CARBON FIBER SUPERSEDES ALUMINUM
SK ANSARI, SEEMA ANSARI
July 12 - 18, 2010
Carbon fiber (alternately called graphite fiber or carbon graphite) is a material consisting of extremely thin fibers and composed mostly of carbon atoms. The carbon atoms are bonded together in microscopic crystals that are more or less aligned parallel to the long axis of the fiber. The crystal alignment makes the fiber incredibly strong for its size.
Several thousand carbon fibers are twisted together to form a yarn, which may be used by itself or woven into a fabric. Carbon fiber has many different weave patterns and can be combined with a plastic resin and wound or molded to form composite materials such as carbon fiber reinforced plastic (also referenced as carbon fiber) to provide a high strength-to-weight ratio material. The density of carbon fiber is also considerably lower than the density of steel, making it ideal for applications requiring low weight.
SYNTHESIS OF CARBON FIBER
A common method of making carbon filaments is the oxidation and thermal pyrolysis of polyacrylonitrile (PAN), a polymer based on acrylonitrile used in the creation of synthetic materials. Like all polymers, polyacrylonitrile molecules are long chains, which are aligned in the process of drawing continuous filaments. A common method of manufacture involves heating the PAN to approximately 300 ∞C in air, which breaks many of the hydrogen bonds and oxidizes the material. The oxidized PAN is then placed into a furnace having an inert atmosphere of a gas such as argon, and heated to approximately 2000 ∞C, which induces graphitization of the material, changing the molecular bond structure. When heated in the correct conditions, these chains bond side-to-side (ladder polymers), forming narrow graphene sheets which eventually merge to form a single, jelly roll-shaped or round filament. The result is usually 93ñ95 per cent carbon. Lower-quality fiber can be manufactured using pitch or rayon as the precursor instead of PAN. The carbon can become further enhanced, as high modulus, or high strength carbon, by heat treatment processes. Carbon heated in the range of 1500-2000 ∞C (carbonization) exhibits the highest tensile strength while carbon fiber heated from 2500 to 3000 ∞C (graphitizing) exhibits a higher modulus of elasticity.
Carbon fibers are a key constituent in advanced composite materials, which are used in demanding defense aerospace applications. From military aircraft, such as the F-22, F/A-18, AV-8B, and B-2, to strategic missiles such as Trident II D5, to space launch vehicles, such as the Titan IV solid rocket motor upgrade, to satellite structures.
Oxidative stabilisation: The polyacrylonitrile precursor is first stretched and simultaneously oxidized in a temperature range of 200-300∞C. This treatment converts thermoplastic PAN to a non-plastic cyclic or ladder compound.
Carbonisation: After oxidation, the fibers are carbonised at about 1000∞C without tension in an inert atmosphere (normally nitrogen) for a few hours. During this process the non-carbon elements are removed as volatiles to give carbon fibers with a yield of about 50 per cent of the mass of the original PAN.
Graphitisation: Depending on the type of fiber required, the fibers are treated at temperatures between 1500-3000∞C, which improves the ordering, and orientation of the crystallites in the direction of the fiber axis.
TYPICAL PROPERTIES OF CARBON WITH COMPARABLE METAL
(Thousand Pound per Sq. inch)
( Million of Pound per Sq. Inch)
METALS Aluminum 80 10 2.76 Titanium 160 16 4.42 Steel 200 30 8.00 COMPOSITES Glass/epoxy 250 8 1.99 Aramid (Kevlar)/epoxy 190 12 1.38 Carbon (graphite)/epoxy 215 21 1.55
CARBON FIBER USAGE ON AEROSPACE SYSTEMS
CARBON FIBER TYPE
. PAN PITCH RAYON MISSILES Strategic Used Used Used Tactical Used Not Used Used SPACE Launch vehicles Used Not Used Used Satellite Used Used Not Used AIRCRAFT Fixed-wing Used Used Not Used Rotary-wing Used Not Used Not Used
PROPERTIES OF CARBON FIBER
In general, it is seen that the higher the tensile strength of the precursor the higher is the tenacity of the carbon fiber. Tensile strength and modulus are significantly improved by carbonization under strain when moderate stabilisation is used.
X-ray and electron diffraction studies have shown that in high modulus type fibers, the crystallites are arranged around the longitudinal axis of the fiber with layer planes highly oriented parallel to the axis. Overall, the strength of a carbon fiber depends on the type of precursor, the processing conditions, heat treatment temperature and the presence of flaws and defects.
With PAN based carbon fibers, the strength increases up to a maximum of 1300oC and then gradually decreases. The modulus has been shown to increase with increasing temperature. PAN based fibers typically buckle on compression and form kink bands at the innermost surface of the fiber. However, similar high modulus type pitch-based fibers deform by a shear mechanism with kink bands formed at 45∞ to the fiber axis. Carbon fibers are very brittle. The layers in the fibers are formed by strong covalent bonds. The sheet-like aggregations allow easy crack propagation. On bending, the fiber fails at very low strain.
The two main applications of carbon fibers are in specialised technology, which includes aerospace and nuclear engineering, and in general engineering and transportation, which includes engineering components such as bearings, gears, cams, fan, blades and automobile bodies.
Recently, some new applications of carbon fibers have been found. Such as bridge rehabilitation, building and construction of industry. Others include: decoration in automotive, marine, general aviation interiors, general entertainment and musical instruments and after-market transportation products.
CHARACTERISTICS AND APPLICATIONS OF CARBON FIBERS
1 Physical strength, specific toughness, light weight Aerospace, road and marine transport, sporting goods 2 High dimensional stability, low coefficient of thermal expansion, and low abrasion Missiles, aircraft brakes, aerospace antenna and support structure, large telescopes, optical benches, waveguides for stable high-frequency (GHz) precision measurement frames 3 Good vibration damping, strength, and toughness Audio equipment, loudspeakers for Hi-fi equipment, pickup arms, robot arms 4 Electrical conductivity Automobile hoods, novel tooling, casings and bases for electronic equipments, EMI and RF shielding, brushes 5 Biological inertness and x-ray permeability Medical applications in prostheses, surgery and x-ray equipment, implants, tendon/ligament repair 6 Fatigue resistance, self-lubrication, high damping Textile machinery, genera engineering 7 Chemical inertness, high corrosion resistance Chemical industry; nuclear field; valves, seals, and pump components in process plants 8 Electromagnetic properties Large generator retaining rings, radiological equipment
APPLICATION TO BUILDING AND CIVIL ENGINEERING FIELDS
In buildings and civil engineering field, the reinforcement of structures among others is the first area of application of Carbon Fibers with favored advantages of lightweight and high strength.
To increase durability of concrete structures such as bridges by covering them with Carbon Fiber sheets is being recognised as an effective reinforcement measure to increase resistance against earthquakes. This technology is used in various part of the world, among others in Japan.
Carbon Fibers are used also for CNG tanks for natural gas cars which are becoming more and more popular now and for hydrogen tanks installed on fuel battery cars which are regarded as the environment friendly favorite in future.
The technology to produce lightweight, high pressure compressed gas tanks with aluminum or plastic liners reinforced by Carbon Fibers winding is becoming popular now, and you can see natural gas cars reinforced by Carbon Fiber in the future.
Carbon Fibers are used also for high-speed railway trains.
Wind power generation which has become popular recently is expected to require bigger and bigger blades to have higher and higher output capacity for each unit. In order to support big size blades, CFRP becomes vitally necessary.
And as the material for high speed rotating body for fly wheels which are also attracting public attention as a technology to store energy effectively based on theory of top spinning, again CFRP is becoming popular.
And fuel batteries, the favorite for clean energy technology, use Carbon Fiber material at their heart parts. Keeping pace with the popularistion of such fuel batteries, the demand of Carbon Fibers are expected to grow up.
On the other hand, in petroleum oil industries, more and more reduction of weight is required for cables supporting offshore constructions and pipes to pump oils up with the development of deep water drilling. Also in this field the use of lightweight, high strength CFRP is under serious study. This is also attracting public attention as a potential big application field.
The 787 is the first commercial airplane to make the change from metal to composite structure. The majority of the 787í major structure is made out of composite material. The Dream liner is the first commercial airplane to be built with a one-piece fuselage. Manufacturing a one-piece fuselage section eliminates 1,500 aluminum sheets and 40,000 to 50,000 fasteners.
A new material - carbon fiber reinforced plastic seems poised to supersede the universal use of aluminum. This material will be used in the wings of the new Airbus A350, A380 and on structural components of the new military lift plane, the A400M. The A350XWB (Xtra Wide Body) will use carbon fiber reinforced plastic (CFRP) panelled fuselage skins that are easier to repair and maintain. Weight saving is achieved via optimum fiber lay-up and skin thickness is tailored to the requirements of the specific location. This all new composite wing design will yield wings spanning 64 meters on a 66.9 meters long body. The maximum take off weight of the A350 is 265 tons.
Airbus introduced composites way back in the late 1970s on secondary structures in the A310 aircraft. By 1985, composites were applied on primary structures and in the innovative drag-reducing wingtip devices on the A310-300. Today, composites are used throughout aircrafts such as the A380. A380 is also the sole commercial plane employing them in the centre wing box and rear fuselage.
The world's first carbon-fiber keel beam for a large aircraft was built for the A340-600, and Airbus' 21st century airliner - the 525-seat A380 - is continuing the tradition of innovation with the increased use of carbon fiber reinforced plastic (CFRP). Airbus has the first application of glass fiber-aluminum laminate on a civil airliner, and was also the first to introduce laser beam welding on a civil aircraft on the A318.