  Equations for dislocation evolution bridge the gap between dislocation
properties and continuum descriptions of plastic behavior of crystalline
materials. Computer simulations can help us verify these evolution equations
and find their fitting parameters. In this paper, we employ discrete
dislocation dynamics to establish a continuum-based model for the evolution of
the dislocation structure in polycrystalline thin films. Expressions are
developed for the density of activated dislocation sources, as well as
dislocation nucleation and annihilation rates. We demonstrate how size effect
naturally enters the evolution equation. Good agreement between the simulation
and the model results is obtained. The current approach is based on a
two-dimensional discrete dislocation dynamics model, but can be extended to
three-dimensional models.
