Re passenger jets - I imagine you are thinking of the Comet 1? That was a more complex failure than is generally known. In brief, they did know about fatigue life at the time, and had ways of retiring aircraft before it was an issue (safe-life design, apparently introduced in the 19C for steam engines despite their being iron and steel). Ok, now you will be thinking "square windows, stress concentrators". Almost all pressurised aircraft use windows with angled corners in the cockpit. There isn't an intrinsic bar to square windows, and in fact the original design would probably have been ok. That used glued installation, avoiding stress concentrators. However a production engineer changed the design to use riveted installation, which caused the well-known problem with hull failure. Still, that would have been discovered if DH had not managed to resist government pressure to do fatigue testing on the pressure hull (because they were racing Boeing to be first to market, and fatigue testing takes time). They actually had the apparatus for repeatedly pressurising the hull in a bath, but only used it for testing static pressure.