The behavior of crack propagation and blunting at different strain rates, temperature and crack geometry in BCC-Fe, W, and FCC-Ni, Cu has been studied by molecular dynamics under load mode I. Large scale atomistic simulations coupled with a structural analysis have been performed to reveal conditions at which dislocations can be emitted from a semi-infinite crack. In FCC metals the crack tip is blunted by the plastic deformation due to the nucleation and emission of dislocations. These processes, however, occur in a different manner in Ni and Cu. It can be concluded that the formation of stacking faults and twins at a crack tip is particularly important for brittle cleavage. In BCC crystals the crack was found to open without plastic deformation, however, the strain work hardening has a significant dependence on temperature. The critical stress intensity factor for brittle cleavage or dislocation emission was calculated from the stress tensor measured on atoms located at the crack tip. The obtained results are discussed and compared with predictions of elasticity theory.