FE Update: Translation Tools Make CAD Models FEA-Ready

Sept. 11, 2008
The sharing of data between CAD, CAE, and CAM has grown to include machining, molding, and PLM.

Abdul Shamma
Chief Technology Officer
Elysium Inc.
100 Galleria Officentre, Suite 426
Southfield, MI 48034
(248) 799-9800

Edited by Leslie Gordon

So true interoperability is a necessity to ensure product-data quality (PDQ) throughout the enterprise.

Interoperability issues even can arise early in the game. For instance, FE analysts need data forms, tolerances, and structures that are different from their counterparts in product development who begin with a CAD model. When there’s a misfit between design data and FEA parameters, meshes generated from CAD contain a host of errors. FE analysts must then spend appreciable time fixing bad data, instead of running actual simulations. Such wasted time reduces development team productivity, particularly as the cycle between design and analysis shrinks and becomes more interactive.

The main reason for a CAD and FEA mismatch is that the 3D model contains too much detail for FEA. Automated algorithms that generate FEA meshes have difficulty accounting for slivers or geometric faces too small to generate well-shaped elements. Run times rise. When the mesh is fed into a solver, the skewed or collapsed elements crash the code. For a solver to work, every single mesh element must meet some parameter of minimum quality.

Make it simple
Simplifying CAD models makes the FE analyst’s job easier and also speeds meshing and brings more accurate physics. Many question whether removal of CAD details will degrade analysis accuracy. The answer: Not functionally. That’s because FEA is an approximate method that provides answers to within 5 or 10% of a theoretical solution. The accuracy provided by a cleaned-up CAD model is more than sufficient for analyzing degree of contact, bending, impact, and other stress information FEA provides.

Furthermore, removed details are generally insignificant for an analysis. For instance, they might be in an area containing no stress concentration, or where details don’t affect the structural integrity of the model. An example of an extraneous detail is a company logo embossed on a part. The logo doesn’t impact the part’s structural integrity and would be difficult to mesh.

Simplified geometry is necessary for CAD-generated FEA models about 80% of the time. When analysts need more detail, such as a contact surface around a bolt hole, the original geometry should be retained.

Hidden geometries
Visible CAD-to-FEA issues are fairly easy but time consuming to fix manually. However, the naked eye cannot see everything that needs simplification. For example, curvature values, surface discontinuities, and tangent continuity can be visually difficult to identify. So can complex geometric errors such as twists and turns that create “surface normal” problems. These also affect FEA (and, ultimately, cutting tools — a surface error can flip a cutting path from inside to outside). Again, gaps or overlapping faces (resulting in two surfaces where there should be only one), small fillets, holes, and bosses, as well as free edges, sharp-edge angles, and intersecting and self-intersecting loops also cause problems.

It’s important to note that “small” in FEA is usually orders of magnitude larger than CAD modeling tolerances. Small edges and faces unintentionally created when a CAD model is built are usually above the modeling tolerance for the CAD designer, who therefore considers the model acceptable. For example, when the tolerance is 0.01 mm and there is no edge, face, or surface in the model less than that dimension, the CAD designer will see the model as “clean.”

But tolerance issues work the opposite way for FEA. For the FE analyst, the 0.01-mm tolerance is much too small. It essentially means, “Place this mesh on this node to within 0.01 mm.” To meet such a miniscule tolerance, meshing algorithms are forced to generate tiny elements which cannot be used without lots of manual correction.

Simplifying CAD data
Dedicated data-translation software provides a useful solution to problems that arise during handoff from CAD to FEA. Translation software essentially deconstructs CAD files mathematically and then reconfigures the data to meet the computational expectations of the FEA program.

Part and parcel with data transformation, translation tools automate the simplification of CAD models where necessary to ensure elements generated by FEA are of usable quality. Translation software also detects tolerance issues and other “bad geometry” such as small holes.

When designers use a data translator, the tool highlights areas that are too small as well as untenable geometries. Designers can then select easy-to-use, semiautomated click, see, accept, or reject options for simplification, removal, or surface healing. The result is a clean, viable model that will let FE analysts create proper meshes.

It can be seen that the advantages of using a dedicated data translator are considerable. For example, simplified geometries from our CADdoctor tool are neutral to any CAD system. In other words, original data can come from any CAD software, be cleaned up, and then be sent to any other CAD system. What’s more, the translator tolerates all intermediate “limbo” states, during data dissection and cleaning, when the actual model becomes invalid to the CAD software.

Alternatives are limited
One alternative to dedicated data translation is the evolving “virtual topology” technology being developed within certain design software programs to facilitate the transition from CAD to FEA. The approach is sound but has limitations, among which are less flexibility and the inability to support intermediate simplification states.

That said, virtual topology works well within its own suite of software tools, but is of no use to an external solver. This could certainly be an issue in a supply chain with lots of different CAD programs, or even within a single, large organization.

Other alternatives to dedicated data translation are complex preprocessors that manually clean up, delete, and merge dirty mesh elements that result from CAD-FEA interface issues. However, such preprocessors are used after the FE model has been created, necessitating the mastery of new commands and more work for analysts.

PDQ for analysts
What FE analysts really need is the same high level of PDQ that has become the Six Sigma holy grail in the manufacturing phase. Yes, companies can go from CAD to meshing and then ask their analysts to hand-edit thousands — or even millions — of erroneous elements. Or firms can save their design teams countless headaches by preparing CAD geometry with a dedicated data translator before sending models to FEA. The cost of fixing CAD-FEA interoperability problems early on in this fashion is small.

This example shows how a CAD-FEA data mismatch results in collapsed elements. The collapsed elements themselves are not visible. However, the degenerate face (the vertices of the yellow triangular element lines should meet along the central edge line, but don’t) is the result of the elements collapsing. The connectivity of the elements appears acceptable topologically but, geometrically, the underlying shape does not support an FEA analysis.

Even a small edge in a CAD model can create problems for FEA. When the model is meshed with triangular FEA elements (left), the resulting tiny edge (highlighted in red by CADdoctor software) produces a stretched or sliver element. The edge forces one side of the element to be short relative to the other sides. A close-up of the same view (right) shows this short side and the resulting elongated element (the lines are not parallel — they converge in the apex of the triangular element off screen). This kind of distorted element is unacceptable for FEA.

An embossed serial number is a part detail that does not need FEA to test the integrity of a design. A dedicated data translation tool would remove the detail before the CAD model was handed off to FE.

A CADdoctor screenshot of a CAD model shows narrow faces (highlighted in red). These kinds of details interfere with accurate FEA meshing.

Small features such as fillets and chamfers in a CAD diagram of a rocker arm are highlighted in pink and gold by CADdoctor (above). The software automatically removes the features to prepare the model for FEA.

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