User Element (Uxxxx)

The user can define his/her own elements. In order to do so he/she has to:

• Give a name to the element. The name has to start with ``U'' followed by maximal 4 characters. Any character from the ASCII character set can be taken, but please note that lower case characters are converted into upper case by CalculiX. Consequently, ``Ubeam'' and ``UBEam'' are the same name. This reduces the character set from 256 to 230 characters.
• specify the number of integration points within the element (maximum 256), the number of nodes belonging to the element (maximum 256) and the number of degrees of freedom in each node (maximum 256) by using the *USER ELEMENT keyword card.
• write a FORTRAN subroutine resultsmech_uxxxx.f calculating the secondary variables (usually strains, stresses, internal forces) from the primary variables (= the solution of the equation system, usually displacements, rotations....). Add a call to this routine in resultsmech_u.f
• write a FORTRAN subroutine e_c3d_uxxxx.f calculating the element stiffness matrix and the element external force vector (and possibly the element mass matrix). Add a call to this routine in e_c3d_u.f
• write a FORTRAN subroutine extrapolate_uxxxx.f calculating the value of the secondary variables (usually strains, stresses..) at the nodes based on their values at the integration points within the element. Add a call to this routine in extrapolate_u.f

An example for a 3D Timoshenko beam element (for static linear elastic calculations and small deformations) according to [81] is implemented as element ``U1'' in CalculiX. It is used in example userbeam.inp in the test suite. The reader is referred to files resultsmech_u1.f, e_c3d_u1.f and extrapolate_u1.f for details on how a user elements is coded.