Fatigue

The phenomenon leading to fracture of a material that is under repeated or fluctuating stresses having a maximum value less than the ultimate tensile strength of the material. Fatigue failure generally occurs at loads which applied statically would produce little perceptible effect. Fatigue fractures are progressive, beginning as minute cracks that grow under the action of the fluctuating stress. There is no obvious warning. A crack can form without appreciable surface deformation, making it difficult to detect. Fractures often start from small nicks or scratches or fillets that cause a localized concentration of stress. Failure can be influenced by a number of factors including size, shape and design of the component, condition of the surface or operating environment.