  Dynamical process where an edge dislocation in fcc copper bypasses an
impenetrable precipitate is investigated by means of molecular dynamics
simulation. A mechanism which is quite different from the Orowan mechanism is
observed, where a dislocation leaves two prismatic loops near a precipitate:
i.e. the Hirsch mechanism. It is found that the critical stress for the Hirsch
mechanism is almost the same with the Orowan stress, while the spatial
inhomogeneity of the shear stress is essential to the Hirsch mechanism. We also
find that the repetition of the Hirsch mechanism does not increase the critical
stress.
