Fem constraint contact

About the development of the FEM module/workbench.

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Jee-Bee
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Re: Fem constraint contact

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bernd
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Re: Fem constraint contact

Post by bernd »

makkemal wrote: Sat Aug 31, 2019 12:16 pm I like the idea but only getting to the code every now an then I once again have problems rebasing my code
https://github.com/makkemal/FreeCAD.git

FEM has some many formatting changes can anybody recommend a editor/settings which which these updates are handled easier ?
Every time I have time to progress I spend a day rebasing code
:oops: That was me ... But we are mostly done with this. I have a rebased version here: https://github.com/berndhahnebach/FreeC ... utocontact But this might be outdated, because you have made changes to the autocontact.

If you still have a not rebased version I can do the rebase for you if you do not mind. Just drop a link here. You could concentrate on your new tools than :mrgreen:

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Re: Fem constraint contact

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Re: Fem constraint contact

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Let's get it into master ...
thschrader
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Re: Fem constraint contact

Post by thschrader »

Good morning!
I must calculate this little clamp fixed on a R88,9x5 mm tube.
antenna_clamp.FCStd
(97.96 KiB) Downloaded 60 times
The goal is to optimize the design and reduce weight, because we need thousands of them.

Question:
Does is make sense to run a contact analysis when you have huge contact-zones?
As far as I understand contact-analysis, you have "hard contact", means little contact zones with
superfine mesh-zone. As shown in the Z88 aurora example below.

I used 1. order elements, 2. order does not converge, ccx runs and runs...
But 1.order gives bad stress-plots. Meshing goes fast, ccx needs 10 min.
clamp.JPG
clamp.JPG (92.45 KiB) Viewed 1684 times
fandaL
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Re: Fem constraint contact

Post by fandaL »

Good afternoon,
I'm not proficient in contact analysis, but I don't think "huge contact-zone" is an issue.

I did some testing with 1st order elements. It is a beam 40x10x2 mm, fixed on one end, loaded with 1000 N on the other, from steel E=210 GPa. In the comparison below, there are maximal von Mises stress and vertical deflection (0.01524 mm from beam theory).

First picture is with only 2 1st order elements through the thickness, which is like a tube in your file. It is obviously too stiff and stress is too low. Stress on the surface is not smooth.
Second picture is with 4 elements as a clamp in your file. It is a bit better, but deflection is still by 19 % lower than theoretical.
Third picture is with 8 elements. Deflection difference 7 %.

2nd order models have difference around 2 % from theory.

Last two pictures are with 1st order hexa mesh imported from Salome. Stress is smooth, but values are similar to tetra elements. Perhaps ccx will calculate faster (I did not measure), but more elements through the thickness are still needed.
bended_plate_mesh_comparison.png
bended_plate_mesh_comparison.png (251.2 KiB) Viewed 1647 times
thschrader
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Re: Fem constraint contact

Post by thschrader »

fandaL wrote: Sat Jun 26, 2021 2:46 pm Good afternoon,
...
First picture is with only 2 1st order elements through the thickness, which is like a tube in your file. It is obviously too stiff and stress is too low. Stress on the surface is not smooth.
...
Hi fandal,
thanks for your analysis.
My mesh is not fine enough for tube thickness. Thats my interpretation.
But pushing up mesh refinement is problematic. Because of the "huge contact zone".
My experience is that ccx has problems above 200000 cells. And I guess you reached this limit with
your beam analysis using 8 cells for thickness.

I use a Laptop with Intel i5 cpu an 16 GB Ram. I can run simulations up to 500000 cells using
https://forum.freecadweb.org/viewtopic.php?f=18&t=58019

At this point the RAM limit is reached.

Why does ccx struggle when using 2.order cells for contact?

Or my model is to complicated. Too large.

So far so good, have a nice sunday.
Thomas
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thickness_mesh.JPG
thickness_mesh.JPG (63.43 KiB) Viewed 1605 times
UR_
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Re: Fem constraint contact

Post by UR_ »

You really should consider to take symmetry into account. :?

If you model only a quarter of tube and clamp, 75% of all these nodes disappears without any loss of accuracy. :roll:
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