Fatigue (material)
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In materials science, fatigue is a process by which a component is weakened by repeated bending or other cyclic stress, often to the point of fracture. The stress can be small enough to be below the ultimate tensile stress, or even the yield stress of the material. However, over a large number of cycles the effect can be catastrophic.

Characteristics of fatigue failures

The following characteristics are common to fatigue in all materials:

• The process starts with a microscopic crack, called the initiation site, which then widens with each subsequent movement, a phenomenon analysed in the topic of fracture mechanics.
• Failure is essentially probabilistic. The number of cycles required for failure varies between homogeneous material samples. Analysis demands the techniques of survival analysis.
• The greater the applied stress, the shorter the life.
• Damage is cummulative. Materials do not recover when rested.
• Fatigue life is influenced by a variety of factors, such as temperature and surface finish, in complicated ways.
• Some materials, for example steel and titanium, exhibit a fatigue limit, a limit below which repeated stress has no effect. Most others, for example aluminium, exhibit no such limit and even infinitesimally small stresses will eventually cause failure.

Famous fatigue failures

De Havilland Comet

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.

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