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# fundamental frequency

## fundamental frequency

(OP)
Good morning all,
I have been focusing only on the first (lowest) frequency and calculate stress and deflection results based on that value as Steinberg indicates that this is where the highest deflection takes place. Why is this the case? If I wanted to calculate the results for higher modes, how would I take the higher frequency value into account? Is the energy dissipated (into heat?) as the frequency is increased? I have not found anything in his book (or others) that addresses this aspect.

Thanks,
Bob

### RE: fundamental frequency

That is, in a general sense, a really good question. Tom Irvine's paper is excellent. This is my answer:

The response of a linear structure to a given transient excitation (which is the most common damage problem) is the sum of the modeshapes. When you hit something most of the response is at the lowest frequency mode, unless you happen to hit the structure at a node for that modeshape. The lower the frequency the greater the displacement and hence strain for a given acceleration.

However, if you are looking at strongly periodic environments, such as I4 engines, where the excitation actually increases with frequency(up to a point), the first few modes may be completely irrelevant, the mode only really matters if it gets excited, so we use what is called a modal alignment chart that maps mode shapes and excitations.

Cheers

Greg Locock

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