foam-extend4.1-coherent-io/applications/solvers/solidMechanics/elasticOrthoSolidFoam/setPlaneStressGradU.H

      //- set gradU.zz() for plane stress
      if(rheology.planeStress())
        {
          forAll(gradU.internalField(), celli)
        {
          gradU.internalField()[celli].zz() =
            (-C.internalField()[celli].xxzz()*epsilon.internalField()[celli].xx()
             - C.internalField()[celli].yyzz()*epsilon.internalField()[celli].yy())
            /
            C.internalField()[celli].zzzz();
        }
          gradU.correctBoundaryConditions();
        }