XFEM Model for Characterizing Fracture Toughness of X52 Pipe Steels

Abstract The extended finite element method (XFEM) has recently emerged as a highly effective tool for analyzing crack propagation in complex structures, but its use in pipeline fracture studies, particularly with cohesive zone models (CZMs), is still developing. Current XFEM fracture criteria are not calibrated for pipeline steels, relying on fixed fracture stress or strain to initiate crack propagation. While the stress-based criterion works for brittle fractures, it fails for ductile ones, either accelerating cracks or preventing them altogether. The strain-based criterion better predicts both fracture types, but its numerical accuracy remains inadequate, highlighting a need for further research. This numerical study explores the use of XFEM to predict crack propagation in standard fracture specimens of single edge notch bending (SENB) made of X52 pipe steels. First, an XFEM-based cohesive zone model was developed to simulate the specimens. The maximum principal strain (MAXPE) and fracture energy (Gc) were selected as key damage parameters to characterize the fracture process, controlling crack initiation and resistance to crack propagation, respectively. These damage parameters were adjusted until the model closely matched experimental results (load–crack tip opening displacement (CTOD)) for different initial notch sizes in SENB specimens. Subsequently, experimental results for CTOD–R and the strain distribution around the crack tip, both at crack initiation and during unstable crack propagation, were compared with the numerical model's predictions to validate the chosen XFEM input damage parameters. The research confirms the effectiveness of XFEM in predicting fracture characteristics (i.e., CTOD and crack growth length), particularly when using the XFEM parameters MAXPE and Gc.