Applicability Indicators and Identification Technique for a Nonlinear Maxwell-Type Elasto-Viscoplastic Model Using Repeated Creep Recovery Tests

Abstract

A physically non-linear Maxwell-type constitutive relation for non-aging elasto-viscoplastic materials is considered. General properties of repeated creep recovery curves generated by the relation under four-step uni-axial loadings consisting of two rectangular pulses of stress and two rest periods at zero stress are studied analytically assuming two material functions of the relation are arbitrary. The analysis reveals several characteristic features of the theoretic creep recovery curves and repeated creep recovery curves that can be employed as the relation applicability (or non- applicability) indicators which are convenient for check using test data of a material. Two effective general identification techniques are developed. The first one is based on a set of creep recovery tests at various stress levels and implies measurement of two strain magnitudes in each test. The second one is based on a set of repeated creep and recovery tests and implies measurement of four strain magnitudes in each test. More complex loading program enables to reduce the number of tests and samples twice and to obtain more information on a material behavior and to check a number of additional applicability indicators. The explicit expressions are derived in each case to determine the material functions values at arbitrarily chosen points in stress domain. The identification techniques enable separate and direct evaluation of the material functions values via test data without error accumulation. A number of the identification technique versions are considered and their advantages and shortcomings are discussed. In the case of materials exhibiting creep rate power dependence on stress, a specific rapid procedure is developed for the model identification in the class of power material functions and additional applicability indicators are found. In this case only one repeated creep recovery test and four measured magnitudes of strain are sufficient to determine four material parameters through the explicit expressions obtained.