Abstract
The Correlation between behavior of small fatigue cracks and variation of cumulative fatigue damage under the multiple repeated two-step stress loading was investigated by using specimens with an initial small crack. Various combinations of the overstress σH and the understress σL cause various values of cumulative fatigue demage. The critical stress σwi for fatigue crack initiation from a small crack is the important measure to evaluate the contribution of σL on fatigue damage.The values of cumulative fatigue damage D based on the Modified Miner Rule became much smaller than 1.0 for a certain combination of σH and σL. The minimum value of D obtained in the present study was D≅0.18. Although a wide scatter of D has been reported by many studies, it is not caused by statistical scatters of a material but caused by various combinations and sequence of σH and σL in relation to crack size at every moment. The possible minimum value of D (Dmin) for a definite cycle ratio of σH and σL under various combinations of sequence of loadings can be predicted by the method proposed in the present study and Dmin will be useful for the design under variable amplitude loadings.
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