COMPOSITE MATERIALS AND STRUCTURES

CHAPTER-1

INTRODUCTION

1.1 NATURAL AND MAN-MADE COMPOSITES

1.2 AEROSPACE APPLICATIONS

1.3 OTHER STRUCTURAL APPLICATIONS

1.3.1 Civil Engineering

1.3.2 Automotive Engineering

1.4 OTHER APPLICATIONS

1.5BIBLIGRAPHY

1.6 EXERCISE

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-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

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 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 (<10µm) have been drawn from almost all materials. They are much stronger and stiffer than the same material in bulk form. The strength and stiffness properties have been found to increase dramatically, when whiskers (i.e., single crystal fibers) are grown from some of these materials. Figure1.1 illustrates the specific tensile strength and the specific tensile modulus properties are obtained by dividing the strength (M Pa) and modulus (G Pa) by either the density (kg m^-3) or the specific gravity of the material. Because of the

superior mechanical properties of fibers, the use of fibers as reinforcements started gaining momentum during the twentieth century. The aerospace industries took the lead in using fiber reinforced laminated plastics to replace several metallic parts. The fibres like glass, carbon, boron and Kevlar, and plastics such as phenolics, epoxies and polyesters caught the imagination of composite designers. One major advantage of using fibre reinforced plastics (FRP) instead of metals is that they invariably lead to a weight efficient design in view of their higher specific modulus and strength properties (Table 1.2).

Composites, due to their heterogeneous composition, provide unlimited possibilities of deriving any characteristic material behavior. This unique flexibility in design tailoring plus other attributes like ease of manufacturing, especially molding to any shape with polymer composites, repairability, corrosion resistance, durability, 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.

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 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 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 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.

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, 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 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.

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 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.

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 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.

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.

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.

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. 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.