Chapter 12: Fatigue

OVERVIEW

Fatigue could be defined as any change in the properties of a material caused by prolonged action of stress or strain, but this general definition would then include creep and stress relaxation. Here, fatigue will be taken to cover only changes resulting from repeated cyclic deformation which means, in effect, long term dynamic testing.

Subjecting a rubber to repeated deformation cycles results in a change in stiffness and a loss of mechanical strength. In some products, even a relatively small change in stiffness can be important, but this measure of fatigue is relatively little used, certainly not in standard tests. It would be relatively straightforward, although perhaps expensive in machine time, to continue a dynamic test as discussed in Chapter 9 over a very long period and monitor the change in modulus. Alternatively, modulus could be measured at intervals after dynamic cycling on a separate apparatus. In many products, notably tyres, it is the loss in strength shown by cracking and/or complete rupture which is considered to be the important aspect of fatigue and this is the measure of fatigue which is normally used in laboratory tests on rubbers.

The manner of breakdown will vary according to the geometry of the component, the type of stressing and the environmental conditions. The mechanisms which may contribute to the breakdown include thermal degradation, oxidation and attack by ozone, as well as the basic propagation of cracks by tearing. In rubber testing, it is normal to distinguish between two types of fatigue...