Non-associated plastic deformation and genuine instability (Q1310701)

An elastic-plastic nonassociated Mohr-Coulomb type of flow rule is considered. The authors show that unless the incremental elastic response (stress work) were to override the plastic one, there would be a genuine but limited instability of configuration for any such model subjected to a triaxial compression test. Also, for the usual simple models, every state of stress in the plastic domain is unstable. The instability exhibited here is in the form of a rotating shear band.