A numerical model to evaluate the effect of hydrogen embrittlement on low-alloy steels

Some extreme working environments are characterised by corrosive conditions, able to develop hydrogen formation. The presence of atomic hydrogen localized in correspondence of plastic strains at the crack tip modifies the steel behaviour and its macroscopical mechanical properties. The phenomenon of hydrogen embrittlement is, indeed, one of the main responsible for the increase in fatigue crack growth rate and life reduction. For this reason, it is important to have validated numerical models able to estimate the mechanical behaviour of material in presence of hydrogen. Aim of this study is to develop a numerical model of two low-alloyed steels used in pipelines applications. Numerical simulations of C(T) specimens are carried out in different steps, considering hydrogen presence/absence combined with local plastic strains of the material. These two parameters are, indeed, responsible for a drop in material toughness, therefore for an increased crack growth rate. Two modelling techniques are used to simulate crack propagation: application of cohesive elements characterised by laws of degradation of mechanical properties, and the virtual crack closure technique (VCCT). Once model is validated by a comparison with experimental toughness measures, final considerations on the most valid simulation technique for the considered case are proposed.