ABSTRACT
A distinctive feature of the mechanical behaviour of polymers is the way in which their response to an applied stress or strain depends upon the rate or time period of loading. This dependence upon rate and time is in marked contrast to the behaviour of elastic solids such as metals and ceramics that, at least at low strains, obey Hooke’s law such that the stress is proportional to the strain and independent of loading rate. On the other hand, the mechanical behaviour of viscous liquids is time dependent. It is possible to represent their behaviour at low rates of strain by Newton’s law whereby the stress is proportional to the strain-rate and independent of the strain. The behaviour of most polymers can be thought of as being somewhere between that of elastic solids and viscous liquids. At low temperatures and high rates of strain they display elastic behaviour, whereas at high temperatures and low rates of strain they behave in a viscous manner, flowing like a liquid. Polymers are, therefore, termed viscoelastic as they display aspects of both viscous and elastic types of behaviour.
