Discussion of finite deformation viscoelasticity laws with reference to torsion loading

In a previous paper two classes of finite deformation viscoelasticity laws, referred to as Model A and Model B, respectively, have been introduced. They were derived as finite deformation counterparts of two well-known spring-dashpot linear solids, the first one being a spring in parallel with a Maxwell element and the second model consisting of a spring in series with a Kelvin element. In particular, two special forms of the free energy function related to Model A (respectively to Model B) were considered, implying two different finite deformation viscoelasticity laws referred to as Model Al and Model A2 (respectively Model B1 and Model B2). In the present paper we discuss predicted responses of these models with reference to simple torsion as well as torsion with free ends. Generally, the investigations of the paper have fundamental character in what concerns the basic concepts in formulating viscoelastic models of rate type.