Abstract
Twc-step variable pp type straining tests were conducted for Mod.9Cr-1Mo Steel at 600°C in air, where pp type strain range level varied from high to low (HL tests) or from low to high (LH tests). It was found that the experimental results of the two-step tests could be predicted by the linear damage rule when both the lower and the higher total strain range levels were greater than 0.8%. When the lower strain range level was smaller than 0.8%, discrepancy was found between the experimental results and the linear damage rule prediction. In HL tests the sum of life ratios was smaller than unity and in LH tests the sum of life ratios was larger than unity. These tests results were well explained by the proposed fatigue life prediction model, in which it was assumed that the crack initiation life is negligible in the straining level larger than 0.8% while it is not negligible in the straining level smaller than 0.8% and that the crack growth curve is described by the linear relation between ln (a/a0) and n/Npp, a being the half crack length, a0 the initial half crack length, n the applied cycles and Npp the total fatigue life. The ratio of the crack initiation life to the total fatigue life, α, was determined quantitatively. The smaller the straining level, the larger the value of α.
