実働荷重下の繰返し塑性ひずみと累積疲労損傷

疲労限度以下の領域における限界修正S-N曲線

抄録

Through the study of the push-pull type programmed fatigue test having the stress amplitude below the fatigue limit with the intermittent high level stress on a medium carbon steel S35C, the authors previously obtained the following primary conclusions: In the constant stress amplitude test, there is the fatigue limit to the plastic strain as there is to the stress. In the programmed test the plastic strain range-pair corresponding to the stress below the fatigue limit becomes larger than that in the constant stress amplitude test, because of the influence of the intermittent high level stress. Moreover, the fatigue limit in terms of the plastic strain disappears and the fatigue damage caused by the plastic strain range-pair below the fatigue limit is linearly accumulated.
In this study, the programmed fatigue test has been conducted more systematically on the same material in order to investigate the effects of some secondary factors on the fatigue damage in the programmed fatigue test.
As a result, it is suggested that there exists the critical modified S-N curve which may always give a conservative estimation of fatigue lives under all kinds of actual loadings, and also this can be obtained by carrying out the test under a special condition on small number of specimens.
The proposal has been examined on the results of programmed fatigue tests of a low carbon steel S25C and is found to be applicable for estimation of fatigue lives under actual loadings. In addition, the programmed fatigue test has been carried out on a high strength alloy steel SNCM 8.