  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 stability of such surfaces is related to stability
outside the surface via a single jump relation that can be regarded as
interchange stability condition. Although this relation appears in the setting
of equilibrium elasticity theory, it is remarkably similar to the well known
normality condition which plays a central role in the classical plasticity
theory.
