foam-extend4.1-coherent-io/applications/solvers/solidMechanics/elasticSolidFoam/writeHistory.H

//- write force displacement to file
if(historyPatchID != -1)
  {
    Info << "Found patch "<<historyPatchName<<", writing y force and displacement to file"
	  << endl;

     //- for small strain or moving mesh
     vector force =
       gSum(mesh.boundary()[historyPatchID].Sf() & sigma.boundaryField()[historyPatchID]);

     vector disp = gAverage(U.boundaryField()[historyPatchID]);

     //- write to file
     if(Pstream::master())
       {
	 OFstream& forceDispFile = *filePtr;
	 forceDispFile << disp.x() << " "
		       << disp.y() << " "
		       << disp.z() << " "
		       << force.x() << " "
		       << force.y() << " "
		       << force.z() << endl;
       }
   }