Aerobatic Fatigue Index
Depending on age, aircraft that were in military service were fitted with fatigue meters. These meters would record individual applications of particular g levels. At the end of a sortie the pilot would enter a code for the nature of that sortie in the tech log. Combining the code and the various fatigue counts would produce a fairly accurate measure of how much fatigue had been used.
Theoretically an aircraft would be dead at 100 Fatigue Index (FI). In practice this would vary and for reasons of economy would often be extended beyond this, or if fleet problems occurred then a modification might be required before reaching a particular FI.
Civilian aircraft and older ex military ones don't have a meter so the calculation of fatigue is a much blunter tool. In some regimes for example a 1 hour trip that includes 5 minutes of aerobatics would require the aerobatic fatigue multiplying factor to be applied for the whole hour.
Depending on aircraft type it might die at a particular number of hours or it might require specific component inspection/ replacement.
Of course if that component is a wing spar then practically speaking it's usually dead.
It's also not unusual for a particular type to suddenly acquire an hours restriction or aerobatics ban following a fatigue failure on a similar aircraft.
In short the point I make is that very careful research is required into the fatigue management of a particular type prior to purchase