[SOLVED] FEM output does not seem to be correct (beginner question)

About the development of the FEM module/workbench.

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ceremcem
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[SOLVED] FEM output does not seem to be correct (beginner question)

Postby ceremcem » Tue Jul 30, 2019 7:06 pm

I started learning FEM by this video. However, my model doesn't seem to be analyzed correctly:

fem-out.png
fem-out.png (86.31 KiB) Viewed 721 times

I would expect the upper "bar" has some stress (some red areas), just like in "Abs displacement" mode:

fem-out2.png
fem-out2.png (84.7 KiB) Viewed 721 times

However, in "Abs displacement" mode I can see no stress on the "lower bar".

I don't know what "Abs displacement" or "Tresca" means (I'm following the French Cafe technique) but I can't obtain a result that is similar to my expectation (which is something like combination of the "Abs displacement" and "Tresca" modes).

Is it my lack of knowledge or are there some missing/wrong steps here?
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fem2-test.FCStd
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Last edited by ceremcem on Fri Aug 02, 2019 11:42 am, edited 2 times in total.
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PrzemoF
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Re: FEM output does not seem to be correct (beginner question)

Postby PrzemoF » Tue Jul 30, 2019 8:50 pm

Looks good to me. Tested on my setup - visually the same results.

My tutorial - FEM in 60 seconds - https://peertube.social/videos/watch/d7 ... 91a69a1446
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bernd
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Re: FEM output does not seem to be correct (beginner question)

Postby bernd » Tue Jul 30, 2019 9:15 pm

looks got to me too ...
screen.png
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ickby
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Re: FEM output does not seem to be correct (beginner question)

Postby ickby » Thu Aug 01, 2019 2:18 pm

what you see is correct. The options show what data of the result is used to color the part. "Abs displacement" means it uses the displacement vector, the absolute value of it. Hence sqrt(dx^2+dy^2+dz^2) where dx and dy is the displacement in x and y direction. So you see no color at the bottom truss as it is closer to the contraint and hence moves way less than the top truss.

So your pictire has nothing to do with the stress in the part. To see the stress you should use "Von Mises stress".
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PrzemoF
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Re: FEM output does not seem to be correct (beginner question)

Postby PrzemoF » Thu Aug 01, 2019 2:50 pm

ceremcem wrote:
Tue Jul 30, 2019 7:06 pm
[..]I would expect the upper "bar" has some stress (some red areas), just like in "Abs displacement" mode[..]
There is a strip of yellow at the top and bottom of the beam if you take a close look. Red is the top of the scale - try to play with a simple beam and different support and different forces - you'll get a feeling quickly what to expect and where in terms of stress and displacement.
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ceremcem
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Re: FEM output does not seem to be correct (beginner question)

Postby ceremcem » Fri Aug 02, 2019 11:41 am

If you all say so, then it's probably my misinterpretation.

ickby wrote:
Thu Aug 01, 2019 2:18 pm

So you see no color at the bottom truss as it is closer to the contraint and hence moves way less than the top truss.
So your pictire has nothing to do with the stress in the part. To see the stress you should use "Von Mises stress".

Well, now I started to get the idea. Moreover I couldn't notice any difference between "Von Mises Stress" and "Max Shear Stress" by the naked eyes. However, taking two screenshots and subtracting one from the other pixel by pixel outputs the slight difference:

Screenshot_2019-08-02_14-18-19.png
Screenshot_2019-08-02_14-18-19.png (255.37 KiB) Viewed 617 times



I'm going to stick with "Von Mises Stress" till I understand what "Tresca" means.

PrzemoF wrote:
Thu Aug 01, 2019 2:50 pm
There is a strip of yellow at the top and bottom of the beam if you take a close look. Red is the top of the scale -
Is there a way to customize the scale where I would like to highlight any area whose stress (or displacement) exceeds a specific limit? (Or better, I'd like to make whole model highly transparent and make the high stress areas more opaque which would provide an X-ray like vision at any angle)
try to play with a simple beam and different support and different forces - you'll get a feeling quickly what to expect and where in terms of stress and displacement.
I got what you suggested. Here is a screenshot that looks like just I would expected:
Screenshot_2019-08-02_14-31-19.png
Screenshot_2019-08-02_14-31-19.png (126.09 KiB) Viewed 617 times
fandaL
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Re: [SOLVED] FEM output does not seem to be correct (beginner question)

Postby fandaL » Sat Aug 03, 2019 11:55 am

ceremcem wrote:
Fri Aug 02, 2019 11:41 am
... Moreover I couldn't notice any difference between "Von Mises Stress" and "Max Shear Stress" by the naked eyes...
"Von Mises Stress" and "Max Shear Stress" are two approaches to answer same question: how to get equivalent stress (one simple number instead of normal and shear stresses in all directions). So they give similar value.
ceremcem wrote:
Fri Aug 02, 2019 11:41 am
Is there a way to customize the scale where I would like to highlight any area whose stress (or displacement) exceeds a specific limit? (Or better, I'd like to make whole model highly transparent and make the high stress areas more opaque which would provide an X-ray like vision at any angle)
Try to play with Pipeline postprocessing, e.g. with “Scalar Clip Filter”. Some hints according to this are on the end of this tutorial https://www.freecadweb.org/wiki/FEM_She ... site_Block
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Re: FEM output does not seem to be correct (beginner question)

Postby PrzemoF » Mon Aug 05, 2019 1:30 pm

ceremcem wrote:
Fri Aug 02, 2019 11:41 am
[..]Well, now I started to get the idea. Moreover I couldn't notice any difference between "Von Mises Stress" and "Max Shear Stress" by the naked eyes.[..]
Image from wikipedia

https://en.wikipedia.org/wiki/Von_Mises_yield_criterion
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ceremcem
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Re: [SOLVED] FEM output does not seem to be correct (beginner question)

Postby ceremcem » Sat Aug 10, 2019 12:09 am

I appreciate all of you for your helps and tips. I could barely understand the difference between Von Mises Stress and Tresca. I suppose choosing either one won't make much difference for me at this stage.
fandaL wrote:
Sat Aug 03, 2019 11:55 am
Try to play with Pipeline postprocessing, e.g. with “Scalar Clip Filter”. Some hints according to this are on the end of this tutorial https://www.freecadweb.org/wiki/FEM_She ... site_Block
I finally managed to follow the tutorial. Here is my result. Thanks!