  The influence of hydrogen on dislocation junctions was analysed by
incorporating a hydrogen dependent core force into nodal based discrete
dislocation dynamics. Hydrogen reduces the core energy of dislocations, which
reduces the magnitude of the dislocation core force. We refer to this as
hydrogen core force shielding, as it is analogous to hydrogen elastic shielding
but occurs at much lower hydrogen concentrations. The dislocation core energy
change due to hydrogen was calibrated at the atomic scale accounting for the
nonlinear inter-atomic interactions at the dislocation core, giving the model a
sound physical basis. Hydrogen was found to strengthen binary junctions and
promote the nucleation of dislocations from triple junctions. Simulations of
microcantilever bend tests with hydrogen core force shielding showed an
increase in the junction density and subsequent hardening. These simulations
were performed at a small hydrogen concentration realistic for bcc iron.
