SELF-CONSISTENT MODELING OF NONLINEAR VISCOELASTIC POLYCRYSTALS - AN APPROXIMATE SCHEME

An approximate self-consistent scheme is presented for calculating the stress evolution in the individual grains of a polycrystal subjected to constant load conditions (creep), from initial loading to steady st ate. This formulation couples elastic, creep and growth strains, and p rovides a very flexible framework for treating the time-dependent resp onse of anisotropic nonlinear visco-elastic polycrystals. It explicitl y includes the anisotropy in the mechanical and thermal properties of the individual grains and of the polycrystal, and it accounts for inte rgranular interactions. The formulation is applied to the problem of i rradiation creep (linear) and of thermal creep (nonlinear) of a reacto r pressure tube, subjected to internal pressure. The effect on the ove rall response of the aggregate, of an irradiation-induced stress-indep endent deformation rate (growth) at the grain level is also considered . In a separate application, the coupling between the creep and the gr owth mechanisms in a non-textured polycrystal is analysed. In the case of nonlinear creep, irradiation growth may substantially enhance the creep rates.