Strange results

trilbytim

I'm trying to add support for Code Aster into FreeCAD (see forum here ), with the eventual aim of being able to model composites and particularly filament winding in FreeCAD. So far I now have it able to read the information from FreeCAD (geometry, mesh, loads, materials, etc), automatically write .comm, .export and .med files, run the analysis and read back the results for some limited cases (shell elements, linear elastic only so far). However I did a quick "sanity check" on the results, and unfortunately they are insane! I've got an extremely simple cantilever beam, just a few elements, just to do a quick cross check. 200mm long, 100mm wide, 1mm thick, 34.5GPa material (short, randomly aligned, isotropic CFRP), 10N force in Z direction, other end fully clamped. Running this case with Calculix through FreeCAD gives a maximum deflection of 4mm, which seems believable. However my Code Aster version gives a maximum deflection of 1091mm, which is not believable!
My first thought was that I'd messed up the units, but the mesh is in mm, the modulus in MPa and the force in N, so these should be compatible, yes? Also it's not just a simple factor of 10 or 1000x wrong, it's about 250x wrong, so that seems like something else. I've attached the .comm, .export and .med files, can someone else run it and confirm they get the same results and see what I'm doing wrong with this please?
test.zip

mf

trilbytim

Hello,

FORCE_ARETE is a force per unit length of the edge where you apply the force. So you have to divide the value by the length of the edge.

Your mesh is not the same size as you described it's 100x50mm. With 0.2N/mm (10N/50mm) in FORCE_ARETE I get 21.821mm in Code_Aster. It is in good agreement with a separate calc done on https://www.schweizer-fn.de/berechnung/festigkeit/biegung/kragtraeger/bieg_kragtraeger.php where I get 23.2mm (but with 10 'elements' rather than only 2)

hope that helps,

Mario.

Austrian87

I have never used the FORCE_ARETE command. However, as far as I know the FORCE commands apply the specified value on each mesh component in the group, i.e., the FORCE_NODALE command applies the specified value on each node. Probably you have to divide by the number of edges in the group.

trilbytim

Thank you, yes, this explains it. I mis-wrote the size of the beam (I used several examples while I was working and forgot which one I posted!). So is there a command to apply a Force distributed evenly across all the nodes, or do I have to manually divide my 10N by the number of nodes and then apply it to each node?

mf

trilbytim
Hello,

just like I did above divide it by the LENGTH of the edge, that is 10/50=0.2N/mm. This value is entered in FORCE_ARETE. 'Distributing evenly across the nodes' is a somewhat confusing term in FEA: if the nodes are unevenly spaced, which in real parts they almost always are, the force is also distributed unevenly across the nodes, but evenly PER LENGTH or PER AREA (if the force is supposed to be constant per length or per area, of course).

Mario.

trilbytim

Thank you, that makes sense, I now just need to work out how to code that into my FreeCAD plugin to write to the .comm file.

mf

Hello,

that I have to leave up to you :-). FreeCAD 1.0 has become quite mature now, I highly recommend anyone to take a look at it. Even an untalented designer like myself can do almost anything with it now (past versions were so-so).

Mario.