Crack front echelon instability in mixed mode fracture of a strongly nonlinear elastic solid

In order to assess the role of elastic nonlinearity in gel fracture, we study the echelon instability in gelatin under mixed mode tensile and antiplane shear loading -i.e. the emergence of segmented crack front structures connected by steps. We evidence the existence of an energyrelease-rate-dependent mode mixity threshold. We show that ' echelons appear via nucleation of localized helical front distortions, and that their emergence is the continuation of the crosshatching instability of gels and rubbers under pure tensile loading, shifted by the biasing effect of the antiplane shear. This result, at odds with the direct bifurcation predicted by linear elastic fracture mechanics, can be assigned to the controlling role of elastic nonlinearity. Copyright (c) EPLA, 2014