ASM Metals HandBook Vol. 17 - Nondestructive Evaluation and Quality Control
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Fig. 9
Flux leakage inspection of a bearing race. (a) Magnetization of inner race. (b) Perturbation in the
magnetic flux at the surface of the inner race. (c) Probe scanning the surface
Finally, the flux leakage method has also been adapted to the inspection of steel reinforcement in concrete beams (Ref 46,
47). The basic function of the magnetic field disturbance (MFD) inspection equipment is to provide maps of the magnetic
field across the bottom and sides of the beam. An electromagnet on an inspection cart, which is suspended on tracks
below the beam, provides a magnetic field that induces magnetization in permeable structures in its vicinity, such as steel
rebars, cables, and stirrups. An array of Hall effect sensors distributed across the bottom and sides of the beam measures
the field produced by magnetized structures within the beam. If a flaw is present in one of these magnetized structures, it
will produce a disturbance of the normal magnetic field pattern associated with the unflawed beam. Thus, the idea behind
the MFD system is to search the surface of the beam for field anomalies that indicate the presence of flaws in reinforcing
steel within the structure.
A flaw, such as a broken wire in a cable or a fractured rebar, produces a distinctive magnetic field anomaly that depends
on the size of the discontinuity and its distance from the sensor. Because the signal shape that results from such an
anomaly is known, flaw detection is enhanced by searching magnetic field records for specific signal shapes, that is, those
that are characteristic of discontinuities in magnetic materials. In the MFD system, this is accomplished by a computer
program that compares signal shapes with typical flaw signal shapes. The program produces a correlation coefficient that
serves as a measure of similarity of the observed signal shape to a typical flaw signal shape. Flaw detection is therefore
not only enhanced by signal shape discrimination but also automated by computer processing of the magnetic field data.
Laboratory tests have demonstrated the ability of the system to detect fracture in steel rebars and cables in a large
prestressed concrete structure (Ref 47). Also planned are field tests of the equipment in the inspection of bridge decks for
reinforcement corrosion damage.
Nondestructive Characterization of Materials. Only two examples of magnetic methods for monitoring material
properties are given because the examples chosen should suffice to illustrate the types of tests that might be employed.
Measurements of magnetic characteristics can, however, provide a wealth of data, and various features of such data can
yield information on different material properties. For example, it has been demonstrated that different features of
magnetic hysteresis data can be interpreted in terms of heat treatment and microstructure, plastic deformation, residual
stress, and mechanical hardness (Ref 25).
An example of the effects of mechanical hardness on hysteresis data is shown in Fig. 10. These data were obtained in the
absence of applied tensile stress with the experimental arrangement shown in Fig. 7. Specimens of different hardness
were prepared by tempering at different temperatures. The grain size (ASTM No. 7) was the same for all four specimens
used in these tests. Other data showed, however, that grain size has little effect on hysteretic behavior for the classes of
alloys studied.
Fig. 10
Effect of mechanical hardness on hysteresis loop data. (a) AISI 410 stainless steel. (b) SAE 4340 steel.
Source: Ref 32
The main point illustrated in Fig. 10 is that the mechanically harder specimens of the same alloy are also harder to
magnetize; that is, the flux density, B, obtained at a large value of H is smaller for mechanically harder specimens than for
softer specimens. For one alloy, AISI 410 stainless steel, the hysteresis loop intersects the B = 0 axis at larger values of H
for the harder specimen than for the softer specimen; that is, the coercive force is greater for the harder material.
However, for the other material, SAE 4340 steel, the coercive force does not change with hardness. This suggests that, for
the two alloys considered here, the saturation flux density provides a more reliable measure of hardness than the coercive
force.
Mayos et al. (Ref 48) used two quite different techniques to measure the depth of surface decarburization of steels. One
method was a variation of a standard eddy current test, with the difference from standard practice being that eddy current
probe response was measured in the presence of a low-frequency (~0.1 Hz) magnetic field. This arrangement provides a
measure of incremental permeability, that is, the magnetic permeability corresponding to changes in the applied field
about some quasistatic value. The second method employed was Barkhausen noise analysis.
Depth of decarburization was analyzed by varying the frequency of the excitation field, thus changing the skin depth in
the material. Experiments were performed with both artificial samples containing two layers of different carbon content
and industrial samples in which carbon concentration varied smoothly with distance from the surface.
It was shown that certain features of both Barkhausen noise and incremental permeability data can be correlated with
depth of decarburization. The Barkhausen noise method showed a somewhat stronger sensitivity to depth, but was useful
over a smaller range of depths than the incremental permeability method. It can be concluded that both methods are
useful, with the optimum choice depending on accuracy requirements and the expected depth of decarburization.
References cited in this section
25.
D.C. Jiles, Evaluation of the Propertie
s and Treatment of Ferromagnetic Steels Using Magnetic
Measurements, in
Proceedings of the Second National Seminar on Nondestructive Evaluation of
Ferromagnetic Materials, Dresser Atlas, 1986
32.
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
35.
P.E. Khalileev and P.A. Grigor'ev, Methods of Testing the Condition of Underground Pipes in Main
Pipelines (Review), Sov. J. NDT, Vol 10 (No. 4), July-Aug 1974, p 438-459
36.
W.M. Rogers, New Methods for In Place Inspection of Pipelines, in
Proceedings of the 16th Mechanical
Working and Steel Processing Conference (Dolton, IL), Iron and Steel Society of AIME, 1974, p 471-479
37.
J.A. Birdwell, F
.N. Kusenberger, and J.R. Barton, "Development of Magnetic Perturbation Inspection
System (A02GS005-1), for CH-
46 Rotor Blades," P.A. No. CA375118, Technical Summary Report for
Vertol Division, The Boeing Company, 11 Oct 1968
38.
J.R. Barton, "Feasibility Investigation for Sun Gear Inspection," P.A. NA-
380695, Summary Report for
Vertol Division, The Boeing Company, 25 July 1968
39.
R.D. Williams and J.R. Barton, "Magnetic Perturbation Inspection of Artillery Projectiles," AMMRC CTR
77-23, Final Report, Contract DAAG46-76-C-
0075, Army Materials and Mechanics Research Center, Sept
1977
40.
M.C. Moyer and B.A. Dale, An Automated Thread Inspection Device for the Drill String, in
Proceedings of
the First National Seminar on Nondestructive Inspection of Ferromagnetic Materials, Dresser Atlas, 1984
41.
H.H. Haynes and L.D. Underbakke, "Nondestructive Test Equipment for Wire Rope," Report TN-
1594,
Civil Engineering Laboratory, Naval Construction Battalion Center, 1980
42.
J.R. Barton, J. Lankford, Jr., and P.L.
Hampton, Advanced Nondestructive Testing Methods for Bearing
Inspection, Trans. SAE, Vol 81, 1972, p 681
43.
F.N. Kusenberger and J.R. Barton, "Development of Diagnostic Test Equipment for Inspection Antifriction
Bearings," AMMRL CTR 77-13, Final Report, Contract Nos. DAAG46-74-C-0012 and DAAG46-75-C-
0001, U.S. Army Materials and Mechanics Research Center, March 1977
44.
J.R. Barton and J. Lankford, "Magnetic Perturbation Inspection of Inner Bearing Races," NASA CR-
2055,
National Aeronautics and Space Administration, May 1972
45.
R.J. Parker, "Correlation of Magnetic Perturbation Inspection Data With Rolling-
Element Bearing Fatigue
Results," Paper 73-Lub-37, American Society of Mechanical Engineers, Oct 1973
46.
F.N. Kusenberger and J.R. Barton, "Detecti
on of Flaws in Reinforcing Steel in Prestressed Concrete Bridge
Members," Interim Report on Contract No. DOTFH-11-8999, Federal Highway Administration, 1977
47.
R.E. Beissner, C.E. McGinnis, and J.R. Barton, "Laboratory Test of Magnetic Field Disturbance
(MFD)
System for Detection of Flaws in Reinforcing Steel," Final Report on Contract No. DTFH61-80-C-
00002,
Federal Highway Administration, 1984
48.
M. Mayos, S. Segalini, and M. Putignani, Electromagnetic Nondestructive Evaluation of Surface
Decarburization on Steels: Feasibility and Possible Application, in
Review of Progress in Quantitative
NDE, Vol 6. D.O. Thompson and D.E. Chimenti, Ed., Plenum Press, 1987
Magnetic Field Testing
R.E. Beissner, Southwest Research Institute
References
1. R.E. Beissner,
G.A. Matzkanin, and C.M. Teller, "NDE Applications of Magnetic Leakage Field
Methods," Report NTIAC-80-1, Southwest Research Institute, 1980
2.
G. Dobmann, Magnetic Leakage Flux Techniques in NDT: A State of the Art Survey of the Capabilities
for Defect Detection and Sizing, in Electromagnetic Methods of NDT,
W. Lord, Ed., Gordon and Breach,
1985
3. P. Höller and G. Dobmann, Physical Analysis Methods of Magnetic Flux Leakage, in
Research
Techniques in NDT, Vol IV, R.S. Sharpe, Ed., Academic Press, 1980
4. F. Förster, Magnetic Findings in the Fields of Nondestructive Magnetic Leakage Field Inspection,
NDT
Int., Vol 19, 1986, p 3
5. F. Förster, Magnetic Leakage Field Method of Nondestructive Testing, Mater. Eval., Vol 43, 1985, p 1154
6. F. Förster, Magnetic Leakage Field Method of Nondestructive Testing (Part 2), Mater. Eval.,
Vol 43,
1985, p 1398
7.
F. Förster, Nondestructive Inspection by the Method of Magnetic Leakage Fields. Theoretical and
Experimental Foundations of the Detection of Surface Cracks of Finite and Infinite Depth, Sov. J. NDT,
Vol 18, 1982, p 841
8.
F. Förster, Theoretical and Experimental Developments in Magnetic Stray Flux Techniques for Defect
Detection, Br. J. NDT, Nov 1975, p 168-171
9. J.R. Barton and F.N. Kusenberger, "Magnetic
Perturbation Inspection to Improve Reliability of High
Strength Steel Components," Paper 69-DE-
58, presented at a conference of the Design Engineering
Division of ASME, New York, American Society of Mechanical Engineers, May 1969
10. R.C. McMaster, Nondestructive Testing Handbook, Vol II, Section 30, The Ronald Press Company, 1959
11. H.J. Bezer, Magnetic Methods of Nondestructive Testing, Part 1, Br. J. NDT, Sept 1964, p 85-
93; Part 2,
Dec 1964, p 109-122
12. F.W. Dunn, Magnetic Particle Inspection Fundamentals, Mater. Eval., Dec 1977, p 42-47
13. C.N. Owston, The Magnetic Leakage Field Technique of NDT, Br. J. NDT, Vol 16, 1974, p 162
14. F. Förster, Non-Destructive Inspection of Tubing and Round Billets by Means of Leakage Flux Probes,
Br.
J. NDT, Jan 1977, p 26-32
15. A. Michio and T. Yamada, Silicon Magnetodiode, in
Proceedings of the Second Conference on Solid State
Devices (Tokyo), 1970; Supplement to J. Jpn. Soc. Appl. Phys., Vol 40, 1971, p 93-98
16. B. Auld and C.M. Fortunko, "Flaw Detection
With Ferromagnetic Resonance Probes," Paper presented at
the ARPA/AFML Review of Progress in Quantitative NDE, Scripps Institution of Oceanography, July
1978
17. F. Förster, Development in the Magnetography of Tubes and Tube Welds, Non-Destr. Test., Dec
1975, p
304-308
18. W. Stumm, Tube Testing by Electromagnetic NDE Methods-1, Non-Destr. Test., Oct 1974, p 251-256
19.
R.E. Beissner, G.L. Burkhardt, M.D. Kilman, and R.K. Swanson, Magnetic Leakage Field Calculations for
Spheroidal Inclusions, in Proceed
ings of the Second National Seminar on Nondestructive Evaluation of
Ferromagnetic Materials, Dresser Atlas, 1986
20. G.P. Harnwell, Principles of Electricity and Magnetism, 2nd ed., McGraw-Hill, 1949
21. C.G. Gardner and F.N. Kusenberger, Quantitative No
ndestructive Evaluation by the Magnetic Field
Perturbation Method, in
Prevention of Structural Failure: The Role of Quantitative Nondestructive
Evaluation,
T.D. Cooper, P.F. Packman, and B.G.W. Yee, Ed., No. 5 in the Materials/Metalworking
Technology Series, American Society for Metals, 1975
22.
M.J. Sablik and R.E. Beissner, Theory of Magnetic Leakage Fields From Prolate and Oblate Spheroidal
Inclusions, J. Appl. Phys., Vol 53, 1982, p 8437
23. J.H. Hwang and W. Lord, Magnetic Leakage Field Signatures of Material Discontinuities, in
Proceedings
of the Tenth Symposium on NDE (San Antonio, TX), Southwest Research Institute, 1975
24. J.F. Bussière, On Line Measurement of the Microstructure and Mechanical Properties of Steel,
Mater.
Eval., Vol 44, 1986, p 560
25.
D.C. Jiles, Evaluation of the Properties and Treatment of Ferromagnetic Steels Using Magnetic
Measurements, in
Proceedings of the Second National Seminar on Nondestructive Evaluation of
Ferromagnetic Materials, Dresser Atlas, 1986
26. R.M. Bozorth, Ferromagnetism, Van Nostrand, 1951
27. C. Kittel and J.K. Galt, Ferromagnetic Domain Theory, in Solid State Physics,
Vol 3, Academic Press,
1970
28. S. Chikazuma and S.H. Charap, Physics of Magnetism, John Wiley & Sons, 1964
29.
Proceedings of the 3rd International Symposium on Nondestructive Characterization of Materials,
Springer-Verlag, to be published
30.
Proceedings of the Third National Seminar on Nondestructive Evaluation of Ferromagnetic Materials,
Atlas Wireline Services, 1988
31. D.O. Thompson and D.E. Chimenti, Ed., Review of Progress in Quantitative NDE,
Vol 7, Plenum Press,
1988
32.
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
33. J.C. McClure and K. Schroeder, The Magnetic Barkhausen Effect, CRC Crit. Rev. Solid State Sci.,
Vol 6,
1976, p 45
34.
G.A. Matzkanin, R.E. Beissner, and C.M. Teller, "The Barkhausen Effect and Its Applications to
Nondestructive Evaluation," Report NTIAC-79-2, Southwest Research Institute, 1979
35.
P.E. Khalileev and P.A. Grigor'ev, Methods of Testing the Condition of Underground Pipes in Main
Pipelines (Review), Sov. J. NDT, Vol 10 (No. 4), July-Aug 1974, p 438-459
36. W.M. Rogers, New Methods for In Place Inspection of Pipelines, in
Proceedings of the 16th Mechanical
Working and Steel Processing Conference (Dolton, IL), Iron and Steel Society of AIME, 1974, p 471-479
37. J.A. Birdwell, F.N. Kusenberger, and J.R. Barton, "Development of Magn
etic Perturbation Inspection
System (A02GS005-1), for CH-
46 Rotor Blades," P.A. No. CA375118, Technical Summary Report for
Vertol Division, The Boeing Company, 11 Oct 1968
38. J.R. Barton, "Feasibility Investigation for Sun Gear Inspection," P.A. NA-38069
5, Summary Report for
Vertol Division, The Boeing Company, 25 July 1968
39.
R.D. Williams and J.R. Barton, "Magnetic Perturbation Inspection of Artillery Projectiles," AMMRC CTR
77-23, Final Report, Contract DAAG46-76-C-0075, Army Materials and Mechanics
Research Center, Sept
1977
40. M.C. Moyer and B.A. Dale, An Automated Thread Inspection Device for the Drill String, in
Proceedings
of the First National Seminar on Nondestructive Inspection of Ferromagnetic Materials,
Dresser Atlas,
1984
41. H.H. Haynes and L.D. Underbakke, "Nondestructive Test Equipment for Wire Rope," Report TN-
1594,
Civil Engineering Laboratory, Naval Construction Battalion Center, 1980
42. J.R. Barton, J. Lankford, Jr., and P.L. Hampton, Advanced Nondestructive Testing Methods for B
earing
Inspection, Trans. SAE, Vol 81, 1972, p 681
43.
F.N. Kusenberger and J.R. Barton, "Development of Diagnostic Test Equipment for Inspection
Antifriction Bearings," AMMRL CTR 77-13, Final Report, Contract Nos. DAAG46-74-C-
0012 and
DAAG46-75-C-0001, U.S. Army Materials and Mechanics Research Center, March 1977
44. J.R. Barton and J. Lankford, "Magnetic Perturbation Inspection of Inner Bearing Races," NASA CR-
2055,
National Aeronautics and Space Administration, May 1972
45. R.J. Parker, "Correlation of Magnetic Perturbation Inspection Data With Rolling-
Element Bearing Fatigue
Results," Paper 73-Lub-37, American Society of Mechanical Engineers, Oct 1973
46. F.N. Kusenberger and J.R. Barton, "Detection of Flaws in Reinforcing Steel in Prestressed Concre
te
Bridge Members," Interim Report on Contract No. DOTFH-11-
8999, Federal Highway Administration,
1977
47.
R.E. Beissner, C.E. McGinnis, and J.R. Barton, "Laboratory Test of Magnetic Field Disturbance (MFD)
System for Detection of Flaws in Reinforcing Steel," Final Report on Contract No. DTFH61-80-C-
00002,
Federal Highway Administration, 1984
48.
M. Mayos, S. Segalini, and M. Putignani, Electromagnetic Nondestructive Evaluation of Surface
Decarburization on Steels: Feasibility and Possible Application, in
Review of Progress in Quantitative
NDE, Vol 6. D.O. Thompson and D.E. Chimenti, Ed., Plenum Press, 1987
Electric Current Perturbation NDE
Gary L. Burkhardt and R.E. Beissner, Southwest Research Institute
Introduction
ELECTRIC CURRENT PERTURBATION (ECP) is an electromagnetic nondestructive evaluation method for detecting
and characterizing defects in nonferromagnetic material. Laboratory evaluations have shown that this method can detect
very small surface and subsurface cracks in both low- and high-conductivity metals (for example, titanium and aluminum
alloys). Results from experiments and an analytical ECP model confirm that linear relationships exist between the ECP
signal amplitude and the crack interfacial area and between the signal peak-to-peak separation and the crack length. This
article will discuss the principles of the ECP method, typical inspection results, and crack characterization results.
Electric Current Perturbation NDE
Gary L. Burkhardt and R.E. Beissner, Southwest Research Institute
Background
The principle by which ECP detects flaws is illustrated in Fig. 1. An electric current density, j
0
, is introduced into the
region to be inspected, thus producing an associated magnetic-flux density, B
0
(Fig. 1a). A flaw perturbs the current flow,
as shown in Fig. 1(b), and the flux density is changed by an amount ΔB. Flaw detection is accomplished by sensing this
change in flux density, ΔB, with a magnetic-field sensing device (Fig. 2). The sensor coils are oriented to detect the x-axis
component of the magnetic flux.
Fig. 1 Electric current density, j, and the associated magnetic flux density, B
(a) with no surface flaw and (b)
with a flaw (for example, a crack) perpendicular to the current flow
Fig. 2
ECP probe configuration and orientation with respect to a flaw in a part surface. The size difference
between the sensor and the induction coil is much greater than shown.
In most applications, the ECP method makes use of an induction coil, much like that of a conventional eddy-current
probe, to provide the unperturbed current density, j
0
, in the region to be inspected. However, unlike conventional eddy-
current methods, the use of a coil to induce an alternating current is not essential. Experiments have in fact used not only
ac injection but also dc injection, with results similar to those obtained by induction. When direct current is injected, a
Hall effect sensor is used instead of coils.
Another difference between ECP and conventional eddy-current methods lies in the orientation of the sensor. The exciter
coil and the sensor coil axes in the ECP method are perpendicular (Fig. 2), which reduces or eliminates direct coupling
with B
0
. In addition, the use of differential sensors tends to cancel the effect of remaining coupling. These decoupling
features allow the excitation to be much stronger in the ECP method, yielding gains in sensitivity. Probe-to-surface
coupling is also minimized, and sensitivity to probe liftoff variations is greatly reduced (Ref 1). Liftoff is a major source
of noise with probes using conventional eddy-current coil orientations. The end result is that ECP probes have excellent
signal-to-noise characteristics, making them well suited to the detection of small flaws. The penalty for this geometry is
directionality: The sensing coils select specific current perturbations and ignore others.
In practice, one noticeable difference between the ECP and conventional eddy-current method is that the frequencies
employed with ECP are almost always much lower. The difference in operating frequencies results from the requirement
for eddy-current probes to use higher frequencies to reduce liftoff noise by separating the liftoff component of the signal
from the flaw component (Ref 2). This is not necessary with ECP because the probe design inherently reduces liftoff
noise. Therefore, while many eddy-current probes designed for surface-flaw detection are configured for frequencies of
several hundred kilohertz to several megahertz, ECP probes are normally used at frequencies below 100 kHz. Using lower
frequencies allows the ECP electronics to be simplified.
A block diagram of a typical ECP system is shown in Fig. 3. A signal generator drives a current source to provide
excitation current for the induction coil in the ECP probe. The sensor output is directed to an amplifier and a phase-
sensitive detector. Devices such as a digitizing oscilloscope or chart recorder can also be used for recording and
displaying ECP signals.
Fig. 3 Block diagram of a typical ECP system
References cited in this section
1.
R.E. Beissner and M.J. Sablik, Theory of Electric Current Perturbation Probe Optimization,
Rev.Prog.
Quant. NDE, Vol 3A, 1984, p 633-641
2.
F. Forster and H.L. Libby, Probe Coil-Detection of Cracks in Nonmagnetic Materials, in
Nondestructive
Testing Handbook, Vol 4, Electromagnetic Testing,
R.C. McMaster, P. McIntire, and M.L. Mester, Ed., 2nd
ed., American Society for Nondestructive Testing, 1986, p 185-187
Electric Current Perturbation NDE
Gary L. Burkhardt and R.E. Beissner, Southwest Research Institute
Typical Applications
The following examples describe the evaluation of the ECP method for detecting flaws in a variety of parts. In these
examples, the evaluation first involved the machining of small notches in the surfaces of the parts. These notches served
as test flaws.
In each of the examples, the induced current, j
0
, was perpendicular to the length of the notches. The ECP sensors scanned
the surface to determine the ECP response to the test flaws. The scan direction was perpendicular to the flaw in Examples
1 and 5, but was parallel to the flaw in Examples 2, 3, and 4. When the scan direction is perpendicular, a unipolar ECP
signal is produced, and when the scan direction is parallel to the flaw, a bipolar ECP signal is produced. The ECP signal
shapes for perpendicular and parallel scans are shown in Fig. 4 and 5.