ure only the compressible material with the relaxation-time ratio s ¼ 1 is plotted alongside the incompressible result. The
long-time dissipated energy over a single cycle is shown in Fig. 4 for a range of frequencies x. It can be clearly observed that
the energy dissipated by the compressible material is greater than that for the neo-Hookean solid. The energy dissipation
appears constant at moderate and higher oscillation frequencies, except for the case with s ¼ 1, which only becomes constant
at very high frequencies. The frequency independence of the dissipated energy at higher frequencies appears to be consistent
with other theory, e.g. Fig. 10 of Lion, 1997 where, for time-harmonic oscillations superposed on an initial
deformation, plots of the temperature against frequency are plotted. One set of curves in that figure does not level off for
the frequency range considered but we expect that at higher frequencies, beyond the range plotted, constant values will
be achieved.
6. Conclusions
This paper has shown that the revised form of Fung’s QLV model, proposed recently by the authors (De Pascalis et al.,
2014), offers an effective and efficient way to model nonlinear viscoelastic materials undergoing simple-shear deformation.
The model is able to incorporate a wide range of behaviours through the choice of instantaneous strain measure (modelled
via an effective hyperelastic stress and underlying strain energy function) and relaxation functions. In this paper we examined
two material models, one incorporating incompressibility proposed by Levinson and Burgess, 1971 and the other a neo-
Hookean (incompressible) material, and chose to take a simple one-term Prony series to account for the fading memory of
the deformation history. It was further assumed that rates of deformation are slow enough that inertial effects can be
neglected; hence, as the deformations are spatially homogenous they automatically satisfy equilibrium.
The major simplifying assumption of QLV is that the relaxation functions are independent of the strain. This may lead to
inaccuracies with some types of materials, but can be expected to offer a reasonable model for many practical purposes, such
as when determining small perturbations about a large deformation, e.g. waves on a pre-stressed body. Separating the relaxation
function from the strain measure in the Boltzmann superposition integral allows one to obtain an explicit relation
between the viscoelastic stress T and the strain, or in the present case, the simple shear kðtÞ. For the models employed