How to Apply Force and Pressure in FreeCAD FEM

Applying force loads and surface pressure in FreeCAD’s Finite Element Method (FEM) workbench is essential for simulating real-world structural stresses on a 3D model. This guide covers how to set up an analysis container, select the target faces, and apply both directional force loads and normal pressure constraints using FreeCAD's built-in FEM tools.

Prerequisites

Before applying loads, ensure your model is ready:

  1. Switch to the FEM Workbench.
  2. Create an Analysis container by clicking the red FEM container icon on the toolbar.
  3. Ensure your solid model has a defined material (e.g., standard steel) added to the Analysis container.

Applying a Force Load to a Surface

A force load applies a total directional load (measured in Newtons, N) distributed across one or more selected surfaces.

  1. Select the Surface: Click on the face or faces of the model where the load should act.
  2. Open the Force Tool: In the top menu, go to Model > Mechanical constraints > Constraint force, or click the Constraint force icon on the FEM toolbar.
  3. Set the Direction:
    • In the task panel on the left, click the Direction button.
    • Click an edge on your model to align the force parallel to that edge, or click a flat face to align the force perpendicular to it.
    • Check the Reverse direction box if the force vector points the wrong way.
  4. Enter Load Magnitude: Input the total force value in Newtons (N).
  5. Confirm: Click OK to apply the constraint. The force constraint will now appear inside your Analysis container in the tree view.

Applying Pressure to a Surface

Unlike a directional force, pressure acts strictly perpendicular (normal) to the chosen surface and is defined as force per unit area (measured in Pa, MPa, or bar).

  1. Select the Surface: Click on the face where the pressure is exerted.
  2. Open the Pressure Tool: Go to Model > Mechanical constraints > Constraint pressure, or click the Constraint pressure icon on the FEM toolbar.
  3. Set the Value: Enter the pressure magnitude in your preferred unit (e.g., 10 MPa or 1000 kPa). FreeCAD automatically converts the value to standard internal units.
  4. Define Normal Direction: Check or uncheck the Reverse direction option to toggle between internal pressure (pushing outwards) and external pressure (compressing the surface).
  5. Confirm: Click OK to save the pressure constraint.

Key Differences to Consider

  • Directionality: Force requires a defined vector direction (via an edge, axis, or planar normal), whereas Pressure always acts automatically normal (perpendicular) to the curved or flat surface.
  • Surface Area Scaling: If you change the geometry size, a pressure load maintains constant stress per area, while a force load spreads the fixed total force over the updated area.