THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

20.6 Techniques for Vibration Control 609and cobalt alloys exhibit such properties but these materials are generally too costlyto use as structural materials and they may not meet the strength criteria.Damping MechanismsFigure 20.12 shows three types of damping mechanisms, namely, the tuned damper,free viscoelastic layer, and constrained viscoelastic layer.The tuned damper of Figure 20.12(a) consists of a mass attached to a pointof vibration through a spring and dashpot or a viscoelastic spring. This device isnot very useful because it functions well at only a single frequency or in a verynarrow frequency band. Moreover, any change in temperature is likely to changethe tuning frequency of the damper.The other two mechanisms of Figure 20.12 entail adding viscoelastic layers toa structure that is to be damped. When a structure consisting of different layerof materials undergoes bending, the layers will extend or deform in shear. Theresultant deformation causes energy dissipation, a phenomenon that constitutesthe basis for the two viscoelastic mechanisms.In the mechanism of Figure 20.12(b), a layer of free (i.e., uncovered) viscoelasticlayer is bonded to the main structure. The damping material thickness should beFigure 20.12. Different types of damping mechanisms: (a) tuned damper, (b) free viscoelasticlayer, and (c) constrained viscoelastic layer.