Mechanism of magnetic memory signal variation in the process of fatigue

Research on the phenomenon of magnetic memory from a microscopic view is the gateway to mechanism study, and the change in microstructure is caused by plastic deformation. Therefore, tensile fatigue tests are designed to study the relation between the variation of the tangential magnetic signal Hpx and the changes of microstructure and plastic strain. The experimental results show that, in the initial stage of fatigue, Hpx enhances quickly, and the plastic deformation is rapid; in the middle stage of fatigue, Hpx enhances gently, the plastic deformation slow down, and the micro cracks generate at the grain boundaries and grow slowly; and in the later stage of fatigue, Hpx has a hop comparing with the middle stage and speeds up, the plastic deformation intensifies, and the macro cracks form and expand quickly. Analysis reveals that, the plastic deformation produces crystal defects such as micro cracks, and blocks the magnetization of material; the area of plastic deformation accumulation thus becomes an interior magnetic source, scatters a field outward and forms the magnetic memory phenomenon. The expression of the magnetic memory field HHp is obtained, and Hp varies with the location of the measurement point and the permeability μ (or plastic strain εpx) in the area of plastic deformation accumulation. The magnetic memory signal along a certain direction at a certain point has a linear variation with μ and a monotone increasing variation with εp. Therefore, Hpx. has different rates of enhancement in three stages of fatigue because of the different rates of εp. And the sensitivity of the magnetic memory signal to εp can be improved by choosing an appropriate measurement point and an appropriate direction of measurement. © 2014 Journal of Mechanical Engineering.