Review of advances in quantitative eddy current nondestructive evaluation

A comprehensive review of advancements in eddy current (EC) modeling is pre sented. This paper contains three main sections: a general treatise of EC t heory, the thin skin EC forward modeling, and the EC inverse problem. (1) T he general treatise of eddy current theory begins with an exposition of the reciprocity formulas for evaluating probe impedance changes, which are der ivable from first principles. Two versions of the reciprocity formulas, one with a surface integral and the other with a volume integral, are given. A ny 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 method s for calculating the field distributions is presented. Third, theory of pr obe/material interactions with various defect types is described. (2) The p aper then proceeds to the forward modeling section, which contains a detail ed treatment of the eddy current forward problem for surface breaking crack s and EDM notches in the thin skin approximation. (3) The inverse problem s ection begins with a general review of commonly used inversion methods, exe mplified by selected references from the literature, followed by more detai led examinations of EC inversions for surface breaking cracks and slots. Th e last part of this section is devoted to the inverse problem for layered s tructures. Although being a review in nature, the paper contains a number o f new accounts for time-domain eddy current interactions. In particular, a modification is proposed to the reciprocity formula in order to take a bett er account of pulsed eddy current signals.