FEM ConstraintContact

Description
Creates a FEM constraint for contact between 2 surfaces.

Usage

 * 1) There are several ways to invoke the command:
 * 2) * Press the button.
 * 3) * Select the option from the menu.
 * 4) Select the master face.
 * 5) Select the slave face.
 * 6) Enter a contact stiffness.
 * 7) 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
 * 3) Due to the fact that multiple meshes are currently not supported, contact must be applied to faces that are separated by (at least) a small distance. If the faces were touching (no gap between them), the result of a boolean union or boolean fragments operation (necessary to avoid having multiple meshes which is not allowed at the moment) would be a continuous mesh and thus no need to use contact anymore. Forum discussion: https://forum.freecadweb.org/viewtopic.php?f=18&t=62307

Tips for modeling

 * 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. (link)


 * A very cool example found in the FreeCAD German subforum. (link)