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 state. This formulation couples elastic, creep and growth strains, and provides a very flexible framework for treating the time-dependent response of anisotropic nonlinear visco-elastic polycrystals. It explicitly includes the anisotropy in the mechanical and thermal properties of the individual grains and of the polycrystal, and it accounts for intergranular interactions. The formulation is applied to the problem of irradiation creep (linear) and of thermal creep (nonlinear) of a reactor pressure tube, subjected to internal pressure. The effect on the overall response of the aggregate, of an irradiation-induced stress-independent deformation rate (growth) at the grain level is also considered. In a separate application, the coupling between the creep and the growth mechanisms in a non-textured polycrystal is analysed. In the case of nonlinear creep, irradiation growth may substantially enhance the creep rates.