A Discrete Dislocation Analysis of Hydrogen-Assisted Mode I Fracture
Hydrogen embrittlement failures are frequently unexpected and sometimes catastrophic and as a result it has been the subject of numerous studies. Up to now a number of mechanisms have been proposed, and many believe that the susceptibility to hydrogen embrittlement is related to the trap population.
We have analyzed mode I crack growth using discrete dislocation analysis in a system which fails due to hydrogen-enhanced decohesion mechanism. The case under study is a two-way coupled diffusion mechanics problem which is based upon: A quasi-static discrete dislocation analysis of mode I crack growth; A quasi-static stress assisted hydrogen concentration formulation; A cohesive law depending on hydrogen concentration. Furthermore, we consider two types of hydrogen diffusion with one assuming fast distribution of hydrogen solutes with respect to the crack growth and the other assuming slow hydrogen diffusion which leads to a constant hydrogen concentration throughout our simulation. Comparing the results of these two methods reveals the impact of hydrostatic stresses on hydrogen embrittlement. In this research we have shown that hydrogen solutes reduce material toughness and the dislocation activity which results in a smaller plastic region at the crack tip. Further increase in hydrogen concentration will lead to an extreme case with few dislocation activity and hence fracture becomes brittle. Moreover, we conclude that in order to be able to model hydrogen-enhanced decohesion mechanism one must consider cohesive strengths as big as 20 Y , which is not possible while employing continuum plasticity.