Normality Rule? Plastic Work Extremals and Related Topics

The task in this chapter is to reexamine the background of the so-called “normality” principle, i.e., the widely accepted assumption that plastic strain rate tensor is perpendicular to a yield surface in the corresponding stress space. In the papers [Dru52, Ili61, Hil68] this conclusion has been drawn from analysis of the plastic work properties. For instance, in [Dru52], rate-independent plasticity was considered and contemplating that all stress increments were admissible normality was concluded from the assumption of plastic work positiveness for arbitrary stress cycle.