[MaL] KV; Add 2D case of fourth order rotation mat

With the assumption of e3 = z-axis being the third
basis vector in the 2D case.

MathLib/KelvinVector.cpp

@@ -180,8 +180,27 @@ kelvinVectorToSymmetricTensor(Eigen::Matrix<double,
         v.size());
 }
 
-KelvinMatrixType<3> fourthOrderRotationMatrix(
-    Eigen::Matrix3d const& transformation)
+template <>
+KelvinMatrixType<2> fourthOrderRotationMatrix<2>(
+    Eigen::Matrix<double, 2, 2> const& transformation)
+{
+    // 1-based index access for convenience.
+    auto Q = [&](int const i, int const j) {
+        return transformation(i - 1, j - 1);
+    };
+
+    MathLib::KelvinVector::KelvinMatrixType<2> R;
+    R << Q(1, 1) * Q(1, 1), Q(1, 2) * Q(1, 2), 0,
+        std::sqrt(2) * Q(1, 1) * Q(1, 2), Q(2, 1) * Q(2, 1), Q(2, 2) * Q(2, 2),
+        0, std::sqrt(2) * Q(2, 1) * Q(2, 2), 0, 0, 1, 0,
+        std::sqrt(2) * Q(1, 1) * Q(2, 1), std::sqrt(2) * Q(1, 2) * Q(2, 2), 0,
+        Q(1, 1) * Q(2, 2) + Q(1, 2) * Q(2, 1);
+    return R;
+}
+
+template <>
+KelvinMatrixType<3> fourthOrderRotationMatrix<3>(
+    Eigen::Matrix<double, 3, 3> const& transformation)
 {
     // 1-based index access for convenience.
     auto Q = [&](int const i, int const j) {

MathLib/KelvinVector.h

@@ -238,8 +238,11 @@ KelvinVectorType<DisplacementDim> symmetricTensorToKelvinVector(
 /// Rotation tensor for Kelvin mapped vectors and tensors. It is meant to be
 /// used for rotation of stress/strain tensors epsilon:Q and tangent stiffness
 /// tensors Q*C*Q^t.
-KelvinMatrixType<3> fourthOrderRotationMatrix(
-    Eigen::Matrix3d const& transformation);
+/// 2D and 3D implementations available.
+template <int DisplacementDim>
+KelvinMatrixType<DisplacementDim> fourthOrderRotationMatrix(
+    Eigen::Matrix<double, DisplacementDim, DisplacementDim> const&
+        transformation);
 
 }  // namespace KelvinVector
 }  // namespace MathLib