ANSYS Modeling and Meshing Guide, Chapter 6
Someday, you will be handed a model to analyze that was not developed with FEA requirements in mind. What can you do ?
When symmetry exists, USE IT (for faster runs, and perhaps higher accuracy).
Make symmetry cuts as the last feature on a geometry model, and make it independent of other features (so it can be deleted or suppressed).
It is normally better to cut the part in the original CAD package, if possible.
Sometimes, cosmetic features ("patch") are needed for placing loads and constraints on the FEA model, or for better mesh size control.
Also, create these features last on the geometry model.
In Pro/ENGINEER, we create features called datum curves or datum points on surfaces of the model.
Datum curves are used to split a face into several regions.
Datum points may be used to define "hard points" i.e., points that must have a node when the model is meshed.
Feature suppression can be a manually, iterative process. After you run a simulation case, you may go back to the CAD program and remove or include features based on the observed response of the model.
Some codes allow you to "collapse" the inner and outer surfaces of the geometry to the mid-plane. For example, CADfix® and Peak Productivity Products offer mid-plane extraction, but you pay an additional licensing cost.
Sometimes, for large thin parts (e.g., auto body panels), you can choose to use an outer surface instead of the mid-plane. You must decide if this approximation is acceptable.
Pro/ENGINEER automatically finds the mid-plane when it is meshing with shell elements.
It is also desirable to do this to the geometry before it is transferred via IGES. In Pro/ENGINEER 2001, under Application: Mechanica (FEM Mode), Structure > Model > Idealizations > Shells > Midsurfaces (detected shells) > Compress (Shells Only) > IGES
In ANSYS, after you Import a solid model via IGES, you MAY have access to "Geom Repair" and "Simplify" in the Preprocessor. These allow you to fix open edges, detect slivers, merge or collapse edges, split areas, etc., but forbid some of the more common construction and Boolean operation procedures.
These are available only with the "Defeature" option of IGES Import. If you use the "No Defeature" option, you still can create entities, overlap, merge, etc. in order to (more manually) repair defects in a CAD model.
Sometimes, you find that the only hope is to delete surfaces and rebuild them in the FEA preprocessor. (See the hints from XANSYS). Other times, the model can be crudely meshed, then later refined to get an acceptable FEA model.