Navtech marine hydraulic piston failure (America's Cup yacht)

Forensic case — 7075-T6 aluminum hydraulic cylinder, 7500 PSI service, 2.5 mm wall, fatigue-initiated fracture running 2/3 through before catastrophic burst. Tom's recommendation: switch to T73 temper (yield 73 → 63 KSI, K_IC 26 → 30 KSI√in, critical flaw 4 mm) for leak-before-break behavior. The signature case for the toughness-versus-strength trade-off in design philosophy.

Any questions on why fracture toughness is important? So I'll now give you a practical example. A graduate of mechanical engineering here at MIT brought this to me. He works for Navtec. Navtec is a company that makes high-end marine hardware for America's Cup yachts. [Tom holds up a small aluminum hydraulic cylinder.] This was a cylinder of aluminum, high-strength aluminum, that they had designed for an America's Cup yacht, and had 7500 PSI hydraulic fluid in it. It's got a wall thickness of about 2 and 1/2 mm. I don't know if we can still see it, this thing's getting a little old, but you have to be able to read a fracture surface. The fracture didn't go all the way through. The initial fatigue crack — this thing did fail in fatigue. If you can read a fracture surface, you'll see lines radiating one way, other lines running this way. The initial fatigue crack is there at the origin, and it ran about 2/3 of the way through before the thing blew up. If you're standing right next to one of these things trying to raise the sails on your America's Cup yacht, and this thing blows up in your face, it's not a good day. At 7500 PSI, you can inject the oil right into your skin and your bloodstream, and it does all kinds of bad things. You end up with hands that die because they get no blood.