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Mar 21, 2018
11:12 AM

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Mar 21, 2018
11:12 AM

Finding transmissibility at a resonant frequency in a structure

I am trying to understand the most efficient way to determine the magnitude of transmissibility at key points within a structure. The structures I design are subject to random vibration base excitation, and so I would like to predict their response. If I can do a “virtual sine sweep” in Creo Simulation, I can optimize the structure without having to do the physical sine sweep testing on a shaker table, and thus save design time.

There appears to be three ways to find the transmissibility using simulation:

1) Run a dynamic random analysis and observe the response at key points (using Measures) as compared to the response at the supports. The difficulty is in interpreting the Measures responses because they are in terms of G-rms²/ Hz.

2) Run a dynamic time analysis in the same way as the dynamic random. Results are in acceleration vs time, and must be converted to acceleration vs frequency via FFT to locate frequencies with the highest amplitude response.

3) Run a dynamic frequency analysis using the same technique as the dynamic random. Results are acceleration vs frequency plots that are same as you would get from a sine sweep.

I believe the dynamic frequency analysis is best for finding transmissibility magnitudes. But I have a couple of questions.

- The help documentation states that the dynamic frequency loads are “oscillating at different frequencies”. Do I interpret the oscillations to be sinusoidal outputs at the specified magnitude for each frequency in the range specified (see attached picture)? In other words, if I set up a table for the base acceleration “Amplitude” function dialog box, and I input a starting frequency and associated amplitude, and an ending frequency and amplitude, the output would be the equivalent of a linear sine sweep between those two frequencies? I don’t know of a way to view the dynamic frequency analysis base excitation (at the supports) in the time domain as a sanity check.
- If I were to use the dynamic random analysis results to determine transmissibility, do I apply Miles Equation to the g²/Hz output from the measures graph to find the G-rms value for the point of interest at a particular frequency? I could then compare that to the G-rms value at the support for that same frequency to determine the transmissibility?

2 REPLIES 2

Mar 21, 2018
02:21 PM

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Mar 21, 2018
02:21 PM

I may not answer all your questions but I made a try at this.

See attached information and demo CREO part on forced frequency response.

The difficulty with correlation will be the damping assumptions.

Mar 22, 2018
11:13 AM

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Mar 22, 2018
11:13 AM

Sweetpeahub,

It was very helpful to see your FFR document. It perfectly verified the process I was trying to describe.

I get frustrated with the lack of background in the help files. Many terms have only a bare minimum of information, and this forces me to look for ways to sanity check every step.

Thanks!

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