foam-extend4.1-coherent-io/applications/solvers/solidMechanics/viscoElasticSolidFoam/calculateSigmaDSigmaCorr.H

{
  scalar t = runTime.value();
  scalar tNext = t + runTime.deltaT().value();

  instantList Times = runTime.times();

  sigma = dimensionedSymmTensor("zero", dimForce/dimArea, symmTensor::zero);
  DSigmaCorr =
    dimensionedSymmTensor
    (
     "zero",
     dimForce/dimArea,
     symmTensor::zero
     );

  for (label i=1; i<Times.size(); i++)
    {
      runTime.setTime(Times[i], i);

      if(runTime.timeIndex() != i)
        {
	  FatalErrorIn(args.executable())
	    << "The strain increment DEpsilon must be stored for "
	    << "each calculated time step. "
	    << exit(FatalError);
        }

      IOobject DEpsilonHeader
        (
	 "DEpsilon",
	 runTime.timeName(),
	 mesh,
	 IOobject::MUST_READ
	 );

      // Check DEpsilon exists
      if (DEpsilonHeader.headerOk())
        {
	  volSymmTensorField DEpsilonOld(DEpsilonHeader, mesh);

	  scalar tau = runTime.value() - m*runTime.deltaT().value();

	  if(tau < 0)
            {
	      sigma += 2.0*rheology.mu(t)*DEpsilonOld
		+ rheology.lambda(t)*(I*tr(DEpsilonOld));

	      DSigmaCorr += 2.0*rheology.mu(tNext)*DEpsilonOld
		+ rheology.lambda(tNext)*(I*tr(DEpsilonOld));
            }
	  else
            {
	      sigma += 2.0*rheology.mu(t - tau)*DEpsilonOld
		+ rheology.lambda(t - tau)*(I*tr(DEpsilonOld));

	      DSigmaCorr += 2.0*rheology.mu(tNext - tau)*DEpsilonOld
		+ rheology.lambda(tNext - tau)*(I*tr(DEpsilonOld));
            }
        }
      else
        {
	  Info << "No DEpsilon" << endl;
        }
    }

  if(Times.size()>=2)
    {
      runTime++;
    }

  scalar tau = runTime.value() - m*runTime.deltaT().value();

  sigma += 2.0*rheology.mu(t - tau)*DEpsilon
    + rheology.lambda(t - tau)*(I*tr(DEpsilon));

  DSigmaCorr += 2.0*rheology.mu(tNext - tau)*DEpsilon
    + rheology.lambda(tNext - tau)*(I*tr(DEpsilon));

  DSigmaCorr -= sigma;
}