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 relation for isotropic time-dependent materials with arbitrary shear and bulk creep compliances are considered. General properties of the creep curve families for volumetric, longitudinal, and lateral strains generated by this linear relation under constant uniaxial tension and constant hydrostatic pressure are studied analytically. It is proved that the linear theory of viscoelasticity is able to describe the effect of (monotonic) expansion of a linear behavior range of material qualitatively as the hydrostatic pressure grows; more precisely, the effect of expansion of a range of axial stress values under which the axial compliance is independent of the stress level. The analysis reveals a number of specific features of the theoretical creep and compliance curves that can be conveniently employed as the applicability or non-applicability indicators of the linear viscoelasticity theory by the data of material creep tests under action of tensile load and hydrostatic pressure.