where sd and sh are the associated deviatoric and compressive relaxation times. Note that we are always able to scale time
on one of these relaxation times and we shall indeed do this shortly by plotting results as a function of t=sd. Hence, we need
only specify the relaxation time ratio s ¼ sd=sh, noting that as s!1 the material becomes more elastic in its hydrostatic
response (typical for rubber-like materials for example). As it will be seen shortly, in the isochoric deformation case considered
here this limit loosely corresponds to the case of incompressibility.
We illustrate the responses below in the case of three different materials. A perfectly incompressible neo-Hookean viscoelastic
material with strain energy function as defined in (21) and a compressible Levinson–Burgess material with strain
energy function as defined in (16). In the former (indicated by a solid line in the forthcoming figures) we are not required to
specify s of course, whereas in the latter we consider the two cases s ¼ 1 and s ¼ 10 (indicated in the figures by dotted and
dashed lines respectively).
5.1. Influence of compressibility on the isochoric shear deformation
As described above, since the simple shear deformation (9) is isochoric, in the perfectly elastic problem compressibility
plays no role in the solution. In contrast, in the viscoelastic problem it does, by virtue of the memory effect of the compressive
relaxation function HðtÞ. We can illustrate this by imposing the simple shear (9) with kðtÞ defined by