I have been doing some modeling of a piezoelectric system and it works well. One can apply a field and see the resulting deformation.  I also get how one can calculate the Cauchy strain and stress using the post-process hook and the terms 'ev_cauchy_strain.2.Omega(u)' and 'ev_cauchy_stress.2.Omega(solid.D, u)'.  But since the constitutive law for the stress is the Cauchy term + the piezo-coupling term, how does one evaluate the stress and output it for given displacements and fields?