"Unhandled unknown exception caught in selection observer" when trying to apply a FEM constraint to a revolved B-spline about freecad

[FEA-eng]

Can you create another model and apply FEM constraints when it is based on a revolution of a sketch with a BSpline, created in FC?

Good point. It turns out that I can't. I attached a FreeCAD file. But it's interesting that the same happens with STEP geometry where B-spline was used in another software.

Then the issue is more general and serious than I expected.

B-spline revolved FEM.zip

[maxwxyz]

how did you model the egg, do you have the FCStd? I'm not sure if this is only FEM related?

[FEA-eng]

how did you model the egg, do you have the FCStd? I'm not sure if this is only FEM related?

I modeled it in another software and can only export it to neutral format and recreate if needed (I've tried it 0.22 too - just downgraded in Draft and converted the wires to a sketch which I then revolved in PD - same errors occur). It seems to be FEM-specific because other operations, e.g. in Part or Part Design, work normally on this geometry.

[maxwxyz]

I have no idea whats wrong, when downgrading and creating a sketch I can create a revolution but get the same error. When trying to change the sketch to a simple triangle I cannot add a revolution or even a pad after I've created an Analysis container. Just a bunch of errors

[FEA-eng]

Yeah, it's really strange. I've even tried with a simple sphere exported to STEP but this one is working correctly. So either the egg shape is so specific and causes issues because of its curvature or the model is broken starting even from the sketch but it shouldn't be the case. I can use this model normally in all other ways and only can't apply FEM constraints to it.

[maxwxyz]

Can you create another model and apply FEM constraints when it is based on a revolution of a sketch with a BSpline, created in FC?

[maxwxyz]

Can you rephrase it then? Seems like FEM constraints can just handle plane or rounded faces, no irregular surfaces?

[FEA-eng]

Seems like FEM constraints can just handle plane or rounded faces, no irregular surfaces?

Not exactly, padded B-spline works. Even with fillets. Maybe because there are more faces with less curvature. This is really strange. I'm surprised it has never been detected.

Since it comes from Selection.cpp, it might be a deeper issue than just related to FEM constraints. I wonder what else could trigger it. I can create e.g. a datum plane or a subshapebinder based on those problematic faces.

[wwmayer]

https://github.com/FreeCAD/FreeCAD/blob/main/src/Mod/Fem/App/FemConstraint.cpp#L433

A null vector cannot be normalized and OCC throws an exception that isn't handled anywhere inside the FEM module.