in whichWj ¼ @W=@Ij and D;H are two scalar (independent) reduced relaxation functions (with Dð0Þ ¼ Hð0Þ ¼ 1). The latter
relate to the inherent viscous processes associated with shear (deviatoric) and compressional (volumetric) deformations,
respectively. Let us consider the homogeneous deformation of simple shear (see (De Pascalis, 2010) for an overview of the
static nonlinear elastic case and related references),