MET 425 Lecture Notes

CAD Modeling for FEA
Reference: Mechanical APDL (formerly ANSYS)
Modeling and Meshing Guide, Chapter 6.1

Some CAD models cannot be used for analysis and some models should not be used for analysis.

Realize that two different models may be needed (CAD vs. FEA)

CAD model (or design geometry) can be for manufacturing, prototyping, detailing, tolerance checking, and maybe FEA

Analysis model (for FEA) has only the features necessary for simulation of the part in its application.

CAD models may be difficult to use for an analysis or may produce meshing failures due to sloppy modeling practices.   Sometimes, its better to start over for FEA.

CLEAN Geometry = a solid model which maximizes the possibility of meshing which can then capture the needed features for correct results.

1. Surfaces having consistent size & shape ratios

2. Simplification is possible without compromising the structure

Feature suppression can be a manual, 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.

Remember, simplifying or suppressing features far from the area of interest is acceptable.  Effects, local to the simplification will most likely not effect the global behavior of the model (St. Venant's Principle)

Beware of:

1. "SHORT" Edges (short, with respect to, model size or mesh size)

2. SLIVER surfaces (faces with high aspect ratio)

3. Voids (holes inside)

4. Complex surface or edges (lofts or blends)

5. Corrupted geometry files (from translators)

6. Parent-Child relations (prohibit simplification)

When SIMPLIFYING the Geometry, consider:

Guidelines for Planning GEOMETRY

1. Delay addition of fillets/chamfers as long as possible

2. Delay definition of draft as long as possible

3. Create parent-child relationship from permanent datums (whenever possible)

4. Never reference new features to fillets/chamfers/draft edges

5. Never leave unwanted or incorrect features in a model (even if they don't show)

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 in Mechanica.

In ANSYS Workbench DesignModeler, we extrude a sketch as "Imprint Faces" to create a surface "patch"

Reading:
Mechanical APDL, Rev 12 (formerly ANSYS)
Modeling and Meshing Guide, Sect. 6.1

This section begins with some discussion on IGES import – generally available, but not always the best (the “Connection” products, direct transfers, are often more successful)

Review this Help System chapter to see examples of poor geometry (slivers, short edges, etc.), topology repair options, and debugging techniques for poor models.

Mechanical APDL Rev. 12 (formerly ANSYS):

 Hints for IGES files written from Pro/ENGINEER: