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

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

     //- calculate force in specified direction on topClamp patch
     vector direction(0, 1, 0);
     
     //- for small strain or moving mesh
     scalar force = gSum(
			    direction &
			    (mesh.boundary()[historyPatchID].Sf() & sigma.boundaryField()[historyPatchID])
			    );
     
     //- for large strain total lagrangian
     // tensorField F = I + gradU.boundaryField()[historyPatchID];
     // vectorField totalForce = mesh.Sf().boundaryField()[historyPatchID] & (sigma.boundaryField()[historyPatchID] & F);
     
     //vector force = sum( totalForce );
     
     //scalar force = force[vector::Y];
     
     //- patchIntegrate utility integrates it this way but this is worng because the sigma tensor should
     //- be dotted with the surface normal to give the actual traction/force
     //- you cannot just take the component of the sigma tensor
     //scalar forcePatchIntegrateMethod = gSum(
     //					    mesh.magSf().boundaryField()[historyPatchID]
     //					    *sigma.boundaryField()[historyPatchID].component(symmTensor::XY)
     //					    );
     
     scalar disp = max(U.boundaryField()[historyPatchID].component(vector::Y));
     
     //- write to file
     if(Pstream::master())
       {
	 OFstream& forceDispFile = *filePtr;
	 forceDispFile << disp << "\t" << force << endl;
       }
   }