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.
fatigue, in physiology, inability to perform reasonable and necessary physical or mental activity. Muscle fatigue, for example, results when the contractile properties of muscle are reduced, and continued exertion is impossible unless the muscle is allowed to rest. In muscle tissue, the depletion of glycogen (stored glucose), a source of energy for muscle cells, and the accumulation of lactic acid, which is produced through the breakdown of glucose, was long thought to the cause of muscle fatigue, but it is now known that the lactic acid produced is used as an energy source as well. A new explanation of muscle fatigue suggests that it is related to the control of the flow of the calcium ions in muscle. The release of those ions causes muscle contraction, while their storage causes relaxation. After prolonged exercise, the channels that control calcium flow become leaky, diminishing the muscle cells ability to contract. In the normal body the damaged channels are repaired after a period of rest. There are some persons in whom fatigue is a chronic state that does not necessarily result from activity or exertion. In some instances this abnormal fatigue may be associated with systemic disorders such as anemia, a deficiency of protein or oxygen in the blood, addiction to drugs, increased or decreased function of the endocrine glands, or kidney disease in which there is a large accumulation of waste products. If excessive fatigue occurs over a prolonged period, exhaustion (marked loss of vital and nervous power) may result. In most persons with chronic fatigue, however, the condition seems to be associated with bipolar disorder. Thorough medical and psychiatric examination may be required.

Weakened condition of metal parts of machines, vehicles, or structures caused by repeated stresses or loadings, ultimately resulting in fracture under a stress much weaker than that necessary to cause fracture in a single application. Fatigue-resistant metals have been developed and their performance improved by surface treatments, and fatigue stresses have been significantly reduced in aircraft and other applications by designing to avoid stress concentrations.

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In engineering, manifestation of progressive fracture in a solid under cyclic loading, as in the case of a metal strip that ruptures after repeated bending back and forth (see metal fatigue). Fatigue fracture begins with one or several cracks that spread in the course of repeated application of forces until complete rupture suddenly occurs when the small unaffected portion is too weak to sustain the load. Seealso ductility, testing machine.

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Sudden debilitating fatigue of unknown cause. It may follow a nonspecific illness with mild fever, tender lymph nodes, sore throat, headaches, weakness, muscle and joint pain, and confusion or difficulty in concentrating. To meet the criteria of CFS, the syndrome must be new, with a definite point of onset, and must persist more than six months. Once dismissed as imaginary, CFS remains controversial. Many authorities question whether it is a distinct disorder, since there is considerable overlap with other conditions such as fibromyalgia and Gulf War syndrome. No diagnostic test for CFS exists. Although a number of theories about the cause of CFS have been advanced, none has been proved. No cure has been found, but most patients improve gradually.

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