Crack propagation in rubber blends : impact of blend morphology and energy dissipation mechanisms around the crack tip

Abstract

This works deals with crack growth investigations of tire tread compounds in laboratory. These measurements are usually performed with the Tear Fatigue Analyzer. In the Tear Fatigue Analyzer notched strip samples are loaded dynamically and the resulting crack growth rate dc/dn is measured. This way the crack growth rate dc/dn in dependence of tearing energy T can be determined for a compound under specific loading condition. The tearing energy T describes the energy available for crack growth and can be calculated for different sample geometries. This energy depends particularly on the stored energy density that is caused by straining the sample. Thereby the crack growth rate depends on the used rubbers. Natural rubber, partially crystallizing under strain, shows clearly lower crack growth than the synthetic butadiene and styrene-butadiene rubbers. Using active fillers like carbon black leads to strongly decreased crack growth due to the multiple interactions between filler and polymer matrix. The crack growth in a polymer blend depends also on the morphology of the distinct polymer phases and the magnitude of the interphase. Also the filler distribution into the different polymer phases and the interphase is influencing the crack growth properties. The dynamic mechanical method used to determine the filler distribution is based on the increase of loss modulus due to the filler but the results are influenced by crystallization of the butadiene rubber. The theoretic description of tearing energy in dependence of crack velocity was developed for steady tearing at constant crack velocity. It can be demonstrated that these concepts can be transferred to the dynamic crack growth of unfilled rubber, partially also to the dynamic crack growth of filled rubber. When applying the tearing energy on elastomers it should be noticed that only a small fraction of this energy is reaching the process zone at the crack tip. The decrease of tearing energy towards the crack tip can be determined by the J-Integral. It turns out that the J-Integral decreases from its plateau value at large integration radii towards very small values for small integration radii. This decrease corresponds to the energy that is lost due to viscoelastic dissipation outside the crack tip and which is not available for crack growth.