  We present a gradient-based theoretical framework for predicting hydrogen
assisted fracture in elastic-plastic solids. The novelty of the model lies in
the combination of: (i) stress-assisted diffusion of solute species, (ii)
strain gradient plasticity, and (iii) a hydrogen-sensitive phase field fracture
formulation, inspired by first principles calculations. The theoretical model
is numerically implemented using a mixed finite element formulation and several
boundary value problems are addressed to gain physical insight and showcase
model predictions. The results reveal the critical role of plastic strain
gradients in rationalising decohesion-based arguments and capturing the
transition to brittle fracture observed in hydrogen-rich environments. Large
crack tip stresses are predicted, which in turn raise the hydrogen
concentration and reduce the fracture energy. The computation of the steady
state fracture toughness as a function of the cohesive strength shows that
cleavage fracture can be predicted in otherwise ductile metals using sensible
values for the material parameters and the hydrogen concentration. In addition,
we compute crack growth resistance curves in a wide variety of scenarios and
demonstrate that the model can appropriately capture the sensitivity to: the
plastic length scales, the fracture length scale, the loading rate and the
hydrogen concentration. Model predictions are also compared with fracture
experiments on a modern ultra-high strength steel, AerMet100. A promising
agreement is observed with experimental measurements of threshold stress
intensity factor $K_{th}$ over a wide range of applied potentials.
