Originally Posted by http://www.bohemianbicycles.com/materials%20science.htm

Ferrous alloys (a.k.a. steel) and titanium have a threshold below which a repeating load may be applied an infinite number of times without causing failure. This is called the fatigue limit, or endurance limit. Aluminum and magnesium don't exhibit an endurance limit, meaning that even with a miniscule load, they will eventually fail after enough load cycles.