Prediction of crack initiation and propagation in X70 pipeline steels

The present work deals with the concept of post-initiation softening applied to ductile crack initiation and propagation problems in thick structural components such as pre-cracked pipes and fracture specimens. The methodology includes a set of calibration steps to determine model parameters of crack initiation and propagation under monotonic quasi static loading condition. The phenomenological Modified Mohr-Coulomb (MMC) fracture criterion is used for fracture initiation along with post-initiation softening for crack extension. A comprehensive experimental/numerical program provides access to the load histories of each specimen geometry at the critical point, where fracture is assumed to occur. This first step enables the construction of the limits of ductility of the material in terms of a weighted von Mises equivalent plastic strain, which strongly dependent on stress triaxiality and Lode angle evolution. Post-initiation softening parameters are then calibrated from tearing analysis built upon resistance curves of single edge notch tension (SENT) specimens. The calibrated parameters model along with detailed FE analyses shows that the present fracture criterion accurately predicts the onset of stable crack growth and propagation along the uncracked ligament of low constraint specimens.