[ConstitutiveLaw] added a member to get equivalent plastic strain increment

MaterialLib/SolidModels/Ehlers.cpp

@@ -46,6 +46,11 @@ namespace Solids
 {
 namespace Ehlers
 {
+template <int DisplacementDim>
+double StateVariables<DisplacementDim>::getEquivalentPlasticStrain() const
+{
+    return std::sqrt(2.0 / 3.0 * Invariants::FrobeniusNorm(eps_p.D.eval()));
+}
 
 /// Special product of \c v with itself: \f$v \odot v\f$.
 /// The tensor \c v is given in Kelvin mapping.

MaterialLib/SolidModels/Ehlers.h

@@ -232,6 +232,11 @@ struct StateVariables
         damage_prev = damage;
     }
 
+    double getEquivalentPlasticStrain() const override;
+
+    static int const KelvinVectorSize =
+        MathLib::KelvinVector::KelvinVectorDimensions<DisplacementDim>::value;
+    using Invariants = MathLib::KelvinVector::Invariants<KelvinVectorSize>;
     using KelvinVector =
         MathLib::KelvinVector::KelvinVectorType<DisplacementDim>;
 

MaterialLib/SolidModels/MechanicsBase.h

@@ -59,6 +59,7 @@ struct MechanicsBase
     {
         virtual ~MaterialStateVariables() = default;
         virtual void pushBackState(){};
+        virtual double getEquivalentPlasticStrain() const { return 0.0; }
     };
 
     /// Polymorphic creator for MaterialStateVariables objects specific for a