8.12.1.1. Understand Your Program and Structure Behavior
If you have not used a particular nonlinear feature before, construct a very simple model (containing
only a few elements), and make sure you understand how to handle this feature before you use it in a
large, complicated model.
Gain preliminary insight into your structure's behavior by analyzing a preliminary simplified model first.
For nonlinear static models, a preliminary linear static analysis can reveal which regions of your model
will first experience nonlinear response, and at what load levels these nonlinearities will come into play.
For nonlinear transient dynamic analyses, a preliminary model of beams, masses, and springs can provide
insight into the structure's dynamics at minimal cost. Preliminary nonlinear static, linear transient dy-
namic, and/or modal analyses can also help you to understand various aspects of your structure's non-
linear dynamic response before you undertake the final nonlinear transient dynamic analysis.
Read and understand the program's output messages and warnings. At a minimum, before you try to
postprocess your results, verify that your problem converged. For path-dependent problems, the printout's
equilibrium iteration record can be especially important in helping you to determine if your results are
valid or not.
8.12.1.2. Simplify Your Model
Keep your final model as simple as possible. For example, if applicable to your analysis, try any or all
of the following:
- Represent your 3-D structure as a 2-D plane stress, plane strain, or axisymmetric model.
- Reduce your model size by using symmetry or antisymmetry surfaces.
If your model is loaded antisymmetrically, however, you can generally not take advantage of antisym-
metry to reduce a nonlinear model's size. Antisymmetry can also be rendered inapplicable by large
deflections.- Omit a nonlinear detail if you can do so without affecting results in critical regions of your model.
- Model transient dynamic loading in terms of static-equivalent loads.
- Consider substructuring the linear portions of your model to reduce the computational effort required
for intermediate load or time increments and equilibrium iterations.
8.12.1.3. Use an Adequate Mesh Density
Recognize that regions undergoing plastic deformation require a reasonable integration point density
(mesh density is particularly important in plastic-hinge regions).
Provide an adequate mesh density on contact surfaces to allow contact stresses to be distributed in a
smooth fashion. Likewise, provide a mesh density adequate for resolving stresses; areas where stresses
or strains are of interest require a relatively fine mesh compared to that needed for displacement or
nonlinearity resolution.
Use a mesh density adequate to characterize the highest mode shape of interest. The number of elements
needed depends on the elements' assumed displacement shape functions, as well as on the mode
shape itself. Also, use a mesh density adequate to resolve any transient dynamic wave propagation
through your structure; if wave propagation is important, then provide at least 20 elements to resolve
one wavelength.
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