Simplified Method for Estimation of Fatigue Life on High-Strength Aluminum Alloy under Two-Step Loading

A simplified method for estimating the fatigue limit and the fatigue life under two-step loading for the stress ratio R = 0 based on the results for R = -1 was proposed. This method required the fatigue test under constant amplitude loading and two-step loading for R = -1. In this paper, fatigue tests were carried out using high-strength aluminum alloy. In two-step loading for R = -1, two stress amplitude 1eve1s were used as the primary stress amp1itude and three cycle ratios were combined. The cycle ratio was defined as the ratio of cycles at the primary stress amplitude to the corresponding fatigue life. The fatigue limit and the fatigue life under the secondary stress amplitude tended to lower, as the cycle ratio under the primary stress amplitude increased. This tendency did not depend on the magnitude of the primary stress amplitude and the results were approximated with the same line. Hence, the method that had proposed for carbon steel under two-step loading could be applied to high-strength aluminum alloy. Then, the results of fatigue test for R = 0 were discussed using the equivalent stress amplitude, which was estimated by considering the stress amplitude and the mean stress. Under constant amplitude loading, the results for R = 0 approached to R = -1 by using the equivalent stress amplitude. For R = 0, the fatigue limits under two-step loading estimated by the simplified method were lower than the experimental ones, whereas the estimated fatigue life was in good agreement with the experimental results.