INFLUENCE OF STRENGTH MISMATCH AND WELD RESIDUAL STRESS ON FATIGUE CRACK GROWTH BEHAVIOR FOR PIPELINE STEEL INCLUDING CLOSURE EFFECTS

The integrity of mechanical components, particularly when they undergo significant fatigue damage for the duration of operating life, can be strongly influenced by the presence of residual stress fields and mechanical heterogeneity. Premature closure of crack flanks greatly influences fatigue crack growth rate. To estimate the crack closure in any welded structure, the residual stresses and strength mismatch in the vicinity of the crack tip should be considered. Extensive elastic-plastic finite element analyses have been carried out to investigate detailed crack closure behavior in heterogeneous compact tension (CT) specimens with three levels of weld strength mismatch and imposed uniform tensile residual stress field. The restriction on uncraked ligament imposed by E-647 is rather unconservative because it does not ensure linear elastic behavior at the crack tip. A relationship between the crack opening loads, the mismatch level and maximum applied stress intensity factor was obtained for small scale yielding (SSY) condition. This equation shall be used for fast estimations of closure effects for welding joints. A homogeneous, soft material has the largest crack opening loads, while a heterogeneous material with 50% overmatch conditions has the smallest opening load under SSY condition. Residual tensile stresses have detrimental effects on the fatigue resistance of the material. On average, residual tensile stresses, with magnitude equal to 0.5 sigma(y) of the base metal, increase the Fatigue Crack Growth Rate (FCGR) by 40% when compared to the case without residual stress under SSY condition. Moreover, overmatch conditions in welded joints have detrimental effects on fatigue crack propagation rate. The fatigue life can be reduced by more than 70% for a condition of 50% overmatch when compared with the evenmatch condition.