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Intro to FreeCAD Part 10: Finite Element Method (FEM) WorkBench Tutorial
2025-02-27 | By ShawnHymel
Performing Finite Element Method (FEM) calculations manually can be incredibly complex and time-consuming, often requiring advanced mathematical and engineering knowledge. Fortunately, FreeCAD comes equipped with an FEM Workbench that allows users to simulate and analyze their designs without needing to solve equations by hand. FreeCAD's FEM Workbench can handle multiple solids, electromagnetics, and fluid calculations. However, for this tutorial, we'll focus on performing a static analysis to determine how a single bracket behaves under a light load.
You can find the FreeCAD files used in this tutorial in this GitHub repository: https://github.com/ShawnHymel/introduction-to-freecad
Overview of FEM
In the realm of mechanical design and engineering, ensuring that components can withstand the forces and stresses they will encounter is crucial. Whether you're designing a simple bracket, a complex machine, or structural elements of a building, understanding how your design will perform under load is essential to prevent failures and optimize material usage.
But how can we predict a part's behavior under various physical conditions without physically testing it? This is where the Finite Element Method (FEM) becomes invaluable. FEM allows engineers and designers to simulate and analyze the behavior of components under different types of loads, including mechanical stress, heat transfer, fluid flow, and electromagnetic fields.
By breaking down complex geometries into smaller, simpler elements—often triangles or tetrahedra—to create a mesh, FEM enables the calculation of approximate solutions to complex physical problems. This process, known as Finite Element Analysis (FEA), helps predict how an object will respond to real-world forces, vibration, heat, and other physical effects.
For example, engineers use FEA to predict how a car would deform during a crash, allowing for safer and more efficient designs. Similarly, FEA can be applied to determine if a custom-designed bracket can support a specific load or if it needs reinforcement.
1. Setting Up Your FreeCAD Document
- Open FreeCAD and switch to the Part Design workbench.
- Click on Create a new document.
- Create a new Body within the document.
2. Designing the Bracket
Create the Base Sketch
- Create a new Sketch on the XY plane.
- Use the Rectangle tool to draw a square centered around the origin.
- Use the Circle tool to draw a circle inside the square.
- Symmetry Constraint: Select two opposite vertices of the square and the origin, then apply symmetry.
- Equal Length Constraint: Ensure all sides of the square are equal.
- Dimension Constraints:
- Set the length and width of the square to 1 inch.
- Set the diameter of the circle to 0.266 inches (a loose fit for a 1/4-20 bolt).
Pad the Sketch
- Close the sketch.
- Use the Pad tool to extrude the sketch by 0.08 inches (approximately 2 mm).
Create the Side Bracket
- Select a side face (from the Pad operation).
- Create a new Sketch on that face.
- Use the External Geometry tool to bring in edges from the original pad.
- Draw another square and circle on this face, aligning them with the external geometry.
- Constrain the new sketch:
- Ensure the new square is concentric with the base.
- Set the dimensions as before.
Pad the Sketch
- Close the sketch.
- Pad the sketch inward by 0.08 inches (use the Reversed option).
At this point, you should have a basic L-shaped bracket model.
3. Preparing for FEM Analysis
Switch Back to Metric Units
For ease of interpretation, we'll perform the analysis in metric units.
- Click on your document in the Model tree.
- If you are not already using the metric system: in the Properties panel, change the Unit System back to Standard (mm/kg/s).
Activate the FEM Workbench
- Switch to the FEM Workbench from the workbench selector.
Create a New Analysis Container
- Click on the New Analysis Container icon.
- This creates an Analysis container that will hold all FEM-related components.
- Right-click on the Analysis object in the tree and select Set active analysis.
- The Analysis container should now be highlighted.
- If you do not see the SolverCcxTools object in your Tree View, click the Solver CalculiX Standard button to add a solver object to your Analysis container. You only need one!
4. Assigning Material Properties
Add Material
- Click on the Material for Solid icon.
- In the material dialog, select PLA-Generic from the list (or your preferred material).
- Click OK to assign the material to your entire model.
5. Defining Constraints and Loads
Fixed Constraint (Boundary Condition)
- We'll fix the bracket at the bolt hole to simulate it being anchored.
- Click on the Fixed Constraint icon.
- In the dialog, click Add and select the inner face of the bolt hole.
- Click OK.
Applying the Load
- Click on the Force Constraint icon.
- Click Add and select the top face of the bracket.
- Reverse the direction of the force if necessary (arrows should point downward into the bracket).
- Set the Force (in N) to 50 (approximately the weight of a 5 kg mass under Earth's gravity).
- Click OK.
6. Creating the Mesh
- Click on your model (e.g., Body) in the tree to ensure it's selected.
- Click on the FEM Mesh from Shape by Netgen icon.
- In the mesh dialog:
- Set Max Size to 5 mm for a balance of resolution and computational complexity.
- Leave Second Order checked for more precise calculations.
- Click Apply and then OK.
7. Running the Solver
- Double-click on the SolverCcxTools object in the tree.
- Ensure Static Analysis is selected.
- Click on Write .inp file to generate the input file for the solver.
- Click on Run CalculiX.
- Monitor the output window for any errors.
- Look for the message "CalculiX done without error!" to confirm successful completion.
- Close the solver dialog in the Task Pane.
8. Viewing and Interpreting Results
Access the Results
- Hide the original Body and Mesh in the Tree View.
- Hide the Pipeline_CCX_Results object in the Tree View.
- Note that CCX_Results and Pipeline_CCX_Results are two different results objects, so you probably want to look at just one at a time.
- Drop-down the CCX_Results object, right-click on CCX_Results_Mesh and select Show.
- Double-click on CCX_Results to open the result dialog.
Displacement Magnitude
- Select Displacement Magnitude under the Result Type in the Tasks Pane to view how much the bracket deforms under load.
- Under the Displacement section, change the Slider Max to 1.0.
- This multiplies our original load amount.
- For example, the slider would go from 0 to 1.0 * 50 N (50 N).
- Use the Slider to visualize the displacement.
- Note the maximum displacement value (e.g., around 3.5 mm for a 50 N force).
Von Mises Stress
- Switch to Von Mises stress under the Result Type to see the distribution of stress within the bracket.
- Areas with higher stress are indicated in red.
- Observe the maximum stress values, especially around the bolt hole.
Important caveat: the von Mises stress is a useful indicator for predicting yielding in ductile, isotropic materials under static loading conditions. For materials that are brittle, anisotropic, or subject to dynamic loads and environmental variations, alternative failure criteria and additional analyses may be necessary to accurately predict performance and ensure safety. For example, 3D printed materials (especially FDM) are anisotropic, as the layers provide different material strength in the X and Y axes (within a layer) versus the Z axis (between layers). As a result, the von Mises stress will likely be quite inaccurate for 3D printed objects.
Alternative Results
- Hide the CCX_Results_Mesh.
- Show the Pipeline_CCX_Results.
- In the Tasks Pane, set the Mode to Surface.
- Change the Coloring field to Displacement or von Mises Stress.
- The color bar on the right side shows the numerical values for the various colors on the object.
9. Improving the Design
To reduce stress and deformation, try reinforcing the bracket. Add angled pads on either side and rerun the FEM analysis to see if that improves the displacement and stress.
Rerun the Analysis
- Switch back to the FEM Workbench.
- Update the mesh:
- Double-click on the existing mesh and click Apply and OK.
- Update the solver:
- Double-click on SolverCcxTools.
- Click Write .inp file.
- Click Run CalculiX.
- Open the updated CCX_Results.
- Observe the reduced displacement (e.g., less than 0.5 mm).
- Note the decreased maximum stress values.
Conclusion
The British statistician George E. P. Box famously said, "All models are wrong, but some are useful." This sentiment applies to finite element analysis. While our simulations may not perfectly represent real-world behavior—especially considering factors like 3D printing anisotropy—they provide valuable insights into potential failure points and overall structural integrity.
By using FreeCAD's FEM Workbench, we've:
- Learned how to set up a basic FEA simulation.
- Assigned material properties to our model.
- Applied constraints and loads.
- Generated a mesh suitable for analysis.
- Interpreted displacement and stress results.
- Improved our design based on the analysis.
Next Steps:
- Experiment Further: Try modifying your design to see how changes affect stress and displacement.
- Explore More Features: FreeCAD's FEM Workbench offers more advanced simulations, including thermal and dynamic analyses.
- Deepen Your Knowledge: If you're designing critical components, consider studying mechanical engineering principles or consulting with a professional.
Additional Resources
- FreeCAD Wiki: Detailed documentation on the FEM Workbench.
- MangoJelly Solutions: An excellent YouTube channel offering FreeCAD tutorials.
- FreeCAD Forum: A helpful community where you can ask questions and share insights.
Thank you for following along with this tutorial series. I hope it has empowered you to harness the capabilities of FreeCAD for your projects. If you've created something exciting using FreeCAD, I'd love to see it! Share your creations on social media and tag @DigiKey with the hashtag #DKFreeCAD.
Have questions or comments? Continue the conversation on TechForum, DigiKey's online community and technical resource.
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