A nondestructive strategy to evaluate deep near-side defects from eddy current signals

This study demonstrates near-side slits much deeper the depth of penetration can be sized using eddy current signals. The key idea is to superpose eddy current distributions and locally realize a unique eddy current distribution that does not decay exponentially and has an abrupt phase change in depth direction. Eddy current testing using an eddy current testing probe designed on the basis of this idea is carried out with an exciting frequency of 100 kHz to gather signals due to 10-20 mm deep near-side slits fabricated in an austenitic stainless steel plate with a thickness of 25 mm. The measured signals show clear differences although all the slits are much deeper than the depth of penetration. Subsequent numerical simulations are conducted to size the slits from the measured signals. The algorithm utilized models a slit as a rectangular region having a constant width and uniform conductivity inside, and evaluates its depth, conductivity, and left and right edge locations using parameter-free genetic algorithm. The simulations evaluate slits with a depth of 12,15, and 20 mm as 12.0, 15.4, and 16.8 mm, respectively.