COMPOSITE MATERIALS

Elective 4-1 Mech

JNTU H

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Composite Materials

Chapter 1 – Introduction to composite MaterialsA composite material can be defined as a combination of two or more materials that results in better properties than those of the individual components used alone. In contrast to metallic alloys, each material retains its separate chemical, physical, and mechanical properties. The two constituents are reinforcement and a matrix. Fibers or particles embedded in matrix of another material are the best example of modern-day composite materials, which are mostly structural.

The main advantages of composite materials are their high strength and stiffness, combined with low density, when compared with bulk materials, allowing for a weight reduction in the finished part.

Classification of composites:

Composite materials are commonly classified at following two distinct levels:

1. The first level of classification is usually made with respect to the matrix constituent

2. The second level of classification refers to the reinforcement form:

Fibre reinforced composites: Fibre Reinforced Composites are composed of fibres embedded in matrix material. Such a composite is considered to be a discontinuous fibre or short fibre composite if its properties vary with fibre length. On the other hand, when the length of the fibre is

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such that any further increase in length does not further increase the elastic modulus of the composite, the composite is considered to be continuous fibre reinforced. Fibres are small in diameter and when pushed axially, they bend easily although they have very good tensile properties. These fibers must be supported to keep individual fibres from bending and buckling.

Laminar Composites are composed of layers of materials held together by matrix. Sandwich structures fall under this category.

Particulate Composites are composed of particles distributed or embedded in a matrix body.

The particles may be flakes or in powder form. Concrete and wood particle boards are examples of this category.

Natural composites

Natural composites exist in both animals and plants.

Wood is a composite – it is made from long cellulose fibers (a polymer) held together by a much weaker substance called lignin.

Cellulose is also found in cotton, but without the lignin to bind it together it is much weaker. The two weak

Substances – lignin and cellulose – together form a much stronger one.

The bone in your body is also a composite. It is made from a hard but brittle material called hydroxyapatite (which is mainly calcium phosphate) and a soft and flexible material called collagen (which is a protein). Collagen is also found in hair and finger nails. On its own it would not be much

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use in the skeleton but it can combine with hydroxyapatite to give bone the properties that are needed to support the body.

Advantages of Composites

Light Weight - Composites are light in weight, compared to most woods and metals. Their lightness is important in automobiles and aircraft, for example, where less weight means better fuel efficiency (more miles to the gallon). People who design airplanes are greatly concerned with weight, since reducing a craft’s weight reduces the amount of fuel it needs and increases the speeds it can reach. Some modern airplanes are built with more composites than metal including the new Boeing 787, Dreamliner.

High Strength - Composites can be designed to be far stronger than aluminum or steel. Metals are equally strong in all directions. But composites can be engineered and designed to be strong in a specific direction.

Strength Related to Weight - Strength-to-weight ratio is a material’s strength in relation to how much it weighs. Some materials are very strong and heavy, such as steel. Other materials can be strong and light, such as bamboo poles. Composite materials can be designed to be both strong and light. This property is why composites are used to build airplanes—which need a very high strength material at the lowest possible weight. A composite can be made to resist bending in one direction, for example. When something is built with metal, and greater strength is needed in one direction, the material usually must be made thicker, which adds weight. Composites can be strong without being heavy. Composites have the highest strength-to-weight ratios in structures today.

Corrosion Resistance - Composites resist damage from the weather and from harsh chemicals that can eat away at other materials. Composites are good choices where chemicals are handled or stored. Outdoors, they stand up to severe weather and wide changes in temperature.

High-Impact Strength - Composites can be made to absorb impacts—the sudden force of a bullet, for instance, or the blast from an explosion. Because of this property, composites are used in bulletproof vests and panels, and to shield airplanes, buildings, and military vehicles from explosions.

Design Flexibility - Composites can be molded into complicated shapes more easily than most other materials. This gives designers the freedom to create almost any shape or form. Most recreational boats today, for example, are built from fiberglass composites because these materials can easily be molded into complex shapes, which improve boat design while lowering costs. The surface of composites can also be molded to mimic any surface finish or texture, from smooth to pebbly.

Part Consolidation - A single piece made of composite materials can replace an entire assembly of metal parts. Reducing the number of parts in a machine or a structure saves time and cuts down on the maintenance needed over the life of the item.

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Dimensional Stability - Composites retain their shape and size when they are hot or cool, wet or dry. Wood, on the other hand, swells and shrinks as the humidity changes. Composites can be a better choice in situations demanding tight fits that do not vary. They are used in aircraft wings, for example, so that the wing shape and size do not change as the plane gains or loses altitude.

Nonconductive - Composites are nonconductive, meaning they do not conduct electricity. This property makes them suitable for such items as electrical utility poles and the circuit boards in electronics. If electrical conductivity is needed, it is possible to make some composites conductive.

Nonmagnetic - Composites contain no metals; therefore, they are not magnetic. They can be used around sensitive electronic equipment. The lack of magnetic interference allows large magnets used in MRI (magnetic resonance imaging) equipment to perform better. Composites are used in both the equipment housing and table. In addition, the construction of the room uses composites rebar to reinforced the concrete walls and floors in the hospital.

Radar Transparent - Radar signals pass right through composites, a property that makes composites ideal materials for use anywhere radar equipment is operating, whether on the ground or in the air. Composites play a key role in stealth aircraft, such as the U.S. Air Force’s B-2 stealth bomber, which is nearly invisible to radar.

Low Thermal Conductivity - Composites are good insulators—they do not easily conduct heat or cold. They are used in buildings for doors, panels, and windows where extra protection is needed from severe weather.

Durable - Structures made of composites have a long life and need little maintenance. We do not know how long composites last, because we have not come to the end of the life of many original composites. Many composites have been in service for half a century.

  Disadvantage of composites  

Even though composites have distinct features over metals, they do have few limitations or drawbacks. So the drawbacks or limitations in use of composites include

High Cost High cost of fabrication of composites is a critical issue. For example, part made of graphite/epoxy composite may cost up to 10 to 15 times the material costs. A finished graphite/epoxy composite part may cost as much as $300 to $400 per pound ($650 to $900 per kilogram). Improvements in processing and manufacturing techniques will lower these costs in the future.

Complex Repair Procedure   Repair of composites is not a simple process compared to that for metals. Sometimes critical flaws and cracks in composite structures may go undetected.   

Mechanical Characterization   Mechanical characterization of a composite structure is more complex than that of a metal structure. Unlike metals, composite materials are not isotropic, that is, their properties are not the same in all directions. Therefore, they require more material parameters. For example, a single layer of a graphite/epoxy composite requires nine stiffness and strength constants for conducting mechanical analysis. In the case of a monolithic material such as

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steel, one requires only four stiffness and strength constants. Such complexity makes structural analysis computationally and experimentally more complicated and intensive. In addition, evaluation and measurement techniques of some composite properties, such as compressive strengths, are still being debated.  

Composite materials don’t break easily, but that makes it hard to tell if the interior structure has been damaged at all. In contrast, aluminum bends and dents easily, making its easy to detect structural damage; the same damage is much harder to detect with composite structures. Repairs can also be more difficult when a composite surface is damaged.

The resin used in composite material weakens at temperatures as low as 150 degrees, making it important for these aircraft to avoid fires. Fires involved with composite materials can release toxic fumes and micro particles into the air. Temperatures above 300 degrees can cause structural failure.

Finally, composite materials can be expensive, but the high initial costs are typically offset by long-term cost savings.

In matrix-based structural composites, the matrix serves two paramount purposes viz., binding the reinforcement phases in place and deforming to distribute the stresses among the constituent reinforcement materials under an applied force.

The demands on matrices are many. They may need to temperature variations, be conductors or

Resistors of electricity, have moisture sensitivity etc. This may offer weight advantages, ease of

Handling and other merits which may also become applicable depending on the purpose for

Which matrices are chosen? Solids that accommodate stress to incorporate other constituents provide strong bonds for the reinforcing phase are potential matrix materials. A few inorganic materials, polymers and metals have found applications as matrix materials in the designing of structural composites, with commendable success. These materials remain elastic till failure occurs and show decreased failure strain, when loaded in tension and compression.

Composites cannot be made from constituents with divergent linear expansion characteristics. The interface is the area of contact between the reinforcement and the matrix materials. In some cases, the region is a distinct added phase. Whenever there is interphase, there has to be two interphases between each side of the interphase and its adjoint constituent. Some composites provide interphases when surfaces dissimilar constituents interact with each other. Choice of fabrication method depends on matrix properties and the effect of matrix on properties of reinforcements. One of the prime considerations in the selection and fabrication of composites is that the constituents should be chemically inert non-reactive.

Role of Matrices in Composites

Holds the fibres together Protects the fibres from environment.

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Transfer stresses between the fibers. Distributes the loads evenly between fibres so that all fibres are subjected to the same amount of strain.

Provide a barrier against an adverse environment. Protect the surface of the fibers from mechanical abrasion. Improves impact and fracture resistance of a component Determine inter-laminar shear strength. Determine damage tolerance of composites. Determine in-plane shear strength. Determine the processibility of composites. Determine heat resistance of composites.

Organic Matrix Composites (OMC)

1. Polymer Matrix Composites (PMC) or Carbon Matrix Composites or Carbon-Carbon CompositesPolymer Matrix Composites (PMC) is the material consisting of a polymer (resin) matrix combined with a fibrous reinforcing dispersed phase. Polymer Matrix Composites are very popular due to their low cost and simple fabrication methods. Polymers make ideal materials as they can be processed easily, possess lightweight, and desirable mechanical properties. It follows, therefore, that high temperature resins are extensively used in aeronautical applications. Two main kinds of polymers are Thermosets and Thermoplastics.

Thermoplastics, in basic terms, are melt-process able plastics (materials that are processed with heat). When enough heat is added to bring the temperature of the plastic above its melt point, the plastic liquefies (softens enough to be processed). When the heat source is removed and the temperature of the plastic drops below its melt point, the plastic solidifies (or freezes) back into a glass-like solid. This process can be repeated, with the plastic melting and solidifying as the temperature climbs above and drops below the melt temperature, respectively. However, the material can be increasingly subject to deterioration in its molten state, so there is a practical limit to the number of times that this reprocessing can take place before material properties begin to suffer. Many thermoplastic polymers are addition-type, capable of yielding very long molecular chain lengths (very high molecular weights).

Reinforcement of Thermoplastics: Resins reinforced with thermoplastics now comprised an emerging group of composites. The theme of most experiments in this area to improve the base properties of the resins and extract the greatest functional advantages from them in new avenues, including attempts to replace metals in die-casting processes. In crystalline thermoplastics, the reinforcement affects the morphology to a considerable extent, prompting the reinforcement to empower nucleation. Whenever crystalline or amorphous, these resins possess the facility to alter their creep over an extensive range of temperature. But this range includes the point at which the usage of resins is constrained, and the reinforcement in such systems can increase the failure load as well as creep resistance.

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A small quantum of shrinkage and the tendency of the shape to retain its original form are also to be accounted for. But reinforcements can change this condition too. The advantage of thermoplastics systems over thermosets are that there are no chemical reactions involved, which often result in the release of gases or heat. Manufacturing is limited by the time required for heating, shaping and cooling the structures.

Thermoplastics resins are sold as moulding compounds. Fiber reinforcement is apt for these resins. Since the fibers are randomly dispersed, the reinforcement will be almost isotropic. However, when subjected to moulding processes, they can be aligned directionally.

There are a few options to increase heat resistance in thermoplastics. Addition of fillers raises the heat resistance. But all thermoplastic composites tend loose their strength at elevated temperatures. However, their redeeming qualities like rigidity, toughness and ability to repudiate creep, place thermoplastics in the important composite materials bracket. They are used in automotive control panels, electronic products encasement etc.

Newer developments augur the broadening of the scope of applications of thermoplastics. Huge sheets of reinforced thermoplastics are now available and they only require sampling and heating to be moulded into the required shapes. This has facilitated easy fabrication of bulky components, doing away with the more cumbersome moulding compounds.

The different types of thermoplastic are: Acrylonitrile Butadiene Styrene (ABS), Acetals,

Acrylics, Cellulosics, Fluorocarbons, Polyamides, Polycarbonates, Polyethylene (PE),

Polypropylenes (PP), Polystyrenes, Polyetheretherketone, Polyvinyl Chloride (PVC), Liquid

Crystal Polymers (LCP), Polyphenylene Sulphide (PPS) and Vinyls.

Molecular structure of thermoplastics

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Applications

Thermoplastics can be used to manufacture the dashboards and car trims, toys, phones, handles, electrical products, bearings, gears, rope, hinges and catches, glass frames, cables, hoses, sheet, and windows, etc.

Thermosets, again in basic terms, are materials that undergo a chemical reaction (cure) and transform from a liquid to a solid. In its uncured form, the material has very small, unlinked molecules (known as monomers). The addition of a second material (catalyst) and/or heat or some other activating influence will initiate the chemical reaction. During this reaction the molecules cross-link and form significantly longer molecular chains, causing the material to solidify. This change is permanent and irreversible. Subsequently, exposure to high heat will cause the material to degrade, not melt. This is because these materials typically degrade at a temperature below where it would be able to melt.

Thermosets have qualities such as a well-bonded three-dimensional molecular structure after curing. They decompose instead of melting on hardening. Merely changing the basic composition of the resin is enough to alter the conditions suitably for curing and determine its other characteristics. They can be retained in a partially cured condition too over prolonged periods of time, rendering Thermosets very flexible. Thus, they are most suited as matrix bases for advanced conditions fiber reinforced composites.

Direct condensation polymerization followed by rearrangement reactions to form heterocyclic entities is the method generally used to produce thermoset resins. Water, a product of the reaction, in both methods, hinders production of void-free composites. These voids have a negative effect on properties of the composites in terms of strength and dielectric properties. Polyesters phenolic and Epoxies are the two important classes of thermoset resins.

Thermosets find wide ranging applications in the chopped fiber composites form particularly when a premixed or moulding compound with fibers of specific quality and aspect ratio happens to be starting material as in epoxy, polymer and phenolic polyamide resins.

Molecular structure of thermosets

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Applications

Thermosets are commonly used for high temperature applications. Some of the common products are electrical equipments, motor brush holders, printed circuit boards, circuit breakers, encapsulation, kitchen utensils, handles and knobs, and spectacle lenses.

Epoxy resins are widely used in filament-wound composites and are suitable for moulding prepress. They are reasonably stable to chemical attacks and are excellent adherents having slow shrinkage during curing and no emission of volatile gases. These advantages, however, make the use of epoxies rather expensive. Also, they cannot be expected beyond a temperature of 140ºC. Their use in high technology areas where service temperatures are higher, as a result, is ruled out.

Polyester resins on the other hand are quite easily accessible, cheap and find use in a wide range of fields. Liquid polyesters are stored at room temperature for months, sometimes for years and the mere addition of a catalyst can cure the matrix material within a short time. They are used in automobile and structural applications.

The cured polyester is usually rigid or flexible as the case may be and transparent. Polyesters withstand the variations of environment and stable against chemicals. Depending on the formulation of the resin or service requirement of application, they can be used up to about 75ºC or higher. Other advantages of polyesters include easy compatibility with few glass fibers and can be used with verify of reinforced plastic accoutrey.

2. Metal Matrix Composites

Metal Matrix Composites (MMCs) are composed of a metal matrix and a reinforcement, or filler material, which confers excellent mechanical performance. Metal Matrix Composites (MMCs) are classified according to whether the reinforcement is continuous (monofilament or multifilament) or discontinuous (particle, whisker, short fibre or other).They can withstand elevated temperature in corrosive environment than polymer composites. Most metals and alloys could be used as matrices and they require reinforcement materials which need to be stable over a range of temperature and non-reactive too. However the guiding aspect for the choice depends essentially on the matrix material. High strength, fracture toughness and stiffness are offered by metal matrices than those offered by their polymer counterparts.Light metals form the matrix for temperature application and the reinforcements in addition to the aforementioned reasons are characterized by high moduli.Most metals and alloys make good matrices. However, practically, the choices for low temperature applications are not many.The principal matrix materials for MMCs are aluminium and its alloys. To a lesser extent, magnesium and titanium are also used, and for several specialised applications.MMCs with discontinuous reinforcements are usually less expensive to produce than continuous fibre reinforced MMCs, although this benefit is normally offset by their inferior mechanical

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properties. Consequently, continuous fibre reinforced MMCs are generally accepted as offering the ultimate in terms of mechanical properties and commercial potential.

Schematic presentation of three shapes of metal matrix composite materials

Applications:

Automotive and heavy goods vehicle- Bracing systems, piston rods, frames, piston, piston pins, valve spring cap, brake discs, disc brake calliper, brake pads, cardan shaftMilitary and civil air travel - Axle tubes, reinforcements, blade-and gear box casing, fan and compressor bladesAerospace industry - Frames, reinforcements, aerials joining elementsOther applications: Super conductor, Carbon brushes, Spot welding electrodes, Bearings

3. Ceramic Matrix Materials (CMM)

Ceramics can be described as solid materials which exhibit very strong ionic bonding in general and in few cases covalent bonding. High melting points, good corrosion resistance, stability at elevated temperatures and high compressive strength, render ceramic-based matrix materials a favorite for applications requiring a structural material that doesn’t give way at temperatures above 1500ºC. Naturally, ceramic matrices are the obvious choice for high temperature applications.High modulus of elasticity and low tensile strain, which most ceramics posses, have combined to cause the failure of attempts to add reinforcements to obtain strength improvement.The purpose

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of developing the ceramic matrix composites (CMCs) is to improve the desirable properties of ceramics with adding reinforcements and limiting their inherent weaknesses.Properties of ceramic composites Typical mechanical, electrical and chemical properties exhibited by ceramic composites are discussed below. Mechanical Properties

Tensile and compressive behavior Fracture toughness Creep R-Curve behavior Fatigue Resistance

Fiber-Reinforced Composites:A fiber-reinforced composite (FRC) is a composite building material that consists of three components:

(i) The fibers as the discontinuous or dispersed phase, (ii) The matrix as the continuous phase, and(iii) The fine interphase region, also known as the interface.This is a type of advanced composite group, which makes use of rice husk, rice hull, and plastic as ingredients. This technology involves a method of refining, blending, and compounding natural fibers from cellulosic waste streams to form a high-strength fiber composite material in a polymer matrix. The designated waste or base raw materials used in this instance are those of waste thermoplastics and various categories of cellulosic waste including rice husk and saw dust. Fiber-reinforced composites are composed of axial particulates embedded in a matrix material. The objective of fiber-reinforced composites it to obtain a material with high specific strength and high specific modulus. (i.e. high strength and high elastic modulus for its weight.) The strength is obtained by having the applied load transmitted from the matrix to the fibers. Hence interfacial bonding is important.

Classic examples of fiber-reinforced composites include fiberglass and wood.

Fiber GeometrySome common geometries for fiber-reinforced composites:

1. AlignedThe properties of aligned fiber-reinforced composite materials are highly anisotropic. The longitudinal tensile strength will be high whereas the transverse tensile strength can be much less than even the matrix tensile strength. It will depend on the properties of the fibers and the matrix, the interfacial bond between them, and the presence of voids. There are 2 different geometries for aligned fibers:

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a) Continuous & AlignedThe fibers are longer than a critical length which is the minimum length necessary such that the entire load is transmitted from the matrix to the fibers. If they are shorter than this critical length, only some of the load is transmitted. Fiber lengths greater that 15 times the critical length are considered optimal. Aligned and continuous fibers give the most effective strengthening for fiber composites.

b) Discontinuous & AlignedThe fibers are shorter than the critical length. Hence discontinuous fibers are less effective in strengthening the material, however, their composite modulus and tensile strengths can approach 50-90% of their continuous and aligned counterparts. And they are cheaper, faster and easier to fabricate into complicated shapes.2. Random: This is also called discrete, (or chopped) fibers. The strength will not be as high as with aligned fibers, however, the advantage is that the material will be istropic and cheaper.

3. Woven:The fibers are woven into a fabric which is layered with the matrix material to make a laminated structure.

Applications:There are also applications in the market, which utilize only waste materials. Its most widespread use is in outdoor deck floors, but it is also used for railings, fences, landscaping timbers, cladding and siding, park benches, molding and trim, window and door frames, and indoor furniture.

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Chapter 2

Reinforcements:

Introduction to Fibres:

Organic and inorganic fibers are used to reinforce composite materials. Almost all organic fibers have low density, flexibility, and elasticity. Inorganic fibers are of high modulus, high thermal stability and possess greater rigidity than organic fibers and notwithstanding the diverse advantages of organic fibers which render the composites in which they are used. Mainly, the following different types of fibers namely, glass fibers, silicon carbide fibers, high silica and quartz fibers, alumina fibers, metal fibers and wires, graphite fibers, boron fibers, aramid fibers and multi phase fibers are used. Among the glass fibers, it is again classified into E-glass, S-glass, A- glass, R-glass etc. There is a greater market and higher degree of commercial movement of organic fibers. The potential of fibers of graphite, silicon carbide and boron are also exercising the scientific mind due to their applications in advanced composites

Fiber Types:

Natural A composite is a material that is formed by combining two or more materials to achieve some superior properties. Almost all the materials which we see around us are composites. Some of them like woods, bones, stones, etc. are natural composites, as they are either grown in nature or developed by natural processes. Natural fibres like straws from grass plants and fibrous leaves were used as roofing materialsSome Examples of Natural fibers.

Eg: Stone axes, daggers, spears with wooden handles, wooden bows, fishing nets woven with vegetable fibers, jewelleries and decorative articles made out of horns, bones, teeth, semiprecious stones, minerals, etc.

Cellulose: Wood is a fibrous material consisting of thread-like hollow elongated organic cellulose that normally constitutes about 60-70% of wood of which approximately 30-40% is crystalline, insoluble in water, and the rest is amorphous and soluble in water.Cellulose fibres are flexible but possess high strengthThe more closely packed cellulose provides higher density and higher strengthThe walls of these hollow elongated cells are the primary load-bearing components of trees and plants. When the trees and plants are live, the load acting on a particular portion (e.g., a branch) directly influences the growth of cellulose in the cell walls located there and thereby reinforces that part of the branch, which experiences more forces.

Bones: Bones contain short and soft collagen fibres i.e., inorganic calcium carbonate fibres dispersed in a mineral matrix called apatite. The fibres usually grow and get oriented in the direction of load. Human and animal skeletons are the basic structural frameworks that support various types of static and dynamic loads.Tooth is a special type of bone consisting of a flexible core and the hard enamel surface. The compressive strength of tooth varies through the thickness. The outer enamel is the strongest with

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ultimate compressive strength as high as 700MPa. Tooth seems to have piezoelectric properties i.e., reinforcing cells are formed with the application of pressure.Man-Made

Types of man made fibers:

1. Glass fibres2. Silica fibres3. Kelvar4. Carbon 5. Metal Fibers6. Ceramic fibers7. Boron Fibers8. Silicon Carbide Fibers

Glass Fibers:

Over 95% of the fibers used in reinforced plastics are glass fibers, as they are inexpensive, easy to manufacture and possess high strength and stiffness with respect to the plastics with which they are reinforced. Fiberglass materials are popular for their attributes of high strength compared to relatively light weight.Their low density, resistance to chemicals, insulation capacity are other bonus characteristics, although the one major disadvantage in glass is that it is prone to break when subjected to high tensile stress for a long time.. However, it remains break-resistant at higher stress-levels in shorter time frames. This property mitigates the effective strength of glass especially when glass is expected to sustain such loads for many months or years continuouslyFiberglass really is made of glass, similar to windows or the drinking glasses. The glass is heated until it is molten, then it is forced through superfine holes, creating glass filaments that are very thin so thin they are better measured in micronsGlass fibers are most commonly used fibers- They come in two forms:

Continuous fibers Discontinuos or “staple” fibers

Principal advantages:

Low cost High strength

Limitations:

Poor abrasion resistance causing reduced usable strength Poor adhesion to specific polymer matrix materials Poor adhiesion in humid environments

Glass fibers are coated with chemicals to enhance their adhesion properties. These chemicals are known as “coupling coupling agents”. Many of coupling agents are silane compounds

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Difference between Glass and Fibre Glass:

Manufacturing processes:

1. Direct Melt Process2. Marble Melt

Direct Melt Process

Step 1: Batching

In the initial stage of glass manufacture, materials must be carefully weighed in exact quantities and thoroughly mixed (batched). More than half the mix is silica sand, the basic building block of any glass.

Step 2: Melting

From the batch house, pneumatic conveyor sends the mixture to a high temperature (≈1400ºC) furnace for melting. The furnace is typically divided into three sections, with channels that aid glass flow. The first section receives the batch, where melting occurs and uniformity is increased, including removal of bubbles. The temperature is so high that the sand and other ingredients dissolve into molten glass. The molten glass then flows into the refiner, where its temperature is reduced to 1370ºC.

Step 3: Fiberization

Glass fiber formation, or fiberization, involves a combination of extrusion and attenuation.In extrusion, the molten glass passes out of the forehearth through a bushing made of an erosion-resistant platinum alloy with very fine orifices, in thousands. Bushing plates are heated electronically, and their temperature is precisely controlled to maintain a constant glass viscosity. Water jets cool the filaments as they exit the bushing at roughly 1204ºC.

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Attenuation is “the process of mechanically drawing the extruded streams of molten glass into fibrous elements” called filaments, with a diameter ranging from 4 μm to 34 μm (one-tenth the diameter of a human hair). A high-speed winder catches the molten streams and, because it revolves at a circumferential speed of ~2 miles/~3 km per minute (much faster than the molten glass exits the bushings), tension is applied, drawing them into thin filaments.

Step 4: Coating

• In the final stage, a chemical coating, or size, is applied. • Size is typically added at 0.5 to 2.0 percent by weight and may include lubricants, binders and/or coupling agents. The lubricants help to protect the filaments from abrading and breaking as they are collected and wound into forming packages and, later, when they are processed by weavers or other converters into fabrics or other reinforcement forms

Step 5: Drying/packaging• Finally, the drawn, sized filaments are collected together into a bundle, forming a glass strand composed of 51 to 1,624 filaments. The strand is wound onto a drum into a forming package that resembles a spool of thread. The forming packages, still wet from water cooling and sizing, are then dried in an oven, and afterward they are ready to be palletized and shipped or further processed into chopped fiber, roving or yarn.

Marble Melt process:

The Marble melt process can be used to form special purpose, for example high strengthfibres. In this process the raw mateirals are melted, and solid glass marbles usually 2 to 3 cm (0.8 to 1.2 in) in dia are formed from the melt. The marbels are remelted (at the same or at a different location) and formed into glass fibers.

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Uses for regular glass fiber Mats and fabrics for thermal insulation, electrical insulation, sound insulation, high-strength

fabrics or heat- and corrosion-resistant fabrics. It is also used to reinforce various materials, such as tent poles, vault poles, arrows, bows

and crossbows, translucent roofing panels, automobile bodies, hockey sticks,surfboards, boat hulls, and paper honeycomb.

It has been used for medical purposes in casts. Glass fiber is extensively used for making FRP tanks and vessels. Open-weave glass fiber grids are used to reinforce asphalt pavement. Non-woven glass fiber/polymer blend mats are used saturated with asphalt emulsion and

overlaid with asphalt, producing a waterproof, crack-resistant membrane. Use of glass-fiber reinforced polymer rebar instead of steel rebar shows promise in areas

where avoidance of steel corrosion is desired

Types of Glass FibreType Description

A Glass

Contains 72% slilica. High Alkali Glass containing (25% Soda and lime). Is transparent, easily formed and most suitable for window glass. Poor resistance to heat (500–600 °C). Used for windows, containers, light bulbs, tableware.

C Glass

Chemical glass—Sodium borosilicate glass (alkali-lime glass) with high boron oxide content, improved durability, making it preferred composition for applications requiring corrosion resistance. Used for glass staple fibers possesses

D glassBorosilicate glasses with improved dielectric strength and low density, developed for improved electrical performance.

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E Glass

An electrically resistant glass fibre.Alumina-calcium-borosilicate glasses. Constitutes the majority of glass fibre production. Used in glass reinforced plastics as general purpose fibres where strength and high electrical resistivity are required

ECR Glass

Calcium aluminosilicate glasses.Modified “E” glass having superior long term resistance to strain crack corrosion in acid conditions.

AR GlassHigh Quality Alkali resistant glasses composed of alkali zirconium silicates used in cement substrates and concrete.

R GlassCalcium aluminosilicate glasses High-strength, high-modulus glass at a lower cost than “S”.

S & S2 GlassMagnesium aluminosilicate glasses (40% higher than E-glass) developed for aerospace applications.

2. Silica fibre:

Silica fiber is a long thin thread made of sodium silicate The thread or strand is actually made of sodium silicate, also known as water glass. In its pure form, it appears as a white powder. It is very stable and through a process of melting and brushing is formed into a series of thin strings or fibers. They can be made such that they are substantially free from non-alkali metal compounds.

These fibers have properties which make them useful in friction-lining materials

Silica fibers made of sodium silicate (water glass) are used in heat protection (including asbestos substitution) and in packings and compensators.

It has many aerospace, electrical and automotive applications due to its high heat resistance

These are also used as optical fibers for long distance telecommunications, sensors and fiber optic medical instruments.Sodium silicate fibers may be used for subsequent production of silica fibers, which is better than producing the latter from a melt containing SiO2 or by acid-leaching of glass fibers.Protection made of this material can withstand temperatures of up to 1832 degrees Fahrenheit (1000 C) for very long periods or even higher temperatures of 3092 F (1700 C) for short periods of time. The silica fibers are useful for producing wet webs, filter linings and reinforcing material.They can also be used to produce silicic acid fibers by a dry spinning method. A silica fiber has an amazingly high mechanical strength against pulling and even bending, provided that the fiber is not too thick and that the surfaces are well prepared. The mechanical strength of a fiber can be further improved with a suitable polymer jacket. Even simple cleaving (breaking) of silica fiber ends can provide nicely flat surfaces with sufficient optical quality.

Kelvar:

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Kevlar is a material formed by combining para-phenylenediamine and terephthaloyl chloride. Aromatic polyamide (aramid) threads are the result. They are further refined, by dissolving the threads and spinning them into regular fibres.

If layers of the woven Kevlar are combined with layers of resin, the resulting ‘rigid’ material is light and has twenty times the strength of steel.

It is also superior to specialist metal alloys. However, Kevlar is expensive due to the demands of the manufacturing process and the

need for specialist equipment. Kevlar’s properties aid in distinguishing it from many other fibers and materials. Kevlar is strong but also very light. The tensile strength of the Kevlar fiber is over eight times stronger than of a steel wire. It

also handles heat very well and can withstand temperatures well above 850ºF. Kevlar will burn but is easily extinguished by removing the heat source. Kevlar is capable of remaining soft and pliable down to -320ºF. and is even slightly stronger

at lower temperatures.

DISADVANTAGES OF KEVLAR:

Kevlar textiles tend to absorb moisture. It must be combined with moisture resistant materials, if there is a need for moisture resistance as a physical property. Consequently, very few general cloths are manufactured with Kevlar.

Kevlar reacts well under a tensile force (stretching force) but badly under a compressive force. It is not used where compression resistance is needed, such as bridge building or the structure of a building.

It is difficult to cut and shape, unless through the use of special tools and equipment. Laminated Kevlar is also difficult to machine and consequently special cutters are required. Special cutting techniques were developed to enable the manufacture of Kevlar parts, for the Euro fighter.

Kevlar reacts badly to UV light (sunlight) unless it is protected / hidden from direct sunlight. Kevlar suffers some corrosion if exposed to chlorine. Long exposure to ultraviolet light will cause discoloration and degradation in its fibers.

Additionally, certain chemicals on the fibers can weaken it

Carbon Fibers:

Carbon fibers are a new breed of high-strength materials. Carbon fiber has been described as a fiber containing at least 90% carbon material consisting of very thin filaments of carbon atoms. When bound together with plastic polymer resin by heat, pressure or in a vacuum acomposite material is formed that is both strong and lightweight.

Carbon fibers have found wide application in commercial and civilian aircraft, recreational, industrial, and transportation markets.

Carbon fibers are used in composites with a lightweight matrix. Carbon fiber composites are ideally suited to applications where strength, stiffness, lower

weight, and outstanding fatigue characteristics are critical requirements. They also can be used in the occasion where high temperature, chemical inertness and high

damping are important.

S.No Characteristic Application

1Physical strength, specific toughness, light weight

Aerospace, road and marine transport, sporting goods

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2

High dimensional stability, low coefficient of thermal expansion, and low abrasion

Missiles, aircraft brakes, aerospace antenna and support structure, large telescopes, optical benches, waveguides for stable high-frequency (GHz) precision measurement frames

3Good vibration damping, strength, and toughness

Audio equipment, loudspeakers for Hi-fi equipment, pickup arms, robot arms

4

Electrical conductivity Automobile hoods, novel tooling, casings and bases for electronic equipments, EMI and RF shielding, brushes

5

Biological inertness and x-ray permeability

Medical applications in prostheses, surgery and x-ray equipment, implants, tendon/ligament repair

6Fatigue resistance, self-lubrication, high damping

Textile machinery, genera engineering

7Chemical inertness, high corrosion resistance

Chemical industry; nuclear field; valves, seals, and pump components in process plants

8Electromagnetic properties Large generator retaining rings, radiological

equipment

Boron Fibers

They are basically composites, in which boron is coated on a substance which forms the substrate, usually made of tungsten.

Boron-tungsten fibers are obtained by allowing hot tungsten filament through a mixture of gases. Boron is deposited on tungsten and the process is continued until the desired thickness is achieved. The tungsten however remains constant in its thickness.

Properties of boron fibers generally change with the diameter, because of the changing ratio of boron to tungsten and the surface defects that change according to size. However, they are known for their remarkable stiffness and strength. Their strengths often compare with those of glass fibers, but their tensile modulus is high, almost four to five times that of glass. Boron coated carbons are much cheaper to make than boron tungsten fiber.

Applications: Boron Fiber composites are in use in a number of U.S. military aircraft, notably the F-14 and F-15, and in the U.S. Space Shuttle.

Increasingly, boron fibers are being used for stiffening golf shafts, tennis rackets, and bicycle frames. One big obstacle to the widespread use of boron Fiber is its high cost compared to that of other fibers. A major portion of this high price is the cost of the tungsten substrate.

Silicon Carbide Fibers:

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Silicon Carbide is the only chemical compound of carbon and silicon. It was originally produced by a high temperature electro-chemical reaction of sand and carbon.

Silicon carbide is an excellent abrasive and has been produced and made into grinding wheels and other abrasive products for over one hundred years. Today the material has been developed into a high quality technical grade ceramic with very good mechanical properties. It is used in abrasives, refractories, ceramics, and numerous high-performance applications. The material can also be made an electrical conductor and has applications in resistance heating, flame igniters and electronic components. Structural and wear applications are constantly developing

The advantages of silicon carbide-tungsten are several and they are more desirable than uncoated boron tungsten fibers. Elevated temperature performance and the fact that they reported only a 35% loss of strength at 1350°C are their best qualities.

Silicon carbide (SiC) can be used in harsh environments due to its thermal, mechanical and chemical

Stability.

Silicon Carbide Properties

Low density High strength Low thermal expansion High thermal conductivity High hardness High elastic modulus Excellent thermal shock resistance Superior chemical inertness

Silicon Carbide Typical Uses

Fixed and moving turbine components Suction box covers Seals, bearings Ball valve parts Hot gas flow liners Heat exchangers Semiconductor process equipment

Boron Carbide (B4C)

Boron Carbide is one of the hardest materials known, ranking third behind diamond and cubic boron nitride. It is the hardest material produced in tonnage quantities.

Boron carbide powder is mainly produced by reacting carbon with B2O3 in an electric arc furnace, through carbothermal reduction or by gas phase reactions. For commercial use B4C powders usually need to be milled and purified to remove metallic impurities.

In common with other non-oxide materials boron carbide is difficult to sinter to full density, with hot pressing or sinter HIP being required to achieve greater than 95% of theoretical density. Even using these techniques, in order to achieve sintering at realistic temperatures (e.g. 1900 - 2200°C), small quantities of dopants such as fine carbon, or silicon carbide are usually required.

As an alternative, B4C can be formed as a coating on a suitable substrate by vapour phase reaction techniques e.g. using boron halides or di-borane with methane or another chemical carbon source.

Key Properties:

Boron carbide is characterised by its:

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Extreme hardness High Melting point Good chemical resistance High Young’s Modulus (it’s a very stiff material) Relatively low thermal expansion and conductivity Good nuclear properties Low density

Applications

Abrasives

Due to its high hardness, boron carbide powder is used as an abrasive in polishing and lapping applications, and also as a loose abrasive in cutting applications such as water jet cutting. It can also be used for dressing diamond tools.

Nozzles

The extreme hardness of boron carbide gives it excellent wear and abrasion resistance and as a consequence it finds application as nozzles for slurry pumping, grit blasting and in water jet cutters

Nuclear applications

Its ability to absorb neutrons without forming long lived radio-nuclides make the material attractive as an absorbent for neutron radiation arising in nuclear power plants. Nuclear applications of boron carbide include shielding, and control rod and shut down pellets.

Ballistic Armour

Boron carbide, in conjunction with other materials also finds use as ballistic armour (including body or personal armour) where the combination of high hardness, high elastic modulus, and low density give the material an exceptionally high specific stopping power to defeat high velocity projectiles.

Other Applications

Other applications include ceramic tooling dies, precision toll parts, evaporating boats for materials testing and mortars and pestles.

Particulate Composites:A composite that consists of tiny particles of one material embedded in another material.

The particulates can be very small particles (< 0.25 microns).

Chopped fibers (such as glass), platelets, hollow spheres, or new materials such as bucky balls or carbon nano-tubes. In each case, the particulates provide desirable material properties and the matrix acts as binding medium necessary for structural applications.

 One form of composites is particulate reinforced composites with concrete being a good example. The aggregate of coarse rock or gravel is embedded in a matrix of cement. The aggregate provides stiffness and strength while the cement acts as the binder to hold the structure together.

Particulate composites offer several advantages. They provide reinforcement to the matrix material thereby strengthening the material.

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The combination of reinforcement and matrix can provide for very specific material properties. For example, the inclusion of conductive reinforcements in a plastic can produce plastics that are somewhat conductive.

Particulate composites can often use more traditional manufacturing methods such as injection molding which reduces cost.

Benefits:

Improved material properties Tailored material properties Manufacturing flexibility

Applications: The most common particulate composite materials are reinforced plastics which are used in a variety of industries.

Consumer Products Many of the plastic components we use in daily life are reinforced in some way. Appliances, toys, electrical products, computer housings, cell phone casings, office furniture, helmets, etc. are made from particulate reinforced plastics.

Automotive Glass reinforced plastics are used in many automotive applications including body panels, bumpers, dashboards, and intake manifolds. Brakes are made of particulate composite composed of carbon or ceramics particulates.

Polymer composites

Polymer composites are any of the combinations or compositions that comprise two or more materials as separate phases, at least one of which is a polymer. By combining a polymer with another material, such as glass, carbon, or another polymer, it is often possible to obtain unique combinations or levels of properties. Typical examples of synthetic polymeric composites include glass-, carbon-, or polymer-fiber-reinforced, thermoplastic or thermosetting resins, carbon-reinforced rubber, polymer blends, silica- or mica-reinforced resins, and polymer-bonded or -impregnated concrete or wood. It is also often useful to consider as composites such materials as coatings (pigment-binder combinations) and crystalline polymers (crystallites in a polymer matrix).

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Chapter 3:

Composite Manufacturing process:Composites can be manufactured in the following methods.

1. Autoclave 2. Tape production3. Moulding

A manufacturer using composite materials has to work directly from the ingredients of fiber and matrix to make the finished product itself.

Relatively thin flat plate or shallow shell with free edges. Normally aerospace components have these types of shapes. These are usually made using the hand-lay-up method. The autoclave is the common tool used for making aerospace composite components having these shapes.

Components of revolution, such as cylindrical or spherical pressure vessels and pipes. These structures usually have no free edges (except for the end openings). These are usually made using the filament winding method.

Components having constant cross section such as tubes, rods, or even components with complex but constant cross section along the length such as door frames. These are usually made using the pultrusion method.

Components having complex 3-D configurations. These can be thick or thin. These are usually made using the liquid composite molding (LCM) method.

Large structures such as boat hulls, wind turbine blades etc.These are made using a modified form of LCM such as vacuum-assisted LCM.

A special process called SCRIMP (seaman composite resin infusion molding process) is usuallyused to make boat hulls.

Small and large components, either without free edges or with free edges. These can be made by the tape or fiber placement method.

The different stages of existence of composite constituents up to the final product:

Stage A: At this stage, the materials appear in raw basic form. For fibers, these consist of fiber either in the form of filaments or fiber bundles. Fibers may also be woven into fabrics or braided into braided perform. For matrix, the material usually appears in liquid form for thermoset resin or in granular form in the case of thermoplastics.

Stage B: At this stage, the fibers and matrix may be combined into a single layer. For the case of thermoset matrix composite, the matrix may appear in a semi-liquid, semi-solid form so that the sheet can hold its shape. For the case of thermoplastic composite, the matrix is solidified. This form for thermoset matrix composites is called prepreg. For thermoplastic composites, it is called towpreg.

Stage C: At this stage, the layers in stage b are stacked on top of each other to make flat plate laminates. This intermediate step is important for the analysis where material properties are tested or calculated. However this step is usually bypassed in the manufacturing process of practical composite parts.

Stage d: This is the final stage where the final product configuration is formed.

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The involvement of these stages in the different manufacturing processes is as follows:

Hand-lay-up (with or without autoclave): Stages a, b and d are involved. Stage c is bypassed. Filament winding: Stages a and d are involved. Stages b and c are bypassed. Pultrusion: Stages a and d are involved. Stages b and c are bypassed. Liquid composite molding: Stages a and d are involved. Stages b and c are bypassed. Thermoplastic composites: Stages a and d are involved.Sometimes stage b and even stage c may be

involved

Autoclave method:

Autoclave processing is commonly used for manufacturing composite components for the aerospace industry. The process produces composite components of high quality, but it requires a considerable amount of time.

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The main steps of the autoclave processing of composites are:

• Prepregs• Tool preparation ( Fig a)• Laying up prepregs on the tool to make the part (Fig b)• Curing of the part (Fig c)• Removal of the part from the tool• Inspection• Finishing steps (Fig d)

Prepregging involves the incorporation of the partially cured resin with the fibers. In the prepregging process,dry fibers are fed from creels through stations of combs where the fibers are spread out. The fibers then enter into a bath of wet resin where they are wetted. Subsequently the fiber/resin combination is heated to change the liquid resin into a partially cured state. The partially cured resin is viscous enough to help keep the fibers in the configuration of flat sheets. This fiber/viscous resin combination is called prepreg. Normally sheets of backing paper are placed on both sides of the prepreg for handing purposes. Then the prepregs are rolled up for storing and shipping. The partially cured resin has about 30% of the cross links already formed. With the incorporation of fibers (such as carbon, glass or Kevlar at about 60% by volume) , prepregs are flexible sheets of fibers about 150 mm thick. This is similar to a sheet of wallpaper except that it is sticky on both sides.

Tool preparationManufacturing using autoclave is a molding process. As such, molds (also called tools) are required. The mold provides the shape and surface finish for the part. As such the size of the mold depends on the size of the part. Large parts require large molds and these can be very expensive. Advanced composites must be cured at about 180°C and at pressures of about 600 kPa; molds would be required to sustain these conditions for periods of several hours. In addition, there are many other considerations when designing and building tools. These include tool cost, life, accuracy, weight, machinability, strength, thermal expansion, dimensional stability, surface finish, and thermal mass and thermal conductivity.Over a wide range of material systems and processing scenarios used for composites, there are many materials suitable for tooling. In general, the choices fall into three categories:1. Reinforced polymers, for low to intermediate temperature ranges2. Metals, for low to high temperatures3. Ceramics and bulk graphite, for very high temperaturesFor production tooling for advanced composites, the choice is usually made between metals, including aluminum, steel, nickel alloys (Invar), electroformed nickel, and graphite/epoxy tooling. Elastomeric tooling is often used as a pressure intensifier and to distribute the applied pressure over a part. For high temperature applications, such as thermoplastic composites, much consideration has been given to bulk graphite and various ceramic systems, including a new material called geopolymere.

Laying up prepregs on the tool to make the part:The laying up of the prepregs on the surface of the mold consists of not only the laying of the fiber prepregs on the mold, but also the placement of ancillary materials for the following purposes:To facilitate the removal of the part after cure (without the problem of the part sticking to the mold)

To allow the compaction of the stack of prepregs using vacuum To prevent excess resin from running within the plane of the stackof fibers, which can distort the

orientation of the fibers

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To provide an escape path for volatiles such as water vapor or gases that are generated during the curing process

To provide materials that can absorb excess resins that ooze out of the laminate during the curing and molding process

To obtain good surface finish on the part

Curing of the partThe resin in the stack of layers of composite in the bag is a viscous liquid, needs to be transformed into a solid to make a useful composite, which requires heat to activate the chemical reaction between the molecules During this transformation of the resin, it is important to assure that the fibers maintain their orientation and that no resin rich area or other defects will exist. Sufficient amounts of pressure need to be applied for this purpose. Also, during the transformation of the resin from the liquid to solid state, volatiles such aswater vapor or other gases may be generated. These need to be removed from the material in order to avoid the occurrence of voids after the resin has become solid.

After curing the part is separated from the tool and subjected for inspection.

Tape/ Fiber Placement method:Fiber placement is a process in which the fibers are placed onto the surface of the mandrel one strip at a time.

The fibers are pushed toward the surface of the mandrel. As such, flexible tows cannot be used for fiber placement. Instead, tapes with a certain degree of rigidity are used there is a pressure applicator that Presses the fibers as they are being placed on the surface of the mandrel. This pressure applicator consolidates the fibers as they are being wrapped around the mandrel. With the pressure applicator, surfaces other than convex can be used.

In the fiber placement process, usually heat is applied at the nip point (point where the fiber bands meet the surface of the mandrel). The application of heat allows the liquefaction of the resin. Combination of heat and pressure provides the drive of flow and consolidation. Due to the presence of heat and pressure the resin systems used for the fiber placement process can be different from those used for filament winding.

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The fiber placement process can be applied to both thermoset and thermoplastic composites without significant change in the machine setup (except for the laying head).

Moulding techniques:

There are two general divisions of composites manufacturing processes: open molding and closed molding. With open molding, the gel coat and laminate are exposed to the atmosphere during the fabrication process. In closed molding, the composite is processed in a two-sided mold set, or within a vacuum bag. There are a variety of processing methods within the open and closed molding categories:

OPEN MOLDING Hand Lay-Up Spray-up Filament Winding

CLOSED MOLDING Compression molding Pultrusion Reinforced Reaction Injection Molding (RRIM) Resin Transfer Molding (RTM) Vacuum Bag Molding Vacuum Infusion Processing Centrifugal Casting Continuous Lamination

Hand lay-up technique Hand lay-up technique is the simplest method of composite processing. The infrastructural

requirement for this method is also minimal. The processing steps are quite simple. First of all, a release gel is sprayed on the mold surface to avoid the sticking of polymer to the

surface. Thin plastic sheets are used at the top and bottom of the mold plate to get good surface finish of the

product. Reinforcement in the form of woven mats or chopped strand mats are cut as per the mold size and

placed at the surface of mold after perspex sheet Then thermosetting polymer in liquid form is mixed thoroughly in suitable roportion with a

prescribed hardner (curing agent) and poured onto the surface of mat already placed in the mold. The polymer is uniformly spread with the help of brush. Second layer of mat is then placed on the polymer surface and a roller is moved with a mild pressure

on the mat-polymer layer to remove any air trapped as well as the excess polymer present. The process is repeated for each layer of polymer and mat, till the required layers are stacked. After placing the plastic sheet, release gel is sprayed on the inner surface of the top mold plate

which is then kept on the stacked layers and the pressure is applied. After curing either at room temperature or at some specific temperature, mold is opened and the

developed composite part is taken out and further processed. The time of curing depends on type of polymer used for composite processing.

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o For example, for epoxy based system, normal curing time at room temperature is 24-48 hours.

This method is mainly suitable for thermosetting polymer based composites. Capital and infrastructural requirement is less as compared to other methods. Production rate is less

and high volume fraction of reinforcement is difficult to achieve in the processed composites. Hand lay-up method finds application in many areas like aircraft components, automotive parts,

boat hulls, dais board, deck etc.

Spray lay-up:

The spray lay-up technique can be said to be an extension of the hand lay-up method. In this technique, a spray gun is used to spray pressurized resin and reinforcement which is in the form of chopped fibers.

Generally, glass roving is used as a reinforcement which passes through spray gun where it is chopped with a chopper gun.

Matrix material and reinforcement may be sprayed simultaneously or separately one after one. Spray release gel is applied on to the mold surface to facilitate the easy removal of component from the mold.

A roller is rolled over the sprayed material to remove air trapped into the lay-ups. After spraying fiber and resin to required thickness, curing of the product is done either at room

temperature or at elevated temperature. After curing, mold is opened and the developed composite part is taken out and further processed

further. The time of curing depends on type of polymer used for composite processing. Spray lay-up method is used for lower load carrying parts like small boats, bath tubs, fairing of trucks

etc. This method provides high volume fraction of reinforcement in composites and virtually, there is no

part size limitation in this technique.

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Filament Winding:

The main components of filament winding are as follows:

1. Fiber creel 2. Resin impregnation system 3. Carriage 4. Rotating mandrel

This process is primarily used for hollow, generally circular or oval sectioned components, such as pipes and tanks.

Fibre tows are passed through a resin bath before being wound onto a mandrel in a variety of orientations, controlled by the fibre feeding mechanism, and rate of rotation of the mandrel.

Fiber tension is critical in filament winding because compaction is achieved through the fiber tension.

The fiber tension affects the percentage of fiber reinforcement and porosity content in the composite which in turn affects the properties of the processed composite product

The fiber tension depends upon the type of fiber, its geometry and the winding pattern required on the rotating mandrel.

The fiber tension should be at optimal level because too high fiber tension may break the fiber completely or initiate fiber fracture at the surface.

Curing of the composite is done with heat, generally in an oven and final composite product is taken out of the mandrel.

To remove the metallic mandrel from the composite part, hydraulic rams may be used. For complex geometry of composite part, the mandrel used may be of soluble plaster which can be washed out after processing or it may be a collapsible rubber and materials having low melting point.

The profile of the mandrel is exactly the same as that of the final product is required. In some cases, mandrel becomes the integral part of the assembly.

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A carriage is used to keep the roving in place and to direct them to the mandrel. A high fiber volume fraction can be achieved in the composite with this processing technique.

Cores may be used in this method but normally, product is in single skin. Now a days, computer controlled machines are used which independently monitor every movement

of the whole process.

Application: 1. Composite products like storage tanks, pipelines, vessels, gas cylinders, fishing rods, missile cases, rocket motor cases, ducting, cement mixture, sail boat mast, aircraft fuselages and golf club shafts are very common to be developed with this method. 2. Now, the application spectrum of filament winding has expanded to complex engineered non-spherical and non-cylindrical composite products with the use of sophisticated machinery and software.

Advantage: 1. High strength to weight ratio is possible to achieve with this process. 2. High degree of uniformity in fiber distribution, orientation and placement. 3. Labor involvement is minimal as it is an automated process. 4. Filament winding method is suitable to process composite parts requiring precise tolerances. 5. Fiber orientation in a specific direction is possible in this process. 6. Cost of the composite part processed through filament winding method is substantially low as compared to other manufacturing methods as this process involves less and low cost material to produce high strength component. 7. Design flexibility in composite part is possible with the change in winding patterns, material and curing option. 8. The size of the component is not restricted. 9. For high production volume, process automation results in cost saving. Disadvantage: 1. Capital investment is relatively high.

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2. Very precise control over the mechanism is required for uniform distribution and orientation of fiber. 3. Composite product configuration be such that it should facilitate in mandrel extraction. 4. It is not possible to produce the reverse curvature (female feature). 5. For some applications, mandrel may be expensive and surface of the composite part may not be satisfactory. 6. Fiber direction cannot be changed within one layer of winding.

CLOSED MOLDING

PultrusionPultrusion is a manufacturing process for producing continuous lengths of reinforced polymer structural shapes with constant cross-sections.Beginning from the left-hand side, the fiber tows drawn from fiber racks are routed through a series of guides. The fibers then traverse through a bath of low viscosity resin for impregnation.

It is a continuous process in which composites in the form of fibers and fabrics are pulled through a bath of liquid resin.

Then the fibres wetted with resin are pulled through a heated die. The die plays important roles like completing the impregnation and controlling the resin. Further,

the material is cured to its final shape. The die shape used in this process is nothing the replica of the final product. Finally, the finished product is cut to length.

In this process, the fabrics may also be introduced into the die. The fabrics provide a fibre direction other 0°. Further, a variant of this method to produce a profile with some variation in the cross-section is available. This is known as pulforming.

The resins like epoxy, polyester, vinylester and phenolic can be used with any fibre.

Applications:Beams and girders used in roof structures, bridges, ladders, frameworks

Advantages: The process is suitable for mass production.

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The process is fast and economic. Resin content can be accurately controlled. Fibre cost is minimized as it can be taken directly from a creel. The surface finish of the product is good. Structural properties of product can be very good as the profiles have very straight fibres.

Disadvantages: Limited to constant or near constant cross-section components. Heated die costs can be high. Products with small cross-sections alone can be fabricated

Resin Transfer Molding (RTM) 

Resin Transfer Molding (RTM) is a low pressure, closed molding process which offers a dimensionally accurate and high quality surface finish composite molding, using liquid thermoset polymers reinforced with various forms of fiber reinforcements. Typically polymers of Epoxy, Vinyl Ester, Methyl Methacrylate, Polyester or Phenolic are used with fiberglass reinforcement. Other reinforcements are offered for more demanding applications such as Arimid, Carbon and Synthetic fibers either individually or in combination with each other.The process consists of arranging the fibres or cloth fabrics in the desired configuration in a preform. These fabrics are sometimes pre-pressed to the mould shape, and held together by a binder. A second matching mould tool is then clamped over the first. Then pressurized resin is injected into the cavity. Vacuum can also be applied to the mould cavity to assist resin in being drawn into the fabrics. This is known as Vacuum Assisted Resin Transfer Moulding (VARTM) or Vacuum Assisted Resin Injection (VARI). The laminate is then cured. Both injection and cure can take place at either ambient or elevated temperature. In this process, the resins like epoxy, polyester, vinylester and phenolic can be used. Further, one use the high temperature resins such as bismaleimides can be used at elevated process temperatures.

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The fibres of any type can be used. The stitched materials work well in this process since the gaps allow rapid resin transport. Some specially developed fabrics can assist with resin flow

Advantages: The process is very efficient. Suitable for complex shapes. High fibre volume laminates can be obtained with very low void contents. Good health and safety, and environmental control due to enclosure of resin. Possible labour reductions. Both sides of the component have a moulded surface. Hence, the final product gets a superior surface finish Better reproducibility. Relatively low clamping pressure and ability to induce inserts.

Disadvantages:

Matched tooling is expensive and heavy in order to withstand pressures. Generally limited to smaller components. Unimpregnated areas can occur resulting in very expensive scrap parts

Applications: The applications include the hollow cylindrical parts like motor casing, engine covers, etc.

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