Applicability Indicators and Identification Techniques for a Nonlinear Maxwell-Type Elastoviscoplastic Model Using Loading-Unloading Curves

A physically nonlinear Maxwell-type constitutive relation with two material functions for nonaging materials is studied analytically in order to reveal the set of basic rheological phenomena it simulates, to establish its application field, to find the necessary phenomenological restrictions that should be imposed on its material functions, and to develop identification techniques. The general properties and characteristic features of the stress-strain curves produced by the relation with arbitrary material functions in quasi-static uniaxial loading and unloading at constant stress rates and a subsequent rest and a constant temperature are analyzed. Based on the results obtained, the characteristic features of theoretical loading-unloading curves that can be employed as applicability indicators of the relation convenient for checking using test data of a material are revealed. Two effective identification techniques are developed. The first one is based on a set of triangular loading-unloading tests with various maximum stresses and loading rates. The second one employs a single triangular test with loading up to the upper bound of the stress range considered and unloading to the zero stress and subsequent rest. Explicit formulas are derived to determine both material functions.