Constrained viscoelastic damping layer

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... 

Figure 3. Free and Constrained Viscoelastic-Layer Damping Treatments...

The Constrained Viscoelastic Laver. The second of our two general damping treatments is the constrained viscoelastic layer shown in Figure 3(b). The complete constrained-layer configuration is a three-layer laminate comprising base layer to be damped, viscoelastic layer, and constraining... 

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. 

The properties desired of an ideal spacer layer are that it be stiff in shear, but that the spacer itself contribute minimally to the bending stiffness of the base structure, shifting the neutral plane as little as possible. We note that for the spaced constrained layer, the combined function of the viscoelastic layer and spacer is to provide a thick, dissipative and appropriately stiff (in shear) layer between the constraining and base layers. Therefore the order of the viscoelastic and spacer elements is arbitrary and they may be subdivided as long as the desired properties are preserved. These possibilities give additional freedom in adapting viscoelastic materials for effective damping. 

FIGURE 4.101 (a) Panel with free layer of viscoelastic (damping) material. (b) Panel with constrained layer of viscoelastic (damping) material. 

FIGURE 14.13 Free and constrained viscoelastic-layer damping treatments, (a) Unconstrained and (b) constrained. (Adapted from Corsaro, R. D. and L. H. Sperling, eds., Sound and Vibration Damping with Polymers, ACS Symposium Series 424, R T. Weissman and R. P. Chartoff, Washington, DC, p. 115, 1990.)... 

Constrained-Layer Treatments. Constrained-layer damping treatments consist of a thin layer (/ m) of viscoelastic material sandwiched between a base material and an outer constraining layer of sheet metal or other stmctural material. Some of these treatments are available with self-adhesives on both sides of the viscoelastic material and act as a bonding agent between the base and constraining layers others have the constraining layer already bonded to the inner layer so they need only be appHed to the base material. 

The damping of the composite structure will be affected by the thicknesses of the various layers, stiffnesses of the base and top plates, and the viscoelastic properties of the constrained layer (12). In the present instance (13), it was desired to develop a a broad-band material to damp a model composite structure consisting of a 2.54 cm. base plate (H ), 0.079 cm. polymer layer (H ), and 0.159 cm. cover (Ho)fi oase and cover were composed oi brass with a modulus of 10 Pa. In this instance, the only variable was the viscoelastic behavior of the polymer layer. A temperature range from 0 to 20 degrees Centigrade and a frequency range from 100 Hz to 10 kHz were desired. 

The development of constrained-layer damping materials through the use of dynamic mechanical testing and mathematical modeling has been described. It has been shown how different types and loadings of fillers will affect the measured viscoelastic properties of chlorobutyl rubbers. It has then been shown how these changes will affect the damping performance of these materials in constrained layer structures. 

Wetton XSl has described the mechanical characteristics for vibration damping materials in terms of the frequency and temperature dependence of the viscoelastic properties of polymeric materials. Use of polymeric materials in free-layer and constrained layer damping configurations has been discussed in the literature by Ungar (10-12). Kerwin (13.14). and others (15.16). 

Another example of constrained layer damping is oil pans for automobile engines, where a layer of polymer is formed between two layers of steel. Constrained layer damping places the viscoelastic material primarily in a shear state of stress during the transverse deflection of the substrate. Thus, the relationship for constrained layer damping is not identical to that for free layer damping. The approximate relationship for constrained layer damping is shown in Equation 14.33 ... 

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