Modal Shape from Experimental FRF 3

[Xtina]

Hi all, I am performing an experimental modal analysis of a beam and I am trying to get the mode shapes from the FRF measured. I have read in many articles and books that the mode shapes (unscaled) may be obtained from the peak amplitudes of the imaginary part of the FRF, as in lightly damped structures the imaginary part reaches maximum values while the real part gets zero values at the resonant frequency. My problem comes when I plot the real and the imaginary parts of the FRF, it seems that they are interchanged, so I can observe peaks in the real part that provide the correct modal shapes and values of zero in the real part for the resonant frequencies. I have also found a thesis that states that for structures with small damping, the undamped modal shapes match with the real part of the FRF. I find the information is contradictory, can someone help me? and what is the reason why any of the parts (real/imaginary) give the modal shapes?

 

[GregLocock]

No, there are lots of methods for dealing with overlapping modes, once you have identified the resonant frequencies.

However every mode shape fitter I have ever seen is happy to create semi infinite number of modes to explain every little bump in every FRF, which is not helpful. The art now is to balance the desire to be able to resynthesise all the FRFs accurately, vs a believable number of modes.

Ideas like bending modes and torsion modes are human constructs, structures don't differentiate between them, unless the structure is very simple or carefully modified. For instance 30 years back cars rarely had a pure torsion mode and a pure bending mode, they typically had a front end torsion with lots of bending, and a rear end torsion with lots of bending, or something like that. Now they are designed with FEA, and we get the pure modes, because that's what we ask for.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[hacksaw]

always a sound bit of advice Mr. Greg