Carbon fiber reinforced plastic (CFRP or CRP), is a very strong, light and expensive composite material or fiber reinforced plastic. Similar to glass-reinforced plastic, sometimes known by the genericised trademark fiberglass, the composite material is commonly referred to by the name of its reinforcing fibers (carbon fiber). The plastic is most often epoxy, but other plastics, such as polyester, vinyl ester or nylon, are also sometimes used. Some composites contain both carbon fiber and other fibres such as kevlar, aluminium and fiberglass reinforcement. The terms graphite-reinforced plastic or graphite fiber reinforced plastic (GFRP) are also used but less commonly, since glass-(fibre)-reinforced plastic can also be called GFRP.
It has many applications in aerospace and automotive fields, as well as in sailboats, and notably in modern bicycles and motorcycles, where its high strength to weight ratio is of importance. Improved manufacturing techniques are reducing the costs and time to manufacture making it increasingly common in small consumer goods as well, such as laptop computers, tripods, fishing rods, paintball equipment, racquet sports frames, stringed instrument bodies, classical guitar strings, and drum shells.
Materials produced with the above-mentioned methodology are often generically referred to as composites. The choice of matrix can have a profound effect on the properties of the finished composite. One method of producing graphite-epoxy parts is by layering sheets of carbon fiber cloth into a mold in the shape of the final product. The alignment and weave of the cloth fibers is chosen to optimize the strength and stiffness properties of the resulting material. The mold is then filled with epoxy and is heated or air cured. The resulting part is very corrosion-resistant, stiff, and strong for its weight. Parts used in less critical areas are manufactured by draping cloth over a mold, with epoxy either preimpregnated into the fibers (also known as prepreg), or "painted" over it. High performance parts using single molds are often vacuum bagged and/or autoclave cured, because even small air bubbles in the material will reduce strength.
The process in which most carbon fiber reinforced plastic is made varies, depending on the piece being created, the finish (outside gloss) required, and how many of this particular piece are going to be produced.
For simple pieces of which relatively few copies are needed, (1–2 per day) a vacuum bag can be used. A fiberglass or aluminum mold is polished, waxed, and has a release agent applied before the fabric and resin are applied and the vacuum is pulled and set aside to allow the piece to cure (harden). There are two ways to apply the resin to the fabric in a vacuum mold. One is called a wet layup, where the two-part resin is mixed and applied before being laid in the mold and placed in the bag. The other is a resin induction system, where the dry fabric and mold are placed inside the bag while the vacuum pulls the resin through a small tube into the bag, then through a tube with holes or something similar to evenly spread the resin throughout the fabric. Wire loom works perfectly for a tube that requires holes inside the bag. Both of these methods of applying resin require hand work to spread the resin evenly for a glossy finish with very small pin-holes. A third method of constructing composite materials is known as a dry layup.
Here, the carbon fiber material is already impregnated with resin (prepreg) and is applied to the mold in a similar fashion to adhesive film. The assembly is then placed in a vacuum to cure. The dry layup method has the least amount of resin waste and can achieve lighter constructions than wet layup. Also, because larger amounts of resin are more difficult to bleed out with wet layup methods, prepreg parts generally have fewer pinholes. Pinhole elimination with minimal resin amounts generally require the use of autoclave pressures to purge the residual gases out.
A quicker method uses a compression mold. This is a two-piece (male and female) mold usually made out of fiberglass or aluminum that is bolted together with the fabric and resin between the two. The benefit is that, once it is bolted together, it is relatively clean and can be moved around or stored without a vacuum until after curing. However, the molds require a lot of material to hold together through many uses under that pressure.
Many carbon fiber reinforced plastic parts are created with a single layer of carbon fabric, and filled with fiberglass. A chopper gun can be used to quickly create these types of parts. Once a thin shell is created out of carbon fiber, the chopper gun is a pneumatic tool that cuts fiberglass from a roll and sprays resin at the same time, so that the fiberglass and resin are mixed on the spot. The resin is either external mix, where the hardener and resin are sprayed separately, or internal, where they are mixed internally, which requires cleaning after every use.
For difficult or convoluted shapes, a filament winder can be used to make pieces.
Civil engineering applications
Carbon fiber reinforced plastic has over the past two decades become an increasingly notable material used in structural engineering applications. Studied in an academic context as to its potential benefits in construction, it has also proved itself cost-effective in a number of field applications strengthening concrete, masonry, steel and timber structures. Its use in industry can be either for retrofitting to strengthen an existing structure, or as an alternative reinforcing (or prestressing material) to steel from the outset of a project.
Retrofitting has become the increasingly dominant use of the material in civil engineering, and applications include increasing the load capacity of old structures (such as bridges) that were designed to tolerate far lower service loads than they are experiencing today, seismic retrofitting, and repair of damaged structures. Retrofitting is popular in many instances as the cost of replacing the deficient structure can greatly exceed its strengthening using CFRP. Due to the incredible stiffness of CFRP, it can be used underneath bridge spans to help prevent excessive deflections, or wrapped around beams to limit shear stresses.
When used as a replacement for steel, CFRP bars are used to reinforce concrete structures. More commonly they are used as prestressing materials due to their high stiffness and strength. The advantages of CFRP over steel as a prestressing material, namely its light weight and corrosion resistance, enable the material to be used for niche applications such as in offshore environments.CFRP is a more costly material than its counterparts in the construction industry, glass fibre reinforced polymer (GFRP) and aramid fibre reinforced polymer (AFRP), though CFRP is generally regarded as having superior properties.
Much research continues to be done on using CFRP both for retrofitting and as an alternative to steel as a reinforcing or prestressing material. Cost remains an issue and long term durability questions still remain. Some are concerned about the brittle nature of CFRP, in contrast to the ductility of steel. Though design codes have been drawn up by institutions such as the American Concrete Institute, there remains some hesitation among the engineering community about implementing these alternative materials. In part this is due to a lack of standardisation and the proprietary nature of the fibre and resin combinations on the market, though this in itself is advantageous in that the material properties can be tailored to the desired application requirements.