Characterization of spontaneous magnetic signals induced by cyclic tensile stress in crack propagation stage

Influenced by the geomagnetic field, crack can induce spontaneous magnetic signals in ferromagnetic steels. The normal component of surface spontaneous magnetic signals of the center-cracked sheet specimens, Hp(y), was measured throughout the tension-tension fatigue tests. The variation of Hp(y) and its maximum gradient Kmax in the crack propagation stage were studied. It shows that Hp(y) began to change its polarity, just right on the crack position, in the intermediate stage of crack propagation. The cause for this phenomenon was also discussed. The peak-to-peak value, ΔHp(y), of the magnetic signal when Hp(y) changing its polarity was collected, and discrete wavelet transform (DWT) was further used to acquire high frequency components of the Hp(y) signal. The results show that the Kmax increased exponentially with the increase of loading cycles; an approximate linear relationship was found between Kmax and crack length 2a in the intermediate stage of crack propagation; and the high-frequency component of Hp(y) can be used to identify the late stage of crack propagation.