Normal Magnetizing-based Eddy Current Testing Method for Surface Crack and Internal Delamination of Steel Plate

As stated in previous research, based on the permeability perturbation mechanism, the normal magnetizing-based eddy current testing (NMB-ECT) probe could effectively detect delamination defects inside the steel plate. However, the detection signal of surface cracks was also observed during practical inspection. Therefore, the signal characteristics of surface cracks detected by the NMB-ECT probe were explored in this paper. Moreover, the feasibility of distinguishing those two types of defects was further demonstrated. Firstly, the detection mechanism of surface cracks using the NMB-ECT probe is theoretically elucidated. Secondly, based on the finite element model, the relationship between detection signals and the surface cracks is revealed. Finally, the feasibility of distinguishing surface cracks and delamination defects is demonstrated by experiments. According to the research, the surface crack and delamination of the steel plate can be distinguished by the NMB-ECT probe based on the signal polarity, which was conducive to identifying the defect types by the threshold method in the automatic inspection.