When a contact condition is defined on interior boundaries by adding an Interior Contact node, a slit condition is defined for the displacement field. All variables and equations are defined for the midsurface of the boundary, and there is thus no distinction made between source and destination boundaries.

The gap function ggeom(X) is here a function of the material coordinates X, and is directly defined by the displacements as

Here uu and ud are the displacements on the “upside” and “downside” of the boundary, respectively. The spatial normal n is always evaluated at the midsurface of the slit boundary. The physical gap distance including offsets is then

A negative offset, doffset, thus implies that there is an initial gap, while a positive offset implies that there is an initial overclosure.

Similarly, the tangential deformation gt is given by

where t is the spatial tangent to the internal boundary. 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.