The strengths and shear moduli of several polycrystalline copper systems ha
ve been calculated with the molecular dynamics method and effective-medium
potential. The grain size varied between 2 and 10 nm, and systems were shea
red beyond the yield point at room temperature. An inverse Hall-Fetch behav
iour was seen: the strength decreased with decreasing grain size. Similar b
ehaviour was seen for the shear modulus. These were caused by the grain bou
ndaries, which allow grain-boundary sliding and have low elastic constants.
Results were compared with the tensile strength, and the Von Mises failure
criterion did hold quite well even on these very small scales. Details of
the potential model are discussed in the appendix.