  Dynamic crack propagation drives catastrophic solid failures. In many
amorphous brittle materials, sufficiently fast crack growth involves
small-scale, high-frequency microcracking damage localized near the crack tip.
The ultra-fast dynamics of microcrack nucleation, growth and coalescence is
inaccessible experimentally and fast crack propagation was therefore studied
only as a macroscale average. Here, we overcome this limitation in
polymethylmethacrylate, the archetype of brittle amorphous materials: We
reconstruct the complete spatio-temporal microcracking dynamics, with
micrometer / nanosecond resolution, through post mortem analysis of the
fracture surfaces. We find that all individual microcracks propagate at the
same low, load-independent, velocity. Collectively, the main effect of
microcracks is not to slow down fracture by increasing the energy required for
crack propagation, as commonly believed, but on the contrary to boost the
macroscale velocity through an acceleration factor selected on geometric
grounds. Our results emphasize the key role of damage-related internal
variables in the selection of macroscale fracture dynamics.
