Fatigue damage evaluation by metal magnetic memory testing

Abstract

Tension-compression fatigue test was performed on 0.45% C steel specimens. Normal and tangential components of magnetic memory testing signals, H _p(y) and H _p(x) signals, with their characteristics, K of H _p(y) and H _p(x)_M of H _p(x), throughout the fatigue process were presented and analyzed. Abnormal peaks of H _p(y) and peak of H _p(x) reversed after loading; H _p(y) curves rotated clockwise and H _p(x) curves elevated significantly with the increase of fatigue cycle number at the first a few fatigue cycles, both H _p(y) and H _p(x) curves were stable after that, the amplitude of abnormal peaks of H _p(y) and peak value of H _p(x) increased more quickly after fatigue crack initiation. Abnormal peaks of H _p(y) and peak of H _p(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 H _p(x)_M ranged from 270 to 300 A/m under the test parameters in this work. After failure, both abnormal peaks of H _p(y) and peak of H _p(x) reversed, K value was 133 A/(m·mm)⁻¹ and H _p(x)_M was −640 A/m. The results indicate that the characteristics of H _p(y) and H _p(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).