Eigen frequency analysis of cantilever beam

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Hi, I want to simulate a simple rectangular cantilever structure and measure the eigen frequencies. I want to measure the change in the eigen frequency upon applying load,and plot the graph for the eigen frequency vs vibration/displacement of the cantilever. But I must have been doing something wrong because the eigen frequency remains the same even if I add the boundary load. For physics, I added the solid mechanics and for study, the eigen frequency. Can someone please help and share your advice on this?


5 Replies Last Post Oct 13, 2022, 2:22 a.m. EDT
Henrik Sönnerlind COMSOL Employee

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Posted: 2 months ago Sep 28, 2022, 2:30 p.m. EDT

You may get some help by looking at the tutorial model bracket_eigenfrequency in Application Libraries.

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Henrik Sönnerlind
COMSOL
You may get some help by looking at the tutorial model bracket_eigenfrequency in Application Libraries.

Acculution ApS Certified Consultant

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Posted: 2 months ago Sep 28, 2022, 2:47 p.m. EDT

Pretty sure loads are omitted by the solver when you run an eigenvalue problem. As it well should be, since you don't have sources in a eigenvalue problem.

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René Christensen, PhD
Acculution ApS
www.acculution.com
info@acculution.com
Pretty sure loads are omitted by the solver when you run an eigenvalue problem. As it well should be, since you don't have sources in a eigenvalue problem.

Henrik Sönnerlind COMSOL Employee

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Posted: 2 months ago Sep 28, 2022, 4:02 p.m. EDT

René,
I think we are talking about a prestressed eigenfrequnecy analysis here since load dependency is to be investigated (that is, the second study in the tutorial I suggested). But you are right in that loads as such are ignored in an eigenfrequency analysis as long as they are not 'follower loads' like pressure.

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Henrik Sönnerlind
COMSOL
René, I think we are talking about a prestressed eigenfrequnecy analysis here since load dependency is to be investigated (that is, the second study in the tutorial I suggested). But you are right in that loads as such are ignored in an eigenfrequency analysis as long as they are not 'follower loads' like pressure.

Acculution ApS Certified Consultant

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Posted: 2 months ago Oct 3, 2022, 5:25 a.m. EDT

Ah yes Henrik, I think you are right. Thought it was a time-varying load of sorts.

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René Christensen, PhD
Acculution ApS
www.acculution.com
info@acculution.com
Ah yes Henrik, I think you are right. Thought it was a time-varying load of sorts.

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Posted: 2 months ago Oct 13, 2022, 2:22 a.m. EDT
Updated: 2 months ago Oct 13, 2022, 8:05 a.m. EDT

Thank you for your replies. It was truly helpful. I have one more question. I added a block at the free end of the cantilever, assigned a blank material with poisson ratio, young modulus and density. Since density (rho) is mass divided by the volume, I was able to give different mass to calculate the rho value, which in turn gave different value of eigen frequency, without making use of any body/boundary load. In the 'eigen frequency, prestressed study' , the load is in terms of force, but I want calculate the change in terms of mass. If I want to measure change in the eigen frequency with different mass, is this the right way to do it? Without making use of any body load. (Since different mass gives differnt values of rho which causes a change in the eigen frequency) The same change in eigen value by changing rho value was also obtained for the 'eigen frequency' where load is not applied. I am confused on how to go about it. Please share your valuable advice/suggestions. Thank you.

Thank you for your replies. It was truly helpful. I have one more question. I added a block at the free end of the cantilever, assigned a blank material with poisson ratio, young modulus and density. Since density (rho) is mass divided by the volume, I was able to give different mass to calculate the rho value, which in turn gave different value of eigen frequency, without making use of any body/boundary load. In the 'eigen frequency, prestressed study' , the load is in terms of force, but I want calculate the change in terms of mass. If I want to measure change in the eigen frequency with different mass, is this the right way to do it? Without making use of any body load. (Since different mass gives differnt values of rho which causes a change in the eigen frequency) The same change in eigen value by changing rho value was also obtained for the 'eigen frequency' where load is not applied. I am confused on how to go about it. Please share your valuable advice/suggestions. Thank you.

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