Fatigue damage evaluation by metal magnetic memory testing

Tension-compression fatigue test was performed on 0.45% C steel specimens. Normal and tangential components of magnetic memory testing signals, Hp(y) and Hp(x) signals, with their characteristics, K of Hp(y) and Hp(x)M of Hp(x), throughout the fatigue process were presented and analyzed. Abnormal peaks of Hp(y) and peak of Hp(x) reversed after loading; Hp(y) curves rotated clockwise and Hp(x) curves elevated significantly with the increase of fatigue cycle number at the first a few fatigue cycles, both Hp(y) and Hp(x) curves were stable after that, the amplitude of abnormal peaks of Hp(y) and peak value of Hp(x) increased more quickly after fatigue crack initiation. Abnormal peaks of Hp(y) and peak of Hp(x) at the notch reversed again after failure. The characteristics were found to exhibit consistent tendency in the whole fatigue life and behave differently in different stages of fatigue. In initial and crack developing stages, the characteristics increased significantly due to dislocations increase and crack propagation, respectively. In stable stage, the characteristics remained constant as a result of dislocation blocking, K value ranged from 20 to 30 A/(m·mm)−1, and Hp(x)M ranged from 270 to 300 A/m under the test parameters in this work. After failure, both abnormal peaks of Hp(y) and peak of Hp(x) reversed, K value was 133 A/(m·mm)−1 and Hp(x)M was −640 A/m. The results indicate that the characteristics of Hp(y) and Hp(x) signals were related to the accumulation of fatigue, so it is feasible and applicable to monitor fatigue damage of ferromagnetic components using metal magnetic memory testing (MMMT).