ASM Metals HandBook Vol. 17 - Nondestructive Evaluation and Quality Control
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Source: Ref 47
Both the photoconductor-liquid crystal sandwich and the ferroelectric photoconductor image camera (FERPIC), a storage
and display device, can also be used in real-time image retrieval from microwave holograms by incorporating them in
schemes such as that shown in Fig. 46.
Microwaves have been used for flaw detection in some articles, such as solid propellant grains, because of the difficulties
involved in their inspection (Ref 54). The resolution of defects has not been particularly successful, especially for
spherical voids. Holography and optical data processing are used for microwave inspection because they permit synthetic
aperture techniques that integrate wide-angle signals from spherical reflectors and scatterers. This system avoids the use
of a spheric correcting lens in CW holograms and in a swept-frequency, side-looking radar, which is useful at very short
range.
Microwaves require a very large hologram to record much detail. The use of two very different wavelengths for recording
and readout, as described previously, causes some severe problems. As a result, the expected advantages of microwave
holography are not realized in terms of a realistic image of the object.
Side-looking radar is another approach. It is more attractive and feasible because there is a method of obtaining depth
resolution at close range. The method uses a swept frequency rather than a pulse echo for depth resolution.
An optical coherent signal processor for A-scan data is illustrated in Fig. 47. It uses a collimator, which is a transform
lens to produce the transform plane. The image lens produces the output, which is photographically recorded. The results
are a series of in-line spots on the film. The first spot is for the front surface. Crack simulations were made and tested at
25, 50, 75, and 102 mm (1, 2, 3, and 4 in.). Each crack was clearly and cleanly defined.
Fig. 47 Coherent processor for optical processing of A-scan data. Source: Ref 47
An A-scan can be readily converted into a B-scan (Ref 54). The horizontal sweep for the oscilloscope, shown in Fig. 48,
is replaced by a sweep synchronizer, and a cylinder lens replaces the spherical transform lens in the optical processor.
This arrangement, along with three simulated delaminations, superficially resembles a side-looking radar system, but it
does not have side-looking radar presentation, and it is further restricted to plane delaminations. Cylindrical flaws were
placed in the simulator, but their round shape was not resolved.
Fig. 48 Optical recording of A-scan data. Source: Ref 47
To convert optically processed B-scans to swept-frequency, side-looking radar scans, all that is needed is to perform the
optical processing in two dimensions (Ref 54). The scan of the antenna in this case synthesizes a hologram similar to that
formed by an incident plane wave and the spherical waves reflected from the object. The incident wave is properly
synthesized and serves as the reference beam of the hologram. The object beam travels twice the path from the antenna to
the object. This is also true of airborne side-looking radars. The only difference is the way in which depth resolution is
obtained. The hologram is composed of one-dimensional interference patterns produced along the scan direction. Their
location in the other dimension is determined by the distance from the antenna to the object. The spacing of the
interference fringes is also a function of distance, and this causes a tilt to the image plane. In airborne radars, a conical
lens is used to make a correction so that all of the object points are imaged in the focal plane.
Fortunately, there is another approach to image formation from this type of hologram. The first step is to produce a
Vander Lugt filter. The hologram of a point is made and used to make the Vander Lugt filter (Ref 54). On the hologram,
an object point is represented by an interference point. If the pattern is recognizable, the object point that produced it can
be identified. The Vander Lugt filter is made with the processor illustrated in Fig. 49. When the filter made in this manner
is placed in Fig. 50, the correctly reconstructed image is formed. The filter is used with the processor shown in Fig. 49,
which was used to make the filter but without the reference beam inserted by the beam splitter in Fig. 49. The forming of
an image from a CW hologram using the Vander Lugt filter is accomplished with the equipment shown in Fig. 50. The
processor shown in Fig. 50 will mark a spot on the output frame for each piece of the hologram contained in the input.
Fig. 49 Making of a Vander Lugt filter for image formation with CW holograms. Source: Ref 54
Fig. 50 Forming an image from a CW hologram using a Vander Lugt filter. Source: Ref 54
References cited in this section
42.
W.E. Kock, Microwave Holography, in Holographic Nondestructive Testing,
R.K. Erf, Ed., Academic
Press, 1974, p 373-403
43.
G.L. Rogers, Gabor Diffraction Microscopy: The Hologram as a Generalized Zone-Plate, Nature,
Vol 166
(No. 4214), 1950, p 237
44.
W.E. Kock and F.K. Harvey, Sound Wave and Microwave Space Patterns, Bell Syst. Tech. J.,
Vol 20, July
1951, p 564
45.
W.E. Kock, Hologram Television, Proc. IEEE, Vol 54 (No. 2), Feb 1966, p 331
46.
G.A. Deschamps, Some Remarks on Radio-Frequency Holography, Proc. IEEE,
Vol 55 (No. 4), April
1967, p 570
47.
N.H. Farhat, Microwave Holography and Its Applications in Modern Aviation, in
Proceedings of the
Engineering Applications of Holography
(Los Angeles), Defense Advanced Research Projects Agency,
1972, p 295-314
48.
J.R. Maldonado and A.H. Meitzler, Strain-
Biased Ferroelectric Photoconductor Image Storage and Display
Devices, Proc. IEEE, Vol 59 (No. 3), March 1971
49.
N.K. Sheridon, "A New Optical Recording Device," Paper presented at the 1970 IE
EE International
Electron Devices meeting, Washington, Institute of Electrical Engineers, Oct 1970
50.
G. Assouline, et al., Liquid Crystal and Photoconductor Image Converter, Proc. IEEE,
Vol 59, Sept 1971, p
1355-1357
51.
T.D. Beard, "Photoconductor Light-
Gated Crystal Used for Optical Data," Paper presented at the 1971
Annual Fall Meeting, Ottawa, Canada, Optical Society of America, Oct 1971
52.
R.B. MacAnally, Liquid Crystal Displays for Matched Filtering, Appl. Phys. Lett.,
Vol 18 (No. 2), 15 Jan
1971
54.
R.W. Cribbs and B.L. Lamb, Resolution of Defects by Microwave Holography, in
Proceedings of the
Symposium on Engineering Applications of Holography,
Defense Advanced Research Projects Agency,
1972, p 315-323
Microwave Inspection
William L. Rollwitz, Southwest Research Institute
References
1. Electromagnetic Testing, Vol 4, 2nd ed., Nondestructive Testing Handbook,
American Society for
Nondestructive Testing, 1986
2. H.E. Bussey, Standards and Measurements of Microwave Surface Impedance, Skin Depth
, Conductivity,
and Q, IRE Trans. Instrum., Vol 1-9, Sept 1960, p 171-175
3. A. Harvey, Microwave Engineering, Academic Press, 1963
4. R.P. Dooley, X-Band Holography, Proc. IEEE, Vol 53 (No. 11), Nov 1965, p 1733-1735
5. W.E. Kock, A Photographic Method for Displaying Sound Wave and Microwave Space Patterns,
Bell
Syst. Tech. J., Vol 30, July 1951, p 564-587
6. E.N. Leith and J. Upatnieks, Photography by Laser, Sci. Am., Vol 212 (No. 6), June 1965, p 24
7. G.W. Stroke, An Introduction to Current Optics and Holography, Academic Press, 1966
8. G.A. Deschamps, Some Remarks on Radiofrequency Holography, Proc. IEEE,
Vol 55 (No. 4), April
1967, p 570
9. G.W. McDaniel and D.Z. Robinson, Thermal Imaging by Means of the Evapograph, Appl. Opt.,
Vol 1,
May 1962, p 311
10. P.H. Kock and H. Oertel, Microwave Thermography 6, Proc. IEEE, Vol 55 (No. 3), March 1967, p 416
11. H. Heislmair et al., State of the Art of Solid-State and Tube Transmitters, Microwaves and R.F.,
April
1983, p 119
12. H. Bierman, Microwave Tubes Reach New Power and Efficiency Levels, Microwave J., Feb 1987, p 26-
42
13. K.D. Gilbert and J.B. Sorci, Microwave Supercomponents Fulfill Expectations, Microwave J.,
Nov 1983,
p 67
14. E.C. Niehenke, Advanced Systems Need Supercomponents, Microwave J., Nov 1983, p 24
15.
W. Tsai, R. Gray, and A. Graziano, The Design of Supercomponents: High Density MIC Modules,
Microwave J., Nov 1983, p 81
16. R.J. Botsco, Nondestructive Testing of Plastic With Microwaves, Parts 1 and 2, Plast. Des. Process.,
Nov
and Dec 1968
17.
M.W. Standart, A.D. Lucian, T.E. Eckert, and B.L. Lamb, "Development of Microwave NDT Inspection
Techniques for Large Solid Propellant Rocket Motors," NAS7-544, Final Report 1117, Aerojet-
General
Corporation, June 1969
18. A.D. Lucian and
R.W. Cribbs, The Development of Microwave NDT Technology for the Inspection of
Nonmetallic Materials and Composites, in
Proceedings of the Sixth Symposium on Nondestructive
Evaluation of Aerospace and Weapons Systems Components and Materials, Western Peri
odicals Co.,
1967, p 199-232
19. L. Feinstein and R.J. Hruby, Surface-Crack Detection by Microwave Methods, in
Proceedings of the Sixth
Symposium on Nondestructive Evaluation of Aerospace and Weapons Systems Components and Materials,
Western Periodicals Co., 1967, p 92-106
20.
F.H. Haynie, D.A. Vaughan, P.D. Frost, and W.K. Boyd, "A Fundamental Investigation of the Nature of
Stress-Corrosion Cracking in Aluminum Alloys," AFML-TR-65-
258, Air Force Materials Laboratory, Oct
1965
21. R.M.J. Cotterill, An Ex
perimental Determination of the Electrical Resistivity of Dislocations in
Aluminum, Philos. Mag., Vol 8, Nov 1963, p 1937-1944
22. D. Nobili and L. Passari, Electrical Resistivity in Quenched Aluminum--Alumina Alloys, J. Nucl. Mater.,
Vol 16, 1965, p 344-346
23. Z.S. Basinski, J.S. Dugdale, and A. Howie, The Electrical Resistivity of Dislocations, Philos. Mag.,
Vol 8,
1963, p 1989
24.
L.M. Clarebrough, M.E. Hargreaves, and M.H. Loretts, Stored Energy and Electrical Resistivity in
Deformed Metals, Philos. Mag., Vol 6, 1961, p 807
25. E.C. Jordan, Electromagnetic Waves and Radiating Systems, Prentice-Hall, 1950, p 237
26. J. Feinleib, Electroreflectance in Metals, Phys. Rev. Lett., Vol 16 (No. 26), June 1966, p 1200-1202
27. A.J. Bahr, Microwave Nondestructive Testing Methods, Vol 1, Nondestructive Testing and Tracts,
W.J.
McGonnagle, Ed., Gordon & Breach, 1982, p 49-72
28. E.E. Collin, Field Theory of Guided Waves, McGraw-Hill, 1960, p 39-40
29. L. Feinstein and R.J. Hruby, Paper 68-321, presented at t
he AIAA/ASME 95th Structures, Structural
Dynamics and Materials Conference, American Institute of Aeronautics and Astronautics/American
Society of Mechanical Engineers, April 1968
30. R.J. Hruby and L. Feinstein, A Novel Nondestructive, Nonconducting Meth
od of Measuring the Depth of
Thin Slits and Cracks in Metals, Rev. Sci. Instrum., Vol 41, May 1970, p 679-683
31. A.J. Bahr, Microwave Eddy-Current Techniques for Quantitative Non-Destructive Evaluation, in Eddy-
Current Characterization of Materials and Structures,
STP 722, G. Birnbaum and G. Freed, Ed.,
American Society for Testing and Materials, 1981, p 311-331
32.
L.A. Robinson and U.H. Gysel, "Microwave Coupled Stripline Surface Crack Detector," Final Report,
Contract DAAG46-72-C-0019, SRI Project 1490, Stanford Research Institute, Aug 1972
33. U.H. Gysel and L. Feinstein, "Design and Fabrication of Stripline Microwave Surface-
Crack Detector for
Projectiles," Final Report, Contract DAAG46-73-C-
0257, SRI Project 2821, Stanford Research Institute,
Sept 1974
34. B.A. Auld, F. Muennemann, and D.K. Winslow, "Observation of Fatigue-
Crack Closure Effects with the
Ferromagnetic-
Resonance Probes," G.L. Report 3233, E.L. Ginzton Laboratory, Stanford University,
March 1981
35. B.A. Auld, et al., Surface Flaw Detection With Ferromagnetic Resonance Probes, in
Proceedings of
DARPA/AFML Review of Progress in Quantitative NDE
(La Jolla, CA), Defense Advanced Research
Projects Agency, July 1980
36. B.A. Auld, New Methods of Detection and Characterization of Surface Flaws, in
Proceedings of
DARPA/AFML Review of Progress in Quantitative NDE
(Cornell University), Defense Advanced
Research Projects Agency, June 1977
37. B.A. Auld, et al., Surface Flaw Detection With Ferromagnetic Resonance Probes, in
Proceedings of
DARPA/AFML Review of Progress in Quantitative NDE
(La Jolla, CA), Defense Advanced Research
Projects Agency, July 1978
38. B.A. Auld, et al., Surface Flaw Detection With Ferromagnetic Resonance Probes, in
Proceedings of
DARPA/AFML Review of Progress in Quantitative NDE
(La Jolla, CA), Defense Advanced Research
Projects Agency, July 1979
39. B.A. Auld and D.K. Winslow, Microwave Eddy Current Experiments With Ferromagnetic Probes, in
Eddy
Current Characterization of Material and Structures, STP 722, G. Birnbaum
and G. Free, Ed., American
Society for Testing and Materials, 1981, p 348-366
40. A.F. Harvey, Properties and Applications of Gyromagnetic Media, in Microwave Engineering,
Academic
Press, 1963, p 352-358
41. S. Segaline, et al., Application of Ferromagne
tic Resonance Probes to the Characterization of Flaws in
Metals, J. Nondestr. Eval., Vol 4 (No. 2), 1984, p 51-58
42. W.E. Kock, Microwave Holography, in Holographic Nondestructive Testing,
R.K. Erf, Ed., Academic
Press, 1974, p 373-403
43. G.L. Rogers, Gabor Diffraction Microscopy: The Hologram as a Generalized Zone-Plate, Nature,
Vol 166
(No. 4214), 1950, p 237
44. W.E. Kock and F.K. Harvey, Sound Wave and Microwave Space Patterns, Bell Syst. Tech. J.,
Vol 20, July
1951, p 564
45. W.E. Kock, Hologram Television, Proc. IEEE, Vol 54 (No. 2), Feb 1966, p 331
46. G.A. Deschamps, Some Remarks on Radio-Frequency Holography, Proc. IEEE,
Vol 55 (No. 4), April
1967, p 570
47. N.H. Farhat, Microwave Holography and Its Applications in Modern Aviation, in Procee
dings of the
Engineering Applications of Holography
(Los Angeles), Defense Advanced Research Projects Agency,
1972, p 295-314
48. J.R. Maldonado and A.H. Meitzler, Strain-
Biased Ferroelectric Photoconductor Image Storage and Display
Devices, Proc. IEEE, Vol 59 (No. 3), March 1971
49.
N.K. Sheridon, "A New Optical Recording Device," Paper presented at the 1970 IEEE International
Electron Devices meeting, Washington, Institute of Electrical Engineers, Oct 1970
50. G. Assouline, et al., Liquid Crystal and Photoconductor Image Converter, Proc. IEEE,
Vol 59, Sept 1971,
p 1355-1357
51. T.D. Beard, "Photoconductor Light-
Gated Crystal Used for Optical Data," Paper presented at the 1971
Annual Fall Meeting, Ottawa, Canada, Optical Society of America, Oct 1971
52. R.B. MacAnally, Liquid Crystal Displays for Matched Filtering, Appl. Phys. Lett.,
Vol 18 (No. 2), 15 Jan
1971
53. G.H. Heilmeier, Liquid Crystal Display Devices, Sci. Am., Vol 222 (No. 4), April 1970, p 100
54. R.W. Cribbs and B.L. Lamb, Resolution of Defects by Microwave Holography, in
Proceedings of the
Symposium on Engineering Applications of Holography,
Defense Advanced Research Projects Agency,
1972, p 315-323
Ultrasonic Inspection
Revised by Yoseph Bar-Cohen, Douglas Aircraft Company, McDonnell Douglas Corporation; Ajit K. Mal, University of California, Los
Angeles; and the ASM Committee on Ultrasonic Inspection
*
Introduction
ULTRASONIC INSPECTION is a nondestructive method in which beams of high-frequency sound waves are introduced
into materials for the detection of surface and subsurface flaws in the material. The sound waves travel through the
material with some attendant loss of energy (attenuation) and are reflected at interfaces. The reflected beam is displayed
and then analyzed to define the presence and location of flaws or discontinuities.
The degree of reflection depends largely on the physical state of the materials forming the interface and to a lesser extent
on the specific physical properties of the material. For example, sound waves are almost completely reflected at metal/gas
interfaces. Partial reflection occurs at metal/liquid or metal/solid interfaces, with the specific percentage of reflected
energy depending mainly on the ratios of certain properties of the material on opposing sides of the interface.
Cracks, laminations, shrinkage cavities, bursts, flakes, pores, disbonds, and other discontinuities that produce reflective
interfaces can be easily detected. Inclusions and other inhomogeneities can also be detected by causing partial reflection
or scattering of the ultrasonic waves or by producing some other detectable effect on the ultrasonic waves.
Most ultrasonic inspection instruments detect flaws by monitoring one or more of the following:
•
Reflection of sound from interfaces consisting of material boundaries or discontinuities within the metal
itself
• Time of transit of a sound wave through th
e testpiece from the entrance point at the transducer to the
exit point at the transducer
• Attenuation of sound waves by absorption and scattering within the testpiece
• Features in the spectral response for either a transmitted or a reflected signal
Most ultrasonic inspection is done at frequencies between 0.1 and 25 MHz--well above the range of human hearing,
which is about 20 Hz to 20 kHz. Ultrasonic waves are mechanical vibrations; the amplitudes of vibrations in metal parts
being ultrasonically inspected impose stresses well below the elastic limit, thus preventing permanent effects on the parts.
Many of the characteristics described in this article for ultrasonic waves, especially in the section "General Characteristics
of Ultrasonic Waves," also apply to audible sound waves and to wave motion in general.
Ultrasonic inspection is one of the most widely used methods of nondestructive inspection. Its primary application in the
inspection of metals is the detection and characterization of internal flaws; it is also used to detect surface flaws, to define
bond characteristics, to measure the thickness and extent of corrosion, and (much less frequently) to determine physical
properties, structure, grain size, and elastic constants.
Note
* Charles J. Hellier, Chairman,
Hellier Associates, Inc.; William Plumstead, Bechtel Corporation; Kenneth
Fowler, Panametrics, Inc.; Robert Gr
ills, Glenn Andrews, and Mike C. Tsao, Ultra Image International;
James J. Snyder, Westinghouse Electric Corporation, Oceanic Division; J.F. Cook and D.A. Aldrich,
Idaho National Engineering Laboratory, EG&G Idaho, Inc.; Robert W. Pepper, Textron Specialty
Materials
Ultrasonic Inspection
Revised by Yoseph Bar-Cohen, Douglas Aircraft Company, McDonnell Douglas Corporation; Ajit K. Mal, University of California, Los
Angeles; and the ASM Committee on Ultrasonic Inspection
*
Basic Equipment
Most ultrasonic inspection systems include the following basic equipment:
• An electronic signal generator that produces bursts of alterna
ting voltage (a negative spike or a square
wave) when electronically triggered
•
A transducer (probe or search unit) that emits a beam of ultrasonic waves when bursts of alternating
voltage are applied to it
• A couplant to transfer energy in the beam of ultrasonic waves to the testpiece
•
A couplant to transfer the output of ultrasonic waves (acoustic energy) from the testpiece to the
transducer
•
A transducer (can be the same as the transducer initiating the sound or it can be a separate one) to accept
and co
nvert the output of ultrasonic waves from the testpiece to corresponding bursts of alternating
voltage. In most systems, a single transducer alternately acts as sender and receiver
• An electronic device to amplify and, if necessary, demodulate or otherwise
modify the signals from the
transducer
•
A display or indicating device to characterize or record the output from the testpiece. The display device
may be a CRT, sometimes referred to as an oscilloscope; a chart or strip recorder; a marker, indicator, or
alarm device; or a computer printout
•
An electronic clock, or timer, to control the operation of the various components of the system, to serve
as a primary reference point, and to provide coordination for the entire system
Ultrasonic Inspection
Revised by Yoseph Bar-Cohen, Douglas Aircraft Company, McDonnell Douglas Corporation; Ajit K. Mal, University of California, Los
Angeles; and the ASM Committee on Ultrasonic Inspection
*
Advantages and Disadvantages
The principal advantages of ultrasonic inspection as compared to other methods for nondestructive inspection of metal
parts are:
• Superior penetrating power, which all
ows the detection of flaws deep in the part. Ultrasonic inspection
is done routinely to thicknesses of a few meters on many types of parts and to thicknesses of about 6 m
(20 ft) in the axial inspection of parts such as long steel shafts or rotor forgings
• High sensitivity, permitting the detection of extremely small flaws
•
Greater accuracy than other nondestructive methods in determining the position of internal flaws,
estimating their size, and characterizing their orientation, shape, and nature
• Only one surface needs to be accessible
•
Operation is electronic, which provides almost instantaneous indications of flaws. This makes the
method suitable for immediate interpretation, automation, rapid scanning, in-
line production monitoring,
and process control.
With most systems, a permanent record of inspection results can be made for future
reference
•
Volumetric scanning ability, enabling the inspection of a volume of metal extending from front surface
to back surface of a part
• Nonhazardous to operations or t
o nearby personnel and has no effect on equipment and materials in the
vicinity
• Portability
•
Provides an output that can be processed digitally by a computer to characterize defects and to
determine material properties
The disadvantages of ultrasonic inspection include the following:
• Manual operation requires careful attention by experienced technicians
• Extensive technical knowledge is required for the development of inspection procedures
• Parts that are rough, irregular in shape, very small or thin, or not homogeneous are difficult to inspect
•
Discontinuities that are present in a shallow layer immediately beneath the surface may not be
detectable
•
Couplants are needed to provide effective transfer of ultrasonic wave energy between transducers and
parts being inspected
• Reference standards are needed, both for calibrating the equipment and for characterizing flaws
Ultrasonic Inspection
Revised by Yoseph Bar-Cohen, Douglas Aircraft Company, McDonnell Douglas Corporation; Ajit K. Mal, University of California, Los
Angeles; and the ASM Committee on Ultrasonic Inspection
*
Applicability
The ultrasonic inspection of metals is principally conducted for the detection of discontinuities. This method can be used
to detect internal flaws in most engineering metals and alloys. Bonds produced by welding, brazing, soldering, and
adhesive bonding can also be ultrasonically inspected. In-line techniques have been developed for monitoring and
classifying material as acceptable, salvageable, or scrap and for process control. Both line-powered and battery-operated
commercial equipment is available, permitting inspection in shop, laboratory, warehouse, or field.
Ultrasonic inspection is used for quality control and materials inspection in all major industries. This includes electrical
and electronic component manufacturing; production of metallic and composite materials; and fabrication of structures
such as airframes, piping and pressure vessels, ships, bridges, motor vehicles, machinery, and jet engines. In-service
ultrasonic inspection for preventive maintenance is used for detecting the impending failure of railroad-rolling-stock
axles, press columns, earthmoving equipment, mill rolls, mining equipment, nuclear systems, and other machines and
components.
Some of the major types of equipment that are ultrasonically inspected for the presence of flaws are:
• Mill components: Rolls, shafts, drives, and press columns
• Power equipment:
Turbine forgings, generator rotors, pressure piping, weldments, pressure vessels,
nuclear fuel elements, and other reactor components
• Jet engine parts: Turbine and compressor forgings, and gear blanks
• Aircraft components: Forging stock, frame sections, and honeycomb sandwich assemblies
• Machinery materials: Die blocks, tool steels, and drill pipe
• Railroad parts: Axles, wheels, track, and welded rail
• Automotive parts: Forgings, ductile castings, and brazed and/or welded components
The flaws to be detected include voids, cracks, inclusions, pipe, laminations, debonding, bursts, and flakes. They may be
inherent in the raw material, may result from fabrication and heat treatment, or may occur in service from fatigue, impact,
abrasion, corrosion, or other causes.
Government agencies and standards-making organizations have issued inspection procedures, acceptance standards, and
related documentation. These documents are mainly concerned with the detection of flaws in specific manufactured
products, but they also can serve as the basis for characterizing flaws in many other applications.
Ultrasonic inspection can also be used to measure the thickness of metal sections. Thickness measurements are made on
refinery and chemical-processing equipment, shop plate, steel castings, submarine hulls, aircraft sections, and pressure
vessels. A variety of ultrasonic techniques are available for thickness measurements; several use instruments with digital
readout. Structural material ranging in thickness from a few thousandths of an inch to several feet can be measured with
accuracies of better than 1%. Ultrasonic inspection methods are particularly well suited to the assessment of loss of
thickness from corrosion inside closed systems, such as chemical-processing equipment. Such measurements can often be
made without shutting down the process. Special ultrasonic techniques and equipment have been used on such diverse
problems as the rate of growth of fatigue cracks, detection of borehole eccentricity, measurement of elastic moduli, study
of press fits, determination of nodularity in cast iron, and metallurgical research on phenomena such as structure,
hardening, and inclusion count in various metals.
For the successful application of ultrasonic inspection, the inspection system must be suitable for the type of inspection
being done, and the operator must be sufficiently trained and experienced. If either of these prerequisites is not met, there
is a high potential for gross error in inspection results. For example, with inappropriate equipment or with a poorly trained
operator, discontinuities having little or no bearing on product performance may be deemed serious, or damaging
discontinuities may go undetected or be deemed unimportant.
The term flaw as used in this article means a detectable lack of continuity or an imperfection in a physical or dimensional
attribute of a part. The fact that a part contains one or more flaws does not necessarily imply that the part is
nonconforming to specification nor unfit for use. It is important that standards be established so that decisions to accept or
reject parts are based on the probable effect that a given flaw will have on service life or product safety. Once such
standards are established, ultrasonic inspection can be used to characterize flaws in terms of a real effect rather than some
arbitrary basis that may impose useless or redundant quality requirements.
Ultrasonic Inspection
Revised by Yoseph Bar-Cohen, Douglas Aircraft Company, McDonnell Douglas Corporation; Ajit K. Mal, University of California, Los
Angeles; and the ASM Committee on Ultrasonic Inspection
*
General Characteristics of Ultrasonic Waves
Ultrasonic waves are mechanical waves (in contrast to, for example, light or x-rays, which are electromagnetic waves)
that consist of oscillations or vibrations of the atomic or molecular particles of a substance about the equilibrium positions
of these particles. Ultrasonic waves behave essentially the same as audible sound waves. They can propagate in an elastic
medium, which can be solid, liquid, or gaseous, but not in a vacuum.
In many respects, a beam of ultrasound is similar to a beam of light; both are waves and obey a general wave equation.
Each travels at a characteristic velocity in a given homogeneous medium--a velocity that depends on the properties of the
medium, not on the properties of the wave. Like beams of light, ultrasonic beams are reflected from surfaces, refracted
when they cross a boundary between two substances that have different characteristic sound velocities, and diffracted at
edges or around obstacles. Scattering by rough surfaces or particles reduces the energy of an ultrasonic beam, comparable
to the manner in which scattering reduces the intensity of a light beam.
Analogy with Waves in Water. The general characteristics of sonic or ultrasonic waves are conveniently illustrated
by analogy with the behavior of waves produced in a body of water when a stone is dropped into it. Casual observation
might lead to the erroneous conclusion that the resulting outward radial travel of alternate crests and troughs represents
the movement of water away from the point of impact. The fact that water is not thus transported is readily deduced from
the observation that a small object floating on the water does not move away from the point of impact, but instead merely
bobs up and down. The waves travel outward only in the sense that the crests and troughs (which can be compared to the
compressions and rarefactions of sonic waves in an elastic medium) and the energy associated with the waves propagate
radially outward. The water particles remain in place and oscillate up and down from their normal positions of rest.
Continuing the analogy, the distance between two successive crests or troughs is the wavelength, . The fall from a crest
to a trough and subsequent rise to the next crest (which is accomplished within this distance) is a cycle. The number of
cycles in a specific unit of time is the frequency, f, of the waves. The height of a crest or the depth of a trough in relation
to the surface at equilibrium is the amplitude of the waves.
The velocity of a wave and the rates at which the amplitude and energy of a wave decrease as it propagates are constants
that are characteristic of the medium in which the wave is propagating. Stones of equal size and mass striking oil and
water with equal force will generate waves that travel at different velocities. Stones impacting a given medium with
greater energy will generate waves having greater amplitude and energy but the same wave velocity.
The above attributes apply similarly to sound waves, both audible and ultrasonic, propagating in an elastic medium. The
particles of the elastic medium move, but they do not migrate from their initial spacial orbits; only the energy travels
through the medium. The amplitude and energy of sound waves in the elastic medium depend on the amount of energy
supplied. The velocity and attenuation (loss of amplitude and energy) of the sound waves depend on the properties of the
medium in which they are propagating.