what are composite materials

1. 13 INTRODUCTION TO COMPOSITE MATERIALS:

Composite materials can be defined as the structures made up of two or more distinct starting materials. The starting materials can be organic, metals or ceramics. The components of composite materials do not occur naturally as an alloy, but are separately manufactured before these are combined together mechanically. Due to this, they maintain their identities, even after a composite material is fully formed. However the starting materials combine to rectify a weakness in one material by strength in another material. Hence composite material exhibits properties distinctly different from those of individual materials used, to make composite. Thus composite material or structure possesses a unique combination of properties such as stiffness, strength, hardness, weight, conductivity, corrosion resistance & high temp. Performance etc. that is not possible by individual materials. Thus the search for materials with special properties to suit some specific stringent conditions of use has given rise to development of materials called “COMPOSITE MATERIALS”.

1.13.1 TYPES OF COMPOSITE MATERIALS:

Composite materials may roughly be classified as:

1)      Agglomerated materials/ Particulate composites.

2)      Reinforced materials.

3)      Laminates.

4)      Surface coated materials.

The particulate composite and reinforced composites are constituted by just two phases, the matrix phase. The aim is to improve the strength properties of matrix material. The matrix material should be ductile with its modulus of elasticity much lower then that of dispersed phase. Also the bonding forces between the two phases must be very strong.

In fact the particulate composite also fall in the category of reinforced composites. Depending upon the nature of reinforced materials (shape and size), the reinforced composites can be classified as

1.      Particle reinforced composites or particulate reinforced composites.

2.      Fiber reinforced composite.

In particulate reinforced composites, dispersed phase is in the form of exi-axed particles, whereas in fibre-reinforced composite, it is in the form of fibers.    

1.13.1.1 AGGLOMERATED MATERIALS:

Agglomerated materials consist of discrete particles of one material, surrounded by matrix of another material. The material is bounded together in an integrated mass to classic eg. Of such composite material are: concrete formed by mixing gravel, sand, cement & water & agglomeration of asphalt & stone particles, that is used for paving the high surfaces. Other eg. Of particulate composite material includes:

1)       Grinding and cutting wheels, in which abrasive particles (Al2O3, Sic, CBN or carbon) are held together by a vitreous or a resin bond.

2)       Cemented carbide, in which particles of ceramic materials such as WC, TaC, TiC & of cobalt & nickel, are bounded together via Powder metallurgy process to produce cutting tool materials. Many powdered metal parts & solid sintering produces various magnetic & dielectric ceramic materials, which requires diffusion.

3)       Electrical contact point from powder of tungsten & silver or copper is process via powder metallurgy method.

4)       Electrical Brushes for motored & heavy duty & frictional materials for brake & clutches by combining metallic & non-metal. Materials.

5)       Cu infilterated iron & silver, tungsten.

6)       Heavy metal (w+6%ni+4%cu)

7)       Electrical resistance welding electrodes from mixture of cu & tung.

8)       Shell moulding sand, using a resin binder, which is polymerized by hot pattern.

9)       Metal polymer structers (metal bearing in filtered with nylon or PTFE).

10)   Particleboard, in which wood chips are held together by suitable glue.

11)   Elastomers & plastics are also reinforced with suitable particle material. The eg. Is addition of 15-30 of carbon black in the vulcanized rubber for automobiles types?

12)   Dispersion strengthened materials: in these materials hard, brittle and fine particles are dispersed in softer or more ductile matrix.

Because of their unique geometry, the properties of particulate composite can be isotropic. This property is very important in many engineering applications

1.13.1.2 REINFORCED MATERIAL:

Reinforced materials from the biggest and most important group of composite materials. The purpose of reinforcing is always to improve the strength properties. Reinforcement may involve the use of a dispersed phase (discussed in the last article) or strong fiber, thread or rod.

Fiber-reinforced materials: in a larger number of applications, the material should have high strength, along with toughness and resistance to fatigue failure. Fiber reinforced materials, offer the solution. Stronger or higher modulus filler, in the form of thin fibers of one material, is strongly bonded to the matrix of another. The matrix material provides ductility and toughness and supports and blinds the fibres together and transmits the load. The toughness of the composite material increases, because extra energy will be needed to break or pull out a fibre. Also, when any crack appears on the surface of a fibre, only that fibre will fail and the crack will not propagate catastrophically as in bulk material. Failure is often gradual, and repairs may be possible.

 Due to the above mentioned desirable properties of the matrix materials, the commonly used matrix materials are; Metals and polymers, such as, Al Cu, Ni etc. and commercial polymers strong fibers in the relatively weak matrix. Like this, it is possible to produce parts where strength control is developing in different directions. if the  part is loaded parallel to the fibers will be much greater than in the matrix . Even if the fibre breaks, the softness of matrix hinders the propagations of crack .The fibre direction are tailored to the direction of loading.

Reinforced Fibers: A good reinforcing fibre should have: high elastic modulus, high strength, low density, reasonable ductility and should be easily wetted by the matrix. Metallic fibres such as patented steel; stainless steel, tungsten and molybdenum wires are used in a metal matrix such as aluminum and titanium. Carbon fibers and whiskers are also used in a metal ultra –high strength composite. Fibres need not be limited to metals. Glass, ceramic and polymer fibers are used to produce variety of composite having wide range of properties .The high modulus of ceramic fibers make them attractive for the reinforced of the metal. The ductile matrix materials can be aluminum magnesium, nickel or titanium and the reinforcing fiber may be of boron , graphite , aluminum or SiC.

Forms of reinforcing fibres: The fibers used for reinforcing materials are available in different forms:

(a)    Filaments: these are very long and continuous single fibres.

(b)   Yarns: this is twisted bundles of filaments.

(c)    Roving: These are untwisted bundles of gathered filaments.

(d)   Tows: These are bundles of thousands of filaments.

(e)    Woven fabrics: These are made from filaments, yarn or roving which have been woven at 90 degree to each other.

(f)    Mats: Fibre form is said to be mat form when the continuous fibre is deposited in a swirl pattern or chopped fibre is deposited in a random pattern.

(g)   Combination mat: Here, one ply of woven roving is bonded to a ply of chopped strand mat.

(h)   Surface mats: These are very thin, monofilament fibre mats for better surface appearance.

(i)     Chopped fibre or roving: These are 3 to 50 mm in length.

(j)     Milled fibres: These are of brittle materials, usually 0.5 to 3 mm in length.

(k)   Whiskers: whiskers are single crystal, in the form of fine filaments, a few microns in diameter and short in length. These single crystal whiskers are the strongest known fibers. Their high strength is due to the high degree of perfection and the absence of dislocation in the structure. Their strength is many times greater than that of the normal metals. For ex The strength of an iron whisker is found to be 13450MN\m2, compared to about 294MPa for a piece of pure iron,. Besides metal whiskers, long non metallic, whiskers and of graphite are being produced. They are introduced in to resin or metallic matrix for the purpose of high strength and high stiffness at high temperatures.

The properties of reinforced materials will depend on:

• The properties of matrix materials.
• The properties of the fibre materials.
• The proportions of the reinforcement in the composite materials. It is never less than 20% and may go up to 80% in oriented structures.
• The orientation of the fibre, relative to the load application and relative to one another.
• The degree of bonding between the fibers and the matrix material.
• The length to diameter ratio of the fibers.There has to be some minimum fibre length, known as, critical length, lc, to get the desired strength and stiffness of the composite materials. It is given as:

L _c =  σ _f .d / ح

Where,      σ_f  =Tensile strength of fibre materials

      d =diameter of fibre

                  ح =shear yield strength of the fibre matrix bond

Fig. 1.6 Reinforcing Fibers

For example, for carbon and glass fibers, the critical length is of the order of 1mm, which may be 20 to150 times the diameter of the fibre.

The fibre reinforcement can be done in three ways:

1. Continuous and aligned, Fig a

2. Discontinuous and aligned, Fig b

3. Continuous and randomly oriented, Fig c

If the fibre length is considerably greater than Lc e.g., 15 times or more, it is called a “continuous fibre”, otherwise it is called “short” or “discontinuous fibre as noted above, the properties of a composite having aligned fibre reinforcements, are highly anisotropic, that is, they depend upon the direction in which these are measured. Their maximum strength is along the direction of alignment. They are very weak in the transverse direction. The arrangement is best suited for application involving multi- direction applied stresses, for e.g., bi-axel stresses in pressure vessel or tube. The same results can be achieved by using bi- axially oriented or cross – ply fibers. It is apparent that the strength of the discontinuous and aligned arrangement will be less than of the continuous and aligned arrangement.

Applications: As discussed in the beginning, composite structures combine the desirable properties of two or more materials. This has greatly expanded the scope of application of all engineering materials. This has greatly expanded the scope of application of all engineering materials. We can produce components with exceptional strength –to –weight and stiffness –to-weight ratio (many composite are stronger than steel, lighter than steel and stiffer than titanium). Also, they have low conductivity, good heat resistance, good fatigue life, adequate wear resistance and are free from corrosion.

Reinforced concrete is a classic example of reinforced materials. Steel rods used in the concrete to reinforce the material take all tensile loads since concrete weak in tension but strong in compression.

1.      Glass- fibre reinforced Plastics: Here, we have glass fibres in a matrix of unsaturated polyester. To get better qualities to use at high temperature, high temperature polyamide resin is used with pure SiO₂ fibres. A special type of glass fibre can be used with cement bond to form flexible type of concrete. Glass fibre reinforced plastics are used to make: boat hulls, car bodies, truck, cabins and aircraft fittings. The other matrix materials can be: vinyl ester and phenolic.

2.      C-C composites: These composites have graphite fibres in a carbon matrix. This material is being used to make: Nose cone and leading edge of missiles and space shuttles, racing car disks brakes, aerospace turbine and jet engine components, rocket nozzles and surgical implants.

3.      Graphite fibre- reinforced epoxy :( Organic or Resin matrix composites): This material is being used to make many parts of a fighter plane: Wing span, outrigger flaring. Overwing flaring, engine access doors, nose cone, forward fuselage. Lid fence and strakes-flap. Flap slot door, aileron seals, Horizontal stabilizer (Full span) and rubber. The other fibre-matrix combination can be: Aramid fibre-Phenolic resin matrix, Boron fibre-Bismaleimide resin matrix.

4.      Automative uses: Body panels, drive shafts, spring and bumpers, Cab shell and bodies, oil pans, fan shrouds, instrument panels and engine covers.

5.      Sports equipment: Golf club shafts, base ball parts, fishing rods, tennis rackets, bicycle frames, skis and pole vaults.

6.      Rubber used for making automobiles tyres is now reinforced with fibres of nylon, rayon steel or Kevlar, to provide added strength and durability. Kevlar is an organic aramid fibre with very high tensile strength and modulas of elasticity. Its density is about half of that of aluminum and it has negative thermal expansion. It is flame retardant to radio signals. This makes it very attractive for military and aerospace applications. It is also being used for making bullet proof jackets. The trade name “Kevlar” is given by Du Pont.

7.      Metal matrix composite (MMC): As already noted, these composites are obtained by impregnating high-strength fibres (of stainless steel, boron, tungusten, molybdenum, graphite, AL₂O₃, SiC and Si₃N₄ etc.) with molten metal ( aluminuim , titanium, Ni and cobalt etc). These composites offer higher strength and stiffness especially at elevated temperatures and lower co-efficient of thermal expansion as compared to metals. And as compared to Organic-matrix composites, these composites offer grater heat resistance and improved thermal and electric conductivity. Hence metal matrix composites are used where operation temperature is high or extreme strength is desired. These will find applications in a variety segments like automobiles and machinery.

Aluminum oxide reinforced aluminum is used for making automotive connecting rods. Aluminum reinforced with SiC whiskers is used to make air craft wing panels. Fibre reinforced super alloys are used for making turbine blades. Graphite fibres in aluminum matrix are used for Satellite, missile, helicopter structures.Graphite fibres in magnesium matrix is used for space and satellite structures. Graphite fibres in lead matrix are used for Strong –battery plates. A graphite fibre in copper matrix is used for bearings and electrical contacts. Other e.g. of MMC is:

(a)    Boron fibre in aluminum: Compressor blades and structural supports.

(b)   ““ “ magnesium : Antenna structures.

(c)    “””” Titanium: Jet-engine fan blades.

(d)   Alumina ““    Lead: Strong-battery plates.

(e)    ““     “     Magnesium: Helicopter transmission structures.

(f)    SiC “‘    Super alloy (Cobalt based): High temperature engine components.

(g)   Tungsten and Molybundum fibres in Super alloy matrix: High temperature engine components.

8.      Ceramic-matrix composites: (CMC): AS already noted, ceramics are strong, stiff, can resist high temperatures, but generally lack toughness. Ceramic matrix materials are: AL₂O₃, SiC and Si₃N₄, and mullite (a compound of Al, Si, and O₂). They can retain their strength upto 1700 degree C, and also resist corrosive environments.

Typical product applications of ceramic matrix composite are: in jet and automotive engines, deep-sea mining equipment, pressure vessels, structural components’ cutting tools, and dies for extrusion and drawing operations.

Composite in development stage:

1.      Advance bismaleinmide resin matrix series for high temperature service.

2.      Polyether ether ketone thermoplastic matrix series for higher temperature service.

3.      Hybrid reinforcements and Knitted/stacked ply fabrics and three-dimensional woven fabric reinforcements.

4.      Selective stitching of collated ply kits.

1.14 LAMINATES:

Laminates or laminar composites are those structures which have alternate layers of materials bonded together in some manner some common examples of laminar composites are given below:

1.      Plywood: it is most common material under this category. Here, thin layer of wood veneer are bonded with adhesives. The successive layers have different orientations of the grain or fibre; Structural parts capable of carrying a load are made of multi-plywood board from 25 to 30 mm thick.

2.      Bimetallic strips used in thermostats & other heat sensing application.

3.      Safety glass

4.      Sandwich material: Here, low density core is placed between thin, high strength

High-density surfaces, for example, corrugated cardboard. Cores of polymer foam or honeycomb structures can be used. Wood substitutes based on red mud polymer have been developed to be used for door shutters, windows, partitions and false ceilings.

5.      Roll cladding (bonding) and explosive cladding (welding) of one metal upon another: The main aim of clad material is to improve corrosion resistance while retaining low cost, high strength and /or lightweight. Mild steel –stainless steel combination, copper stainless steel combination are examples of metal-to-metal laminates. Another example is “Alclad”, which is formed by cladding duralumin with thin sheets of pure aluminium. The material is high strength composite in which aluminium cladding provides galvanic protection for the more corrosive duralumin. The above claddings are done by “hot roll bonding” method.

6.      Laminated Plastic Sheet: This structure is usually made from sheets of paper or       cloth and suitable resin. The resin used includes: phenolics, polyster, silicones and                                                                                                                                                               epoxide. The paper and cloth provides bulk of strength, while the resin acts as a semi rigid binder. Laminated plastic sheet can be machined, drilled, punched and pressed to shaped. It is used in the production of gears, bearings, electrical components, and small cabinets. Laminate fabric base gears have the advantages over metal gears of being silent in operation and stable against the attack of various. Aggressive media. In many cases, laminated fabric base gears have completely replaced nonferrous gears. They are employed to transmit rotation from electric motor in high-speed machine tools; they are mounted on the camshafts of internal-combustion engines etc. In chemical industry, laminate fabric base gears are used in various apparatus & instruments where they resist corrosive attack much more efficiently then gears of bronze brass or leather. In addition to gears, certain other transporting devices: roller, rings etc. are also made of laminated fabric base. Laminated sheets /plates are available in sizes of: 900*900 mm, 900 *1800 mm, and 1200*2400 mm. The minimum thickness of sheet is 0.8 mm & it varies as follows: -

     

Thickness range (mm)    0.8-1.6      1.6-4.8      6.4-9.6       12.8- 19.2      25.6- 38.4

                    Step(mm)           0.4            0.8            1.6                  3.2                  6.4

7. Tufnol: this is a laminated material consisting of layers of woven textiles impregnated with a thermosetting resin. The polymer imparts rigidity, while the woven textile provides great tensile strength. Paper or asbestos may also be used as alternative reinforcements. The material (with woven textile) can be used for making seat covers &carpets.
8. Laminated carbides: In laminated carbides, laminates consisting of a hard thin surface layer TiCand the form of throw away tips, are bonded by epoxy resin to the rake face of a tip body of WC. This increases the crater wear of WC cutting tool.
9. Laminated wood : this sheets of wood (veneer ) , impregnated with special resins & compressed hot , form what is called ‘laminated wood ‘, which find extensive application in textile machinery & electric engineering , as well as substitute for nonferrous metal in bearing of hydraulic machinery &mechanisms operating in abrasive media . Parts of wood are machined in ordinary machine tools &wood working machinery.

Surface coated materials: the surface coating is applied to the materials for various purposes: - protection of the material against corrosion; for decorative, wear resistant &processing purpose. They may also be used to :(i) improve visibility through luminescence & better reflectivity (ii) provide electrical insulation , & (iii) improve the appearance. Surface coating are usually classified as: metallic coatings, inorganic chemical coating & organic chemical coating

1.      Metallic coating: metallic coating of copper, chromium nickel, zinc, lead & tin etc.  are applied by hot dipping , electro- plating or spraying techniques to protect the base metal from corrosion & for other purpose .

2.      Inorganic chemical coating: This surface coating may be divided into: Phosphate coating, oxide coating & vitreous coating. Oxide & phosphate coating are done to make iron or steel surface free from rust & this is done by chemical action. These coating also provide protection against corrosion. Vitreous coating are commonly applied to steel in the form of a powder or frit & are then used to the steel surface by heat. These coating are relatively brittle, but offer absolute protection against corrosion. Enamel is an example of ceramic coating on metal & glaze on tiles is an example of glassy ceramic on crystalline ceramic base. The glazing as a protective coating on porcelain & stoneware ceramic is performed for the purpose of protection from moisture absorption in ceramic materials. Coating of TiC , TiN , Al2o3 or HFN on WC base are examples of ceramics on ceramic & coatings of TiC & TiN on HSS base are examples of ceramics on steel. These coatings increase the life of cutting tools.

3.      Organic Coatings:  It includes paint, varnishes, enamels & lacquers. They                                                           serve to protect the base metal & to improve its appearance.

Polymer coating on paper are used for making milk cartons. Polymer coated textiles are used for making seat covers & carpets. Polymer Coatings on metals act as wire insulation. Polymer coated metals are used for making beverage cans.

1.15 PRODUCTION OF COMPOSITE STRUCTURES

Fabrication of particulate composites: As discussed in above art. A majority of the particulate composites are made via the powder metallurgy route. So, for details readers should refer to chapter 10. However, a few particulate composites are made by dispersing the particles in the matrix materials through introduction into slurry or into a liquid melt (agglomeration of asphalt and stone particles). 

Fabrication of Fibre reinforced Composites:  Many processes have been developed to fabricate fibre-reinforced composite structures. Their aim is to combine the fibre and the matrix into a unified form. The various fabrication techniques depend on: the size and the form of the fibres and their orientation in the matrix material; the shape, size and form of the product. The common fabrication processes are: Open-Mould process, Filament winding, Pultrusion and Matched-die-Moulding, and Laminating. Before these processes are discussed, the following terms should be understood:

·         Prepergs: Prepergs means “Preimpregnated with resin”. It is ready to mould material in the sheet form. Impregnated rovings and mats make these with resin matrix under the condition in which the resin undergoes only a partial cure. These are stored for subsequent use. These are supplied to the fabricator, who lays up the finished shape in stacks, which is subjected to heat and pressure. This completes the curing of the resin into a continues solid matrix. “Lay-up” is positioning of the reinforcement material, sometimes resin-impregnated, in the mould.

·         BMCs:  are “Bulk Moulding Compounds”. These are thermosetting resins mixed with chopped reinforcements or filters and made into a viscous compound for compressing moulding.

·         SMCs: are “Sheet Moulding Compounds”. These comprise chopped fibres and resin in the sheet form approx. 2.5 mm thick. These are3 processed further to fabricate large sheet like parts. They can replace sheet metal, where lightweight, corrosion resistance and integral colour are attractive features.

·         Thick Moulding Compounds: Thick Moulding Compounds (TMC) combines the lower cost of BMC and higher strength of SMC. These are usually injection moulded using chopped fibres of various lengths. Used for electrical components due to their high electrical strength.

1) Open – Mould Process ~ In this process, only one mould (Die) is employed to fabricate the reinforced part. The mould may be made of: wood, plaster or reinforced plastic material. The various techniques in this category are:-

a)      Hand lay-up technique: In this method, the successive layers of reinforcement mat or web (which may or may not be impregnated with resin) are positioned on a mould by hand. Resin in used to impregnate or coat the reinforcement. Curing the resin to permanently fix the shape then follows it. Curing may be at room temperature or heating may speed it up. The technique in which resin-saturated reinforcements are placed in the mould is called   “Wet lay-up”.

b)     Bag Moulding: This is a technique of moulding reinforced plastics composites by using a flexing cover (bag) over a rigid mould. The composite material is positioned in the mould and covered with the plastic film (bag). Pressure is then applied by a : Vacuum, auto-clave, press  or by inflating the bag . An auto-clave is a closed pressure vessel for inducing a resin cure or other operation under heat and pressure.

i)        Vacuum-bag moulding: In this technique for moulding reinforced plastics, a sheet of flexible, transparent material is placed over the lay-up on the mould. After sealing the edges the entrapped air between the sheet and the lay-up is mechanically worked out and removed by the vacuum. Finally, the part is cured.

Fig. 1.7 Vacuum Bag Moulding

ii) Pressure-bag Moulding: It is a process for moulding reinforced plastics in which a tailored, flexible bag is placed over the contact lay-up on the mould, sealed and clamped in placed. Compressed air forces the bag against the part to apply pressure while the part cures.

iii) Spray-up: In this technique, a spray gun supplies resin in two converging streams into which chopped roving fiber is forced with the help of a chopper. The composite material stream is then deposited against the walls of the mould cavity. It is a low-cost method of fabricating medium strength composite structures.

All the above open-mould techniques are extensively used for fabricating parts such as: boats, tanks, swimming pools, ducts and truck bodies.

2) Matched-die moulding: Matched metal dies are used for moulding composite structure when: production quantities are large, tolerances are close and surface quality has to be the best. The dies are heated to complete the curing of the product during the moulding process.

i) Compression Moulding is essentially employed for moulding BMCs.

ii) Resin- Transfer Moulding or Resin Injection Moulding: In this technique (RTM or RIM), two piece matched cavity dies are used with one or multiple injection points and breather holes. The reinforcing material, which is either chopped or continuous strand material is cut to shape and draped in the die-cavity. The die-halves are clamped together and a polyester resin is pumped through an injection port in the die. The pressure used in the die is low, which allows use of low cost tooling. The method is used for moulding small non-load bearing parts.

In a variant of the above technique, instead of the injection of only resin into the die-cavity, the reinforcement (flake glass) is mixed with the resin in a mixing head and the mixture is injected into the closed heated two-piece die. Flake glass is preferred to avoid directionality of reinforcement. This method is known as “Reaction Injection Moulding” and is being increasingly used for BMCs.

iii) SMCs cut to size, are fabricated into parts by methods similar to metal pressing. However, curing of the part takes place outside the press.  

Fig. 1.8 Compression Moulding

                                       

3) Pultrusion: This is the process of extrusion of resin-impregnated roving ( a bundle of fibres ) to manufacture rods, tubes and structural shapes (Channels, I-beams and Z- Sections etc.) o0f a constant cross-section. After passing through the resin-dip tank, the roving is dawn through a heated die (where curing takes place) and cured to form the desired cross-section, as it continuously runs through the machine. After the Puller rolls, a saw cutter cuts the extruded section to the required lengths.

In “ Pulmoulding”, the process begins with pultruding; then the part is placed in a compression mould.

Product applications are: - Golf club shafts, because of their high damping capacity, and structural members for vehicle and aerospace applications.

                                            

Fig. 1.9 Pultrusion

4) Filament Winding: In this process, resin impregnated strands are applied over a rotating mandrel, to produce high strength, reinforced cylindrical shapes. Fibers or tapes are drawn through a resin bath and wound onto a rotating mandrel. The process is relatively slow, but the fiber direction can be controlled and the diameter can be varied along the length of the piece. In a variation, the Fiber bundle (made up of several thousand carbon fibers) is first coated with the matrix material, to make a prepreg tape (endless strip with width equal to several cms, by a meter). With both the fiber and tape winding processes, the finished part is cured in an autoclave and later removed from the mandrel. In axial winding, the filaments are parallel to the axis and in circumferential winding; these are essentially perpendicular to the axis of rotation.

Cylindrical, spherical and other shapes are made by filament winding, for example, pressure bottles, missile canisters, industrial storage tanks and automobile drive shafts C- fibers with epoxy- basin resin composite is used for fabricating strength- critical aerospace structures.

Fig. 1.10 Filament Winding Process

5) Laminating:  In this process, composite parts are produced by combining layers of resin-impregnated material in a press under heat and pressure. The parts include, standard for comparatively flat pieces. Two principal steps in the manufacture of laminated fiber-reinforced composite materials are:-

(a) Lay-up, which consists of arranging fibers in layers.

(b) Curing

We start with a preperg material (partially cured composite with the fibers aligned parallel to each other). A pattern of product’s shape is cut out the preperg material is then stacked in layers into the desired laminate geometry. Curing the stacked pile under heat and pressure in an autoclave makes a final product, or by tool press moulding, winding the impregnated fibre on a mandrel of suitable diameter produces tubes. The assembly is then cured in a moulding press and then the mandrel is removed.

1.16 FABRICATION OF MMC:

Basically, three approaches are followed for fabricating MMC

1. Liquid phase approach: In this technique, the matrix material is the molten phase and the reinforcement is in the solid state. Either one of the conventional casting process can be used to fabricate MMC or “Pressure infiltration casting method “can be used. In this method, a perform is made (usually a sheet or wire) of reinforcing fibres and the liquid metal matrix is forced into it with the help of a pressurized gas.

2. Solid phase technique: Here the Powder Metallurgy route is used to fabricate MMC. The best example is of manufacturing WC tool material where cobalt is used as the matrix material.

3. Two phase Processing: Here the metal matrix contains both the solid and liquid phases. The reinforcing fibres are mixed with the matrix. The mixture is then atomized when it leaves the nozzles and is sprayed and deposited over the surface of a mould cavity to fabricate MMC.

PROCESSING OF CMC

The most common method of fabricating CMC is of “Slurry infiltration”. A perform of reinforcing fibres is prepared which is then hot pressed. Slurry containing matrix powder, a carrier liquid and an organic binder is prepared. The perform is then impregnated with the slurry to fabricate CMC.

1.17 MACHINING CUTTING AND JOINING OF COMPOSITES

Conventional processes and tools are generally not suited for machining, cutting and joining of composites. Therefore, special methods are employed to the final processing operations for the composites.

1. Machining: Machining of composite materials should ensure that there is no splintering, cracking, fraying or delaminating of cured composite edges. Standard machine tools can be used with appropriate modifications. Cutting tools for composites include: drills, reamers, countersinks, cut-of wheels and router bits. Common cutting tool materials are: HSS and WC. However, poly-crystalline diamond insert tool performs satisfactory and is cost effective. Tools must be kept sharp, to provide quality cuts and avoid de-lamination. Tool and its geometry should be carefully selected. Cutting speeds and feeds will depend on the type of composite material, its thickness and the cutting method.

2. Cutting: The conventional methods for cutting uncured composites, such as preperg ply include: manual cutting with Carbide disk cutter, scissors and power shears. For cutting uncured composites, the main techniques are: reciprocating knife cutting, high pressure water jet cutting, ultrasonic knife cutting and laser cutting.

3. Joining: The common joints provided for composites structures are: Bolted joints and Adhesive bonded joints.