Sizing Defects Much Deeper than the Depth of Penetration from Eddy Current Signals

This study demonstrates superposing several eddy current distributions enables one to size defects much deeper than the depth of penetration from eddy current signals. A probe developed in an earlier study of the authors is utilized for the demonstration. The probe consists of four large exciters and one detector; the uniqueness of the probe is to realize an eddy current distribution that does not decay exponentially directly below the detector. As a consequence the probe shows clear difference between signals due to two defects of different depths even though both of them are deeper than the depth of penetration. Inverse problems of evaluating the profiles of defects from measured signals are solved using parameter-free genetic algorithm. A defect is modelled as a rectangular region, and coded as a 24-bit integers representing depth, conductivity, and left and right position of the edge of the defect. Edge-element-based finite element code based on Ar formulation, accelerated by a database approach, is utilized to evaluate signals due to a defects. Numerical simulations show good agreement between true and evaluated near-surface defect profiles even though a defect is 20 mm deep and an exciting frequency of 100 kHz is adopted.