  We investigate dynamic fracture of heterogeneous materials experimentally by
measuring displacement fields as a rupture propagates through a periodic array
of obstacles of controlled fracture energy. Our measurements demonstrate the
applicability of the classical equation of motion of cracks at a discontinuity
of fracture energy: the crack speed jumps at the entrance and exit of an
obstacle, as predicted by the crack-tip energy balance within the brittle
fracture framework. The speed jump amplitude is governed by the fracture energy
contrast and by the combination of rate-dependency of fracture energy and
inertia of the medium, which allows the crack to cross a fracture energy
discontinuity at constant energy release rate. This discontinuous dynamics and
the rate-dependence cause higher effective toughness, which governs the
coarse-grained behavior of these cracks.
