of the impact velocity to the horizontal, was varied between $90^{\circ}$ (normal impact) and $45^{\rm o}$ (oblique impact).

In order to understand the break-up process of discs, we investigated the crack pattern arising both in normal and oblique impacts. Fig. \[snapshots\] presents the time evolution of the crack pattern of normal impact obtained by the simulation of a circular disc of radius $30$cm at $250$cm/sec. When the disc strikes against the hard plate, a high compressive wave is generated at the impact point. Fracture starts from the region of contact point and propagates through the disc. As the time passes, more and more bonds break at the impact region and the area of contact increases progressively. As a result of this primary breakage a cone shaped (triangle shaped in two dimensions) damage area is created whose base corresponds approximately to the area of contact of the specimen and the target (see Fig. \[snapshots\]b) and it is more distinct at the end of the fragmentation process (see Fig. \[snapshots\]d). When the cone is driven into the specimen a large number of cracks are generated starting from the region around the cone (see Fig. \[snapshots\]b). This indicates that a high stress concentration 