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Initial Stress
Posted May 30, 2010, 11:58 a.m. EDT 3 Replies
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Hello All,
I had (and still have) a problem with COMSOL and tried to explain it to the support staff. I describe the problem below:
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I am trying to model the effect of residual (tensile) stress in a fully clapmed circular membrane. The membrane is deflected by a torque applied to an inclusion at the center. Because of symmetry, I modeled a quarter of the membrane and used parametric solver to see the effect of the variable stress on the mode shape (please see the attached file). I entered the value for sigma_x and sigma_y in "intial stress" page of the subdomain settings. Now
- As you can see (postprocessing menu>Domain Plot Parameters>Line/Extrusion> plot the z-deflection for boundaries 2 and 8) the deflection curves are irregular at the boundaries. Why is that?
- It seems the membrane is getting more flexible which is counter intuitive. If the value of the sigma is positive does it mean a tensile or compresive stress in Comsol?
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The answer I got was:
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Unfortunately, it is not really physically accurate to have a constant initial stress. What happens is that the initial stress is not consistent with the boundary conditions and the interface between the softer and the stiffer material. This causes the solution to be somewhat strange. The solution is also mesh dependent; You will see that if you refine the mesh, the solution will be different.
Also, the deformation due to the initial stress is of the same magnitude as the deformation due to the torque. Thus, the increased deformation of the pre-stressed situation is not due to softening, but rather a superposition of the two deformations. You can see this by removing the torque completely and solving for just the initial stress.
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Well, I would like to see if anyone actually has a suggestion to solve this problem.
Nima
I had (and still have) a problem with COMSOL and tried to explain it to the support staff. I describe the problem below:
-------------------------------------------
I am trying to model the effect of residual (tensile) stress in a fully clapmed circular membrane. The membrane is deflected by a torque applied to an inclusion at the center. Because of symmetry, I modeled a quarter of the membrane and used parametric solver to see the effect of the variable stress on the mode shape (please see the attached file). I entered the value for sigma_x and sigma_y in "intial stress" page of the subdomain settings. Now
- As you can see (postprocessing menu>Domain Plot Parameters>Line/Extrusion> plot the z-deflection for boundaries 2 and 8) the deflection curves are irregular at the boundaries. Why is that?
- It seems the membrane is getting more flexible which is counter intuitive. If the value of the sigma is positive does it mean a tensile or compresive stress in Comsol?
--------------------------------------------
The answer I got was:
--------------------------------------------
Unfortunately, it is not really physically accurate to have a constant initial stress. What happens is that the initial stress is not consistent with the boundary conditions and the interface between the softer and the stiffer material. This causes the solution to be somewhat strange. The solution is also mesh dependent; You will see that if you refine the mesh, the solution will be different.
Also, the deformation due to the initial stress is of the same magnitude as the deformation due to the torque. Thus, the increased deformation of the pre-stressed situation is not due to softening, but rather a superposition of the two deformations. You can see this by removing the torque completely and solving for just the initial stress.
--------------------------------------------
Well, I would like to see if anyone actually has a suggestion to solve this problem.
Nima
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3 Replies Last Post Jun 20, 2010, 1:47 p.m. EDT
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Posted:
1 decade ago
usually positive sigma means tensile and negative compression.
I will try to have a look at your file later this week it is an interesting problem you ask. but I dont know if I will come up with anything useful tough...
jf
I will try to have a look at your file later this week it is an interesting problem you ask. but I dont know if I will come up with anything useful tough...
jf
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Posted:
1 decade ago
Well finally I found this back :)
Thanks Jean for your reply, did you get anything?
Really no one has ever tried to include initial stresses in a structure?
Thanks Jean for your reply, did you get anything?
Really no one has ever tried to include initial stresses in a structure?
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Posted:
1 decade ago
Hi
First of all its a force and not a torque, you apply, or have I missed something ?
Second, I would not apply the stress like that as initial "sigma" only on x and y, and identical in both material with a 1:100 young modulus ratio, I would rather apply a static analysis from an initial state to load your model, such as through thermal expansion/compression, this would give you a more realistic stress build-up and values, because I believe too, that yours is not 100% physical, expressed like that.
Have fun Comsoling
Ivar
First of all its a force and not a torque, you apply, or have I missed something ?
Second, I would not apply the stress like that as initial "sigma" only on x and y, and identical in both material with a 1:100 young modulus ratio, I would rather apply a static analysis from an initial state to load your model, such as through thermal expansion/compression, this would give you a more realistic stress build-up and values, because I believe too, that yours is not 100% physical, expressed like that.
Have fun Comsoling
Ivar