The role of stress softening in crack propagation of filler reinforced elastomers as evaluated by the J-Integral

A recently developed micromechanical model of stress softening and hysteresis of filled rubber is used to calculate the energy density- and stress distribution around the crack tip for a given displacement field, which is determined by digital image correlation. This allows for a quantitative evaluation of stress-softening influences on the J-Integral, J(1). In addition, the gradients of stress, strain and energy close to the crack tip are evaluated and compared to predictions of purely elastic material behaviour. The J-Integral is determined for carbon black filled truck fire tread compounds based on natural rubber (NR) and phase separated blends of NR with butadiene rubber (BR) and/or styrene-butadiene rubber (SBR). Dependent on the compound used, J(1) is found to decrease significantly with decreasing contour radius r, indicating that most of the energy flux into the crack tip is dissipated due to stress softening and is not available for crack growth. For larger distances r, when the energy field becomes isotropic, plateau values are obtained for the J-Integral, which coincide with global tearing energies calculated by simple thermodynamics.