Improvement of Random Fatigue Life Prediction in terms of Actual Fatigue Process-Oriented Fatigue Damage Counting Method

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To establish highly reliable fatigue life prediction method under random loading the rain flow method(RFM), originally developed as one of the most useful fatigue damage counting method, was re-interpreted by taking into account the actual fatigue process of crack initiation and propagation. Incounting fatigue damage, either cyclic plastic strain(LCF)or cyclic stress amplitude(HCF)has been used so far. Through the fatigue tests in wide fatigue region, CTOD was found to be a most suitable parameter in estimating fatigue damage. In case of LCF loading, the relationship between CTOD and the small crack growth rate corresponds fairly well to the relationship between the equivalent plastic strain and the small crack growth rate. On the other hand, the presence of the crack closure effect in HCF loading resulted in losing one-to-one correspondence found in LCF loading. This means, the crack growth rate should be estimated by using parameters DELTACTOD and DELTAKeff.