The competition between brittle and ductile modes of failure is analyzed in a double edge-cracked specimen impacted by a projectile on the cracked side. Full transient analyses under plane strain conditions are carried out, with impact simulated by an imposed normal velocity with a small rise time. The material response is characterized by an elastic-viscoplastic constitutive relation for a porous plastic solid, with adiabatic heating due to plastic dissipation and the resulting thermal softening accounted for. The onset of cleavage is assumed to occur when a critical value of the maximum principal stress is attained. Effects of varying the loading rate and the thermal softening characterization of the material are explored. If the thermal softening rate is sufficiently high, a transition from a brittle failure mode at low loading rates to a ductile failure mode at high loading rates is found, which is quite the opposite of the usual brittle-ductile transition behaviour. This preduction for the fracture mode transition is consistent with experimental observations under these loading conditions.