Due to the divergent considerations of design and manufacturing, it becomes very important to understand the behavior of CAD/CAM applications that attempt to bridge the two disciplines.
In Fibersim, we have the ability to pass information back and forth from the FEA system that drives laminate design. At the same time, Fibersim allows for the definition of darts, splicing, automated deposition features, and other objects necessary for manufacturing the part. Not knowing how Fibersim treats some of these manufacturing features during CAE interface can potentially lead to unexpected results.
During Fibersim’s mesh map exports to FEA, each ply’s producibility simulation is converted into a mesh. For each centroid in the input mesh, Fibersim determines which ply meshes can be pierced at that location, then the stackup/PCOMP card is computed. Within this process, however, is a secondary step of filtering out certain components from the stackup, and it’s this filtration that we wish to focus on in this article.
Typically, stress/FEA analysts are concerned only with full, unspliced layers of material. They are certainly aware that splicing and darting are often required, especially for more complex geometry, but besides explicitly defining no-splice regions and other rules that drive manufacturing, those considerations are often left to those engineering the detailed design of the laminate. Given that, when Fibersim encounters two or more plies at the same Sequence-Step level while calculating the stackup at a particular centroid, the software only keeps the thickest component and throws out the extraneous ones of the same Step value. This is done to filter out overlap splices (wherein the individual plies typically have the same Step values) and consider only the uninterrupted layer of fabric in the overall analysis mesh.
This is important to understand because users sometimes assign the same Step value to unrelated plies. Such a practice can lead to some confusion in downstream applications, but it is not prohibited in Fibersim. If two different plies with the same Step value—that are NOT matching spliced pieces of the same layer of coverage—are pierced by a centroid, therefore, the thinner of the two will be filtered out of the stackup for that centroid. This, of course, may not be what the user desires. Unique Step values (for plies not related through a splicing operation) are thus recommended for designs going to FEA.
One further point to add here is that Fibersim actually does not filter the overlaps generated by Wrapped Plies when doing CAE exports. It helps to think of a Wrapped Ply as three individual “subplies”. One subply represents the single-layer region, while the other two subplies represent each layer of coverage in the overlap area. If the centroid is within a Wrapped Ply’s overlap region, both overlapping subplies will be present (and separate) in the PCOMP data. The important thing to be aware of in such cases is that the combined thickness of both subplies will be used to compare against any other plies of the same Step value pierced by that centroid. As such, both subplies will be either present in the PCOMP or both be filtered out by the filtration operation discussed above.
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