Prediction of the fatigue life on the basis of damage progress rate curves

The new concept of a damage progress rate curve (DPRC) is introduced in this paper. These curves were determined experimentally for three grades of structural steels used for high pressure installations working in extreme service conditions. Stress controlled fatigue tests were performed for the steels under investigation and the changes in the inelastic strain were used as the measure of the fatigue damage progress. The results of the investigations are presented in the form of plots showing the rate of the fatigue damage process, dD/dn,as a function of the applied stress amplitude. The applicability of the Palmgren–Miner linear damage rule is restricted to the range of stress amplitudes for which damage is indeed a linear function of the number of the applied load cycles. For higher stress amplitudes damage the progress is not linear — the linear damage rule cannot be applied to predict fatigue life of the construction components. On the basis of experimentally determined DPRCs, the fatigue life was predicted for variable amplitude experiments performed on the tree structural steels investigated. The results were found to be in very good agreement with the prediction in the range of stress amplitudes, for which the linear damage rule is applicable. One exception was HL loading sequence for 15HM steel for which a big discrepancy between the average observed behaviour and the predicted behaviour was observed due to the early activation of Lüders bands.