Simulation of ductile fracture in pipeline steels under varying constraint conditions using cohesive zone modeling

In this study, the effect of non-singular T-stress (a measure of crack tip constraint) on crack growth resistance curves (R-curves) and crack tip opening angle (CTOA) was investigated. The investigation proceeded in two parts; first using a modified boundary layer (MBL) model to systematically assess the effect of T-stress on the R-curves and CTOA, and second comparing the CTOA under different loading modes, namely, bending and tension. Three steels were examined in this work; the first was a typical high strength steel referred to as TH, the second and third were pipeline steels referred to as X70 and X100. Surface-based cohesive zone models with four sets of bilinear traction-separation (TS) laws were used for TH steel. The models of X70 and X100 were computed using element-based cohesive zone modeling with a trapezoidal TS law. All simulations were conducted using the finite element (FE) program ABAQUS. It was assumed in these simulations that there was no effect of T-stress on the TS laws per se. With this assumption, it was found that the T-stress and loading mode have only a small effect (1-2°) on the CTOA for the materials studied.