APPLICABILITY INDICATORS FOR THE NONLINEAR MAXWELL-TYPE ELASTO-VISCOPLASTIC MODEL WITH POWER MATERIAL FUNCTIONS AND TECHNIQUES TO CALIBRATE THEM

A physically non-linear Maxwell-type constitutive relation with two material functions for nonaging elasto-viscoplastic materials is studied analytically in order to examine the set of basic rheological phenomena that it simulates, to enclose its application field, to obtain necessary phenomenological restrictions which should be imposed on its material functions and to develop identification and validation techniques. Characteristic features of loading-unloading-recovery curves family produced by the model with two power material functions (with four parameters) under loading and unloading at constant stress rates and subsequent rest are analyzed in uni-axial case and compared to general properties of stress-strain-recovery curves produced by the constitutive relation with two arbitrary (increasing) material functions (theorems 1 and 2). Their dependences on loading rate, maximal stress and material functions exponents are examined. Power functions are the most popular in creep models, elastoviscoplasticity, polymer rheology, hydrodinamics of non-newtonian fluids and simulation of superplastic flow. The analysis reveals several specific properties of theoretic loading-unloading-recovery curves produced by power model with four parameters that can be employed as the model applicability indicators which are convenient for check using test data of a material. They should be checked in addition to general applicability indicators for the Maxwell-type constitutive relation with two arbitrary material functions. A number of effective calibration procedures for the model in the class of power material functions are developed. They are more rapid and effective than general identification techniques for two arbitrary material functions developed previously. The first procedure employs a pair of stress-strain curves at different stress rates, the second one is based on a pair of loadingunloading- recovery curves with various maximal stress values and loading rates and the third one needs only one loading-unloading-recovery curve. The explicit expressions are derived for four material parameters via test data. They enable separate and direct evaluation of the material parameters without error accumulation. Identification techniques versions are considered and their advantages and shortcomings are discussed. The ways to minimize the error using additional tests are proposed.