[SMApp] Adding 2D truss elements 2N and 3N

applications/StructuralMechanicsApplication/custom_constitutive/truss_constitutive_law.cpp

@@ -156,6 +156,12 @@ double TrussConstitutiveLaw::CalculateStressElastic(
     CalculateValue(rParameterValues,TANGENT_MODULUS,tangent_modulus);
 
     const double current_stress = tangent_modulus*current_strain[0];
+
+    if (rParameterValues.GetOptions().Is(ConstitutiveLaw::COMPUTE_CONSTITUTIVE_TENSOR)) {
+        Matrix& r_const_matrix = rParameterValues.GetConstitutiveMatrix();
+        r_const_matrix.resize(1, 1);
+        r_const_matrix(0, 0) = tangent_modulus;
+    }
     return current_stress;
 }