Metal fatigue is a process by which a metal component is weakened by repeated bending or other cyclic stress, often to the point of fracture. The stress can be quite small, but over a large number of cycles the effect can be catastrophic. The process starts with a microscopic crack, called the initiation site, which then widens with each subsequent movement.
Metal fatigue came strongly to the notice of aircraft engineers in 1954 after three de Havilland Comet passenger jets had broken up in mid-air and crashed within a single year. Investigators from the Royal Aircraft Establishment at Farnborough in England told a public enquiry that the sharp corners around the plane's window openings acted as initiation sites for cracks. All aircraft windows were immediately redesigned with rounded corners.
