Development of a strain-based fracture assessment approach for X80 steel pipe welded girth by modified Mohr-Coulomb model

In the study, a strain-based fracture assessment scheme with the Modified Mohr-Coulomb (MMC) model is established to numerically characterize the tensile strain capacity (TSC) of X80 steel pipe welded girth. The TSC at crack initiation and local instability are predicted based on a lode-dependent MMC model accounting for crack onset and propagation phases. The combined experimental-numerical calibration of the MMC criterion for crack onset is undertaken for X80 pipeline steel, where seven geometrical specimens are designated to represent various stress states from low to high triaxiality, and the parameter determination of the MMC criterion for crack growth is carried on by fitting the single-edge notched tension tests. Finally, the comparison between the present work and PRCI and ExxonMobil in terms of TSC is performed. A relatively good agreement can be achieved, whereas PRCI gives non-conservative predictions of TSC at crack initiation when crack depth a/t is longer than 0.4 and ExxonMobil gives conservative predictions of TSC at local instability when internal pressure is small.