Cumulative Fatigue Damage on Large Steel Specimens Under Axial Programmed Loading With Nonzero Mean Stress

Abstract

Large specimens of an ASTM E-373-58T steel were subjected to zero-tension cyclic loading, hence with nonzero mean stress, under a repeated programmed pattern of nine stress levels. With the major stress twice the minor stress, the levels in the loading pattern were chosen in such a manner that they had approximately a sinusoidal wave as an envelope and the stress cycle at each step was repeated 500 times. The experimental results show a trend of variation of the cycle-ratio summation similar to that obtained on tests with zero mean stress; the higher the number of stress levels in the loading pattern below the original endurance limit, the larger the deviation from unity of this summation. A comparison is made between the predicted lives according to the linear cumulative damage rule and according to an alternative theory which takes into account the variation of the endurance limit (Unified Theory). It is found that the Unified Theory is more conservative and provides better estimates in the high-cycle range in particular for cases where there are, in the stress pattern, several levels lower than the endurance limit of the original material.