THE CHARACTERISTICS OF VISCOPLASTIC EFFECTS IN DYNAMIC CRACK-PROPAGATION IN NUCLEAR-REACTOR PRESSURE-VESSEL STEELS

A small-scale yielding analysis is performed for steady state, rapid crack propagation in order to quantify the influence of viscoplasticity in A533B steel. The Bodner-Partom viscoplastic model is used in this analysis. Because the elastic strain rate dominate the viscoplastic strain rates near the crack tip, an inverse square root singularity in the stress exists at the tip. Estimates for the strength of the crack-tip field in terms of the strength of the remote elastic field and the material properties of the A533B steel are established. The plastic shielding is sufficient to reduce the strength of the crack-tip field to an order of magnitude smaller than that of the remote field. A finite-element analysis is also performed to establish the size of the zone of dominance for the crack-tip field. The crack-tip zone of dominance is typically several orders of magnitude smaller than the extent of the plastic zone. Aside from posing rather severe finite-element modeling problems, this large disparity brings into thc question the relevance of the crack-tip region when compared to the size of the fracture process zone.