The behaviour of cracks in rock due to drag bit cutting is modelled using fracture mechanics principles based on a displacement discontinuity method. A realistic representation of a stress singularity at a crack tip is achieved using a special crack tip element with r 1 2 displacement variation. The stress intensity factors are evaluated using a displacement correlation method. The maximum tensile stress criterion is used for modelling crack behaviour in terms of the crack propagation path and the applied load. This modelling technique is illustrated by a well known angled crack propagation problem in a tensile field. The rock crack propagation, which forms a major chip in the drag bit cutting process, is studied by the proposed technique. The influences of drag bit rake angle on the crack propagation path, the load requirement and the stability of propagation are discussed. �� 1992.