What is the phenomenon of fatigue in materials?

Mechanical fatigue tests are the progressive, localized, and permanent structural change that occurs in materials subjected to fluctuating stresses and strains, which can lead to cracks or fractures after a sufficient number of fluctuations.

Fatigue is a mechanical phenomenon of progressive degradation of material or components subjected to different loads over a period, which can lead to fracture of the material or component. During this fatigue, plastic deformation can be seen on the material. In its smallest form, this deformation can be called microplastic deformation. This damage can magnify with continuous stress and eventually cause the material or component to fracture.