Combining and orienting materials to achieve superior properties is a well proven concept to the progress of humanity. Composites are those materials formed by aligning extremely strong and stiff continuous fibres in polymer resin matrix or binder on a macroscopic scale to form a useful material. The materials in this class have exceptional mechanical properties and are often termed “ Advanced Composites”. These consist of one or more discontinuous phases embedded in a continuous phase. The function of the discontinuous phase, i.e. reinforcement is to impart mechanical properties and toughness to the matrix. Matrix or the continuous phase transfers loads to fibres and between fibres. The main advantage of these advanced materials is their high specific strength and high specific modulus.
Advanced composites were first used in filament wound rocket motor casings and in laminated fibre epoxy composites for air frames. Other important area of applications include aerospace industry, chemical industry, electrical and electronics marine transportation, sports goods, consumer goods etc.
Materials used for composites.
Structural materials require strength, stiffness and toughness. Other properties such as resistance to corrosion, creep, fatique, temperature are also needed in most structural materials. However, these properties are important because of their effect on strength, stiffness, dimensional stability and toughness. Important materials include,
1. Fibres or reinforcements.
3. Other additives.
The first principle of continuous fibre composites is that the fibres must support all main loads and limits deformations acceptably. A striking example is the now famous drawing of an aircraft in which low modulus wings are strained 1.6%.
Most commonly used fibres are Aramid fibres, glass fibres, carbon fibres, boron fibres etc. The important criteria for the selection of various fibres for the manufacturing composites laminates are tensile strength and tensile modulus, compressive strength and compressive modulus, fatique strength, specific gravity etc.
Aramid fibres (KEVLAR)
These fibres are highly oriented organic material derived from aromatic polyamides. These offer the potential for high strength and stiffness fibres. The structure of these fibres is highly crystalline.
Kevlar’s density of 1.44 g/ is among the highest of organics. Long term and fatique properties of Kevlar-49 –epoxy are considerably better than those of glass composites. Kevlar-49 composites are used for filament wound rocket motor cases and pressure vessols. The space shuttle has 17 metal lined Kevlar over wrapped vessols. Kevlar composites are used in interior cabin structures in air craft and also in some secondary exterior fairings. The military has adopted a Kevlar—epoxy helmet. These are also used in boat and kayak hulls.Kevlar fibres are produced from p-phenylene diamine and terephthalic acid coded PPD-T. PPD-T and sulphuric acid form a crystalling complex at a polymer concentration of 20%. This is melted and spun at high speed to get Kevlar fibre. These are having low specific gravity, high strength to weight ratio, impact strength etc. Main problem associated with it is, it difficulty in machining.
Carbon fibres are made by the pyrolytic degradation of fibrous organic pre-cursor. By heating an organic polymeric fibre under tension to high temperature in an inert atmosphere, the volatile components are driven off. The residual carbon atoms tend to orient themselves along the fibre axis into graphitic crystallites and thus form a high strength, high modulus fibre. Industrially these inorganic fibres are made by the pyrolysis of poly acrylonitrile (PAN) and pitch in an inert environment.
They are good conductors of electricity and exhibit unusual thermal expansion characteristics. The thermal co-efficient of expansion is negative along the fibre axis, but positive in the transverse action. These are easier to handil and unusually strong. These are used for ablative applications such as rocket nozzle and ablatives.
Other fibre reinforcements include glass fibre, boron fibre, graphite fibre, silicone carbide fibre etc.
The matrix of an advanced composite material is the continuous phase that binds the fibres together so they can act in concert. The main characteristics required of a matrix material are chemical compatibility with the manufacturing methods used to fabricate the desired advanced composite component and environmental stability under condition of use of the advanced composite component. Thermoplastic and thermosetting resins are available. Thermoplastics are organic materials which are reversible by temperature change. Thermosetting resins which are popularily used in composites show irreversible temperature change. Examples are epoxy resin, polyester etc.
Epoxy resin system is based upon the condensation polymerization formed from Eichlorohydrin and Bisphenol-A and is called Diglycidyl Ether of Bisphenol-A.
Excellent adhesion, excellent resistance to chemicals, low shrinkage, very good electrical and electronics properties are some of its advantageous. Degradation in moisture, high cost, poor non-electrical resistance etc. are some of its disadvantageous.
Epoxy novalacs are used for better thermal and chemical resistance whereas brominated epoxies having good self extinguishing properties. Another important thermosetting resin,polyimide,known as PMR-15 by NASA Lewis Research Centre has a maximum service temperature of 350C.
- Hardners: These substances added to a resin system to promote or control the curing action by taking part in it.Examples are aliphatic amines, aromatic amines, and acid anhydrides.
- Accelerators:These are mixed with a hardner or a resin to speed up the chemical reaction between the hardner and the resin.
- Now-a-days prepregs are used mainly i.e, pre-impregnated woven or unidirectional ply or roving with a resin under closely controlled conditions
Important Manufacturing Techniques
- Vacuum bag moulding
Vacuum bag moulding have been developed for fabricating a variety of aerospace components and structures.The process is principally suited to prepreg material. This method utilizes flexible film of rubber bag that covers the part lay up.
The essential steps in the process are the lay up, preparation of bleeder system and the bagging operations.The required number of plies are cut to size and positioned in a mould. The lay up is covered with a flexible membrane or vacuum bag which is sealed around the edges of the mould by a sealant. An edge bleeder is also placed near the edges of the lay up. Its function is to absorb the excess resign which may flow during curing. Two types of vacuum bag commonly used are:
- Disposable bag of nylon.
- Re-usable silicone rubber bag.
A vacuum is drawn on the lay up which helps in eliminating entrapped air. A maximum pressure of about 15 psi is achieved in this method.
Autoclave moulding is similar to vacuum bag process except that the lay up is subjected to greater pressure and compact parts are produced. It utilizes a pressure chamber to apply heat andpressure to the composite lay up during cure cycle. It is the most commonly used methods for producing composite structural parts of high quality in the air craft industry. Epoxy matrix composite, in general used autoclave cure cycles which involved 85-100psi pressure and 350 F temperature.
Pultrusion is an automated process used to create shapes by pulling rovings through a shaped and heated die. The use of pultruded parts in air craft is limited to specialized application. The process utilizes free impregnated roving or rovings that are pulled through air resin bath to impregnate the fibres. The rovings go through heated die that represents the cross section of the finished part. Curing is accomplished by heating the die. The product is continuously pulled out and as it comes out of the puller mechanism, it is cut to the desired length by an automatic saw.
Other techniques include filament winding, injection moulding, braiding, contact moulding etc.
Advantageous and limitations of composites.
Composite materials have many advantageous there having (1) high specific strength and stiffness, (2) low specific gravity, (3) high fatique strength, (4) corrosion resistance, (5) excellent impact and damage tolerance, (6) low thermal expansion, (7) tooling is cheaper,(8) high dimensional stability, (9) easy of repair, (10) light weight,(11) design flexibility, (12) resistance to crack propagation, (13) tailoring of strength/stiffness properties, (14) possibility of moulding complicated shapes, (15) low thermal conductivity and so on.
Main limitations include: (1) poor erosion resistance, (2) poor electrical resistance, (3) degradation characteristics in moisture.
The properties of composite depend up of a number of factors such as,
- The nature of the separate constituents and the adhesion between them.
- The relative amounts of resin and fibre present in the composite.
- The orientation of the reinforcing fibres. In order to compensate for low transverse properties of unidirectional material (zero degree), lamination of unidirectional fiber may be cross-plied at right angles to one another. ( zero and ninty degree)
The primemost application of advanced composites is in the field of aerospace. Other important field of applications are in marine, industrial and scientific equipment, ground transportation equipment and in consumer products like sporting goods, musical instruments etc.
v In Advanced Light Helicopters (ALH), sliding door, stabilizers, floor boards are made up carbon fibre composite. Graphite – epoxy has been used in military aircrafts. Ninteen percent by weight of the AV – 8B fighter air frame is carbon fibre composite.
Commercial air crafts such as Boeing 757 and 767 employ graphite-epoxy as secondary structural members. Boeing uses 1800 kg/plane, with a net savings of
v A “ pin winder” built for Bell Helicopter for making blade spars. Carrying impregnated S-glass, the carriage moves along the tooling bed rotating 180 degree at each end.
v Production of geodesic air craft fuselage section by filament winding is probably the simplest procedure for composite air craft structures.
v Composites in orbit: A ribbon forming machine was developed for the NASA (Satellite Solar Power System) programme. The 0 + 45 – 45 – 0 graphite – thermoplastic matrix ribbon formed would be installed in a casette in a machine in the cargo bay of the space shuttle. In orbit, the machine would process the ribbon into a triangular truss beam.
Based on the information that I have found, I would say that a majority of the structural components are made out of advanced composites presently, because of the extensive advantageous it offer including weight reduction, cost reduction through parts consolidation, lower life cycle cost and improved system productivity.
v The Advanced Light Helicopter (ALH) has been indigenously designed and developed by HAL with assistance from Eurocopter, Germany find the extensive use of advanced composite materials to the extend of 70% by wet area and 29% by weight.
v Current research works in this field are based on Nano composites and Eco composites. In comparison with conventional polymer-matrix composites, polymer layered silicate nano composites prepared by the intercalation method possess excellent thermal stability, size stability and the composite film possess excellent barrier properties because of the planer orientation of the silicate sheet. Clay nano layers have been shown to be very effective reinforcements in epoxy system to achieve and exfoliated epoxy – clay nano composite structure.
v Natural Fibre Reinforced Composites (NFRP) exhibit low density and the extent of environmental pollution caused is less compared to synthetic fibres. Based on recent report from Germany, door panels in the Mercedes have been made from plastics reinforced with flax fibres. The performance advantage of plant fibres in polymer matrix composites are their chemical properties, high specific stiffness and strength and sound absorption.
v All things considered, advanced composite materials play a vital roll and integral part in the field of advanced engineering applications.