Reaction force request

About the development of the FEM module/workbench.

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poimsm
Posts: 2
Joined: Mon Feb 13, 2017 11:16 pm

Reaction force request

Postby poimsm » Mon Feb 13, 2017 11:32 pm

Any one knows how to calculate reaction forces on FEM FreeCAD 0.17? Thanks!
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makkemal
Posts: 393
Joined: Wed Apr 29, 2015 12:41 pm
Location: South Africa
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Re: Reaction force request

Postby makkemal » Tue Feb 14, 2017 5:46 am

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
Posts: 333
Joined: Thu Jul 24, 2014 8:29 am

Re: Reaction force request

Postby fandaL » Tue Feb 14, 2017 9:28 am

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
Posts: 1872
Joined: Sun Jul 07, 2013 12:08 pm

Re: Reaction force request

Postby ulrich1a » Tue Feb 14, 2017 10:20 am

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