Finite Element Analysis (FEA) is a key step in the design process. In fact, industry now commonly requires a calculation analysis sheet from design departments to approve the final virtual product. Moreover, FEA is an essential tool for developing a mechanism, calculating its lifecycle, and improving it in a number of ways. To speak of its importance would take pages by itself. In this article, we will focus on how to make FEA easier and faster by improving CAD practices.
Although FEA can pretty much handle any shape you throw at it, it will still compute false alarms and/or require a lot of time and attempts if the CAD model is complex. Therefore, below are 10 tips to help you make your CAD models more FEA friendly:
1) Symmetry helps
Make a part symmetrical or mirror its features where possible. This will make reproducing the FE model much easier. Besides, symmetry gives an optimal balance for inertia, and this is always better for a mechanical system.
2) Rotational symmetry helps too!
There is a reason why critical parts in aeronautics and military applications are often axisymmetric. Besides the aerodynamics calculations that favor such shapes, the complexity of creating them and setting them as load cases leads designers to look for shortcuts within the FE model creation. We are not talking about shapes that are easy to mesh, but rather shapes that are modeled in such a way that you will be able to get results on any node of the structure and yet not take weeks to compute the results. In this way, rotational symmetry in CAD allows a beautifully efficient, simple way to create the FE model. One well-drawn section on a 2D plain will generate the whole part.
3) Make space among critical zones
Sometimes you have to crowd together shapes that will each bear a decent amount of stress. Remember to leave space between them to make it possible to mesh and apply nodal connections between them. This can be an intuitive approach as theoretical studies and experience have taught many designers that critical zones shouldn’t be too close together, or it will inhibit the failure of that part’s structure.
4) Make shapes less prismatic
Radical change of slope in surfaces causes useless stress concentrations. To prevent misleading calculations in FE Analysis, shapes and features should be as round as possible. Whenever you see a fillet or a linkage zone, consider using a radius instead of a sharp edge. Sharp edges can fool calculations and are normally useless. However, in some companies engineers “clean” the geometry after the CAD delivery before moving to FE modelling by actually take off the round shapes, holes and tiny edges. So, how do you know whether to use round shapes or “clean” sharp edges? The key difference lies in the role that the zone or feature will play; we clean them when they do not represent a case study of stress and strain, and we prefer rounder shapes for them when they do present a support to the stress within the model. This brings us to the fifth point…
5) Ensure easy modification
We have mentioned this point in other articles and it plays a major role in FEA as well. If you want to suppress features or clean geometry, nothing is worse than going back to the initial sketch and modifying it, then dealing with the cascade of resultant changes that it causes. Besides, once the first calculation iteration is over and analysis delivers results, designers will most likely have to modify the CAD model to improve the lifecycle or resistance of the part. Therefore, making an easy-to-edit CAD model is a time-saver for FEA. Direct modeling has been gaining in popularity thanks to big companies such as Siemens promoting it; with this type of CAD tool, there is no reason to worry about the CAD history/tree, and changes will be quick and efficient. However, this technology is still in its infancy.
6) Prevent unnecessary contacts
This is a classic rookie mistake. Designers often crowd their parts in a way that won’t prevent motion or cause vibration or loads, but which will nevertheless require attention during the meshing phase. During FEA, contacts are given special attention, so unnecessary contacts require unnecessary calculations.
7) Detail the joints between parts
Joining parts within an assembly can be done in different ways. Sometimes, the elements linking the different parts might be neglected. This can cause problems. Designers should model joints in the most accurate way possible. Modeling a screw is not the same as modeling a bolting screw. Mechanically speaking, MIGs are different from punctual welds. Therefore, the links in any assembly should be as detailed as possible, either by providing quantitative descriptions at welds or by putting the convenient MXX screw in its hole with the reference typed on it.
8) Hide construction elements
This is a common procedure among CAD designers but sometimes, after modifying a feature and sending it back, a random surface might be left out. If this happens, the FE designer might not be aware that it’s merely a construction element and actually include it in the FE modeling. This is one of the reasons why designers are more and more becoming multitasking engineers. Such people can prevent these kinds of mistakes.
9) Be a multi-tasker (or at least be aware of everything that needs to be done)
Depending on design departments, tasks might be divided in several ways. Most of the time, you either have a group of designers who will handle only CAD and another group who will handle CAE (Computer Aided Engineering), or you will have a group of multi-taskers that will work on the same project and perform the whole process. In either case, it’s crucial that the designer is aware of the CAE/CAD dynamics. A designer who is versed in all of the design steps will be able to prevent technical problems, understand and answer requests quickly, and bring his own insight into the design.
10) Take manufacturing into account
This is a tricky point since it requires designers to have a consistent knowledge of manufacturing processes and capability. We often find that models that go through the CAD and CAE phases get sent back to the initial step because of the complexity or high costs of manufacturing. Often the designer will have to redo the whole model over again. To prevent this problem, the manufacturing budget and processes should be well-understood before the FEA is completed.