Missing Boundary Condition?

[SteWa]

Hello everyone!

I am working with the FreeCAD-Version 0.19 from 2020/10/3 with the number 22522.

First of all I want to say, that FreeCAD has become a very nice piece of software during the last years!
I was quite amazed to see the development which happend since the last time I tried it.

So as I started to do some FEM-simulations, it seems to me that at least one kind of boundary condition
for mechanical stress simulations might be missing and I hope this is a contribution for pushing FreeCAD further
to comprehensive usability.

I append an example file in order to explain what I mean.

In the file I tried to simulate what happens if one puts a force onto a hollow body lying on a stiff underground.
The stiff underground should simply provide a boundary condition which gives a lower limit for the position of each part
of the hollow body but otherwise not interfere with it. One would expect that the lower part would start to buckle and bend upwards.

The boundary conditions which somehow come close would be "Constraint contact" or "Constraint displacement".

- With the first one, the lower part of the hollow body sticks to the underground and does not buckle at all.
- With the second one with each coordinate one may choose between "Fixed" or "Free", which does not help either.

It migth be a good idea to add the possibility of defining a movement range of some sort to "Constraint displacement".
As far as I see it, it is yet not possible to perform what I am trying to do.
In case it is possible it would be nice if one could give me one or two hints.

A workaround would be simply meshing the underground as well and include it in the simulation this way.
However there seems to be no support for multiple meshed body yet.

Did I express myself in a clear manner?
Somehow my english today is a bit rusty...

Greetings and thanks in advance!
Stefan

[thschrader]

Hi Stefan,
welcome to the FreeCAD-forum and to FEM.
Why a contact analysis? You can use the xyz-displacement boundary conditions
at the edges of the part "ausformung".

Meshing:
For multiple bodies use "boolean-fragments" fusion in part-wb, option compsolid.
Make compound. Mesh the compound.

Hope I have you understood correct.
Thomas

Test-ts.FCStd

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Test-mesh.FCStd

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test_FEM_mesh.JPG (81.82 KiB) Viewed 617 times

[SteWa]

Hello Thomas,

thank you very much for your response!

I actually see, that my first example has not been chosen carefully enough,
as I wanted to point out something different.
So I provide now a new example which shows my point much better.
(I wonder why I didn't come up with it in the first place, I suppose on saturday I have been a bit too tired. Sorry for that!)

The file "Test - Neu" shows an arch lying with the back side on that cylinder I included with the last model, too.
On each of the outer edges I apply a force pressing them downwards towards the cylinder.

So, without any force, the area of support (I mean "Auflagefläche" in german) will be a line in the center as shown right now in the model.
With forces applied to the edges at some point the middle part of the arch
will start to bend upwards such that the area of support will become two lines.
And the position of this lines will be dependent on the applied forces on the edges.

So, in order to simulate this realistically, one should not apply xyz-displacement boundary conditions anywhere,
because the entire geometry of the arch is in flux with respect to the z-coordinate while applying the force on the edges.
What one needs is either the cylinder providing a counter force whereever the lines of support are,
or boundary conditions, which tell the solver, that no part of the arch may have an z-coordinate less than 20mm
(Which is the z-coordinate of the surface of the zylinder), but apart from that there is liberty of action.

How would one implement that with FreeCAD?
Is it possible? If not, that might be a point for a new set of boundary conditions.

Thanks in advance and have a nice weekend!
Stefan

[thschrader]

...
So, in order to simulate this realistically, one should not apply xyz-displacement boundary conditions anywhere,
because the entire geometry of the arch is in flux with respect to the z-coordinate while applying the force on the edges.
...

Thanks for the new model, now I understand what you mean
Yes, no xyz-displacement fixings at the full strip possible.
And using a half strip? Did a little modification at the model.
I am struggling with the ccx-solver settings (time-stepping)?
You dont need to chamfer the loading-zone, you can adjust the
force direction by using edges as reference.
There must be a tiny gap between the bodys (I believe).

half_strip_contact.FCStd

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strip.JPG (41.39 KiB) Viewed 499 times

[SteWa]

Hi Thomas,

using the half strip in combination with a xyz-displacement fixing representing an axial symmetry
might be a workaround for geometries including such a symmetry.
(If I see it right, you fixed the middle part to X=0)
However, in case of a general geometry without such a symmetry it won't be useful, I suppose.
So, the question is, what to do with a general, maybe complex shaped geometry?
There might not be a possibilty to find such a workaround or it is extremely complicated to find.

Thanks for the tip for avoiding the chamfering of the loading zone!

About the seetings of the ccx-solver settings:
I did not do any adjustments to the solver with respect to time stepping.
So I don't know where that came from.

Greetings
Stefan

[thschrader]

...
About the seetings of the ccx-solver settings:
I did not do any adjustments to the solver with respect to time stepping.
So I don't know where that came from.
...

Works, forget the time-steps.
I forgot to set ccx-solver to nonlinear-geometry in data-tab