Originally Posted by mprelaw

After tens of thousands of cycles of pressurization and depressurization. Any one of which puts more stress on an airframe than a bike will experience in two lifetimes. It's not the "propensity" of aluminum to crack and fail, but the propensity of any metal to eventually fail after many, many repeated instances of expansion under pressure, and relaxation after depressurization. Metal fatigue in aluminum airframes wasn't a concern until pressurized cabins became a reality--Google "DeHaviland Comet". More stringent and frequent inspections after a certain level of cycles (a 5 figure threshold) became required after the cabin roof tore off a 737 in Hawaii---it had over 90,000 cycles of pressurization on the airframe when it failed.