Add method that returns a stress function corresponding to the material

The stress function maps a displacement gradient to the corresponding stress
and can be used within the VonMisesStressAssembler

dune/elasticity/materials/geomexactstvenantkirchhoffmaterial.hh

@@ -37,6 +37,8 @@ private:
     typedef GeomExactStVenantKirchhoffFunctionalAssembler<GridType,Lfe> GeomLinearization;
     typedef GeomExactStVenantKirchhoffOperatorAssembler<GridType, Lfe, Lfe> GeomHessian;
     typedef typename GridType::template Codim<0>::Geometry::LocalCoordinate LocalCoordinate; 
+    using Derivative = typename GridFunction::DerivativeType;
+    using StressFunction = std::function<SymmetricTensor<dim, ReturnType>(Derivative)>;
 
 public:
     GeomExactStVenantMaterial() :
@@ -129,6 +131,22 @@ public:
     {
         localHessian_->setConfiguration(displace);
         return *localHessian_;
+
+    static StressFunction stressFunction(ReturnType E, ReturnType nu) {
+      ReturnType lambda{E * nu / (1.0+nu) / (1.0-2.0*nu)};
+      ReturnType twoMu{E / (1+nu)};
+
+      return [=](const Derivative& dispGrad) {
+
+          SymmetricTensor<dim,ReturnType> strain;
+          Dune::Elasticity::strain(dispGrad, strain);
+
+          SymmetricTensor<dim, ReturnType> stress = strain;
+          stress *= twoMu;
+          stress.addToDiag(lambda * strain.trace());
+
+          return stress;
+      };
     }
 
 private: