What is the difference between Carbon fibre and Kevlar?

Kevlar 29 is used for bullet proof vests. Kevlar 49 is used in light weight construction or semi loaded structure, F1, Indy cars and an acceptable material for side protection, for SCCA racing body parts. Kevlar 49 is the preferred material to use on more expensive, modern light weight sail planes or canoes.

Some of the negatives of Kevlar are, it is hard to work with, more set up time is necessary then fiber glass, it is not very easy to cut, it has a tendency to buckle under compression. the simple wet hand lay up process is not preferred, the VARTM or vacuum assisted resin transfer molding lay up process is the preferred method for proper absorption. The positives of Kevlar are it is lighter, then both carbon fiber and fiber glass, its tensile strength is higher then, carbon fiber, but it is not as stiff as carbon fiber. Kevlar is extremely tough and abrasion resistant, Kevlar falls in between S2 fiberglass and Carbon fiber in cost and stiffness. Carbon fiber on the other hand has been used for Race chassis and it is extremely stiff and strong. Once Kevlar has been over loaded it will yield to a degree like steel, this is where Kevlar will show its toughness.

On the other hand the negatives of Carbon fiber are it is very brittle and less tough than kevlar, once the maximum load has been reached for carbon fiber it will disintegrate or shatter under high energy impact literally explode, gone ! So your feet or legs are left to dangle and flip around with the rest of the race car chassis smashing into everything until the chassis comes to a complete stop. This exposure is not good for the human body. The fragments and splinters from Carbon fiber are very painful and may require magnified surgery for removal. The positives of Carbon fiber are it is very stiff and can help to make structures being constructed with it even stiffer and lighter with its use , in combination with EV or HT resins systems it is used as loaded structure. and it is not as expensive or as toxic as Boron fiber. And it reveals a beautiful appearance through clear and transparent resin's.

In England, race car Designer Colin Chapman from lotus started using a Carbon / Kevlar hybrid cloth an 85/15 type ratio where the two fiber strands combined together seemed to make for a wonderful cloth for composites, this combination seemed to help keep the pieces together, even after a few good impacts. But unfortunately due to pressure to compete against one another as race teams do, making race cars faster is race teams job. So a better handling car means greater ft lbs. per degree of stiffness in the chassis, so for some race teams out went the Kevlar for safety and back to the all Carbon fiber chassis.

The current engineer's concept and standard here today is that if they use mostly intermediate modulus carbon fiber and some higher modulus carbon fiber in a fewer key select areas, combined with using the new advancements that have been made to epoxies, combining micro particle's at 100 nm in size to reduce the occurrence of micro cracking during the curing process, these particle's help to increase the composites toughness in the finished stage enough so to withstand against any applied loads to the structure without the Kevlar the carbon fiber then is able to hold up against any impact forces, it may experience during a crash.

So the Carbon fibers toughness becomes primary and the added Kevlar as a safety become's secondary, To exclude Kevlar as a fail safe material from a race chassis design altogether removes a factor of safety that was once realized. possibly a shared approach with the intermediate modulus carbon fiber and Kevlar blend as a cloth with the micro particle's blended in the epoxy resin as for the interior layers just as a fail safe to prevent any protrusion's could be the possible method of thought, as seen in some of the higher end Sports racing classes like GTP and World cup series interior's used for safety. I also believe the improvements made by using micro particle's at 100 nm in size in combination in use with the epoxy, some of the tragic occurrences such as structural failure we've seen over the years on race tracks, will be a thing from the past.

The development and full understanding of these modern composites materials is ongoing and is still in its beginning stage's no one knows everything they can about these materials, so many variables are involved. One should always work towards gaining a better understanding through a scientific approach, from testing and research. These engineering Materials and their manufacturing processes can have an effect or change on the other materials properties once they are combined, this is typical. A clear understanding of their effects upon the other and their properties combined as a whole, should be understood first, before designing anything.