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

 * The contact constraint can only be applied to two faces.
 * Development for multiple contact at once: https://forum.freecadweb.org/viewtopic.php?f=18&t=15699&start=130#p303275
 * Because 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. See Forum discussion.

Tips for modeling

 * From: https://forum.freecadweb.org/viewtopic.php?f=18&p=340874#p340494
 * The use of linear elements is recommended. Otherwise, calculations can be 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 can be estimated as 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 that 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 of different contact types: https://forum.freecadweb.org/viewtopic.php?f=18&t=15699&start=90#p188736
 * Further interesting information:
 * 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)


 * An interesting example found in the FreeCAD German subforum. (link)