Magnetic Permeability Perturbation Testing Based on Unsaturated DC Magnetization for Buried Defect of Ferromagnetic Materials

Magnetic permeability perturbation testing (MPPT) owns the advantages of high sensitivity and stability for buried defect, but the signals are always affected by the externally applied magnetic field. The mechanism of MPPT method for buried defect under unsaturated magnetization is mainly discussed in detail and verified by simulations. A series of experimental results show that the multi-source effect exists in the MPPT detection. The MPPT signal for buried defects under unsaturated magnetization depends on both of the local magnetic permeability perturbation and magnetic leakage field of the defects. The detection results of different plate thickness show that the external magnetization field significantly affected the MPPT signal, presenting a significant "convex-concave" nonlinear feature, which is helpful for the selection of magnetization current in practical detection. Clarifying the dominant mechanism of the MPPT signal for buried defect is of great significance to precision evaluation of cracks and related applications.