Many crystalline solids fail by cleavage at low temperatures and by plastic processes at high temperatures. In the transition region, cleavage failure occurs at stresses increasing with increasing temperature, reflecting a decrease in yield stress and a consequent increase of plasticity around the crack tip. Crack tip plasticity blunts the crack and shields it through the compressive stresses in the plastic zone. This competition between cleavage and plastic flow can be modelled in terms of t...