Chemical Equilibrium Fracture Mechanics – Hydrogen-Induced Crack Growth Initiation

Hydrogen-induced crack growth initiation, in metallic structures, is studied under constant tem-perature and chemical equilibrium, by employing Chemical Equilibrium Fracture Mechanics (CEFM). The conditions of small-scale, contained and large-scale hydrogen embrittlement are in-troduced and the areas of material deterioration, together with the distributions of stresses and hydrogen concentration, including hydride volume fraction, are derived analytically. It is shown that the shape of the material deterioration zone is identical for embrittlement caused either by hydrogen in solid solution or by hydride precipitation; the size depends on the strength of the asymptotic crack-tip field, which develops by the mechanical loading in the hydrogen-free struc-ture, as well as on the average hydrogen content absorbed by the structure. It is also shown that a linear relation exists between a power of the threshold of crack-growth initiation and the logarithm of hydrogen content, depending on the extent of hydrogen embrittlement and material elastic-plastic deformation. The predictions of the present analysis are confirmed by published experimental fracture mechanics measurements on several non-hydride and hydride forming alloys, including α/β hydride forming alloys. The present study facilitates structural integrity assessments, significantly, without relying on complicated coupled numerical analysis of material deformation, hydrogen diffusion and hydride precipitation.