Description of the effect of softening in elastomeric composites observed in tests with increasing strain amplitude

This paper presents the results of complex uniaxial mechanical tests conducted to study polymer nanocomposites with SKU-PPL-2102 prepolymer as a matrix. The unfilled elastomer is a structurally heterogeneous material - it has solid domain structures and therefore can be considered as a nanocomposite exhibiting complex mechanical behavior. Tests were carried out on the samples prepared of polyurethane without filler and with 0.5 phr of few-layer graphene filler under conditions of increasing deformation cycles and long time-delays between changes in the direction of gripper motion. These tests made it possible to monitor the degree of softening in the material subjected to loading and the increase of dissipative losses at different elongation ratios. It was found that, even at low filler content, the mechanical properties of the material changed significantly. In addition, note that the viscoelastic properties of the materials’ samples manifest themselves insignificantly (i.e., the "trained" materials can be considered elastic with a certain degree of accuracy) during repeated deformations. An elastic potential that is based on the notion of the effective behavior of loaded polymer chains was used to describe the elastic properties of the material taking into account the Mullins softening effect. The viscoelastic behavior of the nanocomposite under study was described in the framework of the previously described thermodynamic model. The Mullins softening effect was taken into account in both elastic and dissipative terms of the Cauchy stress tensor. The obtained data demonstrate that the consideration of this effect has a considerable effect on the dissipative component of the stress tensor.