Why is a rip stop doubler used in aircraft?

Answer 1

In the early 1950s, several of the first pressurized jet transport aircrafts broke apart during flight. Most of the aforementioned were in relatively non-turbulent conditions. After much investigations, it was discovered that the main cause of this was "Metal Fatigue" brought about by the flexing of the structure during pressurization and depressurization cycles.

Thus, rip-stop doublers were installed in strategic locations throughout the airplane structure, especially around high stress areas such as windows and doors. These doublers absorbs the stress loads and prevent cracks from extending & ripping through the structure hence the term "Rip-Stop"

Answer 2

It's used to stop cracks in the fuselage from travelling the full length of the aircraft body. Placed at regular intervals along the fuselage, rip-stop doublers are thickened rings of metal which are tougher than the rest of the aircraft's skin.