in various experimental situations [@discrete; @transition; @twodisc; @granulate; @proj]. For more details of the simulation technique see Ref. [@discrete].

Crack pattern {#crack}
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   ![\[snapshots\] Snapshots of the fragmentation process in normal impact of a circular disc of radius 30 cm at $250$ cm/sec. (a) $t=0.00150$ sec, a high compressive wave generated at the impact point causes the primary breakage. (b) $t=0.00300$ sec, cracks start at the periphery of the conical region whose base is the contact line between the disc and the plate. (c) $t=0.00600$ sec, oblique cracks move outwards. (d) $t=0.00996$ sec, the oblique cracks reach at the outer surface. (e) The disc is reassembled at the end of the simulation to observe the final crack pattern. ](crack/90_250_0.00150.ps "fig:"){width="30.00000%"}   ![\[snapshots\] Snapshots of the fragmentation process in normal impact of a circular disc of radius 30 cm at $250$ cm/sec. (a) $t=0.00150$ sec, a high compressive wave generated at the impact point causes the primary breakage. (b) $t=0.00300$ sec, cracks start at the periphery of the conical region whose base is the contact line between the disc and the plate. (c) $t=0.00600$ sec, oblique cracks move outwards. (d) $t=0.00996$ sec, the oblique cracks reach at the outer 