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Eigenvalues of prestressed membrane

Posted 25.06.2012, 15:05 GMT-4 MEMS & Nanotechnology, MEMS & Piezoelectric Devices Version 4.2a 2 Replies

Peter Lerwick
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Hi all,
I want to model how the resonant frequencies of a simple rectangular membrane change with pressure differential across the two sides.

For the life of me can't get the eigenfrequencies to change by applying an initial face load (or any kind of load to be honest).

I've found quite a few discussions on 'stress stiffening' mentioning the solver sequence and how you should first have a stationary study and then a eigenfrequency study. The solution of a stationary study needs to be used as the 'linearization point' of a a eigenfrequency study.

I have tried to implement this advice so far without being able to affect the eigenfrequencies. I've attached my model written in 4.2a, if anyone can help would be greatly appreciated.
Cheers
Peter

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2 Replies Last Post 26.06.2012, 11:14 GMT-4
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 25.06.2012, 15:56 GMT-4
Hi

first of all why 2 Face loads ?

And why not use the Load type Pressure and use "atmos_press" there then the normal direction is looked after for you by COMSOL

(by the way 1[atm] corresponds to your pressure it's already defined in COMSOL)

then if you go to the main shell physics, and replace the shell thickness value from the default 0.01[m] to your value of your "thickness" you will get a different result I assume.
You should always check the result from the stationary study, add a Volume plot of the surface and look at the displacement under the pressure load (select the correct Data-Set 1) then you would have noticed that the deflection was (and remain) really small.

If oyu now aply sour total pressure (you can do it by adding the 2nd load BC but use "pressure" you will get a sall change in frequency, but its really "small" as you deformation is small, but you have about 1 mm^2 of surface, hence 0.1 N force and your window is quite thick for a 10 g force

--
Good luck
Ivar
Hi first of all why 2 Face loads ? And why not use the Load type Pressure and use "atmos_press" there then the normal direction is looked after for you by COMSOL (by the way 1[atm] corresponds to your pressure it's already defined in COMSOL) then if you go to the main shell physics, and replace the shell thickness value from the default 0.01[m] to your value of your "thickness" you will get a different result I assume. You should always check the result from the stationary study, add a Volume plot of the surface and look at the displacement under the pressure load (select the correct Data-Set 1) then you would have noticed that the deflection was (and remain) really small. If oyu now aply sour total pressure (you can do it by adding the 2nd load BC but use "pressure" you will get a sall change in frequency, but its really "small" as you deformation is small, but you have about 1 mm^2 of surface, hence 0.1 N force and your window is quite thick for a 10 g force -- Good luck Ivar
Peter Lerwick
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Posted: 1 decade ago 26.06.2012, 11:14 GMT-4
Hi Ivar,
Thanks a lot for the tip, yes it was the thickness of the membrane that I had wrong. The eigenvalues do increase a bit with increasing pressure.
Cheers
Peter
Hi Ivar, Thanks a lot for the tip, yes it was the thickness of the membrane that I had wrong. The eigenvalues do increase a bit with increasing pressure. Cheers Peter

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