The materials properties defined have all assumed the application of a static load applied in one direction.

In practice, materials are often subject to cyclic loads. This can lead to the process of fatigue. Materials can fail by fatigue at loads well below those required to cause general plastic deformation.

Fatigue occurs by the growth of cracks under the application of cyclic loading. If a crack is able to get large enough to reach the critical size for unstable growth a fracture will occur.

Definition:

Fatigue resistance is the resistance of a material to crack growth under the application of a cyclic load.

There are two important general categories of fatigue problem:

1. Initiation-controlled, when the component does not initially contain a crack and most of the life is used generating a crack before it propagates to failure.
2. Propagation-controlled, when the component already contains a crack, and the life depends on the rate at which it propagates to failure.