Progress in Nondestructive Evaluation of Stress Concentration with MMM Method

It has been proved by experiments that the metal magnetic memory (MMM) NDT method can effectively detect the stress concentration of ferromagnetic metal parts. But it is still hard to evaluate the stress concentration quantitatively because of the confusion in the mechanism of MMM method. In this paper experiments are designed and carried out to investigate the relationship between the MMM signal and its key influencing factors: stress concentration and magnetic field, one part in the mechanism of MMM method. The result shows that the p-p value of MMM signal is almost linear with the maximum stress in an appropriate arrange under the geomagnetic field. But on the other hand, the influence of magnetic environment can’t be ignored; particularly in detection the MMM signal may be quite different with the same stress when magnetic field differs. This result provides a potential method: to get a more reliable evaluation of stress concentration with multimagnetic field in detection. Also, the data presents that the MMM signal during generation is more sensitive with the weak magnetic environment (in the range of geomagnetic level) than strong magnetic field (over several times of geomagnetic field). Magnetic field should be paid much more attention for MMM testing. Keyword: Nondestructive Evaluation, Metal Magnetic Memory, Environmental magnetic field,stress concentration