Most aircraft1 are built primarily out of various aluminium alloys. Aluminium and its alloys have multiple physical and chemical properties that make them terrible choices as structural materials, except in situations (such as the primary structure of aircraft) requiring the excellent strength-to-weight ratio of many aluminium alloys. One of these unpleasant properties is that they have no fatigue limit,2 so that any change in stress magnitude or direction, no matter how small, will produce cumulative damage to the material. When used as part of an aircraft's primary structure, this means that the airframe has a hard limit on how many times it can be stress-cycled3 before the cumulative effects of fatigue damage render the aircraft unsafe to fly unless repaired.
Occasionally, aircraft encounter circumstances that place greater-than-usual stresses on their primary structure, such as hard landings, encounters with extreme turbulence, or overly-aggressive maneuvering. These aggravate fatigue damage in two ways:
- The abnormally-large amplitude of the stress cycle during which the structural overload occurred causes the additional fatigue damage accrued during said stress cycle to be greater than that accrued during a more-normal stress cycle.
- Large overloads can directly produce small-scale cracking and buckling within the aircraft structure, which (as they produce stress concentrations in whatever structural element they occur in) can serve as initiation points for progressive fatigue cracking.
How much of an effect does a structural-overload event have on an airframe's remaining safe fatigue life? Are there any rules of thumb that say that (for instance) a hard landing takes off X number of takeoff/landing cycles from the remaining fatigue life of the hard-landed aircraft? Are there qualitative effects on the airframe's fatigue life that I hadn't thought of?
2: A minimum level of applied cyclic stress below which the material in question does not experience fatigue damage even if stress-cycled indefinitely.
3: Stress-cycling takes different forms for different portions of the airframe; for instance, the wings care about takeoffs and landings, when they have to suddenly start or stop carrying the weight of the rest of the aircraft, while the pressure cabin cares about every time you climb or descend to, from, or between altitudes requiring that it be pressurized, which causes it to start, stop, or change the degree to which it's holding in a lot of air that would really like to get out.