The fracture and toughening mechanism of double-network hydrogel using the network mechanics method

Double-network hydrogels have received considerable attention due to their superior mechanical properties. However, a comprehensive understanding of their fracture and toughening mechanisms is still lacking, primarily due to the complex network structures of double-network hydrogels and the highly non-linear characteristics of the fracture. In this study, we address this gap by developing a mesoscopic model for double-network hydrogels using the network mechanics method, in which the fracture of polymer chains is realized by introducing the stretch criterion to each polymer chain. Through numerical analysis, we find that the stretch criterion ratio of polymer chains in two networks λ₂^c/λ₁^c is the key factor that influences the necking and hardening phenomena of double-network hydrogels. By changing this stretch criterion ratio, the fracture mode of double-network hydrogels can transit between ductile and brittle. When the fracture stretching criteria of the polymer chains in the first and second networks are 1.8 and 6, the necking and hardening phenomena can be observed. It is also found that a more uniform network structure of the first network enhances the toughness of double-network hydrogels. This study sheds light on the fundamental fracture and toughening mechanisms of double-network hydrogels, providing new insights for their further applications.