It finally happened. Yesterday Hume's knee fractured through. Luckily, we noticed the fracture a few months ago when it first started and we designed new knees without the stress focuses (foci?) that caused the fracture. Hume is currently being used in shifts to accommodate multiple researchers running different types of experiments, so we wanted it fixed as quickly as possible and we stayed up until about midnight swapping out the joint.
The knee attaches to the calf with a C-clamp and the original piece contained a square-cornered slot that was really, obviously a bad idea. We couldn't have made a more concentrated stress focus if we tried. The only changes we made were redesigning the clamp to completely remove that slot and machining the knee out of 6061-Al instead of 7075-Al. I would have used 7075 if I had thought about it, but I don't think it will be a problem.
The new knee also weighs about 10% more than the original but that shouldn't be a problem either. Fingers crossed.