Abstract A simulation of the dynamics of dislocation loops emitted at or close to a crack tip in b.c.c. metals is presented. The model simulates the growth of the crack tip plastic zone. The elastic (shielding) interactions between the plastic zone and the crack are calculated as a function of time and temperature, allowing modelling of the increase in fracture toughness with temperature (i.e. the brittle-ductile transition near the lower shelf). The three-dimensional nature of the model allows for the first time study of the differences between dislocations of blunting and non-blunting types, i.e. in configurations where the emitted dislocation glide plane either does or does not contain the crack tip. We found that the blunting configuration induces a localised shielding of the crack close to the dislocation sources, but is not sufficient to give strong toughness increases. Conversely, non-blunting geometries produce more efficient shielding of the crack and are characterised by a slower extension of the plastic zone far from the crack tip. The model also demonstrates the importance of the density of dislocation sources along the crack tip in determining the form of the brittle to ductile transition.