Drucker stability

Summary

Drucker stability is a condition on a material, introduced by Daniel Drucker. It says that an extra load applied by an outside agent does positive work. It also says that the agent cannot extract net work by applying and then removing the load. For elastoplastic materials it implies that the yield surface is convex and that the plastic strain rate is normal to it. It thus gives a mechanical justification of the associated flow rule, and it excludes strain softening.