Link Elements and Rigid Constraints

At times there are very stiff sections in a model and generally it is not good to model these simply by creating very stiff elements as this can lead to ill-conditioning problems. It is better to replace the elements in these regions with link elements or rigid constraints.

The basic idea behind link elements and rigid constraints is that one node is selected as the primary node, and the others are the constrained nodes. The primary node of a link element is taken as the first node in the topology list for the element. The primary node of a rigid constraint must be specified. Using the geometrical relationship between the constrained and primary nodes it is possible to define the displacements and rotations at the constrained node(s) in terms of the displacement and rotation of the primary. Likewise the forces and moments at the constrained nodes can be transferred to equivalent forces and moments at the primary. Thus it is possible to remove the constrained nodes from the solution and avoid the potential ill-conditioning problems.

The link element behaves are a rigid beam or a rigid bar connecting the two ends. For the nonlinear solver static nonlinear solver the link element can also act as tension link, maintaining the geometrical relation ship while the element is in tension; a compression link, maintaining the geometrical relation ship while the element is in compression.

The constraint axes of nodes in a rigid constraint must be the same and may only be Cartesian; not cylindrical or spherical.