Fracture of Stretchable Materials

Abstract

Stretchable materials, like rubbers and gels, have long been used in conventional applications. In recent years, functional polymers and hydrogels are emerging. Especially hydrogels have been significantly toughened for the applications that require materials be soft and stretchable, but capable to resist ruptures, such as soft machines, tissue regeneration and replacement, ionic and stretchable devices, etc. The interest in the mechanics and fracture of stretchable materials has surged. We start with the investigations on the fracture properties, such as the flaw sensitivity, the size dependence of fracture resistance, and the intrinsic strength of stretchable materials. We study the flaw sensitivity of stretchable materials. The stretchability of a stretchable material is insensitive to small cracks, but reduces markedly when the cracks are large. We show that this transition occurs when the length of a crack exceeds a material-specific length, defined by the ratio of the fracture energy measured in the large-crack limit and the work to rupture measured in the small-crack limit. We study the size-dependent fracture resistance of tough hydrogels. As the first time, we measure the fracture resistance curves of tough hydrogels and show their size dependence on the specimen. We study the intrinsic strength of elastomeric polymers, and show that the strength is greatly reduced by the variation of the length of polymeric chains, while the strength of covalent bonds has a limited influence to the strength of material. We then study a particular failure of a natural-rubber balloon with a puncture. We show that the stretchability of a punctured balloon is insensitive to the shape and size of punctures up to a millimeter. We show that the reduction of the stretchability is attributed to the concentrated stretch around the puncture, while the constant reduction is rationalized by the flaw-insensitivity fracture of the natural-rubber sheets. We finally analyze the failure of elastomeric seals used in oil-field. We study the failure of elastomeric seals with the focus on a mode of leak caused by a crack across the length of a long seal. We obtain an analytic solution of large elastic deformation and an analytic expressions for the critical pressure for the onset of crack propagation. We show that the theoretical predictions agree remarkably well with the experiments in the desktop oil-field. An ideal seal needs to be soft to accommodate variations of mating parts and to facilitate installations, and needs to be stiff to enhance sealing capacity. We resolve the controversy by using strain-stiffening materials. We show that the strain-stiffening seals can dramatically enhance the sealing capacity of elastic leak and rupture with low initial stiffness. And the strain-stiffening seals would trend to fail in the mode of elastic leak before damage.