Peak-load hold experiments indicate hydrogen-enhanced creep-like crack-tip deformation in X65 pipeline steel at room temperature

Loading-rate and waveform effects in hydrogen-assisted fracture and fatigue are commonly interpreted in terms of hydrogen transport to the crack tip, but direct measurements of time-dependent crack opening under sustained loading in gaseous hydrogen remain scarce. X65 single-edge-notched tension specimens were tested in air and 40 MPa hydrogen under monotonic loading with and without a 168 h peak-load hold and under cyclic loading with triangular and trapezoidal waveforms. Under monotonic loading at an initial K max of 44.2 MPa·m 1/2 , crack-mouth opening displacement (CMOD) changed little during the hold in air, whereas in hydrogen the CMOD response diverged from an independently measured no-load drift trend. Post-test residual crack-opening displacement (residual COD) was also greater under the hold condition than under the no-hold condition in hydrogen. In the hydrogen hold specimen, no side-surface crack advance was detected optically, while scanning electron microscopy revealed pronounced blunting and a squared-off crack-tip profile. Under cyclic loading at an initial K max of 42.5 MPa·m 1/2 , differences in CMOD and residual COD between the triangular and trapezoidal waveforms were small in air. In hydrogen, however, trapezoidal loading with a 300 s peak-load hold in each cycle showed markedly greater CMOD accumulation and residual COD than did triangular loading. These results indicate a hydrogen-enhanced, hold-time-dependent crack-tip deformation response, referred to here as creep-like deformation. The time spent at peak load should therefore be considered when interpreting fracture and fatigue tests in gaseous hydrogen.