Study on Prediction of Fatigue Crack Propagation Life Considering Welding Residual Stress

Abstract

It is well known that fatigue crack propagation (FCP) rate of welded joints is different from that of the base metal because of the existence of welding residual stress. However, the quantitative analysis for the effects of the residual stresses on FCP has not been made sufficiently.In this paper, the predicting method of the FCP lives of welded joints, regarding the effect of the residual stress as that of the stress ratio, R (=Kmin/Kmax), on base metal, is proposed as follows.(1) Stress ratio considering the residual stress of the welded joint, Rres, is obtained from the equation Rres= (Kmin+Kres) / (Kmax+Kres), where Kres is stress intensity factor caused by the release of the initial residual stress as the crack propagates.(2) The FCP rate of the welded joint, (da/dN) res, is obtained by substituting Rres into U-R and da/dN-ΔKeff relationships on base metal, where U is crack opening load ratio and ΔKeff is U·ΔK.(3) The FCP life of the welded joint, (Np) res, is obtained through integrating (da/dN) res.In order to examine the validity of the method, the FCP tests are carried out on welded joint and base metal of high tensile strength steel, using center cracked specimen.As the result, the FCP behaviors of the welded joint is greatly different from that of the base metal, particularly under zero-compression loading. Even in that case, the predicted FCP behavior of the welded joint is in good agreement with the FCP test result. Thus, it is shown that the proposed method is valid enough to predict the FCP life of welded joint.