pks composite part1
TRANSCRIPT
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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