 The distinctive character of this bifurcation is that it may occur even in an infinite body (or under the extreme constraint of smooth contact with a rigid constraint).Consider an infinite body made up of a nonlinear material, quasi-statically deformed in a way that stress and strain may remain homogeneous.The incremental response of this nonlinear material is assumed for simplicity linear, so that it can be expressed as a relation between a stress increment "formula_1" and a strain increment "formula_2", through a fourth-order constitutive tensor "formula_3" as

where the fourth-order constitutive tensor "formula_3" depends on the current state, i.e.