A comprehensive review of advancements in eddy current (EC) modeling is presented. This paper contains three main sections: a general treatise of EC theory, the thin skin EC forward modeling, and the EC inverse problem. (1) The general treatise of eddy current theory begins with an exposition of the reciprocity formulas for evaluating probe impedance changes, which are derivable from first principles. Two versions of the reciprocity formulas, one with a surface integral and the other with a volume integral, are given. Any particular type of defect, as well as both one-port and two-port probes, can be treated. Second, a brief account of analytical and numerical methods for calculating the field distributions is presented. Third, theory of probe/material interactions with various defect types is described. (2) The paper then proceeds to the forward modeling section, which contains a detailed treatment of the eddy current forward problem for surface breaking cracks and EDM notches in the thin skin approximation. (3) The inverse problem section begins with a general review of commonly used inversion methods, exemplified by selected references from the literature, followed by more detailed examinations of EC inversions for surface breaking cracks and slots. The last part of this section is devoted to the inverse problem for layered structures. Although being a review in nature, the paper contains a number of new accounts for time-domain eddy current interactions. In particular, a modification is proposed to the reciprocity formula in order to take a better account of pulsed eddy current signals.

Review of Advances in Quantitative Eddy Current Nondestructive Evaluation

We attempt a comprehensive review of all published research in nondestructive evaluation (NDE) performed with the superconducting quantum interference device (SQUID) magnetometer since the first work was reported in the mid-1980s The SQUID is the most sensitive detector of magnetic flux known The energy sensitivity of the SQUID may make it the most sensitive detector of any kind The research on SQUIDs for NDE is based on the promise of that sensitivity and on the various other desirable properties developed for SQUID instrumentation in biomagnetism and other fields The sensitivity of SQUID instruments down to very low frequencies allows them to function as eddy-current sensors with unparalleled depth resolution, and to image the static magnetization of paramagnetic materials and the flow of near-dc corrosion currents The wide dynamic range of the SQUID makes it possible to image defects in steel structures and to measure the magnetomechanical behaviour of ferromagnetic materials with high sensitivity In the last decade SQUID instrumentation designed specifically for NDE has appeared and improved the spatial resolution of most work to roughly 1 mm, with promise of another order of magnitude improvement within the next five years Algorithms for flaw detection and image deconvolution have begun to flourish With many talented, industrious people in the field, the future of SQUID NDE looks bright, provided the crucial first niche can be found

http://iopscience.iop.org/article/10.1088/0022-3727/30/3/002/pdf

SQUIDs for nondestructive evaluation

This paper gives a guideline to the literature of magnetic methods for nondestructive evaluation of materials for flaws and defects. This is a sequel to an earlier paper which described magnetic methods for nondestructive evaluation of stress, plastic deformation and microstructure. The present paper discusses magnetic particle inspection, magnetic flux leakage, leakage field calculations and eddy current inspection, including the remote field electromagnetic inspection method. An extensive survey of recent publications in the field is given.

Review of magnetic methods for nondestructive evaluation (Part 2)

Pulsed eddy current (PEC) thermography has been successfully applied to detect cracks in metal alloy and carbon fibre reinforced plastic (CFRP) in previous works. In these applications, an inductor and infrared camera are on the same side of components, named reflection mode. In this work, the transmission mode, where the inductor and infrared camera are on opposite sides of component is investigated for defect characterisation through the analytical analysis and experimental studies. The studies show that the detection mechanisms for impact and delamination in CFRP are totally different. Carbon structure can be observed on the early stage of heating phase and impact leading to decreasing conductivity can be also detected in heating phase. However, delamination can be characterised using late stage of cooling phase. Combing the detection mechanisms, principal components analysis and independent components analysis, image reconstruction method is used to improve the sensitivity.

Defect characterisation using pulsed eddy current thermography under transmission mode and NDT applications

Eddy-current nondestructive evaluation commonly carried out using single frequency time harmonic excitations, but a pulsed excitation offers a simple and effective alternative. The pulse signals have been calculated for a probe coil whose current rises and falls exponentially, approximating a square wave when the exponential time constant is small. Predictions of the induced electromotive force (EMF) across a coil above a half-space conductor and of the magnetic field on the coil axis have been compared with experiments. The comparison shows excellent agreement between theory and experiment.

/pdf/pulsed-eddy-current-response-to-a-conducting-half-space-3sbsavoa42.pdf

Hugo L. Libby

Papers

Introduction to Electromagnetic Nondestructive Test Methods