Constrained viscoelastic 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... 

In contrast to the free layer, the operation of the constrained viscoelastic layer (Fig.9b) involves shear deformation of the layer [47]. Significant dampling can be achieved in the frequency range where there is a balance between the shear stiffness of the viscoelastic layer (2) and the extensional stiffness of the constraining layer (3). In this region the composite loss factor varies approximately as follows. 

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... 

Figure 6. Shear Relaxation in a Constrained Viscoelastic Layer Treatment...

Variations on Constrained Viscoelastic Laver Systems. The constrained viscoelastic layer treatment. as applied to a base member, is at its simplest, a continuous 3-layer laminate. Numerous variations and elaborations have been explored, and a number have found practical application. 

Figure 7. Maximum Loss Factor versus Stiffness Parameter for Constrained Viscoelastic Layer Treatments (Adapted from ref. 10)...

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. 

Other spacer-like geometric manipulations have been considered for constrained viscoelastic layers. These... 

In the particular case of the acoustical requirements mentioned in Table IV, we show in Table VI the technical acoustical requirements specifically for free and constrained viscoelastic layer treatments. This table with its notes summarizes the requirements on material dynamic properties that were presented in the earlier discussion of these broadly useful treatment types. 

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.)... 

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. 

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. 

Cold-Rolled Steel Constraining Layer 3M ISD 112 Viscoelastic Shear Layer... 

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