Most fasteners are essentially springs, and when these fasteners fail it is usually abrupt, "out of nowhere," and caused by fatigue. Fatigue failure is estimated to account for 80-90% of all fastener breaks. However, by its very definition, fatigue damage happens over time, due to repeated loading that stresses an already weakened location. ASTM E1823-10a, Terminology Relating to Fatigue and Fracture Testing, defines fatigue as: "The process of progressive localized permanent structural change occurring in a material subjected to conditions that produce fluctuating stresses and strains at some point or points and that may culminate in cracks or complete fracture after a sufficient number of fluctuations." Preventing fatigue failure is a critical part of engineering designs.
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