The pseudo velocity is calculated as a displacement increment divided by the time increment of the
substep. Therefore, any DOF that tends to be unstable has a large displacement increment causing a
large damping (stabilization) force; this force, in turn, reduces the displacements at the DOF so that
stabilization is achieved.
For the DOFs that are stable, the dashpot elements have little effect on the results because the displace-
ments and the stabilization forces are small relative to the physical forces.
The coefficient used to calculate the damping (stabilization) force is the damping factor. Although it
has the same physical meaning and unit as physical damping, it is purely numerical in nonlinear stabil-
ization. The program calculates a damping factor based on the energy dissipation ratio that you specify,
or you can input the damping factor value directly.
8.11.1.1. Input for Stabilization
The only command necessary for using nonlinear stabilization is STABILIZE. The command activates
or deactivates stabilization from one load step to another, or after a multiframe restart during a load
step.
The program assumes that the first substep of a load step is stable and calculates the basic properties
of the artificial dashpot elements based on this substep. Therefore, the program does not apply stabil-
ization for the first substep unless you specify that it should do so (via the command's SubStpOpt
option).
The following topics describe how to use the STABILIZE command in a nonlinear analysis:
8.11.1.1.1. Controlling the Stabilization Force
8.11.1.1.2. Applying a Constant or Reduced Stabilization Force
8.11.1.1.3. Using the Options for the First Substep
8.11.1.1.4. Setting the Limit Coefficient for Checking Stabilization Forces8.11.1.1.1. Controlling the Stabilization Force
Two methods are available for controlling the stabilization force:
- Applying an energy dissipation ratio (STABILIZE,,ENERGY,,,)
- Applying a damping factor (STABILIZE,,DAMPING,,,)
Energy Dissipation Ratio
The energy dissipation ratio is the ratio of work done by stabilization forces to element potential energy.
The energy dissipation ratio should be between 0 and 1. Because the value is used with predicted en-
ergies, the program allows an input value greater than 1, but use it with caution. The greater the value
of the energy ratio or damping factor, the greater the stabilization force (assuming that the specified
number of substeps and time remain unchanged) so that the system has a stiffer response.
The specified value should be large enough to circumvent the divergence, but small enough to avoid
excessive stiffness. The ideal value is fully dependent on the specific problem, the time of the load step,
and the number of substeps.
You may need a few tries to determine the best value. Generally, use a smaller value for local instability
and a larger value for global instability. The smaller value should be used for solid elements and the
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