Nonlinear features of dislocation emission processes under mode II loads are explored from an atomic scale. Crack tip atom string models coupling with the continuum mechanics analysis are devised. Dynamic analysis shows that the atom motion at the crack tip changes from periodic to chaotic, as the mode II stress intensity factor increases. The chaotic atom motion dictates the dislocation nucleation process at the crack tip. Study on the dislocation emission band reveals the phenomenon of cloudlike drifting of the dislocation core ahead of the crack tip. ©1995 American Institute of Physics.