The "Peierls stress'' for pure metals (evidence that it is negligible)

It is argued that the '' Peierls stress '' for dislocations in pure metal crystals is negligible. The resistance to dislocation motion is not quasistatic but viscous. Direct observations of the viscous behaviour are cited and internal friction measurements are mentioned. The measured viscosity coefficients are very small (less than mPa s). It is pointed out that the yield points of pure metals are not associated with Peierls stresses but with the interaction of the stress dependences of dislocation velocities and the kinetics of dislocation multiplication. The negligible values of the stresses needed to move dislocations are rationalized in terms of the small dependence of the cohesive energies of simple metals on their atomic patterns.