Novel XFEM variable strain damage model for predicting fracture in small-scale SENT and full-scale pipe tests

The tensile strain capacity (TSC) of a welded pipeline under bending and/or axial loads is critical to pipeline integrity assessment, and the fracture toughness expressed as a J-R curve plays a key role in predicting the TSC. The single edge notched tension (SENT) tests have been recommended for fracture toughness measurement of pipelines due to the similar low constraint geometries with the full-scale pipelines. The extended finite element method (XFEM) has been increasingly implemented in predicting the fracture of pipelines from small-scale to full-scale tests. However, current damage criteria based on a fixed critical stress or strain value have not been well calibrated, and limitations have been found. This paper develops a novel XFEM variable strain-based damage model by adopting a critical varying strain profile that accounts for the crack-tip constraint derived from a modified Mohr-Coulomb (MMC) fracture criterion. The novel damage criterion is calibrated by reproducing results like those measured from small-scale SENT tests of X100 pipe steel and full-scale pressurized tests of X52 circumferentially surface-cracked pipeline. Two sets of optimal damage parameters are calibrated specifically for X100 and X52. The initial crack in each model is simulated by cutting a notch in the XFEM domain resembling the original crack pre-machined on the specimen. Previously, researchers inserted a XFEM planar crack into the finite element mesh. The comparisons between results from notched models and planar cracked models reveal a distinct difference in the crack tip constraint and fracture resistance. Predictions from the novel variable damage criterion demonstrate advantages over those from the simple fixed damage criterion.