Stress dependence of the spontaneous stray field signals of ferromagnetic steel

Measuring spontaneous stray field signals provides a promising tool to analyze the stress in ferromagnetic materials. However, strong initial stray field signals on the surface of ferromagnetic materials originating from various manufacturing processes can disturb stress-induced stray field signals. Consequently, it is necessary to conduct a study that will clarify the stress dependence of spontaneous stray field signals by eliminating initial random signals. In the present work, the focus is placed on sheet specimens that have a clean initial magnetic state by means of vacuum heat treatment. Measurements of the normal component H_p(y) signals of stray field were performed during the whole tensile test. The results showed that the stress-induced H_p(y) signal curve had good linearity after loading, i.e., the slope coefficient K_s increased continuously in the elastic deformation stage but decreased slightly during the plastic deformation stage. This phenomenon was discussed and explained from both the stress-induced effective magnetic field and residual compressive stress viewpoints.