A magnetic permeability perturbation testing methodology and experimental research for deeply buried defect in ferromagnetic materials

Abstract

DC magnetization is generally considered to suppress the usual local magnetic permeability variation of ferromagnetic materials but also causes neglected non-uniform magnetic characteristics. This paper proposes a new magnetic permeability perturbation testing (MPPT) method, which uses DC magnetization to excite surface permeability perturbation of deeply buried defects and uses an eddy current probe to sense this surface permeability perturbation. The mechanism of MPPT is analyzed in detail and verified by a series of experiments. The effects of the differential distance of probe, crack size, insulation thickness, and excitation frequency on MPPT are investigated experimentally. Finally, the comparison of the proposed MPPT method with traditional eddy current testing and magnetic flux leakage testing is discussed in detail. The proof-of-concept results indicated that the MPPT testing method can realize the detection of internal defects of a 25 mm thick steel plate. This method is completely feasible and has great practical value as a supplement to conventional nondestructive testing methods.