(page 2)

"Constrained layer" damping, "sandwiched" construction, adding a extra thin metal layer atop a layer of viscoelastic material, more effectively damps than does the material alone. Among other applications, such metallic sandwiches make quiet valve covers and quiet oil pans for automobile engines.

Figure 10-1 Laminated Beam

Visualize clamping two beams to a table. The beams have essentially the same dimensions and the same natural frequencies. One is a viscoelastic "sandwich". The other is solid metal. Both carry small accelerometers whose signals are monitored on an oscilloscope, as in Figure 10-2.

Observe the time histories. The damped, laminated beam "recovers" much more quickly from being plucked.

Figure 10-2 Time Histories - Effect of Damping

That "plucking" resembled a shock test. Now let's attach the two beams back-to-back on a small electrodynamic shaker. Let’s compare the beam responses to continuous, sustained vibration. We see their first, second and third modes (three separate photographs). Note that in each mode, the damped laminated beam has much lesser magnitude of response.

Figure 10-3 Pair of Cantilever Beams in First Three Modes

Here’s how the "constrained layer damping" of Figure 10-1 works: when bending as shown, the fibers in the top metallic layer are in tension. Those in the bottom metallic layer are compressed. When the composite beam bends the other way we have compression on top and tension on the bottom. Thus the surfaces of the viscoelastic layer alternate between tension and shear. Those alternating stresses result in heat. Energy is being extracted. The magnitude of vibration, (this is especially noticeable at resonance) is lessened.