A Theory of Fatigue-Damage Accumulation in Steel

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Abstract
At present, there is no theoretical method for predicting the change in the endurance limit of steel specimens subjected to various numbers of cycles of overstressing at different stress levels. The theoretical and most of the experimental work accomplished thus far has been concerned with studying the change in endurance life at some stress level resulting from the accumulation of various amounts of overstressing at other stress levels. It is the purpose of this paper to present a simple theoretical model for predicting the change in endurance limit resulting from the accumulation of overstressing cycles at moderate levels of overstress and for moderate degrees of fatigue damage. Review of the literature indicates that it is reasonable to assume that fatigue damage occurring prior to the formation of a visible crack is equivalent to an increase in a “stress-concentration factor” associated with the particular defect in the specimen which will form the fatigue nucleus. The application of simple notch-factor concepts and the use of an empirical formula for the S-N curve for steel resulted in an equation for predicting the change in the S-N curve when specimens are subjected to overstressing histories producing moderate amounts of fatigue damage.

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