  Brittle materials under impact loading exhibit a transition from a cracked
solid to a granular medium. Appropriate representation of this transition to
granular mechanics and the resulting initial fragment size and shape
distribution in computational models is not well understood. The current work
provides a numerical model to analyze competitive crack coalescence in the
transition regime and provides insight into the onset of comminution and the
initial conditions for subsequent granular flow. Crack statistics obtained from
initial flaws using a wing crack growth based damage model have been used to
discretely model elliptical cracks in three dimensions, with and without a
minimal intersection constraint. These cracks are then allowed to coalesce with
nearby cracks along favourable directions and the output fragment statistics
have been predicted. The evolving fragmentation offers insight into the onset
of comminution as well as the final transition to granular mechanics and the
resulting initial fragment statistics. A simple phenomenological model has been
proposed that suggests a transition criterion resembling the one obtained from
the numerical model.
