Characterization of the Deformation and Fracture of Tough Double-Network Hydrogels

The remarkable toughness of double-network hydrogels (DNHs) arise from the energy dissipation due to the deformation-induced failure of the stiffer first network, while the softer second network maintains the sample’s overall integrity. In this study, a new approach for strain field analysis suitable for DNHs has been developed by introducing lycopodium spores to create robust speckle patterns on the slippery hydrogel surface. Thus, digital image correlation (DIC) method was feasible for localized strain field analysis during mechanical deformation, i.e. tensile testing, exhibiting inhomogeneous deformation, e.g. during necking. Further, strain field analysis during re-swelling experiments, a straightforward and effective technique to visualize and characterize pre-stretched zones formed during necking in DNHs, has been improved. This DIC-based analysis, performed in a liquid, offers an innovative and complementary approach for internal fracture analysis of DNHs, without requiring complex post-processing, expensive instrumentation or sophisticated chemistry, and provides a deeper understanding of fracture resulting from irreversible damage in DNHs, which is dominated by first-network fracture.