modal damping

[marouaha]

Hello,
Can'I using a modal analysis determine the effect of friction on the modal damping value of a structure?
or I must apply a sinusoidal force and guettig the the damping value correspending to the modal frequency.
thx

 

[GregLocock]

Friction is a very bad thing to have in a linear modal analysis, because it adds third, fifth etc harmonics. However I see no particular reason why, if you can cope with the lousy data, that it cannot be analysed like damping, so long as you make sure the excitation force is enough to always overcome stiction. So I'd be inclined to use swept sine excitation rather than random.

Cheers

Greg Locock


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[amanuensis]

Hi,

So you need a formula between the two kind of damping...
Et voila!
ξ[sub]friction[/sub]=(2η/π)[sup]2[/sup](1-ω[sup]2[/sup]/ω[sub]0[/sub][sup]2[/sup])[sup]2[/sup]/(ω[sup]2[/sup]/ω[sub]0[/sub][sup]2[/sup][ω[sup]4[/sup]/ω[sub]0[/sub][sup]4[/sup]-(4η/π)[sup]2[/sup]])

 

[marouaha]

Thx,
eta is the modal damping?? can you submit please the document or the reference for this equation?
Cheers,

 

[amanuensis]

The formula has been extracted from the "SHOCK AND VIBRATION HANDBOOK" from Harris & Crede, chapter 30.

 

[marouaha]

Thx amanuensis,
but in this book i didn't found this equation ξfriction
and the chapter 30 is about VIBRATION OF STRUCTURES INDUCED BY FLUID FLOW

 

[amanuensis]

I have got the second edition of the book.
The chapter 30 is "Theory of vibration isolation" and the equation is in the section "Rigidly connected coulomb damper"