XFEM-Based Fracture Analysis of Internal Surface Cracks in 09MnNiDR Steel Pipes

The propagation of internal surface cracks in pressure pipelines is one of the primary modes leading to pipeline fracture failure. Based on the Extended Finite Element Method (XFEM), this paper systematically investigates the fracture mechanics behavior of 09MnNiDR cryogenic steel in both cracked plate and pipeline structures. Initially, through cross-validation of the through crack plate and pipeline specimens, it was fully confirmed that the model established using XFEM can accurately capture the stress characteristics at the crack tip of the pressure-bearing pipeline structure and the variation law of the stress intensity factor, verifying the accuracy and reliability of the XFEM model. A novel integration of XFEM with stress triaxiality analysis was employed to elucidate the directional dependency of internal surface cracks-a mechanism not fully reported in prior literature. Building upon this foundation, the study focuses on a model of homogeneous pipelines containing internal surface cracks. It analyzes the evolution trends of stress intensity factors corresponding to cracks of varying angles and sizes under internal pressure alone. Additionally, by integrating the concept of stress triaxiality, the research elucidates the fracture mechanism of pipelines with internal surface cracks and predicts the direction of crack propagation. Research findings indicate that in the pipeline model, the failure risk of a crack is jointly determined by its dimensions and angles. The results show that for 09MnNiDR steel pipelines with a diameter of 508 mm and a wall thickness of 6 mm under internal pressure loading, when the crack Angle is 10°, it should be given special attention as a key Angle. Under varying internal pressures, radial crack propagation in the pipeline consistently precedes axial propagation. This discovery elucidates the complex failure mechanisms and crack-propagation behavior of internal-surface cracks in pipelines, providing a crucial theoretical foundation for the safety assessment and precise detection of 09MnNiDR steel pressure pipelines.