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re: prestressed modal analysis
Posted Jul 21, 2009, 7:09 a.m. EDT 3 Replies
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i wanna do a prestressed modal analysis on mems structure. (i.e) do a static deformation first and include the stress for the eigen value analysis.
the inclusion of stress , i am not able to do . can u help me ??
in ansys there is an option of PSTRES,ON . which take the values of stress from the static deformation and do cont with maodal analysis.
Here is anything like that, if so help me....
the inclusion of stress , i am not able to do . can u help me ??
in ansys there is an option of PSTRES,ON . which take the values of stress from the static deformation and do cont with maodal analysis.
Here is anything like that, if so help me....
3 Replies Last Post Aug 5, 2009, 11:48 a.m. EDT
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Posted:
1 decade ago
Dear Shyamsananth,
when you perform the eigenfrequency analysis go in "Application Mode properties" and set the "large deformation" option to "on".
Then restart the analysis from the static results.
Comsol contains some tutorials about this problem. Look at "Model library/Mems Module/Actuator Models/Resonator folded 2d.
Hi.
Alessandro
when you perform the eigenfrequency analysis go in "Application Mode properties" and set the "large deformation" option to "on".
Then restart the analysis from the static results.
Comsol contains some tutorials about this problem. Look at "Model library/Mems Module/Actuator Models/Resonator folded 2d.
Hi.
Alessandro
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Posted:
1 decade ago
Hi
I took a look at "Model library/Mems Module/Actuator Models/Resonator folded 2d" and I am trying to comprehend the following in the example.
1. First I solve for Eigenvalue when I don't have any load (thermal load). I get somewhere around 14KHz
2. Next I solve for Eigenvalue when I have thermal load (no initial value is taken i.e. no prestressing). I get somewhere around 34KHz.
Now the question is this. How would the loading play a role in Eigen analysis?
I am newbie to FEM as well as Comsol. So please bare with me.
Regards
Shyam Sanath
I took a look at "Model library/Mems Module/Actuator Models/Resonator folded 2d" and I am trying to comprehend the following in the example.
1. First I solve for Eigenvalue when I don't have any load (thermal load). I get somewhere around 14KHz
2. Next I solve for Eigenvalue when I have thermal load (no initial value is taken i.e. no prestressing). I get somewhere around 34KHz.
Now the question is this. How would the loading play a role in Eigen analysis?
I am newbie to FEM as well as Comsol. So please bare with me.
Regards
Shyam Sanath
Please login with a confirmed email address before reporting spam
Posted:
1 decade ago
Hi Shyam Sanath,
the way in which loads are included in an eigenfreq analysis depends on some contributions on the stiffness matrix of the problem.
If you use the "large displacements" option, you are activating a definition of strain and stresses which is more general than the usual. Large displacements corresponds to "finite strain" and not "infinitesimal strain" as in the case of the standard definition of strain.
Enabling that option, the stiffness matrix of your problem includes the effect of the deformation of the structure and therefore, a change of stiffness, generates a change of the resonance frequencies.
The effect of a thermal load (which is a pre-strain, i.e. something different by a pre-stress but yet representing a initially-deformed strate) in the tutorial is first computed by a static analysis and then in an eigenfrequency analysis which uses the previous step as linearization point.
With the large deformation option, you see a quite pronounced shift of the resonance frequencies.
In a modeling problem (i.e. the problem you want to solve) you have always to wonder if the hypothesis of intinitesimal strain is violated or not. According to your answer you choose what is the best option.
Pay attention that, according to my experience, the large strain option does not include the infinitesimal strain option as particular case.
This is something that I have still to verify from a mathematical point of view...
Good luck!
Alessandro
the way in which loads are included in an eigenfreq analysis depends on some contributions on the stiffness matrix of the problem.
If you use the "large displacements" option, you are activating a definition of strain and stresses which is more general than the usual. Large displacements corresponds to "finite strain" and not "infinitesimal strain" as in the case of the standard definition of strain.
Enabling that option, the stiffness matrix of your problem includes the effect of the deformation of the structure and therefore, a change of stiffness, generates a change of the resonance frequencies.
The effect of a thermal load (which is a pre-strain, i.e. something different by a pre-stress but yet representing a initially-deformed strate) in the tutorial is first computed by a static analysis and then in an eigenfrequency analysis which uses the previous step as linearization point.
With the large deformation option, you see a quite pronounced shift of the resonance frequencies.
In a modeling problem (i.e. the problem you want to solve) you have always to wonder if the hypothesis of intinitesimal strain is violated or not. According to your answer you choose what is the best option.
Pay attention that, according to my experience, the large strain option does not include the infinitesimal strain option as particular case.
This is something that I have still to verify from a mathematical point of view...
Good luck!
Alessandro