FreeCad_FEM CalculiX is he calculating right or wrong?

About the development of the FEM module/workbench.

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46Cpi
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FreeCad_FEM CalculiX is he calculating right or wrong?

Postby 46Cpi » Fri May 22, 2020 10:41 am

Testing pressures equivalent to zero displacement stress :
on a simple square plate of 100mm x 100mm x 1mm with 4 supports of 2mm radius placed at 0,5mm from the corners.
Material : 15CrNi6

1- The forces calculated at the supports are respectively in X Y Z
3.126444E-12 -1.695736E-11 1.925470E-01
2.333391E-15 1.570553E-15 1.904788E-01
-3.107160E-15 1.258867E-15 1.911145E-01
-9.094631E-16 2.409700E-15 1.904979E-01
i.e. 0.19115955 N in Z with a deflection of 3.027 microns
Z2a.FCStd
(14.03 KiB) Downloaded 5 times
Calcul Forces.PNG
Calcul Forces.PNG (18.2 KiB) Viewed 276 times
Z2a-init.PNG
Z2a-init.PNG (196.21 KiB) Viewed 276 times
It all seems fair except for the slight asymmetry in the calculated forces...


To find an equivalent and symmetrical deformation while keeping the corner1 fixed, the deformation must be of
14.122 kPa for each of the other 3 corners.
Z2a-1.FCStd
(13.82 KiB) Downloaded 4 times
Z2a-1.PNG
Z2a-1.PNG (226.85 KiB) Viewed 276 times
14.122 kPa applied to the 3 discs of 4mm diameter represents a force of 0.17746 N which is a nice difference with the average force calculated initially. Moreover, with this pressure the deflection increases to 8 microns, i.e. more than twice as much as with the zero displacement Z constraints.


How can these differences in results be explained ?

Cordially
46Cpi
Last edited by 46Cpi on Fri May 22, 2020 3:32 pm, edited 1 time in total.
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bernd
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Re: FreeCad_FEM CalculiX calcule juste ou faux ?

Postby bernd » Fri May 22, 2020 10:51 am

Either would you write in English, or the topic will be moved to the french forum.

the pics look great, :)

cheers bernd
46Cpi
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Re: FreeCad_FEM CalculiX calcule juste ou faux ?

Postby 46Cpi » Fri May 22, 2020 11:30 am

It's translated into English
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bernd
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Re: FreeCad_FEM CalculiX calcule juste ou faux ?

Postby bernd » Sat May 23, 2020 8:02 am

46Cpi wrote:
Fri May 22, 2020 11:30 am
It's translated into English
moved it back ... :D
fandaL
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Re: FreeCad_FEM CalculiX is he calculating right or wrong?

Postby fandaL » Sat May 23, 2020 10:15 am

When boundary condition is on vertical (z) displacement (in the model Z2a.FCStd), it acts on more nodes next to each other due to small circular area. This effectively constrains also rotations around in-plane axes. In fact, reaction forces are not uniform on the constrained surface, so that there are also kind of reaction moments, which are not printed by CalculiX.

Contrary, pressure (in the model Z2a-1.FCStd) does not constrain rotations, so that displacements are much larger and obviously not symmetric, since remaining corner is fixed (constraining both, forces and rotations).
46Cpi
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Re: FreeCad_FEM CalculiX is he calculating right or wrong?

Postby 46Cpi » Wed May 27, 2020 11:43 am

That's it, I think I understand why for a given surface, by replacing the equivalent force calculated by FreeCad with a displacement (null here), the results do not match.
In fact it's logical: FreeCad calculates the total force by summing the forces calculated at each node. And when we apply this force (total result), FreeCad applies this force by distributing it only on a few points that we do not know (probably a few key nodes or only one node, in any case probably not the same nodes as initially and probably not with the same distribution of forces in the starting nodes).
So the result of the deformation of the piece is different at the arrival. There is also an influence on the results of the other displacement constraints.
There is also the influence of the fineness of the mesh, but this is another problem.

Today we can apply a force on a surface or edge, therefore without knowing the force equivalent to a given displacement. This generates a certain amount of deformation.
If we apply this displacement, it generates a different deformation and vice versa.
There is no effective correspondence between these two input conditions, and above all they give different deformations when they are supposed to give identical results.
Wouldn't it be useful to create a tool that allows to apply the true force equivalent to a displacement calculated by FreeCad? Or at least in the form of a list of forces to be applied to nodes in script for example via FEMMeshGmsh.inp

Example of an aberration of forces equivalent to one zone:
Aberration Force equivalente.png
Aberration Force equivalente.png (254.1 KiB) Viewed 107 times
Force equivalente.PNG
Force equivalente.PNG (45.73 KiB) Viewed 107 times
Sum of FreeCad equivalent force : 3,326878 N
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-alex-
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Re: FreeCad_FEM CalculiX is he calculating right or wrong?

Postby -alex- » Wed May 27, 2020 1:26 pm

46Cpi wrote:
Wed May 27, 2020 11:43 am
when they are supposed to give identical results.
No they are not, a force and a displacement constraint have no similar effects on model, please read again the @fandal's answer above and mind it a bit more https://forum.freecadweb.org/viewtopic. ... 49#p401468
Lets talk about that on french forum if you want https://forum.freecadweb.org/viewtopic. ... 18#p402918
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bernd
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Re: FreeCad_FEM CalculiX is he calculating right or wrong?

Postby bernd » Wed May 27, 2020 2:59 pm

I died not have a look at your long test. I am on the way. Just some fast hints. If you mesh with gmsh, set linear middle nodes to false! This should improve a bit, but the general problem resists.

constraint pressure:
the pressure (MPa) is given to the element face, all the rest is done by Calculix. Uses DLOAD key word

constraint force:
Node lodes are calculated acording to the fem element function and the face area. Uses CLOAD key word


In second case the result is exact for the mesh with linear borders. For the first cast I actually do not know if calculix uses bilinear borders to calculate the pressure area. Would be good to know.