Statistical method for decision of S-N curve with fatigue limit

The experimental results of fatigue test near the stress levels around fatigue limit have large scatter. In this paper, we propose the statistical method for decision of the S-N curve considering an effect of them. The procedure of proposed method is a follows. Firstly, all fatigue test data are separated into an inclined part and a horizontal part with an arbitrary number of cycles as a boundary. Secondly, the distributions of fatigue strength are estimated with computational probability paper, and agreement of the distribution of both parts is searched by changing the boundary number of cycles. Finally, the optimum separation is the one for which the agreement of distribution reaches its maximum. Because each fatigue data is transformed into normalized fatigue strength, this method has the advantage point that the distribution of fatigue strength and the fatigue limit can be found by relatively small number of samples. As an example, the proposed approach has been applied to the fatigue test data for some materials. The well accuracy of the proposed S-N curves was proved by comparison of the obtained fatigue limit and calculational result by Probit analysis. Furthermore, the approach can be also applied to the materials which doesn't show obvious fatigue limit, such as CFRP, because non-existence of fatigue limit can be judged.