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Structural Mechanics: Out of plane displacement and buckling

Posted Jun 20, 2016, 4:25 p.m. EDT Modeling Tools & Definitions, Parameters, Variables, & Functions, Studies & Solvers, Structural Mechanics Version 5.2 6 Replies

Thomas Stofko
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I'm trying to model the deformation of a ribbon of SU8 with a pattern of cuts (kirigami) undergoing a 1D tensile loading. The loading is kept in the xy plane, but the force causes a twisting of material and buckling in the Z direction. Attached is an example photo of it occurring to paper with similar conditions. I cannot recreate what is happening physically within COMSOL

I've attempted this as a thin solid as well as a shell, both ending with the same result of only showing strain and deformation within the xy plane. I have assigned one end as a fixed constraint while attempting edge/boundary loading or prescribed displacement. I have also tried conducting a Linear Buckling study, but it doesn't provide me with the results I need I'm assuming because the loading is tensile and not compression. Attached is the file I'm trying to work with. I'm still new to COMSOL so I may be doing something wrong. Are there any suggestions for what may work or that I'm missing?
6 Replies Last Post Jul 5, 2016, 2:23 p.m. EDT
Daniel Connelly
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Posted: 8 years ago Jun 21, 2016, 4:05 p.m. EDT
That photo is fantastic! Impressive.

However, I think a primary issue with simulating it is symmetry. It appears the structure is symmetric, yet the deformation is not. Something needs to break the symmetry: the equations won't do so on their own. This may not be your issue but it's the most noticeable thing I see.
That photo is fantastic! Impressive. However, I think a primary issue with simulating it is symmetry. It appears the structure is symmetric, yet the deformation is not. Something needs to break the symmetry: the equations won't do so on their own. This may not be your issue but it's the most noticeable thing I see.
Thomas Stofko
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Posted: 8 years ago Jun 21, 2016, 4:12 p.m. EDT
Daniel,

Thank you for your input. I'll start trying to take a look at the equations and symmetry.
Daniel, Thank you for your input. I'll start trying to take a look at the equations and symmetry.
Thomas Stofko
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Posted: 8 years ago Jun 28, 2016, 4:51 p.m. EDT
I still can't seem to get any out of plane motion without adding in a small z-direction force. And when I do add the force, the entire geometry acts as if it is a solid member. If anyone would be able to expand on how to work with equations/symmetry or help further I would appreciate it.
I still can't seem to get any out of plane motion without adding in a small z-direction force. And when I do add the force, the entire geometry acts as if it is a solid member. If anyone would be able to expand on how to work with equations/symmetry or help further I would appreciate it.
Henrik Sönnerlind COMSOL Employee

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Posted: 8 years ago Jun 29, 2016, 9:47 a.m. EDT
Hi,

This is a rather difficult analysis, but here are some hints:

1. The most important is that the problem is strongly geometrically nonlinear. So you must activate geometric nonlinearity in the settings for the study step.

2. I would recommend a loading by a prescribed displacement, which is more stable.

3. The load must be applied gradually using an auxiliary sweep.

4. Solving a buckling problem is a numerically very sensitive problem. Please study this example, which deals with how to pass the instability and get into the post-buckling state.

www.comsol.com/model/postbuckling-analysis-of-a-hinged-cylindrical-shell-10257

Along these lines of that example, I think that using sqrt(w^2) as the controlling parameter could be a good idea.

5. You can read more about buckling in general in this blog post:

www.comsol.com/blogs/buckling-structures-suddenly-collapse/

Regards,
Henrik
Hi, This is a rather difficult analysis, but here are some hints: 1. The most important is that the problem is strongly geometrically nonlinear. So you must activate geometric nonlinearity in the settings for the study step. 2. I would recommend a loading by a prescribed displacement, which is more stable. 3. The load must be applied gradually using an auxiliary sweep. 4. Solving a buckling problem is a numerically very sensitive problem. Please study this example, which deals with how to pass the instability and get into the post-buckling state. https://www.comsol.com/model/postbuckling-analysis-of-a-hinged-cylindrical-shell-10257 Along these lines of that example, I think that using sqrt(w^2) as the controlling parameter could be a good idea. 5. You can read more about buckling in general in this blog post: https://www.comsol.com/blogs/buckling-structures-suddenly-collapse/ Regards, Henrik
Thomas Stofko
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Posted: 8 years ago Jul 5, 2016, 10:45 a.m. EDT
Hello Henrik,

Thank you for the guidance and the links to the tutorials for help. I'm still learning as I go and I'm a little confused by what you mean using sqrt(w^2) as the controlling parameter. Does that mean use it as the stop condition within the solver?

Thank you,
Thomas

Hello Henrik, Thank you for the guidance and the links to the tutorials for help. I'm still learning as I go and I'm a little confused by what you mean using sqrt(w^2) as the controlling parameter. Does that mean use it as the stop condition within the solver? Thank you, Thomas
Thomas Stofko
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Posted: 8 years ago Jul 5, 2016, 2:23 p.m. EDT
Never mind, I found what was meant by the controlling parameter. Thank you again.
Never mind, I found what was meant by the controlling parameter. Thank you again.

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