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Rubber materials are characterized by a variety of inelasticities such as softening behavior, hysteresis loops and permanent set. In order to calculate the inelastic material behavior, constitutive models, that describe rubber as a homogeneous continuum, have to make use of damping or friction elements.
On the nanoscale, there is no need to adopt such rheological models. Inelastic material behavior can be explained and simulated by a continuous rearrangement of bonds, in particular, the van der Waals interactions, and by the polymer chains transitioning between cis and trans equilibrium torsion angles. The discrete molecular dynamics simulations presented in this paper are performed in an explicit FEM environment using nonlinear but elastic force field potentials. From a structural mechanics point of view, topological changes of the polymer network can be interpreted as a sequence of local material instability problems due to negative tangential bond stiffnesses.
In order to obtain representative results within reasonable computational time, the model is optimized with respect to the number of atoms and the loading velocity. It is shown that by increasing the model size, the stress–strain curves become independent of both the atoms initial state and the strain amplitudes.
In this paper a practical way for fatigue life prediction of rubber products under multiaxial loads is shown. This is done by means of fracture mechanical concepts and the energy release rate as the failure criterion. Due to a FEA post-processor the potential energy release rate might be calculated at every material point supposed there was a crack. And therefore the risk of failure and with the help of a strain number curve the time to fatigue is able to be calculated by FEA. This concept is applied for an estimation of the life time of a test specimen with tensile loading from fatigue data of a shear loaded specimen of different design. This rather more theoretical concept of the energy release rate is complemented by experimental crack growth data by a Tear Fatigue Analyzer with its great advantage of reduction of testing time and costs compared to those of fatigue tests. For some materials a thorough characterization of crack growth and fatigue behavior is presented and is applied to estimate the time to fatigue by FEA for a real component under multiaxial loads.