Originally Posted by
KonAaron Snake
Someone commented on steel fatigue earlier...
I could be mistaken, and I'm NOT a metalurgist or expert, but I thought steel either bent or it didn't. It didn't fatigue with usage. Aluminum does suffer fatigue (which we learned when aluminum air planes kept crashing).
Correct.
Aluminum has no
fatigue limit, while steel does.
An aluminum structure subjected to repeated stress excursions, no matter how small, will ultimately fail. Because steel has a fatigue limit, however, a steel structure can be subjected to an infinite number of stress excursions below the fatigue limit, and it will not fail from fatigue.
Designers understand this limitation of aluminum as a structural material, and overbuild structures to mitigate the problem.
Pressurized aluminum airplane fuselages are "life-limited" by the number of pressurization/depressurization cycles. After an airframe has experienced the number of cycles designated by the manufacturer as the maximum for that model, it is retired from service and the aluminum is recycled.