Development of an eddy current non-destructive testing method applicable to the evaluation of the depth of a surface breaking crack appearing in a ferromagnetic material

This study develops an eddy current testing probe whose signals due to a surface breaking crack appearing in a ferromagnetic material exhibit clear change with respect to the depth of the crack. Experimental verifications are conducted using a type 430 ferritic stainless plate specimen with five artificial slits whose depths are 1, 3, 5, 10, and 15 mm. Although exciting frequencies adopted were 25, 50 and 100 kHz, there is clear difference between signals due to the 10 mm deep slit and those due to the 15 mm deep one. Subsequent finite element simulations are conducted to discuss the physical background of the difference on the basis of the reciprocity theorem and electromagnetic fields.