Transition mechanism of cycle- to time-dependent acceleration of fatigue crack-growth in 0.4%C Cr-Mo steel in a pressurized gaseous hydrogen environment

•Fatigue crack-growth (FCG) properties of 0.4%C Cr-Mo steel in 90-MPa-hydrogen gas.•Augmented, hydrogen-induced FCG acceleration with increasing strength levels.•Transition mechanism of FCG acceleration from cycle-dependence to time-dependence.•Role of hierarchical martensite boundaries and their statistical aspect. Fatigue crack-growth (FCG) tests were conducted in 90-MPa-hydrogen gas on three martensitic steels with tensile strengths of 811, 921 and 1025 MPa. Increased strength levels resulted in augmented, hydrogen-induced FCG acceleration. In the highest-strength material, the FCG rate per cycle was dependent on test frequency, i.e., the crack-growth distance was proportional to load duration. Several observations and analyses revealed that such time-dependent FCG was due to stress-driven cracking along hierarchical martensite boundaries, stemming from the hydrogen-induced degradation of their cohesive strengths as a result of competition between mechanically-determined crack-tip stress (driving stress) and statistically-distributed boundary strength (resistance stress).