The Comet I aircraft failures in the early 1950s were the result of low-cycle fatigue caused by repeated cabin pressurizations. These are the earliest examples of catastrophic structural failure occuring on airplanes that have not been stressed beyond their limits. These failures demonstrate that because of metal fatigue, a safe load applied once becomes unsafe when applied many times. Most of us use fatigue when we break off a piece of metal wire by repeated bending. The wire breaks because of a low-cycle fatigue fracture.
Fatigue failures are always brittle failures which occur without buckling.
A wing or engine mount should buckle rather than snap. With the pursuit of lightweight vehicles, the advent of new lightweight designs and computer stress analysis, it is possible to create structures which are of minimum weight. As a result, more high-strength steels are used under high stress conditions. Unfortunately, increasing the hardness of steel decreases its ductility and it becomes more subject to brittle fatigue failure under service stress conditions. High strength, high stress and low ductility create catastrophic fatigue failure modes.