  Finite element analysis of stress about a blunt crack tip, emphasizing finite
strain and phenomenological and mechanism-based strain gradient plasticity
(SGP) formulations, is integrated with electrochemical assessment of
occluded-crack tip hydrogen (H) solubility and two H-decohesion models to
predict hydrogen environment assisted crack growth properties. SGP elevates
crack tip geometrically necessary dislocation density and flow stress, with
enhancement declining with increasing alloy strength. Elevated hydrostatic
stress promotes high-trapped H concentration for crack tip damage; it is
imperative to account for SGP in H cracking models. Predictions of the
threshold stress intensity factor and H-diffusion limited Stage II crack growth
rate agree with experimental data for a high strength austenitic Ni-Cu
superalloy (Monel K-500) and two modern ultra-high strength martensitic steels
(AerMet 100 and Ferrium M54) stressed in 0.6 M NaCl solution over a range of
applied potential. For Monel K-500, KTH is accurately predicted versus cathodic
potential using either classical or gradient-modified formulations; however,
Stage II growth rate is best predicted by a SGP description of crack tip stress
that justifies a critical distance of 1 {\mu}m. For steel, threshold and growth
rate are best predicted using high-hydrostatic stress that exceeds 6 to 8 times
alloy yield strength and extends 1 {\mu}m ahead of the crack tip. This stress
is nearly achieved with a three-length phenomenological SGP formulation, but
additional stress enhancement is needed, perhaps due to tip geometry or
slip-microstructure.
