Influence of Gaseous Hydrogen on the Fatigue Crack Propagation Rate in Metals: Experimental Approach and Modeling

The main purpose of this work is to understand the mechanisms that govern hydrogen assisted cracking in metals, based on the experimental analysis of crack propagation data under gaseous hydrogen and the interaction between hydrogen and lattice defects on the one hand, and on the development of a cohesive zone model influenced by hydrogen on the other hand. Fatigue crack propagation tests were performed under high pressure of gaseous hydrogen on the Armco iron. The results show a strong influence of the pressure, the frequency and the ΔK value, on the modification of the failure modes and on the fatigue crack growth rates. In order to identify the physical parameters that govern the changing of the failure modes, a study on the interaction between hydrogen and the crystallographic defects developed during a cyclic loading was performed. We observe an increase in the total absorption of hydrogen with the cumulated plastic deformation, which can be attributed to the increase in the hydrogen trapping by the dislocations generated during the cyclic deformation. These data have to be introduced into a numerical model to reproduce the modification of the hydrogen diffusion at the crack tip, and its effect on plasticity. Moreover, measurements of the out-of-plane plastic deformation at the crack tip in presence of hydrogen have conducted to an improvement of the cohesive zone model by introducing an effect of hydrogen on the plastic behavior of the volume elements. In addition, the study of Krom diffusion law components has shown the importance of the hydrostatic stress gradient on the diffusion and accumulation of hydrogen at the crack tip. The model predicts a strong dependence of the crack propagation with respect to the hydrogen diffusion at the crack tip, and it is able to simulate the propagation under static load, thus validating the cyclic cracking and static cracking superposition, and explaining the transient regime in fatigue crack growth rates experimentally observed. (author)