Early detection and monitoring of fatigue damage in medium carbon steel based on multi-micromagnetic NDT fusion method

Based on stress-controlled fatigue tests of 45 carbon steel notched specimens, an in-situ comprehensive evaluation of fatigue damage evolution was accomplished via micromagnetic non-destructive testing techniques, and various magnetic parameters were then determined to describe the actual fatigue state. In general, the characteristics quantities extracted from magnetic hysteresis, Barkhausen noise and incremental permeability measurements exhibited a trend of initial oscillations during the early stages of fatigue, reflecting fatigue softening. As the load cycle increased, the magnetic parameters became stabilized and experienced drastic changes in the final stage before failure. Evidently, the dynamic dependence of fatigue magnetization behavior was related to the progressive accumulation of damage by the cyclic loading. Among those, the magnetic incremental permeability method was highly sensitive to the fatigue damage before a macroscopic crack was initiated. Particularly, the Mean eigenvalues of incremental permeability signals presented a nonlinear increment behavior, starting approximately from the 80 % of fatigue life, which could be used to assess the degree of damage quantitatively. Furthermore, the unique zero-crossing feature of Mean value at 94.2 % lifetime can serve as a warning for imminent fatigue failure of the component.