In the simplest case, if we impose the deformation kðtÞ (assuming that appropriate relaxation functions have been chosen
and given a specific strain energy function W associated with the instantaneous effective elastic stress) then the equations
(11) immediately determine all the components of stress. Physically, however, it is more common to impose tractions and so,
in this case, Eqs. (11) are of the nonlinear integral Volterra-type, to be solved for the unknown deformation kðtÞ. Analytical
solutions are not possible in general for this viscoelastic deformation. As such it is convenient to implement the numerical
procedure proposed in the appendix of De Pascalis et al., 2014. There attention was restricted to incompressible materials and
so here the procedure is modified for the compressible case. It is convenient to write the Volterra integral equations in the
following separable form