Viscoelastic granular dampers under low amplitude vibration

This paper considers dampers comprising collections of viscoelastic particles that are subjected to vibrations whose amplitude is such that slip between particles is negligible. Energy dissipation occurs primarily by viscoelastic processes within each particle and is maximized when standing waves are set up in the granular medium. In this work, the medium is represented as an equivalent viscoelastic solid and predictions of performance employ models constructed using standard finite element software. Two numerical approaches are considered: one uses the Direct Frequency Response and the other uses standard modal analysis in conjunction with analytical expressions for energy dissipation based on the wave equation. The performance of these prediction techniques is compared with measured behavior from experiments on a box-shaped structure and a hollow composite tube assembly. The computational efficiency of the modal technique allowed a brief investigation of the effects of uncertainties in the actual nature of the granular arrangement. Results show that both prediction methods give a reasonable level of accuracy. Differences between predicted and measured behavior are shown to be of the same order as the uncertainty in the prediction itself. For the systems considered, it is shown that the methods are appropriate for acceleration amplitudes up to almost that of gravity.