fatigue

Fracture surface of an aluminium crank arm from a bicycle. The dark area (due to oil, dirt and fretting) is a slow growth fatigue crack and may contain striations. The bright area is caused by sudden fracture.

In materials science, fatigue is the initiation and propagation of cracks in a material due to cyclic loading. Once a fatigue crack has initiated, it grows a small amount with each loading cycle, typically producing striations on some parts of the fracture surface. The crack will continue to grow until it reaches a critical size, which occurs when the stress intensity factor of the crack exceeds the fracture toughness of the material, producing rapid propagation and typically complete fracture of the structure.