Effect of Pre-Damage 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 pre-damage in the brittle network on the fracture energy Γc of DN hydrogels. The pre-damage of the first network was induced by pre-stretching the DN gels to prestretch ratio λpre. Depending on the λpre in relative to the yielding stretch ratio λy, above which the brittle first network starts to break into discontinuous fragments inside DN gels, two regimes were observed: Γc decreases monotonically with λpre in the regime of λpre < λy, mainly due to the decreasing contribution from the bulk internal damage; while Γc increases with λpre in the regime of λpre > λy, The latter can be understood by the release of hidden length of the stretchable network strands by the rupture of the brittle network, whereby the broken fragments of brittle network could serve as sliding crosslinks to further delocalize the stress concentration near the crack tip and prevent chain scissions.