Normal Magnetizing-Based Eddy Current Testing Method for Multidirectional Cracks on Steel Plate Surface Based on Permeability Perturbation

Conventional-magnetizing-based eddy current testing (CMB-ECT) typically applies a direct current (DC) tangential magnetization field parallel to the surface of the steel plate. The interaction between the tangential magnetization field and the surface crack increases the relative permeability of the region of the steel plate around the crack. The magnitude of the variation in the relative permeability in this area is affected by the angle between the tangential magnetization direction and the crack orientation. Consequently, the CMB-ECT probe cannot guarantee its signal amplitude consistency in detecting multidirectional surface cracks of the same size. Moreover, limited by the magnetizer size, the CMB-ECT probe cannot detect both ends of the in-service steel plate. To solve the problems, the normal magnetizing-based eddy current testing (NMB-ECT) method was proposed in this study. Firstly, the mechanism of the permeability perturbation caused by cracks under tangential and normal magnetization is compared. Then, the effects of crack orientation and magnetization on the permeability perturbation amplitude under tangential and normal magnetization are explored based on the finite element model. Finally, the validity of the theoretical and simulation analysis is verified througqh experiments. It was shown that the NMB-ECT probe could effectively suppress the effect of the crack orientation on the permeability perturbation amplitude, thereby ensuring the signal amplitude consistency in detecting multidirectional surface cracks of the same size.