J. Robert Fricke
Mark A. Hayner
MIT, Rm. 5-218, 77 Massachusetts Ave., Cambridge, MA 02139
Granular materials have been used for many years to damp structural
vibrations. Often these treatments incorporate sand or lead shot. Both are heavy
and provide some of their damping effect through mass loading. This paper
discusses the damping properties of a low-density granular material, 3M glass
microbubbles (tradename Scothlite). A paste was made using water and Scothlite
and placed in an aluminum free-free beam. Resonant peaks of the beam were
reduced by 10 dB, and in some cases more. The specific gravity of the Scothlite
is about 0.1, so mass loading effects cannot account for the damping. Further,
glass is not normally considered to be highly viscoelastic at room temperature.
Rather, the attenuation mechanism is thought to be activated by the low bulk
sound speed of the granular fill. With a low sound speed, the wavelength is
short, and incipient attenuation in the fill becomes important. The mechanism is
a combination of four possibilities: (1) small but finite intrinsic material
attenuation, (2) frictional losses between rubbing grains, (3) nonlinear
hysteresis effects due to the Hertzian contact and deformation relaxation, and
(4) viscous losses of the fluid flow between grains. [Research sponsored by
ARPA/ONR.] [sup