Fatigue damage accumulation around notches

Notched compact tension specimens of type 304 stainless steel (notch tip radius of 1 mm and 4 mm) were low-cycle-fatigued at stress ratio R = −1 and 0.05 in order to investigate the through-thickness fatigue damage accumulation around the notch. The recrystallization-thermal etching technique was used to observe the accumulated plastic zone APZ with the accumulated plastic strain ∊ac above 0.12(APZ0.12) and APZ with ∊ac above 0.02 (APZ0.02) at midthickness, quarter thickness and surface. Both APZ0.12 and APZ0.02 are formed early during the fatigue (before crack initiation) near midthickness and not at less thickness or at the surface. The reason why the fatigue crack initiates near the midthickness of the notch root surface was found to be due to more fatigue damage near midthickness than at the surface. Magnetic permeability(degree of the martensite formation) at midthickness was also higher than at the surface, supporting the above result.