Elastomers Fail from the Edge

Abstract

The performance of soft devices is limited by the fracture resistance of elastomers. Thus, understanding how fracture resistance changes with material and sample geometry is an important challenge. A key observation is that thicker elastomers can be significantly tougher than thinner ones. We show that this surprising toughness enhancement in thick samples emerges from the 3D geometry of the fracture process. In contrast to the classical picture of a 2D crack, failure is driven by the growth of two separate "edge"cracks that nucleate early on at a sample's sides. As loading is increased, these cracks propagate in towards the sample midplane. When they merge, samples reach their ultimate failure strength. In thicker samples, edge cracks need to propagate farther before meeting, resulting in increased sample toughness. We demonstrate that edge-crack growth is controlled by the elastomer's strain-stiffening properties. Our results have direct implications for how to effectively toughen elastomers by controlling their geometry and large-strain mechanical properties.