Low-density granular fill for damping structural vibrations

Abstract

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 hystere viscous losses of the fluid flow between grains. [Research sponsored by ARPA/ONR.] a)E-mail: fricke@mit.edu