started to fix KelvinVoigt damping

src/elasticity/materials/ViscoelasticDamping.hpp

@@ -41,24 +41,22 @@ public:
   };
 
   double Derivative(const Tensor &F, const Tensor &L, const Tensor &H) const override {
-    auto traceEDot = trace(sym(transpose(F) * L));
-    return 2.0 * eta * traceEDot * Frobenius(F, H);
-  };
+    return 2.0 * eta * Frobenius(F, L) * Frobenius(H, L);
+  }
 
   double SecondDerivative(const Tensor &F, const Tensor &L, const Tensor &H,
                           const Tensor &G) const override {
-    return 2.0 * eta * Frobenius(F, H) * Frobenius(F, G);
-  };
+    return 2.0 * eta * Frobenius(H, L) * Frobenius(G, L);
+  }
 
   double Derivative(const Tensor &F, const Tensor &L, const TensorRow &H) const override {
-    auto traceEDot = trace(sym(transpose(F) * L));
-    return 2.0 * eta * traceEDot * Frobenius(F, H);
-  };
+    return 2.0 * eta * Frobenius(F, L) * Frobenius(H, L);
+  }
 
   double SecondDerivative(const Tensor &F, const Tensor &L, const TensorRow &H,
                           const TensorRow &G) const override {
-    return 2.0 * eta * Frobenius(F, H) * Frobenius(F, G);
-  };
+    return 2.0 * eta * Frobenius(H, L) * Frobenius(G, L);
+  }
 };
 
 #endif //VISCOELASTICDAMPING_HPP