Fatigue

The first appearance of a fatigue crack in an item subject to repeated loads, usually based on visual inspection, but sometimes based on the use of nondestructive testing techniques.

The length of a fatigue crack at which the residual strength of the item is no longer sufficient to withstand the specified damage-tolerant load.

1) For an item subject to fatigue, the total time to failure, e.g. crack initiation. 2) For a human, fatigue is weariness, exhaustion, or decreased attention related to labor, exertion, or stress. May also result from lack of sleep, circadian shifts, depression, boredom, or disease. May result in decreased ability to perform mental or physical tasks.