Modeling and Experimental Investigation of Pulsed Eddy Current and Magnetic Saturation for Testing of Ferromagnetic Metals with Sub-surface Pitting

Pulsed eddy current (PEC) technique is commonly used for the detection of sub-surface defects in electrically conductive metals. However, due to the limited penetration depth of eddy currents, the detection of sub-surface defects in ferromagnetic metals is limited while using PEC technique. In order to extend the application of PEC technique for the detection of sub-surface defects in ferromagnetic metals, the penetration depth of eddy currents needs to be increased. For deeper penetration of eddy currents in the material, magnetic saturation of the tested specimen is a useful solution. In magnetic saturation state, the magnetic permeability of the ferromagnetic metal is decreased and stabilized and, as a result, the penetration depth of eddy currents is increased. In this paper, the performance of the PECT for detection of sub-surface pitting defects in the magnetized ferromagnetic specimen has been investigated through finite element modeling (FEM) and experimental studies. The tested specimen is a 10mm-thick steel plate, in which sub-surface pitting defects with various depths have been modeled. A probe consisting of a driver coil, a pickup coil, and a ferrite core is used to measure the time-varying PEC signals. Then, the time domain features of the differential PEC signals are extracted and used to detect the sub-surface pittings. The results indicate that PEC technique together with magnetization can effectively detect sub-surface pitting defects.