The term fatigue refers to the effect of cyclic or intermittent loads. Cyclic loading due to either oscillating mechanical stress or to alternate heating and cooling is more detrimental than static loading. Under cyclic load, cracks initiate at localized sites within the part and these extend in size during cycling. Ultimately they expand and join to such an extent that the material can no longer hold and support the stress. Fractures can be characterized by a series of concentric crack growth bands that grow from the surface initiation site. Cyclic loading can bring about failure in polymer due to: chain scission, built up heat due to hysteresis, recrystallization of material and cumulative crack generation.
