Modal Vibration Analysis in FreeCAD FEM

Modal vibration analysis allows engineers and designers to identify the natural frequencies and corresponding mode shapes of a structure to prevent mechanical resonance. In FreeCAD, this process is carried out within the built-in Finite Element Method (FEM) workbench using the CalculiX solver. By following this guide, you will learn how to set up geometry, assign dynamic material properties, mesh the component, configure the solver for frequency extraction, and evaluate the resulting vibrational modes.

1. Prepare the Geometry

  1. Open FreeCAD and switch to the Part Design or Part workbench.
  2. Model or import the solid part you wish to analyze.
  3. Ensure the model is a clean, single solid object with no non-manifold edges or open seams.

2. Set Up the FEM Analysis Container

  1. Switch to the FEM workbench from the workbench dropdown.
  2. Select your solid part in the tree view.
  3. Click the Analysis container button (yellow capital "A" icon) on the toolbar to initialize the FEM environment.

3. Assign Material Properties

Vibration analysis depends heavily on structural stiffness and mass distribution.

  1. Click the FEM material for solid button (represented by a red/blue sphere).
  2. Choose a predefined material (such as Steel or Aluminum) or input custom values.
  3. Ensure that Young's Modulus, Poisson's Ratio, and especially Density are defined, as mass density is critical for inertial calculations.
  4. Verify the material is assigned to your solid geometry, then click OK.

4. Apply Boundary Conditions

  1. Select the face or edge where the structure will be supported.
  2. Click the Constraint fixed button to clamp the surface (zero displacement/rotation).
  3. Note: If you are testing a free-floating structure to find free-free vibrational modes, do not apply any fixed constraints (the first six modes will represent rigid-body motion with near-zero frequencies).

5. Generate the Mesh

  1. Select your solid model in the tree view.
  2. Choose a mesher: click FEM mesh from shape by Netgen or FEM mesh from shape by Gmsh.
  3. Set the Element dimension to second-order (quadratic) elements, which are required for accurate bending and frequency results.
  4. Set an appropriate maximum element size and click Apply to generate the mesh.

6. Configure CalculiX for Frequency Analysis

  1. Select the CalculiX solver object within your Analysis container in the tree view.
  2. Open the Property panel located at the bottom-left of the screen.
  3. Change the Analysis Type property from Static to Frequency.
  4. Locate the Eigenmode High Limit or Number of Eigenmodes property and define how many natural frequencies you want to compute (e.g., 5 to 10).

7. Run the Analysis

  1. Double-click the SolverCalculiX object in the tree view.
  2. In the task panel, click Write .inp file to generate the solver input.
  3. Click Run CalculiX to execute the modal analysis.
  4. Once the solver outputs "CalculiX done without error", close the task panel.

8. View Natural Frequencies and Mode Shapes

  1. Expand the newly created CCX_Results object in the tree view.
  2. Double-click the results object to view the task panel listing all calculated natural frequencies measured in Hertz (Hz).
  3. Select an eigenmode from the list to display its corresponding displacement field.
  4. Adjust the Displacement scale factor slider to clearly visualize the mode shapes and identify structural bending, torsion, or axial movement patterns.