Defect localization method for ferromagnetic pipes based on stepped magnetization in magnetic permeability perturbation testing

Abstract

The magnetic permeability perturbation testing (MPPT) method is effective for evaluating thick-walled steel pipes. However, determining the burial depth of a defect directly from a single time-domain signal faces challenge. The concept of magnetization layering is introduced, and a defect localization method based on stepped magnetization for ferromagnetic pipes is proposed, which focuses on the change of magnetization layering due to the non-uniform magnetization. The detection signals are acquired under a magnetic field with a step change in intensity, and the defects are localized by the characteristic current $I_{\Delta V}$. The relationship between the characteristic current and the magnetization depth $h$ is verified by equivalent magnetic circuit analysis and finite element simulation analysis. Experiments on different buried defects verify that the characteristic current $I_{\Delta V}$ can reflect the buried depth of defects. Finally, some key dimensional parameters of the magnetizer are discussed and optimized. The method is of great practical value for localizing defects from the signal source, which applies to different ferromagnetic pipes.