Extensional and Constrained Layer Damping

Two types of damping configuration are in common use. Application of a single layer of polymer on the vibrating surface is called extensional 

A common configuration involves sandwiching the polymer between two sheets of metal to make a true composite material. While such composites exhibit optimum damping characteristics, they necessarily have limited form-ability. Alternatively, damping tapes (Wollek, 1965) have found important applications. In these systems, the adhesive serves also as the damping layer, and aluminum foil as the backing multiple layers may be applied with good effect. 

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Table I. Comparison of Extensional and Constrained Layer Damping...

Figure 8.6. Extensional and constrained layer damping of formula G as a function of temperature. ...

Figure 8.6 ° demonstrates the damping capabilities of formula G in both extensional and constrained layer modes as a function of temperature. In the latter mode, a constraining layer material based on reinforced epoxy resin was used. 

Figure 7. Constrained-layer damping of extensional and flexural vibrations of bars and plates.

Sound and vibration damping research with IPNs began in the early 1970 s and resulted in the formation of a constrained layer damping system with the inner damping layer a latex IPN paint (3 ). The constrained layer system results in a shearing effect within the IPN layer along with flexural and extensional motions as the composite panel vibrates. The added shear mechanism, not present in extensional applications, increases the amount of energy that is dissipated in each vibrational cycle. 

Two widely applied damping configurations that use viscoelastic materials are the free viscoelastic layer and the constrained viscoelastic layer, as shown in Fig.9a and 9b. The deformation of the viscoelastic layer is extensional in the first case and shear in the second case. Both these deformations are highly damped by intrinsic absorption in the viscoelastic polymer. In the case of the free viscoelastic layer (Fig.9a) it flexes with the plate participating in the bending stiffness as part of a two-layer beam. The viscoelastic layer must be tightly bonded to the plate and must be continuous over a... 

Incidentally, Kerwin and Smith (2il) have shown that essentially the same segment-length optimization yields best damping of longitudinal or extensional waves in the base plate. The segmented constrained viscoelastic layer is one of the few treatments capable of providing useful damping of such waves, which are troublesome in certain cases. 

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