add documentation to FractureModelBase

MaterialLib/FractureModels/FractureModelBase.h

@@ -18,12 +18,28 @@ namespace MaterialLib
 namespace Fracture
 {
 
+/**
+ * Interface for mechanical fracture material models. Provides updates of the
+ * stress for a given current state and also a tangent at that position.
+ */
 template <int DisplacementDim>
 class FractureModelBase
 {
 public:
     virtual ~FractureModelBase() {}
 
+    /**
+     * Computation of the constitutive relation for specific material model.
+     * This should be implemented in the derived model.
+     *
+     * @param t           current time
+     * @param x           current position in space
+     * @param w_prev      fracture displacement at previous time step
+     * @param w           fracture displacement at current time step
+     * @param sigma_prev  stress at previous time step
+     * @param sigma       stress at current time step
+     * @param C           tangent matrix for stress and fracture displacements
+     */
     virtual void computeConstitutiveRelation(
             double const t,
             ProcessLib::SpatialPosition const& x,