Jacobian Error Woes

About the development of the FEM module/workbench.

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Bimdan
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Jacobian Error Woes

Post by Bimdan »

Howdy

Utter noob here running off Dev 0.20 (also tested on 0.19.4) - Windows 11

Day 1
After spending a day banging my head against the wall as to why I was getting 201 Jacobian errors (I know I know sorry this again) on something as simple as plate steel I found that a combination of removing the angle on countersunk holes to just a through-hole and removing any recesses in combination to moving to a GMSH mesh got me past the post (a solution I read on the forums). Design compromises were nothing major and I was starting to respect FreeCAD to the extent I asked it for a date

Day 2, it ate all the steak, blocked my toilet and slapped my face before I said anything whilst leaving in my car which it then crashed.

You see I'm now working on essentially a glorified tube (single piece hose connector with square mounting plate) so bit by bit I'm removing detail (say goodbye fillets and chamfers) and still I get errors.

Fixing points on the mounting plate are 4 corner through holes (I seem to remember issues in the past where I had to merge the 2 inner faces of the bolt holes to avoid issues but these aren't present on this particular SpaceClaim export but are shown on import)

I've tried simplifying how many and where I put the constraints and forces and tried different STEP versions on the off chance it was that (and IGES as well) but to no avail

I just don't get how FEM in FreeCAD can be useful if you have to remove so much detail from your actual part the results are irrelevant given the differences between the actual model and the FEM mesh.

I don't want to come across as entitled especially when it comes to free software, I've just spent almost a day bashing my head against the wall and I want a beer and cuddle [grabs wife]. Especially as it's probably me doing something wrong lol.
Last edited by Bimdan on Mon May 16, 2022 8:21 am, edited 3 times in total.
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NewJoker
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Re: Which Dark God wants the goat sacrifice?

Post by NewJoker »

Removing small and not important details is something common regardless of which FEM software is used. Even if it's possible to generate the mesh for a model taken straight from CAD, it usually doesn't make sense to include all the details since the analysis would take much longer to complete and the results wouldn't be significantly better. Often, if you can't generate a mesh for a complicated model in FreeCAD (using Netgen or Gmsh), it's just a matter of reducing the element size.

Of course, it all depends on the particular case so it would be good if you could show us your model (at least in form of screenshots, preferably as a .FCStd file). Then we would be able to provide some help.
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Kunda1
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Re: Which Dark God wants the goat sacrifice?

Post by Kunda1 »

Please modify the thread title.
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Please mark thread [Solved]
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Bimdan
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Re: Which Dark God wants the goat sacrifice?

Post by Bimdan »

NewJoker wrote: Fri May 13, 2022 5:13 pm Removing small and not important details is something common regardless of which FEM software is used. Even if it's possible to generate the mesh for a model taken straight from CAD, it usually doesn't make sense to include all the details since the analysis would take much longer to complete and the results wouldn't be significantly better. Often, if you can't generate a mesh for a complicated model in FreeCAD (using Netgen or Gmsh), it's just a matter of reducing the element size.

Of course, it all depends on the particular case so it would be good if you could show us your model (at least in form of screenshots, preferably as a .FCStd file). Then we would be able to provide some help.
Thank you for your feedback

As requested simplified version is here for download (Onedrive share)
https://1drv.ms/u/s!As6F4yuPt_Psgbc1f484g7DfjkPSRQ
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johnwang
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Re: Jacobian Error Woes

Post by johnwang »

You can attach file here if it is not big under the Attachments Tab just below this text input block.
The shape of your object reminds me of this post:
https://forum.freecadweb.org/viewtopic.php?f=18&t=51843

Your shape:
ch.jpg
ch.jpg (46.63 KiB) Viewed 1346 times
hfc series CAE workbenches for FreeCAD (hfcNastran95, hfcMystran, hfcFrame3DD, hfcSU2 and more)
Bimdan
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Re: Jacobian Error Woes

Post by Bimdan »

Certainly and thank you for your time.

Every time I try to add the file as an attachment it states HTTP error?
Bimdan
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Re: Jacobian Error Woes

Post by Bimdan »

Update

It works with a Netgen mesh with second-order disabled.

Now it works with second-order enabled - almost like it was caching bad data (+ idiot factor) :|
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NewJoker
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Re: Jacobian Error Woes

Post by NewJoker »

Bimdan wrote: Tue May 17, 2022 3:24 pm Now it works with second-order enabled - almost like it was caching bad data (+ idiot factor) :|
Isn't that just a matter of setting the proper element size for meshing ? Unfortunately, default settings are neither appropriate for initial guess nor clear for users. You should always know the approximate size of your model and try to figure out a proper element size, starting from a bit higher value and then decreasing it if needed.
Bimdan
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Joined: Thu Mar 10, 2016 12:19 pm

Re: Jacobian Error Woes

Post by Bimdan »

NewJoker wrote: Tue May 17, 2022 6:24 pm
Bimdan wrote: Tue May 17, 2022 3:24 pm Now it works with second-order enabled - almost like it was caching bad data (+ idiot factor) :|
Isn't that just a matter of setting the proper element size for meshing? Unfortunately, default settings are neither appropriate for initial guess nor clear for users. You should always know the approximate size of your model and try to figure out a proper element size, starting from a bit higher value and then decreasing it if needed.
That's some great info that seems to correlate to me playing with the mesh resolution (if that's what you term as element size) - setting a max size to 8000 and the mesh to fine seemed to help.

FreeCAD is so capable but the usability and level of "in the know-ness" makes it feel like you're being slapped in the face at every step lol
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NewJoker
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Re: Jacobian Error Woes

Post by NewJoker »

Bimdan wrote: Wed May 18, 2022 10:55 am FreeCAD is so capable but the usability and level of "in the know-ness" makes it feel like you're being slapped in the face at every step lol
FreeCAD can be quite confusing initially even for people with experience in other (i.e. commercial) CAD software. After all, there are two different workbenches for 3D modeling that seem to share the same operations just named differently, many important tools are missing and so on.

The same applies to the FEM module. Even for experienced FEM analysts, the FreeCAD FEM module's interface can be quite confusing. However, it's still very good (or actually second-best, in my opinion) if you compare it with other free and open-source FEA software. For example, Salome-Meca with code_aster gives a lot of possibilities but it's so unintuitive that I had to write a 4-page guide describing how to prepare a simple linear static analysis and I have to read it each time I go back to this software. The ubiquitous French names and complicated GUI definitely don't make that software easy to use.
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