8.11.1.2. Checking Results Aft er Applying Stabilization
Stabilization can help with convergence problems, but it can also affect accuracy if the stabilization
energy or forces are too large. Although the program automatically reports the stabilization force norms
and compares them to internal force norms, it is still very important to check the stabilization energy
and forces to determine whether or not they are excessive.
Stabilization energy, the work done by stabilization forces, should be compared to element potential
energy. The energies can be output in the .OUT file (via the OUTPR command). You can also access
the energies as follows:
- In POST1, via PRENERGY,PRESOL,PLESOL, and ETABLE commands.
- In POST26 by ENERSOL and ESOL commands.
If the stabilization energy (which could be larger than that specified via the STABILIZE command) is
much less than the potential energy (for example, within a 1.0 percent tolerance), the result should be
acceptable and there should be no need to check the stabilization forces further.
When stabilization energy is large, check the stabilization forces at each DOF for all substeps. If the
stabilization forces are much smaller than the applied loads and reaction forces (for example, within a
0.5 percent tolerance), the results are still acceptable. Such a case could occur when an elastic system
is loaded first, then unloaded significantly. It is possible that the final element potential energy is small
and stabilization energy is relatively large, but all stabilization forces are small. Currently, stabilization
forces are accessible in the .OUT file (via OUTPR ).
Even when both stabilization energy and forces are too large, the results could still be valid. Such a
scenario is possible when a large part of an elastic structure undergoes large rigid body motion (as in
a snap-through simulation). In such a case, the stabilization energy could be large as well as the stabil-
ization force for some DOFs at some substeps, but the results could still be acceptably accurate. Never-
theless, consider the results along with other support data and use your own discretion.
8.11.1.3. Tips for Using Stabilization
You can use nonlinear stabilization to achieve convergence in an analysis of unstable nonlinear problems
such as post-buckling, snap-through simulation, and analyses where material is unstable. Although you
can activate nonlinear stabilization at the beginning of the solution, it is more efficient and accurat e in
most cases to activate stabilization in a multiframe restart.
If you wish to activate stabilization after a restart, do not restart from the last converged substep. Rather,
restart from the next-to-last converged substep or at some other substep prior to the last converged
substep. ( The program needs one substep to prepare the data for stabilization.)
Because it is usually impossible to know when a system will become unstable during loading before
an analysis starts, run the nonlinear analysis as usual while saving restart files for at least the last two
converged substeps. If the analysis fails to converge because of instability, restart the analysis with
stabilization activated from the next-to-last converged substep or at some other substep prior to the
last converged substep. ( The program needs one substep to prepare the data for stabilization.)
If the behavior of a problem is well known from a previous analysis and the structure loses stability
very soon after you begin to apply loads, you can activate stabilization at the beginning of the analysis.
Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential informationNonlinear Structural Analysis