Methods of predicting fatigue crack growth lives in residual stress fields.

Abstract

Two methods of predicting fatigue lives of cracks propagating in residual stress fields were proposed. In these predictions, effective loads near the crack tip in components containing residual stresses are determined by superposition of the stress intensity factors for the residual stresses and for the applied stresses. One method uses the effective stress ratio R and the stress intensity range ΔK of the effective loads to estimate the effective stress range ratio U = ΔKeff/ΔK. The fatigue crack growth lives are predicted by using ΔKeff and the da/dN-ΔKeff relation. The other method replicates simultaniously the variation of both R and ΔK with crack length by using specimens free form initial residual stresses, in order to simulate the mechanical state near the crack tip in the components with residual stresses. The life of the specimen directly provides the predicted value of the component life. Experiments were conducted by using specimens of S 35 C carbon steel with residual stresses. Good agreement was observed between the experimental and predicted values, for cracks emanating either from the tensile residual stress region or from the compressive residual stress region.