A dual-dipole model for stress concentration evaluation based on magnetic scalar potential analysis

Abstract

Experiment shows that the normal component and the amplitude of the spontaneous magnetic signals on the surface of the ferromagnetic part, induced by a stress concentration zone (SCZ) caused by local plastic deformation, have only one peak. These waveform characteristics are precisely opposite of the defect identification criteria proposed by the metal magnetic memory (MMM) method, which include that the normal component of magnetic signals changes to its polarity and the tangential component reaches a peak value. At the beginning, a magnetic dual-dipole model is accordingly proposed to describe and evaluate the stress concentration in ferromagnetic materials. The contour maps of the magnetic scalar potential show that there is a magnetic source generated in the damaged area. This source can be simplified as a dual-dipole. It comes from the two peaks of stress variation at the edges of the SCZ. Furthermore, the maximum potential of magnetic anomalies was obtained. The values are influenced by the applied loading and deflection of the specimen. It decays exponentially with the increase of lift-off values in the air domain. Finally, residual stresses along the scanning line were measured to confirm the theoretical analysis. This dual-dipole model in the MMM technique is feasible to evaluate the stress concentration caused by local plastic deformation.