A Modification of the Extended Tube Model (METM) for the Characterization of Filled Vulcanizates

The aim of this study is to develop a material model for filled vulcanizates that is physically justifiable. This model builds upon the established extended tube model and is incorporated into a finite element program. The research demonstrates that the intrinsic deformation concept is inadequate for describing nonlinear deformation behavior under the assumption of incompressible, isotropic materials. Consequently, an alternative approach is proposed, employing a strain function rather than direct use of principal strains, to characterize reinforcement behavior. This strain function aligns with the first invariant of the right Cauchy-Green strain tensor over a wide deformation range. At minor deformations, the entanglements’ contribution is considered through an additional reinforcement term. The reinforcement function is depicted as a sum of three elements, each representing reinforcement at different strain levels: low, medium, and high. Experimental comparisons show that the Modified Extended Tube Model (METM) effectively captures the stress-strain response of filled systems across all deformation levels. Furthermore, the reinforcement function parameters, derived from fitting the METM to experimental data, offer a quantitative assessment of the fillers’ reinforcing effects, while the extended tube model parameters reflect the network characteristics.