Discontinuous dynamic transition in crack propagation of stretchable elastomers

The fracture of highly stretchable elastomers shows a well-known discontinuous transition, involving crack propagation speed becoming double-valued within a certain window of network stretching. This fracture transition can occur either in continuous stretching or after a sizable step stretching. Crack propagation speed increases sharply with rising temperature, suggesting that dissociation of backbone bonds of network strands is the origin of fracture. Consequently, the nature of transition appears to be related to how the network evolves to redistribute strand tension, getting more strands to participate in the fast crack growth on the upper branch of the transition. Beyond the upper bound of the transition window, sufficiently high stretching leads to only the fast crack growth because load-bearing network strands cannot survive due to the higher bond tension on longer timescales. Conversely, below a minimal degree of stretching, the transition also disappears, and the fracture proceeds with slow crack growth, where very few network strands are ready to undergo chain scission necessary for fracture.