ASM Metals HandBook Vol. 17 - Nondestructive Evaluation and Quality Control
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Fig. 2 Methods of magnetization. (1) Head-
shot method. (b) Magnetization with prods. (c) Magnetization with a
central conductor. (d) Longitudinal magnetization. (e) Yoke magnetization
The flaw leakage field can be detected with one of several types of magnetic field sensors. Aside from the use of
magnetic particles, the sensors most often used are the inductive coil and the Hall effect device.
The inductive coil sensor is based on Faraday's law of induction, which states that the voltage induced in the coil is
proportional to the number of turns in the coil multiplied by the time rate of change of the flux threading the coil (Ref 13).
It follows that detection of a magnetostatic field requires that the coil be in motion so that the flux through the coil
changes with time.
The principle is illustrated in Fig. 3, in which the coil is oriented so as to sense the change in flux parallel to the surface of
the specimen. If the direction of coil motion is taken as x, then the induced electromotive force, E, in volts is given by:
where N is the number of turns in the coil, A is its cross-sectional area, and B is the flux density, in Gauss, parallel to the
surface of the part. Thus, the voltage induced in the coil is proportional to the gradient of the flux density along the
direction of coil motion multiplied by the coil velocity. Figure 4 shows the flux density typical of the leakage field from a
slot, along with the corresponding signal from a search coil oriented as in Fig. 3.
Fig. 3 Flux leakage measurement using a search coil. Source: Ref 13
Fig. 4 Leakage flux and search coil signal as a function of position. Source: Ref 13
Unlike the inductive coil, which provides a measure of the flux gradient, a Hall effect sensor directly measures the
component of the flux itself in the direction perpendicular to the sensitive area of the device (Ref 1). Because the response
of a Hall effect sensor does not depend on the motion of the probe, it can be scanned over the surface to be inspected at
any rate that is mechanically convenient. In this respect, the Hall device has an advantage over the coil sensor because
there is no need to maintain a constant scanning speed during the inspection. On the other hand, Hall effect sensors are
more difficult to fabricate, are somewhat delicate compared to inductive coil sensors, and require more complex
electronics.
Other magnetic field sensors that are used less often in leakage field applications include the flux gate magnetometer (Ref
14), magnetoresistive sensors (Ref 15), magnetic resonance sensors (Ref 16), and magnetographic sensors (Ref 17), in
which the magnetic field at the surface of a part is registered on a magnetic tape pressed onto the surface.
Analysis of Leakage Field Data. In most applications of the leakage field method, there is a need not only to detect
the presence of a flaw but also to estimate its severity. This leads to the problem of flaw characterization, that is, the
determination of flaw dimensions from an analysis of leakage field data.
The most widely used method of flaw characterization is based on the assumptions that the leakage field signal amplitude
is proportional to the size of the flaw (which usually means its depth into the material) and that the signal amplitude can
therefore be taken as a direct measure of flaw severity. In situations where all flaws have approximately the same shape
and where calibration experiments show that the signal amplitude is indeed proportional to the size parameter of concern,
this empirical method of sizing works quite well (Ref 18).
There are, however, many situations of interest where flaw shapes vary considerably and where signal amplitude is not
uniquely related to flaw depth, as is the case for corrosion pits in steel tubing (Ref 19). In addition, different types of
flaws, such as cracks and pits, can occur in the same part, in which case it becomes necessary to determine the flaw types
present as well as their severity. In such cases, a more careful analysis of the relationship between signal and flaw
characteristics is required if serious errors in flaw characterization are to be avoided.
One of the earliest attempts to use a theoretical model in the analysis of leakage field data was based on the analytic
solution for the field perturbed by a spherical inclusion (Ref 20, 21). Two conclusions were drawn from this analysis.
First, when one measures the leakage flux component normal to the surface of the part, the center of the flaw is located
below the scan plane at a distance equal to the peak-to-peak separation distance in the flaw signal (Fig. 5), and second, the
peak-to-peak signal amplitude is proportional to the flaw volume. A number of experimental tests of these sizing rules
have confirmed the predicted relationships for nonmagnetic inclusions in steel parts (Ref 21).
Fig. 5 Dependence of magnetic signal peak separation (a) on the depth of a spherical inclusion (b)
Further theoretical and experimental data for spheroidal inclusions and surface pits have shown, however, that the simple
characterization rules for spherical inclusions do not apply when the flaw shape differs significantly from the ideal sphere.
In such cases, the signal amplitude depends on the lateral extent of the flaw and on its volume, and characterization on the
basis of leakage field analysis becomes much more complicated (Ref 19, 22).
Finally, there has been at least one attempt to apply finite-element calculations of flaw leakage fields to the development
of characterization rules for a more general class of flaws. Hwang and Lord (Ref 23) performed most of their
computations for simple flaw shapes, such as rectangular and triangular slots and inclusions, and from the results devised
a set of rules for estimating the depth, width, and angle of inclination of a flaw with respect to the surface of the part. One
of their applications to a flaw of complex shape is shown in Fig. 6.
Fig. 6 Characterization of a ferrite-
tail type of defect. The dashed line shows the flaw configuration estimated
from the leakage field data.
The promising results obtained from the finite-element work of Hwang and Lord, as well as the analytically based work
on spheroidal flaws, suggest that the estimation of flaw size and shape from leakage field data is feasible. Another
numerical method potentially applicable to flux leakage problems is the boundary integral method, which may prove
useful in flaw characterization. Unfortunately, much more work must be done on both the theoretical basis and on
experimental testing before it will be possible to analyze experimental leakage field data with confidence in terms of flaw
characteristics.
References cited in this section
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. Dobm
ann, 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 Techniqu
es 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
Proceedings 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 Nondestructive 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
Magnetic Field Testing
R.E. Beissner, Southwest Research Institute
Principles of Magnetic Characterization of Materials
Metallurgical and Magnetic Properties. The use of magnetic measurements to monitor the metallurgical properties
of ferromagnetic materials is based on the fact that variables such as crystallographic phase, chemical composition, and
microstructure, which determine the physical properties of materials, also affect their magnetic characteristics (Ref 24, 25,
26). Some parameters, such as grain size and orientation, dislocation density, and the existence of precipitates, are closely
related to measurable characteristics of magnetic hysteresis, that is, to the behavior of the flux density, B, induced in a
material as a function of the magnetic field strength, H.
This relationship can be understood in principle from the physical theory of magnetic domains (Ref 27). Magnetization in
a particular direction increases as the domains aligned in that direction grow at the expense of domains aligned in other
directions. Factors that impede domain growth also impede dislocation motion; hence the connection, at a very
fundamental level, between magnetic and mechanical properties.
Other magnetic properties, such as the saturation magnetization, which is the maximum value B can achieve, or the Curie
temperature at which there is a transition to a nonmagnetic state, are less dependent on microstructure, but are sensitive to
such factors as crystal structure and chemical composition. Interest in the magnetic characterization of materials,
principally steels, derives from many such relationships between measurable magnetic parameters and metallurgical
properties. These relationships are, however, quite complicated in general, and it is often difficult to determine how or if a
particular measurement or combination of measurements can be used to determine a property of interest. Nevertheless,
the prospect of nondestructive monitoring and quality control is an attractive one, and for this reason research on
magnetic materials characterization continues to be an active field.
It is not the purpose of this article to explore such magnetic methods in depth, but simply to point out that it is an active
branch of nondestructive magnetic testing. The more fundamental aspects of the relationship between magnetism and
metallurgy are discussed in Ref 26 and 28. Engineering considerations are reviewed in Ref 24. The proceedings of
various symposia also contain several papers that provide a good overview of the current status of magnetic materials
characterization (Ref 29, 30, 31).
Experimental Techniques. A typical setup for measuring the B-H characteristic of a rod specimen is shown in Fig. 7.
The essential elements are an electromagnet for generating the magnetizing field, a coil wound around the specimen for
measuring the time rate of change of the magnetic flux, B, in the material, and a magnetic field sensor, in this case a Hall
effect probe, for measuring the magnetic field strength, H, parallel to the surface of the part. The signal generator provides
a low-frequency magnetizing field, typically of the order of a few Hertz, and the output of the flux measuring coil is
integrated over time to give the flux density in the material. In the arrangement shown in Fig. 7, an additional feature is
the provision for applying a tensile load to the specimen for studies of the effects of stress on the hysteresis data. When
using a rod specimen such as this, it is important that the length-to-diameter ratio of the specimen be large so as to
minimize the effects of stray fields from the ends of the rod on the measurements of B and H.
Fig. 7 Experimental arrangement for hysteresis loop measurements. Source: Ref 32
Another magnetic method that uses a similar arrangement is the measurement of Barkhausen noise (Ref 33, 34). As the
magnetic field strength, H, is varied at a very slow rate, discontinuous jumps in the magnetization of the material can be
observed during certain portions of the hysteresis cycle. These jumps are associated with the sudden growth of a series of
magnetic domains that have been temporarily stopped from further growth by such obstacles as grain boundaries,
precipitates, or dislocations. Barkhausen noise is therefore dependent on microstructure and can be used independently of
hysteresis measurements, or in conjunction with such measurements, as another method of magnetic testing. The
experimental arrangement differs from that shown in Fig. 7 in that a single sensor coil, oriented to measure the flux
normal to the surface of the specimen, is used instead of the Hall probe and the flux winding.
The review articles and conference proceedings cited above contain additional detail on experimental technique and a
wealth of information on the interpretation of hysteresis and Barkhausen data. However, it should be noted that test
methods and data interpretation are often very specific to a particular class of alloy, and techniques that seem to work well
for one type of material may be totally inappropriate for another. The analysis of magnetic characterization data is still
largely empirical in nature, and controlled testing of a candidate technique with the specific alloy system of interest is
advisable.
References cited in this section
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 o
n 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
Magnetic Field Testing
R.E. Beissner, Southwest Research Institute
Applications
Flaw Detection by the Flux Leakage Method. Perhaps the most prevalent use of the flux leakage method is the
inspection of ferromagnetic tubular goods, such as gas pipelines, down hole casing, and a variety of other forms of steel
piping (Ref 35, 36). In applications in the petroleum industry, the technique is highly developed, but details on inspection
devices and methods of data analysis are, for the most part, considered proprietary by the companies that provide
inspection services. Still, the techniques currently in use have certain features in common, and these are exemplified by
the typical system described below.
The device shown in Fig. 8 is an inspection tool for large-diameter pipelines. Magnetization is provided by a large
electromagnet fitted with wire brushes to direct magnetic flux from the electromagnet into the pipe wall. To avoid
spurious signals from hard spots in the material, the magnetization circuit is designed for maximum flux density in the
pipe wall in an attempt to magnetically saturate the material. Leakage field sensors are mounted between the pole pieces
of the magnet in a circle around the axis of the device to provide, as nearly as possible, full coverage of the pipe wall. In
most such tools, the sensors are the inductive coil type, oriented to measure the axial component of the leakage field
gradient. Data are usually recorded on magnetic tape as the system is propelled down a section of pipe. After the
inspection, the recorded signals are compared with those from calibration standards in an attempt to interpret flaw
indications in terms of flaw type and size.
Fig. 8
Typical gas pipeline inspection pig. The tool consists of a drive unit, an instrumentation unit, and a
center section with an electromagnetic and flux leakage sensors.
In addition to systems for inspecting rotationally symmetric cylindrical parts, flux leakage inspection has been applied to
very irregular components, such as helicopter rotor blade D-spars (Ref 37), gear teeth (Ref 38), and artillery projectiles
(Ref 39). Several of these special-purpose applications have involved only laboratory investigations, but in some cases
specialized instrumentation systems have been developed and fabricated for factory use. These systems are uniquely
adapted to the particular application involved, and in most cases only one or at most several instrumentation systems have
been built. Even in the case of laboratory investigations, special-purpose detection probe and magnetizing arrangements
have been developed for specific applications.
One such system for automated thread inspection on drill pipe and collars is described in Ref 40. The device consists of
an electromagnet and an array of sensors mounted outside a nonmagnetic cone that threads onto the tool joint. The
assembly is driven in a helical path along the threads by a motor/clutch assembly. To minimize the leakage flux signal
variations caused by the threads, signals from the sensor array are compared differentially. The system is capable of
operating in a high field strength mode for the detection of cracks and corrosion pits and also in a residual field mode for
the detection of other forms of damage. At last report, the system was undergoing field tests and was found to offer
advantages, in terms of ease of application and defect detection, over the magnetic particle technique normally used for
thread inspection.
The flux leakage method is also finding application in the inspection of ropes and cables made of strands of ferromagnetic
material. One approach is to induce magnetization in the piece by means of an encircling coil energized by a direct
current (dc). With this method, one measures the leakage field associated with broken strands using a Hall effect probe or
an auxiliary sensor coil. A complementary method with alternating current (ac), which is actually an eddy current test
rather than flux leakage, is to measure the ac impedance variations in an encircling coil caused by irregularities in the
cross-sectional area of the specimen. Haynes and Underbakke (Ref 41) describe practical field tests of an instrumentation
system that utilizes both the ac and dc methods. They conclude that instrumentation capable of a combination of
inspection techniques offers the best possibility of detecting both localized flaws and overall loss of cross section caused
by generalized corrosion and wear. They also present detailed information on the practical characteristics of a
commercially available device that makes use of both the ac and dc methods.
Another area in which the flux leakage method has been successfully implemented is the inspection of rolling-element
antifriction bearings (Ref 42, 43, 44, and 45). A schematic illustration of the method as applied to an inner bearing race is
shown in Fig. 9. In this application, the part is magnetized by an electromagnet, as indicated in Fig. 9(a). The race is then
rotated by a spindle, and the surface is scanned with an induction coil sensor. Typically, the race is rotated at a surface
speed of about 2.3 m/s (7.5 ft/s), and the active portion of the raceway is inspected by incrementally indexing the sensor
across the raceway. Magnetizing fields are applied in the radial and circumferential orientations. It has been shown that
radial field inspection works best for surface flaws, while circumferential field inspection shows greater sensitivity to
subsurface flaws (Ref 43). Data have been collected on a large number of bearing races to establish the correlation
between leakage field signals and inclusion depths and dimensions determined by metallurgical sectioning (Ref 44, 45).