fatigue, in engineering, microscopic cracking of materials, especially metals, after repeated applications of stress. Fissures may be formed within pieces of metal during their manufacture when, while cooling from the molten state, they shrink and tensile stresses arise. Once a crack has started it spreads under repeated stress until the metal ruptures. Examples of fatigue are found in steel rails, beams, and girders. Metallic fatigue resulted in the catastrophes encountered by many of the Liberty ships built during World Wars I and II and the crashes of a number of the earliest jet aircraft constructed. Materials used in construction are tested for fatigue strength, or endurance limit, by being subjected mechanically to cyclic applications of stress. Steel parts are sometimes treated by shot blasting to increase their fatigue resistance.