Numerical study of hydrogen influence on void growth at low triaxialities considering transient effects

Assuming that hydrogen enhances localised plasticity, as one of the leading mechanisms proposed in the literature, the void growth and coalescence are modified by local softening and ductile failure features depend on hydrogen accumulation. It is anticipated that strain rate plays an important role in hydrogen-informed void mechanisms, however, coupling voids, transient hydrogen diffusion, rate-dependent hydrogen-material interactions and intrinsic hardening, remains a challenge. In this study, the simulation of a void unit cell in a hydrogen pre-charged material is reconsidered here for the first time to incorporate transient effects, i.e. the kinetic redistribution of hydrogen around a void subjected to a high strain rate and a constant stress triaxiality. A coupled diffusion-mechanics scheme is implemented in a set of ABAQUS subroutines in order to analyse the interaction of hydrogen with the material response. The influence of strain rate is also considered when defining the cell boundary conditions through the limiting cases of equilibrium and insulated unit cells. The competition between the two inherent mechanisms, namely, hydrogen softening and strain rate hardening, is studied with the implemented framework. Results show that transient effects determine hydrogen concentrations and strongly dictate failure mechanisms: shearing might occur due to the hydrogen induced softening for moderate strain rates even though the cell is insulated. However, for very fast loading it is demonstrated that the fast creation of traps due to plastic deformation results in hydrogen depletion and necking failure is observed.