Equivalent stress approach to predicting the effect of stress ratio on fatigue threshold stress intensity range

A previously proposed exponential stress function for predicting the effect of mean stress on fatigue life of smooth specimens is employed to account for the effect of stress ratio on the fatigue stress intensity range for crack-like members. By means of equivalent stress that is expected to yield the same damaging effect as combined cyclic and mean stresses, a relation between fatigue threshold stress intensity range ΔKth and stress ratio R is derived. It is indicated that the R-sensitivity on ΔKth is controlled by a single parameter, which appears to depend on the degree of crack closure exhibited by the material under the loading conditions. Predictions of the relation are compared with published experimental data and agreement is found to be very good. The R-effect on ΔKth is briefly discussed in terms of crack closure.