On the capability of linear viscoelasticity theory to describe the effect of extending region of material linearity as the hydrostatic pressure grows

Applicability indicators of the linear viscoelasticity constitutive equation for isotropic materials with an arbitrary shear and bulk creep compliances are considered. General properties of the creep curves for volumetric, longitudinal and lateral strains generated by the linear equation under constant tensile load and constant hydrostatic pressure are studied analytically. It is proved that the linear theory is able to describe the effect of expansion of a material linear behavior range with hydrostatic pressure growth. The analysis revealed a number of specific features of the theoretic creep and compliance curves that can be employed as the applicability or non-applicability indicators of the linear viscoelasticity theory and are convenient to check using data of a material creep tests under various levels of pressure and tensile stress.