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Fatigue
From WikiPedalia
Fatigue (metal fatigue) is the property that causes metal to break after repeatedly being bent or flexed. This is a common cause of spoke breakage, and can also affect frames, handlebars and other parts.
A familiar example of metal fatigue is experienced if you open a pop-top beverage can then flex the top back and forth a few times.
Aluminum is more prone to metal fatigue than steel is, so aluminum parts have to be designed a bit stronger to make up for this characteristic.
Note, this is a very simplistic description of a rather complicated process.
