 A composite material is made by combining two or more materials to give a unique combination of properties one of which is made up of stiff, long fibres, and the other, a binder or 'matrix' which holds the fibres in place. The fibres are strong and stiff relative to the matrix and are generally orthotropic. Composites are a versatile and valuable family of materials that can solve problems of different applications, improve productivity, lower cost and facilitate the introduction of new properties in material. This article deals with fibre-reinforced composites properties and its uses in machine, equipment and apparatus construction, medical technology and tensile behaviour in knitted fabrics.
Composites consist of two or more phases that are usually processed separately and then bonded, resulting in properties that are different from those of either of the component materials. Polymer matrix composites generally combine high-strength, high-stiffness fibres (graphite, Kevlar, etc). Composites involve two or more component materials that are generally combined in an attempt to improve material properties such as stiffness, strength, toughness, etc. Fibre-reinforced composites often aim to improve the strength to weight and stiffness to weight ratios (ie, desire lightweight structures that are strong and stiff). Fibres are available in 3 basic forms.
Types of composite
The three basic types of composites are generally identified as:
- Particle-reinforced (Aggregates).
- Fibre-reinforced (Continuous fibre or Chopped fibre).
- Natural composites (Examples: Wood and Bone).
Properties of composite materials
Lightweight, high-strength, corrosion resistant, flexibility, low cost, low mass, in aeronautics. Composite materials have six properties giving them an edge over standard materials: Longer life cycle thanks to high fatigue strength, corrosion resistance, improved fire resistance, easier design because of function integration, possibility of complex shapes and lightweight components.
Properties of 3D textile composites
The plain weft knitted fabric reinforced composite material investigated in this study is assumed to have only reinforcement fibre yarns and polymer matrix. For analysis purposes, a unit cell representing the complete knitted fabric composite is identified. A geometric model is proposed to determine the orientation of yarn in the composite. Outline the procedure for estimating the fibre volume fraction of the composite. The unit cell is divided into four representative volumes, also called a 'crossover model'. The crossover model is further divided into sub-volumes, which are considered as transversely isotropic unidirectional fibre-reinforced composites. A new micro-mechanical model is used to predict all the five independent elastic constants of the unidirectional fibre-reinforced composites. By considering the contributions of both the fibres and net matrix material, the compliance/stiffness matrix of each sub-volume in the material co-ordinate system is calculated using the new formula. This compliance/stiffness matrix of each sub-volume is then transformed to the global co-ordinate system. A volume averaging scheme has been applied to obtain the overall compliance/stiffness matrix of the knitted fabric composite. The effects of fibre content and other parameters of knitted fabric on the elastic properties of the composite material are identified.
Application of composites
Composite materials have long been utilised in various industries including automotive, heavy truck, aerospace, civil infrastructure, marine and durable goods.
- Fibres for Conveyor Belt.
- Fibres-Reinforced Conveyor Belt Application.
- Pipes, Tanks & Vessels Applications.
- Composite Pipe Work Application.
- Aerospace Application.
- Aircraft Application.
- Rocket and Missiles Applications.
- Marine Application.
- Tensile Behaviour of Knitted Fabrics Composite.
Fibres for conveyor belt
 These physical and chemical properties of Kevlar fibres allow using it as the reinforcement material for conveyor belts. By using this it is possible to design light and linger conveyor belts. This design feature is a major advantage in applications that require a high modulus, lightweight belt as a replacement to steel cords, thus providing for the use of narrow, fast belts of high-strength, which can be employed efficiently over long distances. The use of these type of belt led to lower manufacturing and installation costs, reduced energy consumption, no sparking and non-flammability giving improved safety, better corrosion, better impact resistance and longer life.
Fibre-reinforced conveyor belt application
Conveyor belts are flexible composite plates that when interconnected it forms an endless entity and can efficiently transfer bulk materials or parcels from one place to another. Conveyor belt systems are composed of endless belt, pulleys, idlers, electrical motors, counter weight, rigid structure and other accessories. Conveyor belts have to meet rather different requirements, depending on the particular application. The basic properties required include:
- High strength and flexibility.
- Low extension in service.
- Resistance to abrasion.
- Impact and tearing resistance.
- Resistance to moisture, oils and chemicals.
- Typical conveyor belt system.
Pipes, tanks and vessels application
 Fibre-reinforced composites have more number of end applications and significant areas in different fields, one of the applications were composites are being employed for fluid transport and storage. The most important material here is glass reinforced epoxy (GRE), which has been used onshore for both low and high-pressure applications with a wide variety of fluids, including hydrocarbons. The chemical resistance of glass reinforced epoxy and the maximum use of temperature in a particular fluid depends on the type of resin and hardener used. Glass reinforced epoxy tubes are largely immune to the effects of hydrogen sulphide and carbon dioxide. The most damaging chemical component is often water, rather than oil, although some highly aromatic species such as toluene and xylene can be damaging.
Composite pipe work application
 The most successful offshore applications for composites have been in pipework for aqueous liquids. The most important material here is glass reinforced epoxy (GRE), which has been used onshore for both low and high-pressure applications with a wide variety of fluids, including hydrocarbons. By contrast, the main offshore applications have been confined so far to relatively low pressure aqueous services, of the type shown in Figure. The chemical resistance of glass reinforced epoxy tubes and the maximum use temperature in a particular fluid depends on the type of resin and hardener used. Glass reinforced epoxy tubes are largely immune to the effects of hydrogen sulphide and carbon dioxide. The most damaging chemical component is often water, rather than oil, although some highly aromatic species such as toluene and xylene can be damaging.
Joining techniques
Several joining techniques are used for thermosetting pipes. The lengths of glass reinforced epoxy pipe may be joined to fittings or to each other by;
- Adhesive bonding.
- Laminating (butt and wrap joints) or
- By mechanical means, such as the rubber seal joint or the threaded joints.
Adhesively bonded joints may be of the taper-taper, socket and spigot or parallel (Quick-Lock) type. In each case, the socket may be either filament wound or moulded. Alternatively the socket may be directly moulded into the end of a straight length of filament wound pipe. The spigot end of the pipe is prepared by machining or shaving to the required dimensions and shape. For field or on-site joints, special shaving tools are provided for this. The joint is made by coating with adhesive (usually epoxy), assembly and elevated temperature curing using a heating blanket. Joints of this type are very common in the oil industry, both onshore and offshore. Bonding is also used to assemble flanges onto glass reinforced epoxy pipe work for subsequent attachment to other parts of the system. Flange joints are often used where easy disassembly is required.
Butt and wrap joints
Butt and wrap joints are provided by many pipe manufacturers. Here, the plain pipe ends are brought together, after abrading the outer surface, the joint being made by over-laminating with glass fibre and resin. Although cheaper from a materials viewpoint than adhesive joints, butt and wrap joints are more labour-intensive and difficult to make on-site. Both adhesive and butt and wrap joints, when properly made, provide a generous margin of safety. It has been shown (Cowling et al, 2000) that such joints are highly defect tolerant when assembled according to the correct procedures. The principal means of ensuring integrity is the hydro test, usually carried out at 1.5 times working pressure. Good results have been reported overall with these systems; provided a number of simple rules are followed and responsible personnel have been properly trained. It is also necessary to maintain an auditable chain of responsibility from the pipe manufacturer through to the personnel who carry out the jointing procedure.
Rubber seal joint
 Rubber seal systems are used commonly at lower pressures because they provide for rapid assembly (and disassembly). The hydraulic seal is achieved by means of one or more rubber O-rings inserted into grooves in the male pipe end. The axial load is supported by a cylindrical key, often of nylon, inserted through a tangential hole in the socket wall, into preformed tangential grooves in the male and female end of the tube. The Ameron Key-lock™ system is a good example.
Rubber seal joints enable long pipe runs to be laid with low handling and labour costs. The joint itself can accommodate a few degrees of flexure, making it possible to lay the system over ground with minimal preparation. The pressure rating of the joint can be improved by the use of more than one key. For higher-pressure applications, socket and spigot joints with moulded threads are successfully used, sometimes in conjunction with a thread sealant or adhesive. The thread design is often similar to the API tapered threads used with steel tubing.
Aerospace applications
The very important topic in my paper is this aerospace applications topic. Applications of these composites in aerospace engineering being aerospace composites parts, we must be giving a layout of aircraft. Without possibilities of amalgamating the above explanations and lightweight panel can be produced by using fibre-reinforced composites. All the components parts can be produced by using fibre-reinforced composites panels and door parts.
Aircraft application
The advanced composites in the construction of aircraft and helicopters, weight savings of 20 - 30% are achieved as compared to conventional materials. Fairings, landing gears, engine cowls, rudder, fin boxes, doors, floor boards and many other interior gadgets are made of advanced composites in combination with metallic and non-metallic honey comb cores and metals. The recently launched prototype of Advanced Light Helicopter (ALH) is said to have as much as 60% of the surface area made up of composite components including advanced fibre components and metal sandwich structures.
Space crafts: Two factors, high specific modulus and strength, and dimensional stability during large changes in temperature in space make composites the material of choice in space applications. Examples include the graphite/epoxy-honeycomb payload bay doors in the space shuttle. Weight savings over conventional metal alloys translate to higher payloads which cost as much as $1000/lb ($2280/kg) also, for the space shuttles remote manipulator arm, which deploys and retrieves payloads, graphite/epoxy was chosen primary for weight savings and for small mechanical and thermal deflections. Antenna ribs and struts in satellite systems use graphite/epoxy for their high specific stiffness and its ability to meet the dimensional stability requirements due to large temperature excursions in space. Remember "aerodynamic heating" during re-entry should also be taken into concern.
Rocket and missiles
Rocket motor cases and liners are made using composites of carbon, aramid and glass. Formulated epoxies, phenolics and polyimide materials are being used. Carbon-carbon composites are used for re-entry nose tips and heat shields. These applications, which require a lower ablation rate, higher bulk density and superior mechanical strength, are possible with carbon-carbon composites compared to monolithic graphite. Carbon-carbon composite items are successfully made from 3-D fabrics followed by densification process.
Marine application
Development of composite propulsion shafts for naval vessels is being investigated to replace the massive steel shafts that comprise up to 2% of the ship's total weight. Composite shafts of glass and carbon reinforcing fibres in an epoxy matrix are projected to weigh 75% less than the traditional steel shafts and offer the advantages of corrosion resistance, low bearing loads, reduced magnetic signature, higher fatigue resistance, greater flexibility, excellent vibration damping and improved life-cycle cost.
Tensile behaviour of knitted fabrics composite
Composites are fabricated by impregnating knitted fabric of reinforcement fibre yarns with the matrix polymer. For a given knitted fabric structure, the mechanical behaviour of composite material depends on the properties of the constituent fibre and matrix materials. Typical tensile stress-strain curves of three different kinds of knitted fabric composites. These curves are obtained from tensile testing in the wale direction of the composite. The tensile stress-strain curve of composite made from knitted glass fibre fabric and epoxy matrix is grossly linear with a small failure strain 1.3%.
Knitted Glass fibre-reinforced composites for epoxy resin, polypropylene and polyurethane fibres.
 In the case of knitted glass fibre fabric reinforced polypropylene composite material, the stress-strain curve changes from an initial linearly elastic relationship to a significantly nonlinear relationship with an intermediate ultimate failure strain of 8.5%. The matrix polymer used in these composite materials mainly causes this difference.
At the other end of the spectrum, a highly flexible stress-strain behaviour could be achieved by reinforcing elastomeric material with a knitted fabric. A typical stress-strain curve of knitted polyester fibre fabric reinforced polyurethane elastomeric is shown in the Figure. The stress-strain behaviour is characterised by a small initial linear elastic relationship, followed by nonlinear behaviour with large ultimate failure strain of 60%. In other words, by selecting the type of matrix and reinforcement materials, the mechanical characteristics of a knitted fabric composite can be tailored from rigid to flexible. This chapter mainly concerns the mechanical behaviour of the knitted glass fibre fabric reinforced epoxy composites, in which the stresses and strains are connected by fixed linear relationships.
The stress-strain curve is linear up to the knee point, which occurred at approximately 0.45% strain. Above the knee point, the material deformation and microfracture processes in the specimen cause the nonlinearity. A schematic representation of a typical fracture process in a knitted fabric composite is shown in the figure. Above the knee point, debonding of yarns oriented normal to the testing direction occurs.
The cracks nucleated from the deboned sites propagate into resin-rich regions and coalesce into large transverse cracks. Unfractured yarns bridge the fracture plane. The ultimate fracture of the tensile specimen occurs upon the fracture of bridging yarns. In other words, the tensile strength of composite material is determined mainly by the fracture strength of yarns bridging the fracture plane.
Conclusion
Fibre-reinforced composites often aim to improve the strength to weight and stiffness to weight ratios. These properties will reduce the weight of the components produced by the fibres and therefore fibres used for composite materials will have high strength, high flexibility and it is most widely used for textiles and other major fields. Fibre-reinforced composites are mostly widely used in the concretes, marine, aircraft, conveyor belts and other manufacturing applications. The future scope for the reinforced fibre is more and its application is spreading in all the fields of science and engineering. Still researches are going under this to improve their properties and applications of different fibres in various fields.
Original Articles were published in http://www.indiantextilejournal.com/articles/FAdetails.asp?id=5096
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