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Eigenfrequency analysis with load

Posted Apr 11, 2011, 10:39 a.m. EDT Studies & Solvers, Structural Mechanics Version 3.5a 2 Replies

Alessandro Tocchio
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Hi to all,

I am new in Comsol ( Comsol 3.5a ) and I am facing problem to find out eigenmodes in a simple structure WITH A LOAD APPLIED on a boundary. It will be very helpful if someone can give some advice.

I am trying to find out eigenfrequencies of Poly-Si surface(2D) in vacuum with a load applied , so to get acquainted with that kind of problem I started solving a very simple problem , I drew a rectangle of poly-Si ( width = 500e-6 , height= 5e-6 ) , I fixed the upper and lower boundary and after setting a load on one of free boundary I solved the model.

I get as result same eigenfrequencies I obtain when I solve the model without load.

I read others discussions and I understood it's not the right way, so I tried performing a static analysis , where I apply load , creating a moving mesh ( with all subdomain settings ... ) and then performing a eigenfrequency analysis on the deformed mesh ( after creating geometry from mesh..) .

When I solve results change but not as I expected, so I would like to know if , this time, it's the right way.

Thanks in advance for the answer.

Kind regards

Alessandro
2 Replies Last Post Apr 11, 2011, 2:48 p.m. EDT
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Apr 11, 2011, 11:19 a.m. EDT
Hi

in 3.5 you need to use Matlab to save the static load case and use it as a linearizion point for the eigenfrequency analysis (no need for matlab in V4). See the older threads on this subject

--
Good luck
Ivar
Hi in 3.5 you need to use Matlab to save the static load case and use it as a linearizion point for the eigenfrequency analysis (no need for matlab in V4). See the older threads on this subject -- Good luck Ivar
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago Apr 11, 2011, 2:48 p.m. EDT
Hi Alessandro,

The mesh movement is not the main reason frequencies of structures change with load. The main reasons are usually geometrically nonlinear effects, changes in material stiffness (such as plasticity) and changes in boundary conditions (such as contact). You should include geometric nonlinearity in the material model then perform the static analysis followed by the eigenfrequency analysis. The “Vibrating String” example in the COMSOL documentation is a good starting point, even though in this case the initial stress is explicitly provided instead of resulting from a static analysis. That’s all based on V4.1, but I guess there is an equivalent procedure using V3.5.

Nagi Elabbasi
Veryst Engineering
Hi Alessandro, The mesh movement is not the main reason frequencies of structures change with load. The main reasons are usually geometrically nonlinear effects, changes in material stiffness (such as plasticity) and changes in boundary conditions (such as contact). You should include geometric nonlinearity in the material model then perform the static analysis followed by the eigenfrequency analysis. The “Vibrating String” example in the COMSOL documentation is a good starting point, even though in this case the initial stress is explicitly provided instead of resulting from a static analysis. That’s all based on V4.1, but I guess there is an equivalent procedure using V3.5. Nagi Elabbasi Veryst Engineering

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