Accurate Prediction of Fatigue Life under Random Loading

This study focuses on the method of predicting the fatigue life of materials subjected to random loading. Since random stress is rigorously expressed in frequency domain as stress power spectral density (PSD), the fatigue life should be predicted with the stress PSD. In this article, we propose a method of improving the accuracy of the fatigue life prediction with stress PSD in frequency domain. Although the method proposed by Dirlik is widely used for predicting the fatigue life in frequency domain, it often overestimates fatigue damage caused by large stress amplitude. To avoid this overestimation, we estimate the expected value of maximum stress amplitude from stress PSD and then use the expected value to adjust the probability density function of stress amplitude evaluated using Dirlikﾕs method. The proposed adjustments were applied to the fatigue life prediction in the cases of typical random stresses observed on mechanical products, and as a result, it worked well to improve the accuracy of the prediction. By evaluating fatigue strength against random stress accurately, we can design lightweight and mechanically reliable trains, construction machinery, automotive products, etc.