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Generalization Of Hysteresis Modeling To Anisotropic Materials, A. Ramesh, David C. Jiles, Y. Bi Apr 1997

Generalization Of Hysteresis Modeling To Anisotropic Materials, A. Ramesh, David C. Jiles, Y. Bi

Materials Science and Engineering Publications

An extension to the model of hysteresis has been presented earlier which included the effect of anisotropy in the modeling of the anhysteretic magnetization curves of uniaxially anisotropic single crystalline materials. Further exploration of this extension shown here considers different kinds of crystal anisotropy in materials. Theory considers that the differential susceptibility at any given field is determined by the displacement of the prevailing magnetization from the anhysteretic magnetization. Thus, it has been shown that the effect of anisotropy on magnetic hysteresis in materials can be incorporated into the model of hysteresis through the anisotropic anhysteretic. This extension is likely ...


Barkhausen Effect In Steels And Its Dependence On Surface Condition, Anthony P. Parakka, David C. Jiles, H. Gupta, M. Zang Apr 1997

Barkhausen Effect In Steels And Its Dependence On Surface Condition, Anthony P. Parakka, David C. Jiles, H. Gupta, M. Zang

Center for Nondestructive Evaluation Publications

Micromagnetic Barkhausen signals from magnetic materials originate from the discontinuous changes in magnetization under the action of a changing applied magnetic field.Barkhausen emissions that are detected by a sense coil come predominantly from a surface layer. In iron based materials this layer is about 500 μm thick. The Barkhausen signal is affected by changes in material microstructure and the presence of residual stress, since these affect the dynamics of domain wall motion. The selective attenuation of high frequency components of the Barkhausen signal due to eddy currents in electrically conducting materials is used to evaluate changes in material condition ...


Magnetomechanical Effect In Nickel And Cobalt, Michael K. Devine, David C. Jiles Apr 1997

Magnetomechanical Effect In Nickel And Cobalt, Michael K. Devine, David C. Jiles

Ames Laboratory Publications

The change in magnetization as a result of applied uniaxial stress has been measured in nickel and cobalt. Both tensile and compressive stresses were applied up to 125 MPa. Magnetostriction and anhysteretic magnetization as a function of stress were also measured. The change in magnetization with stress depended on the applied stress and the displacement between the prevailing magnetization and anhysteretic. At the loop tips, nickel showed a +6 mT (compression) and −6 mT (tension) magnetization change while cobalt displayed a +15 mT (compression) and −15 mT (tension) magnetization change. At remanence,nickel decreased in magnetization by 45 mT under ...


Multifrequency Eddy Current Signal Analysis, Avanindra Jan 1997

Multifrequency Eddy Current Signal Analysis, Avanindra

Retrospective Theses and Dissertations

This thesis presents a novel procedure for representing and processing multifrequency eddy current signals. Multifrequency eddy current NDE methods are used extensively for the inspection of steam generator tubes in nuclear power plants. Existing methods utilize computationally expensive time domain procedures to process the data. The procedure outlined in the thesis uses frequency domain methods to minimize the computational effort significantly. Two different approaches are evaluated. The first method uses the Fourier descriptor to represent the signal. The Fourier coefficients are utilized to obtain the rotation, scaling and translation parameters required for mixing. The second approach uses the cosine transform ...


Signal Processing And Image Restoration Techniques For Two-Dimensional Eddy Current Nondestructive Evaluation , Bing Wang Jan 1997

Signal Processing And Image Restoration Techniques For Two-Dimensional Eddy Current Nondestructive Evaluation , Bing Wang

Retrospective Theses and Dissertations

This dissertation presents a comprehensive study on the forward modeling methods, signal processing techniques, and image restoration techniques for two-dimensional eddy current nondestructive evaluation. The basic physical forward method adopted in this study is the volume integral method. We have applied this model to the eddy current modeling problem for half space geometry and thin plate geometry. To reduce the computational complexity of the volume integral method, we have developed a wavelet expansion method which utilizes the multiresolution compression capability of the wavelet basis to greatly reduce the amount of computation with small loss in accuracy. To further improve the ...


Nonlinear Response Of Superconductors To Alternating Fields And Currents , Jason R. Mcdonald Jan 1997

Nonlinear Response Of Superconductors To Alternating Fields And Currents , Jason R. Mcdonald

Retrospective Theses and Dissertations

Nonlinearities caused by vortex pinning and weak links in superconductors are considered. The effects of vortex pinning will be treated in both the parallel and perpendicular geometries using the critical state model. The results for the parallel geometry will be used to calculate the nonlinear surface impedance of a hard type-II superconductor. The results for the perpendicular geometry will be used to treat harmonic generation (HG) and intermodulation distortion (IM) in superconducting transmission lines and resonators. Nonlinearites due to weak links will be modelled using both the resistively shunted junction model (RSJ) for small junctions, and the damped sine-Gordon equation ...


Two-Dimensional Pancake Vortices In A Finite Stack Of Magnetically Coupled Thin Superconducting Films , Thomas Lim Pe Jan 1997

Two-Dimensional Pancake Vortices In A Finite Stack Of Magnetically Coupled Thin Superconducting Films , Thomas Lim Pe

Retrospective Theses and Dissertations

This dissertation investigates the structure and behavior of two-dimensional (2D) pancake vortices in a stack of N Josephson-decoupled superconducting thin films under a magnetic field applied prependicular to the layers. This system of magnetically coupled superconducting films is proposed as a model for the multilayered high-Tc superconductors. Starting with the properties of one pancake vortex located in any of the N superconducting layers, an analytical expression is derived for the magnetic coupling force between pairs of 2D pancake vortex lattices residing in different layers, each having the same lattice structure, but displaced (not rotated) relative to each other. Using this ...


Design, Analysis, And Modeling Of Giant Magnetostrictuve Transducers , Frederick Theodore Calkins Jan 1997

Design, Analysis, And Modeling Of Giant Magnetostrictuve Transducers , Frederick Theodore Calkins

Retrospective Theses and Dissertations

The increased use of giant magnetostrictive, Terfenol-D transducers in a wide variety of applications has led to a need for greater understanding of the materials performance. This dissertation attempts to add to the Terfenol-D transducer body of knowledge by providing an in-depth analysis and modeling of an experimental transducer. A description of the magnetostriction process related to Terfenol-D includes a discussion of material properties, production methods, and the effect of mechanical stress, magnetization, and temperature on the material performance. The understanding of the Terfenol-D material performance provides the basis for an analysis of the performance of a Terfenol-D transducer. Issues ...


A New Instrument For The Detection Of Fatigue Cracks Under Airframe Rivets, Buzz Wincheski, Ron Todhunter, John Simpson Jan 1997

A New Instrument For The Detection Of Fatigue Cracks Under Airframe Rivets, Buzz Wincheski, Ron Todhunter, John Simpson

Review of Progress in Quantitative Nondestructive Evaluation

During the past several years the electromagnetics laboratory at NASA Langley Research Center has focused on the Aging Aircraft Program. A major goal of this program has been the development of easy to use yet highly accurate inspection methods for the detection of flaws in airframe fuselage structures. A major breakthrough in this research came with the discovery of the Self-Nulling Probe Effect in November of 1992 [1]. It was clear that the unambiguous flaw signature of the probe could be developed into a low cost and easy to use fatigue crack detection device. Work toward this goal proceeded quickly ...


Hts-Squid Magnetometer With Digital Feedback Control For Nde Applications, E. Zimmermann, G. Brandenburg, U. Clemens, H. Rongen, H. Halling, H. -J. Krause, R. Hohmann, H. Soltner, D. Lomparski, M. Gruneklee, K. -D. Husemann, H. Bousack, A. I. Braginski Jan 1997

Hts-Squid Magnetometer With Digital Feedback Control For Nde Applications, E. Zimmermann, G. Brandenburg, U. Clemens, H. Rongen, H. Halling, H. -J. Krause, R. Hohmann, H. Soltner, D. Lomparski, M. Gruneklee, K. -D. Husemann, H. Bousack, A. I. Braginski

Review of Progress in Quantitative Nondestructive Evaluation

Superconducting Quantum Interference Devices (SQUIDs) are extremely sensitive detectors for the measurement of magnetic flux. Especially the current High Temperature Superconductor (HTS) SQUIDs, which can be operated at liquid nitrogen temperature with easy cryogenic requirement, are well suited for the practical use [1]. Nowadays this HTS-SQUIDs are used for applications like the detection and localization of currents within the human heart, for nondestructructive evaluation of materials and for geological exploration.


Recent Developments Of Eddy Current Inspection Simulator, Norio Nakagawa, M. Garton, J. C. Moulder, J. H. Rose, J. Xu Jan 1997

Recent Developments Of Eddy Current Inspection Simulator, Norio Nakagawa, M. Garton, J. C. Moulder, J. H. Rose, J. Xu

Review of Progress in Quantitative Nondestructive Evaluation

This paper reports on development of simulation software for eddy current (EC) inspections.


Progress In Eddy Current Modeling Via The Boundary Element Method, Norio Nakagawa, S. Ghanekar, D. Lehther, Y Liu, J. C. Moulder, A. N. S. Prasad, F. Rizzo, J. C. Chao Jan 1997

Progress In Eddy Current Modeling Via The Boundary Element Method, Norio Nakagawa, S. Ghanekar, D. Lehther, Y Liu, J. C. Moulder, A. N. S. Prasad, F. Rizzo, J. C. Chao

Review of Progress in Quantitative Nondestructive Evaluation

For the past several years, we have been developing an eddy current model, using the boundary element method (BEM). Last year, in particular, a BEM algorithm based on the Hertz potential approach was found and shown to be effective in dealing with complex part geometry, while keeping the computational resource requirement to a minimum [1–3]. This paper concerns a further extension of the model to include cracks.


Fast Eddy Current Forward Models Using Artificial Neural Networks, Bing Wang, John P. Basart, John C. Moulder Jan 1997

Fast Eddy Current Forward Models Using Artificial Neural Networks, Bing Wang, John P. Basart, John C. Moulder

Review of Progress in Quantitative Nondestructive Evaluation

Eddy current testing is a widely used nondestructive evaluation (NDE) technique in which flaw information is extracted from the impedance change of a coil placed above a metal testpiece. Typical applications of eddy current NDE are the inspection of heat-exchanger tubes in steam generators of nuclear power plants and detection of hidden corrosion in the lap-splices of aircraft skins. To obtain quantitative information about flaw size and shape, we would like to have a forward model which is able to predict the impedance change of a coil for different flaws in the test geometry. Analytical solutions exist for simple test ...


Automatic Flaw Detection System For Eddy Current Inspection Of Steam Generator Tubes, Sheng-Fa Chuang, John P. Basart, John C. Moulder Jan 1997

Automatic Flaw Detection System For Eddy Current Inspection Of Steam Generator Tubes, Sheng-Fa Chuang, John P. Basart, John C. Moulder

Review of Progress in Quantitative Nondestructive Evaluation

Eddy current testing is a widely used nondestructive testing (NDT) method, particularly for inspecting the heat-exchanger tubes in steam generators. Due to the complex nature of the eddy current technique, the analysis of such inspection data is a difficult task which requires a huge amount of work by experienced human analysts [1]. This is time consuming and expensive. Human nature itself will cause some variance in analysis. Also, the variation of many different properties in the eddy current signal makes it very difficult to analyze. To overcome these obstacles, an eddy current automatic analysis system is needed to aid the ...


Electromagnetic Microscope For Deep, Pulsed, Eddy Current Inspections, Walter Podney, John Moulder Jan 1997

Electromagnetic Microscope For Deep, Pulsed, Eddy Current Inspections, Walter Podney, John Moulder

Review of Progress in Quantitative Nondestructive Evaluation

Superconductive quantum interference devices (SQUIDS), patterned in copper oxide superconductor, offer new technology for eddy current evaluation of materials using pulsed currents. Their high sensitivity at low frequency1 enables penetration of 15 mm or so of aluminum, through multiple layers, to identify millimeter fatigue cracks and material loss of a few percent from corrosion in underlayers[1][2]. Pulsed eddy currents[3] provide a three dimensional view of defects in sublayers and enable tomographic sectioning of multilayer structures. Together with SQUIDS, they would give an ELECTROMAGNETIC MICROSCOPE for examining fatigue damage and corrosion in underlayers of structures.


Design Considerations For The Remote Field Eddy Current Probe For Inspecting Ferromagnetic Flat Structures, S. Nath, Y. S. Sun, M. Mina Jan 1997

Design Considerations For The Remote Field Eddy Current Probe For Inspecting Ferromagnetic Flat Structures, S. Nath, Y. S. Sun, M. Mina

Review of Progress in Quantitative Nondestructive Evaluation

Traditionally the remote field eddy current (RFEC) phenomenon has been applied to the inspection of ferromagnetic tubes in heat exchangers, boilers etc. The RFEC probe has an exciter and sensor coil spaced such that most of the magnetic field received by the sensor is due to the field that has diffused through the pipe wall. The phase difference between the exciter and sensor signals is indicative of the defect dimensions. This same idea has been applied to the design of a new RFEC probe for inspecting flat ferromagnetic structures [1–3] and thick aluminum plates [4].


Basic Study Of A New Ect Probe Using Uniform Rotating Direction Eddy Current, Kiyoshi Koyama, Hiroshi Hoshikawa Jan 1997

Basic Study Of A New Ect Probe Using Uniform Rotating Direction Eddy Current, Kiyoshi Koyama, Hiroshi Hoshikawa

Review of Progress in Quantitative Nondestructive Evaluation

Recently, the development and the research of high performance new ECT probe have been conducted aiming at high precision ECT flaw testing [1–3]. The authors devised a new ECT probe named the Hoshi Probe using uniform rotating direction eddy current. The Hoshi Probe has the following features: 1) self-nulling, 2) self-differential, 3) lift-off noise free, and 4) different signal generation depending on the direction and the position of defect. The authors reported on these basic characteristics of the Hoshi Probe [4].


A Distinctive Featured Optimization Approach For Ect Probes, Z. Chen, K. Miya, M. Kurokawa Jan 1997

A Distinctive Featured Optimization Approach For Ect Probes, Z. Chen, K. Miya, M. Kurokawa

Review of Progress in Quantitative Nondestructive Evaluation

The conventional ways to evaluate the detectability of ECT probes are usually based on some type of Maxwell equations with use of the FEM or BEM method[1]. Though these approachs can give relatively accurate eddy current field and corresponding impedance, the numerical calculation needs a lot of computer memory and CPU time. This causes them a drawback in ECT probe optimization procedures. Moreover, probe optimization is not only related to the best choice of some parameters of a given probe configuration, but the configuration parameters such as the number and arrangement of exciting and pick-up coils also need to ...


Thick-Walled Aluminum Plate Inspection Using Remote Field Eddy Current Techniques, Y. S. Sun, W. Lord, L. Udpa, S. Udpa, S. K. Udpa, K. H. Ng, S. Nath Jan 1997

Thick-Walled Aluminum Plate Inspection Using Remote Field Eddy Current Techniques, Y. S. Sun, W. Lord, L. Udpa, S. Udpa, S. K. Udpa, K. H. Ng, S. Nath

Review of Progress in Quantitative Nondestructive Evaluation

The detection of defects that are located deep in thick walled ( >12 mm ) aluminum plates is of interest to both the aircraft and space industries. Conventional eddy current (EC) techniques are limited to the inspection of surface and subsurface anomalies. Newly developed high sensitivity magnetic sensors, such as magnetoresistive elements and superconducting quantum interface devices (SQUIDs) have enhanced the EC technique’s capability. Such sensors can be used to detect flaws that are located deep in aluminum plates. However, inspection of a defect located 12 mm to 25 mm below the surface of an aluminum plate is beyond the ability ...


Eddy Current Scanning For Testing Aircraft Structures; Requirements For Data Processing For An Application Spectrum From Riveted Al-Structures To Heat Damaged Cfrp, Wolfgang J. Bisle Jan 1997

Eddy Current Scanning For Testing Aircraft Structures; Requirements For Data Processing For An Application Spectrum From Riveted Al-Structures To Heat Damaged Cfrp, Wolfgang J. Bisle

Review of Progress in Quantitative Nondestructive Evaluation

Crack detection in riveted fuselages, sizing of corrosion areas and testing CFRP structures for heat damage initiated our working group step a little deeper into the problems of scanning with eddy current (EC). EC and especially scanning is mainly a method for in service inspections. Practical, light weight, effordable, easy to use solutions are required


Optimization Of Self-Nulling Eddy Current Probe For The Detection Of Shallow Fatigue Cracks, Buzz Wincheski, Shridhar Nath, Sarit Sharma, John Simpson Jan 1997

Optimization Of Self-Nulling Eddy Current Probe For The Detection Of Shallow Fatigue Cracks, Buzz Wincheski, Shridhar Nath, Sarit Sharma, John Simpson

Review of Progress in Quantitative Nondestructive Evaluation

The Self-Nulling Eddy Current Probe has been the focus of much research during the past several years [1–7]. Developed under NASA’s Airframe Structural Integrity Program, past research has focused on applying the Self-Nulling Probe technology to the inspection of damage to thin aluminum airframe skins. As a result of this work prototype fatigue crack detectors, single and multi-layer thickness gauges, and a system for the detection of cracks under installed fasteners have been developed[l–2,5–7]. The probe has also been successful at detecting surface flaws in thick bulk materials, for which a commercial instrument has ...


Mobile Hts Squid System For Eddy Current Testing Of Aircraft, H.-J. Krause, R. Hohmann, H. Soltner, D. Lomparski, M. Gruneklee, M. Banzet, J. Schubert, W. Zander, Y. Zhang, W. Wolf, H. Bousack, A. I. Braginski, M. L. Lucia, E. Zimmermann, G. Brandenburg, U. Clemens, H. Rongen, H. Halling, M. I. Faley, U. Poppe, H. Buschmann, G. Sporl, A. Binneberg Jan 1997

Mobile Hts Squid System For Eddy Current Testing Of Aircraft, H.-J. Krause, R. Hohmann, H. Soltner, D. Lomparski, M. Gruneklee, M. Banzet, J. Schubert, W. Zander, Y. Zhang, W. Wolf, H. Bousack, A. I. Braginski, M. L. Lucia, E. Zimmermann, G. Brandenburg, U. Clemens, H. Rongen, H. Halling, M. I. Faley, U. Poppe, H. Buschmann, G. Sporl, A. Binneberg

Review of Progress in Quantitative Nondestructive Evaluation

In Non-Destructive Evaluation (NDE), eddy current techniques are commonly used for the detection of hidden material defects in metallic structures. Conventionally, one works with an excitation coil generating a field at a distinct frequency. The eddy currents are deviated by materials flaws and the resulting distorted field is sensed by a secondary coil. Because of the law of induction, this technique has its limitations in the low frequency range. This leads to a decrease of the Probability of flaw Detection (POD) in larger depths.


Finite Element Modeling Of Eddy Current Testing Of Steam Generator Tube With Crack And Deposit, Toshiyuki Takagi, Junji Tani, Hiroyuki Fukutomi, Mitsuo Hashimoto Jan 1997

Finite Element Modeling Of Eddy Current Testing Of Steam Generator Tube With Crack And Deposit, Toshiyuki Takagi, Junji Tani, Hiroyuki Fukutomi, Mitsuo Hashimoto

Review of Progress in Quantitative Nondestructive Evaluation

Eddy current testing (ECT) is a type of non-destructive testing and effective for detecting surface cracks or flaws in conducting materials. A typical application of the ECT is non-destructive testing for heat exchanger tubes of steam generators (SG) of chemical plants and nuclear power plants. Higher accuracy is needed for external very small cracks from the practical point of view. Recently there are some requests not only for the location of cracks but also for the characterization of crack shape and the type of cracks. In order to improve the accuracy of the ECT it is considered to be important ...


Recent Developments In Eddy-Current Modeling, Harold A. Sabbagh, R. Kim Murphy, Anthony Chan, Elias H. Sabbagh, Lai Wan Woo, Michael C. Warnes Jan 1997

Recent Developments In Eddy-Current Modeling, Harold A. Sabbagh, R. Kim Murphy, Anthony Chan, Elias H. Sabbagh, Lai Wan Woo, Michael C. Warnes

Review of Progress in Quantitative Nondestructive Evaluation

In [1] we developed a volume-integral eddy-current model that is applicable to steam generator tubing. The model is now implemented in VIC-3D1, and in this paper we present some results computed with it.


A Novel Linear Array System For Inspection Of Large Metal Surfaces, R. F. Mostafavi, D. Mirshekar-Syahkal Jan 1997

A Novel Linear Array System For Inspection Of Large Metal Surfaces, R. F. Mostafavi, D. Mirshekar-Syahkal

Review of Progress in Quantitative Nondestructive Evaluation

Recently, much effort has been focused on the development of arrays for the non destructive testing (NDT) of metal structures in offshore, nuclear and aerospace industry, eg: [1–2]. The main feature of these arrays is electronic scanning which reduces (or eliminates) mechanical noise and makes rapid scan of large areas possible. Some arrays can also provide opportunity for direct signal processing of data. This paper is concerned with a linear array using the non-uniform field ac field measurement (ACFM) technique which is also known as the surface magnetic field measurement (SMFM) method [3].


Eddy Current Exam Simulation Using Coupled Finite Element/Volume Integral Or Finite Element/Boundary Element Method, Edith A. Creek, Robert E. Beissner Jan 1997

Eddy Current Exam Simulation Using Coupled Finite Element/Volume Integral Or Finite Element/Boundary Element Method, Edith A. Creek, Robert E. Beissner

Review of Progress in Quantitative Nondestructive Evaluation

The ability to model complex probes and simulate eddy current examinations is critical to our eddy current inspection system design efforts. The increasing demand for specialized probes creates the need to solve more complex problems than in the past. In this paper, we present details of an effort to improve our modeling capabilities by combining several techniques in order to provide greater flexibility in probe design. The result is a fast and efficient eddy current exam simulation capability for designing probes or probe arrays with complex geometry and material.


Barkhausen Noise And Eddy Current Microscopy (Bemi): Microscope Configuration, Probes And Imaging Characteristics, J. Bender Jan 1997

Barkhausen Noise And Eddy Current Microscopy (Bemi): Microscope Configuration, Probes And Imaging Characteristics, J. Bender

Review of Progress in Quantitative Nondestructive Evaluation

On the basis of the approved nondestructive testing methods Barkhausen noise measurement and eddy current testing a microscope for both techniques has been built up. With this Barkhausen Noise and Eddy Current Microscope (BEMI) high resolution images of the distribution of the maximum of Barkhausen noise and the eddy current coil impedance can be obtained by scanning a sensor with a precise manipulation unit over a test specimen [1, 2]. The aim is the characterization of magnetic and mechanical properties of ferromagnetic materials (e.g. thin films). In addition to a suitable manipulation and signal processing system probes with high ...


Development Of A Hand-Held, Flexible Eddy Current Probe For Inspection Of Curving Surfaces, T. Patton, R. Filkins, J. Fulton, K. Hedengren, J. Young, C. Granger, T. Hewton Jan 1997

Development Of A Hand-Held, Flexible Eddy Current Probe For Inspection Of Curving Surfaces, T. Patton, R. Filkins, J. Fulton, K. Hedengren, J. Young, C. Granger, T. Hewton

Review of Progress in Quantitative Nondestructive Evaluation

To enable GE Aircraft Engines to meet stringent engine inspection requirements a new eddy current inspection technology has been developed — the Eddy Current Array Probe (ECAP) [1]. The ECAP is based upon flexible micro-fabricated eddy current sensor elements that are capable of conforming to complex geometries. The technology has been used quite successfully by GE Aircraft Engines to inspect fracture critical aircraft engine components in a manufacturing environment.


An Eddy Current Model Based On Parametric Description Of Induced Current Loops, R. La, B. Benoist, B. De Barmon, R. Lengelle, P. Gaillard, J. Reuchet Jan 1997

An Eddy Current Model Based On Parametric Description Of Induced Current Loops, R. La, B. Benoist, B. De Barmon, R. Lengelle, P. Gaillard, J. Reuchet

Review of Progress in Quantitative Nondestructive Evaluation

Our objective is to design a “forward model” for steam generator tube flaw characterization, using eddy current technique. An investigation of the existing forward models was made.


Finite Element Modeling Of Eddy Current Probes For Edge Effect Reduction, Sarit Sharma, Ibrahim Elshafiey, Lalita Udpa, Satish Udpa Jan 1997

Finite Element Modeling Of Eddy Current Probes For Edge Effect Reduction, Sarit Sharma, Ibrahim Elshafiey, Lalita Udpa, Satish Udpa

Review of Progress in Quantitative Nondestructive Evaluation

Eddy current methods are a widely used technique in the nondestructive inspection of aircraft structures and parts. The method consists of inducing eddy-currents in the material being tested using a probe coil. The magnetic field produced by these eddy-currents opposes that of the probe coils (Lenz’s law) and the net effect is a reduced magnetic flux linking the coil. The presence of defects in the material under test disturbs the distribution of eddy-currents which in turn disturbs the net field. This change in the field is detected as a change in the impedance of the coil. The changes in ...