- In some cases, you can use imposed displacements instead of applied forces. This approach can be used
to start a static analysis closer to the equilibrium position, or to control displacements through periods
of unstable response (for example, snap-through or postbuckling). - Another technique that can be effective in circumventing problems due to initial instability is running a
static problem as a "slow dynamic" analysis (that is, using time-integration effects in an attempt to prevent
the solution from diverging in any one load step). - You can also apply temporary artificial stiffness to unstable DOFs, using control elements (such as COM-
BIN37), or using the birth and death option on other elements. The idea here is to artificially restrain the
system during intermediate load steps in order to prevent unrealistically large displacements from being
calculated. As the system displaces into a stable configuration, the artificial stiffness is removed.
8.12.2.9. Use the Rezoning Feature
If the solution fails to converge and the mesh is severely distorted, consider using rezoning, which allows
you to repair the distorted mesh and continue the simulation. For more information, see Rezoning in
the Advanced Analysis Guide.
8.12.2.10. Dispense with Extra Element Shapes
The program provides "incompatible" modes" formulation (also referred to as "extra shapes") for mod-
eling bending applications. If your problem is predominantly bulk deformation, then you may choose
to turn extra shapes off to reduce CPU/storage requirements and enhance convergence. However, doing
so precludes the ability to model any bending.
8.12.2.11. Using Element Birth and Death Wisely
Realize that any sudden change in your structure's stiffness matrix is likely to cause convergence
problems. When activating or deactivating elements, try to spread the changes out over a number of
substeps. (Use a small time step size if necessary to accomplish this.) Also be aware of possible singu-
larities (such as sharp reentrant corners) that might be created as you activate or deactivate elements.
Such singularities can cause convergence problems.
8.12.2.12. Read Your Output.
The program performs a nonlinear analysis as a series of linear approximations with corrections. The
program printout gives you continuous feedback on the progress of these approximations and corrections.
(Printout either appears directly on your screen, is captured on Jobname.OUT, or is written to some
other file (/OUTPUT).) You can examine some of this same information in POST1, using the PRITER
command, or in POST26, using the SOLU and PRVAR commands. You should make sure that you un-
derstand the iteration history of your analysis before you accept the results. In particular, do not dismiss
any program error or warning statements without fully understanding their meaning. A typical nonlinear
output listing is shown in Figure 8.16:Typical Nonlinear Output Listing (p. 271).
Figure 8.16: Typical Nonlinear Output Listing
SOLVE ***** ANSYS SOLVE COMMAND *****
com-
mand
echoRelease 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential informationGuidelines for Nonlinear Analysis