We experimentally and theoretically investigate dynamic fracture of two-dimensional periodic heterogeneous materials. We create a model material using multi-material additive manufacturing of polymers. We measure the crack front position and near tip displacement field by means of integrated digital image correlation at high temporal resolution, from which we infer the elastic energy release rate. We provide direct evidence of jumps in crack speed at the boundary of obstacles. 
We show that such intermittent crack dynamics results not only from the inertia of the solid but also from the rate dependence of fracture energy. The crack dynamics are governed by the energy balance at the tip, where the dynamic energy release rate must equal the rate-dependent fracture energy. The implication of these results on the homogenized toughness of periodic brittle materials is investigated and this mechanism is shown to lead to an increase in resistance to failure. Overall, our work highlights the central role played by rate dependency on crack evolution, an effect that is not usually included in equilibrium theory of fracture.

Effective toughness of periodic heterogeneous materials: the role of rate-dependent fracture energies

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