 In all other cases, there is a divergence between the actual and calculated stored energy in the material, which is maximum in the case of pure hydrostatic loading, where, according to the von Mises criterion, no energy is stored.This paradox is resolved if a second constitutive equation is introduced, that relates hydrostatic pressure p with the volume change formula_3.These two curves, namely formula_4 and (p-formula_3) are essential for a complete description of material behaviour up to failure.