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

Abstract 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 Δ K th and stress ratio R is derived. It is indicated that the R -sensitivity on Δ K th 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 Δ K th is briefly discussed in terms of crack closure.