Reaction force request

About the development of the FEM module/workbench.

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hex41434
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Re: Reaction force request

Post by hex41434 »

this is a simple fem I made on a cube in FreeCAD. and the .dat file (I changed the file extension from .dat to .txt to upload here!) Surprisingly I checked it on Linux and got the same thing... (where am I doing wrong?!)

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fandaL
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Re: Reaction force request

Post by fandaL »

The output in *.dat file is a sum of reaction forces (with components Fx, Fy, Fz). E.g. if you have a beam fixed at both ends and you use one Fixed Constraint object for both ends, total reaction force (as printed) will correspond to external load. If you will use one Fixed Constraint object for one end and second Constraint for the other, *.dat file will contain information for one and for the other.

Otherwise Calculix can print reaction forces per each node, if you change *.inp file from

Code: Select all

*NODE PRINT, NSET=ConstraintFixed, TOTALS=ONLY
RF
to

Code: Select all

*NODE PRINT, NSET=ConstraintFixed, TOTALS=YES
RF
where ConstraintFixed is name corresponding to appropriate constraint object.


Side note: CalculiX calculates reaction forces without forces applied directly to constrained nodes, so that your reaction force is slightly lower than applied load 3000 N.
hex41434
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Re: Reaction force request

Post by hex41434 »

wow thanks a lot. I was really confused! now I get it! :D

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bernd
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Re: Reaction force request

Post by bernd »

Do we need to change something in the FreeCAD FEM inp file in this regard?
cad1234
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Re: Reaction force request

Post by cad1234 »

fandaL wrote: Fri Jul 30, 2021 11:15 am
Very helpful, I've been looking for that for a long time! Many thanks!
fandaL
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Re: Reaction force request

Post by fandaL »

bernd wrote: Sat Jul 31, 2021 10:25 am Do we need to change something in the FreeCAD FEM inp file in this regard?
No, if I understood the hex41434’s question correctly, it was only about understanding what is in ccx_dat_file object. We can rather write some note about reaction forces to documentation. I have just mentioned it at https://wiki.freecadweb.org/FEM_Calculi ... _interface
Reaction forces can be found in ccx_dat_file which contains reaction force components (fx, fy, fz) for each Fixed Constraint.
cad1234
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Re: Reaction force request

Post by cad1234 »

@fandal and bernd
this is a great feature for fixed constraints!
Is it possible to let it work , if one side is x,y,z fixed, rotation free, and the other side y,z fixed and rotation free?
I was not able to do for non fixed constaints..
OS: Windows 10 (10.0)
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fandaL
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Re: Reaction force request

Post by fandaL »

cad1234 wrote: Sat Aug 07, 2021 8:28 pm Is it possible to let it work , if one side is x,y,z fixed, rotation free, and the other side y,z fixed and rotation free?
I have looked at it. This commit
https://github.com/fandaL/FreeCAD/commi ... 88df9caf6e
makes the same for Constraint displacement. I added if condition

Code: Select all

if not femobj["Object"].xFree or not femobj["Object"].yFree or not femobj["Object"].zFree:
so it should ask for reaction forces only if Constraint displacement constrains translation DOFs, because CalculiX does not print reaction moments, so if you constrain only moments, then reaction forces will be ~0.

@bernd
It should work on FEM examples, but I got to little trouble with definition of free and fixed DOFs in FEM example:
Constraints -> Displacement -> RC Wall 2D
In the property editor, there are zFix and zFree checked simultaneously. Does it have some meaning? In the Constraint displacement GUI, when I click zFix, it unchecks zFree and opposite, but property editor enables to switch on both simultaneously. Should the behaviour be that one is checked and second unchecked as allowed by GUI?
RC_Wall_2D.png
RC_Wall_2D.png (6.11 KiB) Viewed 2017 times
I would use only zFix and zDisplacement, omitting zFree parameter.
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bernd
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Re: Reaction force request

Post by bernd »

good catch :D git commit a0d860679ef6

this should never happen. IMHO the data model of constraint displacement could be better, but I never spent the time to change anything in this regard.

cheers bernd
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bernd
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Re: Reaction force request

Post by bernd »

fandaL wrote: Mon Aug 09, 2021 7:57 pm
cad1234 wrote: Sat Aug 07, 2021 8:28 pm Is it possible to let it work , if one side is x,y,z fixed, rotation free, and the other side y,z fixed and rotation free?
I have looked at it. This commit
https://github.com/fandaL/FreeCAD/commi ... 88df9caf6e
makes the same for Constraint displacement. I added if condition
...
Will you go for a PR? Some unit tests would need to be adapted to get this into master.

cheers bernd
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