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Time now for the last in our blog series on FEA. I’ve previously discussed how to set up CAD for FEA, how to mesh that CAD model and the different types of analysis that FEA programs can run. The next step in the process is generating results from the FEA simulation, like the displacement illustration in Figure 1. These are fantastic tools for generating useful reports, and are also very useful in helping an FEA consultant or engineer check the accuracy of the own results.
As a reminder, for an example I’ve been using a recent project we worked on involving punching shear in reinforced concrete. We were examining the effectiveness of reinforcing a column-supported concrete slab against the possibility of that column punching through the concrete.
After iteratively solving partial differential equations for displacement, stress and strain in every finite element, the FEA program retains the final values for these parameters at every element. These values can be used to calculate a wide variety of different stress and strain results, depending on the type of scenario the engineer is evaluating and the kind of results they’re looking for. These include common parameters like the von Mises stress, maximum and minimum principle stresses, engineering strain, or overall displacement or displacement in a specific X, Y or Z direction, as shown in the toolbar in Figure 2.
FEA programs can output any of the above in a few different useful formats. For numerical purposes, that output might be a simple spreadsheet showing the values at selected nodes, or a line graph showing the change in stress values over an area (shown in Figure 3). In the scenarios involving physical deformation and movement, the FEA program can warp and twist the CAD model as a physical sample would under the same loads; often this has to be scaled up, since deformations are usually small compared to the actual size of the part.
Our concrete slab, for instance, had a maximum displacement of about 1 mm in the worst case scenario; this obviously wouldn’t be visible if it was kept to the same scale as the 6 meter-long sides. Figure 4 shows the shear stress on our concrete slab, shown side-on, with the displacement scaled up by a factor of five. The column is supporting the concrete slab at the bottom left corner, and with the displacement scaled up it is easy to see how this will be the most troublesome spot in the model.
The most eye-catching and commonly-seen outputs from FEA programs are the color contour plots. For any of the above parameters (displacement, stress, strain, etc), the program will overlay colors across the model. Usually programs use blue for low values and red for high values, so that areas with high stress concentrations are highly visible. In Figure 4, the color scale immediately shows us that the shear reinforcement bars are functioning as expected. All three bars visible in the image (3 of the 6 bars present in this one-eighth section) are “hotter” than the surrounding concrete, meaning they are under a higher shear stress. This is exactly as expected, and means that the bars are helping the concrete slab resist shear failure.
For Impressive and Useful FEA Reports
Autodesk Simulation offers a wide variety of tools for automatically generating simulation reports, and most other FEA suites offer the same. For an FEA consultant or mechanical engineer, these tools can be invaluable. Not only do they help the engineer check their own work, but they help the operator communicate their results to people that might not have the technical background to interpret esoteric mathematical prose. Whether you’re returning results to a client who hired you to FEA stress-test their product or you’re looking to impress customers who might purchase yours, a clear and comprehensive set of graphics in a report can be a life-saver.
Glew Engineering has done FEA consulting on a variety of projects, from semiconductor chambers and optical systems to vehicle lifts, heat exchanges and smart phone headsets. Most of these we can’t post for proprietary reasons. Take a look at our finite element analysis consulting services, and let us know how we might help you.