Many industrial granular processes involve desired or undesired fragmentation of grains. However, despite experimental measurements and numerical modeling approaches, the mechanisms of single grain fragmentation and its effects on the behavior of granular materials are still poorly understood. In this work, we investigate the fracture and fragmentation of a single grain due to impact using three- dimensional Contact Dynamics simulations. The grains are assumed to be perfectly rigid but modeled as an assembly of bonded polyhedral Voronoï cells. The strength of the bonds represents the internal cohesion of the grain along normal and tangential directions. The inter-cell joints can open either in tension or by slip when the fracture strength is reached. A series of simulations for a range of different values of parameters (number of cells, cohesion, impact velocity) were performed. The efficiency of the fragmentation process was defined as the ratio of the energy dissipated by fracture to the kinetic energy at impact. It find that the efficiency increases with the number of cells. We also show that the efficiency is inversely proportional to the internal cohesion. Finally, the impact velocity maximizing the efficiency is found to be 0.08 m/s for two grains tessellated with 20 and 100 cells.