Pks Composite Part1

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COMPOSITE MATERIALS AND

STRUCTURES

P. K. Sinha


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COMPOSITE MATERIALS AND STRUCTURES

P. K. Sinha

Published by Composite Centre of Excellence, AR & DBDepartment of Aerospace Engineering

I.I.T. Kharagpur


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DedicatedToMy Family MembersIncluding Students


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CHAPTER-1

INTRODUCTION

1.1 NATURAL AND MAN-MADE COMPOSITES

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. 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 strength. The more closely packed cellulose provides higher density and higher

strength. The 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. This self-strengthening mechanism is something unique that can also be observed

in the case of live 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 ultimate compressive strength as high as 700MPa. Tooth seems to have

piezoelectric properties i.e., reinforcing cells are formed with the application of pressure.

The most remarkable features of woods and bones are that the low density, strong and

stiff fibres are embedded in a low density matrix resulting in a strong, stiff and

lightweight composite (Table 1.1). It is therefore no wonder that early development of

aero-planes should make use of woods as one of the primary structural materials, and

about two hundred million years ago, huge flying amphibians, pterendons and pterosaurs,

with wing spans of 8-15 m , could soar from the mountains like the present–day hang-

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gliders. Woods and bones in many respect, may be considered to be predecessors to

modern man-made composites.

Early men used rocks, woods and bones effectively in their struggle for existence

against natural and various kinds of other forces. The primitive people utilized these

materials to make weapons, tools and many utility-articles and also to build shelters. In

the early stages they mainly utilized these materials in their original form. They gradually

learnt to use them in a more efficient way by cutting and shaping them to more useful

forms. Later on they utilized several other materials such as vegetable fibres, shells, clays

as well as horns, teeth, skins and sinews of animals.

Table 1.1 Typical mechanical properties of natural fibres and natural composites

Materials Density Tensile modulus Tensile strength

Kg/m3

GPa MPa

Fibres

Cotton 1540 1.1 400

Flax 1550 1 780

Jute 850 35 600

Coir 1150 4 200

Pineapple leaf 1440 65 1200

Sisal 810 46 700

Banana 1350 15 650

Asbestos 3200 186 5860

Composites

Bone 1870 28 140

Ivory 1850 17.5 220

Balsa 130 3.5 24

Spruce 470 11 90

Birch 650 16.5 137

Oak 690 13 90

Bamboo 900 20.6 193

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Woods, stones and clays formed the primary structural materials for building

shelters. Natural fibres like straws from grass plants and fibrous leaves were used as

roofing materials. 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. were but a few examples that

illustrate how mankind, in early days, made use of those materials. The limitations

experienced in using these materials led to search for better materials to obtain a more

efficient material with better properties. This, in turn, laid the foundation for development

of man-made composite materials.

The most striking example of an early man-made composite is the straw-

reinforced clay which molded the civilization since prehistoric times. Egyptians, several

hundred years B.C., were known to reinforce the clay like deposits of the Nile Valley

with grass plant fibres to make sun baked mud bricks that were used in making temple

walls, tombs and houses. The watchtowers of the far western Great Wall of China were

supposed to have been built with straw-reinforced bricks during the Han Dynasty (about

200 years B.C.). The natural fibre reinforced clay, even to-day continues to be one of the

primary housing materials in the rural sectors of many third world countries.

The other classic examples are the laminated wood furniture used by early

Egyptians (1500 B.C.), in which high quality wood veneers are bonded to the surfaces of

cheaper woods. The origin of paper which made use of plant fibres can be traced back to

China (108 A.D.). The bows used by the warriors under the Mongolian Chief Djingiz

Chan (1200 A.D.) were believed to be made with the adhesive bonded laminated

composite consisting of buffalo or anti-lope horns, wood, silk and ox-neck tendons.

These laminated composite bows could deliver arrows with an effective shoot in range of

about 740 m.

Potteries and hydraulic cement mortars are some of the earliest examples of

ceramic composites. The cloissone ware of ancient China is also a striking example of

wire reinforced ceramics. Fine metallic wires were first shaped into attractive designs

which were then covered with colored clays and baked. In subsequent years, fine metallic

wires of various types were cast with different metal and ceramic matrices and were

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utilized in diverse applications. Several other matrix materials such as natural gums and

resins, rubbers, bitumen, shellac, etc. were also popular. Naturally occurring fibres such

as those from plants (cotton, flux, hemp, etc.), animals (wool, fur and silk) and minerals

(asbestos) were in much demand. The high value textiles woven with fine gold and silver

threads received the patronage from the royalty and the rich all over the world. The

intricate, artful gold thread embroidery reached its zenith during the Mughal period in the

Indian subcontinent. The glass fibres were manufactured more than 2000 years ago in

Rome and Mesopotamia and were abundantly used in decoration of flower vases and

glass wares in those days.

The twentieth century has noticed the birth and proliferation of a whole gamut of

new materials that have further consolidated the foundation of modern composites.

Numerous synthetic resins, metallic alloys and ceramic matrices with superior physical,

thermal and mechanical properties have been developed. Fibres of very small diameter

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adaptability, cost effectiveness, etc. have attracted the attention of many users in several

engineering and other disciplines. Every industry is now vying with each other to make

the best use of composites. One can now notice the application of composites in many

disciplines starting from sports goods to space vehicles. This worldwide interest during

the last four decades has led to the prolific advancement in the field of composite

materials and structures. Several high performance polymers have now been developed.

Substantial progress has been made in the development of stronger and stiffer fibres,

metal and ceramic matrix composites, manufacturing and machining processes, quality

control and nondestructive evaluation techniques, test methods as well as design and

analysis methodology. The modern man-made composites have now firmly established as

the future material and are destined to dominate the material scenario right through the

twenty-first century.

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Table 1.2 Comparative mechanical properties of some man-made structural

composites and metallic alloys

Materials Specific Tensile Tensile Compressive Specific Specific Specific

gravity modulus strength strength tensile tensile compressive

modulus strength strength

S E Xt Xc E/S Xt/S Xc/S

G Pa M Pa M Pa G Pa M Pa M Pa

Unidirectional Fibre Reinforced Plastics

GFRP 2.0 40 1650 1400 20.00 825.0 700.0

CFRP 1.6 140 1450 1050 87.50 906.3 656.3

KFRP 1.5 90 1650 300 60.00 1100.0 200.0

Metals

Steel 7.8 206 400-2500 400-2500 26.40 50-320 50-320

Ti alloy 4.5 103 360-1400 360-1400 22.90 80-310 80-310

Al alloy 2.8 69 55-700 55-700 24.60 20-250 20-250

Mg alloy 1.8 47 150-300 150-300 25.00 83-166 83-166

Beryllium 1.8 303 400 400 168-33 222 222

1.2 AEROSPACE APPLICATIONS

One of the primary requirements of aerospace structural materials is that they

should have low density and, at the same time, should be very stiff and strong. Early

biplanes used wood for structural frameworks and fabrics for wing surfaces. The fuselage

of World War I biplane fighter named Vieux Charles was built with wire braced wood

framework. The monoplane, Le Monocoque, had an unusually smooth aerodynamic

design. Its fuselage was made with laminated tulip wood, where one layer was placed

along the length of the fuselage, the second in a right-hand spiral and the third in a left

hand spiral around the fuselage. This laminated single shell wood construction provided

highly polished, smooth surfaces. There was a significant reduction in the drag, and the

plane could achieve a high speed of 108 mph. It won the Gordon Bennett speed race in

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Chicago in 1912. Almost all biplanes and monoplanes, with very few exceptions, were

built of wood during the first quarter of the twentieth century. Lighter woods like balsa,

poplar, spruce, tulip, etc. were more popular. The five-seater Lockheed Vega (first flight

in 1927) also had highly polished, smooth, streamlined fuselage made of strips of spruce

wood bonded together with resin. The Vegas were considered to be the precursor of the

modern transport airplane and had the distinction of successfully completing many major

flights such as crossing the American continent non-stop from Los Angels to New York,

over-flying the Atlantic, encircling the globe and succeeding in several other long distant

flights and races. Soaring planes, in those days, also had highly polished thin plywood

fuselages.

The thirties and forties noticed a gradual shift from wood to aluminium alloy

construction. With the increase in the size and speed of airplanes, the strength and

stiffness requirements for a given weight could not be met from wooden construction.

Several new structural features, e.g., skin-stringer construction, shear webs, etc. were

introduced. The aerospace grade aluminium alloys were made available. Two important

airplanes Northrop Alpha and Boeing Monomail, which were forerunners in the

development of several other aircraft, had aluminium alloy monocoque fuselage and a

wood wing. These aircraft were introduced in 1930, although they were not the first to

use metals. The switch over to light aluminium alloys in aircraft construction was no

doubt, a major step in search for a lightweight design. The trend continued till fifties, by

which almost all types of airplanes were of all-metal design.

However, the limitations of aluminium alloys could be assessed as early as fifties

with the speed of the aircraft increasing sharply (significantly more than the speed of

sound), the demand for a more weight optimized performance, the fuel-efficient design

and so on. The aluminium was stretched to its maximum limit. The search for newer and

better materials was the only alternative. Continuous glass fibres, which were

commercially available since thirties, are found to be very strong, durable, creaseless,

non-flamable and insensitive to weathering. The glass fibres coated with resin can be

easily moulded to any complex curved shape, especially that of a wing root and fuselage

intersection and can be laid layer wise with fibres aligned in a desired direction as in the

case of the three-layered wood fuselage of Le Monocoque. Fibreglass fabrics were

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successfully used in a series of Todai LBS gliders in Japan during the mid-fifties. Todai

LBS-1 had spoilers made from fiberglass fabrics. Todai LBS-2 had a wood wing and a

sandwich monocoque fuselage whose wall consisted of a balsa wood core sandwiched

between two glass fibre reinforced composite face skins. The wing skin of another

important glider, the Phoenix (first flight in 1957), developed in Germany was a

sandwich with fiberglass-polyester faces and balsa wood core. The other successful glider

SB-6, first flown in 1961, had a glass fibre-epoxy shell and a glass fibre composite-balsa

sandwich box spar. The remarkable feature of all these gliders is that they exhibited

superior flight performance and thus became the trend-setters in the use of glass fibre

reinforced plastics.

Glass fibres are strong, but not stiff enough to use them in high sped aircraft. The

search for stiffer fibres to make fibre reinforced composite started in the fifties in several

countries. The laboratory scale production of high-strength carbon fibres by Royal

Aircraft Establishment, Farnborough, U.K. was reported in 1952. In USA, Union Carbide

developed high-modulus continuous carbon fibres in 1958. High-strength graphite fibres

were developed at the Government Industrial Research Institute of Osaka, Japan in 1959.

Before the end of sixties the commercial production of carbon fibres (PAN based) started

in full scale. Very high modulus boron fibres were also introduced during this time. High

strength, low density organic fibres, Kevlar 49, were also marketed by Dupont, USA

during early seventies. A host of synthetic resins, especially structural grade epoxy resins,

were also commercially available. All these advanced materials provided the much-

needed alternatives to less efficient aluminium alloy and fiberglass composites. The

switch-over from the aluminium and GFRP to advanced composites in airframe

construction was, however, very slow at the initial stage (Fig. 1.2). It started with the F-

14 fighter and the F-111 fighter bomber around 1972, but in the period of about two and a

half decades, there were quite a few airplanes, in which almost all structures are made of

composites (Table 1.3). Similar trend in material uses can be observed in the

development of helicopters as well. As early as 1959-60, the Vetrol Company, now

Boeing Helicopters, developed helicopter rotor blades with glass-epoxy faces and

aluminium honeycomb core. In course of time several structural parts such as horizontal

stabilizer, vertical pylon, tail cone, canopies, fuselage, floor board, rotor hub and landing

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gears were developed with various composites, which later culminated in the

development of the all composite helicopter, Boeing Model 360 which was flight tested

in 1987.

The manufacturers of passenger aircraft soon realized the significance of using

composites in the airframe structure. CFRP, KFRP and hybrid composites were

extensively used throughout the wing, fuselage and tailplane sections of Boeing 767.

Although the application of composites in civilian aircraft is relatively less, this trend is

likely to change by the turn of the twenty-first century with the introduction of supersonic

civil transports. Some of the future transport planes may fly at a very high speed (Mach

2-5). Significant advancement has been made in several high technology areas such as

supersonic V/STOL flights, lightweight air superiority fighters with thrust vectoring,

supersonic interceptors and bombers with high Mach number, advanced lightweight

helicopters with tilt rotors, aerospace planes and hypersonic vehicles with multi-mode

trans atmospheric cruise capability such as take-off and landing with a turbine engine,

accelerating to Mach number 10-12 with a Scramjet and achieving an orbital velocity

with a rocket engine. The composite materials will provide an increased number of

choices to meet the tight weight budget and the critical performance level for all these

advanced flight vehicles.

Table 1.3 Advanced composites in selected aerospace applications

Vehicles Components Composites

Sailplanes

SB-10 Middle portion of the wing CFRP

SB-11, SB-12

Ventus, Nimbus,

AS-W22 All composite CFRP

Aeroplanes

F-14 Stabilator BFRP

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F-15, F-16 Horizontal and Vertical tail skins BFRP

Speed brakes CFRP

A-4 Flap, Stabilator CFRP

F-5 Leading edge CFRP

Vulcan Airbrakes CFRP

Mirage 2000 Rudder Boron/carbon/epoxy

AV-8B Wing skin, Control surfaces,

Front fuselage CFRP

Rafale Wing structure CFRP

Boeing 757 Control surface, Cowlings, Under

&767 carriage doors, Fairings CFRP

A 310-300 Fin box CFRP

Lear Fan 2100 All composite CFRP

Voyager All composite CFRP

Starship All composite CFRP

Airbus, Concorde,

Delta 2000, Brake discs Carbon/carbon

Falcon 900

DC-10 Aft pylon Boron/aluminium

C-5A Wing box SiC/aluminium

F-111 Fuselge segment Boron/aluminium

Rockets and

Space Vehicles

Tactical Nose cone, Inlet Quartz/polyimide

Missiles fairing, Fins Carbon/polyimide

Polaris,

Minuteman, Rocket cases KFRP, GFRP

Poseidon,

Trident

Tomahawk Shaft for turbofan Borosic/titanium

PSLV Upper stage solid motor case KFRP

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ARIANE Dual-launch structures, Fairings CFRP

Hubble Space High-gain antenna boom Graphite/6061 Al

Telescope

INTELSAT Antennas, CFRP & KFRP

Antenna support structure,

Multiplexers, Solar array wings CFRP

Viking Parabolic antenna reflector CFRP sandwich

Boom CFRP

Voyager Parabolic reflector, Subreflector CFRP

support structure

Dichroic subreflector KFRP

INSAT,

ARABSAT, ITALSAT Antenne reflectors CFRP

OLYMPUS

EUTELSAT Dual-grided reflectors CFRP & KFRP

TDF-1 Solar array wing CFRP

EURECA Micro-gravity spacecraft

platform truss structure CFRP

Space shuttle Main frame and rib-truss struts,

Frame stabilizing braces, Nose landing Boron/aluminium

gear and drag-brace struts

The materials for the next-generation aeroengines will go a sea-through change in

view of much hotter running engines to increase the thermal efficiency and enhance the

thurst-to-weight ratio. It is envisaged that, for the future military aircraft, the thrust-to-

weight ratio will double, while the fuel consumption will reduce by 50%. Metal-matrix

composites (MMCs) and ceramic-matrix composites (CMCs), which are thermally stable

and can withstand loads at high temperatures (Fig.1.3) will be of immense use in such

applications. Carbon-carbon composites, which are ceramic composites can withstand

load beyond 20000C. The use of these advanced materials in aeroengines is likely to pick

up in the first decade of the twentifirst century (Fig.1.4). Fan blades, compressor blades,

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vanes and shafts of several aeroengines are now either employing or contemplating to use

in the near future metal matrix composites with boron, borsic, boron carbide, silicon

carbides or tungsten fibres and aluminium, titanium, nickel and super alloy (e.g. NiCrAly

or FeCrAly) matrices. S-glass, quartz

And carbon fibre reinforced polyimides have recently been used on radomes and fins

operating at high temperatures for short and long duration, because polyimides have high

temperature strength retention properties compared to epoxies and phenolics. Carbon-

carbon composites have been successfully employed in the brake discs of aircraft, rocket

nozzles and several other components operating in extreme thermal environments.

The material menu for rockets, missiles, satellite launch vehicles, satellites and

other space vehicles is quite extensive and diverse. The trend is to design some of the

upper stage structural components like payload structures, satellite frame works and

central cylindrical shells, solar panel wings, solar booms, antennas, optical structures,

thermal shields, fairings, motor cases and nozzles, propellant tanks, pressure vessels, etc.

with composite materials to derive the maximum weight benefit. All space vehicles of

recent origin have several composite structural systems. CFRP is the obvious choice

because of its excellent thermo-mechanical properties, i.e., high specific stiffness and

strength, higher thermal conductivity and lower coefficient of thermal expansion. The

future large space stations are likely to be built with CFRP. Although BFRP has several

positive features, it is mainly used for stiffening purposes. Both GFRP and KFRP are

favoured for design of pressurized systems for their superiority in strength and cost-

effectiveness. Beryllium, although not a composite, possesses highly favourable

properties (Table 1.2) but it is sparingly used due to safety hazards, especially during

fabrication. The examples of space applications of composites are too many. One of the

early major application is the graphite-epoxy mesh grid off-set parabolic antenna

reflector developed by Hughes Aircraft Company for the Canadian ANIK satellite which

was launched in 1972. The European Remote Sensing Satellite ERS-I has several

composite parts plus a large 10m long metallised graphite-epoxy radar antenna array. The

Voyager spacecraft contains a large 3.7m diameter CFRP parabolic antenna reflector.

The fairing of ARIANE 4 is a graphite composite stiffened shell structure of maximum

4m diameter and 8.6m height. A few other typical examples are listed earlier in Table

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1.3. The composite application in the aerospace industries is a process of continuous

development in which newer and more improved material systems are being utilized to

meet the critical design and flight worthiness requirements.

1.3 OTHER STRUCTURAL APPLICATIONS

1.3.1 Civil Engineering

The interest in the use of glass fibre reinforced polyesters in building structures

started as early as sixties. The beautiful GFRP dome structure in Benghajj was

constructed in 1968. The other inspiring example is the GFRP roof structure of Dubai

Airport. This was built in 1972 and is comprised of clustered umbrella like hyperbolic

paraboloids. Several GFRP shell structures were erected during seventies. Another

striking example is the dome complex at Sharajah International Airport, which was

constructed during early eighties. The primary advantage of using composites in shell

structure is that any complex shell shape, either synclastic, anticlastic or combination of

both, which is of architectural significance and aesthetic value, can be easily fabricated.

The composite folded plate system and skeletal structures also became popular. The roof

of Covent Garden Flower Market at Nine Elms, London covering an area of 1ha is an

interesting example which was based on a modular construction. In this, pyramidal

square modules were connected at their apices and bases to two-way skeletal grids. The

modular construction technique helps to build a large roof structure which is normally

encountered in the design of community halls, sports complexes, marketing centres,

swimming pools, factory sheds, etc. Several other applications, where GFRP has been

successfully used, include movable prefabricated houses, exterior wall panels, partition

walls, canopies, stair cases and ladders, water tanks, pipes and drainages and led to its

wide use in radomes and antenna towers. In one particular construction, the top 100 ft of

a radar microwave link tower was built with GFRP and the guys were Kevlar fibres (also

radio transparent) to reduce unwanted disturbances in air traffic control radar signals.

Considering the future prospects of composites in civil structural application, ASCE

Structural Plastics Research Council, as early as seventies endeavoured to develop design

methods for structural plastics, both reinforced and unreinforced. However, the major

deterrent for the popularity of composites in civil engineering structures is the material

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cost. But, in many applications, GFRP and KFRP may be cheaper considering the

cumulative cost. The low structural weight will have direct bearing in lowering the cost

of supporting skeletal structures and foundation. Moreover, ease of fabrication and

erection, low handling and transportation cost, less wear and corrosion, simpler

maintenance and repairing procedures, non-magnetic properties, integrity and durability

as well as modular construction will cumulatively reduce the cost in the long run. The

Living Environment house, developed by GE plastics in 1989, is an illustrative example

of the multipurpose use of composites in a building.

1.3.2 Automotive Engineering

Feasibility studies were carried out, since early seventies, to explore the

possibilities of using composites in the exterior body panels, frameworks/chassis,

bumpers, drive shafts, suspension systems, wheels, steering wheel columns and

instrument panels of automotive vehicles. Ford Motor Co. experimented with the design

and development of a composite rear floor pan for an Escort model using three different

composites: a vinyl-ester-based SMC and XMC and a glass fibre reinforced

prolypropylene sheet material. Analytical studies, static and dynamic tests, durability

tests and noise tests demonstrated the feasibility of design and development of a highly

curved composite automotive part. A composite GM heavy truck frame, developed by the

Convair Division of General Dynamics in 1979, using graphite and Kevlar fibres (2:1 by

parts) and epoxy resin (32% by wt) not only performed satisfactorily but reduce the

weight by 62% in comparison to steel for the same strength and stiffness. The hybrid

glass/carbon fibre composite drive shafts, introduced around 1982 in Mazdas, provided

more weight savings, lower maintenance cost, reduced level of noise and vibration and

higher efficiency compared to their metal counterparts. The more recent pickup truck

GMT-400 (1988 model) carries a composite driveshaft that is pultruded around a 0.2cm

thick and 10cm diameter aluminium tube. The composite driver shaft is 60% lighter than

the original steel shaft and possesses superior dampening and torsional properties.

Chevrolet Corvette models carry filament wound composite leaf springs (monoleaf) in

both rear suspension (1081) and front suspension (1984). These springs were later

introduced during 1985 on the GM Chevrolet Astro van and Safari van. Fibre glass

reinforced polypropylene bumper beams were introduced on Chevrolet Corvette Ford and

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GM passenger cars (1987 models). Other important applications of composites were the

rear axle for Volkswagen Auto-2000, Filament wound steering wheels for Audi models

and composite wheels of Pontiac sports cars. Composites are recognized as the most

appropriate materials for the corrosion resistant, lightweight, fast and fuel efficient

modern automobiles, for which aerodynamics constitute the primary design

considerations. All major automotive components like space frames, exterior and interior

body panels, instrument panel assemblies, power plants, power trains, drive trains, brake

and steering systems, etc. are now being fabricated with a wide variety of composites that

include polymer, metal and ceramic matrix composites. The latter two composites will be

of significance in heated engine components and brake pads. The pistons and connecting

rods of modern diesel and IC engines are invariably made of composites with alumina

fibres and aluminium or magnesium alloy matrices. The Ford`s probe V concept car is a

classical example of multiple applications of composites in an automobile car. The

present trend is to use composites even in the design of large size tankers, trailers,

delivery vans and passenger vehicles.

1.4 OTHER APPLICATIONS

Strong, stiff and light composites are also very attractive materials for marine

applications. GFRPs are being used for the last 3-4 decades to build canoes, yatchs, speed

boats and other workboats. The hull of a modern racing yatch, New Zealand, is of

sandwich construction with CFRP faces. There is currently a growing interest to use

composites, in a much larger scale, in ship industries. A new cabin construction material

that is being tried in the Statendam-class ship building is a metallic honeycomb sandwich

with resin-coated facing, that may lead to substantial weight saving. The Ulstein water jet

has a long moulded inlet tract for better control of dimensional accuracy. The

carbon/aluminium composite has been used for struts and foils of hydrofoils, and the

silicon carbide/aluminium composite has been employed in pressure hulls and torpedo

structures. The composites are also being increasingly used in the railway transportation

systems to build lighter bogeys and compartments. The other important area of

application of composites is concerned with fabrication of energy related devices such as

wind-mill rotor blades and flywheels.

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The light artificial limbs and external bracing systems made of CFRP provide the

required strength, stiffness and stability in addition to lightness. Carbon fibres are

medically biocompatible. Composites made with carbon fibres and biocompatible metals

and polymers have been found to be suitable for a number of applications in

orthopaedics. A carbon–carbon composite hip joint with an aluminium oxide head has

performed satisfactorily. Matrices such as polyethylene, polysulfone and

polyaryletherketone reinforced with carbon fibres are also being used to produce

orthopaedic implants.

Composites also have extensive uses in electrical and electronic systems. The

performance characteristics of CFRP antennas are excellent due to very low surface

distortion. Composite antenna dishes are much lighter compared to metallic dishes.

Leadless ceramic chip carriers are reinforced with Kevlar or Kevlar-glass coweave

polyimides to reduce the incidence of solder joint microcracking due to stresses induced

by thermal cycling. The stress level is reduced by matching the low coefficient of

thermal expansion of ceramic chip carriers with that of tailored composites.

Composites are, now-a-days, preferred to other materials in fabrication of several

important sports accessories. A light CFRP golf shaft gives the optimum flexural and

torsional strength and stiffness properties in terms of accuracy and the distance travelled

by the ball. All graphite and graphite hybrid composite archery bows and arrows enhance

arrow speed with a flattened trajectory and increased efficiency. The reduction in weight

of a CFRP bobsleigh permits ballast to be added in the nose of the sleigh and thereby

improves the aerodynamic characteristics due to the change in the position of the centre

of gravity with respect to the centre of aerodynamic pressure. There are several other

interesting composite leisure time items such as skis, tennis and badminton rackets,

fishing rods, vaulting poles, hockey sticks, surf boards, and the list is likely to be endless

in the twenty-first century. The day is not far when common utility goods will be made

with composites. A few such examples are illustrated in Figs.1.5.

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1.5 BIBLIGRAPHY

1. N.J.Hoff, Innovation in Aircraft Structures-Fifty years Ago and Today, AIAA

Paper No. 84-0840,1984.

2. R.J. Schliekelmann, A Soft and Hard Future - A Look into Past and Future

Developments of Structural Materials, AIAA International Annual Meeting on Global

Technology 2000, Baltimore, 1980.

3. S.M. Lee (Ed.), International Encyclopedia of Composites, Vols.1-6, VCH

Publishers, New York 1990-1991.

4. J.W. Weeton, D.M. Peters and K.L. Thomas (Eds.) Engineer`s Guide to

Composite Materials, American Soceity of Metals, Metals Park, Ohio,1987.

1.6 EXERCISE

1. What are the special features of a structural composite? Compare between

natural and man-made structural composites.

2. Why composites are favoured in engineering applications? Write a brief note

on their uses in various engineering disciplines.

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CHAPTER-2

COMPOSITE MATERIALS

2.1 INTRODUCTION

From this chapter onwards we restrict our attention primarily to man-made

modern composites that are used in structural applications. The main constituents of

structural composites are the reinforcements and the matrix. The reinforcements, which

are stronger and stiffer, are dispersed in a comparatively less strong and stiff matrix

material. The reinforcements share the major load and in some cases, especially when a

composite consists of fibre reinforcements dispersed in a weak matrix (e.g., carbon/epoxy

composite), the fibres carry almost all the load. The strength and stiffness of such

composites are, therefore, controlled by the strength and stiffness of constituent fibres.

The matrix also shares the load when there is not much difference between the strength

and stiffness properties of reinforcements and matrices (e.g., SiC/Titanium composite).

However, the primary task of a matrix is to act as a medium of load transfer between one

reinforcement to the other. It also holds the reinforcements together. In that regard, the

matrix plays a very vital role. Besides, the matrix may considerably influence the hygral,

thermal, electrical, magnetic and several other properties of a composite. For example, to

obtain a good conducting composite with SiC fibres one may choose an aluminium

matrix rather than a titanium matrix. It may be noted that both the SiC fibres and the

titanium matrix possess very poor thermal conductivities.

The classifications of composites are commonly based on either the forms of

reinforcements or the matrices used. There are two major forms of reinforcements: fibres

(including whiskers) and particles (having various shapes and sizes). Accordingly, there

are two broad classes of composites – fibre reinforced composites and particle reinforced

composites (or simply particulate composites). On the other hand, there are three

important groups of matrices, namely, polymers, metals (and their alloys) and ceramics.

The composites made using these matrices are classifies as polymer matrix composites

(or polymer composites), metal matrix composites and ceramic matrix composites.

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Composites are also grouped in several other ways. One important class of composites is

termed as laminar composites. They are also called laminated composites or laminates. A

laminate usually consists of two or more layers of planar composites in which each layer

(also called lamina or ply) may be of the same or different materials. Similarly, a

sandwich laminate is a composite construction in which a metallic or composite core

layer is sandwiched between two metallic or composite face layers. The composite face

layers may also be in the form of laminates. Laminated and sandwich composite

structures are very strong and stiff, and are commonly recommended for lightweight

structural applications.

2.2 REINFORCEMENTS

2.2.1 Fibres

Fibres constitute the main bulk of reinforcements that are used in making

structural composites. A fibre is defined as a material that has the minimum 1/d ratio

equal to 10:1, where 1 is the length of the fibre and d is its minimum lateral dimension.

The lateral dimension d (which is the diameter in the case of a circular fibre) is assumed

to be less than 254 µm. The diameter of fibres used in structural composites normally

varies from 5µm to 140µm. A filament is a continuous fibre with the l/d ratio equal toinfinity. A whisker is a single crystal, but has the form of a fibre.

Common low density fibres are manufactured from lighter materials especially

those based on elements with low atomic number (e.g., H, Be, B, C, N, O, Al, Si, etc.).

The cross-section of a fibre may be circular, for example as in the cases of glass, boron

and Kevlar fibres, but some fibres may have regular prismatic cross-sections (e.g.,

whiskers) or arbitrary cross-sections (e.g., PAN, rayon and special pitch based carbon

fibres). The irregularity in the cross-section may introduce anisotropy in the fibre. Thetypical microstructural morphology of common fibres are shown in Fig.2.1.

From the micro-structure point of view, fibres can be either amorphous (glass),

polycrystalline (carbon, boron, alumina, etc.) or single crystals (silicon carbide, alumina,

beryllium and other whiskers). The strength and stiffness properties of a fibre are

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significantly higher compared to those of the bulk material from which the fibre is

formed. Most of the common fibres are brittle in nature. The tensile strength of bulk

brittle material is considerably lower than the theoretical strength, as it is controlled by

the shape and size of a flaw that the bulk material may contain. As the diameter of a fibre

is very small, a flaw, it may contain, must be smaller than the fibre diameter. The smaller

flaw size, in turn, reduces the criticality of the flaw and thereby the tensile strength is

enhanced. For example, the tensile strength of an ordinary glass (bulk) may be as low as

100-200 MPa, but that of a S-glass fibre may be as high as 5000 MPa. However, the

tensile strength of a perfect glass fibre, based on intermolecular forces, is 10350 MPa.

Further, the orientation of crystallites along the fibre direction also helps considerably in

improving the strength properties. A whisker, being a single crystal, is not prone to

crystal defects unlike polycrystalline fibres and provides very high strength and stiffness

properties. The tensile strength and tensile modulus of a graphite whisker are as high as

25000 MPa and 1050 GPa, respectively. These values are quite significant compared to

those of commercial fibres. The typical longitudinal tensile properties of a commercially

available PAN based T300 fibre are 2415 MPa (strength) and 220 GPa (modulus).

Typical thermomechanical and thermal properties of common fibres are listed in Tables

2.1 and 2.2, respectively.

Both inorganic and organic fibres are used in making structural composites.

Inorganic fibres (including ceramic fibres) such as glass, boron, carbon, silicon carbide,

silica, alumina, etc. are most commonly used. The structural grade organic fibres are

comparatively very few in number. Aramid fibres are the most popular organic fibres.

Another recent addition is a high strength polyethylene fibre (Spectra 900) which has a

very low density and excellent impact resistant properties. The carbon fibres may also be

grouped with organic fibres, although they are more often considered as ceramic

(inorganic) fibres. Inorganic fibres in general are strong, stiff, thermally stable and

insensitive to moisture. They exhibit good fatigue resistant properties, but low energy

absorption characteristics. Organic fibres, on the other hand, are cheaper, lighter and

more flexible. They possess high strength and better impact resistant properties.

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Table 2.1 Typical mechanical properties of selected fibres

Fibre Material Density kg/m3

Tensile

strength MPa

Tensile

modulus GPa

Diameter

µm

Glass 2550 3450-5000 69-84 7-14

Boron 2200-2700 2750-3600 400 50-200

Carbon 1500-2000 2000-5600 180-500 6-8

Kevlar 1390 2750-3000 80-130 10-12

Polyethelyne 970 2590 117 38

Silica (SiO2) 2200 5800 72 35

Boron carbide

(B4C)

2350 2690 425 102

Boron nitride 1910 1380 90 6.9

Silicon carbide

(SiC)

2800 4500 480 10-12

TiB2 4480 105 510 -

TiC 4900 1540 450 -

Zirconium oxide 4840 2070 345 -

Borsic(SiC/B/W) 2770 2930 470 107-145

Alumina(Al2O3) 3150 2070 210 17

Alumina FP 3710 1380 345 15-25

Steel 7800 4140 210 127

Tungsten 19300 3170 390 361

Beryllium 1830 1300 240 127

Molybdenum 1020 660 320 127

Quartz whisker 2200 4135 76 9

Fe whisker 7800 13,800 310 127

SiC whisker 3200 21,000 840 0.5-10

Al2O3 whisker 4000 20,700 427 0.5-10

BeO whisker 2851 13,100 345 10-30

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Fibre Material Density kg/m3

Tensile

strength MPa

Tensile

modulus GPa

Diameter

µm

B4C whisker 2519 13,790 483 -

Si3 N4 whisker 3183 13,790 379 1-10

Graphite whisker 2100 20,800 1000 -

Table 2.2: Typical thermal properties of selected fibres

Fibre Melting point0C

Heat

Capacity

kJ/(kg.K)

Thermal

conductivity

W/(mK)

Coefficient of thermal

expansion 10-6

m/mK

Glass 840 0.71 13 5

Boron 2000 1.30 38 5

Carbon 3650 0.92 1003 -1.0

Kelvar 49 250 1.05 2.94 -4.0

SiC 2690 1.2 16 4.3

Steel 1575 0.5 29 13.3

Tungsten 3400 0.1 168 4.5

Beryllium 1280 1.9 150 11.5

Molybdenum 2620 0.3 145 4.9

Fe whisker 1540 0.5 29 13.3

Al2O3 whisker 2040 0.6 24 7.7

Quartz Whisker 1650 0.963 10 0.54

Organic fibres as well as glass, silica, quartz and carbon fibres are commercially

available in the form of strands, tows or yarns. A strand (or end) is a collection of

filaments. A tow (or roving) consists of several ends or strands. A yarn is a twisted

strand. Some twist is preferred for compactness and for making a composite with higher

fibre content. However, an excessive twist should be avoided, as that may not permit the

matrix to penetrate and wet all the fibres. These fibres are also used to make woven

rovings and woven fabrics (clothes). The weave styles in the fabrics may be

unidirectional (uniaxial), bidirectional (biaxial 2D and biaxial 3D) and multidirectional

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(multiaxial). In a uniaxial fabric wrap fibres are yarns that are laid along the roll. Fill/pick

fibres, which constitute only a small percentage of the wrap fibres, are placed in the weft

direction which is transverse to the roll direction. The weaving design methodology and

weaving techniques, in most cases, adapt similar procedures that are followed in textile

technology. A few typical weave styles are illustrated in Figs. 2.2 and 2.3. Commercial

woven rovings are made with a simple plain weave style, whereas fabrics for carbon-

carbon composites may adopt complex multidirectional weaving patterns. Hybrid fabrics

may also be produced by mixing of various fibres in the warp and weft directions. Woven

rovings and fabrics are quite often preimpregnated with resin to make prepregs that are

convenient to use in the fabrication of composite parts.

Glass Fibres

Glass was first made by man in 3000 BC in Asia Minor. Continuous glass fibres

were known to be used for decorative purposes in ancient times in Syria and Venice. The

industrial manufacturing of glass fibres started in 1930`s for use in filters and insulations.

Glass fibres currently comprise more than 90% of fibres used in polymer composites.

There are five major types of glass used to make glass fibres. These are A glass (high

alkali), C glass (chemical), D glass (low dielectric constant), E glass (electrical) and Sglass (high strength), out of which the last two types, due to their superior mechanical

properties, are most widely used in composite roofings, pressure vessels, containers,

tanks, pipes, etc.

E glass is a low alkali, aluminium borosilicate glass and is based on a mixture of

alumina, boric acid, calcium carbonate and magnesia. S-glass is based on a mixture of

silica, alumina and magnesia. For the manufacture of glass fibres, glass is premixed and

formed into glass marbles or beads. The glass marbles or beads are then melt, and themolten glass is gravity fed, under a controlled temperature, through a platinum bushing

containing a large number of very small orifices. The melt vitrifies within the bushing,

and the filaments are simultaneously cooled and drawn rapidly to a small diameter.

Figure 2.4 presents a schematic view of a fibre drawing process. The surfaces of drawn

glass fibres are normally treated with appropriate sizing materials to promote adhesion

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with the resin matrix used, to facilitate weaving without causing mechanical damage to

the fibre or to improve certain properties like, toughness and impact resistance. The

cross-section of glass fibre is circular in nature and the diameter is usually in the range of

7-14 µm. A glass fibre exhibits isotropic properties. Glass fibres are cheap, nonmagnetic,

x-ray transparent, chemically inert, biocompatible, insensitive to moisture and

temperature as well as possess high specific strength (strength to density ratio). However,

a long duration loading under certain environmental conditions, may reduce the load

carrying capacity of fibres by about 25%. This behaviour is known as static fatigue.

Silica Fibres

The silica (Si02) content of silica fibres ranges from 95 to 99.4% and is usually

much higher compared to that of glass fibres. The glass fibres contain only 55 to 75%

silica. The silica fibres are produced by treating glass fibres with acids so as to remove all

impurities. A quartz fibre is a ultra-pure silica fibre. Quartz fibres are made from natural

quartz crystals, in which the silica content is as high as 99.95%. There are a few other

methods for producing high silica or quartz fibres. In one method, a polymer of silicon

alkoxide is spun using a sol-gel process and subsequent heating of the fibre to 10000c

yields a 99.999% pure quartz fibre. Silica and quartz fibres have superior thermal

properties compared to glass fibres. They have extremely low thermal conductivities and

thermal expansion coefficients. They can withstand extreme changes in thermal

environments. They can be heated to a very high temperature without causing any

damage. These properties make them ideal materials for application in highly heated

structures such as thermal shields, nose cones, rocket nozzles, exit cones, etc.

Boron Fibres

Boron fibres with consistent and good mechanical properties were first

manufactured in the 1960`s. Boron is a multiphase fibre. A boron fibre is produced by

depositing boron on a thin substrate by a chemical vapour deposition process. Substrates,

which are thin filaments, usually made of tungsten or carbon. The substrate (of dia.8-

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12µm) is fed in a plating chamber containing a mixture of hydrogen and boron trichloride

and is electrically heated (Fig. 2.5). The boron is deposited in an amorphous form on the

surface of the substrate filament due to the chemical reaction as given by

2BCl3 + 2H2→

2B + 6HCl

The thickness of the deposited boron depends on the rate at which the substrate is passed

through the plating chamber. The boron coated fibre is normally fed to successive plating

chambers to increase the diameter of the fibre. The boron fibres are often treated

chemically to remove surface defects, thermally to reduce residual stresses or by

providing thin coatings (SiC, B4C or BN) to increase oxidization resistance and to make

compatible with metal matrices.

The boron fibre is marketed as a single filament. The boron filaments are now

available in diameters 50µm, 100µm, 125µm,140µm and 200µm. The boron fibre with a

tungsten substrate is costlier than that with a carbon substrate. A carbon substrate also

reduces the density of the fibre. Boron fibres are usually impregnated with a resin to form

tapes, as they are too stiff to weave. Boron fibres exhibit excellent stiffness properties,

because of which they are used for stiffening of structural parts in aerospace applications.

Their tensile strength is also quite good. The thermal properties are, however, in the

intermediate range, although the melting point temperature is on the higher side.

Silicon Carbide and Boron Carbide Fibres

Like boron, there are several other multiphase fibres such as silicon carbide and

boron carbide. Silicon carbides are also vapour deposited on tungsten or carbon

filaments. The silicon carbide vapours are deposited when various chlorosilanes or their

mixtures are used as reactants. A typical reaction is given as

CH3SiCl3 → SiC + 3HCl

The fibres are currently made in diameters of 100µm and 140µm. Silicon carbide fibres

in general exhibit good high temperature characteristics. They are compatible with

several lightweight alloys e.g., aluminium, nickel and titanium alloys. Silicon carbide on

a carbon substrate has several other merits over its counterpart (silicon carbide on a

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tungsten substrate). It is cheaper and lighter. No reaction takes place between the

deposited silicon carbide and the carbon substrate at a high temperature. The tensile

strength is also on the higher side. The tensile strength and tensile modulus of a SiC

whisker is 21000 MPa and 840GPa, respectively. SiC whiskers are grown by combining

silicon and carbon at 1200-15000C under special conditions.

A boron carbide vapour deposited mantle on a tungsten filament substrate can be

formed using mixture of boron trichloride and methane or carboranes. A boron carbide

fibre can also be produced by heating carbon fibres in a chamber containing boron halide

vapour. The melting point of a boron carbide fibre is 24500C, and the fibre retains proper

ties at a temperature higher than 10000C.

Alumina Fibres

The commercial grade alumina fibre developed by Du Pont is known as alumina

FP (polycrystalline alumina) fibre. Alumina FP fibres are compatible with both metal and

resin matrices. These fibres possess a high melting point temperature of 20400C. They

also withstand temperatures up to 10000C without loss of strength and stiffness

properties. They exhibit high compressive strengths, when they are set in a matrix.

Typical longitudinal compressive strengths of alumina FP/epoxy composites vary from2275 to 2413 MPa. Alumina whiskers exhibit the tensile strength of 20700 MPa and the

tensile modulus of 427 GPa.

Carbon Fibres

Carbon fibres are also commonly known as graphite fibres, although there are

some basic differences between the two types. Graphitization takes place at a much

higher temperature compared to the temperature at which carbonization takes place. The

carbon content in the graphite fibre is also higher and is usually more than 99%. The

manufacture of carbon fibres in the laboratory scale started in the early fifties. However,

carbon fibres were made commercially available only during mid-sixties. They are made

after oxidizing and carbonizing the organic textile fibre precursors at high temperatures.

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There are three common types of precursors: polyacrylonitrile (PAN), rayon and

petroleum pitch. A typical fibre fabrication process based on a precursor is shown in Fig.

2.6. The molecular orientation already present in a precursor is formed along the fibre

axis. Carbonization takes place at lower temperature (at about 10000C). The carbonized

fibre is then treated at higher temperature to facilitate the graphitization process. The

degree of graphitization can be enhanced by raising the temperature further. High-

modulus PAN and Rayon based graphite fibres need as high as 25000C for proper

graphitization. High strength PAN-based carbon fibres are treated at about 15000C for

required carbonization. The carbon fibre microstructural morphology changes

considerably with the precursor used (Fig.2.1). This also affects the fibre-matrix interface

characteristics. The fibres in general exhibit anisotropic behaviour. The average

diameters of commercial fibres range from 6-8µm.

Carbon fibres are produced in a variety of tensile strengths and tensile moduli.

They are accordingly designated as ultrahigh, very high, high or intermediate modulus

and high strength. The tensile strength and tensile modulus of carbon fibres may be as

high as 5600 MPa and 500 GPa, respectively. Typical properties of some high modulus

and high strength fibres are presented in Table 2.3. Carbon fibres have many other

positive attributes, for which they are most popular in aerospace applications. They can

withstand extremely high temperatures without loss of much strength and stiffness. The

thermal conductivity is high and at the same time the coefficient of thermal expansion is

almost negligible. These thermal characteristics make them outstanding candidate

materials for high temperature applications. Further, carbon fibres are non-magnetic, x-

ray transparent, chemically inert, bio-compatible and insensitive to moisture to a great

extent.

Carbon fibres are much costlier compared to glass and other organic fibres. Their

application is, therefore, limited to strategic structural components, expensive sport goodsand biological implants.

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Table 2.3: Typical properties of high performance carbon fibres

Type Density

Kg/m3

Tensile strength

MPa

Tensile modulus

GPa

M40 1800 2740 390

M46 1900 2350 450

M50 1900 2450 490

T40 1800 5650 280

T75 1830 2620 545

T300 1770 3240 231

T400 1800 4500 250

T500 1800 5600 241

T800 1800 5600 290

Ultra high Modulus 1800 2240-2410 690-827

Aramid Fibres

These are aromatic polyamide fibres. These are based on polymers formed by

condensation of aromatic diacid derivatives with aromatic diamines. Kevlar is the trade

mark for the commercially available aramid fibres marketed by Du Pont first in the early

seventies. There are three types of Kevlar fibres: Kevlar, Kevlar 49 and Kevlar 29.

Kevlar 49 and Kevlar 29 fibres posses the same strength, but Kevlar 29 has the two-third

of the tensile modulus of Kevlar 49. Kevlar 29 is used for reinforcing rubber cordage and

belting. Kevlar is similar to Kevlar 49, but is designed for tyre reinforcement. Kevlar 49

fibres are commonly termed as Kevlar fibres and find extensive uses in pressure vessels,

motor cases and other structures where strength is the major design criterion. The

diameter of Kevlar fibres range from 8-12µm.

Kevlar fibres, being organic in composition, are susceptible to hygral and thermal

environments. They are easily attacked by alkalis and acids. Each fibre is fibrillar in

nature and consists of several long, stiff fibrils (aligned along the fibre axis) embedded in

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a softer matrix. Because of this microstructural morphology (Fig.2.1), Kevlar fibres are

very weak in compression (due to buckling of fibrils), but exhibit good impact resistance.

They are cheaper, non-magnetic, x-ray transparent and bio-compatible and resistant to

flame, organic solvents, fuels and lubricants.

2.2.2 Particulates

Particulates of various shapes and sizes are used as reinforcing particles. The

shapes vary from a simple sphere (e.g., glass beads) to a complex polyhedron (e.g.,

crystals). The size ranges from a few microns to several hundred microns. Particles of

various inorganic and organic materials are employed to make particulate composites.

However, they should be compatible with the matrix system used. Materials like talc,

clay, mica, calcium carbonate, calcium sulphate, calcium silicate, titanium oxide, wood

dust, sand, silica, alumina, asbestos, glass beads, metal flakes, metal powder, carbon

powder, ceramic grains and several polymeric particles are normally used. Besides

strengthening the composite, particles also serve other purposes. They act as additives to

modify the creep, impact, hygral, thermal, electrical, chemical and magnetic properties as

well as wear resistance, flammability and such other properties of the composite. They

may as well be utilized as fillers to change the matrix content and density of the

composite. The strength, stiffness and other properties of the composite are dependent on

the shape, size, distribution and blends of various particles in a given matrix and also on

the particle-matrix interface condition. Depending on the composite`s end use, the

volume content of the reinforcement may go up to 40-50%, or more.

Short fibres are discontinuous fibres and may also be treated as particles with

cylindrical shapes. Flakes/platelets are also commonly used. They are less expensive than

short fibres, and can be aligned to obtain improved in plane directional properties

compared to those of short fibres. Metal flakes (say, aluminium) can be used to improve

the thermal and electrical conductivity of the composite, whereas mica flakes can be

added to the matrix to increase the resistivity.

Solid glass microspheres, silicate-base hollow microspheres and ceramic

aluminosilicate macrospheres are used in reinforcing polymer matrices. The particle

diameter for solid glass microspheres ranges from 5 to 50µm, whereas for hollow

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properties. Both thermoplastics and thermosets are employed in making reinforced

plastics. Polyethelene, polystyrene, polyamides, nylon, polycarbonates, polysulfaones,

etc. are common thermoplastics whereas thermosets are epoxy, phenolic, polyester,

silicone, bismaleimide, polyimide, polybenzimidazole, etc.

2.3.1 Thermoplastics

Although thermosets are commonly used in structural composites due to their

higher strength and stiffness properties, there is a growing interest in recent years to use

thermoplastics as well. The development of several high performance thermoplastics has

been primarily responsible for this new trend. The main advantage with thermoplastic

polymers is that they can be repeatedly formed by heat and pressure. A thermoplast is a

collection of high molecular weight linear or branched molecules. It softens upon heating

at temperature above the glass transition temperature, but regains its strength upon

cooling. The increase in temperature activates the random motion of the atoms about their

equilibrium positions and results in breakage of secondary bonds. The thermoplast

softens and results in breakage of secondary bonds. The thermoplast softens and flows

when pressure is also applied. When the temperature is lowered, new secondary bonds

are formed and the polymer reverts to its original structure. The process of softening at

higher temperature and regaining rigidity upon cooling is thus reversible in the case of a

thermoplastic polymer. This characteristic behaviour helps it to be recast and reused

several times. The repair of a damaged part also becomes simpler. The scrapage rate is

also reduced. All these make thermoplasts very much cost effective. A thermoplastic

polymer softens, but does not decompose unless the temperature is high enough to break

the primary covalent bonds.

Table 2.4 provides the typical thermo-mechanical properties of a couple of

structural grade thermoplastic resins. These high performance resins are thermally stable

at higher temperatures. They normally achieve high glass transition temperature Tg due

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Table 2.4 Typical properties of some high performance thermoplastics

Properties Polyether

ether

ketones

(PEEK)

Polyamide-

imide

Polyether-

imide

Polysulfone Polypheny

lene

sulfide

Density, kg/m3

1300 1400 1270 1240 1340

Tensile strength,

MPa

104 138 115 70 76

Tensile modulus,

GPa

4.21 4.48 3.38 2.48 3.31

Poisson’s ratio 0.35 0.35 0.35 0.35 0.35

Coefficient ofthermal expansion,

10-6

m/m K

- 56 50 86.40 88

Maximum service

temperature, K

630 - 490 490 565

to their relatively stiff, linear chains and high molecular weight. They are also strong and

stiff and exhibit good creep resistant properties. They are relatively tougher and less

sensitve to moisture. The structures of these resins are illustrated in Fig. 2.7. All resins

are found to contain a high proportion of aromatic rings that are linked by a stable

heteroatom or group (-CO-, SO2-, -O-, etc.). This provides a high degree of chain rigidity

and thereby results in a higher Tg. In addition, the low aliphatic hydrogen (C-H) content

enhances the thermal stability of all these resins at high temperatures. PEEK and

polyphenylene sulfide are essentially crystalline polymers, and other resins shown in

Table 2.4 are amorphous. PEEK has received considerable attention since its inception.

The rigid rings, connected by fairly chemically inert groups (-o- and –c-) make PEEK

highly crystalline. The melting point and chemical resistance of PEEK are also

considerably enhanced. PEEK has a Tg of 1430C and a melting point of 332

0C. It is

soluble only in concentrated sulphuric acid. The processing temperature ranges 300-400

0C. The moisture absorption limit is very low. The fracture toughness is

comparatively higher. All these features of PEEK make it a highly attractive

thermoplastic resin for application in reinforced composites. Graphite/PEEK composite

prepregs are commercially available. Polysulfones reinforced with glass, aramid and

carbon fibres have also found several applications.

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2.3.2 Thermosets

Thermoset polymers are formed from relatively low molecular weight precursor

molecules. The polymerization process in a thermoset resin is irreversible. Once cured,

they do not soften upon heating. They, however, decompose before softening uponfurther heating. Cross-linked and interlinked reactions lead to formation of chain

molecules in two and three-dimension arrays. Because of three dimensional network of

covalent bonds and cross links, thermosetting resins are listed in Table 2.5. At high

temperature, the covalent bonds may break leading to destruction of the network structure

and the polymer decomposes. Thermosetting resins vary widely with Tg values varying

from 45-3000C and elongations ranging from 1% to more than 100%.

Table 2.5 Typical properties of some thermosetting resins

Properties Epoxies Polyesters Phenolics Polyimides

Density, kg/m3

1100-1400 1200 1200-1300 1400

Tensile strength, MPa 35-100 50-60 50-60 100-130

Tensile modulus, GPa 1.5-3.5 2-3 5-11 3-4

Poisson’s ratio 0.35 0.35 0.35 0.35

Coefficient of thermalexpansion, 10

-6m/mK 50-70 40-60 40-80 30-40

Service temperature, K 300-370 330-350 440-470 550-750

The most commonly used thermosets are epoxy, polyester and phenolic resins, among

which polyster resins are most widely used in various common engineering goods and

composite applications. However, epoxy resins constitute the major group of thermoset

resins used in composite structures and adhesives, as they are stronger and stiffer.

Phenolic resins are rich in carbon and possess good thermal properties and are normally

used in high temperature applications especially as an ablative material in thermal

protection systems. Silicone, bismaleimide, polyimide, polybenzimidazol, etc. are in fact,

high temperature polymers that can perform at higher temperature ranging from 200-

4500C. The structures of some thermosetting resins are illustrated in Fig. 2.8.

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Epoxy resins in general possess good thermomechanical, electrical and chemical

resistant properties. They are so called, because they contain two or more epoxide groups

in the polymer before cross-linking. This epoxide group is a three membered cyclic ether

O

C C which reacts with several reagents. It is commonly found in glycidyl

ethers and amines which are the major sources for epoxies in composite applications.

The common epoxy is synthesized by condensing epichlorohydrin with bisphenol A in

the presence of sodium hydroxide. Several other hydroxyl-containing compounds can

replace bisphenol A. A wide variety or special purpose resins can thus be prepared.

Some aerospace grade epoxy systems are based on an aromatic amine (glycidyl

amines) instead of a phenol to increase the epoxy functionality leading to high cross-

link density in the cured resin. Epoxy resins are cured using suitable curing agents or

appropriate catalysts. The major curing agents are aliphatic amines, aromatic

polyamines and polyanhydrides. Curing is the processs of reaction (ionic reaction,

usually polyadditions) between the epoxide and the curing agent in which many

epoxide groups are formed. Aliphatic amines are relatively strong bases and therefore

react with aromatic amines to achieve cure at room temperature. The reaction is highly

exothermic, and the pot life is shorter. This epoxy resin is useful for contact moulding,

but not for prepregging and filament winding. Aromatic polyamines are normally

solids and require high temperature (100-1500C) for mixing and curing. Anhydrides

need higher thermal exposure (150-2000C) for a longer duration (8-16 hours) for

proper curing. The reaction is low exothermic, but the pot life is longer. Both

polyamines and anhydrides are suitable for prepreg manufacturing and filament

winding. These epoxy resins are characterized by comparatively high thermal stability

and chemical resistance. Catalysts can also be used along with curing agents to

accelerate the curing process. Catalytic agents that are often used as curing agents to promote homopolymerisation of epoxide groups may be Lewis acids or bases. The

commonly used catalytic curing agent is boron trifluoride blocked with ethyl amine (a

typical Lewis acid). It is also used as a catalyst with aromatic amines to accelerate

curing at a temperature of 150-2000C. Lewis bases are normally used as accelerators

with anahydrides. Both Lewis acids and bases provide long pot lives.

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A polyester resin is comprised of an unsaturated backbone polymer dissolved in a

reactive monomer. The polyester backbone polymer is formed by condensation of a

mixture of diabasic acids (saturated and unsaturated) and one or more glycols. The

components of the most commonly used polyester resin is phthalic anhydride (saturated

acid), maleic anhydride (unsaturated acid) and propylene glycol. The backbone polymer

is then diluted in styrene monomer (about 35% by weight). The solution is then blended

with an inhibitor such as hydroquinone to prevent premature polymeisation. The process

of curing is initiated by adding a source of free radicals (e.g., benzoyl peroxide or

hydroperoxide and catalysts (e.g., organic peroxides such as cobalt naphthenate or alkyl

mercaptans). Curing takes place in two stages: a soft gel is first formed and this is

followed by a rapid polymerization with generation of heat. A higher proportion of

unsaturated acid in the backbone polymer yields a more reactive resin, while with a

higher quantity of saturated acid the reaction becomes less exothermic. During curing, the

styrene monomer reacts with the unsaturated sites of the backbone polymer to form a

three-dimensional cross-linked network. A small amount of wax is often added to the

solution before curing to facilitate proper curing of the surface of a laminate. Wax, during

curing, exudes to the surface to form a thin protective layer that reduces loss of styrene

from the surface and prevents oxygen which inhibits reaction to come in contact with the

radicals. Several types of polyester resins are commercially available. Vinyl-ester resins

are high performance polyester resins, which are acrylic esters of epoxy resins dissolved

in styrene monomer. Polyester resins can be reinforced with almost all types of

reinforcements to make polyester composites. Polyester resins are cheaper and more

versatile, but inferior to epoxy resins in some respects. Their use in advanced structural

composites is therefore limited. However, they have been widely used in boat hulls, civil

engineering structures, automobile industries and various engineering products and

appliances.

The commonly used phenolic (phenol-formaldehyde) resins are divided into two

groups: resoles and novolacs. Resoles are one-stage resins which are synthesized with

formaldehyde/phenol ratio greater than one (1.25:1) in presence of an alkaline catalyst.

The polymerization process is not fully completed. It is stopped by cooling to obtain a

reactive and soluble polymer which is stored at low temperature. The final

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2.4 METALS AND METAL MATRIX COMPOSITES

Polymer composites are used normally up to 1800C, but rarely beyond 350

0C. The

high temperature capabilities of inorganic reinforcements cannot be realized, when

polymers are employed as matrix materials. Metal matrices, on the other hand, can widenthe scope of using composites over a wide range of temperatures. Besides, metal matrix

composites allow tailoring of several useful properties that are not achievable in

conventional metallic alloys. High specific strength and stiffness, low thermal expansion,

good thermal stability and improved wear resistance are some of the positive features of

metal matrix composites. The metal composites also provide better transverse properties

and higher toughness compared to polymer composites.

Table 2.7 provides the list of some metal matrices and associated reinforcing

materials. The reinforcements can be in the form of either particulates, or short fibres or

continuous fibres. Cermets constitute an important group of metal matrix composites in

which ceramic grains of sizes greater than 1 µm are dispersed in the refractory metal

matrix. A typical example is the titanium carbide cermet which comprises of 70% TiC

particles and 30% nickel matrix and exhibits high specific strength and stiffness at very

high temperatures. The thermo-mechanical properties of some common matrices are

presented in Table 2.8. The aluminium matrices include several alloys such as AA 1100,

AA 2014, AA6061, AA 7075, AA5052, etc. The composites with aluminium matrices are

relatively lightweight, but their applications are limited to the lower temperature range

Table 2.7: Metal matrices and reinforcements

Matrix Reinforcements

Aluminium and alloys C, Be, SiO2, B, SiC, Al2O3, Steel,

B4C, Al3 Ni, Mo, W, Zr O2

Titanium and alloys B, SiC, Mo, SiO2, Be,ZrO2

Nickel and alloys C, Be, Al2O3,SiC, Si3 N4, steel, W,Mo, B

Magnesium alloys C, B, glass, Al2O3

Molebdenum and alloys B, ZrO2

Iron and Steel Fe, Steel, B, Al2O3, W, SiO2,ZrO2

Copper and alloys C,B, Al2O3, E-glass

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Table 2.8: Typical thermomechanical properties of some metal matrices

Matrices Density

kg/m

3Tensile

strengthMPa

Tensile

modulusGPa

Coefficient

thermalexpansion

10-6

m/mk

Thermal

conduct--vity

W/(mk)

Heat

capa--city

KJ/

(kg.k)

Melting

point0C

AA6061 2800 310 70 23.4 171 0.96 590

Nickel 8900 760 210 13.3 62 0.46 1440

Ti-6AL-4V 4400 1170 110 9.5 7 0.59 1650

Magnesium 1700 280 40 26 100 1.00 570

Steel 7800 2070 206 13.3 29 0.46 1460

Copper 8900 340 120 17.6 391 0.38 1080

because of its low melting point. Titanium and nickel can be used at a service

temperature of up to 1000-11000C. There are several systems such as engine components

which are exposed to high level of temperature. Titanium and nickel composites are ideal

for such situations, as they retain useful properties at 1000-11000C. Ti-6AL-4V is the

commonly used titanium matrix material. The other alloys of titanium include A-40Ti, A-

70Ti, etc. Nickel matrices are comprised of a series of Ni-Cr-W-Al-Ti alloys. Super

alloys, NiCrAlY and FeCrAlY, are also used as matrices because of their high oxidation

resistance properties. Molybdenum is a high temperature matrix and fibre material. Iron

and steel matrices are cheaper and can be used at high temperatures, if the weight is not

the major concern. Figure 2.9 exhibits a fractograph of the Al2O3 fibre reinforced Mg

alloy ZM21 composite. A scanning electron micrograph of the Al2O3 fibre reinforced AA

2014 composite is shown in Fig. 2.10.

The high temperature applications of metal matrix composites are listed in Table

2.9. The material cost is the major problem that currently limits their uses, otherwise

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most of the metallic structural parts can be replaced with metal matrix composite parts to

gain advantages.

Fig.2.9

Fig.2.10

Table 2.9: High temperature application of composites

Composites Applications

MMCs Aeroengine Blades, Combustion Chamber, Thrust

Chamber, Nozzle Throat, Exit Nozzle, Engine

Valves, Fins.

CMCs Re-entry Thermal Shields and Tiles, Nozzle Throat,

Nozzle Linings, Pump Seal Rings, Break Linings,

Extrusion Dies, Valves, Turbochargers, Turbine

Blades, Cutting Tools

Carbon-Carbon Nose Cones of Re-entry Vehicles, Combustion and

Thrust Chambers, Nozzle Throats, Exit Nozzles,

Leading Edges of Re-entry Structures, Brake Discs.

2.5 CERAMIC MATRIX COMPOSITES

Ceramic provide strength at high temperature well above 15000C and have

considerable oxidation resistance. They possess several desirable attributes like high

elastic modulus, high Peierl`s yield stress, low thermal expansion, low thermal

conductivity, high melting point, good chemical and weather resistance as well as

excellent electromagnetic transparency. However, the major drawback of ceramics is that

they exhibit limited plasticity. This low strain capability of ceramics is of major concern,

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as it, quite often, leads to catastrophic failure. For this reason ceramics are not considered

as dependable structural materials. But such limitations may not exist with ceramic

matrix composites, as suitable reinforcements may help them to achieve desirable

mechanical properties including toughness. The ceramic matrices are usually glass, glass

ceramics (lithium aluminosilicates), carbides (SiC), nitrides (SiN4, BN), oxides (Al2O3,

Zr 2O3, Cr 2O3, Y2O3, CaO, ThO2) and borides (ZrB2, TiB2). The reinforcements which are

normally high temperature inorganic materials including ceramics, may be in the form of

particles, flakes, whiskers and fibres. The commonly used fibres are carbon, silicon

carbide, silica and alumina. The current resurgence in the research and development of

ceramic matrix composites is due to their resistance to wear, creep, low and high cycle

fatigue, corrosion and impact combined with high specific strength at high temperatures.

The cutting rate of an alumina-SiC whisker cutting tool is ten times higher than that of

conventional tools. The use of ceramic composites in aero-engine and automotive engine

components can reduce the weight and thereby enhance the engine performance with

higher thrust to weight ratios due to high specific strength at high temperatures.

Automotive engines exhibit greater efficiency because of their low weight, better

performance at high operating temperatures and longer life time due to excellent

resistance to heat and wear. Several high temperature applications of ceramic matrix

composites are presented in Table 2.9.

Carbon-carbon composites are the most important class of ceramic matrix

composites that can withstand temperatures as high as 30000C. They consist of carbon

fibres distributed in a carbon matrix. They are prepared by pyrolysis of polymer

impregnated carbon fibre fabrics and preforms under pressure or by chemical vapour

deposition of carbon or graphite. The polymers used are of three types: thermosets

(furfurals, phenolics), thermoplastic pitches (coal tar based and petroleum based) and

carbon-rich vapours (hydrocarbons such as methane, propane, acetylene, benzene).

Phenolic resins are more commonly used in the manufacturing process of carbon-carbon

composites. The phenolic resin impregnated carbon fibre preforms, on pyrolysis, converts

the phenolic resin to a high proportion of amorphous carbon char. The composite material

is found to be porous after the first pyrolysis. It is further impregnated with the phenolic

resin and pyrolised, usually under vacuum and pressure, and the process is repeated

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several times to reduce the void content and realize the optimum density of the material.

The major advantage of carbon-carbon composite is that various fabrics and shapes of

preforms with multidirectional fibre alignments can be impregnated with resins and

pyrolised to yield a wide class of one directional (1D), two directional (2D), three

directional (3D) and multidirectional composite blocks of various shapes and sizes, which

can be machined to produce the desired dimensions. Excellent wear resistance, higher

coefficient of friction with the rise of temperature, high thermal conductivity, low thermal

expansivity and high temperature resistance make them useful materials in high

temperature applications. In absence of oxygen, carbon-carbon composites can withstand

very high temperatures (30000C or more) for prolonged periods. They are also used in

prosthetics due to excellent biocompatibility.

2.6 LAMINATE DESIGNATION

The structural applications of composites are mostly in the form of laminates.

Laminates provide the inherent flexibility that a designer exploits to choose the right

combination of materials and directional properties for an optimum design. A lamina is

the basic building block in a laminate. A lamina may be made from a single material

(metal, polymer or ceramic) or from a composite material. A composite lamina, in which

all filaments are aligned along one direction parallel to each other, is called a

unidirectional lamina. Some unidirectional laminae are illustrated in Fig. 2.11. Here the

fibres (continuous) are oriented along a direction parallel to the x1 axis. Note that the x1',

x2' axes are the material axes, and the x1, x2 axes are the reference axes. The orientation of

the fibre with respect to the reference axis (i.e., x1 axis) is known as the fibre angle and is

denoted by Ø (in degrees). A unidirectional lamina is designated with respect to the fibre

angle Ø. For example, a 00 lamina corresponds to Ø=0

0, a 90

0 lamina corresponds to

Ø=900 and so on.

Fig. 2.11

A laminate is designated by the manner laminae are stacked to form the laminate.

For example, a (00/±45

0/90

0) laminate (Fig. 2.12) is one in which one 0

0 lamina is placed

at the top, one 900 lamina is placed at the bottom and one +45

0 lamina and one -45

0

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lamina are kept at the middle. Unless it is specified, it is normally assumed that all the

laminae in a laminate possess the same thickness. A cross-ply laminate consists of only

00 and 90

0 laminae. An angle-ply laminate, on the other hand, contains only ±Ø laminae.

A laminate may be considered symmetric, antisymmetric or unsymmetric, in case there

exists, with respect to the middle surface, any symmetry, antisymmetry or unsymmetry,

respectively. Figures 2.13 and 2.14 illustrate several cross-ply and angle-ply laminates. It

should be further noted that a [ (00/90

0)n ] laminate is an antisymmetric cross-ply laminate

consisting of n numbers of repeating two-layered (00/90

0) cross-ply laminates. The total

number of laminae in a [ (00/90

0)n ] laminate is 2n. However, a [ (0

0/90

0)ns ] laminate is a

symmetric cross-ply laminate. It has symmetry about the midsurface of the laminate. The

top half of the laminate contains n number of repeating of repeating two-layered (00/90

0)

cross-ply laminates. The bottom half consists of n number of two-layered (900/00) cross-

ply laminates so that the symmetry about the mid-surface is maintained. Note that the

subscript `s` stands for symmetry and the number of laminae in a [ (00/90

0)ns] laminate is

4n. The laminates containing repeating (±Ø) angle-plies can also be identified in a similar

way.

A general unsymmetric laminate may contain 00 laminae, 90

0 laminae and/or Ø

laminae stacked in an arbitrary manner. For example, [ (00)4 / (90

0)2], [(90

0)2/ (30

0)2] and

[00

/900

/300

/600

] are all general unsymmetric laminates. An unsymmetry may also be

introduced by stacking laminae made of different composites. A [00c/90

0g /0

0k] laminate

consists of a top layer of 00

carbon fibre reinforced composite, a middle layer of 900

glass

fibre reinforced composite and bottom layer of 00

Kevlar fibre reinforced composite and

is an unsymmetric cross-ply hybrid laminate. Various such hybrid laminates can be

prepared for practical applications choosing various combinations of layers of metallic

materials, polymer composites, metal-matrix composites and ceramic composites. The

“ARALL” is a hybrid laminate consisting of alternate layers of aramid/epoxy co

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