THE INFLUENCE OF THE EQUILIBRIUM STRESS GROWTH LAW FORMULATION ON THEMODELING OF RECENTLY OBSERVED RELAXATION BEHAVIORS

Novel and consistent relaxation behaviors have recently been found wit h several steels at homologous temperatures less than 0.5, on a Titani um alloy, on an Aluminum alloy and on the polymer Nylon 66 at room tem perature. A strong dependence of the relaxation rate on prior loading rate was observed. At equal relaxation times the stress at the end of the relaxation period associated with fastest (slowest) prior strain r ate has the smallest (largest) magnitude. It is shown that the stress rate term in the evolution law of the equilibrium (back) stress enable s the viscoplasticity theory based on overstress (VBO) to naturally mo del this newly found relaxation behavior. Ii the stress rate term is a bsent, as is the case for other state variable theories, this relaxati on behavior cannot be modeled without modifications. Analyses valid at the beginning of and during the relaxation tests and numerical experi ments illustrate the properties of VBO and other ''unified'' state var iable models at low and high homologous temperatures.