Textbook of Production Technology PDF

692 A Textbook of Production Technology For example, for carbon and glass fibres, the critical length is of the order of 1 mm, which may be 20 to 150 times the diameter of the fibre. The fibre reinforcement can be done in thre...

692 A Textbook of Production Technology For example, for carbon and glass fibres, the critical length is of the order of 1 mm, which may be 20 to 150 times the diameter of the fibre. The fibre reinforcement can be done in three ways : 1. Continuous and aligned, Fig. 14.1a. 2. Discontinuous and aligned, Fig. 14.1b. 3. Continuous and randomly oriented, Fig. 14.1c. 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 strength of the randomly oriented reinforcements, lies between the above two extremes. This arrangement is best suited for applications involving multi-directional applied stresses, for example, bi-axial stresses in pressure vessels or a tube. The same results can be achieved by using bi-axially oriented or cross-ply fibres. It is apparent that the strength of the discontinuous and aligned arrangement will be less than that 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. We can produce components with exceptional strength-to-weight and stiffness- to-weight ratios (many composites are stronger than steel, lighter than aluminuim 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 the tensile load since concrete is 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 temperatures, 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 : Vinylester 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 the missiles and space shuttles, racing car disk 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 rudder. The other fibre-matrix combination can be : Aramid fibre-Phenolic resin matrix, Boron fibre-Bismaleimide resin matrix. 4. Automative uses : Body panels, drive shafts, springs and bumpers, Cab shells and bodies, oil pans, fan shrouds, instrument panels and engine covers. 5. Sports equipment : Golf club shafts, baseball parts, fishing rods, tennis rackets, bicycle frames, skis and pole vaults. 6. Rubber used for making automobile 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 Composite Materials and Their Processing 693 very high tensile strength and modulus of elasticity. Its density is about half of that of aluminuim and it has negative thermal expansion. It is flame retardant and transparent 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 composites (MMC) : As already noted, these composites are obtained by impregnating high-strength fibres (of stainless steel, boron, tungsten, molybdenum, graphite, Al2O3, SiC and Si3N4 etc.) with molten metal (aluminuim, titanuim, 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 greater heat resistance and improved thermal and electrical 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. Aluminium oxide reinforced aluminium is used for making automotive connecting rods. Aluminium reinforced with Si C whiskers is used to make air craft wing panels. Fibre reinforced superalloys are used for making turbine blades. Graphite fibres in aluminium matrix is used for Satellite, missile, and helicopter structures. Graphite fibres in magnesium matrix is used for space and satellite structures. Graphite fibres in lead matrix is used for Storage-battery plates. Graphite fibres in Copper matrix is used for bearings and electrical contacts. Other examples of MMC are: (a) Boron fibre in Aluminium : Compressor blades and structural supports. (b) ’’ ’’ ’’ Magnesium : Antenna structures. (c) ’’ ’’ ’’ Titanium : Jet-engine fan blades. (d) Alumina ’’ ’’ Lead : Storage-battery plates. (e) ’’ ’’ ’’ Magnesium : helicopter transmission structures. (f) SiC ’’ ’’ Super alloy (Cobalt based) : High-temperature engine components. (g) Tungsten and Molybdenum fibres in Superalloy 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, Si3N4, and mullite (a compound of Al, Si, and O2). They can retain their strength upto 1700°C, and also resist corrosive environments. Typical product applications of Ceramic-matrix composites are : in jet and automotive engines, deep-sea mining equipment, pressure vessels, structural components, cutting tools, and dies for extrusion and drawing operations. In Japan, Toyota have made automotive engine pistons and connecting rods from a hybrid MMC. This material has fine particulates, ceramic whiskers or both on continuous SiC or Carbon fibres that then are cast in an aluminium matrix. The resulting MMC is twice as strong as conventional SiC or Carbon-reinforced aluminium composite materials. Los Alamos National Lab. (U.S.A.) has developed a SiC-reinforced molybdenum disilicide intermetallic that at temperatures greater than 1200°C is 15 times stronger than current intermetallic alloys. Composites in the Development Stage : The following fibre reinforced composites are in the developmental stage :- (i) Advanced bismaleimide resin matrix series for high temperature service. (ii) Polyether etherketone thermoplastic matrix series for higher temperature service. (iii) Hybrid reinforcements and Knitted/stacked ply fabrics and three-dimensional (special shape) woven fabric reinforcements. (iv) Selective stiching of Collated plykits. 694 A Textbook of Production Technology In hybrid reinforced composites, two or more different types of fibres are used in a common matrix. Such composites exhibit better properties as compared to a single fibre reinforced composite. A very common example is of glass- and carbon-fibre reinforced resins. They combine the strength, stiffness and low density of carbon fibres with low cost of glass fibres. Such hybrid composites possess low weight, toughness, higher impact resistance and are of reduced cost. Hybrid composites are finding use in light weight transport (land, water or air), light weight orthopeadic components, sports equipment and structural components. In the hybrid composites, the reinforcing fibres can be arranged in the following ways : 1. Reinforcing fibres positioned in alternate layers, known as, interply. 2. Mixed reinforcing fibres in the same layer (intraply). 3. Combination of 1 and 2, that is, inter-ply-intra-ply. 4. Selected layup. Here, costlier fibre is used where it is necessary. 5. Stiching the plies of different reinforcing materials together, known as interply knitting. 14.2.3. 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 is the most common material under this category. Here, thin layers 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 thermostat and other heat sensing applications. 3. Safety glass : Discussed under Art. 13.7.3. 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 also be used. Wood substitutes based on red mud polymer have been developed to be used for door shutters, windows, partitions and false ceilings. The sandwich or honeycomb structures possess high specific strength and specific stiffness, that is, strength to weight ratio and stiffness-to-weight ratio, and high resistance to bending forces. The corrugated cardboard is extensively used in packaging for consumer and industrial goods. Due to the above mentioned properties, the honeycomb structures are used for aircraft and aerospace components such as wings, fuselage and tailplane skins. For these honeycomb structures, aluminium alloys are the most commonly used materials. 5. Roll cladding (bonding) and explosive cladding (welding) of one metal upon another : See chapter 5, Art. 5.5. The main aim of clad materials is to improve corrosion resistance while retaining low cost, high strength and/or light weight. 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 a 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 a suitable resin. The resins used include : phenolics, polyesters, silicones and epoxides. The paper or cloth provides the bulk of the strength, while the resin acts as a semirigid binder. Laminated plastic sheet can be machined, drilled, punched and pressed to shape. 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, laminate fabric base gears have completely replaced nonferrous gears. They are employed to transmit rotation from electric motors in high speed machine tools, Composite Materials and Their Processing 695 they are mounted on the camshafts of internal-combustion engines etc. In chemical industry, laminate fabric base gears are used in various apparatus and instruments where they resist corrosive attack much more efficiently than gears of bronze, brass or leather. In addition to gears, certain other transmitting devices : rollers, rings, etc are also made of laminate fabric base. Laminated sheets/ plates are available in sizes of : 900 × 900 mm, 900 × 1800 mm, 1200 × 2400 mm. The minimum thickness of sheet is 0.8 mm and it vaires 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 and carpets. 8. Laminated carbides : In laminated carbides, laminates consisting of a hard thin surface layer of TiC and in 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 (See chapter 7). 9. Laminated wood : Thin sheets of wood (veneer), impregnated with special resins and compressed hot, form what is called ‘laminated wood’, which finds extensive application in textile machinery and electrical engineering, as well as a substitue for nonferrous metals in bearings of hydraulic machinery and mechanisms operating in abrasive media. Parts of wood are machined in ordinary machine tools and wood working machinery. 14.2.4. Surface Coated Materials : The surface coatings are applied to the materials for various purposes :- protection of the material against corrosion; for decorative, wear resistant and processing purposes. They may also be used to : (i) improve visibility through luminescence and better reflectivity (ii) provide electrical insulation, and (iii) improve the appearance. Surface coatings are usually classified as : Metallic coatings, Inorganic chemical coatings and Organic chemical coatings. 1. Metallic coatings : Metallic coatings of copper, chromium, Nickle, Zinc, Lead and tin etc. are applied by hot dipping, electro-plating or spraying techniques to protect the base metal from corrosion and for other purposes (for details, refer to Art. 12.5.1). 2. Inorganic chemical coatings : These surface coatings may be divided into : Phosphate coating, Oxide coating and Vitreous coatings. Oxide and phosphate coatings are done to make iron or steel surfaces free from rust and this is done by chemical action. These coatings also provide protection against corrosion (For details, refer to Art. 12.5.1). Vitreous coatings are commonly applied to steel in the form of a powder or frit and are then fused to the steel surface by heat. These coatings are relatively brittle, but offer absolute protection against corrosion. Enamel is an example of a ceramic coating on metal and glaze on tiles is an example of a glassy ceramic on crystalline ceramic base. The glazing as a protective coating on porcelain and stoneware ceramic is performed for the purpose of protection from moisture absorption in ceramic materials. Coatings of TiC, TiN, Al2O3 or HFN on WC base are examples of ceramics on ceramic and coatings of TiC and TiN on HSS base are examples of ceramics on steel. These coatings increase the life of cutting tools (For details, refer chapter 7 and Art. 12.7). 3. Organic coatings : Organic coatings include paints, varnishes, enamels and lacquers. They serve to protect the base metal and to improve its appearance (for details refer to Art. 12.5.2). Polymer coatings on paper are used for making milk cartons. Polymer coated textiles are used for making seat covers and carpets, Polymer coatings on metals act as wire insulation. Polymer coated metals are used for making beverage cans. 696 A Textbook of Production Technology 14.3. PRODUCTION OF COMPOSITE STRUCTURES 14.3.1. Fabrication of Particulate Composites : As discussed under Art. 14.2. a majority of the particulate composits 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 meterials through introduction into a slurry (Concrete) or into a liquid melt (agglomeration of asphalt and stone particles). 14.3.2. 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 form of the fibres and their orientation in the matrix material; the shape, size and the quantity of the product. The common fabrication processes are : Open-Mould proceses, Filament winding, Pultrusion and Matched-die-Moulding, and Laminating. Before these processes are discussed, the following terms should be understood : (i) Prepregs : Prepregs means “Preimpregnated with resin”. It is a ready-to-mould material in sheet form. These are made by impregnating rovings and mats 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 continuous solid matrix. “Lay-up” is positioning of the reinforcement material, sometimes resin-impregnated, in the mould. (ii) BMCs - are “Bulk Moulding Compounds”. These are thermosetting resins mixed with chopped reinforcements or fillers and made into a viscous compound for compression moulding. (iii) SMCs - are “Sheet Moulding Compounds”. These comprise chopped fibres and resin in sheet form approximately 2.5 mm thick. These are processed further to fabricate large sheet like parts. They can replace sheet metal, where light weight, corrosion resistance and integral colour are attractive features. (iv) Thick Moulding Compounds. Thick moulding compounds (TMC) combine 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 dielectric 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 is used to impregnate or coat the reinforcement. It is then followed by curing the resin to permanently fix the shape. Curing may be at room temperature or it may be speeded up by heating. The technique in which resin-saturated reinforcements is placed in the mould is called “Wet lay-up”. (b) Bag Moulding : This is a technique of moulding reinforced plastics composites by using a flexible 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 byá: 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 Composite Materials and Their Processing 697 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. 14.2.) (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 place. Compressed air forces the bag against the part to apply pressure while the part cures. Oven or autoclave Heat Pressure (Vacuum) Atm Wet or prepreg resin and fabric Sheet Sealant tape (Periphery) Mould Fig. 14.2. Vacuum Bag Moulding. (c) Spray-up : In this technique, a spray gun supplies resin in two converging streams into which chopped roving fibre 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 meduim 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. [See Fig. 14.3. for “Compression moulding” of composite parts.] PRESSURE Top die Material Bottom die Time Heat Fig. 14.3. Compression Moulding. (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 698 A Textbook of Production Technology 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 pre- ferred to avoid directionality of reinforcement. This method is known as "Reaction Injection Moulding", and is being increasingly used for BMCs. Puller rolls Multipleply Fibre Resin wetout tray Preheater Heated Die Strands Fig. 14.4. Pultrusion. (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. 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.) of a constant cross-section. After passing through the resin-dip tank, the roving is drawn through a heated die (where curing takes place) and cured to form the desired cross-section, as it continuously runs through the machine, (Fig.14.4.) 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, vehicle drive shafts, because of their high damping capacity, and structural members for vehicle and aerospace applications. Unit traverses horizontally Continuous Resin dip-cum strand squeeze unit (roving) Rotating mandrel Resin dipped strand wound over mandrel Fig. 14.5. Schematic Diagram of Filament Winding Process. 4. Filament winding : In this process, resin-impregnated strands are applied over a rotating mandrel, to produce high strength, reinforced cylindrical shapes. Fibres or tapes are drawn through Composite Materials and Their Processing 699 a resin bath and wound onto a rotating mandrel Fig. 14.5. The process is relatively slow, but the fibre direction can be controlled and the diameter can be varied along the length of the piece. In a variation, the Fibre bundle (made up of several thousand carbon fibres) is first coated with the matrix material, to make a prepreg tape (endless strip with width equal to several cms, to a metre). With both the fibre 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-fibres with epoxy- basin resin composite is used for fabricating strength-critical aerospace structures. 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 structural shapes, plates, sheets, angles, channels, rods, tubes etc. However, mainly it is used for comparatively flat pieces. Two principal steps in the manufacture of laminated fibre-reinforced composite materials are :- (a) Lay-up which consists of arranging fibres in layers, (b) Curing We start with a prepreg material (partially cured composite with the fibres aligned parallel to each other). A pattern of product's shape is cut out, and the prepreg material is then stacked in layers into the desired laminate geometry. A final product is made by curing the stacked pile under heat and pressure in an autoclave, or by tool press moulding. Tubes are produced by winding the impregnated fibre on a mandrel of suitable diameter. The assembly is then cured in a moulding press and then the mandrel is removed, See Fig. 14.5. 14.3.3. Fabrication of MMC. Basically, three approaches are followed for fabricating MMC: 1. Liquid Phase Approach. In this technique, the matrix material is in the molten phase and the reinforcement is in the solid state. Either one of the conventional casting processes can be used to fabricate MMC or “Pressure infiltration casting method” can be used. In this method, a preform 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. 14.3.4. Processing of CMC. The most common method of fabricating CMC is of “Slurry infiltration”. A preform of reinforcing fibres is prepared which is then hot pressed. A slurry containing matrix powder, a carrier liquid and an organic binder is prepared. The preform is then impregnated with the slurry to fabricate CMC. 14.3.5. Fabrication of Laminates : As already discussed, the two common methods of fabricating laminar composites, are : Roll welding or Roll bonding or Roll cladding and Explosive welding or cladding (Chapter 5, Art. 5.5). In Roll welding (a cold process), two or more sheets of similar or dissimilar metals are joined together by simultaneously passing them through a rolling mill. The method is also used for cladding one material onto another and for producing bimetallic structures (Fig. 5.42 a). Wide plates and dissimilar materials with large difference in mechanical properties are fabricated by Explosive welding or bonding (Fig. 5.41). Composite coins are also made by these two methods. The aim is to save more costly high nickle material and at the same time retain the desired lustre and corrosion resistance. Another method of fabricating laminates has been discussed in the last article. 700 A Textbook of Production Technology There are two ways of stacking the plastic sheets in layers (The thickness of sheet may be as small as 0.05 to 0.l2 mm). In one, the direction of continuous and aligned fibre reinforcements is the same in all the sheets, whereas in the second, the two consecutive sheets are stacked with the fibre direction at right angles to each other. In the first arrangement, the structure will be very strong if loaded in the direction of fibre alignment and it will be very weak in the transverse direction. However, in the second case, the structure will be equally strong in the two dimensional normal planes. For example, adjacent wood sheets in plywood are aligned with the goain directions at right angles to eachother. Honeycomb Structures and Sandwich Panels To fabricate honeycomb structures, the first step is to fabricate the core (honeycomb panel or the sandwich panel). Two methods are used for this : 1. Corrugated Process. The flat sheet is passed through a pair of specially designed rolls (just similar to straight tooth spur gears). The flat sheet is converted into corrugated sheet. The continuous corrugated wheet coming out of the rolls is cut into desired lengths. The individual cut corrugated sheets are stacked together with adhesive applied at nodal points. The complete assembly is then cured to produce a honeycomb block. Face Sheet Honeycomb Adhesive Face sheet Fabricated sandwich panel (a) (b) Fig. 14.6. Honeycomb Structure. 2. Expansion Process. Flat sheets are cut from a roll. Adhesive is applied at intervals along nodal lines. The individual sheets are stacked together and are then cured in a furance. Strong joints will be created along nodal lines. The c ured sheet is then cut into slices of desired dimensions. These are then stretched or expanded to get the honeycomb block. To fabricate the honeycomb structure, the face sheets are attached with adhesives or by brazing, to the top and bottom surfaces of the honeycomb block, Fig. 14.6. The face materials can be : plywood, aluminium alloys, stainless steels, nickle alloys, titanium and fibre reinforced plastics (such as aramid-epoxy). The honecomb block (core) materials include : synthetic rubbers, foamed polymers and inorganic cements etc. 14.3.6. Fabricating Surface-Coated Composite Structures : For this, the readers should refer to Art. 12.5.3, 12.5.4 and Art. 12.7. 14.4. 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 delamination of cured composite edges. Standard machine tools can be used Composite Materials and Their Processing 701 with appropriate modifications. Cutting tools for composites include : drills, reamers, countersinks, cut-off wheels and router bits. Common cutting tool materials are :- HSS and WC. However, poly- crystalline diamond insert tool performs satisfactorily 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 prepreg ply include : manual cutting with Carbide disk cutter, scissors and power shears. For cutting cured 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 composite structures are : Bolted joints and Adhesive bonded joints. PROBLEMS 1. What are composite materials? 2. What are the various types of composite materials ? 3. What are Fibre-reinforced materials ? 4. What are the common reinforcing materials ? 5. Give the application of :- (a) Glass-fibre reinforced plastics. (b) C-C composites. (c) Resin-matrix composites. 6. What are Laminates ? Write about the common laminates. 7. What are surface coated materials ? 8. Write about the various techniques of fabricating fibre reinforced composites. 9. What are : (a) Prepregs (b) BMCs (c) SMCs (d) TMCs“ 10. Write about the following processes :- (a) Resin Injection Moulding. (b) Pultrusion. (c) Filament Winding. (d) Laminating 11. What are MMC and CMC ? 12. How are MMC and CMC fabricated ? 13. How a honeycomb structure is fabricated ?. 14. List the advantages of composite materials. 15. Distinguish between alloys and composite materials. 16. What is a hybrid composite? 17. What is the range of length and diameter of reinforcing fibres? 18. What are the basic functions of a honeycomb structure? What are its basic components? 19. Explain the functions of the matrix and the reinforcing fibres. What is the fundamental difference in the characteristics of the two? 20. Compare the advantages and disadvantages of MMC, CMC and reinforced plastic composites. 21. List the product applications of “Pultrusion” method. 22. List all possible applications for filament wound plastics. 23. List the product applications of open-mould process. 24. What are the various forms of reinforcing fibres?