The effect of temperature on the tearing of rubber

The tearing behaviour of rubber is typically characterised using an energy balance approach where the rubber has a geometrically independent relationship between the crack growth rate and the strain energy release rate. This approach works well during steady tearing, such as that encountered with an unfilled, non-strain crystallising rubber such as styrene butadiene rubber. The rate of tearing is both easy to measure experimentally and to then interpret. However, this approach is made more complicated under conditions where the rubber exhibits unsteady crack growth such as stick slip tearing or knotty tearing, which is often encountered with a strain crystallising rubber such as natural rubber or when reinforcing fillers are used. Under these conditions it is not easy to measure the crack growth rate as the value adopted is usually the average of a very rapid tearing rate and a zero velocity tearing rate. The approach adopted in this study characterises the rate of increase in the strain energy with time, T˙, at the crack tip just before the onset of tearing. The relation between this and the critical strain energy release rate T* for the crack propagation is examined over a wide range of tear rates and temperatures for several different rubber materials, including natural rubber (NR), epoxidized natural rubber with half of the isoprene units being epoxidized (ENR-50) and polybutadiene rubber (BR). It is apparent that the critical strain energy release rate, T* is surprisingly independent of the rate of strain at the crack tip for the majority of materials and over a wide range of temperatures. Only when tearing has become very rapid do additional viscoelastic mechanisms result in significant toughening of the rubbers.