  Brittle materials exhibit sharp dynamical fractures when meeting Griffith's
criterion, whereas ductile materials blunt a sharp crack by plastic responses.
Upon continuous pulling ductile materials exhibit a necking instability which
is dominated by a plastic flow. Usually one discusses the brittle to ductile
transition as a function of increasing temperature. We introduce an athermal
brittle to ductile transition as a function of the cut-off length of the
inter-particle potential. On the basis of extensive numerical simulations of
the response to pulling the material boundaries at a constant speed we offer an
explanation of the onset of ductility via the increase in the density of
plastic modes as a function of the potential cutoff length. Finally we can
resolve an old riddle: in experiments brittle materials can be strained under
grip boundary conditions, and exhibit a dynamic crack when cut with a
sufficiently long initial slot. Mysteriously, in molecular dynamics simulations
it appeared that cracks refused to propagate dynamically under grip boundary
conditions, and continuous pulling was necessary to achieve fracture. We argue
that this mystery is removed when one understands the distinction between
brittle and ductile athermal amorphous materials.
