The control of vibrations with viscoelastic materials

Abstract

The trend towards lightweight and continuous structures means that vibrations are transmitted very readily. Excessive noise and the malfunctioning of equipment can occur at points far removed from the source of the vibrational energy, particularly when a state of resonance exists in some parts of the structure. The resonant response of beams and plates excited in bending modes of vibration can be reduced by increasing the damping, either by applying an unconstrained viscoelastic layer or by making a multilayer sandwich construction. In both techniques the optimum damping at the lower modes of vibration is dependent upon the frequency and the thickness ratio of the layers. A resonance magnification (Q factor) as low as 2·5 times can be achieved, but at frequencies of excitation away from the optimum much larger responses can occur and the damping effectiveness with random vibration input can be disappointing unless special care is taken in design. This loss in damping effectiveness can be avoided for the three-layer sandwich construction by using a viscoelastic material with a shear modulus which increases in value linearly with frequency. The frequency range for optimum damping can be broadened considerably by making a four-layer sandwich with a core consisting of two layers of viscoelastic material with very different values for the shear modulus. The results of computation for geometrically symmetrical beams with three and five layers, and for the four-layered beam construction are presented. It is shown that the unconstrained layer technique can be equal in damping effectiveness to the four-layer construction with the same total thickness of damping layers only when the modulus of elasticity of the unconstrained layer is of the same order of magnitude as for metals. Little benefit is obtained from a uniformly applied damping layer when the structure is not in resonance. The transmission through the structure of vibrational energy can then be reduced by introducing viscoelastic materials at discrete points such as at the edges of panels. The requirements for edge type damping are a low stiffness and a high loss factor.