Introduction

In classic elasticity, there is no time delay between application of a force and the deformation that it causes. However, for many materials there is additional time-dependent deformation that is recoverable. This is called viscoelastic or anelastic deformation. When a load is applied to a material, there is an instantaneous elastic response but the deformation also increases with time. This viscoelasticity should not be confused with creep (Chapter 8), which is time-dependent plastic deformation. Anelastic strains in metals and ceramics are usually so small that they are ignored. However, in many polymers viscoelastic strains can be very significant.

Anelasticity is responsible for the damping of vibrations. A large damping capacity is desirable where vibrations might interfere with the precision of instruments or machinery and for controlling unwanted noise. A low damping capacity is desirable in materials used for frequency standards, such as in bells and in many musical instruments. Viscoelastic strains are often undesirable. They cause the sagging of wooden beams, denting of vinyl flooring by heavy furniture, and loss of dimensional stability in gauging equipment. The energy associated with damping is released as heat, which often causes an unwanted temperature increase. The study of damping peaks and how they are affected by processing has been useful in identifying mechanisms. The mathematical descriptions of viscoelasticity and damping will be developed in the first part of this chapter, then several damping mechanisms will be described.