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 Hp(y) signals of stray field were performed during the whole tensile test. The results showed that the stress-induced Hp(y) signal curve had good linearity after loading, i.e., the slope coefficient Ks 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.