# # 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 action of link elements and rigid constraints is substantially the same, the main difference being that a rigid constraint can include many nodes while a link element can only include two nodes, but has some additional linkage options.

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. 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 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 or a bar maintaining the fixed length of the link but not applying any constraint on the rotation of the link. These link types can only be used in the static nonlinear solver.

A custom link works in the same way as a joint by simply coupling degrees of freedom at the constrained node to the same degree of freedom at the primary. As this does not take account of the geometry of the element it is possible to create a custom link which does not enforce equilibrium.

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.

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