[BGRa] Changed one member name and improved the documentation

Documentation/ProjectFile/material/solid/constitutive_relation/CreepBGRa/t_a.md

-Parameter A.
\ No newline at end of file
+A parameter determined by experiments. Its unit is 1/[time unit].

Documentation/ProjectFile/material/solid/constitutive_relation/CreepBGRa/t_n.md

-Parameter n.
\ No newline at end of file
+A dimensionless parameter determined by experiments.

Documentation/ProjectFile/material/solid/constitutive_relation/CreepBGRa/t_q.md

-Parameter Q.
\ No newline at end of file
+Activation energy. Its unit is J/mol.

Documentation/ProjectFile/material/solid/constitutive_relation/CreepBGRa/t_sigma0.md

-Parameter sigma_0.
+A stress scaling factor. Its unit is the stress unit.

MaterialLib/SolidModels/CreepBGRa.cpp

@@ -45,7 +45,7 @@ CreepBGRa<DisplacementDim>::integrateStress(
 
     const double b =
         dt * _coef *
-        std::exp(-_Q / (MaterialLib::PhysicalConstant::IdealGasConstant * T));
+        std::exp(-_q / (MaterialLib::PhysicalConstant::IdealGasConstant * T));
     auto const& deviatoric_matrix = Invariants::deviatoric_projection;
 
     double pow_norm_s_n1_n_minus_one_2b_G = 0.;
@@ -101,10 +101,10 @@ double CreepBGRa<DisplacementDim>::getTemperatureRelatedCoefficient(
     double const T, double const deviatoric_stress_norm) const
 {
     return 2.0 * _coef *
-           std::exp(-_Q /
+           std::exp(-_q /
                     (MaterialLib::PhysicalConstant::IdealGasConstant * T)) *
            this->_mp.mu(t, x) * std::pow(deviatoric_stress_norm, _n - 1) * dt *
-           _Q / (MaterialLib::PhysicalConstant::IdealGasConstant * T * T);
+           _q / (MaterialLib::PhysicalConstant::IdealGasConstant * T * T);
 }
 
 template class CreepBGRa<2>;

MaterialLib/SolidModels/CreepBGRa.h

@@ -52,7 +52,7 @@ public:
 
     std::unique_ptr<
         typename MechanicsBase<DisplacementDim>::MaterialStateVariables>
-    createMaterialStateVariables() override
+    createMaterialStateVariables()
     {
         return LinearElasticIsotropic<
             DisplacementDim>::createMaterialStateVariables();
@@ -66,7 +66,7 @@ public:
           _nonlinear_solver_parameters(std::move(nonlinear_solver_parameters)),
           _n(n),
           _coef(A * std::pow(1.5, 0.5 * (1 + _n)) / std::pow(sigma0, _n)),
-          _Q(Q)
+          _q(Q)
     {
     }
 
@@ -93,10 +93,10 @@ public:
 private:
     NumLib::NewtonRaphsonSolverParameters const _nonlinear_solver_parameters;
 
-    const double _n;
+    const double _n; /// parameter determined by experiments.
     /// $fA\left(\frac{3}{2}\right)^{n/2+1}/\sigma_{eff}^n $f
     const double _coef;
-    const double _Q;
+    const double _q; /// Activation energy
 };
 
 extern template class CreepBGRa<2>;

web/content/docs/benchmarks/CreepBGRa/CreepBRGa.md

@@ -72,10 +72,10 @@ $$\begin{gathered}
 where
 $\mathbf{C}:= \lambda \mathcal{J} + 2G \mathbf I \otimes \mathbf I  $ 
 with $\mathcal{J}$ the forth order identity, $\mathbf I$ the second order identity,
-$\lambda$ the Lame constant,  $G$ the shear module, and $\otimes$  the tensor
+$\lambda$ the Lamé constant,  $G$ the shear modulus, and $\otimes$  the tensor
 product notation.
 
-is a forth order tensor. Substituting equation and the expression of $C$
+is a fourth order tensor. Substituting equation and the expression of $C$
 into the stress rate expression, equation , we have
  $$\begin{gathered}
 \dot { \mathbf \sigma}=  \mathbf{C} \dot { \mathbf \epsilon}- 2bG {\left\Vert{\mathbf s}\right\Vert}^{m-1}