Strain gradient plasticity modeling of hydrogen diffusion to the crack tip

Abstract

In this work hydrogen diffusion towards the fracture process zone is examined accounting for local hardening due to geometrically necessary dislocations (GNDs) by means of strain gradient plasticity (SGP). Finite element computations are performed within the finite deformation theory to characterize the gradient-enhanced stress elevation and subsequent diffusion of hydrogen towards the crack tip. Results reveal that GNDs, absent in conventional plasticity predictions, play a fundamental role on hydrogen transport ahead of a crack. SGP estimations provide a good agreement with experimental measurements of crack tip deformation and high levels of lattice hydrogen concentration are predicted within microns to the crack tip. The important implications of the results in the understanding of hydrogen embrittlement mechanisms are thoroughly discussed.

On the suitability of slow strain rate tensile testing for assessing hydrogen embrittlement susceptibility

“…A nearly identical concentration profile is observed if the local notch-enhanced stress profile is accounted for in the numerical simulation. The limited effect of the hydrostatic stress was not unexpected given the reduced triaxiality associated with a blunt notch, as compared to a sharp crack tip where strong effects of stress are observed on the diffusion behavior [55,76]. In all material heats, the model results show that the H ingress is below 50% of the intergranular fracture depth (indicated by a vertical dashed line); ∼ 30% for NRL LS at -1100 mV SCE .…”

A cohesive zone framework for environmentally assisted fatigue

“…However, as noted by Turnbull [17], such scheme may oversimplify the electrochemistry-diffusion interface and the use of generalized boundary conditions is particularly recommended for materials with high hydrogen diffusivity. Here, we follow Martínez-Pañeda et al [18] and adopt Dirichlet-type boundary conditions where the lattice hydrogen concentration at the crack faces depends on the hydrostatic stress. Hence, the lattice hydrogen concentration at the crack faces equals,…”

Finite element simulation of hydrogen transport during plastic bulging of iron submitted to gaseous hydrogen pressure

“…In the following, the material is considered as elastic-plastic in the FE simulations of the disk bulging for sake of simplicity. Only standard plasticity is used, even if promising results on hydrogen concentration have been obtained in recent works using strain-gradient plasticity [23] in a small-scale yielding configuration. The influence of hydrogen on mechanical behavior was not considered.…”