Experimental and numerical study on hydrogen-induced failure of X65 pipeline steel

Hydrogen-induced fracture of X65 pipeline steel under in-situ electrochemical charging is investigated by using slow strain-rate tensile (SSRT) test, hydrogen diffusion test, fractography analysis, and finite element simulation. Smooth and notched tensile specimens with varying notch radii are utilized to ascertain the impact of stress triaxiality on hydrogen embrittlement (HE) susceptibility. A fully coupled model, H-CGM+, capable of simulating the synergy between hydrogen-enhanced plasticity and decohesion, is employed. The simulation proficiently replicates both the global stress-strain trajectories and the local failure initiation sites of the in-situ SSRT tests. The findings indicate a predominance of dislocation trapping hydrogen mechanism in HE, with crack inception at the notch surface where local plastic strain peaks, subsequently advancing towards the center of the specimen. Notably, an inverse relationship is observed between HE susceptibility and stress triaxiality. A hydrogen-induced failure criterion, defined as a critical combination of local hydrogen concentration and plastic strain, is derived. The failure criterion is found to be independent of stress triaxiality, which serves as a good reference for safety assessment of hydrogen pipelines.