FEM ConstraintContact

Description
Creates a FEM constraint for contact between 2 surfaces.

How to use

 * 1) Click on [[Image:FEM ConstraintContact.png|32px]] or choose → Mechanical Constraints →   from the top menu.
 * 2) Select the master face.
 * 3) Select the slave face.
 * 4) Enter a contact stiffness.
 * 5) Enter a friction coefficient.

Limitations

 * 1) The contact constraint can only be applied to two faces.
 * 2) development for multiple contact at once: https://forum.freecadweb.org/viewtopic.php?f=18&t=15699&start=130#p303275

Some tipps how to model?

 * from https://forum.freecadweb.org/viewtopic.php?f=18&p=340874#p340494
 * Better use linear elements otherwise calculations are very time consuming
 * master/slave assignment:
 * The larger of the two surfaces should act as the master surface.
 * If the surfaces are of comparable size, the surface on the stiffer body should act as the master surface.
 * If the surfaces are of comparable size and stiffness, the surface with the coarser mesh should act as the master surface.

CalculiX

 * The contact stiffness should be 10 times the young's modulus of the material for hard contact. The higher the value for contact stiffness, the harder the contact between surfaces.
 * The slave face is the face which penetrates into the master face, and therefore experiences more deformation.
 * The *CONTACT PAIR card is used for modeling contact in CalculiX. The constraint uses Face-to-Face penalty contact and the contact formulation is explained in detail at http://web.mit.edu/calculix_v2.7/CalculiX/ccx_2.7/doc/ccx/node112.html
 * Overview for different contact types: https://forum.freecadweb.org/viewtopic.php?f=18&t=15699&start=90#p188736
 * Further interesting informations:
 * https://forum.freecadweb.org/viewtopic.php?f=18&t=23102#p180709 and following posts !!!
 * https://forum.freecadweb.org/viewtopic.php?f=18&t=20276
 * https://forum.freecadweb.org/viewtopic.php?f=18&t=21331
 * https://forum.freecadweb.org/viewtopic.php?f=18&t=15699 (initial contact topic)


 * A very detailed CalculiX contact example:
 * http://dip28p.web.fc2.com/calculix/netgen2calculix/index.html


 * A very cool example too. In German supforum.
 * https://forum.freecadweb.org/viewtopic.php?f=13&t=39663&start=10#p337254