Free 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... 

The Free Viscoelastic Layer. As we see in Figure 3, the free viscoelastic layer is bonded to the plate to be damped. As the plate vibrates in bending, the viscoelastic layer is deformed principally in extension and compression in planes parallel to the plate surface. Such damping layers have long been known, and at first were applied more-or-less empirically. In the early 1950s Oberst 3.,D and Lienard (, ) published analyses describing quantitative analytical models of free layer behavior. 

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. 

The damping performance of a free layer treatment for plate bending waves is shown in Figure 4 (lH,UL) This chart, which is Oberst s result, gives the system loss factor T relative to T 2/ the loss factor of the viscoelastic material, as a function of the thickness ratio H2/H1 (viscoelastic layer to plate). Each of the several curves corresponds to a particular value of the relative Young s storage modulus E2/E1 (viscoelastic layer to plate). 

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

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.)... 

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). 

The vitreous humor is a viscoelastic connective tissue composed of small amounts of glycosaminoglycans, including hyaluronic acid, and of such proteins as collagen.The collagen fibrils are anchored directly to the basal lamina, which forms the boimdaries of the lens, the ciliary body epithelium, and the neuroglial cells of the retina. Although the anterior vitreous is cell free, the posterior vitreous contains a few phagocytic cells, called hyalo-cytes, and is sometimes termed the cortical tissue layer. 

The otoliths are an overdamped second-order system whose structure is shown in Figure 64.1a. In this model the otoconial layer is assumed to be rigid and nondeformable, the gel layer is a deformable layer of isotropic viscoelastic material, and the fluid endolymph is assumed to be Newtonian fluid. A small element of the layered structure with surface area dA is cut from the surface and a vertical view of this surface element, of width Ax, is shown in Figure 64.1b. To evaluate the forces that are present, free body diagrams are constructed of each elemental layer of the small differential strip. See the nomenclature table for a description of all variables used in the following formulas (for derivation details see Grant et al., 1984 and 1991). 

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