void elasticitySolver::computeEffectiveStiffness(std::vector<double> stiff)
{
  double st[6] = {0., 0., 0., 0., 0., 0.};
  double volTot = 0.;
  for(std::size_t i = 0; i < elasticFields.size(); ++i) {
    double E = elasticFields[i]._e;
    double nu = elasticFields[i]._nu;
    SolverField<SVector3> Field(pAssembler, LagSpace);
    for(groupOfElements::elementContainer::const_iterator it =
          elasticFields[i].g->begin();
        it != elasticFields[i].g->end(); ++it) {
      MElement *e = *it;
      double vol = e->getVolume() * e->getVolumeSign();
      int nbVertex = e->getNumVertices();
      std::vector<SVector3> val(nbVertex);

      double valx[256];
      double valy[256];
      double valz[256];
      for(int k = 0; k < nbVertex; k++) {
        MVertex *v = e->getVertex(k);
        MPoint p(v);
        Field.f(&p, 0, 0, 0, val[k]);
        valx[k] = val[k](0);
        valy[k] = val[k](1);
        valz[k] = val[k](2);
      }

      double gradux[3];
      double graduy[3];
      double graduz[3];
      SPoint3 center = e->barycenterUVW();
      double u = center.x(), v = center.y(), w = center.z();
      e->interpolateGrad(valx, u, v, w, gradux);
      e->interpolateGrad(valy, u, v, w, graduy);
      e->interpolateGrad(valz, u, v, w, graduz);

      double eps[6] = {gradux[0],
                       graduy[1],
                       graduz[2],
                       0.5 * (gradux[1] + graduy[0]),
                       0.5 * (gradux[2] + graduz[0]),
                       0.5 * (graduy[2] + graduz[1])};

      double A = E / (1. + nu);
      double B = A * (nu / (1. - 2 * nu));
      double trace = eps[0] + eps[1] + eps[2];
      st[0] += (A * eps[0] + B * trace) * vol;
      st[1] += (A * eps[1] + B * trace) * vol;
      st[2] += (A * eps[2] + B * trace) * vol;
      st[3] += (A * eps[3]) * vol;
      st[4] += (A * eps[4]) * vol;
      st[5] += (A * eps[5]) * vol;
      volTot += vol;
    }
  }
  for(int i = 0; i < 6; i++) stiff[i] = st[i] / volTot;
}
PView *elasticitySolver::buildStressesView(const std::string postFileName)
{
  double sti[6] = {0., 0., 0., 0., 0., 0.};
  double str[6] = {0., 0., 0., 0., 0., 0.};
  double volTot = 0.;
  std::cout << "build stresses view" << std::endl;
  std::map<int, std::vector<double> > data;
  for(std::size_t i = 0; i < elasticFields.size(); ++i) {
    double E = elasticFields[i]._e;
    double nu = elasticFields[i]._nu;
    SolverField<SVector3> Field(pAssembler, LagSpace);
    for(groupOfElements::elementContainer::const_iterator it =
          elasticFields[i].g->begin();
        it != elasticFields[i].g->end(); ++it) {
      MElement *e = *it;
      double vol = e->getVolume() * e->getVolumeSign();
      int nbVertex = e->getNumVertices();
      std::vector<SVector3> val(nbVertex);

      double valx[256];
      double valy[256];
      double valz[256];
      for(int k = 0; k < nbVertex; k++) {
        MVertex *v = e->getVertex(k);
        MPoint p(v);
        Field.f(&p, 0, 0, 0, val[k]);
        valx[k] = val[k](0);
        valy[k] = val[k](1);
        valz[k] = val[k](2);
      }

      double gradux[3];
      double graduy[3];
      double graduz[3];
      SPoint3 center = e->barycenterUVW();
      double u = center.x(), v = center.y(), w = center.z();
      e->interpolateGrad(valx, u, v, w, gradux);
      e->interpolateGrad(valy, u, v, w, graduy);
      e->interpolateGrad(valz, u, v, w, graduz);

      double eps[6] = {gradux[0],
                       graduy[1],
                       graduz[2],
                       0.5 * (gradux[1] + graduy[0]),
                       0.5 * (gradux[2] + graduz[0]),
                       0.5 * (graduy[2] + graduz[1])};

      double A = E / (1. + nu);
      double B = A * (nu / (1. - 2 * nu));
      double trace = eps[0] + eps[1] + eps[2];
      double sxx = A * eps[0] + B * trace;
      double syy = A * eps[1] + B * trace;
      double szz = A * eps[2] + B * trace;
      double sxy = A * eps[3];
      double sxz = A * eps[4];
      double syz = A * eps[5];

      std::vector<double> vec(9);
      vec[0] = sxx;
      vec[1] = sxy;
      vec[2] = sxz;
      vec[3] = sxy;
      vec[4] = syy;
      vec[5] = syz;
      vec[6] = sxz;
      vec[7] = syz;
      vec[8] = szz;

      data[e->getNum()] = vec;

      for(int k = 0; k < 6; k++) str[k] += eps[k] * vol;
      sti[0] += sxx * vol;
      sti[1] += syy * vol;
      sti[2] += szz * vol;
      sti[3] += sxy * vol;
      sti[4] += sxz * vol;
      sti[5] += syz * vol;
      volTot += vol;
    }
  }
  for(int i = 0; i < 6; i++) {
    str[i] = str[i] / volTot;
    sti[i] = sti[i] / volTot;
  }
  printf("effective stiffn = ");
  for(int i = 0; i < 6; i++) printf("%g ", sti[i]);
  printf("\n");
  printf("effective strain = ");
  for(int i = 0; i < 6; i++) printf("%g ", str[i]);
  printf("\n");

  PView *pv = new PView(postFileName, "ElementData", pModel, data, 0.0);
  return pv;
}