Many applications exist where a material is subjected to a repeated cyclic stress. These materials are found to fail after a large number of applications of the stress even though the cyclic stress that is applied is below their yield strength. This mode of failure is termed fatigue.
In these cases, the material fails due to progressive stepwise crack growth through the sample. Cracks create stress concentrations that raise the level of stress to a level above the yield strength of the material in the vicinity of the crack. Repeated cyclic stresses propagate the crack until failure.
Fatigue fracture occurs as a three-stage process.
Initially, a crack is nucleated, usually at the surface of the component.
Subsequently, the crack propagates over time until a critical crack length is reached.
Finally, the crack is large enough that the material cannot support the nominal applied stress and the material undergoes fast fracture.