A Morphology-Based Analysis of Crack Initiation, Propagation and Arrest Behaviors in Pre-Defected CO2 Pipelines

Crack initiation, propagation and arrest in high-pressure CO2 pipelines are strongly affected by decompression behavior, defect geometry and material resistance. In this study, nine DN100-scale CO2 pipe burst tests with prefabricated axial defects were analyzed from a morphology-based perspective. Instead of evaluating the tests only by whether crack propagation or arrest occurred, the post-test macroscopic fracture morphologies were used to reconstruct the rupture sequence. The observations from the present test series show that the initial leakage in all tests occurred in the middle region of the prefabricated defect, confirming that the defect controlled the initial failure location. However, the subsequent crack evolution differed significantly. Four rupture modes were identified: circumferential full-bore tearing, axial running followed by circumferential instability, arrested axial tearing, and leakage-dominated local opening without crack propagation. Among them, the axial running mode exhibited the strongest sustained propagation capability, with a significant axial crack extension before final full-bore fracture. Fish-mouth-shaped openings were associated with ductile tearing, crack-tip blunting and energy dissipation, whereas slit-like openings indicated leakage-dominated local failure. The proposed morphology-based classification provides a useful supplementary framework for interpreting crack evolution in the tested range of conditions.