Subcritical, critical, and supercritical directions of stress-rate in finite rigid plasticity (Q1085670)

During loading of elastic-plastic materials, it has been shown previously [the author and \textit{H. H. Lin}, J. Méc. Théor. Appl. 5, 685-702 (1986)] that the direction of the Lagrangian strain-rate tensor can be either subcritical, critical, or supercritical. These three situations generalize the three types of softening behavior (subcritical, critical, and normal) observed in uniaxial compression tests on rocks. For rigid- plastic materials, the direction of the strain-rate during loading is fixed by a constitutive equation, and, as shown below, it is now the direction of the symmetric Piola-Kirchhoff stress tensor that can be either subcritical, critical, or supercritical.