de Havilland Comet

Square windows as stress concentrators. Alan Wells worked out the cause. Used to anchor stress-concentration intuition immediately after Griffith's equation.

Alan Wells — he's up here because he wrote a letter for my tenure case — also worked in the 1950s. Ever heard of the Comet aircraft? He was the one who figured out why the Comets came down. The Comet was one of the first jets to fly across the Atlantic, and they were just falling out of the sky. They were at the bottom of the ocean and no one could figure out why. Alan Wells came along and said: square corners, not a good idea — stress concentrations, you get cracks. They had to prove it, but they did.

Mid-1950s mid-Atlantic breakups. Alan Wells (British Welding Institute) identified square window stress concentration. Water-filled pressure tank reproduced the cracking. Tom uses it to introduce stress concentration around a hole in a plate (3-sigma result, 1883).

Another one is the Comet aircraft, in the middle of the 1950s. Anybody hear about the Comet?

Student: Yeah, low cycle fatigue.

It was low cycle fatigue, but the problem was, they were falling apart in the middle of the Atlantic, and no one could get the evidence to find out what was causing it. They just break up in the middle of the Atlantic Ocean, and there's no evidence. Finally Alan Wells, who was head of the British Welding Institute and a fracture mechanics guy, looked at it and realized the windows were square.

Student: They had a big water-filled tank, they pressurized and depressurized it for months, and they actually grew the fatigue cracks from there in one day.

Yeah. It's not all that different than Aloha Airlines, except Aloha was 37,000 cycles, because you were in Hawaii and it was all 15, 20, 30-minute flights from island to island. The plane had 40,000 hours, where a typical Boeing aircraft would go 100,000 hours. But in terms of ground-air-ground cycles — takeoffs and landings — it was 250,000 hours equivalent compared to everything else in terms of low cycle fatigue.