Normality Condition in Elasticity

Strong local minimizers with surfaces of gradient discontinuity appear in variational problems when the energy density function is not rank-one convex. In this paper we show that the stability of such surfaces is related to the stability outside the surface via a single jump relation that can be regarded as an interchange stability condition. Although this relation appears in the setting of equilibrium elasticity theory, it is remarkably similar to the well-known normality condition that plays a central role in classical plasticity theory.