We have designed a new experimental setup able to investigate fracture of soft materials at small scales. At high crack velocity, where energy is mostly dissipated through viscoelastic processes, we observe an increasingly large high strain domain in the crack tip vicinity. Taking advantage of our ability to determine where linear elasticity breaks down, we derive a simple prediction for the evolution of the energy release rate with the crack velocity.

In a recent work, Lefranc and Bouchaud (LB) showed that the Mode I crack opening displacement in a thin sheet of Agar gel deviates significantly from the square root profile predicted by linear elastic fracture mechanics. In this letter we reexamine this problem and show that the experimentally measured crack opening displacement is consistent with the prediction based on finite elastostatics and a hyperelastic strain hardening model.

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