America's Cup hydraulic cylinder stress corrosion cracking

7075-T6 aluminum hydraulic cylinder at 7,500 psi exploding from SCC initiated outside. Solution: 3 mm → 4 mm wall thickness, sacrificing strength to double SCC threshold and convert failure mode to leak-before-break. Used to introduce the America's Cup design philosophy: if your hardware doesn't fail, you've made it too heavy.

You've got to worry about some of the corrosion resistance of aluminum. Since one of you is interested in sailing, I brought one of my America's Cup projects. [Tom produces a fractured hydraulic cylinder.] I don't know if you can see it. It was always hard to see — I never etched it. This is a fractured hydraulic cylinder from America's Cup. The fracture origin is right here, but over time it's hard to see unless I put it in a microscope.

MIT Mech E alum runs Connecticut company supplying America's Cup hardware; brought Tom a failed 7,500 psi hydraulic cylinder. Pairs with the "design philosophy" cluster — if it doesn't fail occasionally, you over-built it.

When you get to one of my classes — I can't remember which one — I bring in a part of America's Cup. A graduate of Mechanical Engineering over here runs a company down in Connecticut, makes specialty hardware for fancy yachts. He brought me a broken little hydraulic cylinder. They use 7,500 psi hydraulics on the America's Cup yachts. Even on the V-22 Osprey they went to 3,000 psi hydraulic systems to run the controls, which was a big jump from the typical 1,500 or so psi. The higher the pressure, the lighter the weight. If you run a system at higher pressure, it can be thinner, smaller diameter tubes, and lighter weight overall. Well, they go to 7,500 psi, and if you have a fracture in one of those things, you've got hydraulic fluid that could come out and just cut some person — cut their arm off. Take a high-pressure washer at 3,000 psi — you can slice your finger off. Don't run your finger through the high-pressure jet. But at 7,500 you could kill one of those people.