Reaction force request

About the development of the FEM module/workbench.

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poimsm
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Reaction force request

Post by poimsm »

Any one knows how to calculate reaction forces on FEM FreeCAD 0.17? Thanks!
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makkemal
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Re: Reaction force request

Post by makkemal »

FreeCAD does not read reaction forces into results yet
You need to add RF to the *Node file card and the ccxinputwriter.py
http://web.mit.edu/calculix_v2.7/Calcul ... de159.html

Then on the freecad side read the results in in the ccxfrdreader.py file and pass them to VTK
This is not a difficult one for you to add yourself
Thanks for the contribution
fandaL
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Re: Reaction force request

Post by fandaL »

Hi Poimsm, welcome to the forum.

You can request reaction forces in .frd file by

Code: Select all

*NODE FILE
RF
or in .dat file - where it is practical to limit them on some nodeset

Code: Select all

*NODE PRINT,NSET=FemConstraintFixed
RF
this prints reaction forces in x, y, z for all nodes. For me it is practical to ask for total reaction forces

Code: Select all

*NODE PRINT,NSET=FemConstraintFixed, TOTALS=ONLY
RF
You can easily add this into your .inp file after similar lines when you generates it in FreeCAD, when you insert nodeset(s) which names you can find under *BOUNDARY card in the generated .inp file - something like "FemConstraintFixed".

if TOTALS=YES then you get it in nodes and also totals

However, there are some difficulties with reaction forces. I doubt it is possible to get reaction moments. From http://web.mit.edu/calculix_v2.7/Calcul ... #nodeprint
The external forces are the sum of the reaction forces, concentrated loads (*CLOAD) and distributed loads (*DLOAD) in the node at stake. Only in the absence of concentrated loads in the node and distributed loads in any element to which the node belongs, the external forces reduce to the reaction forces. Forces induced by multiplpoint constraints are not calculated. Since single point constraints defined in transformed coordinates are converted into multiple point constraints in the global rectangular system, the force in a node in which a SPC is defined in local coordinates are not correctly delivered upon using the RF key in combination with the *NODE PRINT keyword card.
So if you apply loads through rigid body elements, then reaction forces are wrong. If you apply load on fixed node, then this load (e.g. portion of pressure on element face) is not counted to reactions.
ulrich1a
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Re: Reaction force request

Post by ulrich1a »

The newer manuals of ccx like version 2.11 have an extra section regarding reaction forces. Have a look here: http://www.feacluster.com/CalculiX/ccx_ ... de177.html

Ulrich
julieng
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Re: Reaction force request

Post by julieng »

Hello,

I am not very familiar with FEM FreeCAD and Calculix. I use FreeCAD 0.18 on Windows 10
I try to compute the reaction force on fixed constraint nodes using

RF

Code: Select all

*NODE PRINT,NSET=FemConstraintFixed, TOTALS=ONLY
RF
as FandaL said.

First question: After generating the .inp file with the write command, I can edit the file directly in FreeCAD , but how to save it directly in FreeCAD?

So I search in my "C:\Users\julien\AppData\Local\Temp\" folder and save the file manually.

Second question:

I calcule the modified .inp file but I don't know where to see the reaction force results. I don't anaything under the CalculiX_static_results node in FreeCAD

Sorry for the basic question.

Julien
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bernd
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Re: Reaction force request

Post by bernd »

see https://forum.freecadweb.org/viewtopic.php?f=18&t=12212

If you would use 0.19 the dat file will be written. The dat file contents will be in the tree in FreeCAD in this dat file content will be the reaction forces.

cheers bernd
julieng
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Joined: Sun Nov 25, 2018 8:57 pm

Re: Reaction force request

Post by julieng »

Thank you bernd,

Yes I see by myself the improvement with the dat file. This is very good.

Does the reaction force is automatically computed on each BC with fixed constraints condition in 0.19 ? Because I have not add any command in the .inp file and I can see a result.

Is it possible to add some command in the .inp file and have the results in the dat file in FreeCAD?
How can I save directly modification when editing the file .inp via FreeCAD?

Julien
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bernd
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Re: Reaction force request

Post by bernd »

Yes reaction forces are added automatically in 0.19

If you use the solver task panel you can edit the inp file before run the inp file. Furthermore the use of the splitted inp file option makes sense in this regard.

No the edit can not be saved ATM.
hex41434
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Re: Reaction force request

Post by hex41434 »

bernd wrote: Tue Nov 10, 2020 7:40 pm Yes reaction forces are added automatically in 0.19

If you use the solver task panel you can edit the inp file before run the inp file. Furthermore the use of the splitted inp file option makes sense in this regard.

No the edit can not be saved ATM.
Hi all! I have freecad 0.19 on My Macbook and in .dat file I have only these 2 lines (instead of reaction force for all nodes)

total force (fx,fy,fz) for set FEMCONSTRAINTFIXED and time 0.1000000E+01

1.885456E-01 -3.685454E-02 -3.500000E+03


might be a difference between .dat file in Linux and Mac?

Best
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bernd
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Re: Reaction force request

Post by bernd »

hex41434 wrote: Fri Jul 30, 2021 8:37 am
bernd wrote: Tue Nov 10, 2020 7:40 pm Yes reaction forces are added automatically in 0.19

If you use the solver task panel you can edit the inp file before run the inp file. Furthermore the use of the splitted inp file option makes sense in this regard.

No the edit can not be saved ATM.
Hi all! I have freecad 0.19 on My Macbook and in .dat file I have only these 2 lines (instead of reaction force for all nodes)

total force (fx,fy,fz) for set FEMCONSTRAINTFIXED and time 0.1000000E+01

1.885456E-01 -3.685454E-02 -3.500000E+03


might be a difference between .dat file in Linux and Mac?

Best
in FreeCAD the file is just read, means there is no difference. If there is one in ccx run is difficault to say since I have never run ccx on mac. But I would not exclude it ...

If you post the file I could test on Linux and Windows.
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