Hello.
I have question about the FRF result.
I performed the vibration test and got some FRF data.
As can be seen the following figure, the FRF result is oscillated from 70Hz to 75Hz.
This phenomenon seem to be shown like noise signal. But I have guessed, there are so many modes between 70 and 75Hz.
Why does this phenomenon occur in FRF?
From the given FRF data, it is apparent that the measured signal shows some noise which affects the signal amplitude and/or phase characteristics.
However, another possible reason could be due to high non-linearities exhibited by the system and damping forces at that frequency range.
Mode shapes are different from this phenomena since they are unique for an entire frequency range and exhibit finite amplitude . Resonant mode shapes, on other hand usually show an infinite amplitude at specific frequencies.
Does the accelerometer sit truly fixed or is it possible that it rattles?
How about the cable?
Interference in some sort can give results of dips and peaks like this. But they are very dense. Might be related to something slow in the time history?
If it is possible, you can make a simulation by using FEM and make a comparison. this could help you to know if this phenomena is a noise, a measurement error or the real behaviour of the structure
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