Characterization of fracture toughness and damage zone of double network hydrogels

Double-network (DN) hydrogels have received extensive attention owing to their excellent mechanical properties such as high fracture toughness. It is widely accepted that the toughening mechanism of DN gels is based on the mutual interaction of two contrasting interpenetrating networks. However, the quantitative interpretation of this toughening mechanism during fracture is still lacking; there are also some contradictions regarding the fracture toughness. In this study, we developed a quantitative framework to decompose the fracture toughness and feature size of the crack-tip field. Through extensive tearing tests with varying free widths of the tearing specimens, we propose an exponential function to describe the relationship between the apparent fracture energy and free width. Using the proposed function, the fracture energy and feature size of the dissipation zone were decomposed into their different components, and their quantitative values were obtained. Tearing tests were also conducted on prestretched DN gels, which showed that the intrinsic fracture energy is related to the historical deformation. Finally, we established an intrinsic fracture model with a clear physical meaning by considering the complex interactions between the two interpenetrating networks of DN gels. The model revealed the physical mechanism behind the dependence of the intrinsic fracture energy of DN gels on the historical loading. The contributions of the two interpenetrating networks of DN gels to the intrinsic fracture energy were quantitatively decomposed by the model, and the feature size of the intrinsic field was also obtained. The study reveals the physical inconsistencies in some opinions about DN gel fracture and resolves some paradoxes on the toughness of DN gels.