An eddy current probe suitable to gain information about the depth of near-side flaws much deeper than the depth of penetration

This study proposes an approach to gain information about the depth of near-side flaws using eddy currents. The approach utilizes only two coils, one of which works as an exciter and the other as a detector, like conventional eddy current testing using a transmitter–receiver probe. The uniqueness of this approach is that signals obtained by this approach change significantly with the depth of a flaw even though the flaw is much deeper than the depth of penetration. After the physical background of the approach is explained, its validity is confirmed in experiments. The experiments utilize a 25 mm thick austenitic stainless steel plate with five artificial rectangular slits of 40 mm length, 0.5 mm width, and 1, 5, 10, 15, and 20 mm depth. The experiments confirm clear differences between signals generated by the five slits even although the exciter is driven at 50 kHz at which the depth of penetration is approximately 2.0 mm. Subsequent finite element simulations are carried out to confirm the validity of the experimental results and to support discussion about the physical background of the approach.