Controlling factors and mechanisms of fatigue crack growth influenced by high pressure of gaseous hydrogen in a commercially pure iron

This paper proposes an extended discussion of experimental results previously published by the authors and from literature about fatigue crack growth influenced by high pressure of gaseous hydrogen in a commercially pure iron. Firstly a summary of these results obtained for different exposure conditions in terms of gas pressure and loading frequency is presented. Two characteristic regimes, the first at low Delta K values characterized by a moderated fatigue crack growth enhancement and a nearly fully intergranular mode, and a highly-accelerated regime with a quasi-cleavage failure mode with the formation of coarse striations. The mechanisms controlling the different regimes of Hydrogen-Assisted Fatigue Crack Growth are discussed on the basis of observations of fracture surfaces and of deformation substructures within the plastic zone while assessing the capability of different models from literature to account for these observations. In a last section, the dependency of the transition between the different regimes as well as the crack growth rate enhancement in the highly-accelerated regime with respect to the hydrogen gas pressure and loading frequency is analyzed.