ASM Metals HandBook Vol. 17 - Nondestructive Evaluation and Quality Control
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Fig. 5 Dependence of acoustic emission during the magnetization of low-
carbon steel on stress. Total gain: 80
dB. Magnetic field strength (rms value): 13,000 A/m (160 Oe) for curve A; 6400 A/m (80 Oe) for curve B.
Source: Ref 13
Capabilities and Limitations. Because of the eddy current screening, the inductively detected Barkhausen noise
signals reflect the activity occurring very near the surface of the specimen to a depth of approximately 0.1 mm (0.004 in.).
Therefore, the Barkhausen noise technique is suitable for measuring near-surface stresses. The effective stress-
measurement range is up to about 50% of the yield stress of the material because the change in the Barkhausen noise with
stress becomes saturated at these high stress levels.
Barkhausen measurements can usually be made within a few seconds. Continuous measurements at a slow scanning speed
( 10 mm/s, or 0.4 in./s) are possible. Preparation of the surface of a part under testing is generally not required. Portable,
field-usable Barkhausen instruments are available.
The results of Barkhausen noise measurements are also sensitive to factors not related to stress, such as microstructure,
heat treatment, and material variations. Careful instrument calibration and data analysis are essential for reliable stress
measurements. As can be seen in Fig. 4, the Barkhausen amplitude at zero stress is approximately isotropic and shows no
dependence on the relative orientation of the magnetic field and stress directions. When the specimen is subjected to a
stress, the Barkhausen noise exhibits dependence on the magnetic field direction and becomes anisotropic. This stress-
induced anisotropy in the Barkhausen noise is effective for differentiating stress from nonstress-related factors whose
effects are approximately isotropic. The accuracy of the technique is about ±35 MPa (±5 ksi).
The acoustic Barkhausen noise technique can be used in principle to measure bulk stresses in materials because the
acoustic waves travel through materials. However, practical application of this technique is currently hampered by the
difficulty in differentiating acoustic Barkhausen noise from other noise produced from surrounding environments.
The inductive Barkhausen noise technique has been used for measuring welding residual stresses (Ref 20, 22), for
detecting grinding damage in bearing races (Ref 23), and for measuring compressive hoop stresses in railroad wheels (Ref
24).
References cited in this section
1. M.R. James and O. Buck, Quantitative Nondestructive Measurements of Residual Stresses, CRC Crit.
Rev.
Solid State Mater. Sci., Vol 9, 1980, p 61
2.
C.O. Ruud, "Review and Evaluation of Nondestructive Methods for Residual Stress Measurement," Final
Report, NP-1971, Project 1395-5, Electric Power Research Institute, Sept 1981
7. R.M. Bozorth, Ferromagnetism, Van Nostrand, 1951
8.
G.A. Matzkanin, R.E. Beissner, and C.M. Teller, "The Barkhausen Effect and Its Applications to
Nondestructive Evaluation," State of the Art Report, NTIAC-79-
2, Nondestructive Testing Information
Analysis Center, Southwest Research Institute, Oct 1979
9. J.C. McClure, Jr., and K. Schroder, The Magnetic Barkhausen Effect, CRC Crit. Rev. Solid State Sci.,
Vol
6, 1976, p 45
10.
R.L. Pasley, Barkhausen Effect--An Indication of Stress, Mater. Eval., Vol 28, 1970, p 157
11.
S. Tiitto, On the Influence of Microstructure on Magnetization Transitions in Steel, Acta Polytech. Scand.,
No. 119, 1977
12.
R. Rautioaho, P. Karjalainen, and M. Moilanen, Stress Response of Barkhausen Noise and Coercive Force
in 9Ni Steel, J. Magn. Magn. Mater., Vol 68, 1987, p 321
13.
H. Kusanagi, H. Kimura, and H. Sasaki, Acoustic Emission Characteristics During Magnetization of
Ferromagnetic Materials, J. Appl. Phys., Vol 50, 1979, p 2985
14.
D.C. Jiles, Review of Magnetic Methods for Nondestructive Evaluation, NDT Int., Vol
21 (No. 5), 1988, p
311-319
15.
K. Ono and M. Shibata, Magnetomechanical Acoustic Emission of Iron and Steels, Mater. Eval.,
Vol 38,
1980, p 55
16.
M. Shibata and K. Ono, Magnetomechanical Acoustic Emission--
A New Method for Nondestructive Stress
Measurement, NDT Int., Vol 14, 1981, p 227
17.
K. Ono, M. Shibata, and M.M. Kwan, Determination of Residual Stress by Magnetomechanical Acoustic, in
Residual Stress for Designers and Metallurgists, L.J. Van de Walls, Ed., American Society for Metals, 1981
18.
G
.L. Burkhardt, R.E. Beissner, G.A. Matzhanin, and J.D. King, Acoustic Methods for Obtaining
Barkhausen Noise Stress Measurements, Mater. Eval., Vol 40, 1982, p 669
19.
K. Ono, "Magnetomechanical Acoustic Emission--A Review," Technical Report TR-86-02, Uni
versity of
California at Los Angeles, Sept 1986
20.
G.L. Burkhardt and H. Kwun, "Residual Stress Measurement Using the Barkhausen Noise Method," Paper
45, presented at the 15th Educational Seminar for Energy Industries, Southwest Research Institute, April
1988
21.
H. Kwun, Investigation of the Dependence of Barkhausen Noise on Stress and the Angle Between the Stress
and Magnetization Directions, J. Magn. Magn. Mater., Vol 49, 1985, p 235
22.
L.P. Karjalainen, M. Moilanen, and R. Rautioaho, Evaluating the
Residual Stresses in Welding From
Barkhausen Noise Measurements, Materialprüfung, Vol 22, 1980, p 196
23.
J.R. Barton and F.M. Kusenberger, "Residual Stresses in Gas Turbine Engine Components From
Barkhausen Noise Analysis," Paper 74-GT-51, presented at
the ASME Gas Turbine Conference, Zurich,
Switzerland, American Society of Mechanical Engineers, 1974
24.
J.R. Barton, W.D. Perry, R.K. Swanson, H.V. Hsu, and S.R. Ditmeyer, Heat-
Discolored Wheels: Safe to
Reuse?, Prog. Railroad., Vol 28 (No. 3), 1985, p 44
Electromagnetic Techniques for Residual Stress Measurements
H. Kwun and G.L. Burkhardt, Southwest Research Institute
Nonlinear Harmonics
Because of the magnetic hysteresis and nonlinear permeability, the magnetic induction, B, of a ferromagnetic material
subjected to a sinusoidal, external magnetic field, H, is not sinusoidal but distorted, as illustrated in Fig. 6. This distorted
waveform of the magnetic induction contains odd harmonic frequencies of the applied magnetic field. Mechanical
stresses greatly influence the magnetic hysteresis and permeability of the material (Ref 7). An example of the stress
effects on the hysteresis loops is shown in Fig. 7. Accordingly, the harmonic content of the magnetic induction is also
sensitive to the stress state in the material. With the nonlinear harmonics technique, these harmonic frequencies are
detected, and their amplitudes are related to the state of stress in the material (Ref 26, 27).
Fig. 6 Distortion of m
agnetic induction caused by hysteresis and nonlinearity in magnetization curve. The curve
for magnetic induction, B, is not a pure sinusoid; it has more rounded peaks.
Fig. 7
Hysteresis loops of an AISI 410 stainless steel specimen having ASTM No. 1 grain size and a hardness of
24 HRC under various levels of uniaxial stress. x-axis: 1600 A/m (20 Oe) per division; y-axis: 0.5 T (
5 kG) per
division. Source: Ref 25
Instrumentation. The nonlinear harmonics technique is implemented with the arrangement shown schematically in
Fig. 8. The magnetic field is applied to a specimen with an excitation coil, and the resulting magnetic induction is
measured with a sensing coil. A sinusoidal current of a given frequency is supplied to the excitation coil with a function
generator (or oscillator) and a power amplifier. The induced voltage in the sensing coil is amplified, and the harmonic
frequency content of the signal is analyzed. The amplitude of the harmonic frequency, typically the third harmonics, is
used to determine the stress.
Fig. 8 Block diagram of nonlinear harmonics instrumentation
Stress Dependence. The harmonic amplitudes are dependent on the stress as well as the relative orientation between
the stress and the applied magnetic field directions. Like the stress dependence of the Barkhausen noise amplitude
illustrated in Fig. 4, the harmonic amplitude for materials with a positive magnetostriction increases with tension when
the direction of the stress and the applied field are parallel (Ref 27). When the directions are perpendicular, the opposite
result is obtained. As with Barkhausen noise, the nonlinear harmonics depend on strain and can be used to determine
stress.
Capabilities and Limitations. The nonlinear harmonics technique can be used to measure near-surface stresses, with
sensing depth approximately equal to the skin depth of the applied magnetic field. Because the skin depth is a function of
the frequency of the applied magnetic field, the depth of sensing can be changed by varying the frequency. Therefore, the
technique can potentially be used to measure stress variations with depth.
The results of nonlinear harmonic measurements are sensitive to factors not related to stress, such as microstructure, heat
treatment, and material variations. The stress-induced anisotropy in the harmonic amplitude has been shown to be
effective for differentiating stress from factors not related to stress (Ref 27). When the stress-induced anisotropy is used
for stress determination, the accuracy of the technique is about ±35 MPa (±5 ksi). The range of stress to which the
technique is effective is up to about 50% of the yield stress of the material, with the response becoming saturated at
higher stress levels. With this technique, it would be feasible to measure stress while scanning a part at a high speed ( 10
m/s, or 30 ft/s); therefore, this technique has potential for rapidly surveying stress states in pipelines or continuously
welded railroad rails (Ref 28).
References cited in this section
7. R.M. Bozorth, Ferromagnetism, Van Nostrand, 1951
25.
H. Kwun and G.L. Burkhardt, Effects of Grain
Size, Hardness, and Stress on the Magnetic Hysteresis Loops
of Ferromagnetic Steels, J. Appl. Phys., Vol 61, 1987, p 1576
26.
N. Davis, Magnetic Flux Analysis Techniques, in Research Techniques in Nondestructive Testing,
Vol II,
R.S. Sharpe, Ed., Academic Press, 1973, p 121
27.
H. Kwun and G.L. Burkhardt, Nondestructive Measurement of Stress in Ferromagnetic Steels Using
Harmonic Analysis of Induced Voltage, NDT Int., Vol 20, 1987, p 167
28.
G.L. Burkhardt and H. Kwun, Application of the Nonlinear Harmo
nics Method to Continuous Measurement
of Stress in Railroad Rail, in
Proceedings of the 1987 Review of Progress in Quantitative Nondestructive
Evaluation, Vol 713, D.O. Thompson and D.E. Chimenti, Ed., Plenum Press, 1988, p 1413
Electromagnetic Techniques for Residual Stress Measurements
H. Kwun and G.L. Burkhardt, Southwest Research Institute
Magnetically Induced Velocity Changes (MIVC) for Ultrasonic Waves
Because of the magnetoelastic interaction, the elastic moduli of a ferromagnetic material are dependent on the
magnetization of the material. This phenomenon is known as the E effect (Ref 7). Consequently, the velocity of the
ultrasonic waves in the material changes when an external magnetic field is applied to the material. This MIVC for
ultrasonic waves is characteristically dependent on the stress as well as the angle between the stress direction and the
direction of the applied magnetic field (Ref 29, 30, 31, and 32). This characteristic stress dependence of the MIVC is used
for stress determination (Ref 33, 34, 35, and 36).
Instrumentation. Figure 9 shows a block diagram of instrumentation for measuring MIVC. An electromagnet is used
to apply a biasing magnetic field to the specimen. The applied magnetic field is measured with a Hall probe. An
ultrasonic transducer is used to transmit ultrasonic waves and to detect signals reflected from the back surface of the
specimen. For surface waves, separate transmitting and receiving transducers are used. The shift in the arrival time of the
received ultrasonic wave caused by the velocity change due to the applied magnetic field is detected with an ultrasonic
instrument. Because MIVC is a small effect (of the order of only 0.01 to 0.1%), the measurements are typically made
using the interferometer principle, called the phase comparison technique, in the ultrasonic instrumentation (Ref 33).
Fig. 9 Block diagram of instrumentation for measuring MIVC for ultrasonic waves
Stress Dependence. A typical stress dependence of the MIVC is illustrated in Fig. 10. At zero stress, the MIVC at
first generally increases rapidly with the applied magnetic field, H, and then gradually levels off toward a saturation
value. When the material is subjected to stress, the magnitude of the MIVC decreases, and the shape of the MIVC curve
as a function of H changes. Under tension, the shape of the MIVC curve remains similar to that at zero stress, but with
reduced magnitude in proportion to the stress level. Under compression, the MIVC curve exhibits a minimum, which
drastically changes the shape and reduces the magnitude. The magnitude of the minimum and the value of H where the
minimum occurs increase with stress level.
Fig. 10 Schematic showing the change in ultrasonic velocity, V, with magnetic field H
under various stress
levels,
The detailed stress dependence of MIVC, however, varies with the mode of ultrasonic wave used (longitudinal, shear, or
surface) and with the relative orientation between the stress and the magnetic field directions (Ref 29, 30, 31, 32, 33, 34,
and 35). The stress dependence shown in Fig. 10 holds for longitudinal waves in materials with a positive
magnetostriction coefficient (Ref 31, 33). With this technique, the stress state in the material, including the magnitude,
direction, and sign (tensile or compressive) of the stress, is characterized by analyzing the shape and magnitude of the
MIVC curves measured at two or more different magnetic field directions.
Capabilities and Limitations. The MIVC technique can be used to measure bulk and surface stresses by applying
both bulk (shear or longitudinal) and surface ultrasonic waves. A measurement can be made within a few seconds.
Because the magnitude of MIVC depends on material type, reference or calibration curves must be established for that
material type prior to stress measurements. However, this technique is insensitive to variations in the texture and
composition of nominally the same material. The accuracy in stress measurements is about ±35 MPa (±5 ksi). This
technique has been used to measure residual welding stresses (Ref 34) and residual hoop stresses in railroad wheels Ref
36).
A relatively large electromagnet is needed to magnetize the part under investigation and may be cumbersome to handle in
practical applications. Because of difficulty in magnetizing complex-geometry parts, the application of the technique is
limited to simple geometry parts.
References cited in this section
7. R.M. Bozorth, Ferromagnetism, Van Nostrand, 1951
29.
H. Kwun and C.M. Teller, Tensile Stress Dependence of Magnetically Induced Ultrasonic Shear Wave
Velocity Change in Polycrystalline A-36 Steel, Appl. Phys. Lett., Vol 41, 1982, p 144
30.
H. Kwun and C.M. Teller,
Stress Dependence of Magnetically Induced Ultrasonic Shear Wave Velocity
Change in Polycrystalline A-36 Steel, J. Appl. Phys., Vol 54, 1983, p 4856
31.
H. Kwun, Effects of Stress on Magnetically Induced Velocity Changes for Ultrasonic Longitudinal Waves
in Steels, J. Appl. Phys., Vol 57, 1985, p 1555
32.
H. Kwun, Effects of Stress on Magnetically Induced Velocity Changes for Surface Waves in Steels,
J. Appl.
Phys., Vol 58, 1985, p 3921
33.
H. Kwun, Measurement of Stress in Steels Using Magnetically Indu
ced Velocity Changes for Ultrasonic
Waves, in Nondestructive Characterization of Materials II,
J.F. Bussiere, J.P. Monchalin, C.O. Ruud, and
R.E. Green, Jr., Ed., Plenum Press, 1987, p 633
34.
H. Kwun, A Nondestructive Measurement of Residual Bulk Stresse
s in Welded Steel Specimens by Use of
Magnetically Induced Velocity Changes for Ultrasonic Waves, Mater. Eval., Vol 44, 1986, p 1560
35.
M. Namkung and J.S. Heyman, Residual Stress Characterization With an Ultrasonic/Magnetic Technique,
Nondestr. Test. Commun., Vol 1, 1984, p 175
36.
M. Namkung and D. Utrata, Nondestructive Residual Stress Measurements in Railroad Wheels Using the
Low-Field Magnetoacoustic Test Method, in
Proceedings of the 1987 Review of Progress in Quantitative
Nondestructive Evaluation, Vol 7B, D.O. Thompson and D.E. Chimenti, Ed., Plenum Press, 1988, p 1429
Electromagnetic Techniques for Residual Stress Measurements
H. Kwun and G.L. Burkhardt, Southwest Research Institute
References
1. M.R. James and O. Buck, Quantitative Nondestructive Measurements of Residual Stresses, CRC Crit.
Rev.
Solid State Mater. Sci., Vol 9, 1980, p 61
2.
C.O. Ruud, "Review and Evaluation of Nondestructive Methods for Residual Stress Measurement," Final
Report, NP-1971, Project 1395-5, Electric Power Research Institute, Sept 1981
3. W.B. Young, Ed., Residual Stress in Design, Process, and Material Selection,
Proceedings of the ASM
Conference on Residual Stress in Design, Process, and Materials Selection, Cincinnati, OH, April 1987,
ASM INTERNATIONAL, 1987
4. M.R. James and J.B. Cohen, The Measurement of Residual Stresses by X-
Ray Diffraction Techniques, in
Treatise on Materials Science and Technology--Experimental Methods,
Vol 19A, H. Herman, Ed.,
Academic Press, 1980, p 1
5. Y. H. Pao, W. Sachse, and H. Fu
kuoka, Acoustoelasticity and Ultrasonic Measurement of Residual
Stresses, in Physical Acoustics: Principles and Methods,
Vol XVII, W.P. Mason and R.M. Thurston, Ed.,
Academic Press, 1984, p 61-143
6. D.S. Hughes and J.L. Kelly, Second-Order Elastic Deformation of Solids, Phys. Rev.,
Vol 92, 1953, p
1145
7. R.M. Bozorth, Ferromagnetism, Van Nostrand, 1951
8.
G.A. Matzkanin, R.E. Beissner, and C.M. Teller, "The Barkhausen Effect and Its Applications to
Nondestructive Evaluation," State of the Art Report, NTIAC-79-
2, Nondestructive Testing Information
Analysis Center, Southwest Research Institute, Oct 1979
9. J.C. McClure, Jr., and K. Schroder, The Magnetic Barkhausen Effect, CRC Crit. Rev. Solid State Sci.,
Vol
6, 1976, p 45
10. R.L. Pasley, Barkhausen Effect--An Indication of Stress, Mater. Eval., Vol 28, 1970, p 157
11. S. Tiitto, On the Influence of Microstructure on Magnetization Transitions in Steel, Acta Polytech. Scand.,
No. 119, 1977
12. R. Rautioaho, P. Karjalainen, and M. Moilanen, Stress Respo
nse of Barkhausen Noise and Coercive Force
in 9Ni Steel, J. Magn. Magn. Mater., Vol 68, 1987, p 321
13.
H. Kusanagi, H. Kimura, and H. Sasaki, Acoustic Emission Characteristics During Magnetization of
Ferromagnetic Materials, J. Appl. Phys., Vol 50, 1979, p 2985
14. D.C. Jiles, Review of Magnetic Methods for Nondestructive Evaluation, NDT Int.,
Vol 21 (No. 5), 1988, p
311-319
15. K. Ono and M. Shibata, Magnetomechanical Acoustic Emission of Iron and Steels, Mater. Eval.,
Vol 38,
1980, p 55
16. M. Shibata and K. Ono, Magnetomechanical Acoustic Emission--
A New Method for Nondestructive
Stress Measurement, NDT Int., Vol 14, 1981, p 227
17.
K. Ono, M. Shibata, and M.M. Kwan, Determination of Residual Stress by Magnetomechanical Acoustic,
in Residual Stress for Designers and Metallurgists,
L.J. Van de Walls, Ed., American Society for Metals,
1981
18.
G.L. Burkhardt, R.E. Beissner, G.A. Matzhanin, and J.D. King, Acoustic Methods for Obtaining
Barkhausen Noise Stress Measurements, Mater. Eval., Vol 40, 1982, p 669
19. K. Ono, "Magnetomechanical Acoustic Emission--A Review," Technical Report TR-86-
02, University of
California at Los Angeles, Sept 1986
20.
G.L. Burkhardt and H. Kwun, "Residual Stress Measurement Using the Barkhausen Noise Method," Paper
45, pre
sented at the 15th Educational Seminar for Energy Industries, Southwest Research Institute, April
1988
21.
H. Kwun, Investigation of the Dependence of Barkhausen Noise on Stress and the Angle Between the
Stress and Magnetization Directions, J. Magn. Magn. Mater., Vol 49, 1985, p 235
22.
L.P. Karjalainen, M. Moilanen, and R. Rautioaho, Evaluating the Residual Stresses in Welding From
Barkhausen Noise Measurements, Materialprüfung, Vol 22, 1980, p 196
23. J.R. Barton and F.M. Kusenberger, "Residual Stresse
s in Gas Turbine Engine Components From
Barkhausen Noise Analysis," Paper 74-GT-
51, presented at the ASME Gas Turbine Conference, Zurich,
Switzerland, American Society of Mechanical Engineers, 1974
24. J.R. Barton, W.D. Perry, R.K. Swanson, H.V. Hsu, and S.R. Ditmeyer, Heat-
Discolored Wheels: Safe to
Reuse?, Prog. Railroad., Vol 28 (No. 3), 1985, p 44
25.
H. Kwun and G.L. Burkhardt, Effects of Grain Size, Hardness, and Stress on the Magnetic Hysteresis
Loops of Ferromagnetic Steels, J. Appl. Phys., Vol 61, 1987, p 1576
26. N. Davis, Magnetic Flux Analysis Techniques, in Research Techniques in Nondestructive Testing,
Vol II,
R.S. Sharpe, Ed., Academic Press, 1973, p 121
27. H. Kwun and G.L. Burkhardt, Nondestructive Measurement of Stress in Ferromagnetic
Steels Using
Harmonic Analysis of Induced Voltage, NDT Int., Vol 20, 1987, p 167
28.
G.L. Burkhardt and H. Kwun, Application of the Nonlinear Harmonics Method to Continuous
Measurement of Stress in Railroad Rail, in Proceedings of the 1987 Review of Progr
ess in Quantitative
Nondestructive Evaluation, Vol 713, D.O. Thompson and D.E. Chimenti, Ed., Plenum Press, 1988, p 1413
29.
H. Kwun and C.M. Teller, Tensile Stress Dependence of Magnetically Induced Ultrasonic Shear Wave
Velocity Change in Polycrystalline A-36 Steel, Appl. Phys. Lett., Vol 41, 1982, p 144
30.
H. Kwun and C.M. Teller, Stress Dependence of Magnetically Induced Ultrasonic Shear Wave Velocity
Change in Polycrystalline A-36 Steel, J. Appl. Phys., Vol 54, 1983, p 4856
31. H. Kwun, Effects of
Stress on Magnetically Induced Velocity Changes for Ultrasonic Longitudinal Waves
in Steels, J. Appl. Phys., Vol 57, 1985, p 1555
32. H. Kwun, Effects of Stress on Magnetically Induced Velocity Changes for Surface Waves in Steels,
J.
Appl. Phys., Vol 58, 1985, p 3921
33.
H. Kwun, Measurement of Stress in Steels Using Magnetically Induced Velocity Changes for Ultrasonic
Waves, in Nondestructive Characterization of Materials II,
J.F. Bussiere, J.P. Monchalin, C.O. Ruud, and
R.E. Green, Jr., Ed., Plenum Press, 1987, p 633
34.
H. Kwun, A Nondestructive Measurement of Residual Bulk Stresses in Welded Steel Specimens by Use of
Magnetically Induced Velocity Changes for Ultrasonic Waves, Mater. Eval., Vol 44, 1986, p 1560
35. M. Namkung and J.S. Heyman, Residual
Stress Characterization With an Ultrasonic/Magnetic Technique,
Nondestr. Test. Commun., Vol 1, 1984, p 175
36.
M. Namkung and D. Utrata, Nondestructive Residual Stress Measurements in Railroad Wheels Using the
Low-Field Magnetoacoustic Test Method, in Pr
oceedings of the 1987 Review of Progress in Quantitative
Nondestructive Evaluation, Vol 7B, D.O. Thompson and D.E. Chimenti, Ed., Plenum Press, 1988, p 1429
Eddy Current Inspection
Revised by the ASM Committee on Eddy Current Inspection
*
Introduction
EDDY CURRENT INSPECTION is based on the principles of electromagnetic induction and is used to identify or
differentiate among a wide variety of physical, structural, and metallurgical conditions in electrically conductive
ferromagnetic and nonferromagnetic metals and metal parts. Eddy current inspection can be used to:
• Measure or identify such conditions and properties as ele
ctrical conductivity, magnetic permeability,
grain size, heat treatment condition, hardness, and physical dimensions
• Detect seams, laps, cracks, voids, and inclusions
• Sort dissimilar metals and detect differences in their composition, microstructure, and other properties
•
Measure the thickness of a nonconductive coating on a conductive metal, or the thickness of a
nonmagnetic metal coating on a magnetic metal
Because eddy currents are created using an electromagnetic induction technique, the inspection method does not require
direct electrical contact with the part being inspected. The eddy current method is adaptable to high-speed inspection and,
because it is nondestructive, can be used to inspect an entire production output if desired. The method is based on indirect
measurement, and the correlation between the instrument readings and the structural characteristics and serviceability of
the parts being inspected must be carefully and repeatedly established.
Note
* V.S. Cecco, Atomic Energy of Canada L
imited, Chalk River Nuclear Laboratories; E.M. Franklin, Argonne
National Laboratory, Argonne-
West; Howard E. Houserman, ZETEC, Inc.; Thomas G. Kincaid, Boston
University; James Pellicer, Staveley NDT Technologies, Inc.; and Donald Hagemaier, Douglas Aircr
aft
Company, McDonnell Douglas Corporation
Eddy Current Inspection
Revised by the ASM Committee on Eddy Current Inspection
*
Advantages and Limitations of Eddy Current Inspection
Eddy current inspection is extremely versatile, which is both an advantage and a disadvantage. The advantage is that the
method can be applied to many inspection problems provided the physical requirements of the material are compatible
with the inspection method. In many applications, however, the sensitivity of the method to the many properties and
characteristics inherent within a material can be a disadvantage; some variables in a material that are not important in
terms of material or part serviceability may cause instrument signals that mask critical variables or are mistakenly
interpreted to be caused by critical variables.
Eddy Current Versus Magnetic Inspection Methods. In eddy current inspection, the eddy currents create their
own electromagnetic field, which can be sensed either through the effects of the field on the primary exciting coil or by
means of an independent sensor. In nonferromagnetic materials, the secondary electromagnetic field is derived
exclusively from eddy currents. However, with ferromagnetic materials, additional magnetic effects occur that are usually
of sufficient magnitude to overshadow the field effects caused by the induced eddy currents. Although undesirable, these
additional magnetic effects result from the magnetic permeability of the material being inspected and can normally be
eliminated by magnetizing the material to saturation in a static (direct current) magnetic field. When the permeability
effect is not eliminated, the inspection method is more correctly categorized as electromagnetic or magnetoinductive
inspection. Methods of inspection that depend mainly on ferromagnetic effects are discussed in the article "Magnetic
Particle Inspection" in this Volume.
Eddy Current Inspection
Revised by the ASM Committee on Eddy Current Inspection
*
Development of the Inspection Process
The development of the eddy current method of inspection has involved the use of several scientific and technological
advances, including the following:
• Electromagnetic induction
• Theory and application of induction coils
• The solution of boundary-
value problems describing the dynamics of the electromagnetic fields within
the vicinity of induction coils, an
d especially the dynamics of the electromagnetic fields, electric current
flow, and skin effect in conductors in the vicinity of such coils