Designing Titanium Carbon Uprights for racecar

blixxa

Hello,

For my University project I am trying to design uprights for a race car. Currently we are using TIG welded 4130, I am trying to push the Titanium carbon direction.

I have many questions!

The idea of the upright is to use a titanium core (manufacture from laser sintering) then skin it with carbon fiber. This should mean the core will have good compressive strenght and the skin will have a high tensile strenght.

One of the quierys I have is what the structure of the core should look like. The sintering machine can do a repeated pattern to build up a 3d object, What is the best pattern for the core? I was thinking a honey comb structure.

Please help,

Thank you.

Mech_Engineer

What makes you think titanium and carbon are a good match mechanically? High-strength Titanium alloys have high yield strengths, and relatively low density and elastic modulus. This means they tend to bend farther than steel for a given force but have a high strength to weight ratio compared to steel. carbon fiber composites on the other hand have very high modulus or elasticity and high ultimate strength; they bend very little under force but tend to fail catastrophically when pushed to the limit.

My thinking is if you want a very light and strong structure, just go with straight carbon fiber or maybe a carbon-fiber composite paired with some other high strength fiber like kevlar. Adding the titanium core in the middle isn't gaining you anything a novel geometry can't (or even just alunimum or magnesium which are lighter), and the dissimilarities between the materials will play havoc with bonding and thermal expansion.

BTown

I agree with Mech_Engineer on using straight carbon fiber instead.

carbo-titanium alloys have been around for a while, but are scarcely used because of their high cost and bonding problems. NASA did a study on this back in the 70's, but I am unsure what the results were.

If you do decide to do it, then yes the honeycomb pattern will do just fine.

Mech_Engineer

When you say "carbo-titanium alloys" do you mean composites or alloys? If composites, do such composites use alternating meshes of CF and Ti or do they utilize a bulk Ti base with CF surface layering?

BTown

I was referring to a composite with alternating meshes. Reading the initial post again, I realize that this is not what blixxa was referring to. Apologies...

Nonetheless, I still would be a strong advocate of straight CF as you suggested, especially considering the application.

Mech_Engineer

That's what I thought as well. Carbon-fiber composites can have different alternating meshes in it to gain certain required performance characteristics (for example layering with Invar to reduce thermal expansion), but I don't know about "coating" a part in carbon fiber like the OP is proposing. I suspect he thinks CF composites are inherently weak in compression, but that depends more on the layer angles, resin matrix, and the direction the force is applied w.r.t. the geometry and composite matrix.

When it comes down to it, the large mismatch in modulus of elasticity between CF and Ti make what the OP is proposing a bad idea; the portion that deflects the least will take the majority of the force in tension or compression. In the end, the design would end up something like a bad combination of a bimetallic strip and statically indeterminate structure.

BTown

Exactly, and going back to your original post, the titanium effectively doesn't buy you anything except a few extra pounds, which is definitely something to be avoided for a racing vehicle.

I wonder, would the opposite gain you anything (CF w/ Ti coating)? The titanium could offer flexibility, and a greater resistance to impact forces (may be applicable in a racing situation), while allowing the overall structure to be lighter weight than straight Ti.

Mech_Engineer

In my estimation the only thing a coating on a CF component gets you is more abrasion resistance; no need for that to be titanium though it could be any number of abrasion-resistant materials (aluminum or HDPE for example).

LOTS of race vehicles utilize all sorts of carbon-fiber components. Usually the only thing in a race car's chassis that can't be made out of CF is the roll cage for toughness and energy absorption concerns.

BTown

Good point, a much more cost effective solution as well, especially considering the large variety of Al alloys out there now (prefer 6061 T6 myself, but not sure of how it performs when acting as a coating).