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The Present and Future of Eddy Current Testing

by Robert C. McMaster

 

 

Eddy current nondestructive testing (NDT) employs time varying electromagnetic fields as a probing medium to explore the properties, discontinuities or variations in geometry and dimensions of test materials. The varying electromagnetic fields are usually created by a flow of periodic (alternating or pulsating) electric currents in coils or arrays of conductors called probes. These probes are placed in close proximity to the surfaces of the test materials. They induce within the test materials a flow of electrical currents known as eddy currents. The intensity of eddy current flow is greatest at the excited surface of the test parts. Within the material, the eddy current density decreases exponentially with depth below this surface. The eddy currents, induced at the surface of the test material, are also time varying and have magnitude and phase.

 

PRINCIPLES AND APPLICATIONS OF EDDY CURRENT TESTING

Eddy Current Test Indications and Material Properties and Discontinuities
Eddy current tests respond specifically only to the electrical conductivity, magnetic permeability and geometric properties of test objects and to the spatial relationship of the test probes to the surfaces of test objects. The electrical conductivity and magnetic permeability of the test material directly affect the magnitude of eddy current flow and the flow paths of the eddy currents can be restricted or distorted by the test material geometry and the presence of discontinuities within the test material. Many other material properties can be related to these primary eddy current test measurements, but proof of such correlation must be obtained for each specific case. Thus, eddy current tests can respond to variations in electrical conductivity related to many different metallurgical factors: alloy microstructure, temperature variations, hardness changes, hot or cold working and other processing steps. All of these conditions may influence material conductivity or permeability. These tests can also detect effects of processing and shaping operations during production, as well as corrosion damage or cracking induced in service, for most nonmagnetic metals and alloys.

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Eddy current testing is an important and widely used method within the broad field of NDT. 

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In addition, with ferromagnetic materials such as steels, effects of thermal or mechanical processing and heat treatments (which change the magnetic permeability, elastic properties, tensile strength, hardness or ductility of the test materials) can be detected. However, with ferromagnetic materials, magnetic anomalies produced by handling, working or prior magnetization (as by use of magnetic lifting cranes) can interfere with the interpretation of material properties. Residual magnetism within steels and ferromagnetic materials can affect the eddy current test indications. Sometimes it is extremely difficult to separate desired from undesired effects (for example, meaningful from spurious indications).

Typically, it is possible that some characteristics, not related to the evaluation of a test object's serviceability, may result in signals larger than those of critical properties that are vital to the prediction of serviceability. Fortunately, techniques are available to reduce these undesired effects in eddy current tests for many applications. The science of eddy current NDT is essentially that of establishing optimum test conditions which respond clearly to the desired effects and which reduce or eliminate the undesired effects.

 

Typical Industrial Applications of Eddy Current Tests
Applications of eddy current tests in industry are numerous and widespread and the total number of test measurements made annually by this NDT method may exceed that of all other types. Although the eddy current test responds only to material conditions that influence the material geometry, electrical conductivity and magnetic permeability in the region excited by the magnetizing field, it is highly versatile and serves a number of functions, including:

• measuring the thickness of metallic foils, sheets, plates, tube walls and machined parts from one side only by noncontacting means

• measuring the thickness of coatings over base materials, where the coating and base material have significantly different electrical or magnetic properties

• identifying or separating materials by composition or structure where these influence electrical or magnetic properties of the test material

• detecting material discontinuities (such as cracks, seams, laps, score marks, plug cuts, drilled and other holes and laminations at cut edges of sheet or plate) that lie in planes transverse to the flow paths of the eddy currents

• identifying and controlling heat treatment conditions and evaluating fire damage to metallic structures

• determining depths of case hardening of steels and some ferrous alloys

• locating hidden metallic objects such as underground pipes, unexploded buried bombs, ore bodies and metallic objects accidentally packaged in foodstuffs

• timing or locating the motions of hidden parts of mechanisms or machinery

• counting metallic objects on conveyor lines and detecting metallic missiles in flight

• precise dimensional measurements of symmetric, machined or ground and polished metallic parts (such as bearings and bearing races), small mechanism components and others.

 

Metallurgical Process Applications of Eddy Current Tests
Electromagnetic induction tests find application in all stages of forming, shaping and heat treating metals and alloys, where the effectiveness of the processing steps can be quickly evaluated. Materials damaged during processing can be detected and removed from production without incurring further processing costs. Thermal treatments such as annealing, normalizing, hardening, case hardening and other heat treating processes can be monitored directly in many instances. Effects of mechanical processing, such as machining, drilling, rolling and hot or cold deformation can be measured and the development of cracks or damage to mechanical properties can be assessed. Small, portable forms of eddy current test instrumentation provide simple and rapid means for:

• manual quality tests by individual operators

• mechanized test systems to sort mixed lots of materials

• monitoring deterioration of materials and equipment in service.

 

Advantages of Commercially Available Eddy Current Test Systems
Eddy current testing is an important and widely used method within the broad field of NDT. Like other nondestructive methods, eddy current tests permit either measurements of material properties and dimensions or detection of discontinuities. Eddy current test systems have many advantages which justify their present wide usage.

In general, eddy current tests provide nearly instantaneous measurements, often in a fraction of a second. Consequently, they can be used in production lines to test swiftly moving bars, tubes, sheets, plates, welds and other symmetric parts. These parts either pass through test coils or are scanned by moving test probes.

Modern eddy current and electromagnetic test techniques offer low cost methods for high speed, large scale testing of metallic materials such as those used in nuclear, aerospace, marine, high pressure, high temperature and high speed engineering systems where premature failures could represent economic disasters or the endangering of human life. More recently, the method's special suitability for testing of automobiles, engines, machine parts and consumer products has been recognized. The automation of eddy current testing and test data evaluation permits mass testing of similar parts at high rates, with economies not attainable by other commonly used NDT methods.

Absolute conductivity meters and instruments designed for thickness measurements of specific metals and alloys are often quantitative, with precision of 1% or better. Comparison instruments permit heightened sensitivities for detection of discontinuities and of variations in material geometries or properties.

One final and great advantage is the reproducibility of measurements. With stable reference specimens, tests can be repeated with a high degree of confidence. Instruments with phase and amplitude signal capabilities (which consistently duplicate phase plane data) permit a wide range of interpretations to be made, depending upon the strategic test conditions selected. Phase separation to suppress unwanted signals and provide desired signals without interfering effects are especially valuable where consistency of geometry and physical properties of test materials permit their use. The general use of reference standards with drilled holes, milled or electrodischarge machined slots, stepped wall thickness and certain natural discontinuities provides a quick means of ensuring proper operation during testing or of calibration and adjustment of control settings at the beginning of test sequences, on objects of a particular type or material. These advantages generally accrue with nonferromagnetic test materials and symmetrical, simply shaped test objects; they cannot always be attained with magnetizable test materials or with parts of complex geometry where positioning may not be reproducible.

 

Detection and Analysis of Eddy Current Intensities and Flow Patterns
The output signals from eddy current test coils and probes are typically time varying, alternating current voltages or currents. Such signals can be analyzed by circuits and instrumentation analogous in principle to those used in analyzing alternating current circuit impedance in engineering and industry.

The eddy currents induced in a material generate their own resultant magnetic field. The magnitudes, time lags, phase angles and flow patterns of the eddy currents within the test materials are detected by measuring resultant magnetic fields with another set of sensing coils or solid state magnetic field detectors (hall effect devices, which detect the normal component of the magnetic field directly), usually housed within the eddy current test probes.

In some cases, the magnetizing coils also serve as signal pickup devices or detectors of the eddy current field reactions. Most commonly, the magnetizing coil system and the detectors (pickup devices) are combined into a single probe system, to provide a one side, noncontacting test probe arrangement.

For testing thin sheets, it is also possible to place the magnetizing coil system on one side of the test material, while the pickup element (receiver) is placed on the opposite side of the material to provide a through transmission eddy current test system. Similar results can be obtained with each of these variations in the test coil and detector arrangements.

Often, the magnetizing coil and pickup coil systems in the eddy current test probe are of nearly identical size, shape and location. However, it is also possible to provide two or more magnetizing or pickup coils in different locations. These may provide differential arrangements that are sensitive to small, local variations in test material properties, dimensions or discontinuities. Such differential eddy current test systems are widely used in locating heterogeneities, discontinuities and weld discontinuities in tubes, bars and plates during manufacturing. In systems where the eddy current reaction fields are measured by hall devices, a multiplicity of such pickup elements can be associated with a single magnetizing coil. This multiple detector array can be placed within or outside the circumference of the magnetizing coil.

In many eddy current systems, signal amplitudes and phase angles can be measured with high precision so that quantitative analysis of eddy current test signals are feasible. Such signals are appropriate for comparison, manipulation and analysis. They could also be readily digitized for display, permanent records and manipulation by digital computers of all types.

 

Selection of Eddy Current Test Equipment
Portable test instruments are often small, compact and battery operated and can be easily carried in one hand. Large, complex, fixed systems provide for mechanized handling of large test objects (such as billets and rounds in steel mills), automatic discontinuity detection, marking and recording of test data for computer analysis and control systems. Costs of eddy current test systems are often lower than for other NDT methods. Many variations in signal display and analysis systems are also available in commercial test instruments and systems. Simple indicating meters, digital displays or cathode ray tube readouts are used for tests monitored by inspectors. Complex, fully automated computer systems operating automatic sorting systems for test objects are used in production line applications for mass testing of similar products. Choice of a system is often dictated by economic considerations, the volume of testing required per day, the material properties or discontinuities to be detected and analyzed, the geometry of the test objects and the locations (laboratory, production line or the field) at which the test must be made.

 

Automation of Analysis of Eddy Current Test Signals
The information derived from eddy current tests can be optimized for automation of test systems, sorting of test parts, control of manufacturing processes and automatic production of processing control charts and statistical quality control records. The speed of eddy current tests and of modern signal analysis systems permits such analysis to be performed in real time. Materials evaluation test data can be made available as rapidly as test objects can be fed through the eddy current test system. Such features often make eddy current testing preferable to other NDT methods that require evaluation of data by human operators. Delays in deciding the disposition of parts or materials can be eliminated by automated eddy current test systems.

 

Selection of Eddy Current Test Frequencies
By selection of test frequencies, the same eddy current test system can be used for a multiplicity of differing measurements. The frequencies used in eddy current test systems vary from below 10 Hz to many megahertz. However, most industrial eddy current tests are made in the frequency range between 5 Hz and 10 MHz. Most types of eddy current test equipment provide either variable frequency oscillators or several fixed frequency steps.

These test frequencies are those of the excitation current applied to the magnetizing coils of the eddy current test probes. The output signal from a detector is usually of a frequency identical to that applied to the magnetizing coil system. Thus, signal to noise ratios in the signal analysis circuits can be optimized for very high signal detection sensitivities.

Thus, appropriate test frequencies can be readily selected by the user to meet special test requirements (such as to test for a particular material property). For example, it is often desired to measure only the electrical conductivity of nonmagnetic alloys (for sorting materials by alloy content, detection of impurity effects in electrical copper alloys, monitoring of effects of heat treatment or mechanical working, detection of intergranular corrosion in service or similar uses).

In conductivity measurements, the test frequency can be selected to be sufficiently high that eddy current penetration is limited to only a fraction of the test material thickness. In this case, the material properties can often be measured independently of the thickness or geometry of the test material. Many eddy current conductivity inspectors, for example, use test frequencies in the range of 64 kHz so that conductivity can be measured reliably in materials whose total thickness exceeds perhaps 3 mm (0.1 in.).

Alternatively, where test object thickness and deep lying discontinuities are to be measured, it is desirable to select test frequencies sufficiently low to ensure penetration of eddy currents throughout the total thickness of the test parts. This condition can be readily checked by placing a piece of copper or some other highly conductive material on the back side of the test material, to see if it affects the eddy current test indication. Where such an effect is observed, it is evident that the magnetic field of the test probe has penetrated completely through the thickness of the test material. In this case, the metal thickness and discontinuities near the farthest surface from the test probe can probably be detected.

More precisely, the penetration depths of eddy currents can be calculated in advance from material conductivities, test frequency and test object geometry and then can be analyzed by impedance plane analysis techniques.

 

Penetration
One of the most distinguishing features of eddy current tests with alternating current excitation is the tendency for the eddy currents to be concentrated near the material surfaces adjacent to the magnetizing coils or sources of induction. High test frequencies can be used for selective examination of near surface regions, for testing of thin materials and for testing of materials having low electrical conductivities. Low frequencies of excitation are used to penetrate deeper within a conducting test material.

High sensitivity to electrical conductivity in nonferromagnetic materials has been attained with small probe coil test instruments typically operating in the range of 64 kHz test frequencies. Such small coil probes tend to be sensitive to liftoff, and liftoff compensation systems such as those developed by Friedrich Förster are often used to correct liftoff effects over a small range.

Similarly, small differential coil or field detector systems provide high sensitivity to surface cracks in both nonmagnetic and ferromagnetic materials, but manual positioning and scanning with these fine probes are usually required for such crack detection on nonsymmetrical part surfaces or materials in service structures and machines. There is no inexpensive means for total testing for cracks on parts with complex surfaces, such as those for which liquid penetrant tests (or magnetic particle tests) provide overall surface testing at high speed and low costs.

With ferromagnetic materials, the high relative magnetic permeability of the test material acts to concentrate the eddy currents within very shallow layers of the near surface regions of the test objects. In steel and other ferromagnetic alloys, this effect can be used to advantage in detection of seams and laps or of service fatigue cracks. Where deeper penetration of eddy currents into ferromagnetic materials is desired, tests can be made at very low test frequencies such as 5 Hz or the test material can be subjected to additional magnetization by steady, direct current magnetizing coils to reduce the relative magnetic permeability.

Although the detection of discontinuities in surface layers of steels is well developed, measurement of physical or metallurgical properties of steels is generally not feasible by eddy current tests. One basic difficulty is the sequential use of sheets, tubes or rounds from different mills or different heats in rapid succession on production lines. Although the chemical and physical properties of these steels from different sources may meet manufacturing requirements adequately, no effort is made to control the magnetic permeability properties of these steels to any type of calibrated standard. As a consequence, random variations in magnetic permeability prohibit the development of reproducible correlations between absolute measurements of eddy current test signals and the actual physical or metallurgical structures of the test objects.

By use of magnetic bias (or saturation magnetization), depths of penetration of eddy currents and alternating current magnetic fields into ferromagnetic materials can be greatly increased. Many simple detectors of surface discontinuities operate quite effectively during automatic scanning, despite difficulties due to surface roughness or to variations in hardness or in magnetic permeabilities of test objects. Large scale through coil test systems for smaller diameter rounds and tubes and orbiting probe coil systems for rods, welded tubes and even rectangular billets have been developed to a high degree of ruggedness, serviceability and reliability for use in steel mills and on large volume testing applications. Automatic marking of discontinuity locations permits salvage by grinding out and welding repair (if the latter is needed) on production line operations.

 

Limitations and Disadvantages of Eddy Current Testing
Limitations of eddy current tests are a direct consequence of the specific nature of the test and of the response of conducting test materials to the externally applied time varying magnetic fields used to excite eddy current flow. In general, eddy current tests are applicable only to test materials with significant electrical conductivity, such as metals and alloys, or to composites with conducting layers or reinforcing fibers. Eddy current tests can, however, measure the thickness of nonconducting layers on the surface of conducting metallic materials by the liftoff effect, in which the coating separates the test probe from the conducting material by the thickness of the nonconducting coating or sheet material.

Eddy current tests provide maximum test sensitivity for the surface and nearsurface layers of the test material adjacent to the source of excitation. In some cases, it may be difficult or impossible to penetrate to the center of thick specimens because of skin effect and attenuation of the electromagnetic field at certain depths below the surface. Eddy currents tend to flow only in paths paralleling the surface to which the exciting field is applied and usually do not respond to laminar discontinuities that lie parallel to this surface. They do tend to respond, however, to discontinuities that lie transverse to the flow of eddy currents within test materials, where these discontinuities interrupt, lengthen or distort the current flow paths.

The primary disadvantage of eddy current test systems is the fact that they are less effective in stimulating management and worker comprehension and corrective action than the graphic images provided by processes such as liquid penetrant, magnetic particle or X-radiographic testing. Because eddy current tests fail to produce a clear, visible, interpretable image of discontinuities, an almost instantaneous recognition of nature, shape, size or location is not obvious to all observers. Also, when these tests are used to measure material properties or dimensions, the quantitative displays of signal amplitudes, component values or phase angles have no direct meaning to the untrained observer. When numbers have to be looked up in a book or chart to find the real meaning of test indications, the opportunity exists for human errors. In addition, because the same book or chart would not be valid for materials other than the specific material for which the chart is designed, untrained observers will question the results. If eddy current tests provided clear, informative images or direct readouts in numbers of a specific dimension, property or service characteristic, their use could be multiplied indefinitely.

The second disadvantage of some eddy current test systems is that they are limited by constraints in design and use. There have been very few fundamental changes from the basic designs which Förster provided in 1955, nor in the methods for interpreting test signals. Advancement to the next era of eddy current testing will occur when responsible and active engineers, management and test personnel develop systems using the full capabilities of the method and use these systems for effective control of processes and products.

A third disadvantage of eddy current test systems is that they are limited in penetration depths (often to less than 5 to 10 mm [0.2 to 0.4 in.]) and in magnetizing coil and detector adaptability to rough or contoured test material surfaces. Few probes or test coils are designed to fit into a sharp inside corner or intimately to the outer edge of a sheet material. Many probe coils are on rigid forms and cannot conform to irregular contours on test objects. It is also often assumed that small diameter probe coils must be used to measure fine discontinuities or the properties of small areas of test objects. Yet, because the coil field in air is proportionately small, small coils ensure lack of deep penetration of the magnetic field into metals or alloys.

The fourth disadvantage of eddy current systems is their insensitivity to local conditions or discontinuities, which produce only small distortions in eddy current flow paths. Most systems do not detect discontinuities which lie outside the perimeter of the test coils. They are also typically insensitive to small discontinuities which lie on the centerline of the test coils. In fact, test coils usually integrate all the magnetic flux lines that their winding turns enclose. With discontinuities which are small compared to the coil diameter, the signals are submerged in a large average coil signal so that highly sensitive detector circuits are needed to detect the minute changes in amplitude or phase. Even worse, coil type detectors are insensitive to the tilt or angle of magnetic flux lines encircled by the coils. They simply measure the time rate of change of the total magnetic flux enclosed by the test coil. This often loses signal magnitude by ratios as great as 100 to 1. Finally, the use of coils for detection of signals limits the most minute area detectable to that roughly corresponding to the pickup coil diameter (or the diameter of a ferromagnetic core within the pickup coil).

The fifth disadvantage of eddy current testing is its limitation to higher test frequencies and to tests at larger phase angles on the complex plane. The voltage signal amplitude provided by a pickup coil is proportional to the test frequency. If an effort is made to operate at very low test frequencies (to attain deeper penetration), the signal can become too low to detect in the presence of normal noise signals. Even if amplification permits signal display, the low frequency test condition leads to signal points on the upper left portion of the locus curves of response on the complex plane. Here, the signal closely approaches the empty coil signal in both amplitude and phase. The small contribution of the eddy current losses to this test signal also imply lack of test sensitivity.

The sixth disadvantage of some eddy current test systems is the variation of magnetizing current amplitude with test frequency. Higher test frequencies require higher power supply voltages to provide a given magnitude of current in the test coils. If variations in test frequency result in inverse changes in magnetizing current, tests may be made on ferromagnetic test parts at widely different levels of maximum magnetization and at different test frequencies. This can create difficulties with harmonic signal generation and nonlinear response characteristics in eddy current test measurements. Alternatively, if true constant current magnetization levels cannot be provided, as frequency varies over a wide range, the designer may limit the test instrument to one or a few discrete test frequencies, for which constant current levels can be ensured. Even when multifrequency tests are made at these few frequencies, a loss of information at other intermediate frequencies results.

A final limitation of some eddy current test systems is their use of sinusoidal continuous alternating current excitations. A useful signal thus lost is that of magnetic retentivity and its relation to eddy current pulse decay characteristics. Square wave or spike excitation can provide both retentivity signals and decay curves for eddy currents within the test materials. The use of coil pickups prevents detection of the direct current components of test signals which could be generated with pulse or rectangular wave shapes because response is zero to steady state magnetic flux conditions.

 

THE FUTURE

Unnecessary Constraints
Development of new or different forms of eddy current test systems might be hindered by constraints in thinking about novel approaches, perhaps because these new concepts are not fully documented in the history of eddy current testing. For example, circular test coils were selected for initial investigations because they were easy to build and because many test objects had circular symmetry. Theory also has been directed to circular test coils because the solutions of Maxwell's equations for the electromagnetic field could be attained more easily with symmetric circular boundary conditions (such as can be solved with Bessel's equation and its modifications).

Actually, however, test coils can be wound around square, triangular or spherical forms. They could be made highly flexible and made to conform to surfaces of any shape. In all cases, advantages accrue in eddy current testing if the magnetizing coil liftoff can be minimized. Flexible magnetizing coils with stranded conductors imbedded in rubber like sheets or tubes might offer considerable advantages. Applied under pneumatic or other pressure, such flexible sheet magnetizing coils could be fitted to gently curved test parts with essentially zero liftoff. If the detector coil could also be in intimate contact with the curved surface of the test object, maximum test sensitivity and elimination of nonuniform liftoff conditions could be attained.

A further constraint lies in the assumption that the magnetizing coil and the pickup coil should be either one and the same coil or of identical diameter and coincident in position. True, the literature describes such simple arrangements redundantly. However, the pickup coil could be of any diameter (preferably smaller than the magnetizing coil) and placed at any angle and in any desired position with respect to the magnetizing coil. For example, the pickup coil could be located at any point and in any orientation, either within or completely outside the annulus of the magnetizing coil or even at a point directly under only one point of the magnetizing coil winding.

In fact, if the pickup coil is replaced by a semiconductor magnetic field detector, total freedom exists with respect to the number, positions and angulations selected for the individual semiconductor detector elements. For example, an array of semiconductor detectors could be placed anywhere within, under or outside the magnetizing coil windings to provide a multiplicity of input signals with only one magnetizing coil. Ideally, such an array should cover the entire area enclosed within the magnetizing coil or should be extended over an area much larger than the magnetizing coil to provide total test information concerning the entire eddy current test field created by the magnetizing coil. If this detector array could be interrogated in sequence by rapid techniques such as those used to read computer memories or to digitize images, the resultant multichannel data could be analyzed by digital techniques and displayed in any desired image format (including two or three dimensional images on a television screen).

A particularly desirable change would be to use very large diameter magnetizing coils closely fitting test object contours, to ensure deep geometrical penetration of the magnetizing field. For example, a 0.25 m (9.8 in.) diameter test coil could easily project strong magnetic fields 50 to 75 mm (2 to 3 in.) in front of the coil face. Used with lower test frequencies, such a coil might provide penetration through 25 to 50 mm (1 to 2 in.) of nonmagnetic test material (particularly in cases of materials with electrical conductivities less than about 10% International Annealed Copper Standard). With arrays of semiconductor magnetic field detectors, detail sensitivity to near surface discontinuities might become sufficient to provide good, recognizable images of typical discontinuities.

Alternatively, a linear array of magnetic field detectors within or next to the magnetizing coil might scan linearly across the field or be rotated to provide a circular scan of the field. The instantaneous appearance of a recognizable eddy current image of discontinuities would greatly increase demand for eddy current tests. The repeatability of such images, as coil and probes are moved over test surfaces (or as tests are repeated after a time period), would do much to establish confidence in the reliability of such images. In general, the instantaneous character of eddy current images and the ease with which depth sensitivity could be changed or eddy current flow could be polarized might compare favorably with X-radiographic or ultrasonic test images of welds or with fluorescent penetrant or magnetic particle tests of surface cracks, seams and laps.

Another potentially attractive technique is that of using differential probe signal pickups (preferably by detecting imbalance in a four detector array, analogous to a wheatstone bridge) for a direct map of eddy current flow below test surfaces. The reality of eddy current flow paths and of their deviations caused by discontinuities could then be visualized. Because local detectors in the vicinity of crack ends, for example, can have surprisingly large test signals (compared to those of large area pickup coils), unique opportunities exist for precise measurements of crack lengths and crack extension rates. These topics are of special interest in fracture mechanics, where analyses are made to determine a crack's ability to propagate under service stresses.

 

Development of Deep Penetration Eddy Current Test Systems
The future should see a vast improvement in the depth capabilities of eddy current test systems. At present, most eddy current tests are used for surface and near surface testing, where they provide high test sensitivities. Good performance can be attained with thin wall test objects, namely those whose metal wall thickness is a small fraction of the eddy current penetration depth.

Eddy current penetration depth is the metal depth at which the eddy current density J is reduced to about 37% of its value at the test material surface closest to the magnetizing coil. At a depth of three times this penetration depth, the eddy current density is only about 5% of the surface current density. At five times the penetration depth, the eddy current density is negligible, at less than 0.5% of its surface intensity. (The standard penetration depth is an inverse function of the square root of the product of test frequency, material conductivity and/or material magnetic permeability. In highly ferromagnetic test materials, the penetration depths are typically reduced by a factor of 10 to 100, compared to a nonmagnetic test material.)

Improvements in penetration depth are attainable by lowering the test frequency or saturating ferromagnetic materials to lower their relative magnetic permeability. The first technique, lowering test frequency, was limited in the past by the difficulty of detecting low test frequencies with pickup signal coils. With semiconductor detector systems or with the addition of integrating operational amplifiers to pickup coils to process the test signals, it should be possible to work at much lower test frequencies. If, for example, it were feasible to lower a conventional 10 kHz eddy current test frequency to 1 Hz, the standard penetration depth should increase by 100 times. However, at such a low test frequency, it would take 1 s to complete one cycle of alternation. With modern electronic integration systems, such frequencies are not out of the range of feasible measurements. In fact, low frequency oscillators and analysis systems should be able to handle frequencies as low as 10 MHz.

However, increasing the penetration depth by lowering test frequency is of no value if the magnetizing coil diameter is such that the magnetizing field (in air) is reduced geometrically, allowing very few or no flux lines to reach the new penetration depth limits. The answer here is to employ large diameter magnetizing coils (although the eddy current detectors can be as small as desired). An example of a large diameter magnetizing coil is the metal detector used to test passengers before they board aircraft in the US. Such large size test coils should also conform to surface contours of test objects where feasible and should provide adequate levels of test frequencies low enough to meet testing requirements.

 

Microwave NDT
At very high frequencies, electromagnetic fields can be concentrated into beams and propagated through space. When such a beam pulse strikes a conducting metallic surface, for example, it is reflected and may return as an echo to the site of the original pulse transmitter or to other detectors as in radar detection. In dielectric materials, microwaves can be subject to rotations and phase shifts, as well as to attenuation due to dielectric hysteresis losses. In many ways, microwave NDT systems are analogous in performance applications to immersion ultrasonic test systems. By Maxwell's theory of the electromagnetic field, microwaves are reflected like light waves by eddy currents induced in the surface layers of highly conducting metallic materials. Thus, microwaves appear to have the capacity to apply high frequency eddy current tests to a metallic surface from a distance and perhaps to scan such surfaces to detect discontinuities which change the pulse reflection patterns.

The theory and design of microwave generators, horns, antennas, detectors and display systems had been developed for long distance ranging in radar. Many textbooks present the electromagnetic theory of microwaves in terms readily used by electrical engineers and microwave system components and electron tubes are commercially available. However, electrical engineers are rarely aware of the needs of NDT engineers and few NDT engineers are familiar with microwaves.

Limited research sponsored by government agencies has indicated the possibility for crack detection from a distance. Slots and wires simulating discontinuities in metallic test object surfaces can be detected under proper conditions of microwave pulse reflection testing.

The theory of microwave antennas and of time domain reflectometry of microwaves in tubes, passing along wires, reflecting and refracting in dielectric layers, offer many indications of potentially valuable NDT applications. Because microwaves can be focused, microwave systems could potentially be designed that are analogous to optical instruments and test systems, as well as ultrasonic test systems.

 

Development of Time Domain Reflectometry Eddy Current Tests
Time domain reflectometry has been used for detection of discontinuities in high frequency electromagnetic field transmission lines, telephone lines and telegraph lines. Similar time domain reflectometry techniques are used in ultrasonic NDT, particularly in immersion testing. Short electromagnetic pulses of high frequency wave trains or a single step or square wave pulse can be used. It is also possible to use short electromagnetic pulses in through transmission electromagnetic testing. For example, Paul Gant of Shell Development Laboratories in Emoryville, Calfornia, used such a system many years ago to transmit electromagnetic pulses along oil well drill pipe and steel tubes. Encircling coils used as transmitters and receivers permitted detection of larger discontinuities and of zones of reduced wall thickness. To establish distances to metallic sheet and other reflectors, Richard Hochschild also used radar type echo ranging with his microwave test equipment.

Time domain reflectometry and standing wave analyses are widely used in high frequency electronic engineering analyses. Microwave parts and fixtures are available from electronic equipment manufacturers for construction of such systems. Time domain reflectometry plug in hardware is available for high quality cathode ray oscilloscopes, which can be used directly for time domain reflectometry eddy current tests. In such tests, microwave pulses are transmitted along metallic or dielectric rods, tubes or sheets. Where impedance mismatch conditions are encountered, reflections occur. These systems are entirely analogous to ultrasonic pulse reflection tests. Where the electromagnetic waves travel in dielectrics or in air around a metallic conductor, reflection can result from liquid or solid dielectrics (such as ceramics) or from metal surfaces (which typically act as total reflectors).

Such techniques might apply to rapid testing of metallic material moving at high speeds in a rolling mill or perhaps to testing of dielectric coatings being applied to wires, tubes or sheets under fast transport conditions. The travel speed of waves encountered in typical electromagnetic time domain reflectometry on metallic structures is perhaps 0.67 times the speed of light (or about 2 x 10⁵ km/s [1.2 x 10⁵ mi/s]). Thus, echoes would return from a reflector 1 m (3.3 ft) from the source in a time of 10 ns. Precise, fast response, high resolution electronic signal detection and analysis equipment (cathode ray oscilloscopes or digital systems) would be needed in most cases, except when standing wave resonance conditions are present.

Further development is still needed to provide practical systems that use microwave beams to interrogate metal surfaces at a distance or to detect conditions such as slots, cracks, highly conducting surface coatings, dielectric surface coatings, projections and surface irregularities. In a similar sense, use of microwave distance measuring devices to detect movements of structures such as large tanks or bridges during earthquakes or under service loading might also be feasible. The still higher frequency laser beams used to range distances in surveying are similar, because their electromagnetic waves are still shorter in wavelengths than microwaves. As optical wave guides and signal transmission systems improve, it may be possible that these will also be used for analysis of electrical and magnetic properties of materials and so join the ranks of practical NDT systems.

 

Intelligent Materials with Microwave Trouble Signals
The ultimate goal for all forms of NDT system development should be developing intelligent engineering materials which detect troubles by themselves and transmit suitable alarm signals in time to permit human intervention and prevention of disastrous failures. The technique of acoustic emission NDT is an example of signals emanating from materials under mechanical stresses or subject to damaging events such as stress corrosion or fatigue damage. Scientists have not tried very hard to record the many kinds of signals emitted by natural and artificial materials. Recent interest has been directed to the prediction of earthquakes and dangerous storms.

It seems likely that not many NDT engineers have bothered to listen to the microwave signals emitted by metallic surfaces and structures under stress, vibration or surface attack. Yet engineers often have to work very hard to muffle or destroy these signals when they interfere with intentional human microwave or radio transmissions. For example, it has long been known that railway axles rotating in journal bearings create radio noise, which is considered objectionable. In fact, copper straps are applied to short circuit these emissions and ensure that they do not interfere with railway signal systems or other communications.

Anyone who drives a car with a radio has had an opportunity to observe the microwave signals from large trucks, bridges and machinery. If the car radio is tuned between broadcasting stations to receive only static noise signals, variations in these signals can be quickly found as his car passes large trucks with metallic bodies or drives across older iron or steel bridges with loose bolts or connections. If long distance static is screened out (as in a shielded room), the radio signals from contacts dragging across metal surfaces, from rotating bearings or from loosely bolted joints undergoing vibration can be heard distinctly. In fact, if while wearing gloves one taps a knife and fork together while walking in the vicinity of the radio receiver, he can send Morse code or any other sequence of signals through the radio loudspeaker.

When two metals rub together, enormous sounds can be heard, as if the metals complain of the damage their surfaces are undergoing. When ball or roller bearings rotate under heavy load or with inadequate lubrication, each metal to metal contact can be announced by clicks and distinct signals. Often the same sequence of signals is broadcast with each rotation of the shaft or of a ball or roller with a damaged surface. In all cases, the intensity of these signals can be greatly increased by connecting one of the metal surfaces to the antenna lead of the radio (preferably through a shielded cable). The other metal surface may be grounded or allowed to stand insulated from all other surfaces. On the other hand, short circuiting the two pieces of metal together at the point of signal generation generally extinguishes the radio signals' broadcast.

The triboelectric effect (electrification by friction), known by the Greeks 2000 years ago, illustrates the basis of such microwave emissions. During rubbing, one material steals electrons from the other material, particularly when contact is broken. Because the electron cloud within conducting metals constitutes a plasma, the sudden removal or injection of charge locally may create plasma oscillations. If one of the metal pieces is insulated from the other, it is possible that such oscillations result in electromagnetic waves traveling through the metal. It then serves as an antenna to broadcast these waves into the space around it. These weak signals can be easily lost in static conditions. Tests in a shielded room permit their clear identification and their correlation with material surface characteristics. The author has found these signals to approximate white noise, in that they can be detected at all frequencies, from those of audio amplifiers, through those of AM and FM radio broadcasting, to frequencies of 100 MHz or higher. This could be expected from the short time involved in the robbery of electrons from a metal surface.

Thus, the ultimate in service monitoring process for metallic systems and machines may well be formed of microwave monitors of electron emissions. When the electron charges are removed from a metal, the eddy current reaction is one of high frequencies, capable of being transmitted through the metal antenna and from it to detectors at moderate distances. Increased stressing or rubbing of contacts across contaminated (oxidized) metal surfaces results in enhanced microwave distress signals. These same signals can be used to create television images of metal surfaces, including geometric features such as scratches, chemical features such as oxides and other corrosion, contaminants such as fingerprints, or even the effects of absorbed gas layers and amorphous coatings. A low voltage electron beam scanning such surfaces (as in a Vidicon television camera) does not damage the surface features and can reproduce their images faithfully over long periods of time. In this special case, conditions reflected by eddy current reactions, as electrons transit metal surfaces, can be imaged with remarkable clarity. Typical images enlarged 30 times show detail approaching a few micrometers in dimension.

 

Closing
It is evident that the NDT community has only begun to explore the many configurations and applications available for eddy current equipment. The future for this method should be exciting. The Nondestructive Testing Handbook, second edition: Volume 4, Electromagnetic Testing contains many articles and detailed information on eddy current testing, as well as microwave and flux leakage testing.

 

Acknowledgments
This article was originally published in the November 1985 (Vol. 43, No. 12) issue of Materials Evaluation, pages 1512-1521. The text appearing in the current issue has been edited for length and clarity.

 

Copyright © 2002 by the American Society for Nondestructive Testing, Inc. All rights reserved.

[ Materials Evaluation ]

 

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