Natural frequency of two different materials in one system

[mecmert]

Hi all,
I am working on a truck part which consists of one composite and one steel hollow part. I have the critical range of vibration frequency and I am able to calculate the frequency of steel part. However, composite part will be designed and mounted to the steel part. Therefore, I try to get some values to decide on the content of the composite part. I am confused about the natural frequency of the composite part. Should it be around the natural frequency of steel part or should complete system's frequency be out of the critical range?
Thanks for your help.

 

[JBlack68]

if you can have the natural frequencies of both parts out of the critical range then you are less likely to exist them in a (random) vibration environment

 

[mecmert]

Thanks for your reply JXB0809. Do you think whether mounted parts should have similar natural frequencies separately? Or does not it make any sense?

 

[drawoh]

mecmert,

You have two vibration problems. The composite part has its own vibration modes. It sounds like your steel part is a spring with the mass of your composite part fixed to it, and this has a natural circular frequency [ω]=sqrt(k/m).

--
JHG

 

[GregLocock]

If you have two separate SDOF systems with their own natural frequencies, when you bolt them together then the frequencies will change. I realise you are working on top secret components that nobody else has ever seen, but if you could give us some idea of what you are actually trying to do then I dare say I can be more helpful.

Cheers

Greg Locock


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[mecmert]

Basically there is a steel pipe. Composite part will be overmolded on the steel pipe to connect both parts instead of using bolts. Then connected parts will be assembled to the system with the connection interface on the composite part. Since truck is exposed to harsh vibration due to rough road and part carries some weight which causes bending, I try to be sure that new design can withstand these contidions theoretically. Therefore, as a beginning, I decided to make some calculation related to natural frequencies to limit the required properties of the composite. Then I will try to plan next steps.

 

[drawoh]

mecmert,

Here is another failure scenario.

You have two pieces connected by a moulded composite fitting. The pieces are bolted at opposite ends. If the opposite ends move, how strong is your moulded composite to metal interface? How flexible is your composite?

--
JHG

 

[IRstuff]

From a philosophical perspective, I don't see why one would intentionally design an operational system where the natural frequencies of the subcomponents are intentionally aligned, except for the case where you actually want violent oscillations.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[JBlack68]

agree with IRstuff. The key input to consider/bear in mind is the freq. range of operation. If say it's 10-100Hz then it doesn't matter if the natural frequencies of the subcomponents are identical what is the chance of that!) if these are much higher than 100Hz (say 400Hz). Mecmert would need to clarify this.

 

[GregLocock]

" I don't see why one would intentionally design an operational system where the natural frequencies of the subcomponents are intentionally aligned"

That is of course the exact ideal tune for a harmonic damper.

Cheers

Greg Locock


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[rob768]

If i understand well, when they are moulded together, it is one stiffness, and one mass, it won't behave as two different systems.