A micromechanical model representing two adjacent grains is developed. Rapid crack propagation from one grain into another driven by a constant global stress state is simulated. The normal of the crack face in the grain where the micro-crack initiates coincides with the principle loading direction. In the adjacent grain, the propagation direction changes and separation occurs in a mixed way, involving both normal and shear separation. The largest grain size that can arrest a rapidly propagating micro-crack is defined as the critical grain size. The effects of the global stress state and temperature on the critical grain size is examined. The influence of the mismatch in lattice orientation between two neighboring grains is qualitatively described. The influence of temperature is modeled by a temperature dependent viscoplastic response.