The stress field near the tip of a plane stress crack in a gel consisting of chemical and physical cross-links.

We study the time-dependent asymptotic stress fields near the tip of a mode I plane stress crack in a hydrogel. The analysis is based on a three-dimensional continuum model which describes the viscoelastic behaviour of a hydrogel gel with permanent and transient cross-links. The viscoelasticity results from the breaking and healing of the transient cross-links in the gel network. We show that the crack tip fields satisfy a local correspondence principle-that is, the spatial singularities of these fields are identical to a hyperelastic cracked body with the same but undamaged networks. Asymptotic results compare very well with finite-element simulations on a single-edge crack specimen loaded under constant stretch rate. We also compare the theoretical results (crack opening profile and crack tip strain field) with experiments and find excellent agreement.