Abstract
A comprehensive study on the interfacial fracture behaviors of hydrogen-bonded double network (DN) hydrogels is performed using a novel fracture specimen configuration and J-integral approach under static mode-I (opening) and mixed-mode (50% opening and 50% shear) loading conditions. Amylopectin/polyacrylamide (Amy/PAAm) and Amy/P(N-hydroxyethyl acrylamide) (Amy/PHEAAm) are used as model DN gels. The effect of monomer concentration (20%, 25%, and 30% for Amy/PAAm and 20%, 22.5%, and 25% for Amy/PHEAAm) on both mode-I and mixed-mode fracture initiation toughness is determined. Digital image correlation is used to obtain the strain fields around the crack tip to understand the opening and shear strain field within the fracture process zone and to correlate with fracture toughness values. Results reveal that for both fracture loadings, decreasing monomer concentrations in both hydrogel types lead to significant enhancement of fracture initiation toughness. For Amy/PAAm hydrogels, the fracture toughness increased as the concentration decreased, with 20 wt.% showing the highest fracture toughness value of 4.73 J/m2, reflecting a 78% increase compared to the 30 wt.%. A similar trend is observed for Amy/PHEAAm hydrogels, where 20 wt.% shows a 2.15-fold improvement in fracture toughness compared to 25 wt.%. Overall, under mixed-mode loading, the fracture toughness of both hydrogel types is enhanced compared to mode-I loading.