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How to make free-body cuts of your model in Abaqus

Written by Miguel

2 Apr, 2021

1. What are free-body cuts?

Free-body cuts are a mechanical tool to analyze forces and bending moments on a section of a structure, such as beams in a frame or continuum solids in volumetric parts. Structure mechanics courses teach how to analytically solve this free-body cuts through equilibrium equations and in some cases applying kinematic conditions (e.g. in hyperstatic structures). However, in general and more complex situations, it might be convenient to use numerical methods.

2. What do we need in Abaqus?

In Abaqus, we can display free-body cuts including free-body forces in the post-processing stage, that is within the odb.

Before running the model, we need to do is to ask for the variable NFORC in the Field output. These are nodal forces that will be used in the post-process stage to compute the forces on free-body cuts. NFORC variable is required in solid elements, such as C3D8, CPS4, CPE4… However, if our model is made of beam elements we have to ask for the variable SF (section forces and bending moments) in the Field output.

3. Visualization of free-body cuts in the post-process

To display free-body cuts in your model, you need to solve it, as usual, submitting your job. And then, in the Visualization module, click on View Cut Manager to select the cutting plane, as shown in the figure below.

In the image we have selected the X-plane as cutting plane and to display free-body forces on the cut,  must be enabled. Then, we will see two arrows on the cut of our model: one representing the resultant force (in red) and another the resultant bending moment (in blue).

Display format of free-body forces and moments can be customized in:

Free Body Cut Manager > Options

Some of the most insteresting options are to display components (not resultants), edit colors of each of the components, and customize vector labels for better visualization.

4. Advanced feature: Custom CSYS

The components of forces and moments displayed in a view cut, by default follows the global coordinate system (CSYS-Global). However, if we want this decomposition to follow any other CSYS, we can create a new one and select it in:

View Cut Manager > Options

Check out the examples in this video

To have a more visual explanation of how to make free-body cuts in Abaqus check the following video in which we will show this by means of two examples: a solid beam assuming plane stress conditions and a frame made of beam elements. You can download the script to create the frame model in the following link.

There are many other features related to view cuts and free-body cuts in Abaqus. If you are interested about it, feel free to ask in the comments section below.

I hope you find this useful!

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