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Aug 29, 2018
06:35 AM

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Aug 29, 2018
06:35 AM

Restrict modal analysis

Hi,

I have a Beam idealization helt at three points, at the ends and in the middle. I then perform a modal analysis on this. All is fine.

Assuming the hold point in the middle (in real life) fails such that the analysis shows a lower first mode in say Y direction and thus higher displacement. However, the beam, in real life, is restricted such that it can be displaced in the +Y direction, but will "hit" something in the -Y direction and thus not be allowed to move there. How do I simulate this? I was thinking first with a spring, but how can it be restricted such that it doesn't affect the beam when it is on the +Y side and fully stops it when the beam is on the -Y side?

Maybe there is a better way of doing this?

Thanks

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Aug 30, 2018
05:23 AM

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Aug 30, 2018
05:23 AM

i don't have workaround

your problem is non-linear, but

modal analysis in simulate is linear.

Aug 30, 2018
08:53 AM

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Aug 30, 2018
08:53 AM

Yes this is a non-linear case. I think it is the simplest non-linear because it is the sum of two linear systems. (piece-wise-linear?)

Your model has two different modal states with their own unique modes. If you know the exact damping and can predict the amplitudes, and come up with the "duty cycle" or % of time each is true my theory is that you can add the two systems using superposition. You would need to at a minimum run modal analysis on both states.

It depends on what you need as output. If it is just frequencies then you can list the frequencies from both systems as possible modes. If you need a FRF (frequency response function) then you need to determine how much to weight each state. This will be difficult because it will depend on the input and output amplitudes which include the illusive damping factors.

Good luck, I hope this helps some.