The Boltzmann-Volterra linear constitutive equation for isotropic non-aging visco-elastic materials (with an arbitrary shear and bulk creep compliances) is studied analytically in order to find out its capabilities to provide an adequate qualitative description of rheological phenomena related to creep under uni-axial loading combined with constant hydrostatic pressure and to outline the material functions governing abilities, to indicate application field boundaries of the relation and to develop identification and verification techniques. The constitutive equation doesn’t involve the third invariants of stress and strain tensors and implies that their hydrostatic and deviatoric parts don’t depend on each other. It is governed by two material functions of a positive real argument (that is shear and bulk creep compliances); they are implied to be positive, differentiable, increasing and convex up functions. General properties and characteristic features of the creep curves for volumetric, longitudinal and lateral strain generated by the linear equation under constant tensile load and constant hydrostatic pressure are investigated. Their dependences on pressure and tensile stress levels and the material functions qualitative characteristics, the conditions for creep curves monotonicity and for existence of extrema and sign changes of strains are in the focus of attention. In particular, it is proved that the linear relation is able to simulate non-monotone behavior and sign changes of lateral and axial strains. The analysis reveals a number of specific features and quantitative characteristics of the theoretic creep curves which can be employed as the applicability or non-applicability indicators of the linear viscoelasticity theory. They are convenient to check using data of a material creep tests with various levels of pressure and tensile stress. More detailed and precise indication of phenomenological restrictions for the linear viscoelasticity theory and enclosure of linear behavior range of a rheonomic material is also significant for identification of non-linear models and simulation of a material behavior in non-linear range. A simple and effective identification technique is developed. It is based on two creep tests with different pressures and implies measurement of axial compliance in each test (i.e. axial creep curve divided by the stress level). The explicit formulas are derived to determine the shear and bulk creep compliances via experimental axial compliances.

axial and volumetric compliances, IDENTIFICATION, linear range boundaries, longitudinal and lateral creep curves, non-applicability indicators, VISCOELASTICITY, volumetric creep, вязкоупругость, граница области линейности, идентификация, индикаторы неприменимости, кривые осевой и поперечной ползучести, объемная ползучесть, осевая и объемная податливости

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