Environmentally-Assisted Fatigue Crack Growth in ARMCO Iron Under High Pressure of Gaseous Hydrogen

The issue of fatigue crack propagation assisted by gaseous hydrogen is here addressed in the case of a simple microstructure, namely a commercially pure ARMCO iron under a gas pressure of 35MPa at different load frequencies. Three characteristic regimes were identified in the da/dN curves. At low ΔK value, the exposure to gaseous hydrogen immediately induced a change in fracture mode characterized by numerous intergranular facets with markings, without any concomitant effect on growth rates. In contrast, a highly enhanced regime is observed at high ΔK values in association with a transgranular fracture mode with coarse brittle striations. A saturation of the crack growth enhancement is noticed in this regime. These crack growth regimes are discussed along a competition between mechanically induced crack growth and hydrogen diffusion at the crack tip.