XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX''"> Working with Imported CAD Models
In some cases, you can use imported CAD geometry without modification. However, models are not always prepared in CAD systems with the ability to analyze them in mind. It is common to modify imported geometry, to remove extraneous detail, and to construct missing surfaces needed for topological congruency.
Some of the most common problems with imported CAD data, and suggested solutions within Patran, are as follows.
Remove Excess Detail
There may be features in the imported CAD geometry data that add greatly to the complexity of a model, such as rows of small bolt holes in a component. If you allow numerous small extraneous features that are not critical to performing an analysis to remain in the geometric model, they will greatly increase the number of finite elements required in an analysis model and will cause a substantial increase in the run time of the analysis. Here you must use your engineering judgment to determine which features are less important and which must be retained for accuracy.
Use the following Patran features to implement your choices for simplifying your model and removing excess detail:
• Plan ahead in your CAD modeling, and use layers and/or colors to separate out entities that you want to exclude from the analysis model.
• Use filtering options to restrict input to specific types of entities. For example, bring in solids only, or curves only, to rebuild the model in Patran. The filtering options are found on a subordinate form nested under the Import form (e.g., when the Source file type is set to IGES, an IGES subordinate form provides filtering options).
• Use imported CAD geometry as a base to construct simpler analysis geometry in Patran. Here the vertices, curves and/or bounding surfaces of the original CAD model are used as a basis for constructing geometry using features available in Patran's Geometry menu options.
• There may be cases where a larger surface or solid is represented by several smaller regions, adding to its complexity. You can select Create/Surface/Trimmed and Create/Solid/B-Rep to simplify complex geometry by basing it on outside edges or faces.
• Alternatively, in the case of many adjacent entities, consider approaches for merging adjacent surfaces such as the action Create/Surface/Composite, or other construction methods. Whichever approach you choose, a certain amount of simplification can have a major impact in both the time spent generating a finite element mesh, and the computing time required for running an analysis.
Add Missing Surfaces
Solid CAD models may not have to be closed 3D solids for drafting or layout purposes; hence, surfaces which enclose the solid may be missing. To determine these missing surfaces, use the Verify/Surface/Boundary action.
• For cases where a regular number of edges (e.g., 3 or 4) will enclose the region, select a Geometry action such as Create/Surface/Edge to fill in missing surfaces by specifying the edges of adjacent surfaces.
• When a region is bounded by more than 4 sides, select the Geometry action Create/Surface/Trimmed In this case, you must first create a composite curve representing the exterior boundary using either the Create/Curve/Chain option or the Auto Chain button on the Create/Surface/Trimmed form. An alternative to this is to break the region into several simple biparametric surfaces.
Repair Incomplete Entities
Popular CAD data formats such as the popular IGES format are often extremely broad standards that include many different types of geometric data. IGES is a very rich standard, and not all entity types in IGES are supported by every CAD system. This may result in simplifications such as curves being translated as a series of points, or a regular solid being represented by its boundary surfaces.
In cases like this, you must use Patran geometry actions to convert these incomplete geometries to entities that are clearly defined.
For example, use the Create/Curve/Spline option for fitting a curve to a set of points, or use the Create/Solid/B-rep option for creating a bounded solid from its surrounding surfaces.