Question about meshing for two intersecting solids

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mitu7362
Posts: 25
Joined: Sat Dec 14, 2019 3:55 pm

Question about meshing for two intersecting solids

Post by mitu7362 » Sat Dec 21, 2019 8:08 pm

Hello everyone,

I have a problem for the mesh of the hook spring. The hook spring has generally a initial stress inside the coils, which means it will not suffer from any deformation until the external force is greater than the total internal force. Now this initial stress need to be simulated with the FE-Analysis.

My plan for the simulaiton at the moment ist to make the pitch of the spring negativ. Under this circumtance there is no initial stress in the coils any more, a situation that can not be reached in the real life since the coils of the hook spring cannot "go inside" to the other in reality. Then the boundary condition of displacement will be added, making it reach its original length, so that the value of the initial stress could be calculated.

However, in this case the solids of the coils will encounter intersection, making it difficult for generating volume mesh. I've already tried the Netgen and Gmsh in FreeCAD, no volume mesh can be generated.

I've learnd that the function of partrition in Salome could deal with this situation. Is there a similar function in FreeCAD that could handle it? Of course it is better if everything could be done in just one software nicely...

Thanks a lot:)
Attachments
Intersection of the coils.FCStd
(19.49 KiB) Downloaded 6 times
Intersection of the coils.png
Intersection of the coils.png (91.67 KiB) Viewed 252 times
chrisb
Posts: 21222
Joined: Tue Mar 17, 2015 9:14 am

Re: Question about meshing for two intersecting solids

Post by chrisb » Sat Dec 21, 2019 9:55 pm

I am not sure if you are right with the initial stress inside of the spring - beyond the forces which hold the whole world together.
mitu7362
Posts: 25
Joined: Sat Dec 14, 2019 3:55 pm

Re: Question about meshing for two intersecting solids

Post by mitu7362 » Sat Dec 21, 2019 11:02 pm

chrisb wrote:
Sat Dec 21, 2019 9:55 pm
I am not sure if you are right with the initial stress inside of the spring - beyond the forces which hold the whole world together.
Hallo!

Here is the excerpt from the book <FEM for Springs> written by Shimoseki and so on, which shows the initial force in the spring that I mean perfectly. What I want to do is try to mesh with the negative pitch.
Attachments
FEM for springs.png
FEM for springs.png (57.64 KiB) Viewed 227 times
chrisb
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Re: Question about meshing for two intersecting solids

Post by chrisb » Sat Dec 21, 2019 11:20 pm

So this doesn't hold for extension springs in general, only if the coils touch; is this right?
UR_
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Joined: Tue Jan 03, 2017 8:42 pm

Re: Question about meshing for two intersecting solids

Post by UR_ » Sun Dec 22, 2019 8:35 am

chrisb wrote: :bell:
chrisb, would you be that nice to move this posts to FEM forum. So it would much easier for mitu7362 to gather needed attention.


@mitu7362:
IMO, two things are needed to get a solution by ccx:
  1. Self contact (master and slave face are the same) to prevent self intersection caused by pretension.
    please see here
    http://mecway.com/forum/discussion/813/ ... y-calculix
    http://mecway.com/forum/discussion/812/ ... th-sharing
  2. Applying pretension (torsion) to spring's main body by introducing a external accumulative torque.
mitu7362
Posts: 25
Joined: Sat Dec 14, 2019 3:55 pm

Re: Question about meshing for two intersecting solids

Post by mitu7362 » Sun Dec 22, 2019 9:23 am

UR_ wrote:
Sun Dec 22, 2019 8:35 am
chrisb wrote: :bell:
chrisb, would you be that nice to move this posts to FEM forum. So it would much easier for mitu7362 to gather needed attention.


@mitu7362:
IMO, two things are needed to get a solution by ccx:
  1. Self contact (master and slave face are the same) to prevent self intersection caused by pretension.
    please see here
    http://mecway.com/forum/discussion/813/ ... y-calculix
    http://mecway.com/forum/discussion/812/ ... th-sharing
  2. Applying pretension (torsion) to spring's main body by introducing a external accumulative torque.
Thanks a lot for the tips and the help. Sorry I am brand new to the forum so I did not think of posting in the "Development Area"... :oops:
chrisb
Posts: 21222
Joined: Tue Mar 17, 2015 9:14 am

Re: Question about meshing for two intersecting solids

Post by chrisb » Sun Dec 22, 2019 9:40 am

One further remark on posting: We like to see quotes, but we like to see them to be reduced to a minimum, showing exactly what you are referring to. They usually don't have to show the complete previous post, possibly even containing further quotes.
mitu7362
Posts: 25
Joined: Sat Dec 14, 2019 3:55 pm

Re: Question about meshing for two intersecting solids

Post by mitu7362 » Sun Dec 22, 2019 10:10 am

chrisb wrote:
Sat Dec 21, 2019 11:20 pm
So this doesn't hold for extension springs in general, only if the coils touch; is this right?
At first thanks for your remark on the posting, I will pay attention to it next time. In this post, however, I think it is better if I explain it here in detail. Sorry for the uncomfort for reading.

The diagramm from the book holds for most of the extension springs, since they need the initial force to save the axial space. That means even if no external lasts work on the spring, there is already stress inside the coils, which is quite different from a compression spring.

Or it could be expressed this way: If the coils could "go inside" each other like what is showed in my screenshot, i.e. it could have the negative pitch, the spring would then be shorter.

What I want now is to do the FEM for the spring with this imaginay negative pitch, then recover it to its original length (make it look like what it should be in the real life), so that this initial force inside can be simulated. But since the coils must have self intersection under this circumtance, the mesh is very difficult.
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