A numerical model to assess the role of crack-tip hydrostatic stress and plastic deformation in environmental-assisted fatigue cracking

To better understand the mechanics of environmentally assisted cracking, and particularly hydrogen embrittlement, a correct description of the hydrostatic stress field is indispensable. The concentration of hydrogen in the proximity of the crack tip is indeed dependent of the hydrostatic stress effect on the microstructural lattice of the material. The overall parameters of the hydrostatic stress, including peak value, its location, gradient, and distribution size are fundamental to assess the effect on hydrogen distribution near the crack tip, specifically considering hydrogen-enhanced decohesion mechanism, or the HEDE mechanism. Hydrostatic stress is hence widely analysed in studies related to hydrogen embrittlement contribution in stress corrosion cracking or corrosion fatigue of metallic alloys. However, recent studies highlighted that the hydrogen-enhanced local plasticity (HELP) mechanism can be more relevant than HEDE in hydrogen-assisted fatigue failure of metallic alloys. In order to investigate the contribution of the HELP mechanism, detailed finite element modelling is reported for notched Ti-6Al-4V specimens, based on experimental fatigue data. The material is modelled with elastic-perfectly plastic behaviour, reproducing actual geometry of the notches and the fatigue crack, from measurements and replicas conducted during testing. Strain data are obtained in initial and final crack configuration, to discuss the HELP contribution on environmentally assisted cracking, and compare it with HEDE contribution linked to hydrostatic stress.