Carbon fiber reinforced plastic
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Carbon fiber reinforced plastic or (CFRP or CRP), is a very strong, light and expensive composite material or fiber reinforced plastic. Similar to glass-reinforced plastic, which is sometimes simply called 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 fiberglass reinforcement. Less commonly, the term graphite-reinforced plastic is also used.
It has many applications in aerospace and automotive fields, as well as in sailboats, and notably in modern bicycles, where these qualities are of importance. It is becoming increasingly common in small consumer goods as well, such as laptop computers, tripods, fishing rods, racquet sports frames, stringed instrument bodies, classical guitar strings, and drum shells.
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[edit] Composite
The choice of matrix can have a profound effect on the properties of the finished composite. One common plastic for this application is epoxy, and materials produced with this methodology are often generically referred to as composites. One way 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 carefully selected to optimize the strength and stiffness properties of the resulting material. In demanding applications, all air is evacuated from the mold, but in applications where cost is more important than structural rigidity, this step is skipped. The mold is then filled with epoxy and is heated or air cured. The resulting stiff panel will not corrode in water and is very strong, especially for its weight. If the mold contains air, small air bubbles will be present in the material, reducing strength. Most composite parts are manufactured by draping cloth over a mold, with epoxy either preimpregnated into the fibers (also known as prepreg), or "painted" over it. Hobby or cosmetic parts are often made this way, as are high performance aerospace parts. High performance parts using single molds are often vacuum bagged and/or autoclave cured.
The large amount of (often manual) work required to manufacture composites has hitherto limited their use in applications where a high number of complicated parts is required.
The chemistry and manufacturing techniques for thermosetting plastics like epoxy are often poorly suited to mass-production. One potentially cost-saving and performance-enhancing measure involves replacing the epoxy matrix with a thermoplastic material such as nylon or polyketone. Boeing's entry in the Joint Strike Fighter competition included a delta-shaped carbon fiber reinforced thermoplastic wing, but difficulties in fabrication of this part contributed to Lockheed Martin winning the competition.
[edit] Process
The process in which most CFRP 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 that relatively few copies are needed of (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 (pre-preg) 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 gasses 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 CFRP 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 impossible shapes (such as a tube) a filament winder can be used to make pieces.
[edit] Automotive uses
CFRP is used extensively in automobile racing, especially in Formula One and IndyCar racing. The high cost of carbon fiber is mitigated by the material's unsurpassed strength-to-weight ratio, and low weight is essential for high-performance automobile racing. Racecar manufacturers have also developed methods to give carbon fiber pieces strength in a certain direction, making it strong in a load-bearing direction, but weak in directions where little or no load would be placed on the member. Conversely, manufacturers developed omnidirectional carbon fiber weaves that apply strength in all directions. This type of carbon fiber assembly is most widely used in the "safety cell" monocoque chassis assembly of high-performance racecars.
Several supercars over the past few decades have incorporated CFRP extensively in their manufacture, using it for their monocoque chassis as well as other components. Examples include the Koenigsegg CCR, Koenigsegg CCX, McLaren F1, Bugatti Veyron, Bugatti EB110, Pagani Zonda, Enzo Ferrari and Porsche Carrera GT.
Until recently, the material has had limited use in mass-produced cars because of the expense involved in terms of materials, equipment, and the relatively limited pool of individuals with expertise in working with it. Recently, several mainstream vehicle manufacturers such as General Motors and BMW have started to use carbon fiber technology in everyday road cars.
Chevrolet is using carbon fiber in a special version of its flagship sports car, the Corvette. The Z06, a special high performance version of the Corvette, includes carbon fiber front bodywork for reduced weight and added rigidity instead of glass-reinforced plastic bodywork found in the standard Corvette.
BMW produces carbon fiber reinforced plastics in its Landshut plant. To make the roof of the BMW M3 CSL, for example, five layers of carbon fiber cloth are placed in an 1,800 ton press, where epoxy is resin transfer molded and heat-cured in a robot-automated process. The resulting roof is half the weight of an equivalent steel roof.
Use of the material has been more readily adopted by low-volume manufacturers like TVR who use it primarily for creating body-panels for some of their high-end cars due to its increased strength and decreased weight compared with the glass-reinforced plastic they use for the majority of their products.
Often street racers or hobbyist tuners will purchase a CFRP hood, spoiler or body panel as an aftermarket part for their vehicle. However, these parts are rarely made of full carbon fiber. They are often just a single layer of carbon fiber laminated onto fiberglass for the "look" of CF. It is common for these parts to remain unpainted to accentuate the look of the carbon fiber weave.
[edit] Civil engineering applications
CFRP has recently become somewhat of a hot topic in the field of Structural Engineering, surprisingly enough, due to cost-effectiveness. For example, many old bridges in the world were designed to tolerate far lower service loads than they are subject to today, and compared with the cost of replacing the bridge, reinforcing it with CFRP is quite cheap. Due to the incredible stiffness of CFRP, it can be used underneath spans to help prevent excessive deflections, or wrapped around beams to limit shear stresses. As of 2005, the Westgate Bridge in Melbourne, is the largest bridge in the world to be reinforced with carbon fiber laminates [1].
Much research is also now being done using CFRP as internal reinforcement in concrete structures, such as beams and bridge decks. The material has many advantages over conventional steel, mainly that it is much stiffer and corrosion resistant. There is, however, some hesitation among the engineering community about implementing these new materials until more real-world evaluation has been done.
[edit] Other applications
An area where CFRP has found good use is in the manufacture of bicycles, especially high-end racing bicycles. The vibration absorbing properties of CFRP make for a less harsh ride, while offering weight reduction compared to traditional bicycle tubing materials such as aluminum or steel. The choice of weave can be carefully selected to maximize stiffness. Exploitation of the variety of shapes CFRP can be built into has further increased stiffness and also allowed aerodynamic considerations into tube profiles. CFRP frames, forks, handlebars, seatposts and crank arms are becoming commonplace on medium- and higher-priced bicycles. CFRP forks are used on most new racing bicycles.
Another widespread use of carbon fiber is in the manufacture of fishing rods. Its high flexibility and low weight make it ideal to feel every bite.
Most modern rowing shells are made of carbon fiber, which significantly lowers the weight of the boat.
A Resin Transfer Moulding system has been developed by Wilson Benesch that enables Advanced Composite Structures to be created. This manufacturing technology creates the stiffest and lightest loudspeaker cabinet. The damping properties are exceptional, not dis similar to the same vibration absorbing abilities found in high performance cycles. The combination of these properties makes it possible to achieve the lowest possible sonic output from the cabinet. The absence of cabinet borne distortions makes for a more accurate transducer.
[edit] Recycling
An important usage concern involves the material's entire lifecycle, as carbon fiber reinforced plastics have an almost infinite lifetime. Some companies [2] are succeeding in recycling this carbon fiber. The recycling strategy centers on milling, compounding or shredding the reclaimed carbon fiber, and finding use for this end product in various industrial applications (including carbon fiber applications less stringent than those required by, say, the aerospace industry). It is also commonly used in electronics, such as laptops, to lower the weight load and to improve durability.
[edit] Supply constraints
The market price of CFRP saw a 150% increase during 2005, primarily due to increased use in the civil aerospace industry. Allocation and contracts to Boeing Commercial Airplanes' 787 and Airbus' A350 XWB and A380 caused supply constraints, and suppliers such as Toray Industries raised prices to suit. As the aerospace companies were locked-in with long-term contracts, users of CFRP in other applications have suffered the brunt of the price hike.[1]
[edit] References
- ^ "In Composite Race, Jets May Beat Out Bikes," The Wall Street Journal, January 24, 2006.
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