FreeCAD FEM and avian bones: simulating parts with void spaces

About the development of the FEM module/workbench.

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Juvuzux
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FreeCAD FEM and avian bones: simulating parts with void spaces

Post by Juvuzux »

Hello,

I have been trying to assess whether I can use the FEM functionality of FreeCAD to simulate the mechanical performance of avian bones for a biomechanical study. I am working with STL meshes that were created from micro CT scanned volumes. One of the features of these meshes (and of avian bones generally) is that they contain void spaces, and this seems to cause issues when I attempt to follow my usual FEM workflow.

This is the workflow I've been using:

1. Open STL file in FreeCAD.
2. Part menu > create shape from mesh.
3. Part menu > convert to solid.
4. FEM menu > FEM mesh from shape by Gmsh
5. FEM menu > Analysis container, material for solid, constraints, forces etc.

I created two simple STL meshes to test this workflow on; one mesh with void spaces and one without. With the standard mesh (no void spaces) there were no problems and I was able to create a working analysis. With the second mesh (identical, but with a void space in the middle) I would get this error: "Cannot convert Femur_Box_Model_larger_hollow001 because Shape is not a shell." at step 3.

I also tried skipping step 3, and creating the FEM mesh from the shape produced in step 2. This did result in a new mesh, and I was able to set up an analysis, but upon attempting to run it I then got this error: "CalculiX can not be started due to missing prerequisites: FEM mesh has no volume elements, either define a shell thicknesses or provide a FEM mesh with volume elements. No material object defined in the analysis."

And upon inspection, the mesh did indeed have 0 volumes, as opposed to the first mesh without void spaces, which had 349.

So it seems the problem is I am unable to produce a mesh with volumes from one that contains a void space. Does anyone know if there a way to do this?

Any help would be greatly appreciated! Thank you.
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NewJoker
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Re: FreeCAD FEM and avian bones: simulating parts with void spaces

Post by NewJoker »

For moderators: this should be moved to the FEM subforum.

Can you share the file with a void for testing ?
chrisb
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Re: FreeCAD FEM and avian bones: simulating parts with void spaces

Post by chrisb »

NewJoker wrote: Thu Mar 09, 2023 7:51 am this should be moved to the FEM subforum.
Done.
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thschrader
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Re: FreeCAD FEM and avian bones: simulating parts with void spaces

Post by thschrader »

Quick and dirty:
Maybe you can reduce your problem to a 2D case to get some qualitative results.
Meshing a 3 case based on an STL is a true challenge. If even possible.
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-alex-
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Re: FreeCAD FEM and avian bones: simulating parts with void spaces

Post by -alex- »

Juvuzux wrote: Thu Mar 09, 2023 1:25 am I am working with STL meshes that were created from micro CT scanned volumes. One of the features of these meshes (and of avian bones generally) is that they contain void spaces,
Yes, micro topographie produces meshes with void spaces, non-manifold triangles, shells and solids mixed.
Really not easy to get a mesh compilant with FEM analysis.
Few years ago this topic (in french) was similar with yours, but no easy fix: viewtopic.php?t=39240
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Re: FreeCAD FEM and avian bones: simulating parts with void spaces

Post by bernd »

for such complicated meshs a standalone meshing tool is highly recommended
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