America's Cup hydraulic cylinder stress corrosion cracking

Cited as a sample focused-failure topic appropriate for a student presentation: a 7,500 psi hydraulic cylinder that failed and had to be redesigned to fail safe.

State what the problem is. That's one overhead. The other nine are basically what's the solution. Things students have done: pole-vaulting poles — one year I had two students do pole-vaulting poles, and it's an interesting composite material, has to be very flexible, has to be very strong, has to be very light. Tennis racquets. Gothic arches like the flying buttresses on cathedrals from 800 years ago in Europe. Sailing hardware. I don't want how to build an America's Cup yacht — that's too complex. But you could have, like one of my lectures, the failure of a 7,500 psi hydraulic cylinder that failed and how we had to design it to fail safe. Another student was interested in bayonets. Anything you want, because if it's of interest to you you'll do a good job.

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.

Worked fracture-mechanics example. 7075-T6 piston cylinder in a yacht winch hydraulic system, operating at 7,500 psi, failed by stress corrosion cracking from saltwater exposure. Critical flaw size calculation showed ~1 mm — confirmed by examination of the fracture surface. Solution: switch to 7075-T73 (over-aged, lower yield, higher toughness and corrosion resistance) with slightly thicker wall, achieving leak-before-break.

I got this from a guy who graduated in the department, Jerry — I might be empty-headed on his last name. He was working for a company called Navtech. Navtech used to be here in Massachusetts; they've moved to Connecticut. They make specialty marine hardware for fancy yachts and America's Cup boats — pretty expensive stuff. [Tom holds up the fractured piston cylinder.] This is a piston cylinder that operated part of a winch system that fractured. In 2002 he came to me. It was 7075-T6 alloy. They'd been using it at a yield strength of 73 ksi. K-1c — fracture toughness — was 26 ksi √inch. Wall thickness was about an inch. And it operated at a pressure of 7,500 psi. I wouldn't want to stand next to that and let go of a hydraulic line. Well, neither did the guys working the winches for the sails on the America's Cup, and this one let go. They were concerned. The problem was stress corrosion cracking, which I always tend to find in saltwater environments.

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.