foam-extend4.1-coherent-io/applications/solvers/solidMechanics/elasticOrthoNonLinULSolidFoam/checkPlaneStress.H

if(rheology.planeStress())
  {
    //- add higher order terms
    volScalarField higherTerms = -0.5*volTensorField(gradDU&gradDU.T()).component(tensor::ZZ);

    forAll(gradDU.internalField(), celli)
      {
    gradDU.internalField()[celli][tensor::ZZ] =
      ((-C.internalField()[celli][symmTensor4thOrder::XXZZ]*DEpsilon.internalField()[celli][symmTensor::XX]
        - C.internalField()[celli][symmTensor4thOrder::YYZZ]*DEpsilon.internalField()[celli][symmTensor::YY])
                    /
       C.internalField()[celli][symmTensor4thOrder::ZZZZ])
      -higherTerms.internalField()[celli];
      }
    forAll(gradDU.boundaryField(), patchi)
      {
    forAll(gradDU.boundaryField()[patchi], facei)
      {
        gradDU.boundaryField()[patchi][facei][tensor::ZZ] =
          ((-C.boundaryField()[patchi][facei][symmTensor4thOrder::XXZZ]*DEpsilon.boundaryField()[patchi][facei][symmTensor::XX]
        - C.boundaryField()[patchi][facei][symmTensor4thOrder::YYZZ]*DEpsilon.boundaryField()[patchi][facei][symmTensor::YY])
                        /
           C.boundaryField()[patchi][facei][symmTensor4thOrder::ZZZZ])
          - higherTerms.boundaryField()[patchi][facei];
      }
      }
  }