FRFs comparison 1

[GMarsh]

Hi all,

I am comparing two FRFs acquired on a cantilever with two different holding systems. I am comparing in receptance format so that I can find out the compliance (or stiffness) of holding system.

System A shows lower amplitude than System B on receptance plot, indicating that System A holding stiffness is better than that of System B.

But I also noticed that the cantilever natural frequency is lower while holding with System A. How can this be explained ? I am expecting an increase in natural frequency as well (due to increased stiffness). What does the change in natural frequency indicate in terms of holding system effectiveness ?

I am attaching the FRF picture here.

Thank you and Merry Christmas !!

Geoff

 

[GregLocock]

I don't much like the red curve - why does it drop below the stiffness line ? why haven't you included all the data down to 0 Hz? why is the second mode of the red one so heavily damped?

Cheers

Greg Locock


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[GMarsh]

Hi Greg,

Thank you for the response. Actually this is the data converted to receptance from inertance. So I see a big spike at 0 Hz (could be due to DC component) and hence while displaying I shifted the graph a bit !! Sorry for this. Please find attached full graph.

I am not sure about the second mode. Actually these are two FRFs acquired on a cantilever with two different holding systems. I am trying to evaluate which one is holding better. So I did a quick tap test on both configurations.

From displacement amplitude, I can see the system with blue color FRF is good. But I am not understanding why frequency shifted to left. Generally better the stiffness of holding, higher should be frequency of object that is held. Is this true ?

Thank you very much.

Geoff

 

[GregLocock]

Last bit first yes, generally, but there is at least one special case where adding fixity to a system apparently reduces the first measured frequency.

If that is driving point data then you've definitely got some problems with test technique, phase should be in a 180 degree band.





Cheers

Greg Locock


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[GMarsh]

Hi Greg,

Thank you so much for the reply. Can you elaborate a bit about that 'special case' ? I see it can happen if the fixity is so strong that it acts like mass addition, then wn = (k/m)^0.5 and this may lead to reduction in natural frequency. Could there be any other possibility ?

Reg phase, I have that doubt as well ! But I didn't want to mix up the things. Actually the inertance plot shows phase of -180 to +180 deg. Please see attached figure. But when I convert to receptance, this is happening. I am not sure why. I am puzzled at it as well. Any comments ?

Thanks a lot. Merry Christmas.

Geoff

 

[GregLocock]

Hmm, something very screwy there, the phase should just be offset by 180 degrees after two integrations. I suspect you may have radians rather than degrees in your receptance plots.

Can you do driving point measurements rather than these?

Cheers

Greg Locock


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[GMarsh]

Hi Greg,

Thank you for the response. I also believe that the software is making some mistake in phase calculation while converting from inertance to receptance.

Unfortunately this is the only measurement I have for this structure. I just did a quick check to compare effectiveness of holding. However I will now do a drive point measurement as well.

Can you comment on that special case of fixity which you were referring to ? Does my assumption of strong fixing adds mass and hence reduction in natural frequency correct ?

Thank you.

Geoff

 

[GregLocock]

A free free beam has a higher first flexural mode than a simply supported one. Normally one would expect that adding constraints would stiffen a structure and increase resonant frequencies.

The reason is that as you've suggested, coupling an infinite mass to the beam has a larger effect than the constraints. The apparent paradox is resolved when you bear in mind that the free free beam's first mode is at 0 Hz.

I suggest you do a proper modal on the structure to get an idea of what is going on, looking at single FRFs is too confusing.



Cheers

Greg Locock


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

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[GMarsh]

Thank you Greg. I am now planning to do a complete modal test on the structure.

Geoff