Originally Posted by asmallsol

I'm a materials enginEEring student (senior so I have a little credibility). Carbon fiber parts are a fiber reinforced plastic. The typical plastic being used is epoxy. Carbon itself is extremely stiff (modulus of elasticity is about 700 GPa compared to Reynolds 531 which is like AISI 4130 steel has a modulus of 205 GPa {when normalized to 870°C and air cooled.}) and an extremly low density (1.78-2.15 in fiber form, compared to 7.9 g/cm^3 for a high strength steel.

Now the problem. Carbon itself is EXTREMELY brittle. Think of pencil lead. Same stuff really. However, if you make the fibers small enough, then there isn't a large enough internal bending moment to fracture the fiber. Its the same principle for fiber glass (truly just normal silicon glass that is made extremely thin.) In essence, you can not fracture these fibers by bending them, however, you can bend them easily so thats not the best for supporting human life with. However, in the direction of the fiber length, it can support a lot of force. In a perpendicular weave orientation (typical looking checkerboard style) you can support a lot of force in both directions and when the force is applied at an angle the vector is resolved to each direction in the fiber.

Now there is epoxy. It is not really stiff, nor light. However, pre set, it flows alright at normal temperatures (I work with it a lot with the only protection being rubber gloves because the stuff is kinda messy) so it can impregnate the carbon weave relatively easily by using vacuums. Once set, it holds the carbon weave in place. In reality, this is the major role the matrix plays. Using more only decrease the specific strength of the carbon fiber composite (makes heavier without increasing the strength) Lately companies (easton is a big leader in this) have been using carbon nano tubes in the matrix to displace some of the epoxy (use less epoxy) because the CNT's are less dense then the matrix they displace.

In a composite, many times the strength of one material is reduced and weight increased, (carbon) while the flexibility of the other is reduced (epoxy) to create a material with a compromise in strength and weight. Looking in my old Callister intro to MSE book, there is a great figure that shows this however, I can't find the same figure online (figure 15.9 for any one who's ever taken intro to MSE and used this book which almost every intro MSE class does) The result is a material who's specific strength is multiple time higher then steel.

Now why not PVC. PVC alone is weak, brittle, worthless for structural purposes. Now could it be used as the matrix of a composite. Yes, and strength could be increased. However PVC is a thermoplastic that has a processing temp around 400*F/200*C. Heating a liquid to this high is expensive, and makes it much more difficult to work with. Epoxy is a product of mixing two liquids resin and hardener together, and within about 8 hours (less if baked) become a solid. This makes it much easier to work with. Thats not to say carbon fiber and thermoplastics do not meet. I do materials testing for an auto-company. One of the things I see a lot is impregnating tiny (as in you cant see without an SEM microscope) carbon fibers into plastics to increase their electrical conductivity.

And before anyone jumps on me for saying blah blah blah, I am simply giving a very brief overview of composites. The science behind them is pretty interesting.