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

Dynamic fracture of heterogeneous materials is investigated by measuring displacement fields as a rupture propagates through a periodic array of obstacles of controlled toughness. We provide direct evidence of crack speed jumps at the boundary of obstacles. Our experiments reveal that such a discontinuous dynamics emerges from rate-dependent fracture energy, which combined with inertia, allows the crack to cross a toughness discontinuity at constant energy release rate. The rate-dependency, a direct consequence of the out-of-equilibrium nature of fracture phenomenon that is often neglected, plays a central role in setting up homogenized toughness, leading ultimately to increased resistance to failure.