  Dynamic fracture experiments were performed in PMMA over a wide range of
velocities and reveal that the fracture energy exhibits an abrupt 3-folds
increase from its value at crack initiation at a well-defined critical
velocity, below the one associated to the onset of micro-branching 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 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/quasi-brittle transition studied here.
