Fatigue

The tendency of a metal component to break when it is subjected to a great number of repeated stress cycles, even when the applied stress is considerably below the tensile strength of the material.

Failure normally occurs after a large number of stress cycles – usually several million – and therefore rotating parts, such as shafts which rotate at high speed, are the most-commonly affected components.

When a load is applied to a metal part, the maximum stress is usually located at the surface. Therefore, any treatment that increases the surface strength, such as carburising, nitriding and shot peening will increase the fatigue life of the component.