This idea that aircraft continue to fly due to their over-engineering to begin with is far from the whole truth. Yes, aircraft like the B52 were over engineered and that has led to their ability to remain relevant over the decades they have been flying. The real reason they are still in the air is due to a rigorous maintenance and inspection schedule that identifies and addresses issues before they become catastrophic failures. The idea that the air frame is the original “skeleton” when the aircraft was originally built is IMO a fallacy. They are fixing and replacing parts that are end of life or failed due to stress on a regular basis. Are there some parts that are original? No doubt. But to imply that no structural repairs have been needed is completely false.
http://www.afgsc.af.mil/news/story.asp?id=123455217
http://www.asee-prism.org/icon-of-the-air-dec/
The heavyweight bomber’s continued airworthiness stems from a rigorous schedule of inspections and maintenance
Every four years, a B-52 makes its way to Tinker Air Force Base in Oklahoma for what’s called “depot maintenance,” or a nondestructive teardown, at a nearby Boeing facility. Parts that can be disassembled are taken apart and inspected, with repairs or replacements made as needed. “It’s stripped down, inspected, fixed, and put back together,” explains Scot Oathout, Boeing’s B-52 program director. That’s a big chore, given that the planes are nearly 160 feet long and have wingspans of 185 feet. Inspectors look for “material degradation of the structure,” says Gregory Shoales, director of the Center for Aircraft Structural Life Extension, the U.S. Air Force Academy’s largest research program for cadets and part of its mechanical engineering department. Corrosion and cracks are not two separate things but intrinsically linked, he says. “The majority of fatigue cracks start at a corrosion pit, and crack sites are places where corrosion can take root.” Besides visual inspections, engineers use a variety of technologies to assess how healthy a part is, including ultrasonics, X-rays, and surface eddy current; electromagnetic induction can detect below-surface corrosion in some materials. Meanwhile, the B-52s also undergo regular flight-line inspections and maintenance at their home bases. “The two inspections go hand in hand,” Oathout says. The crews keep precise flight records – data from sensors are used to check such things as stress levels and past areas of concern. For instance, if a hairline crack is found during depot maintenance, it may not initially be worrisome, but inspectors will keep track of it during flight-line checks to see if it begins to turn nasty.