Linear constitutive relations in isotropic finite viscoelasticity

The linear constitutive relations (1a) and (1b) for incompressible viscoelastic materials give identical response for infinitesimal simple shearing and infinitesimal simple extension deformations. However, for finite deformations, they predict different response. In stress-relaxation tests, the slope of the stress isochrone is an increasing function of the deformation measure for the constitutive relation (1a) but a non-increasing function of the deformation measure for the constitutive relation (1b).