Problems with FEM analysis on imported model

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peto2006
Posts: 1
Joined: Sat Mar 04, 2017 5:34 pm

Problems with FEM analysis on imported model

Postby peto2006 » Sat Mar 04, 2017 6:25 pm

Hi,

I'm quite new to FreeCAD. I tried to import .stl from blender and run FEM analysis on it, but every time I get error similar to this (when running CalculiX):

Code: Select all

*ERROR in solidsections: *SOLID SECTION can
not be used for beam or shell elements
Faulty element: 46228
I have tried making mesh manifold using blenders Print3D plugin before import (hoping this would help).
Created FEM mesh looks weird. (see pic2) It looks like some faces between edges are missing. (Part of mesh not shown on picture looks good.)

I have tried Part / Refine shape, and change some random settings I found in forums. Nothing worked.
Picture 1 shows another result. I can see yellow faces from inside of "cabin" (it's hard to see in picture).

It would be great, if there are some steps, which could automagically transform mesh to same looking, but correctly working mesh.

(I have tried same steps with simple extruded cube (also imported from Blender), and everything worked, so I guess I have CalculiX and Netgen properly installed.)

I attached .stl with low face count, so operations are bit faster.

Please help. I'm trying to fix it for few days. Also, if you find solution, please share also debugging tips how you found it.

Here is info about my instance of freecad (I'm using newest from PPA):

Code: Select all

OS: Linux Mint 18.1 Serena
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.16.6707 (Git)
Build type: None
Branch: releases/FreeCAD-0-16
Hash: 5465bc47c95db45e0be85dc0e2872419efadce0f
Python version: 2.7.12
Qt version: 4.8.7
Coin version: 4.0.0a
OCC version: 6.8.0.oce-0.17
Attachments
pic2.png
pic2.png (40.57 KiB) Viewed 1350 times
pic1.png
pic1.png (47.98 KiB) Viewed 1350 times
quadcopter_1-Cube.stl
(336.12 KiB) Downloaded 36 times
fandaL
Posts: 325
Joined: Thu Jul 24, 2014 8:29 am

Re: Problems with FEM analysis on imported model

Postby fandaL » Sun Mar 05, 2017 2:08 pm

Hi
From your figures and also from the CalculiX error I expect that you get only surface mesh.
peto2006 wrote:Created FEM mesh looks weird. (see pic2) It looks like some faces between edges are missing. (Part of mesh not shown on picture looks good.)
That is due to reversed normal of these elements (FreeCAD displays bottom side of FEM mesh shell elements transparent by default, which can be switched in the View panel "Backface Culling")

I have checked the imported mesh from .stl. In mesh module, "Meshes/Analyse/Evaluate and repair mesh". It seems there are some self-intersections, but automatic reparation leads to holes in the mesh. Perhaps check it before exporting from Blender.

After you create a FEM mesh (not the same as mesh from stl) check if it contains volume elements.
checking_volumes.png
checking_volumes.png (37.93 KiB) Viewed 1331 times
btw: development version 0.17 contains a lot of improvements in the FEM module. This might help with installation https://freecadweb.org/wiki/Download#Ub ... A_Packages
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bernd
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Location: Zürich, Switzerland

Re: Problems with FEM analysis on imported model

Postby bernd » Sun Mar 05, 2017 6:54 pm

peto2006 wrote: Version: 0.16.6707 (Git)
if you would like to work with FEM in FreeCAD it is highly recommended to use latest development versions 0.17.xxxx. There have been tons of fixes and improvements in FEM WB in 0.17 version of FreeCAD.



BTW: Would be cool if you could generate a stl which has no errors in FreeCAD mesh workbench.

In FreeCAD you need to convert the stl into a solid. This solid you should test with geometry check in Part Workbench. If this check has no errors, you should start making a FEM mesh out of the solid.
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sgrogan
Posts: 5126
Joined: Wed Oct 22, 2014 5:02 pm

Re: Problems with FEM analysis on imported model

Postby sgrogan » Sun Mar 05, 2017 7:00 pm

bernd wrote: BTW: Would be cool if you could generate a stl which has no errors in FreeCAD mesh workbench.
I'll try to wash the stl thru Ansys tomorrow. It's auto mesh repair capability is quite good.
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bernd
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Re: Problems with FEM analysis on imported model

Postby bernd » Mon Mar 06, 2017 11:58 am

I have been playing a bith the stl too. I realized there are inner faces. See attached picture. @peto2006: you should get rid of this inner faces first before you export to stl in blender.

bernd
screen.jpg
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bernd
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Re: Problems with FEM analysis on imported model

Postby bernd » Mon Mar 06, 2017 2:19 pm

I deleted all inner facets, used the mesh repair and analyse tools of FreeCAD mesh wb and eventually got an solid without error and a FEM mesh. At the moment CalculiX is solving. I'm curious if it will be solved ...

BTW: does someone know the difference between Mesh --> file holes and Mesh --> close hole
screen.jpg
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bernd
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Re: Problems with FEM analysis on imported model

Postby bernd » Mon Mar 06, 2017 2:23 pm

:D
screen.jpg
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bernd
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Re: Problems with FEM analysis on imported model

Postby bernd » Mon Mar 06, 2017 2:36 pm

only the solid shape, ca 3 MB --> https://pro.woelkli.com/s/f4qgM4jzXPJ7ywN

with FEM mesh and a result, ca 50 MB --> https://pro.woelkli.com/s/SDJhe34R8aHezve