114
Figure 82: Through transmission presentation
7.4.3.2.
Pitch-Catch Methods- Tandem
« The tandem method, the examination is normally carried out using two similar 45°angle probes, one probe operating as the transmitter and the other probe as receiver. For wall thicknesses greater than approximately 160 mm, probes with different transducer sizes are preferred in order to ensure approximately the same beam diameters in the examination zone. The use of probe angles other than 45° may be necessary to comply with particular geometrical conditions. Probe angles that give rise to mode conversions shall be avoided. The probes are located in a line with their acoustic axis in the same direction. In this way the sound beam from the rear probe will, after reflection from the opposite
115
surface, intersect the sound beam from the front probe at the center of the examination zone. » Extract from: EN 583-4 Non-destructive testing - Ultrasonic examination - Part 4: Examination for discontinuities perpendicular to the surface.
Figure 83 shows the relationship between the spacing of the probes (y) and the examination depth of the cross point (tm) and the height of the examination zone (tz). When examining objects with plane parallel surfaces the distance between the probes can be defined using the following equation: y = 2 tan α (d – tm) or 2 tan α (bottom depth)
Figure 83: Basic principle of tandem technique
116
Figure 84: Distance between Transmitter / Receiver Probes
7.5.
I mmersion testing
Immersion testing techniques are mainly used in the laboratory and for large installations doing automatic ultrasonic testing. It has the advantage that uniform couplant conditions are obtained and longitudinal and transverse waves can be generated with the same probe simply by changing the incident beam angle. The three basic techniques used in immersion testing are the immersion technique, the bubbler technique and the wheel transducer technique. In the immersion technique both the probe and the test specimen are immersed in water. The ultrasonic beam is directed through the water in to the test specimen, using either a normal beam technique (Figure
117
85a) for generating longitudinal waves or an angle beam technique (Figure 85b) for generating transverse waves. When the normal beam technique is being used the water path distance must always be longer than the distance S in the following equation:
=
thickness specimen ∗ sound velocity in water Sound velocity in specimen
When the specimen is steel the water path distance must be longer than 1/4 steel thickness otherwise the 1st back wall echo overlaps the 2nd surface echo and defects near the back wall may not be seen. In the bubbler or squirter technique, the ultrasonic beam is directed through a water column in to the test specimen (Figure 86). This technique is usually used with an automated system for high speed scanning of plate, sheet, strip, cylindrical forms and other regularly shaped forms. The ultrasonic beam is either directed in a perpendicular direction (i.e. normal direction) to the test specimen to produce longitudinal waves or is adjusted at an angle to the surface of the test specimen for the production of transverse waves. In the wheel transducer technique the ultrasonic beam is projected through a water-filled tire in to the test specimen. The probe, mounted on the wheel axle, is held in a fixed position while the wheel and tire rotate freely.
118
The wheel may be mounted on a mobile apparatus that runs across the specimen, or it may be mounted on a stationary fixture, where the specimen is moved past it (Figure 87 a and b). The position and angle of the probe mounted on the wheel axle may be constructed to project normal beams, as shown in Figure 87 a and b or to project angled beams as shown in Figure 88
Figure 85: Normal beam & angle beam immersion techniques
Figure 86: Bubbler and wheel transducer techniques
119
Figure 87: stationary and moving wheel transducer
Figure 88: Wheel transducer angular capabilities
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8. Ultrasonic Testing Equipment 8.1.
Basic pulse echo instrument
The basic electronic instrument used in pulsed ultrasonic testing contains a source of voltage spikes (to activate the sound source-that is, the pulser) and a display mechanism that permits interpretation of received ultrasonic acoustic impulses. Figure 89 shows a block diagram of the basic unit. The display can be as simple as a digital meter for a thickness gage or a multidimensional representation of signals over an extended area of interest. The timer circuitry triggers the pulser (activates the transducer) and the sweep generator forces the electron beam within the display to move horizontally across the screen. Other special circuits triggered as needed include markers, sweep delays, gates, distance amplitude correction and other support circuits. Pulse signals from the receiver transducer are amplified to a level compatible with the display. The term pulse id used in two contexts in ultrasonic testing. The electronic system sends an exciting electrical pulse to the transducer being used to emit the ultrasonic wave. This electrical pulse is usually a unidirectional spike with a fast rise time. The resulting acoustic wave packet emitted by the transducer is the ultrasonic pulse with both a positive and negative excursion. It is characterized by a predominant central frequency at the transducer’s natural thickness resonance.
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Figure 89: Basic pulse-echo instrument
The received signals are often processed to enhance interpretation with filters (that limit spurious background noise and smooth the appearance of the pulse), rectifiers (that change the oscillatory radio frequency signals to unidirectional video spikes) and clipping circuits (that reject low level background signals). The final signals are passed on the vertical displacement circuits of the display unit and produced the time delayed echo signals interpreted by the operator. This type of display is commonly referred to as an A-scan (signal amplitude displayed as a function of time).Most functions are within the control of the operator, and their collective settings are the setup of the instrument. Characteristics of the initial ultrasonic, radio frequency pulse (shape and frequency content) are carried forward throughout the system, to
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the test object, back to the transducer the receiver (amplifier), the gate and display. In essence, the information content of the initial electrical pulse is modified by each of these items. It is the result of this collective signal processing that appears on the screen. The initial pulse may range from 100 to 500 V and have a very short rise time. In some systems, the initial pulse may represent a portion of a sinusoidal oscillation, tuned to correspond to the nature frequency of the transducer. The sinusoidal driving pulses are needed to help penetrate highly attenuative materials such as rubber and concrete. Signals from the receiving transducer (usually in the millivolt range) maybe too small to be directly sent to the display unit. Both linear and logarithmic amplifiers are used to raise signal levels needed to drive the display. These amplifiers, located in the receiver sections of A-scan units, must be able to produce output signals linearly related to the input signals and which supply signal processing intend to assist the operator in interpreting the display signals. Amplifiers may raise incoming signals to a maximum level, followed by precision attenuators that decrease the signal strength to levels that can be positioned on the screen face- capable of changing amplification ratios in direct response to the gain control. Discrete attenuators (which have a logarithmic response) are currently used because of their ease of precise construction and
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simple means for altering signal levels beyond the viewing range of the screen. Their extensive use has made decibel notation a part of the standard terminology used in describing changes in signal levels, such as changes in receiver gain and material attenuation. The ratio of two pulse amplitudes (A2 and A1) can be expressed in decibel notation (Ndb) A2
Ndb = log10 1
8.2.
Control functions and uses
Timing and synchronization
The operation of a basic instrument is timed and synchronized by the so-called timing section, which control the system’s pulse repetition frequency. The timing section also generates the internal sweep rate signals which determine the separation between the received ultrasonic signals on the instrument’s display.
The pulse repetition frequency timing signals are fed directly to a pulser that drives the ultrasonic transducer through a manually selectable diplexer. Diplexers are also known as transmit/receive switches.
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Following a propagation delay corresponding to the ultrasonic time of flight between the transducer and an internal reflector, the back scattered ultrasonic signals are received by the same transducer. These signals are then directed by the receiver preamplifier. However before the received signals can be processed and displayed, additional signal processing steps are needed. Processed signals are displayed for evaluation by inspectors or automatic detectors. It is assumed that the travel time is long enough to keep signals of different transmitter pulses from overlapping. Receiver gain adjustment
After preamplification to help establish the best electrical signal-tonoise ratio, the amplitudes of the received signals on the display can be adjusted using a combination of fixed and variable attenuators. The overall gain of the amplifier can be selected by switching in two or three 20 dB gain circuits. Generally, this selection can be accomplished using controls at the front panel of the instrument. Receiver gain might be adjusted also through an external controller. After preamplification, band pass filtering and video detection (rectification and low pass filtering), the signals are amplified again by the video amplifier. This amplification is often followed by an adjustable low pass filter and the output of the filter is then applied to the vertical axis of the display, as function of time (horizontal axis of the display). The final detected and filtered signal is called the
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video display or A-scan. In some designs, it is possible to display radiofrequency waveforms directly. Sweep, signal filtering and display
The horizontal axis of the display device is driven by the weep signal generated in the system’s timing section. Generally, the start of each
sweep signal is delayed with respect to the transmitter pulse or by an interface trigger. This delay is used to offset the start of the display to some convenient interface echo. The amplitude of the displayed signals are determined principally by the receiver gain and frequency filter settings. They can also be affected by the low pass filter in detector circuit. In addition, the setting of the transmitter pulse amplitude and pulse damping controls can affect the amplitude and the appearance of the displayed ultrasonic signals. Signal Gating and threshold selection
Among the essential functions of a basic instrument are the signal gate and the alarm threshold controls. These functions enable the operator to isolate a specific portion of the received signal train and to compare its peak amplitude with a preset threshold level. The signal gate delay, width parameters and alarm threshold level typically can be selected from the front panel. To ensure reliable results, receiver gain levels and the alarm threshold level within the
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gate interval should be adjusted before the test using an appropriate ultrasonic reference standard and an instrument calibration procedure. Pulse repetition frequency
Battery powered discontinuity detectors can be operated at relatively high pulse repetition frequencies (500 Hz and higher) to ensure a bright display. Thickness gages can achieve even higher pulse repetition frequencies. However high repetition frequencies use more power and so make the instrument less portable. High pulse repetition frequencies can cause interference of ultrasonic signals generated by different transmitter pulses, in turn producing undesirable fluctuations in signal amplitude. Interference must be avoided because of its detrimental effect on test reliabilities. In many advanced instruments, fast digital sampling, storage techniques and advanced display technologies increase display brightness while reducing power consumption. In these designs, pulse repetition frequencies can be as low as 40 Hz. Lower frequencies could result in perceptible flicker and make real time scanning inadvisable because of wide intervals between adjacent pulses. Pulse amplitude and shape control
Most portable ultrasonic instruments use relatively simple pulse circuitry. In the twentieth century, spike pulser designs were
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common. In the twenty-first century many designs incorporate square wave pulsers. If the instrument uses a spike pulser, then the operator may be able to modify the pulse amplitude by adjusting the energy of the pulse. Pulse is adjusted by selecting the value of the energy storage capacitor. In addition, an adjustment of the damping resistor value may be made to minimize transducer ringing. If the instrument uses a square wave pulser, the operator is generally required to adjust pulse width individually for each transducer to exactly match the frequency characteristics. In addition, the value of the damping resistor should be adjusted to match the impedance characteristics. To protect the transducer from the effects of voltage overdrive, pulser voltages seldom exceed 400V. Avoidance of receiver saturation
Most ultrasonic testing procedures require the operator to adjust the gain of the input amplifier and attenuator to ensure that none of the components in the receiver amplifier chain are in saturation. Typically, the maximum displayed signal level is adjusted to the saturation value, about 80 percent of the full display. Such an adjustment can be made using front panel controls. The overall gain of typical receiver may be adjustable over a range of 100 dB in discrete steps of 1, 2, 6 and 10 dB Signal gate and alarm level settings
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The gain adjustment and signal gate functions are important because they can be used to control accept/reject threshold. If the amplitude of the signal in a discontinuity gate exceeds a pre-established threshold,
then
the
discontinuity
alarm
is
activated.
The
discontinuity alarm is usually built-in and can be audible or visual.
Operation in pitch catch or through-transmission modes
If the diplexer or transmit/receive switch is set in the open position, then it is possible to operate in pitch catch or through-transmission modes. In this configuration, separate transducers are used to generate and receive the ultrasonic signals.
8.3.
I nformation displays: A scan, B scan, C scan, digi tal readouts
The A-Scan Method
The ultrasonic A-scan presents one-dimensional data showing the response along the beam path at a specific location of the test object. Such scans can produce detailed information about discontinuities in the scanned material. The depth of discontinuities is indicated by the time-of-flight as measured from the time base of the display screen. The size of discontinuities can be estimated from the amplitude of the reflected signal. The type of discontinuity can be determined by
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analysis of the amplitude and phase information. The A-scan method is the most widely used and can be displayed on most standard ultrasonic instruments.
Figure 90: Diagram of pulse echo A-scan system
The B-Scan Method
With the ultrasonic B-scan, the test object is scanned along one axis to produce a presentation of its cross section. The location along the scanning path is shown on the X axis and time-of-flight values are
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shown along the Y axis. Because a cross section is produced, the B-scan is not used where large volumes of material must be inspected. The B-scan is popular for medical diagnosis where cross-sectional views are very useful. In medical applications, the angular manipulation of the transducer is monitored to prevent image distortion and the display is adjusted to account for changes in the beam angle along the cross section of the examined area.
Figure 91: Diagram of typical ultrasonic B-scan presentation
The C-Scan Method
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The ultrasonic C-scan is applied to the test object in a raster pattern and presents a view of the discontinuity's area as seen from above. Discontinuity location and size data are available from changes in amplitude as a function of position. These are displayed on a screen or recorded on paper. Modern C-scan systems use computers to control the transducer position and to acquire, display, document and store the test results. The computer synchronously acquires the digitized position of the transducer and the associated reflected signal or the value of a specific ultrasonic parameter. The position can be obtained by various means,
including
Computerized
C-scan
optical
encoders
systems
can
or
acquire
sonic
digitizers.
several
ultrasonic
parameters as a function of position. In most cases, the parameter is time of flight or the amplitude of reflection or transmission amplitude at a certain time range. The parameters are digitized with aid of an analog-to-digital converter. In immersion ultrasonic testing, the C-scan systems can scan at speeds up to 500 mm.s-l (20 in..s-l) or higher. Speeds must be kept at a level that does not induce water turbulence, which introduces noise and degrades the reliability of the test.
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Figure 92: Diagram of simple ultrasonic C-scan presentation
Multiple transducers can be used in C-scan tests. A multiplexer is used to sequentially trigger the transducers in a predetermined order during scanning. As an alternative to mechanical scanning, these transducer arrays are operated synchronously and their scanning location is indicated by the cursor position on the computer monitor. Ultrasonic C-scan systems are large in size and most are limited to onsite testing conditions. With the increasing availability of inexpensive microprocessors, scanners have also been used for field applications.
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The limitations of this generation of field systems are: (l) high cost, (2) small scanning area and (3) reliability for testing objects with simple geometries. Crawlers have been developed to perform field scanning of structures. Current crawlers are tethered to provide power. Air coupled systems make it possible to test parts of complex geometry.
8.4. C ompute omputerr enha nhanced di di splays Scientists and engineers have developed a new family of ultrasonic testing systems based on miniaturized electronic modules. In combination with a modular designed software architecture and suitable PC hardware, these new products permit custom configurations for a wide range of client-specific applications, from simple PC-aided manual ultrasonic inspections through fully automated inspections using compact portable systems with up to four channels or very sophisticated multi-channel systems for the ultrasonic inspection of heavy components. Highly integrated electronic circuits and the powerful processing capabilities of today’s PC systems allow the integration of electronic components even for multi-channel systems into portable computers, thus providing compact and simple to operate instruments to the ultrasonic inspector in the field. The inspection software contains modules for setup, examination, analysis, and reporting. Various database modules provide substantial information on inspection parameters such as inspection procedure
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requirements,
component
geometry
and
history,
material
characteristics, heat treatment, operating temperature, and pressure. The integration of the synthetic aperture focusing technique (SAFT) analysis module provides three-dimensional views of the inspected zones in various cross-sections. This tool helps the qualified technician to accurately determine type, location, and size of detected material discontinuities important for fracture and fatigue mechanics analysis; for example, for use in the assessment of a component's lifetime.
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9. Ultrasonic Reference Blocks 9.1. Calibration Methods Calibration refers to the act of evaluating and adjusting ad justing the precision and accuracy of measurement equipment. In ultrasonic testing, several forms of calibration must occur. First, the electronics of the equipment must be calibrated to ensure that they are performing as designed. This operation is usually performed by the equipment manufacturer and will not be discussed further in this material. It is also usually necessary for the operator to perform a "user calibration" of the equipment. This user calibration is necessary because most ultrasonic equipment eq uipment can be reconfigured reconf igured for use in a large variety of applications. The user must "calibrate" the system, which includes the equipment settings, the transducer, and the test setup, to validate that the desired level of precision and accuracy are achieved. The term calibration standard is usually only used when an absolute value is measured and in many cases, the standards are traceable back to standards at the National Institute for Standards Stand ards and Technology. In ultrasonic testing, there is also a need for reference standards. Reference standards are used to establish a general level of consistency in measurements and to help interpret and quantify the information contained in the received signal. Reference standards are used to validate that the equipment and the setup provide similar results from one day to the next and that similar results are produced
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by different systems. Reference standards also help the inspector to estimate the size of flaws. In a pulse-echo type setup, signal strength depends on both the size of the flaw and the distance between the flaw and the transducer. The inspector can use a reference standard with an artificially induced flaw of known size and at approximately the same distance away for the transducer to produce a signal. By comparing the signal from the reference standard to that received from the actual flaw, the inspector can estimate the flaw size. This section will discuss some of the more common calibration and reference specimen that are used in ultrasonic inspection. Some of these specimens are shown in the figure above. Be aware that there are other standards available and that specially designed standards may be required for many applications. The information provided here is intended to serve a general introduction to the standards and not to be instruction on the proper use of the standards. 9.2.
I ntroduction to the Common Standards
Calibration and reference standards for ultrasonic testing come in many shapes and sizes. The type of standard used is dependent on the NDE application and the form and shape of the object being evaluated. The material of the reference standard should be the same as the material being inspected and the artificially induced flaw should closely resemble that of the actual flaw. This second requirement is a major limitation of most standard reference samples. Most use drilled holes and notches that do not closely represent real flaws. In most
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cases the artificially induced defects in reference standards are better reflectors of sound energy (due to their flatter and smoother surfaces) and produce indications that are larger than those that a similar sized flaw would produce. Producing more "realistic" defects is cost prohibitive in most cases and, therefore, the inspector can only make an estimate of the flaw size. Computer programs that allow the inspector to create computer simulated models of the part and flaw may one day lessen this limitation. 9.3. Area amplitude blocks Area-amplitude blocks are also usually purchased in an eight-block set and look very similar to Distance/Area-Amplitude Blocks. However, area-amplitude blocks have a constant 3-inch metal path distance and the hole sizes are varied from 1/64" to 8/64" in 1/64" steps. The blocks are used to determine the relationship between flaw size and signal amplitude by comparing signal responses for the different sized holes. Sets are commonly sold in 4340 Vacuum melt Steel, 7075-T6 Aluminum, and Type 304 Corrosion Resistant Steel. Aluminum blocks are fabricated per the requirements of ASTM E127, Standard Practice for Fabricating and Checking Aluminum Alloy Ultrasonic Standard Reference Blocks. Steel blocks are fabricated per the requirements of ASTM E428, Standard Practice for Fabrication and Control of Steel Reference Blocks Used in Ultrasonic Inspection.
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9.4.
Distance amplitude blocks
Distance-amplitude blocks also very similar to the distance/areaamplitude blocks pictured above. Nineteen block sets with flat bottom holes of a single size and varying metal path distances are also commercially available. Sets have either a #3 (3/64") FBH, a #5 (5/64") FBH, or a #8 (8/64") FBH. The metal path distances are 1/16", 1/8", 1/4", 3/8", 1/2", 5/8", 3/4", 7/8", 1", 1-1/4", 1-3/4", 21/4", 2-3/4", 3-14", 3-3/4", 4-1/4", 4-3/4", 5-1/4", and 5-3/4". The relationship between the metal path distance and the signal amplitude is determined by comparing signals from same size flaws at different depth. Sets are commonly sold in 4340 Vacuum melt Steel, 7075-T6 Aluminum, and Type 304 Corrosion Resistant Steel. Aluminum blocks are fabricated per the requirements of ASTM E127, Standard Practice for Fabricating and Checking Aluminum Alloy Ultrasonic Standard Reference Blocks. Steel blocks are fabricated per the requirements of ASTM E428, Standard Practice for Fabrication and Control of Steel Reference Blocks Used in Ultrasonic Inspection.
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9.5.
I nternational I nstitute of Welding (I I W) block
Figure 94: IIW reference block
The standard shown in the above figure is commonly known in the US as an IIW type reference block. IIW is an acronym for the International Institute of Welding. It is referred to as an IIW "type"
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reference block because it was patterned after the "true" IIW block but does not conform to IIW requirements in IIS/IIW-23-59. "True" IIW blocks are only made out of steel (to be precise, killed, open hearth or electric furnace, low-carbon steel in the normalized condition with a grain size of McQuaid-Ehn) where IIW "type" blocks can be commercially obtained in a selection of materials. The dimensions of "true" IIW blocks are in metric units while IIW "type" blocks usually have English units. IIW "type" blocks may also include additional calibration and references features such as notches, circular groves, and scales that are not specified by IIW. There are two full-sized and a mini versions of the IIW type blocks. The Mini version is about one-half the size of the full-sized block and weighs only about one-fourth as much. The IIW type US-1 block was derived the basic "true" IIW block and is shown below in the figure on the left. The IIW type US-2 block was developed for US Air Force application and is shown below in the center. The Mini version is shown on the right.
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Figure 95: IIW Type US-1
Figure 96: IIW Type US-2
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Figure 96: IIW Type Mini
9.6. Miniature angle beam calibration block Also known as a “Rompas” block, this ASTM and U.S. Air Force
miniature angle beam block is a substitute for the DSC block for distance, beam index, refracted angle and sensitivity calibration. Contains a 1.0” radius opposite a 2.0” radius, and a 5/64” diameter x .750” deep flat-bottom hole. In accordance with ASTM E164 and U.S.
Bureau of Public Roads, Type B specificati ons. Dimensions: 1.0” thick. Metric version available .
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Figure 97: Miniature angle beam block
9.7.
Uses of artificial reflectors compared
The uncertainties in evaluation can be reduced when there is a reference block available which is made of the same material as the object to be tested and which also contains artificial reflectors whose echoes can be directly compared to the discontinuity echoes from the test object. The application of the reference block method is, in practice, made in two different ways:
9.7.1. Comparison of echo amplitudes
The test object is tested with a high gain setting by which the smallest detectable reflector is displayed. An echo indication is peaked, i.e. the maximum echo indication is achieved by careful movement of the probe and the echo peak set by adjustment of the gain to a predetermined height, e.g. 80% CRT screen height (reference height).
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Figure 98: Test object with a flaw: echo at 80% (reference height)
Using the same settings, the reflector from the reference block is scanned which is approximately positioned at the same distance as the discontinuity.
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Figure 99: Reference block: reference echo at 30%
The quantative unit for evaluation is now the gain change of the ultrasonic instrument which is necessary to set the reference echoto the reference height.
Figure 100: References block: reference echo to reference height
Result : The discontinuity echo is 8 dB higher than the reference echo
because the gain must be increased by 8 dB (from 34 dB to 42 dB). The recording limit normally corresponds to the echo height of the reference reflector whose size is to be determined, the same as the DGS method, before the ultrasonic test.
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9.7.2. Distance amplitude curve
All reflectors in the reference block are scanned before the test, their maximum echo heights marked on the attachment scale of the display and joined by a curve.
Figure 101: Reference block wiht side drilled holes and resulting echoes
The curve produced is called the Distance Amplitude Curve, or DAC for short. When a discontinuity echo appears, an immediate assessment can be made whether or not the discontinuity echo exceeds the DAC. In addition to this a determination is made, by a corresponding gain change, to see by how many dBs an echo exceeds the curve. This excess recording echo height (EREH) is our reproducible measure for the evaluation and reporting of the discontinuity. The advantages of the reference block method with a DAC are: 1. That it is no longer necessary to compare each discontinuity echo with the corresponding reference echo from the reference block but to directly make the evaluation with the DAC.
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2. That the heavy reference block need not be transported to the testing location. 3. That the recording of a DAC for certain applications is only required once because the curve is documented on a transparency or in the memory of a modern ultrasonic test instrument. By recording the curve using reflectors in a test object comparable to the work piece, this curve contains all the influences in the test object (distance law, sound attenuation, and surface losses). Corresponding corrections are therefore not necessary. Regarding the evaluation results, we must understand here that the effect of the discontinuity (geometry, orientation and surface quality) is not taken a great deal into account the same as the DGS method. Therefore, the result of a discontinuity evaluation with the reference block method has the same uncertainty as the
DGS method. The preference regarding which method to use is subjective. The corresponding national test specifications normally state the test method to be used so that the operator is not able to make his own decision. If no data is available, the test situation should be analyzed in order to decide which method be best used: Firstly, it must be established whether a reference block exists which corresponds to the test object. If yes, then the test can be carried out simply and reproducibly with the reference block method. If no reference block is available then the DGS method can be used, or a reference block must be subsequently produced comparable to the test
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object. However, in many cases the DGS method can be used without difficulty, namely when the test object is made of low alloy steel, has a simple geometry, a low sound attenuation and an even surface quality. The test should be carried out with a narrow band standard probe with a frequency between 1 MHz and 6 MHz for which there is a DGS diagram or a DGS scale. The new computer controlled instruments normally support the program controlled recording of DACs. With the USD 10 the recorded DAC is automatically converted to a horizontal line. This is known as time corrected gain (TCG).
Figure 102: DAC of the reference echoes (left) and with time corrected gain (right)
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10. Inspected Part Variations An important variable is that of surface condition. The differences in surface finish can result in large variations in the results of an examination. Paint or other coatings can have similar effects. 10.1. E ffect of surface roughness What is surface roughness?
It is clear that materials have natural properties such as density, conductivity and elastic modulus. Surfaces, representing material boundaries have perhaps rather more insubstantial properties but we still think of some of these properties are natural, like color. There are other properties, however, which are easy to define but whose value seems to depend on the technique or scale of measurement: hardness, for instance. Roughness seems to be such a property, with the added difficulty that is not always so easy to define as a concept. The fact is that roughness is the natural state of surfaces, and left to its own devices, nature will make sure they are rough. The roughness of a surface is a measure of its lack of order. Disorder is entropy under another name, and if a solid surface is considered as a closed system then the Second Law of Thermodynamics predicts that its entropy will tend to a maximum. To reduce its roughness, its entropy must be reduced, and the Second Law tells that it can only be done this by doing work. Thus if the axes of the well-known figure are
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transposed which relates machining time to roughness, it can easily seen that, it is nothing but an entropy diagram.
Figure 103: Relationship of surface texture to production time (b) the same figure replotted as work reducing entropy
Terminology on Surfaces and Profiles Types of Surfaces
Surface: A surface is a boundary that separates an object from another
object or substance. Nominal Surface: A nominal surface is the intended surface. The shape
and extent of a nominal surface are usually shown and dimensioned on a drawing. The nominal surface does not include intended surface roughness.
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Real Surface: A real surface is the actual boundary of an object. It
deviates from the nominal surface as a result of the process that created the surface. The deviation also depends on the properties, composition, and structure of the material the object is made of. Measured Surface: A measured surface is a representation of the real
surface obtained with some measuring instrument. This distinction is made because no measurement will give the exact real surface. Later portions describe many different types of measuring instruments. Form: Form refers to the intentional shape of a surface which differs
from a flat line. Surface Finish Imperfections
Form Error : Form error encompasses the long wavelength deviations
of a surface from the corresponding nominal surface. Form errors result from large scale problems in the manufacturing process such as errors in machine tool ways, guides, or spindles, insecure clamping, inaccurate alignment of a work piece, or uneven wear in machining equipment. Form error is on the dividing line in size scale between geometric errors and finish errors. Texture: Surface texture is the combination of fairly short wavelength
deviations of a surface from the nominal surface. Texture includes roughness, waviness, and lay, that is, all of the deviations that are shorter in wavelength than form error deviations.
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Figure 104: An Exaggerated Surface Shape
Roughness: Roughness includes the finest (shortest wavelength)
irregularities of a surface. Roughness generally results from a particular production process or material condition. Waviness: Waviness includes the more widely spaced (longer
wavelength) deviations of a surface from its nominal shape. Waviness errors are intermediate in wavelength between roughness and form error. The distinction between waviness and form error is not always made in practice, and it is not always clear how to make it. New standards are emerging that define this distinction more rigorously. Lay: Lay refers to the predominant direction of the surface texture.
Ordinarily lay is determined by the particular production method and geometry used. Turning, milling, drilling, grinding, and other cutting tool machining processes usually produce a surface that has lay: striations or peaks and valleys in the direction that the tool was drawn across the surface. The shape of the lay can take one of several forms as shown below. Other processes produce surfaces with no characteristic direction: sand casting, spark erosion and grit blasting.
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Sometimes these surfaces are said to have a non-directional, particulate, or protuberant lay. Several different types of lay are possible depending on the manufacturing and machining processes. Lay (or the lack thereof) is important for optical properties of a surface. A smooth finish will look rough if it has a strong lay. A rougher surface will look more uniform if it has no lay (it will have more of a matte look). How the surface roughness affects the results of the ultrasonic test
Surface roughness will have several possible effects on the inspection of a test piece. In contact testing, roughness on a gross scale results from: weld spatter, plate scale, dirt (sand) and rough cast surfaces from sand casting and different rough surfaces occurred from various machining operations. These irregularities will cause some points of contact to push away the couplant and force it into the lower areas around the probe. If the couplant is not sufficiently viscous it will drain away quickly and fail to couple the probe to the test piece.
Figure 105: Poor coupling results due to rough surface and thin couplant
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In addition to reduced coupling, which will reduce signal amplitudes, the rough surface increases the rate of wear on the probe. On an otherwise smooth surface isolated sticky regions such as weld spatter can hinder or stop probe motion or in the case of mechanized systems there may be sufficient force to move the probe past the obstruction but this could result in damaging the probe by either tearing it from its mounting or severely scoring the plastic wedge. When the dirt on the test piece is very fine (similar to a flour texture) coupling can be prevented due to surface tension preventing the liquid couplant penetrating to the metal. Unless a transfer value has been established between test piece and calibration piece, this could go undetected. In addition to affecting coupling, surface roughness tends to reduce signal amplitude by scattering and focusing the beam. This applies to both contact and immersion testing. Whether uniform or irregular, a rough surface has the potential to present a scattering effect at an interface where a beam impinges. The degree of scattering is based on the ratio of roughness to wavelength. When roughness is less than about 1/10 a wavelength, scatter will be negligible. To reduce signal losses due to scattering an operator can select a lower frequency probe. In addition to signal reduction another effect of surface irregularities is to redirect and mode convert some energy which when returned to the probe can be the source of spurious signals. In contact testing false indications from standing waves
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resulting from scatter on rough surfaces will normally have short sound paths. They can be eliminated as true flaws by failing to locate any trace of indication from the full skip or from the opposite side. Unless done properly, removal of surface roughness by mechanical means can result in further scattering problems. Small curved gouges left by a grinding wheel used to remove spatter or machining grooves can form small lenses. The effect of grinding can be unpredictable. Some of the lensing may concentrate the beam thereby increasing signal amplitude, or, the lens effect may be a de-focusing of the beam, again resulting in lower than expected signal amplitudes. Uniform surface preparation by sand or shot blasting usually provides a good surface for ultrasonic testing. Removal of excess metal by a hand held grinding wheel is commonly used on weld caps and roots. When a pipe weld has had its root ground flush and inspection can only be performed from the outside diameter, quality of grinding can result in unnecessary repair calls if grinding has been along the weld axis. The small grooves made by the grinding wheel run parallel to the root edge and are easily confused with lack of fusion, missed edge or undercut defects. 10.2. E ffect of surface coatings Surface coatings are added to protect a surface from corrosion or to enhance its appearance. Thin films, such as oxide layers, anodizing layers or electroplated finishes, and the slightly thicker coatings of paint or lacquer are usually well bonded to the surface. Quality of bond may
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be compared to the uncoated reference block by a simple transfer value. Even a slight loss due to the coating may be preferable to removing the coating and trying to inspect on the rough surface it hides. Tightly adhering coatings on the surface generally allow good transfer of the energy. Loose or flaking coatings are undesirable and should be removed prior to conducting the examination. When calibrating the equipment for reference sensitivity on critical applications, it is essential to evaluate the component for any energy losses due to surface condition and apparent attenuation variations. The procedure for this is fairly simple and is performed by using two transducers in a “pitch– catch” technique.
10.3. Surface curvature In ultrasonic pulse/echo immersion inspection, the transducer beam transmits through the water-solid interface and is reflected back by flaws in the part, producing a flaw signal that can be used to find, locate, and size flaws. In many applications, for example, the monitoring of the cleanliness of steel billet and the detection of flaws in pipes, interfaces between water and parts are not flat, but rather curved. Curved surfaces can modify flaw signals, making the performance of an inspection process more difficult to predict and the inspection results more difficult to interpret. A solution to this problem is to use physics-based simulation tools to design and assess the performance of inspections. In this work, we present a particular example, the use of models to aid in the characterization of the cleanliness of steel billet.
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To accurately predict the effects of curved surfaces on pulse/echo immersion inspection, it is necessary to have a set of models for the transducer radiation field, the flaw response signal, and the microstructure induced noise. Over the years, many fast and reasonably accurate models have been developed in a number of laboratories. For the transducer radiation field, the Gauss-Hermite beam model produces good results for transducer radiation into liquids and for transmission of those fields through flat or curved liquid-solid interfaces. By taking advantage of the paraxial approximation, the model runs much faster, by many orders-of-magnitude, than other numerical methods. This "beam" model enables the transducer wave fields on the solids to be rapidly computed for a series of transducers and focal lengths as a part of the inspection design process. For the flaw response signal, a "flaw signal" model has been developed that uses a modified born approximation and requires a numerical integration of the incident pressure field over the flaw volume. This model can predict time domain flaw signals that can be compared, on an absolute level, to actual measurements. As for the beam models, use of an appropriately selected approximation allows the code to run much faster than purely numerical methods such as the finite element method. For the microstructure-induced noise, a backscattered noise model has been developed that uses a single-scattering approximation and assumes that the observed RF noise is an incoherent summation of echoes from all insonified grains. The model can predict the backscattered grain noise
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level for a given inspection scenario in a computationally efficient manner. In this work we describe the use of these physics-based models of an ultrasonic
pulse/echo
inspection
process
to
guide
technique
optimization for detecting inclusions through cylindrical surfaces in steel bar stock. Models are used for three purposes. The beam model is used to select transducer focal properties that will be minimally affected by the cylindrical geometry of the billet surface, which acts to defocus the beam in a plane perpendicular to the billet axis. The flaw response model is used to predict the strength of echoes from undesirable inclusions that can lead to premature failure. The microstructural response model is used to predict backscattered grain noise levels for microstructures of a given type and grain size. The flaw response and noise predictions combine to yield estimates of the minimum detectable inclusion size. As an example of the integrated use of these tools, we discuss two competing immersion designs for a system to inspect 3-inch diameter cylindrical billet used for the manufacture of gears. One design uses a standard, spherically focused transducer; and the other uses a bicylindrically focused transducer, designed to better inspect the zone where gear teeth will later be machined. We will assume that the inclusions are hypothetical "stringers" whose elongation direction is aligned with the billet axis. Our main goal is to estimate stringer detectability for both the standard and optimized transducers.
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10.4. Grain size Small crystallites (also called grains) in polycrystalline metals can scatter a propagating ultrasonic wave in various directions, leading to an attenuation of the beam and producing backscattered noise that can mask the signals reflected from small flaws in pulse/echo inspections. Attenuation and backscattering are thus important in studying the flaw detectability of a given material. Attenuation and backscattering are also fundamental properties of ultrasonic wave propagation in polycrystalline materials, and can be directly related to microstructure of the material. Hence, the study of the relationships between microstructure and ultrasonic attenuation and backscattering properties are foundations for understanding both flaw detection and material characterization. Ultrasonic attenuation of polycrystalline metals has been extensively studied, especially for single-phase materials. Papadakis provided a review of much of the early work, Important recent contributions include the work of Rokhlin, Hirsekorn, and Stanke and Kino, The theory of Stanke and Kino is of particular interest, since it uses a twopoint correlation function to characterize microstructure and is claimed by the authors to be accurate for all frequencies, even for materials with texture and grain elongation. Stanke and Kino developed a general formalism and then applied it explicitly to the case of untextured materials having equiaxed grains. More recently,
160
Ahmed and Thompson, and Panetta have extended the theory to treat polycrystalline materials with texture and elongated grains. The theoretical calculations of attenuation in this study use their extended methods since we mainly deal with elongated grains. Backscattering is another ultrasonic quantity that is closely related to material microstructure. Previous work has been of two kinds. First, several researchers have developed the independent scattering model for predicting the rms noise, as influenced by the details of a measurement system in terms of a material property known as the backscattering coefficient. They assumed single scattering and that the total scattered power would be equal to the sum of the power scattered by individual crystallites. This model can be used to deduce the backscattering coefficient from measured grain noise in pulse/echo inspections by providing a basis for removing the effects of measurement systems. Rose has also developed a formal theory for relating the backscattering coefficient of longitudinal waves to the microstructure, based on the assumptions of single scattering and the Born approximation, Given material and microstructural properties, in terms of density, single-crystal elastic constants, and the two-point correlation function of elastic constants, Rose's theory can calculate backscattering coefficients. Ahmed and Thompson have extended Rose's theory to treat single-phase polycrystalline materials with texture and elongated grains.
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Han and Thompson have extended Rose's theory to treat duplex titanium alloys. Rose's theory and related extensions will be used in this work to analyze backscattering of longitudinal waves. It is also desirable to extend the theory to calculate backscattering for transverse waves since it can provide additional valuable information about the microstructure. In addition to their importance in flaw detection, ultrasonic attenuation and backscattering can also be used to characterize material microstructure. One of the important microstructural parameters is the grain size that influences mechanical properties such as yield and tensile strength. Grain size is usually determined from optical photomicrographs. Ultrasonic methods are advantageous since they are nondestructive and fast. Much work has been done in using ultrasonic attenuation and/or backscattering to determine grain size, However, that work was primarily based on empirical correlations, and almost always assumed equi-axcd grains and used a single mean grain size to describe microstructural dimension. A more general approach, good for elongated grains, is highly desirable. Based on our studies of the relationships between attenuation and backscattering and elongated grain structure, we developed three approaches to simultaneously determine grain size and shape orientation. One was based on a simultaneous measurement of the attenuation and backscattering of waves propagating collinearly and the second was based on
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measuring longitudinal wave backscattering from three orthogonal directions. The third one was based on measuring longitudinal and transverse wave backscattering from a single surface.
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11. Discontinuity Variations 11.1. Sizing methods A discontinuity is best evaluated when its size (extension) is known. The operator's wish to accurately know the "real reflector size" is understandable therefore it is expected that an nondestructive testing method, such as ultrasonic testing, give this information. However, due to the fact that on the display only the echo can be interpreted, this means the reflected sound coming from the discontinuity, it is very often difficult, and in some cases even impossible, to reliably assert the size of the reflector. In fact, the echo height plays the decisive part when evaluating discontinuities during manual Ultrasonic Testing. 11.1.1. Scanning method
In ultrasonic evaluation one is frequently able to come near to the true reflector size as long as the discontinuity is large compared to the diameter of the sound field. The discontinuity then reflects the complete impacting energy back, Figure 106.
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Figure 106: A large reflector in the sound beam
By scanning the boundaries of the discontinuity, reliable information can be obtained about its extension. The ultrasonic operator normally observes the height of the discontinuity echo. The probe position on the test object at which the echo drops by exactly half indicates that the discontinuity is only being hit by half the sound beam, Figure 107.
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Figure 107a: Straight beam probe on the reflector boundry
This means that the acoustic axis is exactly on the boundary of the discontinuity. The probe position is marked and the operator determines further boundry points until a contour of the discontinuity is formed by joining the marked points together, figure 107b.
Figure 107b: Top view with reflector for extension
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11.1.2. E valuation of small discontinuities: The DGS method
A reflector which is completely contained within the sound beam is regarded as a small reflector. If such a reflector is evaluated by scanning then it is not the size of the reflector which is obtained as a result but the diameter of the sound beam! Therefore, the scanning method is not practical in this case. We have noticed previously that the height of a reflector echo will become greater the larger the sound beam area is which covers the reflector. This feasible behavior can be used on small reflectors: their echo heights increase with their areas, Figure 108.
Figure 108: Reflectors with different areas and their echoes
Under optimal conditions, e.g. drill holes with flat bottoms and at equal depths, this law can be confirmed:
167
The echo heights are proportional to their area or The echo heights are proportional to the square of their diameter
Example: The flat-bottom hole with a diameter vof 2 mm has an echo
which is 4 times that of a 1 mm flat-bottom hole because the area has quadrupled. However, if the echoes from two drill holes at different depths are compared then an additional distance dependence of the echo heights is established, figure 109.
Figure 109: Reflectors at different depths and their echoes
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With accurate tests using flat-bottom holes at different depths a simple law can be found, at least in the far field of the applied sound beam: The echo heights inversely reduce to the square of their distance
This does not normally apply to the nearfield of the sound beam! Here, the test results show that the echo heights within the focus reach their highest amplitude and are reduced again at shorter distances, figure 110.
Figure 110: Distance amplitude curve of a 2 mm – disk reflector
If such curves are put on transparent scales having the CRT format then we immediately have the possibility to comparatively evaluate echoes from unknown reflectors and those from natural reflectors, i.e. the echo height of the discontinuity is compared to that of a circular disk. The discontinuity in figure 111 reflects the sound waves the same as a circular disk having a diameter of 4 mm.
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Figure 111: Evaluation of a discontinuity (F) using evaluation curves
Due to the fact that we can only assess the sound reflected from the discontinuities we must of course not equate the diameter of 4 mm with the "true size" of the discontinuity. We therefore refer to them as an equivalent disk-shaped reflector or as equivalent reflector size (ERS). The equivalent reflector size only correspondsto the true reflector size of a discontinuity in an ideal case which is when it is circular and exactly hit vertical to the acoustic axis. In practise this almost never occurs which means that the true size of a discontinuity is normally larger than the equivalent reflector size. A law for this cannot be derived because the echo height is strongly dependent on the characteristics of the discontinuity, this means its geometry, orientation to the sound beam and the surface quality. For example, a pore (spherically shaped gas inclusion) with a diameter of 2 mm has an equivalent reflector size of 1 mm; an angled flat reflector 5 mm long gives, according to orientation, a result of ERS 0 (not detectable) to perhaps ERS 2. This uncertainty in the evaluation of the discontinuity is however neutralized when other possibilities and techniques in ultrasonic testing
170
are used to inspect detected discontinuities closer. An experienced ultrasonic operator can, without additional expense, accurately give information about the discontinuity which he has detected. Scanning the discontinuity from different directions, assessing the echo shape and the behavior of the display when moving the probe (echo dynamics) are just a few techniques which can be successfully applied. Despite the remaining uncertainty with evaluation of natural discontinuities theabove method of discontinuity evaluation is applied in many countries due to the fact that the method is based on well proven laws in the sound field. It is therefore reproducible, i.e. the evaluation results are independent of testing device and operator. The socalled DGS scales or discontinuity evaluation can be obtained from the probe manufacturer for many probes and various calibration ranges. DGS means that the scale is allocated an echo at the Distance, with correctly set Gain and (equivalent reflector) Size. However, the modern version of the DGS scale would need some explanation because it was developed to fulfill the requirements of the most common specifications in practical testing: If, on a certain test object whose purpose and therefore stress values are known, an ultrasonic test is to be carried out then firstly, if necessary with destructive testing, it should be established how large the permitted material flaw should be. Of course, the position of such a flaw in the material and its rate of occurance play a part. If a permitted flaw size has been determined then this size is multiplied with the safety factor which, amongst others, also takes the evaluation
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uncertainty of the ultrasonic test into account. The corresponding echo amplitude curve for this size is now of importance for the ultrasonic test. The ultrasonic operator scans the test object with the probe and only needs to record the indications which exceed this recording curve, figure 112.
Figure 112: DGS scale for the probe B 4 S
Consequently, only one curve is necessary for the evaluation. Due to the fact that, depending on the application, different recording limits occur, it must be possible to allocate other equivalent reflector sizes to this curve. This allocation is shown by a table positioned at the top right of the scale: starting from a defined default setting of the instrument, the auxiliary gain is taken from the table which belongs to the required recording value and added to the gain controls. If the correct range calibration has been made then test object scanning can now begin. When an indication from the test object exceeds the recording curve then this result is to be recorded in writing and evaluated. If required,
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the test instructions provide the following measures: rejection, repair or further tests for exact assessment of the discontinuity (diagnosis). Figure 113 shows testing of a forged part. The recording curve corresponds to Equivalent Reflector Size 3. The detected discontinuity, at a depth of 110 mm, exceeds the curve, i.e. all reflector data must now be recorded into a predetermined form.
Figure 113: Discontinuity evaluation with a DGS scale
11.2. Shape Discontinuities are interruptions in the typical structure of a material. These interruptions may occur in the base metal, weld material or "heat affected" zones. Discontinuities, which do not meet the requirements of the codes or specification used to invoke and control an inspection, are referred to as defects.The following discontinuities are typical of all types of welding:
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Crack is tight linear separations of metal that can be very short to
very long indications. Cracks are grouped as hot or cold cracks. Hot cracks usually occur as the metal solidifies at elevated temperatures. Cold cracks occur after the metal has cooled to ambient temperatures (delayed cracks).
Figure 114: Cracks
Lack of fusion (Cold Lap) is a condition where the weld filler metal
does not properly fuse with the base metal or the previous weld pass material (inter pass cold lap). The arc does not melt the base metal sufficiently and causes the slightly molten puddle to flow into base material without bonding.
Figure 115: Lack of fusion
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Porosity is the result of gas entrapment in the solidifying metal.
Sometimes porosity is elongated and may have the appearance of having a tail This is the result of gas attempting to escape while the metal is still in a liquid state and is called wormhole porosity. All porosity is a void in the material.
Figure 116 : Porosity
Cluster porosity is caused when flux coated electrodes are
contaminated with moisture. The moisture turns into gases when heated and becomes trapped in the weld during the welding process. Cluster porosity appear just like regular porosity but the indications will be grouped close together (Please verify the applicable norme of the project to know more about indications groupement).
Figure 117: Cluster Porosity
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Slag inclusions are nonmetallic solid material entrapped in weld
metal or between weld and base metal.
Figure 118: Slag inclusion
Incomplete penetration (IP) or lack of penetration (LOP) occurs
when the weld metal fails to penetrate the joint. It is one of the most objectionable weld discontinuities. Lack of penetration allows a natural stress riser from which a crack may propagate.
Figure 119: Incomplete penetration
Root or Internal concavity concavity or suck back is is condition where the weld
metal has contracted as it cools and has been drawn up into the root of the weld.
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Figure 120: Internal concavity
Internal or root undercut is is an erosion of the base metal next to the
root of the weld. Undercutting is not as straight edged as LOP because it does not follow a ground edge.
Figure 121: Root undercut
is an erosion of the base metal next to External or crown undercut is the crown of the weld.
Figure 122: Crown undercut
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Offset or mismatch are terms associated with a condition where two
pieces being welded together are not properly aligned. The difference in density is caused by the difference in material thickness. The dark, straight line is caused by failure of the weld metal to fuse with the land area.
Figure 123: Offset
Inadequate weld reinforcement is an area of a weld where the
thickness of weld metal deposited is less than the thickness of the base material.
Figure 124: Inadequate weld reinforcement
is an area of a weld that has weld metal Excess weld reinforcement is added in excess of that specified by engineering drawings and codes. cod es. A visual inspection will easily determine if the weld reinforcement is in excess of that specified by the engineering requirements.
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Figure 125: Excess weld reinforcement
11.3. D i stance stance fr om ent entrr y surface sur face Flaw Position Estimation
The flaw position is used for both region of interest (ROI) selection the depth normalisation (which is required for the feature extraction performed later). later). It is therefore important that the position estimation estimation is accurate and robust. The current method to find the flaw position is based on fitting an hyperbolic function to the flaw response in Bscan data. The algorithm operation is illustrated at Fig 126. Consider the A-scan 48 mm from fro m the centre of the weld, marked with a vertical line in Figure 126b (also included in the box in the same figure). The maximum response yielded by the algorithm is approx. R max max= 45mm.
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Figure 126: Illustration of defect position estimation.
Depth Normalisation
Due to the lobe characteristics (cone-beam geometry) of the probe, a defect located close to the transducer will be seen in a fewer Ascans in a B-scan than a similar defect detected further away from the probe. Echo-dynamics of the flaw close to the transducer will have a narrower shape than the other echo-dynamics corresponding to the remote flaw. A simple way to normalise is to re-sample the echo-dynamics (or wavelet coefficients) in some angle interval. That is, the feature vector (or matrix) is re-sampled in an angular scale instead of the original linear scale. This is illustrated in Figure 127, where the two horizontal arrows indicate the distances where the flaws f1, and f2, are inside the ultrasonic beam. The depth normalisation procedure consists of re-sampling the features for a
180
suitable angular range given the depth of the flaw. This implies interpolating features from flaws located close to the probe, and down sampling features for flaws that lie further away from the probe.
Fig 127: Illustration of the effect of the cone beam geometry for two defects at different depths. Defect f1 will be seen in fewer A-scans than defect f2.
ROI Selection
Region of interest (ROI) for defect characterisation is an important issue since all further processing relies on it. If the analysing window for ROI is not positioned correctly the features fed to the classier will vary between different measurements, giving inconsistent results. It appears however, that accurate positioning of the analysing window in ultrasonic
B-scans
is
not
a
trivial
task.
Ideally, a hyperbolic analysing window should be positioned around a flaw response in a B-scan, and the exact position of the window should be determined based on the estimated position of the flaw. The estimation procedure described above is, however, not accurate enough for the precise horizontal positioning required here. In the previous studies the echo-dynamics (max amplitude variation) of the flaw response
was
used
for
181
horizontal
positioning.
However, for realistic defects the echo-dynamics curves may be skewed, have more than one peak, etc. The approach used previously was to smooth the echo-dynamics, with low-pass filtering, which partially solves the problem. This method was suitable for the simulated and artificial defects. Experiments have been performed using entre-ofmass calculations in order to find a robust estimate for the echodynamics centre point. This approach was, however, too sensitive to long tails with high amplitude (energy) in the echo-dynamics. Therefore, the previously used algorithm was adopted here as well. The algorithm includes: low-pass filtering and finding the maximum of the echo-dynamics in the first step, and selecting a number of A-scans centred around the A-scan corresponding to max amplitude in the second step. Classical Features
The perhaps most commonly used features for classification of defects during ultrasonic inspection is the rise time, pulse duration and fall time. These three features are calculated from the A-scan envelope using time instants corresponding to the 10% and 90% amplitude levels. These basic features are reliable provided that the ultrasonic pulses (echoes) are well defined. However, reliability of these features may be considerably impaired for realistic defects since they often result in pulses with envelopes that cannot be approximated by well-defined bell curves. Smoothing (low-pass filtering) partly alleviates these
182
problems, but at the expense of some loss of information. Figure 128 shows three examples of envelopes corresponding to different types of defects.
Fig 128: Envelopes of A-scans corresponding to three different flaws. The pulse duration and the raise- and fall times are similar despite the very different pulse shapes.
In spite of the very different shape of the waveforms, the rise time, pulse duration, and fall time are rather similar for all of the signals in Figure 128. It is evident that more powerful features are needed if the classification should be feasible for this type of signals.
Conclusions
During the evaluation of ultrasonic data acquired from the V-welded steel blocks it became evident that the characterisation task is much more complex than for simulated and artificial flaw signals. The feature space of possible flaw signals is also considerably larger for the real defects than for the artificial counterparts, i.e., the variation of the
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ultrasonic signals within one type (class) of defects is much larger for real than for artificial defects. Our goal was to separate soft (or volumetric) defects from the sharper ones (crack-like defects), but if one studies the echo-dynamics and the pulse shapes (i.e. the envelope) it becomes apparent that some sharp and soft defect types are very hard to separate. This implies that overlapping feature regions are encountered, especially when using classical features (fall/raise times, pulse duration and echo-dynamics). To avoid overlapping class boundaries, more powerful feature extraction algorithms are needed to achieve a good classification performance. High variation of the ultrasonic signals also has two further consequences: flaw position estimation (needed for feature extraction) may be poor and the amount of data needed to construct reliable classifiers large. The following is recommended during design of a self-learning classifier:
Ensure that the measurements are informative enough to distinguish between different types of defects.
The features must be representative. The features that are fed to the classifier must preserve the information needed for successful classification.
The number of representative training examples must be sufficient. As a rule of thumb at least ten times as many examples as the parameters in the classifier are needed, to avoid that the classifier learns the training examples and performs poor on unseen examples.
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The first recommendation may be not fulfilled using single B-scan measurements only. A common practice is to combine measurements from several transducers (with different angles, centre frequencies, etc.) and TOFD measurements. The second condition is clearly not fulfilled if only classical features are used and therefore more powerful methods are needed.
The third condition is a common problem in NDT, it can be hoped that mathematical modelling can contribute to the a priori knowledge and thus lead to decreasing the required number of examples. This knowledge can also take the form of expert knowledge acquired from experienced operators. A reasonable approach to defect characterisation is to concentrate on improving flaw imaging and leave the classification tasks to experienced operators.
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Appendix 1: Locating a discontinuity Calibration of the instrument
The location of a discontinuity can be instantly determined using its echo if the instrument is correctly calibrated. Calibration means, linear display, from the zero point on the scale, of a certain distance range of the object to be tested. The zero point on the scale corresponds to the surface of the test object and the 10th scale graduation the maximum distance, e.g. 100 mm steel, 10 mm aluminum, 25 mm brass etc. When specifying the calibration rangethe naming of the material is also important because the displayed distance of the echo, sound path s, is always deduced from the time of flight t of the pulse and the sound velocity c according to the equation: s = sound path [mm] c = sound velocity [km/s] t = transit time [ms] This relationship is not unimportant for the ultrasonic operator but it is not required for the sequence of calibration. The rule simply says: Use a work piece of the same material as the test object whose dimensions are known. By coupling the probe onto an object of known thickness t an echo sequence appears on the display, Fig. 39. The associated sound paths correspond logically to the paths being travelled in the test object, for example with a straight-beam probe it is the multiple of the test object's thickness t, therefore:
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1st
Echo
=
t,
2nd
Echo
=
2t,
3rd Echo = 3t, etc. We must now adjust 2 of these echoes on the corresponding scale graduation to the required calibration range. The instrument is then calibrated, i.e. by Fig. 39 USK 7: Backwall reading off the scale position T the echo sequence with a sound path s (distance) of the associated straight-beam probe reflector can be determined (location of reflectors, wall thickness measurement). Calibration with a straight-beam probe
The reference piece used for calibration is called the Calibration Block , or Standard Calibration Block , if the block used is
standardized. The Standard Calibration Block 1, also simply referred to as V1 block (according to BS 2704 - A2), has a thickness of exactly 25 mm and is made of low-alloyed fine grained steel so that it can be used for nearly all types of calibration when similar steels are to be tested. Example 1: Calibration range 100 mm steel (longitudinal waves)
The 10 scale graduations on the horizontal display scale are to have a range of 0 to 100†mm steel, Fig. 40. One scale graduation therefore
corresponds to 10 mm in the test object. We say: the scale factor k
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(display scale) is 10 mm per scale graduation. We couple the straight beam probe to the V1 block (laying flatwise), Fig. 39. The backwall echo sequence now comes from the 25 mm Fig. 40 Calibration range: 0steel path. The allocation of sound 10mm paths si to the corresponding scale positions Ti is carried out using the calibration table: Echo-No Sound path si Scale factor k Skalen-position Ti i
[mm]
[mm/scale grad.] [scale grad.]
1
25
10
2.5
2
50
10
5.0
3
75
10
7.5
4
100
10
10.0
The corresponding scale position Ti is calculated by using the formula: si = sound path of umpteenth echoes Ti = scale position of the umpteenth echo k= scale factor
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Fig. 41 USK 7: Calibration in the 100 Fig. 42 USK 7 D: Consideration of mm range
the probe delay
The exact adjustment of echoes from the calibration block, as in Fig. 41, is made with analog ultrasonic flaw detectors using the controls pulse shift (or delay) as well as coarse and fine ranges. In doing this, the adjustments must be alternately carried out at these points until the echo flanks are at the correct scale positions. With modern digital instruments the calibration range of 100 mm and the sound velocity of 5920 m/s are firstly entered. After coupling the probe to the calibration block, the function delay or probe delay is changed until the echoes are correctly positioned, Fig. 42. Example 2: Calibration range of 250 mm in aluminum
10 scale graduations correspond to 250 mm in aluminum: k = 25 mm/graduation. We couple the straight-beam probe to an aluminum test block which is 80 mm thick, i.e. a backwall echo sequence is produced from this thickness (t = 80 mm), Fig. 43. The calibration table now looks like this:
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Echo-No Sound path si Scale factor k Skalen-position Ti i
[mm]
[mm/scale grad.] [scale grad.]
1
80
25
3.2
2
160
25
6.4
3
240
25
9.6
Exact reflector location is only possible after correct calibration of a test instrument. The ultrasonic operator moves the probe over the test object. In a normal case, i.e. when a discontinuity does not exist, only the initial pulse and the backwall echo are visible on the display. As soon as a discontinuity is within the area of the sound beam, an additional echo appears between the initial pulse and the backwall echo, Fig. 44, e.g. an echo at scale graduation 1.4. With calibration in the 250 mm range the distance to the reflector s is therefore 1.4 x 25 = 35 mm.
Fig. 43 USK 7 D: Calibration of
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a 250 mm range with an 80 mm Fig. 44 USK 7 D: Sound path aluminum path Calibration
measurement. with
a
TR
probe
For technical reasons, the calibration with a TR probe can only be made to a certain extent using a backwall echo sequence from a comparison object. Due to the slight angular beaming, Fig. 35, transverse waves occur with the TR probe which cause strong interference behind the 1st backwall echo so that the 2nd backwall echo is often unable to be identified. Therefore, a stepped calibration block is used for the adjustment of both echoes, alternately going between two steps (2 point calibration). Example 3: Calibration range for 10 mm steel
Step block VW (steel: 1 - 8 mm). The 3 mm and 6 mm steps should be used for calibration. The step selection depends on the depth range of the expected reflectors. Here the echo from 3 mm must be adjusted to the 3rd scale graduation and the echo from 6 mm to scale graduation 6, Fig. 45a+b. a) Firstly, we couple the TR probe to the 3†mm step and use the delay control for adjusting the echo flank to the 3rd scale graduation.
191
b) Now we couple the probe to the 6 mm step and bring the echo to the 6th scale graduation with the range control. c) Steps a) and b) are alternately repeated until both echo flanks are exactly on the 3rd and 6th scale graduations, Fig. 45a+b. The main application of TR probes are wall thickness measurements, but they are also suitable for the detection of near-to-surface discontinuities, Fig. 46a+b.
Fig. 45a The two positions (3 mm and 6 mm step) of the TR probe on the stepped calibration block VW
Fig. 46a Probe DA 312 on a Fig. 45b Calibration echo at the speciemen with a side drilled hole 3rd graduation (top) Calibration in
192
a
depth
of
1
mm.
echo
at
the
6th
graduation
(bottom)
Fig. 46 b Detection of the drill hole from Fig. 46 a Calibration
with
an
angle-beam
probe
For calibration of the test instrument with an angle-beam probe the standard calibration block 1, Fig. 47a, and the calibration block V2 (according to BS 2704 - A4), Fig. 47b, are almost exclusively used because no backwall echo sequence is received due to the angular beaming from a plane-parallel calibration block.
Fig.
47a
Calibration
WB
60-2E Block
on 1 Fig. 48 Different probe angels at V1
193
block
Fig. 49a Sound path in the V1 block without angle reflection
Fig.
47b
MWB
45-4E
on
Calibration Block 2
Fig. 49b Sound path in the V1 block with angle reflection. The advantage with echoes from the circle segment of the calibration block is that the same sound path is always given independent of the probe angle, Fig. 48. When the angle-beam probe is exactly coupled in the center of the circle segment, a first echo is exactly received from 100 mm out of V1 block. According to the reflection law, the sound waves coming out of the arc are reflected away from the coupling surface to the back, this means away from the arc, Fig. 49a. A second echo out of the arc, needed for the calibration sequence, cannot therefore be produced. For this, there are two saw cuts made in the center of the quarter circle: in the edges, which these saw cuts form with the surfaces, the sound waves are reflected back within themselves due
194
to double reflection (angle reflection effect) so that they go back to the arc, Fig. 49b. Because the radius of the circle segment is exactly 100 mm we will regularly receive an echo sequence with distances of 100 mm, 200 mm, 300 mm etc. with which we are able to carry out calibration of the test instrument the same way as the straight-beam probe. Fig. 50 shows calibration of the 250 mm range.
Fig. 50 Range: 250 mm with a Fig. 51a Path of a sound wave in a WB 60-2 on V1 block
V2 block, radius 50 mm
For the miniature angle-beam probe one uses the considerably smaller and lighter Standard Calibration Block 2 (V2 block). This has, as opposed to the V1 block, two circle segments with a common center point, however it does not have saw cuts. The required echo sequence is produced here by the alternating reflection of the sound waves, Fig. 51a+b.
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Fig. 51b Path of a sound wave in a Fig. 52 Range: 100 mm calibrated V2 block, radius 25 mm
on V2, radius 25 mm.
The corresponding echo sequence is produced according to whether the probe beams into the 25 mm radius or the 50 mm radius. No echoes appear with sound paths by which the sound pulses from the "wrong" direction meet at the center point because these pulses are absorbed by the front damping element of the probe. Fig. 52 shows calibration of the 100 mm range by scanning into the 25 mm radius of Standard Calibration Block V2. Locating reflectors with an angle-beam probe
The echo of a discontinuity on the instrument display does not now give us any direct information about its position in the material. The only available information for determination of the reflector position is the scale position and therefore the sound path s, this means the distance of the discontinuity from the index point(sound exit point) of the probe, Fig. 53.
196
The mathematics of the rightangled triangle helps us to evaluate the
Surface
Distance
and
the Depth of a reflector which are both important for the ultrasonic test, Fig. 54a. We therefore now have the possibility to instantly Fig. 53 Scanning a reflector using mark a detected flaw's position on an angle beam probe the surface of the test object by measurement of the surface distance from the sound exit point and to give the depth. For practical reasons, the reduced surface distance is used because this is measured from the front edge of the probe. The difference between the surface distance and the reduced surface distance corresponds to the x-value of the probe, this is the distance of the sound exit point to the front edge of the probe, Fig. 54b.
Fig. Fig. 54a The flaw triangle
54b
Reduced
surface
distances and x-value
With ultrasonic instruments having digital echo evaluation these calculations are naturally carried out by an integrated microprocessor
197
and immediately displayed so that the operator does not need to make any more time-consuming calculations, Fig. 55. This is of great help with weld testing because with the calculation of the flaw depth an additional factor must be taken into account, namely: whether the sound pulses were reflected from the opposing wall. If this is the case then an apparent depth of the reflector is produced by using the depth formula which is greater than the
Fig. 55 USN 50: A
hole
being
scanned with the probe MWB 604E
thickness T of the test object. The ultrasonic operator must acertain whether a reflection comes from the opposite wall and then proceed with calculating the reflector depth, Fig. 56b.
Fig. 56b The real reflector depth Fig. 56a The apparent depth Locating
reflectors
after sound reflection with
an
angle-beam
probe
The echo of a discontinuity on the instrument display does not now give us any direct information about its position in the material. The only available information for determination of the reflector position is the scale position and therefore the sound path s, this means the distance of
198
the discontinuity from theindex point (sound exit point) of the probe, Fig. 53. The mathematics of the right-angled triangle helps us to evaluate the Surface Distance and the Depth of a reflector which are both important for the ultrasonic test, Fig. 54a. We therefore now have the possibility to instantly mark a detected flaw's position on the surface of the test object by measurement of the surface distance from the sound exit point and to give the depth. For depth of the reflector is produced by using the depth formula which is greater than the thickness T of the test object. The ultrasonic operator must acertain whether a reflection comes from the opposite wall and then proceed with calculating the reflector depth, Fig. 56b.
Appendix 2: Vocabulary of the Ultrasonic testing
199
A-scan — A method of data presentation on an ultrasonic display utilizing a horizontal baseline, that indicates distance, and a vertical deflection from the baseline, that indicates amplitude. A-Scan presentation — A method of data presentation utilizing a horizontal baseline to indicate distance, or time, and a vertical deflection from the baseline to indicate amplitude. Amplitude — The vertical height of a signal, usually base to peak, when indicated by an A-scan presentation. Angle beam — A term used to describe an angle of incidence or refraction other than normal to the surface of the test object, as in angle beam examination, angle beam search unit, angle beam longitudinal waves, and angle beam shear waves. Area amplitude response curve — A curve showing the relationship between different areas of reflection in an material and their respective amplitudes of ultrasonic response. Attenuation — A factor that describes the decrease in ultrasound intensity or pressure with distance. Normally expressed in decibels per unit length. B-scan presentation — A means of ultrasonic data presentation that displays a cross section of the specimen, indicating the approximate length (as detected per scan) of reflectors and their relative positions. Back reflection — An indication, observed on the display screen of a UT instrument, that represents the reflection from the back surface of a reference block or test specimen. Back echo — See back reflection. Back surface — The surface of a reference block or specimen that is opposite the entrant surface. Beam spread — A divergence of the ultrasonic beam as it travels through a medium. Bubbler — A device using a liquid stream to couple a transducer to the test piece. C-scan — An ultrasonic data presentation that provides a plan view of the test object and discontinuities.
200
Collimator — A device for controlling the size and direction of the ultrasonic beam. Contact testing — A technique in which the transducer contacts directly with the test part through a thin layer of couplant. Couplant — A substance, usually a liquid, used between the transducer unit and test surface to permit or improve transmission of ultrasonic energy. Critical angle — The incident angle of the ultrasonic beam beyond which a specific refracted wave no longer exists. DAC — Distance amplitude correction. Electronic change of amplification to provide equal amplitude from equal reflectors at different depths. Also known as swept gain, time corrected gain, time variable gain, etc. DAC curve — A curve (usually drawn on the screen) derived from equal reflectors at different depths. Damping, search unit — Lmiting the duration of a signal from a search unit subject to a pulsed input by electrically or mechanically decreasing the amplitude of successive cycles. dB control — A control that adjusts the amplitude of the display signal in decibel (dB) units. Dead zone — The distance in the material from the surface of the test specimen to the depth at which a reflector can first be resolved under specified conditions. It is determined by the characteristics of the search unit, the ultrasonic instrumentation, and the test object. Decibel (dB) — Logarithmic expression of a ratio of two amplitudes or intensities. (UT) dB = 20 log 10 (amplitude ratio). Delay line — A column of material such as Plexiglas that is attached to the front of a transducer. It behaves similarly to a water path and allows the initial pulse to be shifted off the scree. This often improves “near surface resolution.” Delay sweep — An A-scan or B-scan presentation in which an initial part of the time scale is not displayed.
201
Discontinuity — A lack of continuity or cohesion; an intentional or unintentional interruption in the physical structure or configuration of a material or component. Distance amplitude, compensation (electronic) — The compensation or change in receiver amplification necessary to provide equal amplitude on the display of an ultrasonic instrument for reflectors of equal area that are located at different depths in the material. Distance amplitude, response curve — See DAC. A curve showing the relationship between the different distances and the amplitudes of an ultrasonic response from targets of equal size in an ultrasonic transmitting medium. Distance linearity range — The range of horizontal deflection in which a constant relationship exists between the incremental horizontal displacement of vertical indications on the A-scan presentation and the incremental time required for reflected sound to pass through a known length in a uniform transmission medium. Doppler effect — The change in frequency of a sound wave due to movement of the reflector. Movement toward or away from the sound will result in a change in frequency (e.g., the tone of a train whistle changing as the train passes). Dual search unit — A search unit containing two elements, one a transmitter, the other a receiver. Dynamic range — The ratio of maximum to minimum reflective areas that can be distinguished on the display at a constant gain setting. Entrant surface — The surface of the material through which the ultrasonic waves are initially transmitted. Far field — The zone of the beam (beginning at the Y 0 point) where equal reflectors give exponentially decreasing amplitudes with increasing distance. Flaw — A discontinuity in a material or component that is unintentional. Flaw characterization — The process of quantifying the size, shape, orientation, location, growth, or other properties of a flaw based on NDT response.
202
Frequency (examination) — The number of cycles per second (Hz). Frequency, pulse repetition — The number of times per second that a search unit is excited by the pulser to produce a pulse for ultrasonic imaging. This is also called pulse repetition rate or pulse repetition frequency (PRF). Gate — An electronic means of selecting a segment of the time range for monitoring, triggering an alarm, or further processing. Immersion testing — An ultrasonic examination technique in which the search unit and the test part are submerged (at least locally) in a fluid, usually water. Impedance, acoustic — A mathematical quantity used in computation of reflection characteristics at boundaries. It is the product of wave velocity and material density Indication — A response or evidence of a response disclosed through an NDT that requires further evaluation to determine its full and true significance. Initial pulse — The response of the ultrasonic system display to the transmitter pulse (sometimes called “main bang”). Lamb wave — A specific mode of propagation in which the two parallel boundary surfaces of the material under examination (such as a thin plate or wall of a tube) establish the mode of propagation. The Lamb wave can be generated only at particular values of frequency, angle of incidence, and material thickness. The velocity of the wave is dependent on the mode of propagation and the product of the material thickness and the examination frequency. Linearity, amplitude — A measure of the proportionality of the amplitude of the signal input to the receiver and the amplitude of the signal appearing on the display of the ultrasonic instrument or on an auxiliary display. Linearity, time or distance — A measure of the proportionality of the signals appearing on the time or distance axis of the display and the input signals to the receiver from a calibrated time generator or from multiple echoes from a plate or material of known thickness.
203
Longitudinal wave — A wave in which the particle motion of the material is essentially in the same direction as the wave propagation. (also called compressional wave). Metal path — See Sound path Mode — The type of ultrasonic wave propagating in the material as characterized by the particle motion (e.g., longitudinal, transverse, etc.) Mode conversion — Phenomenon by which an ultrasonic wave that is propagating in one mode refracts at an interface to form ultrasonic wave(s) of other modes. Multiple back reflections — Successive signals from the back surface of the material under examination. Near field — The region of the ultrasonic beam adjacent to the transducer having complex beam profiles and intensity variations. Also known as the Fresnel zone. Noise — Any undesired signal (electrical or acoustic) that tends to interferes with the interpretation or processing of the desired signals. Normal incidence (also see Straight beam) — A condition in which the axis of the ultrasonic beam is perpendicular to the entrant surface of the part being examined. Penetration depth — The maximum depth in a material from which usable ultrasonic information can be obtained and measured. Probe — See Search unit. Pulse-echo technique — An examination method in which the presence and position of a reflector are indicated by the echo amplitude and time. Pulse length — A measure of the duration of a signal as expressed in time or number of cycles. Range — The maximum distance that is presented on a display. Rayleigh wave — An ultrasonic surface wave in which the particle motion is elliptical and the effective penetration is approximately one wavelength. Reference block — A block of material that includes reflectors. It is used both as a measurementscale and as a means of providing an ultrasonic reflection of known characteristics.
204
Reflector — An interface at which an ultrasonic beam encounters a change in acoustic impedance and at which at least part of the sound is reflected. Reject, suppression — A control for minimizing or eliminating lowamplitude signals (electrical or material noise) so that true signals are emphasized. Relevant indication — An indication caused by a discontinuity that requires evaluation. Scanning — The movement of a transducer relative to the test part in order to examine a volume of the material. Search unit — An electroacoustic device used to transmit and/or receive ultrasonic energy. The device generally comprises a piezoelectric element, backing, wearface and/or wedge. Sometimes known as a “probe” or “transducer.” Sensitivity — A measure of the smallest reflector that produces a discernible signal on the display of an ultrasonic system. Shear wave — wave motion in which the particle motion is perpendicular to the direction of propagation (transverse wave). Sound path — The path of the sound energy from the time that it leaves the transducer and reflects back to the transducer. Skip distance — In angle beam testing, the distance along the test surface from sound entrant point to the point at which the sound returns to the same surface. It can be considered the top surface distance of a complete “vee” path of sound in the test material. Transducer — A piezoelectric element used to produce ultrasonic vibrations. Through-transmission technique — A technique in which ultrasonic waves are transmitted by one search unit and received by another at the opposite surface of the material being examined. Vee path — The angle beam path in materials starting at the search-unit examination surface, through the material to the reflecting surface, continuing to the examination surface in front of the search unit, and
205
reflecting back along the same path to the search unit. The path is usually shaped like the letter V. Water path — The distance from the transducer to the test surface in immersion or water column testing. Wedge — In angle beam examination by the contact method, a device used to direct ultrasonic energy into the material at an angle. Wheel search unit — An ultrasonic device incorporating one or more transducers mounted inside a liquid-filled flexible tire. The beam is coupled to the test surface through the rolling contact area of the tire. Also known as a “wheel probe” or “roller search unit.”
Appendix 3: Materials acoustic characteristics Metals
Longitudinal Velocity
Shear Velocity
206
Surface Velocity
Density g/cm3
Acoustic Impedance
g/cm2-sec x105
cm/µs
in/µs
cm/µs
in/µs
cm/µs
in/µs
Aluminum
0.632
0.2488
0.313
0.1232
N/A
N/A
2.70
17.10
AL 1100-0 (2SO)
0.635
0.25
0.310
0.122
0.290
0.114
2.71
17.20
AL (14S)
0.632
0.2488
0.307
0.1209
N/A
N/A
2.80
17.80
AL 2024 T4 (24ST)
0.637
0.2508
0.316
0.1244
0.295
0.116
2.77
17.60
AL 2117 T4 (17ST)
0.650
0.2559
0.312
0.1228
N/A
N/A
2.80
18.20
Babbitt Bearing
0.230
0.0906
N/A
N/A
N/A
N/A
7.4 11.0
Beryllium
1.29
0.5079
0.888
0.3496
0.787
0.310
1.82
23.50
Bismuth
0.218
0.0858
0.110
0.0433
N/A
N/A
9.80
21.40
Brass
0.428
0.1685
0.230
0.0906
N/A
N/A
8.56
36.70
0.383
0.1508
0.205
0.0807
N/A
N/A
8.10
31.02
Brass, Naval
0.443
0.1744
0.212
0.0835
0.195
0.0770
8.42
37.3
Bronze, Phospho
0.353
0.139
0.223
0.0878
0.201
0.0790
8.86
31.28
Cadmium
0.278
0.1094
0.150
0.0591
N/A
N/A
8.64
24.02
Cesium (28.5oC)
0.0967
0.0381
N/A
N/A
N/A
N/A
1.88
1.82
Columbium
0.492
0.1937
0.210
0.0827
N/A
N/A
8.57
42.16
Constantan
0.524
0.2063
0.104
0.0409
N/A
N/A
8.88
46.53
Copper
0.466
0.1835
0.233
0.0890
0.193
0.0760
8.93
41.61
Gallium
0.274
0.1079
N/A
N/A
N/A
N/A
5.95
16.3
Germanium
0.541
0.213
N/A
N/A
N/A
N/A
5.47
29.59
Gold
0.324
0.1276
0.120
0.0472
N/A
N/A
19.32
62.6
2014
Brass, Hard
Half
207
-
23.20
Hafnium
0.384
0.1512
N/A
N/A
N/A
N/A
N/A
N/A
Inconel
0.572
0.2252
N/A
N/A
0.279
0.110
8.25
47.19
Indium (156oC)
0.222
0.0874
N/A
N/A
N/A
N/A
7.30
16.21
Iron
0.590
0.2323
0.323
0.1272
0.279
0.110
7.70
45.43
Iron, Cast
0.480
0.189
0.240
0.0945
N/A
N/A
7.80
37.44
Lead
0.216
0.085
0.070
0.0276
0.0630
0.0248
11.4
24.62
Lead 5% Antinomy
0.217
0.0854
0.081
0.0319
0.0740
0.0291
1.9
23.65
Magnesium
0.631
0.2484
N/A
N/A
N/A
N/A
1.74
10.98
Magnesium (AM-35)
0.579
0.228
0.310
0.122
0.287
0.113
1.74
10.07
Magnesium (FS-1)
0.547
0.2154
0.303
0.1193
N/A
N/A
1.69
9.24
Magnesium (J-1)
0.567
0.2232
0.301
0.1185
N/A
N/A
1.70
9.64
Magnesium (M)
0.576
0.2268
0.309
0.1217
N/A
N/A
1.75
10.08
Magnesium (O-1)
0.580
0.2283
0.304
0.1197
N/A
N/A
1.82
10.56
Magnesium (ZK-60A-TS)
0.571
0.2248
0.305
0.1201
N/A
N/A
1.83
10.45
Manganese
0.466
0.1835
0.235
0.0925
N/A
N/A
7.39
34.44
Molybdenum
0.629
0.2476
0.335
0.1319
0.311
0.122
10.2
64.16
Monel
0.602
0.237
0.272
0.1071
0.196
0.0772
8.83
53.16
Nickel
0.563
0.2217
0.296
0.1165
0.264
0.104
8.88
49.99
Platinum
0.396
0.1559
0 .167
N/A
N/A
N/A
21.4
84.74
Plutonium
0.179
0.0705
N/A
N/A
N/A
N/A
N/A
28.2
208
Plutonium (1% Gallium)
0.182
0.0717
N/A
N/A
N/A
N/A
N/A
28.6
Potassium (100oC)
0.182
0.0717
N/A
N/A
N/A
N/A
0.83
1.51
Radium
0.0822
0.0324
0.111
0.0437
0.103
0.0404
5.0
4.11
Rubidium
0.126
0.0496
N/A
N/A
N/A
N/A
1.53
1.93
Silver
0.360
0.1417
0.159
0.0626
N/A
N/A
10.5
37.8
Silver, Nickel
0.462
0.1819
0.232
0.0913
0.169
0.0665
8.75
40.43
Silver, German
0.476
0.1874
N/A
N/A
N/A
N/A
8.70
41.41
0.566
0.2228
0.312
0.1228
0.312
0.123
8.03
45.45
0.574
0.226
0.309
0.1217
N/A
N/A
7.91
45.4
0.539
0.212
0.299
0.118
0.216
0.085
7.67
56.68
Steel, 1020
0.589
0.2319
0.324
0.1276
N/A
N/A
7.71
45.41
Steel, 1095
0.590
0.2323
0.319
0.1256
N/A
N/A
7.80
46.02
Steel, 4150, Rc14
0.586
0.2307
0.279
0.1098
N/A
N/A
7.84
45.94
Steel, 4150, Rc18
0.589
0.2319
0.318
0.1252
N/A
N/A
7.82
46.06
Steel, 4150, Rc43
0.587
0.2311
0.320
0.126
N/A
N/A
7.81
45.84
Steel, 4150, Rc64
0.582
0.2291
0.277
0.1091
N/A
N/A
7.80
45.4
Steel, 4340
0.585
0.2303
0.319
0.1259
N/A
N/A
7.80
45.63
Tantalum
0.410
0.1614
0.114
0.0449
N/A
N/A
16.6
68.06
Thallium (302oC)
0.162
0.0638
N/A
N/A
N/A
N/A
11.9
19.28
Steel, Cres
302
Steel, Cres
347
Steel, Cres
410
209
Thorium
0.240
0.0945
0.156
0.0614
N/A
N/A
11.3
27.12
Tin
0.332
0.1307
0.167
0.0657
N/A
N/A
7.29
24.2
Titanium
0.607
0.239
0.331
0.1303
N/A
N/A
4.50
27.32
Titanium Carbide
0.827
0.3256
0.516
0.2031
N/A
N/A
5.15
42.59
Tungsten
0.518
0.2039
0.287
0.113
0.265
0.104
19.25
99.72
Uranium
0.338
0.1331
0.196
0.0772
N/A
N/A
18.9
63.88
Uranium Dioxide
0.518
0.2039
N/A
N/A
N/A
N/A
6.03
31.24
Vanadium
0.600
0.2362
0.278
0.1094
N/A
N/A
6.03
36.18
Zinc
0.417
0.1642
0.241
0.0948
N/A
N/A
7.10
29.61
Zircaloy
0.472
0.1858
0.236
0.093
N/A
N/A
9.03
42.6
Zirconium
0.465
0.1831
0.222
0.0874
N/A
N/ A
6.48
30.1
References:
210
1. C.G. Windsor, Can we train a Computer to be a Skilled Inspector, Insight, 37, January 1995, p. 36. 2. A. MacNab and I. Dunlop, Review of Artificial Intelligence Applied to Ultrasonic Defect Evaluation, Insight, 37, January, 1995, p 11. 3. K. Fukanaga, Introduction to Statistical Pattern Recognition, Academic Press, 1990. 4. B. Eriksson and T. Stepinski, Ultrasonic Characterization of Defects, Part 2. Theoretical Studies, SKI Report 95:21, Swedish Nuclear Power Inspectorate, 1995. 5. B. Eriksson, T. Stepinski and B. Vagnhammar, Ultrasonic Characterization of Defects, Part 3.Experimental Verification, SKI Report 96:75, Swedish Nuclear Power Inspectorate, 1996 6. F. Lingvall, Efficient Data Representations for Eddy Current and Ultrasonic Applications, Lic. Thesis, Uppsala University, Signals and Systems, May 2000. 7. Friedrich Förster: On the way from the "know - how" to the "know - why" in the magnetic leakage field method of nondestructive testing (Part Two). Materials Evaluation. October 1985. 43 (10): 1399 - 1404 8. Kai - Hua Zhao, Xi - Mo Chen: Electromagnetism. Higher Education Publisher, Beijing. Apr. 1978 9. Wei - Chang Zhong: Theoretical fundamentals of magnetic dipole for longitudinal magnetization of a square steel component. (Chinese Journal of Non - Destructive Testing. 1997.19 (4): 95 - 98;
Anaheim, California, USA, March 1998: 67 - 70; Materials Evaluation. 1999.57 (9): 937 - 939 10.Wei - Chang Zhong: Linear magnetic charge density along all edges of a cuboid steel magnetized longitudinally. Santou, China. Oct. 1999: 641 11.Xi - Chun Huang ed. Electromagnetic energy and electromagnetic force. People's Education Publisher, Beijing. 1983 12.Ci - Zhang Feng ed. Static electromagnetic field. Xi - an Jiao - Tong University Publisher, Xi - an, China. 1985 13.De - Fu Wan, Shi - Hua Luo ed. Magnetic physics. Electronic Industry Publisher, Beijing. 1987 14.ULTRASONIC TESTING OF STEEL CASTINGS by J. D. Lavender Research Manager. Quality Assurance Group Steel Castings Research & Trade Association Sheffield. England
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