Status of "Rotations are only valid for Beam and Shell elements" issue

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aomanchuria
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Status of "Rotations are only valid for Beam and Shell elements" issue

Postby aomanchuria » Wed Apr 07, 2021 9:46 pm

Yesterday I realized that I could not actually apply a moment force or a displacement rotation by an angle via FreeCAD.

What's the current .inp file workaround?

What's the status on implementing that? I read several posts about it and they are now like a year old or older. from what I gather there was some work done but it hasn't gone into the main branch. So if you want to play with that you have to compile your own FreeCAD. It was a long long read :).
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aomanchuria
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Re: Status of "Rotations are only valid for Beam and Shell elements" issue

Postby aomanchuria » Wed Apr 07, 2021 9:59 pm

using the inp editor from launcher3.4 it looks like this is a limitation existing in Calculix

** directions <dir> can be 1,2,3 for bodies
** and 1 to 6 for shells and beams (4,5,6 - to fix rotation)
**
*BOUNDARY
<nod.group>,<dir>,<dir>,<value>

** repeat last line, if needed
Jee-Bee
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Re: Status of "Rotations are only valid for Beam and Shell elements" issue

Postby Jee-Bee » Thu Apr 08, 2021 11:24 am

I think the issue is more general.
FreeCAD don't allow enough constraints in general FC only alow fixed. and maual you are able to remove the fix in some directions. but i mis some of them. We have only the right one
Inklemming.PNG
Inklemming.PNG (7.95 KiB) Viewed 293 times
The figure comes from :https://nl.wikipedia.org/wiki/Oplegging since i don't know the english version of this
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aomanchuria
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Re: Status of "Rotations are only valid for Beam and Shell elements" issue

Postby aomanchuria » Thu Apr 08, 2021 6:53 pm

I thought the displacement constraint allowed one to allow movement in only one axis so that you could use it as a sliding constraint. but then you are right, the pivoting constraint is missing. But again it seems to be a Calculix limitation. some how one has to work around it
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aomanchuria
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Re: Status of "Rotations are only valid for Beam and Shell elements" issue

Postby aomanchuria » Fri Apr 09, 2021 12:04 am

for example this simple beam loading. the bottom is fixed, the top is where I would like to add some kind of moment, I apply 0.5mm displacement in the two plane directions and I get the same as if I applied 0.5mm plus a rotation of 45degrees, exactly the same image, the 45 degrees doesn't even come up in the inp file, so just as others have noted.Image
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Screenshot 2021-04-08 170115.png
Screenshot 2021-04-08 170115.png (51.07 KiB) Viewed 227 times
Jee-Bee
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Re: Status of "Rotations are only valid for Beam and Shell elements" issue

Postby Jee-Bee » Fri Apr 09, 2021 6:02 pm

Displacement describes a displacement.
i want that movement is a certain direction but not aloud in others(this can currently be done by manual editting the .inp)
The other one is a hinge like constraint. what allows rotation around a axis or a( circular) surface
fandaL
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Re: Status of "Rotations are only valid for Beam and Shell elements" issue

Postby fandaL » Fri Apr 09, 2021 7:39 pm

Volume elements typically does not have rotation DoF, so that direct constraints on their nodes will constrain only translations at each node. Results will contain also only translational displacements. If you want to apply a moment load, you can simply prescribe one force on one edge and second force on the other so you will simulate a moment.
Calculix has tools (*COUPLING card) which can distribute a moment from one node to the surface - doing something similar you would do by prescribing forces to edges as mentioned above.
Example of prescribing moment load:
https://github.com/mkraska/CalculiX-Exa ... iaxBending
Example of defining a pin connection:
https://github.com/mkraska/CalculiX-Exa ... est/Joints
Jee-Bee
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Re: Status of "Rotations are only valid for Beam and Shell elements" issue

Postby Jee-Bee » Sat Apr 10, 2021 2:52 pm

I kind of understand that elements don't have a rotation. but objects have.
I create a simple model.
Screenshot 2021-04-10 at 13.30.26.png
Screenshot 2021-04-10 at 13.30.26.png (148.56 KiB) Viewed 95 times
On face is fixed. at one face a force is presented and the green surface is part of a hinge and should rotate based on the force that is put on the object. I would say it a basic fem problem what is not possible in FC.

Now i'm busy i also created a example of the other missing constraint
Screenshot 2021-04-10 at 15.31.26.png
Screenshot 2021-04-10 at 15.31.26.png (78.44 KiB) Viewed 95 times
One side is fixed the other side is free to move in length direction but fixed in the height. (From what i understand you can see this as a bridge where they use this principle to compensate for thermal changes)
Attachments
simple_example_sliding.FCStd
(15.47 KiB) Downloaded 2 times
simple_example_rotation.FCStd
(19.54 KiB) Downloaded 2 times
fandaL
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Re: Status of "Rotations are only valid for Beam and Shell elements" issue

Postby fandaL » Sat Apr 10, 2021 6:39 pm

Sliding example:
Use https://wiki.freecadweb.org/FEM_ConstraintDisplacement where you can prescribe separately which DoFs should be constrained. I have added a horizontal force, so that deformation is more visible, if you remesh and rerun the example.
simple_example_sliding.FCStd
(18.55 KiB) Not downloaded yet

Rotation example:
Use again ConstraintDisplacement to constrain X displacement in the hole
Use https://wiki.freecadweb.org/FEM_ConstraintTransform (oups documentation is missing) for Cylindrical transform by selecting the previously constrained face.
simple_example_rotation.FCStd
(23.09 KiB) Downloaded 2 times

For more examples look at:
Utilities -> Open FEM examples -> displacement -> ...
Utilities -> Open FEM examples -> transform -> ...