Check this thread: Calculix Contact Surface
You also have to specify facet numbers:
Element faces are identified by the surface label Sx where x is the number of the face. The numbering depends on the element type.
for triangular plane stress, plane strain and axisymmetric elements:
- Face 1: 1-2
- Face 2: 2-3
- Face 3: 3-1
- Face N: in negative normal direction (only for plane stress)
- Face P: in positive normal direction (only for plane stress)
