I am performing a frequency response analysis of a structure. The bode plot (frequency vs displacement plot) doe not show peaks at all resonant frequencies only two of them ( I have extracted 20 modes). while I am convinced that this is acceptable, can any one cite some literature regarding this. Thanks in advance.
Regards
A Bode diagram (amplitude and phase) is a graphical representation of the transfer function between an input and an output. If you want all the modes to appear you need to properly excite them.
If in the frequency range of your analysis you expect 20 modes and you observe only 2, I would suggest reconsidering the locations of your exciting points and/or your measurement points as they might be placed in some nodal points.
Hope this helps.
Best,
G.
This book by Chris Fuller discusses structural vibration in detail. https://books.google.com/books?hl=en&lr=&id=HGP4iGWhAdEC&oi=fnd&pg=PP2&dq=info:OhZruiwzwAAJ:scholar.google.com&ots=z8QediIPMm&sig=gEZ1agFmb9LmD4cgM8LjpfZcVvE#v=onepage&q&f=false
In addition to the Dr. Pennisi's answer, I would like to suggest you to re-perform the frequency response analysis of that structure with a finer frequency step. Also, It is necessary to plot the diagram of frequency vs displacement in log-scale which can give a better indication of modes with low-magnitude.
Best
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