The interaction of rate and history-dependent effects and its significance for slow cyclic inelastic analysis at elevated temperature

Life prediction for creep and low-cycle fatigue interaction and the analysis of ratcheting phenomena are of great importance in the design of future high-temperature nuclear reactors. These problems involve slow cyclic load application, inelastic deformation, and are inherently nonlinear. Fortunately, finite element computer programs with their time following and nonlinear capabilities are now available for the efficient solution of these complicated problems. The task is now to ensure that a proper representation of the material deformation behavior, the constitutive equation, is used in these computer programs. A systematic study of structural metal deformation behavior in slow cyclic laboratory tests such as cyclic creep, cyclic relaxation, low-cycle fatigue with and without hold-time showed that rate and history dependence interact. A few examples of such interactions are given in this paper. It is further shown that constitutive equations based on the additive use of elastic, plastic, and creep strains are not capable of reproducing these interaction phenomena on principal grounds. New inelastic constitutive equations for metals must be developed and some presently pursued approaches are discussed briefly.