Brittle-quasibrittle transition in dynamic fracture: an energetic signature

Dynamic fracture experiments were performed in polymethylmethacrylate over a wide range of velocities and reveal that the fracture energy exhibits an abrupt threefold increase from its value at crack initiation at a well-defined critical velocity, below the one associated with the onset of microbranching instability. This transition is associated with the appearance of conics patterns on fracture surfaces that, in many materials, are the signature of damage spreading through the nucleation and growth of microcracks. A simple model allows us to relate both the energetic and fractographic measurements. These results suggest that dynamic fracture at low velocities in amorphous materials is controlled by the brittle-quasibrittle transition studied here.