Grain size effect on metal magnetic memory signal for stress damage evaluation of low carbon steel

Based on the magneto-mechanical effect of ferromagnetic material, the grain size effect on stress damage evaluated with metal magnetic memory (MMM) technology was discussed in this paper. To collect the normal component of MMM signal – Hp(y) signal of sample, the three dimensional electrically controlled displacement system was employed to keep constant scanning speed and lift-off of sensor probe, and the magnetic intensity gradient of Hp(y) signal, as characteristic parameter for stress damage evaluation, was determined with least square fitting method, then the relationship between slope of magnetic intensity gradient and grain size was discussed. The results show that 1.0 mm is the most optimal lift-off of sensor probe, and the influence of detection line on Hp(y) signal can be ignored. As stress increases, the Hp(y) signals change linearly, and all the Hp(y) signals intersect at the only zero crossing point, while when stress exceeds yield strength, the Hp(y) signals change irregularly as stress increases further, at last the polarity of Hp(y) signal is opposite at the position of fracture of sample. As grain size increases, the magnetic intensity gradient decreases slightly, and the deformation capacity affected by grain size is seen as the main reason for above experimental results. Based on polynomial function, the relationship between slope of magnetic intensity gradient and grain size is determined, and then the correction method for grain size effect on stress damage evaluation is proposed. Finally, the theoretical discussion is verified by fracture morphology observation.