Effect of Predamage on the Fracture Energy of Double-Network Hydrogels

Double-network (DN) hydrogels are tough soft materials, and the high fracture resistance can be attributed to the formation of a large damage zone (internal fracture of the brittle first network) around the crack tip. In this work, we studied the effect of predamage in the brittle network on the fracture energy Γ_c of DN hydrogels. The prestretch of the first network was induced by prestretching the DN gels to prestretch ratio λ_pre. Depending on the λ_pre in relative to the yielding stretch ratio λ_y, above which the brittle first network starts to break into discontinuous fragments inside DN gels, two regimes were observed: Γ_c decreases monotonically with λ_pre in the regime of λ_pre < λ_y, mainly due to the decreasing contribution from the bulk internal damage, while Γ_c increases with λ_pre in the regime of λ_pre > λ_y. The latter can be understood by the release of the hidden length of the stretchable network strands by the rupture of the brittle network, whereby the broken fragments of the brittle network could serve as sliding cross-links to further delocalize the stress-concentration near the crack tip and prevent chain scissions.