Estimation of defect depth on plates by eddy-current coil array

Eddy-current (EC) testing is one of the contactless nondestructive testing techniques that has been frequently applied to evaluate the defects on metallic objects. In this paper, an estimation method of defect depth on metallic plates is proposed based on the approximate analytical model. An EC coil array sensitive to various defect orientation is designed to scan the plate surface. The independent inductance from various ferrite-core coil pairs can be found by the approximate analytical model. The influence of defect on measurements, including depth and edges, is regarded as the variation of probe lift-off in the tested region, defined as the equivalent lift-off. Based on the approximate analytical model, when defect edges have relatively smaller influence, the defect depth on the plate surface can be approximately obtained by estimating the equivalent probe lift-off through optimisation. In numerical simulations and experiments, the proposed method is evaluated by testing the aluminum and stainless-steel plates with cracks by the coil array. The reconstructed equivalent lift-off distribution closely relates to the position and dimensions of defects on plates.