Weight simulation with multiple separable contact faces

About the development of the FEM module/workbench.

Moderator: bernd

Post Reply
Ningrich3
Posts: 12
Joined: Mon Jul 18, 2022 4:18 am

Weight simulation with multiple separable contact faces

Post by Ningrich3 »

Hello everyone,

despite elaborate online research I'm unable to simulate an object with ConstraintSelfWeight and 4 or more support points that are constrained in -Z but free in +Z, such as a four-legged table, for example.
I tried ConstraintDisplacement with the Z axis locked, but this restrains the contact faces in both + and - Z, as would be the case when such a table is screwed to the floor, which does not meet my requirements.
Another option I tried is the Constraint Contact, but this is currently restricted to one master and one slave face, while I need four or more slave faces.

Any idea of how to proceed with this case?
Thank you very much in advance!
User avatar
NewJoker
Veteran
Posts: 3014
Joined: Sun Oct 11, 2020 7:49 pm

Re: Weight simulation with multiple separable contact faces

Post by NewJoker »

Contact works in both ways, to model one-way support you would have to use GAP elements available in CalculiX but not supported in FreeCAD so manual keyword edits would be necessary. However, for cases like a table regular boundary conditions are usually used instead.
Ningrich3
Posts: 12
Joined: Mon Jul 18, 2022 4:18 am

Re: Weight simulation with multiple separable contact faces

Post by Ningrich3 »

@NewJoker Thank you very much for your reply, great hint!

I'd be fine with hacking some code, but I can't seem to find where to define boundary conditions other than in the GUI where the options apparently don't meet the conditions I'm after. Would you have any further hint for me or a discussion that has more info?
User avatar
NewJoker
Veteran
Posts: 3014
Joined: Sun Oct 11, 2020 7:49 pm

Re: Weight simulation with multiple separable contact faces

Post by NewJoker »

You have to edit CalculiX keywords, they are very user-friendly, no need for any programming skills. Just edit the input file: FEM SolverControl. The syntax can be found in the CalculiX documentation: http://www.dhondt.de/ccx_2.20.pdf

Basically, you will need something like this:

Code: Select all

*NODE
number, coord_1, coord_2, coord_3
*ELEMENT, TYPE=GAPUNI, ELSET=elset_name
number, node_1, node_2
*GAP, ELSET=elset_name
initial_clearance, gap_direction_1, gap_direction_2, gap_direction_3, , compression_stiffness, tensile_force
Those elements have to be placed between each node of the support and corresponding manually created node not belonging to the mesh. It’s just like with spring element definition.
Ningrich3
Posts: 12
Joined: Mon Jul 18, 2022 4:18 am

Re: Weight simulation with multiple separable contact faces

Post by Ningrich3 »

Beautiful, thank you for the information.
I'll check this out and report back on my progress.

Cheers, mate.
Ningrich3
Posts: 12
Joined: Mon Jul 18, 2022 4:18 am

Re: Weight simulation with multiple separable contact faces

Post by Ningrich3 »

Hello again,

I set up the simplest application of a plain square plate and tried to rest it on four gap units in each corner (the content of the .inp file attached).
For this purpose I manually created 4 nodes (lines 1422-1426) 1mm under the mesh and applied fixed boundary conditions to them (lines 2021-2025, lines 2151-2152).
I formed 4 Gapunit elements with each respective mesh corner point above and grouped them in an element set GapElset.
On this set I carried out the Gap condition with d=0.5mm and the direction in Z (lines 2114-2115).
Calculix does compile, however what I end up with is a displacement of 413m (while my intention was to have the plate resting on the 10^12 N/mm spring portion as a hard stop and being unrestrained in the opposite direction).

Also what I'm not clear about yet is the role of the GAP's d parameter. From my understanding the gap width is formed by the distance of the two nodes forming the Gapunit element, which is apparently not the case?

Thank you very much for any assistance! :)
Attachments
FEMMeshGmsh.txt
(89.34 KiB) Downloaded 24 times
Ningrich3
Posts: 12
Joined: Mon Jul 18, 2022 4:18 am

Re: Weight simulation with multiple separable contact faces

Post by Ningrich3 »

I myself have not come to a conclusion in this matter.
However, due to a concept change I'm currently not searching for a solution in this gap related concern anymore.
Thanks! :)
User avatar
NewJoker
Veteran
Posts: 3014
Joined: Sun Oct 11, 2020 7:49 pm

Re: Weight simulation with multiple separable contact faces

Post by NewJoker »

Well, maybe someone else will make use of this in the future. I attached the test file for gap elements. Clearance is usually set to zero.

d_gaps.txt
(1.12 KiB) Downloaded 26 times
Post Reply