A research note on certain loading directions in rate independent plasticity

Starting with the strain space formulation of rate independent elastic-plas tic materials, the notion of "work-instability", i.e. the possibility of an elastic plastic material undergoing spontaneous plastic deformation in a w orkless manner, is discussed. Some simple criteria for a material to be wor k stable are presented. An example of a material that satisfies Prandtl-Reu ss type equations with strain softening is presented and it is demonstrated that for plane homogeneous deformations, the material shows the possibilit y of spontaneous plastic flow even if the in-plane strains are held fixed, if it softens beyond a critical amount. (C) 1999 Elsevier Science Ltd. All rights reserved.