Strain gradient plasticity modeling of hydrogen diffusion to the crack tip

• Crack tip hydrogen diffusion is examined through strain gradient plasticity.
• Finite element results reveal a significant influence of geometrically necessary dislocations.
• Good agreement with experimental measurements of crack tip deformation is observed.
• Very high levels of lattice hydrogen concentration are attained close to the crack tip.
• The implications in the understanding of H embrittlement mechanisms are thoroughly discussed.

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.