The fracture toughness of metals can be viewed as being derived primarily from the shielding of the crack tip by the attendant plastic zone. Rapid crack propagation in metals can lead to strain rates that are estimated to be on the order of 10 3 S 1 to l0 5 S -I or higher near the crack tip. Tough ductile materials such as A533B steel can exhibit significant viscoplastic effects at these strain rates. These viscoplastic effects can substantially influence the crack-tip plastic zone and, hence, the fracture resistance.
A small scale yielding analysis is performed for steady state, rapid crack propagation in an elastic-viscoplastic material. The viscoplasticity is characterized by the Bodner-Partom material model. Both cleavage and ductile fracture models are developed. In the cleavage model, fracture is assumed to be governed by a critical crack-tip stress intensity factor whereas fracture in the ductile model is based upon a critical value of the effective plastic strain ahead of the crack tip. These models are used to predict the dynamic fracture toughness as a function of crack speed for A533B steel when its initiation and arrest toughnesses are prescribed. Cleavage fracture is only possible at high crack speeds whereas ductile growth can only occur at the lower crack speeds. The implications of these predictions relative to the brittle-ductile transition observed in dynamic fracture tests are discussed.