Need help, Titanium / Carbon Fiber / Aluminum

[R.C.]

We were wondering, which substance is lighter, stronger, and flexes back to original position the best. A list would be extremely helpful. Also what are your personal experiences with these materials.

Thanks

[geeknerd99]

This all depends on its application. I'll use bike frames and components, as that's what I have the most experience with involving these materials.

Ti is extremely lightweight, but also pretty darn expensive. It is probably the strongest of the 3, but also has benefical vibration dampening features. It's been said that it has the ride quality of steel, the weight of carbon fiber, and the durability to the elements of aluminum. I wouldn't flex it though.

Carbon is really light, but often has no crush resistance. It might be a decent choice if flexing (within reason) is required.

Aluminum is light enough, strong, but does not fatigue well at all. When aluminum goes, it goes suddenly and often catastrophically.

I would generally pick Ti for the weight/strength ratio, but the addition of flex in your query makes it difficult. Could you elaborate more on what you need this data for?

[sanddrag]

Originally Posted by geeknerd99

Ti is extremely lightweight, but also pretty darn expensive. It is probably the strongest of the 3.

It actually is not extremely lightweight at all. The density of titanium is approximately 0.16 lb/in^3 where the density of nearly all aluminum alloys is approximately 0.10 lb/in^3. By comparison, titanium is quite heavy. However, with higher grade titaniums (Grade 5 and Grade 9) you can get away with less material thickness in your part. However, grade 2 titanium is actually quite weak (40ksi yield). Some of the aluminum alloys such as 7075 and 7068 can have quite high yield strengths, in the range of about 63ksi and higher.

For your application though, I might suggest polycarbonate (Lexan).

[MrForbes]

Do you have some pictures of the hand so we can see just what it is you're working on? It's a lot easier to offer useful suggestions if we can see just what the problem is.

[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,

[clydefrog88]

http://www.chiefdelphi.com/media/photos/20217

Here's an example of what I was talking about with the custom fabrication, our 2005 bot. My dad and I made a mold specifically for the second stage of the arm (c-shape). We laid two different configurations: a 3lb heavy duty, and a 1.5lb light duty. We had the weight, so we ended up using the heavier one. the arm though is indestructable. It's great to see the reactions people have to me hitting the light arm against a brick wall without it even scratching.

[R.C.]

here's the pic
http://www.chiefdelphi.com/media/img...7ba7059d_l.jpg

So is titanium(Grade 5/9) lighter than aluminum? I thank you all for the advice, we want to use titanium or carbon fiber for the fingers on the hand ( the part where the ball rests). What do you guys think? Also you have to consider the fact that at San Diego our robot hand got pounded and is bearly in one piece and that is why we want to upgrade. That is why I asked if titanium or carbon fiber flexes.

[clydefrog88]

i think that unless you have the ability to make several sets, carbon would be a bad choice. though carbon would be significantly stronger, it would ultimately have the potential to break. for that application i think fiberglass is your friend. it offers greater flexibility and is easier to acquire.

[Rick TYler]

I am not a materials engineer. I'm not a mechanical engineer. What I am is a hobbyist who has built several boats out of composite materials and a few robots. Here are some things you might want to think about in addition to "flexibility."

- Titanium is a pain to work with. It's hard to cut, nearly impossible to bend, and has to be welded in an inert gas environment. It is springy, though, which meets one of your criteria. The chance of your robotics team being able to work with titanium is low, however, unless you have a mentor with real experience with the metal. I wouldn't consider it.

- The reason people like carbon fiber composites is that they are incredibly stiff, and light for the strength and stiffness. The thing that carbon fiber composites are best at -- stiffness -- is exactly what you aren't looking for. It's probably not your best choice.

- I see a lot of stuff on this board where "fiberglass" is used in addition to "composites." What is usually known as "fiberglass" is glass fiber matt, cloth, or biax held together in a polyester resin matrix. Generically, fiberglass is just one type of composite material. The way you normally experience it, fiberglass is not very stiff, not particularly strong, and heavy for the strength and stiffness. The advantage to fiberglass in boat building, for example, is that it can be laid up by fairly unskilled labor, and doesn't cost much.

More advanced composites use epoxy (or sometimes, vinylester resins, but let's keep it simple) to bind a material together. The materials used include glass fiber in a variety of orientations and compositions, kevlar, and carbon fiber. One of the best things about glass fibers is that they are available in a variety of pre-made types -- woven glass (in the US, measured in ounces per square yard of fabric -- 6 ounce is lightweight, 10 or 12 ounces is heavy), bundles of glass fiber held together in crossing orientations of different weights (called "biaxial"), and other things like mat or chopped fibers which are generally not used with epoxy. Matt and chopped fibers are used to bulk up thicker laminations used with cheap polyester resins.

Since epoxy is expensive, very strong, and impervious to moisture, it is usually laid up with a fiber over a core material. We won't go over cores in detail here, but for amateur use it's very popular to use lightweight, high-quality plywood as a core for structural use. Something like BS1088 meranti or okoume plywood is stiff, strong, and lightweight. Some very expensive, very impressive sail- and motor-boats are built up with multiple layers of biaxial fiberglass laid up in epoxy over a core of 1088 plywood.

For really, truly expensive boats (see "America's Cup") they will lay up carbon fibers over a foam core with a thermo-setting epoxy to hold the whole thing together. If you want a mind-blowing experience, start pricing things like carbon-fiber spinnaker poles for 80-foot racing boats. For us mortals, carbon fiber is not a great choice for large assemblies -- it's picky, brutally hard to cut, messy, and demanding.

With one exception, if you want to lay up a composites structure for a robot, I would stick to either woven or biaxial glass fibers laid up in epoxy. If you need to make a long, strong pole, you might follow the plans like this: http://boatbuildercentral.com/products.php?cat=36 to build a pole, which can then be bonded to attachment pads for assembly to your robot. This is what I wanted to do with the tetra game a few years ago, but the students didn't want to mess with composites. By the way, it's the availability of carbon fiber "socks" that makes this possible for simple CF composite poles. (That link is to a commercial Website to which I have no affiliation, other than as a happy customer.)

Probably more than you wanted to know...

[artdutra04]

Titanium will eat through most general tooling like there is no tomorrow if you aren't very careful when working with it.

And there is a small chance, that if you aren't milling it correctly, it can actually catch on fire.

Although if you want "springiness", you can also try PVC pipe, a flat sheet of Lexan (polycarbonate), or even use aluminum with a pivot point and surgical tubing to give the springiness.

[Jack Jones]

Originally Posted by artdutra04

Titanium will eat through most general tooling like there is no tomorrow if you aren't very careful when working with it.

And there is a small chance, that if you aren't milling it correctly, it can actually catch on fire.

Although if you want "springiness", you can also try PVC pipe, a flat sheet of Lexan (polycarbonate), or even use aluminum with a pivot point and surgical tubing to give the springiness.

Mr. McGuire: I want to say one word to you. Just one word.
Benjamin: Yes, sir.
Mr. McGuire: Are you listening?
Benjamin: Yes, I am.
Mr. McGuire: Plastics.
Benjamin: Just how do you mean that, sir?

[dlavery]

Originally Posted by Rick TYler

- Titanium is a pain to work with. It's hard to cut, nearly impossible to bend, and has to be welded in an inert gas environment. It is springy, though, which meets one of your criteria. The chance of your robotics team being able to work with titanium is low, however, unless you have a mentor with real experience with the metal. I wouldn't consider it.

Originally Posted by artdutra04

Titanium will eat through most general tooling like there is no tomorrow if you aren't very careful when working with it.

And there is a small chance, that if you aren't milling it correctly, it can actually catch on fire.

Titanium is surrounded by this wonderful mythology that seems to supercede reality. It actually is not all that tough to work with. It just requires some appropriate knowledge and slightly different practices when you are working with it to get the results that you want. HSS tooling will work just fine, and carbide tools will do even better. Keep surface speeds low, feed rates high, and use lots of coolant. You want to keep the cutting edge very sharp, cool, and constantly moving to prevent work hardening and/or galling. Titanium machines extremely well (you can basically treat it just like any other high-performance alloy like austenitic stainless steel), and finishes beautifully.

-dave

[Tom Bottiglieri]

A Uni-directional weave of carbon bent correctly will give you high stiffness in one bend direction, but a "spring" action in the other. Pre-preg is usually easier to work with, but has its own set of caveats in comparison to laying up your own.

[Travis Hoffman]

Slightly off topic:

What's the going rate for Titanium tube?

We had 4-5 15-foot lengths of 1/2" diameter tube up in our storage area, as well as a 1" dia. tube. It was given to us back in the day by somebody.

Some of our kids looked up the info for the codes printed on the outside of the tubing. The cost they found was somewhere around $144 per foot!

Is that anywhere near a sensible number?

It might be better if we tried to sell the stuff as a fundraiser lol.

[David Brinza]

Something to remember when discussing structural materials properties:

The terms strength and stiffness are not interchangeable.

Strength is the resistance to breakage, whereas stiffness is resistance to deformation. A nylon rope is incredibly strong, but not at all stiff. A ceramic rod is quite stiff, but has little strength.

Stiffness of structure depends on material and geometry. For instance, a 1/8" x 2" aluminum bar is not nearly as stiff as 1/8" x 1" aluminum angle, but their tensile strength is almost the same.