About strain and Stress

[JeffWitz]

Dear FreeCAD FEM users,

I try to use FreeCAD/Calculix and I really don't understand why there is no possibility to have the strain and stress tensor in the results.

The Strain/Stress tensor are a symmetric second order tensor, they have 6 coordinates that are written in Mandel notations (Exx,Eyy,Ezz,sqrt(2)*Eyz,sqrt(2)*Exz,sqrt(2)*Exy) and in Voigt notations (Exx,Eyy,Ezz,2*Eyz,2*Exz,2*Exy). Calculix use Voigt while Code_Aster and Cast3m use Mandel (which is theoretically better).

In FreeCAD I only get acces to ex,ey,ez or sx,sy,sz. This is clearly not enough and frankly I don't understand what I am looking at. Is it just the first component of the Voigt vector ? Is it the strain in there principal frame ?

We really want to use FreeCAD for mechanical courses but without a simple access to the global tensor it is not possible.

If someone know how to do this, it will really help me.

[thschrader]

Dear FreeCAD FEM users,

I try to use FreeCAD/Calculix and I really don't understand why there is no possibility to have the strain and stress tensor in the results.

The Strain/Stress tensor are a symmetric second order tensor, they have 6 coordinates that are written in Mandel notations (Exx,Eyy,Ezz,sqrt(2)*Eyz,sqrt(2)*Exz,sqrt(2)*Exy) and in Voigt notations (Exx,Eyy,Ezz,2*Eyz,2*Exz,2*Exy). Calculix use Voigt while Code_Aster and Cast3m use Mandel (which is theoretically better).

In FreeCAD I only get acces to ex,ey,ez or sx,sy,sz. This is clearly not enough and frankly I don't understand what I am looking at. Is it just the first component of the Voigt vector ? Is it the strain in there principal frame ?

We really want to use FreeCAD for mechanical courses but without a simple access to the global tensor it is not possible.

If someone know how to do this, it will really help me.

Hi Jeff,
after running the FEM-analysis, you can convert the xxx.frd result file to vtk format.
Use this tool, written by mammah (see post from UR_, the ccxfrd2vtk.zip file).
https://forum.freecadweb.org/viewtopic. ... 4&start=10

Extract the zip, copy frd-file into ccxfrd2vtk/../bin/release folder.
Drag frd file with mouse over the mmh_ccx2vtk.exe, conversion starts.
Import the vtk file into paraview to see tensor components.
BTW: can you give some info about your mechanical courses?
regards Thomas

done_with_FC017_Roland.JPG (44.92 KiB) Viewed 1913 times

screenshot_stress_tensor.jpg (61.79 KiB) Viewed 1913 times

[bernd]

Which version of FreeCAD do you use? My one has all six stress and strain components in Result object.

[JeffWitz]

@thschrader
This is not an option, it is too complicated for student use, and not suitable with the simplicity if FreeCAD.
It is for optional modules where it is needed to use FEM with simplicity, and the idea is to avoid Abaqus. Salome-Meca is too complex for such task.

@Bernd
FreeCAD 0.18, Libs: 0.18R16093 (Git)
I use snap from vejmarie as the PPA in ubuntu 16.04 break my database, I don't succeed in installing it even after purging everything.
(I post in the french forum about this)
So if you have your six components how do you call them in the result dialbox ?
I only have ex,ey,ez and sx,sy,sz. I use the vtk export and in paraview I have the same ...
If you can point me to an appimage with netgen, calculix and the complete tensors, it would be great !
Or I can wait for the PPA to be fixed.

[bernd]

any latest 0.19dev would do the trick. They are available for Linux and Windows. On Linux you could use the AppImage if you get not installed the ubuntu one, or just compile it yourself. This is not difficault.

https://github.com/FreeCAD/FreeCAD/blob ... py#L50-L74

https://github.com/FreeCAD/FreeCAD/blob ... #L665-L696

Be aware we only have Node data no element data. This is on the TODO as it would be very useful for nonlinear materials.

[polymer]

Maybe you will try PrePoMax, I like it.
http://lace.fs.uni-mb.si/wordpress/boro ... page_id=41

Edit:
Ah, you are on Linux, that is a Problem...