Multiphysics, ANSYS Mechanical, and ANSYS Structural products only. See the Element Reference for
details.
8.10.1. Element Birth and Death
Sometimes, an element's status changes between "existent" and "nonexistent." The birth and death
options (EKILL,EALIVE,ESTIF) (Main Menu> Solution> Load Step Opts> Other) can be used to de-
activate or reactivate selected elements in such cases. The birth and death feature is discussed in detail
in Element Birth and Death in the Advanced Analysis Guide.
8.11. Unstable Structures
A structure can become unstable when a load reaches its buckling value or when nonlinear material
becomes unstable. It is more common in slender structures than in bulky structures. The instability
could be global (such as a snap-through of a plate) or local (such as failure of a stiffener). Instability
problems usually pose convergence difficulties and therefore require the application of special nonlinear
techniques.
You can apply the following techniques to solve instability problems:
- Nonlinear stabilization
A tool for dealing with local instabilities as well as global instability. You can use it together with
nearly any other nonlinear solution technique, such as line search and automatic time stepping (al-
though not with the arc-length method).- Arc-length method
This method can circumvent global instability when forces are applied. More importantly, it can
simulate the negative slope portion of a load-vs.-displacement curve.- Running a static problem as a "slow dynamic" analysis
This method is not strictly a different technique; rather, you use a dynamic effect to prevent divergence.
This method is not especially easy to use because the analysis type changes, so you must input mass,
apply a damping factor if necessary, and use proper time-integration parameters. ANSYS, Inc. therefore
recommends trying nonlinear stabilization or the arc-length method first.Alternative methods are available to help achieve convergence. For example, you could apply displace-
ments instead of forces, if applicable. Or, you could apply artificial small stiffness to the unstable degrees
of freedom. These stiffnesses could be either permanent or activated/removed at some load step by
applying the stiffness using the control element COMBIN37. However, such methods may be unreliable,
impractical, or simply not applicable.
8.11.1. Using Nonlinear Stabilization
Convergence difficulty due to an unstable problem is usually the result of a large displacement for
smaller load increments. Nonlinear stabilization can be understood as adding an artificial damper or
dashpot element at each node of an element that supports this technique.
To better conceptualize the artificial dashpot element, think of it as having two nodes: one is the node
of the FE model that you create, the other is fixed on the ground. The program calculates the damping
force such that it is proportional to the relative pseudo velocity of the two nodes of the artificial element,
which is equal to the velocity of the node belonging to the FE model.
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