OVERVIEW

Creep, stress relaxation and set are all methods of investigating the result of an applied stress or strain as a function of time. Creep is the measurement of the increase of strain with time under constant force; stress relaxation is the measurement of change of stress with time under constant strain and set is the measurement of recovery after the removal of an applied stress or strain. It is important to appreciate that there are two distinct causes for the phenomena of creep, relaxation and set, the first physical and the second chemical. The physical effect is due to rubbers being viscoelastic, as discussed in Chapter 9, and the response to a stress or strain is not instantaneous but develops with time. The chemical effect is due to ageing of the rubber by oxidative chain scission, further crosslinking or other reaction.

In practice, it is often rather difficult to distinguish between the two causes and it could be argued that if tests are made under the same conditions as in service it matters relatively little what caused, for example, the creep as long as it can be measured. However, if any form of accelerated conditions, such as increased temperature, is used it follows that results could be very misleading. Generally, distinction is helped by the fact that physical effects are dominant at short times and low temperatures and chemical effects more apparent at longer times and higher temperatures.

Apart from being simply measures of how much a rubber creeps,...