CARBON FIBRE REINFORCED POLYMER

CARBON FIBRE REINFORCED POLYMER
The main impetus for development of carbon fibres has come from the aerospace industry with its need for a material with combination of high strength, high stiffness and low weight. Recently, civil engineers and construction industry have begun to realize that this material (CFRP) have potential to provide remedies for many problems associated with the deterioration and strengthening of infrastructure. Effective use of carbon fibre reinforced polymer could significantly increase the life of structures, minimizing the maintenance requirements.
Carbon fibre reinforced polymer is a type of fibre composite material in which carbon fibres constitutes the fibre phase. Carbon fibre are a group of fibrous materials comprising essentially elemental carbon. This is prepared by pyrolysis of organic fibres. PAN-based (PAN-poly acrylo nitrile) carbon fibres contains 93-95 percentage carbons, and it is produced at 1315°C (2400°F). Carbon fibres have been used as reinforcement for albative plastics and for reinforcements for lightweight, high strength and high stiffness structures. Carbon fibres are also produced by growing single crystals carbon electric arc under high-pressure inert gas or by growth from a vapour state by thermal decomposition of a hydrocarbon gas.
CFRP materials possess good rigidity, high strength, low density, corrosion resistance, vibration resistance, high ultimate strain, high fatigue resistance, and low thermal conductivity. They are bad conductors of electricity and are non-magnetic.
Carbon fibre reinforced polymer (CFRP) is currently used world wide to retrofit and repair structurally deficient infrastructures such as bridges and buildings. Using CFRP reinforcing bars in new concrete can eliminate potential corrosion problems and substantially increase a member’s structural strength. When reinforced concrete (RC) members are strengthened with externally bonded CFRP, the bond between the CFRP and RC substrate significantly affects the members load carrying capacity.
Strengthening measures are required in structures when they are required to accommodate increased loads. Also when there are changes in the use of structures, individual supports and walls may need to be removed. This leads to a redistribution of forces and the need for local reinforcement. In addition, structural strengthening may become necessary owing to wear and deterioration arising from normal usage or environmental factors.
The usage of composite materials like CFRP is still not widely recognized. The lack of knowledge of technology using CFRP and the simplicity of it will make some people hesitant to use it. Carbon Fibre strengthening system includes laminate and sheet products for increasing the strength of concrete structures. Carbon, Aramid and E-glass can be used to solve problems from change of use to increased load carrying capacity to bomb blast protection. Simple installation, minimal increase of deadload and ability to deal with complex strengthening requirements make this a problem solve.

Strengthening of a structure may be necessary if increases in loads, changes in structural articulation or intended use occur, and also, after accidental damages. Depending on the location of structure, access to the repair area, or the time allow for the repair, several materials and technique have to be considered. Carbon Fiber Reinforced Polymer Strip is an external strengthening system that can be used on structural elements comprised of concrete, wood, or steel. This system consists of a pultruded, pre-cured carbon fiber reinforced polymer (CFRP) strip and a high modulus/high strength epoxy gel. The strips are adhered using epoxy gel to structural elements to increase flexural capacity, fatigue resistance and reduce deflection.

Carbon Fiber Reinforced Polymer (CFRP) is a Polymer Matrix Composite material reinforced by carbon fibers.
The reinforcing dispersed phase may be in form of either continuous or discontinuous carbon fibers of diameter about 0.0004” (10 mkm) commonly woven into a cloth.
Carbon fibers are very expensive but they possess the highest specific (divided by weight) mechanical properties: modulus of elasticity and strength.
Carbon fibers are used for reinforcing polymer matrix due to the following their properties:
• Very high modulus of elasticity exceeding that of steel;
• High tensile strength, which may reach 1000 ksi (7 GPa);
• Low density: 114 lb/ft³ (1800 kg/m³);
• High chemical inertness.
The main disadvantage of carbon (Graphite) fibers is catastrophic mode of failure (carbon fibers are brittle).
The types of carbon fibers are as follows:
• UHM (ultra high modulus). Modulus of elasticity > 65400 ksi (450GPa).
• HM (high modulus). Modulus of elasticity is in the range 51000-65400 ksi (350-450GPa).
• IM (intermediate modulus). Modulus of elasticity is in the range 29000-51000 ksi (200-350GPa).
• HT (high tensile, low modulus). Tensile strength > 436 ksi (3 GPa), modulus of elasticity < 14500 ksi (100 GPa).
• SHT (super high tensile). Tensile strength > 650 ksi (4.5GPa).
Carbon fibers are also classified according to the manufacturing method:
1. PAN-based carbon fibers (the most popular type of carbon fibers).
In this method carbon fibers are produced by conversion of polyacrylonitrile (PAN) precursor through the following stages:
• Stretching filaments from polyacrylonitrile precursor and their thermal oxidation at 400°F (200°C). The filaments are held in tension.
• Carbonization in Nitrogen atmosphere at a temperature about 2200 °F (1200°C) for several hours. During this stage non-carbon elements (O,N,H) volatilize resulting in enrichment of the fibers with carbon.
• Graphitization at about 4500 °F (2500°C).
2. Pitch-based carbon fibers.
Carbon fibers of this type are manufactured from pitch:
• Filaments are spun from coal tar or petroleum asphalt (pitch).
• The fibers are cured at 600°F (315°C).
• Carbonization in nitrogen atmosphere at a temperature about 2200 °F (1200°C).
The most popular matrix materials for manufacturing Carbon Fiber Reinforced Polymers (CFRP) are thermosets such as epoxy, polyester and thermoplastics such as nylon (polyamide).
Carbon Fiber Reinforced Polymers (CFRP) materials usually have laminate structure, providing reinforcing in two perpendicular directions.
Carbon Fiber Reinforced Polymers (CFRP) are manufactured by open mold processes, closed mold processes and Pultrusion method.
Carbon Fiber Reinforced Polymers (CFRP) are characterized by the following properties:
• Light weight;
• High strength-to-weight ratio;
• Very High modulus elasticity-to-weight ratio;
• High Fatigue strength;
• Good corrosion resistance;
• Very low coefficient of thermal expansion;
• Low impact resistance;
• High electric conductivity;
• High cost.
Carbon Fiber Reinforced Polymers (CFRP) are used for manufacturing: automotive marine and aerospace parts, sport goods (golf clubs, skis, tennis racquets, fishing rods), bicycle frames.
Properties of some Carbon Fiber Reinforced Polymer Composites
(Materials Data)
• Epoxy Matrix Composite reinforced by 70% carbon fibers
• Epoxy Matrix Composite reinforced by 50% carbon fibers
• Polyether Ether Ketone B Matrix Composite reinforced by 30% carbon fibers