An Experimental Comparison of Several Current Viscoplastic Constitutive Models at Elevated Temperature

Four current viscoplastic models are compared experimentally for Inconel 718 at 593°C. This material system responds with apparent negative strain rate sensitivity, undergoes cyclic work softening, and is susceptible to low cycle fatigue. The models used include Bodner’s anisotropic model, Krieg, Swearengen, and Rhode’s model, Schmidt and Miller’s model, and Walker’s exponential model. Schmidt and Miller’s model and Walker’s model correct for negative strain rate sensitivity response. A correction similar to Schmidt’s is applied to the models of Bodner and Krieg et al. A series of tests has been performed to create a sufficient data base from which to evaluate material constants. A method to evaluate the constants is developed which draws on common assumptions for this type of material, recent advances by other researchers, and iterative techniques. A complex history test, not used in calculating the constants, is then used to compare the predictive capabilities of the models. The combination of exponentially based inelastic strain rate equations and dynamic recovery is shown to model this material system with the greatest success. The method of constant calculation developed in this work was successfully applied to the complex material response encountered. Backstress measuring tests were found to be invaluable and warrant further development.