Cumulative fatigue damage of circumferentially cracked 0.35% carbon steel specimens under two step alternating stresses was investigated in rotating bending. Virgin cracked specimens of 0.16mm depth were made of sharp notched specimens by machining away the notch and annealing in vacuum after the fatigue crack was initiated at the notch root. The fatigue limit of the virgin cracked specimens σ
1w, based on the net area, was 17.8kg/mm
2 (the fatigue strength reduction factor β=1.44). The fatigue crack propagation in the virgin cracked specimens at three first stresses σ
1=23.5, 21.0 and 18.5kg/mm2 that were higher than σ
1w was examined, and the virgin cracked specimens were subjected to
n1 cycles at σ
1, a process known as overstressing, until the crack propagated from 0.16 to 0.30mm depth. Then, the numbers of cycles of life remaining
n2 and the fatigue limits σ
2w of the cracked specimens of 0.30mm depth at various second stresses σ
2 were examined. The results obtained are as follows:
(1) The cumulative cycle ratio Σ
n/N has values of 1.32∼0.71 when σ
2>σ
1w. Though Σ
n/N appears to be greater than 1 when σ2>σ1 and less than 1 when σ
2<σ
1, we could say that the linear damage law is nearly substantiated. (2) The fatigue limit σ
2w is reduced by the first stressing (overstressing effect) because the crack propagates at the first stress σ
1. Though it appears that the higher is σ
1, the smaller is the reduction of σ
2w,
i.e. σ
2w=15.6 when σ
1=23.5, σ
2w=14.6 when σ
1=21.0 and σ
2w=14.3kg/mm
2 when σ
1=18.5kg/mm
2 for the same 0.30mm crack depth, σ
2w is evaluated by means of the crack depth if σ
1 is not so high.
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