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Re: < Damping Coefficient >

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On Oct 9, 2004, at 3:22 PM, Anantha Narayan C.K. wrote:

I want to know if there was a means of finding the damping coefficient
for the device based on these data.
In general, no. You might be able to fake it, by comparing the dynamic loading for a given displacement to the static loading for the same displacement. If you assume that the difference is due to damping (it isn't--the assumption requires that the inertia loading is small), you can determine the variation of this force with the imposed velocity. If the variation is more or less linear you can back calculate the damping coefficient for your test as the slope of the force-velocity curve. That's your damping coefficient, but it only applies to the test you ran.

This process is loaded with holes. First, if your test specimen and the loading doesn't resemble the prototype, there's no way to extrapolate the results. Unless the 'device' is actually a simple system the mass and stiffness of the system won't be apparent, so there's no way to determine the system stiffness and mass and therefore no way to figure the critical damping ratio, As a result you really won't have characterized the system, and you can't apply the data to anything useful. In the real world viscous damping is an idealization--the actual energy loss will vary with maximum deformation and frequency, so again you can't generalize the damping coefficient, beyond the test whose results you already know.

The very best way to get an idea of the damping involved is to do a sine sweep testing, which will give you an idea of the load and frequency dependence. You can do a pluck test and measure the log decrement, which will give you an idea of the damping ratio for the the dominant modes, but not for anything else. You can get a handle on the displacement (and stress) variation by plucking the 'device' harder or more lightly.

Christopher Wright P.E. |"They couldn't hit an elephant at
chrisw(--nospam--at)    | this distance" (last words of Gen.
...................................| John Sedgwick, Spotsylvania 1864)

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