Mode shapes in experimental modal analysis

[GMarsh]

Hi,

I am doing experimental modal analysis of a cylindrical casing. I have 64 FRFs. I have curve fitted dominant modes and obtained a synthesised FRF. Now when I see the animation of mode shapes corresponding to each of the frequencies in FRF, I am finding a not-so-significant (amplitude wise) mode in FRF is having the highest vibration amplitude on relative scale.

To explain in detail a bit, I have nearly 65 modes in frequency range 1200 - 15000 Hz. As expected the initial bending modes are showing greater magnitude in FRF. But when I see mode shapes, a small mode at 4900 Hz is vibrating heavily. This is validated through experiment as well. In operational condition, 4900 Hz and its harmonics clearly stand out in FFT of vibration signal.

How to identify the strength of a mode from FRF ? And is it possible that a low amplitude mode in FRF has heavy vibrating amplitude in mode shape.

I am attaching the file with FRFs and mode shape.

Thank you in advance for your time and help.

Rgds
Geoff

 

[JoshPlumSE]

I don't know how to define the "strength of a mode" from frequency response functions.

I'm no expert. But, to me, this would be dependent very much on the frequency of the excitation force. Whether one mode dominates over others depends greatly on what your input forcing function is.

The mechanical / experimental world deals with vibrations in a very different sense than the typical structural engineer. Therefore, I'm not sure that you're going to get a whole lot of responses to this question in this forum... Most of us are just too used to dealing with the dynamics for earthquakes and civil structures... This is greatly simplified in comparison to aerospace structures or dynamic equipment.

 

[hetgen]

Josh is right; post your question on "Mechanical Acoustics/Vibration engineering Forum”
If you are lucky you will get detailed response from vibration experts like “GregLocock"…

 

[GMarsh]

Hi Joshplum, hetgen:

Thank you for your comments.

There are two issues here:

1. A mode with less amplitude in FRF showing highest relative vibration amplitude in mode shape.

2. Identification of mode that excites in operational condition.

The concept of input forcing function addresses the second issue. While I have some problems with this as well, presently I am focusing on first issue.

I have now posted this question in Mechanical Acoustics / Vibration Engineering Forum and hoping to receive some useful comments.

Thank you.

Regards
Geoff