When a crack is defined by an interior boundary, or is located on a symmetry boundary, it is possible to consider contact between the crack faces by adding a Crack closure subnode. When such a node is added, all boundaries selected in the Crack node are applicable and selected in the Crack closure subnode as well, which means that contact is added between all crack faces.

Contact between crack faces is implemented by the penalty method as described in Contact Analysis Theory in the Structural Mechanics Theory chapter. By default, frictionless contact is considered. However, if the crack is defined by interior boundaries, you can also take friction into account.

The kinematics of the contact condition between crack faces differs from that used in general contact analysis and the Contact node. It is fully defined by the displacement discontinuity defined by the slit condition of the crack, and thus involves no contact search algorithm. The gap gn is then given by

where n is the spatial normal to the interior boundary, and uu and ud are the displacements on the “upside” and “downside”, respectively. Similarly, the tangential deformation gt is given by

where t is the spatial tangent to the interior boundary. In 3D, there are two components of gt given by t1 and t2, respectively. Friction is based on an incremental formulation, and the incremental slip is given by

where gt,old is the tangential deformation at the previous converged increment. For a symmetric crack uu = 0 and ud = u. Note that no tangential deformation is computed in such cases. Given that the gap and the slip are computed from the displacement jump across an interior boundary, the contact formulation is only valid for small sliding since the same nodal points are always connected even as the bodies deform.