The stress amplitude of most of service loads is below the fatigue limit. In the previous paper, we examined the effect of such stress amplitude on the fatigue life by the program fatigue tests, and found that the fatigue life can be shorter or longer than that estimated by Miner's law. In the present paper, two step and two fold program fatigue tests were carried out and the crack propagation behavior was observed to clarify the effect of fatigue crack propagation by stress amplitudes below the fatigue limit on the fatigue life.
The results reveal that the fatigue crack is propagated by the stress amplitude below the fatigue limit, and that the crack propagation rate in the first period of the repetition of the stress amplitude below the fatigue limit is higher than the estimated value obtained by extrapolating the result in the range of stress amplitudes above the fatigue limit to the range of stress amplitudes below the fatigue limit. It seems that this acceleration effect of the crack propagation is caused by the preloading due to the stress amplitude above the fatigue limit. On the other hand, the crack propagation by the stress amplitude above the fatigue limit in the first period of that repetition is retarded by the preloading due to the stress amplitude below the fatigue limit. As a crack propagates, this retardation effect disappears and the crack propagation rate gradually recovers to that of the constant stress.
Therefore, the fatigue life under service loads should be estimated by considering both the acceleration and retardation behaviors in crack propagation by the stress amplitudes below the fatigue limit.
