**Bin Liu 1,\*, Peng Fu 1, Ruifeng Li 1, Peng He <sup>2</sup> and Shiyun Dong <sup>3</sup>**


Received: 9 October 2019; Accepted: 2 December 2019; Published: 4 December 2019

**Abstract:** Based on the magneto-mechanical effect, the influence of crack size on stress evaluated with metal magnetic memory (MMM) technology was discussed in this paper. Based on equivalent theory, the regular rectangular grooves, with different widths and depths, were precut in the surface of an experimental sample for simulating surface crack, and a three dimensional electrically controlled displacement system was used to collect the *H*p(*y*) signal of the sample under different stresses, and the fracture morphology was observed by using scanning electron microscopy (SEM). The results show that the influence of detection line on *H*p(*y*) signal can be ignored; as stress increases, the *H*p(*y*) signal turns counterclockwise around zero-crossing point and its mutation, corresponding to the location of groove, becomes distinct gradually. When groove depth is constant, the magnetic intensity gradient changes in the form of quadratic polynomial as groove width increases, and when the groove width is the same, the magnetic intensity gradient is a linear function of groove depth. When stress reaches the yield strength of the material, the magnetic intensity gradient decreases gradually as stress increases further, and the orientation of magnetic domain is seen as the main reason for that result. At last, the experimental results are discussed based on the piezomagnetic effect and leakage magnetic field theory of finite depth slit model, and the change of magnetic domain orientation is considered to be the main reason.

**Keywords:** non-destructive evaluation; crack effect; metal magnetic memory; magnetic intensity gradient; stress
