[Jimmy Cao]

Well, i'm not a materials expert, but here's what I believe.

There are differant types of stresses. Compression, torsion, and lateral load (i dont know what it's official name is or w/e).

Titanium outperforms both CF and Alu in all three fields. It's also the lightest (if used properly but it should be VERY close to CF). That comes at a price however.... lots and lots of $$$. Titanium is far from cheap... but it's extremely strong! We used it one year on our electrical board (we actually used a titanium tennis racket.. and it worked great). To make an entire robot from this would exceed the cost limitations.

Carbon Fiber, or CF, is also extremely strong in compression and lateral load. I dont think it's very good at withstanding torsion though. Again, though, CF isnt cheap, and it isnt healthy. Cutting CF puts small fibers into the air that destroy your lungs =D.

Lastly, aluminum. Aluminum is the most heavilly used material in FIRST, by far. It's lightweight, and relatively strong. It can withstand tremendous compression forces. It can also hold up to torsion quite well (well, at least Alu box and tube can). It will give quite easilly, though, if it is subjected to lateral loads with a long lever arm. Best of all, Aluminum is cheap, and easy to use. Additionally, it can be welded under reasonable conditions. Welding titanium requires an EXTREMELY controlled environment or the metal will burn. Welding CF is well... impossible. Welding Aluminum is easy (compared to the others. From what I hear, welding Aluminum is still significantly harder than welding steel because aluminum will still melt with relative ease).

For my team, we've always used mostly (prob 95%+) aluminum on our robots. We cannot afford to use Titanium, and we don't see the benefits of CF. Another alternative, steel, which is also cheap and strong, is extremely heavy, so we cannot afford (the weight) to use it.

As for using expanded (extruded, i assume) aluminum, it will not flex back to the original position after it flexes to pick up the ball. CF wont flex either. Titanium will flex, and it has pretty strong memory, but it's very expensive. Maybe you can try using some plastic material (like polycarbonate or delrin)

I hope that was useful (and correct). Take care,