  Dislocation dynamics simulation is used to investigate the effect of grain
size and grain shape on the flow stress in model copper grains. We consider
grains of 1.25-10 $\mu$m size, three orientations (<135>, <100> and <111>) and
three shapes (cube, plate and needles). Two types of periodic aggregates with
one or four grains are simulated to investigate different dislocation flux at
grain boundaries. It is shown that in all cases the flow stress varies linearly
with the inverse of the square root of the grain size, with a proportionality
factor varying strongly with the grain orientation and shape. Simulation
results are discussed in the light of other simulation results and experimental
observations. Finally, a simple model is proposed to account for the grain
shape influence on the grain size effect.
