For an example, I am talking about the Southwest Airlines Flight 812 where there was a small tear in the fuselage section caused as a result of metal fatigue.
In the 1980's, manufacturers thought that they had solved the problems that plagued earlier models like the De Havilland Comet. Then Aloha Airlines Flight 243 had a large section of upper fuselage come off during flight. Manufacturers realized that the fuselage could accumulate more damage more quickly than expected, and when this damage went undetected, it resulted in catastrophic failure.
The result was both regulators and manufacturers emphasizing a focus on both damage tolerance and fail safety when designing these structures.
Damage tolerance means that the structure can tolerate a certain amount of damage while still performing its function. Inspections should be performed soon enough that they will detect the damage before it compromises the structure.
Fail safety means that even if a feature fails, the overall structure can still perform its function. One option for this is redundancy. In the fuselage, this means stopping cracks from propagating too far. The cracks in the Aloha Airlines incident were able to continue growing until a large portion of structure failed.
These ideas mean that the structure is designed so that any damage is caught before it becomes critical, but in the event that this is not sufficient, items can fail without endangering the safety of flight.
An example is what happened to Southwest Flight 812. A section of fuselage ripped open in flight, which resulted in a rapid depressurization but a safe landing. A crack formed in the fuselage and was not detected. However, the crack was contained in a limited section of the skin. This allowed the cracked skin to fail while the rest remained intact.
