Granular dynamics (DEM) simulations of a polydisperse cuboidal agglomerate impacting with a target wall are reported. The paper focuses on identifying the physical processes that lead to fracture. It is shown that fracture is the result of the manner in which strong interparticle forces are transmitted into the agglomerate and the consequent development of a heterogeneous distribution of primary particle velocities. This heterogeneous velocity field produces strong velocity discontinuities along which shear weakening occurs. Consequently, the strong velocity discontinuities become the potential fracture planes. (C) 2004 Elsevier B.V. All rights reserved.