Reinforced concrete FEA shear stress

FEA Consulting Part 6: Analyzing Results

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Reinforced concrete FEA shear stressFigure 1: Shear stress results for our concrete slab simulation.  The red area of high compression is where the corner of the square-shaped supporting column holds up the slab.
© Glew Engineering Consulting, 2016

Welcome to the final entry in our finite element analysis (FEA) blog series, in which I’ll discuss a little about analyzing and evaluating FEA results.  Over the course of this blog series, I’ve covered tips on setting up the model in CAD and in the FEA program, configuring the analysis, and generating results.  The final step in the process is the analysis of the results, both to get the answers to whatever problem inspired the FEA simulation and to ensure that the final results are accurate and representative of the system’s actual physical behavior.

Analyzing Results

These analysis tools and graphics generators aren’t just useful for generating pretty reports.  One of their most important purposes is that they let the engineer easily check their own work.  Since FEA software is very sensitive to incorrect inputs or problems in setup, it’s vital that the engineer make sure the model has solved in a way that makes sense.  There are a few simple things to check that can show if results are accurate, or if the mesh needs to be refined and the simulation re-evaluated.

Checking the largest values for the maximum and minimum principle stresses is the first quick check that an engineer can perform (these can by highlighted in Autodesk™ Simulation using the toolbar shown in Figure 3).  These should have reasonable values and be located at points that make sense to the engineer; otherwise, a value far past the failure point for that material or a stress concentration in a single unexpected spot might indicated that the mesh was not generated accurately.  For a higher level sanity check in systems involving deformation, it can also be useful to check the sign on the principle stresses across the model, indicating whether sections are in tension or compression.  It’s usually easy to predict whether an area should be in tension or compression, so this is a fast way to confirm that the loads and boundary conditions were applied correctly.

For instance, since our concrete slab example was bowing under its own weight, we would expect that the top surface is generally being compressed, while the bottom surface is under tension as it is stretched out.  Since our simulation results matched our predictions, we were confident that we had created an accurate model.  The shear stress for our model is shown in Figure 1.

Fix Errors in the FEA Process

It takes a lot of experience with FEA programs to get a simulation right on the first try.  Even if a simulation converges to a solution without aborting or running out of system memory, there’s a good chance that upon viewing the results the user might find unexpected or incorrect mechanical behavior.  Depending on the problems discovered, the user will have to work backwards through the process, ensuring that the analysis parameters are correct, that the mesh was generated appropriately, and that the CAD file does not contain any interferences, errors, or extraneous complexities.  After iterating through changes to the setup and review of the results, one can be more certain that they have the most accurate simulation for the situation.

The Whole Process

Getting all the way from a CAD model to a set of accurate and useful FEA results can be a long process, and frustrating for an inexperienced FEA user.  One must be thorough as they go through the process, since finding a mistake in the results might mean many more hours of mesh generation and analysis.

  1. Clean up the CAD model
    1. Remove interferences
    2. Simplify or remove complex geometries
    3. Take advantage of symmetry
  2. Mesh the model
    1. Set up the CAD model to guide the mesh generation
    2. Check that the mesh is fine (dense) where it needs to be
  3. Set boundary conditions
    1. Set appropriate loads
    2. Set appropriate translation and rotation constraints
  4. Run the analysis
    1. Choose the correct analysis type
    2. Set the convergence conditions or time scales as necessary
  5. Generate your results
    1. Determine whether you need to know stress, strain, displacement, or another parameter
    2. Determine the correct scale to display your results
  6. Check the results
    1. Check that maximum and minimum stresses make sense
    2. Check that displacements makes sense
    3. Check that sections of the model are correctly in tension or compression
  7. Generate the final report

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.


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