Presentation at Department of Energy Workshop on Time-Dependent Fracture of Materials at Elevated Temperature, Germantown, Md., 15-16 February 1979: PLASTIC CREEP FLOW PROCESSES IN FRACTURE AT ELEVATED TEMPERATURES

This paper discusses recent theoretical developments on fracture at elevated temperature in the presence of overall plastic (dislocation) creep. Two topics are considered: 1. Stress fields at tips of macroscopic cracks in creeping solids: Here consideration is given to the transient development of an effectively steady state of creep in a cracked body, following sudden load application for which the short-time material response is elastic. At long times (steady creep state) .the severity of the near-crack-tip deformation rate field can be characterized in terms of a path-independent integral C* , which is a generalization for non-linearly viscous materials of the J integral for rate-independent materials. Based on recent work by Riedel and Rice, it is shown that the short time transient field of creep flow has the same functional form at the crack tip, but that its amplitude parameter C* is replaced, approximately, by G/(l+n)t , where t is time since load application, n is the exponent in a power-law creep relation e « a , and G is Irwiu's elastic energy release rate (calculated in terms of the stress intensity K as if the body were elastic). Hence G/(l+n)C* can be identified as a characteristic time for stress redistribution in attaining the steady creep state. Macroscopic creep crack growth is discussed in terms of these concepts and associated analyses. 2. Diffusive growth of microscopic grain boundary cavities in creeping solids: Previous analyses of this problem are based on an assumption that grains adjoining the cavitating boundary separate in an effectively rigid manner. However, important interactions between cavitation, by surface and grain boundary diffusion, and plastic creep processes are observed to occur when a state of overall dislocation creep prevails. These arise from two