Filler reinforced elastomers have a wide range of applications, the technological most import one is the tire. The outstanding wear and fatigue properties of these materials are connected to the interaction between polymer and nano-structured fillers like carbon black and silica. While an unfilled elastomer behaves nearly ideally hyperelastic, the filling of the polymer leads not only to a reinforcement but increases also the hysteresis and stress-softening.
For many applications compounds are made from blends of different elastomers balancing the properties of the single polymers. Filled blends of natural rubber (NR), polybutadiene rubber (BR) and styrene-butadiene rubber (SBR) are commonly used for truck tire treads. In this study we consider the dynamic crack growth behavior of NR compounds blended with BR and/or SBR and filled with carbon black. Different blends are evaluated giving hint to polymer specific behavior, like the effect of strain-induced crystallization of the NR. We focus on the evaluation of tearing energies by referring to the J-Integral concept that considers the displacement and stress fields around the crack tip. The displacement fields in notched samples are recorded with an ARAMIS system by the evaluation of the displacements of an airbrushed pattern. From the displacements the energy density and stress distribution is calculated by referring to the Dynamic-Flocculation-Model (DFM). The DFM is a micro-mechanical model including the stress softening of elastomers. According to this model stress softening appears because the loading breaks stiff filler clusters so that an amount of polymer gets released and can now participate in the deformation.
The J-Integral, an energy flux integral integrated over a closed contour, represents the amount of elastic energy flowing through the contour of the integral. It is calculated from the energy density and stress distribution. For purely elastic materials the J-Integral is path-independent but as filled elastomers show significant energy dissipation the J-Integral is path-dependent. By integrating the J-Integral over different contours it is possible to determine the dependence of the integral from the distance to the crack tip.