Abstract
It is well known that the stress amplitudes below the fatigue limit can damage the materials under the programmed fatigue tests or in service loadings. But its proper estimation has not yet so far been established. The authors have found that the linear cumulative fatigue damage law in terms of the plastic strain range-pair gave good estimation of the fatigue lives in low and high cycle fatigue nearly up to the fatigue limit with the low or medium carbon steels. From such a view point, the push-pull type programmed fatigue tests having the stress amplitude below the fatigue limit with intermittent high level stress were carried out with unnotched specimens of the normalized medium carbon steel S35C, and the behavior of the plastic strain was investigated.
In the constant stress amplitude test, it is observed that there is fatigue limit to the plastic strain as there is to the stress. In the programmed tests the plastic strain range corresponding to the stress below the fatigue limit is found to be larger than that of the constant stress amplitude, due to the influence of intermittent high level stress. Moreover, it is found that the fatigue limit in terms of the plastic strain has disappeared, and that the fatigue damage caused by the plastic strain range below it is linearly accumulated.
