The impact of cyclic stresses in a pressure hull, relatively speaking, weren't however.
The impact of cyclic stress was well known from many railroad desasters. The appearance of cracks was quite well known, what was unknown at that times was what we use to call "fracture mechanics" today, the growth mechanism for cracks which may be explosive (instable) under certain conditions. Especially in a thin, highly stressed pressurized skin, the energy released from the skin which is unloaded due to the crack is higher than the energy required to enlarge the crack. It is the same phenomena that makes a balloon go pop if you inflate it high enough (enough energy stored in the rubber skin).
And of course it was the mechanism of coldworking by overstess not understood at that time.