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.

Regards
Geoff

 

[Achy47]

Hi Geoff,

Thank you for sharing your findings and data. It is quite common to observe a discrepancy between the amplitude of a mode in the frequency response function (FRF) and its corresponding mode shape. This is because the FRF is a measure of the system's response to a specific excitation frequency, while the mode shape represents the shape and amplitude of vibration at a specific frequency.

In order to identify the strength of a mode from the FRF, you can look at the peak amplitude of the mode in the FRF plot. This will give you an idea of the magnitude of the mode compared to others in the frequency range. However, it is important to note that the FRF is not a direct measure of the mode's strength, but rather an indication of its influence on the overall system response.

It is definitely possible for a low amplitude mode in the FRF to have a high amplitude in the mode shape. This can happen due to several reasons, such as the mode being excited by a specific harmonic of the excitation frequency, or the mode being highly localized at a specific location in the structure.

In order to validate your findings, it is important to compare your experimental results with analytical or numerical models. This will help you understand the behavior of your system and provide a more comprehensive understanding of the observed mode strengths.

I hope this helps. Best of luck with your analysis.

Regards,