mancini tube upper control arm failure....
That crack is from rust. I have also seen stock arms crack in the same location from bad handling (as I suspect - monkeys doing the R&R on the bushings). I have also seen bent stock arms (easy to spot is you sight through the bushings).
That's definitely a
stress crack, not a rust issue at all! Almost looks like the bushing pressed in was a little oversize, appears that the outer part of the wall is pushed out. Maybe just a bad installation. Either way, stock parts fail too! They rust, they flex, and over time they fatigue. All your stock parts have 40 years of metal fatigue built in now, even if they don't look bad it doesn't mean they can't fail.
The CAP A-arm that failed is right at the weld. Its probably just a bad weld. If it had failed in the middle of the tube or somewhere else, I'd say bad design. But if its just a cold weld,it has nothing to do with design at all. All the aftermarket A-arms have threaded ends welded onto tubes, not sure if all of them are just butt-welded though, a better design would be for it to have a shoulder on the end that fits into the tube. But even that design still wouldn't make up for a bad weld, and there's no reason a butt-weld can't be strong enough to support the front end. So yes, there are stronger designs, but it doesn't mean its a bad design.
Also, the fact that it failed while the owner was driving down the road at 45mph doesn't mean it was caused by regular driving. A hard launch and landing could have cracked the arm at the weld, but not enough for immediate failure. Drive around for a few months, hit a few potholes, and the crack completes itself and the arm fails. Don't get me wrong, I have CAP a-arms on my car and I'm concerned, but it may not have been entirely on the manufacturer. It certainly might be a poor weld that slipped through and failed under normal conditions, but it also might have been damaged on the car.
Yes, it shouldn't happen, but even the best QC in the world can't prevent all failures. If you get a bad tube for example, how would you know? If it was heated improperly and all the carbon migrated to one place making it brittle, how would you know? Without an detailed engineering analysis (which would destroy the part btw), you wouldn't.
Tubular a-arms in general are a much better design than the stockers. They will be stronger, lighter, and less prone to flexing if built out of the right materials. But unlike the stockers they depend on welds for the strength of the part, so if you have a crappy weld, you have a crappy part.
On that note, we all love powder coated parts. But realistically they hide cracks. If parts are left bare, you can see even small cracks begin to form. Of course, then you have a corrosion issue to deal with, and rust will hide cracks too.
