Effect of Cyclic Stresses Below the Endurance Limit on the Fatigue Life of 40Cr Steel

The effect of cyclic stresses below the endurance limit on the fatigue life of 40Cr medium-strength carbon steel is studied. Conventional constant-amplitude cyclic tests and specially designed variable-amplitude ones are conducted under torsional loading at the stress ratio R = 0.1. The results show that the strengthening effect of cyclic stresses below the endurance limit can be reached if they are applied prior to the exceeding ones. Moreover, the stress amplitude, number of cycles and load sequence are found to be the three major strengthening effect-controlling factors. Generally, different cyclic stress levels exhibit different strengthening effect values, whereas the respective fatigue strength will initially rise and then drop with the number of loading cycles. The cyclic stresses of a 85% endurance limit are established to provide the maximum strengthening effect. Under multi-level cyclic loadings, the strengthening effect at different cyclic stress levels is nonlinearly growing, and the total fatigue life is strongly related to the last level of stress amplitude in the load sequence. The scatter in fatigue lives is highly dependent on the loading conditions and strengthening effect The coefficient of variance (COV) of fatigue life values exhibit the tendency to a decrease with the strengthening effect, which may improve the uniformity of the material fatigue strength characteristics.