Request for help meshing and and analysing a part {Solved}

[Laurie Hartley]

I have run into 4 "jac" errors meshing this part and analysing it. (I don't know what they are to be quite frank)
I am a FEM novice and no doubt I am making an elementary mistake.
Any help pointing out where I am going wrong and pointers in the right direction will be greatly appreciated.

[NewJoker]

It’s just hard to obtain a sufficient quality solid mesh for this thin-walled part. The error that you get means that your mesh has nonpositive jacobians. Try with finer mesh or another mesher (Netgen). If this doesn’t work, consider simplifying this part and using shell elements.

Update: It seems that Gmsh yields correct mesh and the analysis runs to completion when max element size is set to 3 or 4.

[bernd]

Have you tried to set second order linear? This helps a lot and you do not need to make a very fine mesh.

https://wiki.freecadweb.org/FEM_MeshGms ... _Jacobians


if you use the run button the non positives are green.if there are only a few a local finer mesh or improving the geometry could be an option as well.

bernd

[Laurie Hartley]

Update: It seems that Gmsh yields correct mesh and the analysis runs to completion when max element size is set to 3 or 4.

Thanks for your response - I will do that tomorrow as well as explore Bernd’s advice below. I will post the outcomes

[Laurie Hartley]

Have you tried to set second order linear? This helps a lot and you do not need to make a very fine mesh.

https://wiki.freecadweb.org/FEM_MeshGms ... _Jacobians


if you use the run button the non positives are green.if there are only a few a local finer mesh or improving the geometry could be an option as well.

bernd

Thanks Bernd - I will try this as well as new jokers suggestion above and post the outcomes tomorrow.

I tried both methods and was able to get a clean mesh but encountered another problem probably best explained by uploading some screen shots - see below. (No modes found in Frd file)
For New Jokers suggestion I set gmsh "from shape" the maximum node size to 3mm and min to automatic.
For you suggestion bernd I set gmsh "2D" and the shell geometry thickness to 4mm. (not sure if I have interpreted your instructions correctly)
I ran an analysis on another part successfuly so I don't think I have broken FreeCAD.

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OS: Debian GNU/Linux 10 (buster) (XFCE/xfce)
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.19.24291 (Git) AppImage
Build type: Release
Branch: (HEAD detached at 0.19.2)
Hash: 7b5e18a0759de778b74d3a5c17eba9cb815035ac
Python version: 3.8.8
Qt version: 5.12.9
Coin version: 4.0.0
OCC version: 7.4.0
Locale: English/Australia (en_AU)

Operating System:-

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[thschrader]

For you suggestion bernd I set gmsh "2D" and the shell geometry thickness to 4mm.

Here is a sim with 2D-shell elements. I eliminated the fillets in the MSH-profile
with the defeaturing-wb. With fillets I get a weird mesh.
For 2D-meshing you must produce a 2D-surface from your solid.

hartley_strut.FCStd

(188 KiB) Downloaded 741 times

strut_2D_shells.JPG (48.41 KiB) Viewed 41912 times

[thschrader]

How to produce the 2D-body from the solids?
Steps:

Defeaturing-wb
1+2+3+4: select inner fillets of strut
5: press button, faces are added to list
6: press button, fillets are removed

Arch-wb:
Select all inner faces from the MSH-solid, tube-solids
Press facebinder-tool

defeatured_strut.JPG (89.55 KiB) Viewed 41896 times

[cad1234]

Hi thomas,
which FC Version did you use for your simulaton?

[thschrader]

Hi thomas,
which FC Version did you use for your simulaton?

LP-built
OS: Windows 10 Version 1909
Word size of FreeCAD: 64-bit
Version: 0.20.25065 (Git)
Build type: Release
Branch: master
Hash: 4de2ad46b2b9c78ac2721a0dbea7c19306261095
Python version: 3.8.6+
Qt version: 5.15.2
Coin version: 4.0.1
OCC version: 7.5.0
Locale: German/Germany (de_DE)

[NewJoker]

How to produce the 2D-body from the solids?
Steps:

…

Arch-wb:
Select all inner faces from the MSH-solid, tube-solids
Press facebinder-tool

It’s a really nice way for solid to shell FEA model conversion in general. However, one has to keep in mind that offset is needed for it to be actual midsurface shell modeling. Is it possible to do it in FreeCAD ? Facebinder’s offset option seems to work differently.

Btw. Facebinder in Draft workbench is the same tool, right ?