Elastic–viscous transition in tear fracture of rubbers

There is a widely reported transition in the relationship between the critical strain energy release rate and a critical crack growth rate during the tearing of rubber materials. This transition is explained here for the first time in terms of an elastic–viscous transition phenomenon. The transition thus characterized depends on the balance of elasticity and viscosity of the material and hence the proximity of the test to the materials' glass transition temperature. Therefore two factors dominate the transition behavior, the cross-link density and the visco-elastic energy dissipation. A new elastic–viscous transition diagram is introduced to elucidate the mechanism of this phenomenon, where the diagram consists of three zones, each with a different fracture mode. These are an elastic–brittle fracture zone, a viscous–ductile fracture zone and an intermediate transition zone between the elastic and viscous zones. The transition zone characterized by stick–slip motion is caused in mechanics as results from unstable fluctuations of crack growth rate due to the energy dissipation near the glass transition temperature. The mechanism for the transition to be generated at the crack front is investigated both theoretically and experimentally with a consideration of surface roughness formation and stick–slip tear motion together with frictional sliding of the rubber at the crack tip.