I recorded the displacement response of a concrete beam. I excited it with harmonic excitation with 30Hz frequency (less than resonant frequency which is 43Hz).
In the figure attached, the graph shown in red rectangle is the frequency domain response after fft. Non-linear analysis was performed. The response in frequency domain should have only 1 peak at 30Hz, i don't know why the second peak is showing at ~37Hz.
I'm very new to signal processing, so please forgive my lack of knowledge in this regard. And guide me with the possible reason for the second peak.
I am also attaching the displacement response in excel sheet.
I will really appreciate your help
Thank you
~usman
Try running a modal analysis and watch the first couple of modes. Perhaps what you are seeing a longitudinal, rotational mode or something like that (do not know the dimensions or your setup, so just guessing. Or just watch (animate) the displacements that you already have, and check what is going on.
Thank you Rene Christensen, I will do it. and will come back to you with developments.
If you are recording after the input (free vibration) you can be seen the natural frequency modified due to some softening (is a nonlinear analysis). Also, to check the modes is a good idea. And a final guess is that you have to considered that the response of a system subjected to an harmonic input is the sum of a transient and a stationary state. With the frequency of the system and the input respectively. So, it is not wear to find two frequencies assuming that you are applying the FFT to the entire signal. You can try with short FFT to find when do you change from one predominant frequency to the other... or just make the input longer so you will be sure that the transient will disappear.
- Badges
- Science topic