Originally Posted by
Scooper
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.
This is why for all practical purposes, bike frames made from aluminum have a VERY marginal weight advantage over steel. In order to make it strong enough so as to not deflect and fatigue, it has to be nearly as heavy as the steel component (assuming they're of similar quality) would be.
You CAN make an aluminum frame that won't break from fatigue, but it'll be nearly as heavy as a comparable steel frame, and from a frame standpoint, less comfortable to ride. That can be mitigated with wheel/tire choice, seatpost choice, etc., but in my not-so-humble opinion, a large steel frame that flexes just gives a better ride.