Nondestructive testing method for internal defects in ferromagnetic materials under weak bias magnetization.

• Proposing a weak magnetization-based dynamic permeability testing method for internal defects in ferromagnetic materials. • Thorough analysis of the detection principle and performance validation through simulations and experiments. • Insightful investigation of key factors affecting detection signal through experiments. • The smaller magnetization (current) than that of conventional magnetic flux leakage testing under a similar detection capability. Magnetic flux leakage (MFL) testing is effective in detecting internal defects in ferromagnetic materials. However, it requires the saturation magnetization of materials, which is not always practicable in complex inspection environments. Therefore, this study presents a novel dynamic permeability testing (DPT) method, in which internal defects will induce dynamic permeability variations on the surface of weakly magnetized ferromagnetic materials, and the variations are then detected by alternating current detection coils with magnetic cores, enabling the identification of internal defects. The principle of DPT is analyzed through simulations, which are subsequently verified by experiments. Furthermore, the effects of excitation frequency, probe lift-off, and defect size on the DPT signal are investigated experimentally. Finally, the comparison with conventional MFL demonstrates that the DPT method can effectively identify the internal defect using a weak magnetization current of 0.1 A (about 0.25 T in the steel plate), whereas MFL requires 0.5 A (about 1.32 T) to achieve a similar detection capability under the same probe lift-off values. This new method has significant practical value, especially in situations where saturation magnetization is not feasible. [ABSTRACT FROM AUTHOR]