Abstract
Hydrogen-induced fracture of X65 pipeline steel under in-situ electrochemical charging is investigated by using in situ slow strain-rate tensile (SSRT) test, hydrogen diffusion test, fractography analysis, and finite element simulation. Smooth and notched tensile specimens with a range of notch radii, are tested to reveal the influence of stress triaxiality on hydrogen embrittlement (HE) sensitivity. A fully coupled model, H-CGM+ implemented in ABAQUS, capable of simulating the interplay between hydrogen-enhanced plasticity and decohesion, is employed. Both the global stress-strain curves and the local failure initiation sites of the in situ SSRT tests are well captured by the simulation. It is found that HE is dominated by dislocation trapping hydrogen, with crack initiation occurring at the notch surface where local plastic strain is maximized, followed by propagation towards the sample center. Surprisingly, HE susceptibility decreases with increasing stress triaxiality. A hydrogen-induced failure criterion, as a critical combination of hydrogen concentration and local plastic strain is derived. The hydrogen-induced failure criterion is independent of stress triaxiality, which can be a good reference for the safety assessment of hydrogen pipelines.