an introduction to composite materials

AN INTRODUCTION TO COMPOSITE MATERIALSby Anthony Kelly

The term composite originally arose in engineeringwhen two or more materials were combined in orderto rectify some shortcoming of a particularly usefulcomponent. For example, cannons which had barrelsmade of wood were bound with brass because a hollowcylinder of wood bursts easily under internal pressure.The early clipper sailing ships, which were said to be ofcomposite construction, consisted of wood plankingon iron frames, with the wood covered by copperplates to counter the attack of marine organisms onthe wood.

For the purposes of this Encyclopedia, a compositematerial can be defined as a heterogeneous mixture oftwo or more homogeneous phases which have beenbonded together. Provided the existence of the twophases is not easily distinguished with the naked eye,the resulting composite can itself be regarded as ahomogeneous material. Such materials are familiar:many natural materials are composites, such as wood;so are automobile tires, glass-fiber-reinforced plastics(GRP), the cemented carbides used as cutting tools,and paper-a composite consisting of cellulose fibers(sometimes with a filler, often clay). Paper is essentiallya mat of fibers, with interfiber bonding being providedby hydrogen bonds where the fibers touch oneanother.

It is sometimes a little difficult to draw a distinctionbetween a composite material and an engineeredstructure, which contains more than one material andis designed to perform a particular function. Thecombination is usually spoken of as a compositematerial provided that it has its own distinctiveproperties, such as being much tougher than any of theconstituent materials alone, having a negative thermalexpansion coefficient, or having some other propertynot clearly shown by any of the component materials.

In many cases, the dimensions of one of the phasesof a composite material are small, say between 10nmand a few micrometers, and under these conditionsthat particular phase has physical properties ratherdifferent from that of the same material in the bulkform; such a material is sometimes referred to as ananocomposite.

The breaking strength of fibers of glass, graphite,boron and pure silica, and of many whisker crystals, ismuch greater than that of bulk pieces of the samematerial. In fact, it may be that all materials are attheir strongest when in fiber form. In order to utilizethe strength of such strong fibers, they must be stucktogether in some way: for example, a rope is appro-priate for fibers of hemp or flax, and indeed carbonand glass fibers are often twisted into tows. However,for maximum utilization, a matrix in which to embedsuch strong fibers is required, in order to provide a

strong and stiff solid for engineering purposes. Theproperties of the matrix are usually chosen to becomplementary to the properties of the fibers: forexample, great toughness in a matrix complements thetensile strength of the fibers. The resulting combi-nation may then achieve high strength and stiffness(due to the fibers), and resistance to crack propagation(due to the interaction between fibers and matrix).

Nowadays, the term advanced composite meansspecifically this combination of very strong and stifffibers within a matrix designed to hold the fiberstogether. This type of composite combines the extremestrength and stiffness of the fibers and, due to thepresence of the matrix, shows much greater toughnessthan would otherwise be obtainable.

1. Prediction of Physical PropertiesAn important question, both for engineering design ofa composite material and for scientific understandingof its properties, is that of how the overall properties ofthe composite depend upon those of the individualconstituents.

Properties of composite materials can be consideredunder two headings: (a) those that depend solely uponthe geometrical arrangement of the phases and theirrespective volume fractions, and not at all upon thedimensions of the components. For this to be so, thesmallest dimension of each phase must usually begreater than 10nm, and sometimes much greater thanthis; (b) those that depend on structural factors such asperiodicity of arrangement or the sizes of the pieces ofthe two or more component phases.

1.1 Properties Determined by Geometry-Additive PropertiesThe geometrical arrangement of the phases in a com-posite material can often be described in simple terms(Fig. 1).

The simplest physical property of a composite-namely its density pc-is given by the volume-weighted average of the densities of the components:

Pc = PI VI + P2V2 (1)where V is the volume fraction and subscripts 1and 2 refer to the components. If there are no voids,Vt + V2 = 1. More complicated physical properties,e.g., those described by a second-rank tensor, relatetwo vectors: either a solenoidal vector and an irro-tational vector (as with magnetic or electric suscepti-bility), or else a flux vector and the gradient of a scalarfunction (as with diffusivity, and electric and thermalconductivity). The relations derived for these proper-ties for a composite material in terms of the same

xvii

An Introduction to Composite Materials

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Figure 1Composite geometries: (a) random dispersion of spheres ina continuous matrix; (b) regular array of aligned filaments;(c) continuous laminae; and (d) irregular geometry (afterHale 1976)

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properties measured in a piece of the submaterialoutside the composite. In addition, the composite istaken to be statistically homogeneous in the sense that,if we extracted small elements of the material, thesewould have the same physical properties as the wholesample. In addition, the implicit assumptions are madethat there are no voids, that space-charge effects andpolarization are absent (in the case of electrical con-ductivity) and that there is no discontinuity in temp-erature at the interface (in the case of heat flux).

The prediction of the elastic properties of particu-late composites is discussed in the article ParticulateComposites; for fiber composites see Fibrous Com-posites: Thermomechanical Properties.

Thermal expansion coefficients of composites in-volve both the elastic constants and the thermalexpansion coefficients of the individual phases. Animportant function of reinforcing fillers and fibers inplastics is the reduction and control of thermal ex-pansion. For example, with dental filling materials, adifference in thermal expansion between the fillingmaterial and the tooth substance can lead to a mar-ginal gap, and hence composite filling materials aredesigned to have a thermal expansion coefficient veryclose to that of the tooth substance.

With some components, especially at low fibervolume fractions, the transverse thermal expansioncoefficient of, for example, a glass-fiber-epoxy-resincomposite, can be greater than that of the matrix. Thiseffect is particularly noticeable with fibers of highmodulus and low expansion coefficient (e.g., boron orcarbon) in a low-modulus matrix.

Fibers are often used in laminated arrangementsbecause properties parallel to the fibers are verydifferent from properties perpendicular to the fibers.The effective in-plane thermal expansion coefficientsfor angle-ply laminates (in which the fibers are ar-ranged at plus and minus an angle to a particulardirection) show that in such laminates a scissoring orlazy-tongs type of action can occur, and, with appro-priate values of , can lead to a zero or even negativethermal expansion coefficient in one direction.

1.2 Special Property Combinations-Product PropertiesComposite materials can, in principle, be thought of asmaterials which produce properties unobtainable in asingle material: for example, by combing a piezoelec-tric material with a material showing magnetostric-tion, the composite should show a magnetoelectriceffect-that is, an applied magnetic field would inducean electric dipole moment; or a material could beproduced in which an applied magnetic field producedoptical birefringence, by coupling a material whichshows strain-induced birefringence with one showingmagnetostriction.

This idea lead van Suchtelen (1972) to classify sucheffects as product properties of composites and so now


there are considered to be two different types ofphysical property of composite materials.

The first is the type discussed so far in Sect. 1.1.These are sum or additive properties where the com-posite property is related to that same property of eachof the components and so depends on the geometry ofarrangment of the two components. The geometry ofarrangement includes of course the volume fraction.Examples are elastic stiffness, relating applied stressand measured strain, or electrical resistance, relatingapplied electric field and measured current density, orthe simple example of mass density. The value of thephysical property of the composite in general liesbetween those of the components.

There is a subclass of additive properties in whichthe value of the property of the composite can lie welloutside the bounds set by the values of the property ofthe components; examples are Poisson's ratio invol-ving the ratio of two compliance coefficients under theaction of a single applied stress, or acoustic wavevelocity, which depends on the ratio of elastic modulusto density. Here, because the elastic modulus and thedensity can follow quite different variations with vol-ume fraction, e.g., elastic modulus following Eqn. (3)and density following Eqn. (1), the acoustic wavevelocity of the composite can lie well outside the valuefor either component.

A further, but less obvious, example is the thermalexpansion coefficient, which depends on the thermalexpansion coefficients and the elastic constants of thetwo phases. Newnham (see Nonmechanical Propertiesof Composites) distinguishes this last set of propertiesand calls them combination properties (distinct fromsum or additive properties) when the composite prop-erty lies outside the bounds of the same property ofthe two or more constituents. However, since a valuelying outside the range of the constituents' propertiescan in principle occur for all composite properties(even mass density) it seems best to view all cases inwhich the same property of the composite and of thecomponents is considered as cases of additive or sumproperties.

Product properties are those cited at the beginningof this section: the property of the composite dependsspecifically on the interaction between its components.Any physical property can be considered as the actionof a physical quantity X resulting in physical quantityY giving the X - Y effect. Product properties are thosein which an X - Y effect in submaterial 1 produces aY-Z effect in submaterial 2, producing in the com-posite an X-Z effect. A good example is the one citedof a magnetoelectric effect in a composite materialhaving one magnetostrictive and one piezoelectricphase. Application of a magnetic field produces achange in shape in the magnetostrictive phase whichthen stresses the piezoelectric phase and hence gener-ates an electric field. In this case the coupling is mech-anical but the coupling could also be electrical, optical,magnetic, thermal or chemical (van Suchtelen 1972).

xix


Viewed in this way, sum properties are those inwhich an X - Y effect in submaterial 1 and the sameX - Y effect in submaterial 2 combine to give an X - yeffect in the composite.

Table 1 classifies some physical properties orphenomena according to the input-output parametersX and Y. A small selection of possible product proper-ties is given in Table 2. Practical examples of theseeffects are given in Nonmechanical Properties of Com-posites.

1.3 Properties Dependent on Phase Dimensionsand Structural PeriodicityIn Sects. 1.1 and 1.2 the physical properties of thecomponents or submaterials have been assumed to beunaltered by the incorporation of the components intothe composite. This is usually not the case. The effectsof phase changes or chemical reactions during fabri-cation are of importance in a wide range of com-posites. Excessive shrinkage of one component canresult in high internal stresses, which may lead topremature failure or even preclude successful fabri-cation. Matrix shrinkage also has an important indi-rect effect on the mechanical behavior of fiber com-posites, since the resulting internal stresses can deter-mine the frictional forces at the fiber-matrix interface.

The dimensions and periodicity of a compositestructure also have an important effect on the proper-ties when they become comparable with, for example,the wavelength of incident radiation, the size of amagnetic domain or the thickness of the space-chargelayer at an interface. The dimensions are also particu-larly important when the properties of a materialdepend on the presence of defects of a particular size(e.g., cracks). The strength of a brittle solid containinga crack depends on the square root of the length of thecrack. Since very small particles cannot contain longcracks, and because the surface region of most solidsbehaves differently from the interior (a small fiber orsphere contains proportionately more surface materialthan a large piece), the breaking strength depends onthe dimensions of the piece tested. This is particularlyimportant when considering mechanical strength andtoughness of composites.

Many of the optical effects which depend on struc-tural dimensions also depend in a very complex wayon other factors, and optical effects produced bydispersion of a second phase within a material areoften used to determine the distribution of that phase(e.g., determination of the molecular weights of poly-mers or of the size of crystal nuclei in glasses), ratherthan regarding the material as a composite withspecial optical properties. However, a composite ma-terial containing aligned elongated particles of anoptically isotropic material in an optically isotropicmatrix may exhibit double refraction as a straight-forward consequence of the relationship between di-electric constant and refractive index.

xx

In a ferromagnetic material at temperatures belowthe Curie point, the electronic magnetic momentsare aligned within small contiguous regions-themagnetic domains-within each of which the localmagnetization is saturated. Magnets of very highcoercivity can be obtained by using very fine particles:a particle with a diameter of between 10- 1 and10- 3 urn would be magnetized almost to saturation,since the formation of a flux-closure configurationwould be energetically unfavorable. Magnetizationreversal cannot take place in a sufficiently small single-domain particle by boundary displacement; it mustoccur by a single jump against the magnetocrystallineanisotropy and the shape factor. Fine-particle per-manent magnets of rare-earth alloys such as SmCo5'enveloped in an inert matrix such as tin, have beendeveloped which have superior properties; forexample, the energy product can be up to 30 MGOe(1 GOe = 79.6 x 10- 4 TAm - 1 = 8 J m - 3). The prep-aration of these fine-particle magnets shows that at-tention to small size alone is not enough, and that caremust be taken to prevent domain-wall nucleation atthe surfaces of the particles.

Considerable attention has also been paid to rod-type eutectics as permanent magnetic materials. If thepermanent magnet is long and thin, the demagnetiz-ation energy can be very large as a consequence of theshape anisotropy, even in the absence of highmagnetocrystalline anisotropy. This is the concept ofthe elongated single domain. In the Bi-Mn, Bi system,very high coercive forces (e.g., 24 kOe = 1.9 x 106Am -1) can be obtained but, because the volumefraction of magnetic material is low, so is the energyproduct.

Composite principles are also used in the con-struction of powerful electromagnets containing fila-mentary superconducting composites: for example,high-performance electromagnets producing field den-sities of 16 T with a Nb-Sn (intermetallic) winding, or8 T with Nb-Ti. One of the problems here is instabilityin the interaction between the superconductor and themagnetic field, which causes quenching of supercon-ductivity. When this occurs, a very large current mustbe carried by other means, and one solution to thisproblem is to surround the conductor with, say, tentimes its volume in copper. The copper can then coolthe superconductor and return it to the superconduc-ting state. The energy dissipated is proportional to r 2 ,where r is the radius of the wire, and the cooling isproportional to r. The ratio of surface area to cross-sectional area per unit length varies as l/r and there-fore small wires are more effectively cooled.

The theory of how instability is produced alsoshows that it can be almost eliminated by ensuringthat the diameter of the individual superconductingfilaments is reduced below approximately 20-50 urn.This size effect arises because instability occurs when-ever a small rise in temperature reduces the shieldingcurrent (proportional to the superconducting current),

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Table 2Product properties of composite materials (after van Suchtelen 1972)

X-y-Z Property of phase Property of phase 2 Product property(Table 1) (X-Y) (Y-Z) (X-Z)

1-2-3 Piezomagnetism Magnetoresistance PiezoresistancePhonon drag

1-2-4 Piezomagnetism Faraday effect Rotation of polarization by mechanicaldeformation

1-3-4 Piezoelectricity Electroluminescence Piezoluminescence1-3-4 Piezoelectricity Kerr effect Rotation of polarization by mechanical

deformation2-1-3 Magnetostriction Piezoelectricity Magnetoelectric effect2-1-3 Magnetostriction Piezoresistance Magnetoresistance

Spin-wave interaction2-5-3 N ernst - Ettingshausen Seebeck effect Quasi-Hall effect

effect2-1-4 Magnetostriction Stress-induced Magnetically induced birefringence

birefringence3-1-2 Electrostiction Piezomagnetism Electromagnetic effect3-1-3 Electrostriction Piezoresistivity } Coupling between resistivity and electric3-4-3 Electroluminescence Photoconductivity field (negative differential resistance,quasi-Gunn effect)3-1-4 Electrostriction Stress-induced Electrically induced birefringence

birefringence light modulation4-2-1 Photomagnetic effect Magnetostriction Photostriction4-3-1 Photoconductivity Electrostriction Photostriction4-3-4 Photoconductivity Electroluminescence Wavelength changer (e.g., infrared-visible)4-4-3 Scintillation Photoconductivity Radiation-induced conductivity (detectors)4-4-4 Scintillation, FIuorescence Radiation detectors, two-stage fluorescence

fluorescence

Figure 3Tensile strengths and stiffness of a variety of fibers

The strength and stiffness of the same fibers divided bytheir density and by the acceleration due to gravity isshown in Fig. 4. The resulting physical unit is a lengthand, as far as the strengths are concerned, represents(intriguingly) the greatest length of a uniform rodwhich could in principle be picked up from one end onthe surface of the Earth without breaking.

Quantities such as specific strength and stiffness areoften quoted in units such as strength divided by

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causing a flux change which in turn leads to heating,and so on. The energy liberated per flux jump issmaller if the superconducting element is narrower,and the time to cool the superconductor from outsideis smaller if the diameter is smaller. Both effectsproduce an upper limit to the diameter of the super-conductor for effective stabilization. For ac appli-cations it is also necessary to reduce the eddy-currentlosses in the copper. This can be done by using amaterial with a high electrical resistivity to decouplethe wires.

To produce the very highest magnetic fields at-tainable in the laboratory (50-100 T)pulsed-field sys-tems are used. These require the conductor to showhigh mechanical strength as well as very good electri-cal conductivity; a composite has the ideal geometryfor producing this combination of properties.

2. Advanced Fiber CompositesThe best known and most striking example of amodern composite material is as we have said, the fibercomposite, designed for high strength, high stiffnessand low weight.

The breaking strength and the stiffness of typicalspecimens of a number of modern fibers are shown inFig. 3. Steel wire and glass provide the strongestmaterials and pitch-based carbon fibers the stiffest.

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specific gravity, in which case units of stress are used,or else as stress divided by density, the resulting unitbeing a velocity squared. In the case of stiffness thevelocity represents that of longitudinal sound waves inthe material. In Fig. 4 a unit of specific stiffness, Ej pg(or of specific strength, (Jjpg) of 107 em is equivalentto a value of stiffness, Ej p, (or of strength, (J jp) of9.8 x 109 cm 2 s - 2 (E is Young's modulus and (J istensile strength).

Figure 3 shows that the stiffest materials are cer-amics composed of elements from the first rows of theperiodic table (B, C, SiC, A1 2 0 3 , etc.). This is becausethese elements contain the highest possible packing ofstrong covalent bonds directed in three dimensions.Such materials also each have a high melting point, alow coefficient of thermal expansion and a low density.All of these are very desirable engineering properties.The strongest materials are the glasses and the metals.

In all structural applications, the weight of thestructure necessary to bear a given load and to do sowith a minimum elastic deflection is important, and sostrong fibers are compared on the basis of strengthdivided by density. The fibers shown in Fig. 3 arecompared in this way in Fig. 4. When this is done thestiffest materials on a weight-for-weight basis remainthe ceramics but the strongest are the plastics (PEt,PBT, Kevlar) and the glasses. The metals make a verypoor showing on either stiffness or strength. However,fibers cannot be used directly and must be boundtogether within a matrix.

A set of aligned fibers in a pliant matrix is onlyuseful as an engineering material under direct tensileforces parallel to the fibers. The salient properties arerepresented by those of a pocket handkerchief which isstiff in the direction of the warp and weft (parallel tothe fibers) but shears easily parallel to these. Hence theproperties of fiber composites are very directional (see

Figure 5Measured variation of tensile strength with angle c/> forspecimens consisting of a number of alternate layers offibers. The fibers in each layer are parallel andcontinuous. Alternate layers are at + and at - to thetensile axis. Open circles represent data for a volumefraction of 40A> of silica fibers in aluminum. Full circlesrepresent data for a volume fraction of 66% of E-glassfiber in an epoxy resin

Fig. 5),being very strong and stiff parallel to the fibers,but rather weak in shear parallel to the fibers (becausethis property depends principally upon the shearproperties of the matrix), and very weak indeed intension perpendicular to the fibers. In order to over-come this, the fibers are arranged in laminae, eachcontaining parallel fibers, and these are stuck together(Fig. 6) so as to provide a more isotropic material witha high volume loading of fibers. Figure 6 shows thespecific strengths and stiffness values of laminatedforms of fibrous composites made of some of the fiberswhose properties are shown in Figs. 3 and 4. Theadvantage over conventional isotropic constructionalmaterials represented by the metal alloys is seen to beconsiderable but is much less marked than in Figs. 3and 4.

Alternatively, the fibers may be randomly arrangedin a plane or in three dimensions; such arrangementslimit the obtainable fiber packing density. Fibers arealso often woven into mats before incorporation intothe composite because this aids the handling of fiberarrays.

When carbon- or other fiber-reinforced plastics aremade with woven fabric rather than nonwoven ma-terial, distortion of the load-carrying fibers parallel tothe applied stress reduces the tensile strength further

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properties of the individual laminae to the propertiesof the fibers and to take into account the interactionsin terms of bending-twisting coupling between thevarious laminae. Initially, of course, these compli-cations were viewed as a great disadvantage of thematerial, since the engineer was not happy with aniso-tropy. Now the problem of how to use it and make avirtue, not so much out of necessity, but of a supposedvice has been solved. The latest Grumman X29 aircraftmakes use of bending-twisting coupling so that, as theloads on an aircraft wing increase, the structure candeform but remain tuned to the aerodynamic require-ments. The material possesses almost a form of"gearing," so that it changes its shape automaticallywithout the need for sensors and levers.

Figure 6Specific strength and stiffness of some isotropicmaterials and of fiber composites. The designation 1 onthe composites means the following arrangements offibers: 50% at 0; 40% at 45 and 10% at 90 to thestress; 2 denotes balanced laminates with equalproportions at 45, 90 and 135;0 indicates alignedfibers in the specified matrix. The volume fraction offibers in the various composites are not the same inthe different systems. They vary between 40 and 60%.The metal alloys are those without such designation

and reduces stiffness and toughness. When, however,woven fabric is oriented at 45 to the load directionthese properties compare favorably with those fornonwoven 45 material. Indeed, the residualstrengths after impact can be greater. At presentbetween one third and one half of the market ofcarbon-fiber-reinforced plastic (CFRP) consists ofwoven fabric; the rest consists of aligned material.

The anisotropy of the properties of advanced fibercomposites represents a completely new feature inengineering design. The nonisotropy can be varied; infact it is possible now to place the fibers preciselywhere they are needed in a structure to bear the loads.In fact it is also possible, for instance, to vary theamount and stacking of the fibers in various directionsalong the length of a beam so as to vary the stiffnessand torsional rigidity of the material.

This is precisely what is needed in helicopter rotorblades, and within a few years' time all rotor bladesexcept perhaps those for heavy-lift helicopters will bemade from GRP or combinations ofGRP and CFRP,despite the fact that such composites were not appliedto helicopter blades until the mid-1960s.

The AV 8B, the latest version of the Harrier verticaltakeoff and landing aircraft, employs composites intorque boxes, auxiliary flaps, the forward fuselage andcone structure, the horizontal stabilizers and in theammunition and gun pods.

It would be impossible to make use of the inherentadvantages of anisotropic materials such as CFRPwithout the invention of computer-aided methods.The reiterations and the number of variables involvedrequire computer programmes both to relate the

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3. Applications of CompositesHigh-performance composites use fibers in order toattain the inherent properties of the fiber, coupling thiswith a judicious choice of matrix so that toughness andimpact damage are not lost.

Strong fibers, whether they be stiff or not, have thegreat advantage of restraining cracking in what arecalled brittle matrices. It is this use of fibers which isoften referred to when showing how the ancientsemployed composite materials. Quotations from theBible are made, such as that from the Book of Exodus,Chapter V, verses 6 et seq., which refer to the difficultyof making of bricks without straw. Fibers restraincracking because they bridge the cracks. The principleis very simple. It has been used in asbestos-reinforcedcement and the principle of reinforced concrete is nottoo far from the same idea. The principle is illustratedin Fig. 7. A crack passing through a brittle materialmay not enter the fibers but leaves a crack straddled byfibers so that a material which would normally havebroken with a single crack now has to be cracked in a

Without fibers

With fibers

Figure 7 . .Demonstration of how cracks are prevented from running Ina brittle material because of the fibers in their path

large number of places before the fibers themselves failor pull out. This is utilized in fiber-reinforced cement,which is based on the same principles as reinforcedconcrete, but since the fibers are very much finer thanreinforcing bars and are often not visible to the nakedeye, a homogeneous material is effectively producedwhich can show quasiplastic deformation.

This judicious use of fibers enables us to thinknowadays in terms of using brittle materials for con-struction purposes, because if they are cracked thefibers will render the cracks harmless. Building panelsand pipes, of course, are made of such material. Glass-reinforced cement is becoming commonplace. How-ever, a much greater prize appears ahead. Metals havelimited high-temperature capability. The ceramic ma-terials described above as providing the best fibers alsoprovide the materials of highest melting point. For theconstruction of prime movers at high temperaturethese will have to replace metals. They cannot do thisas monolithic pieces because, as we have seen, thesewould be easily broken. However, they can resistcracking if they contain fibers. The composite prin-ciple then leads to suggesting that even fibers of thesame material as the matrix may give marked resi-stance to cracking and fracturing. Such is true ofcarbon-carbon composites which are used in brakesfor Concorde and in certain ballistic missile andreentry vehicle applications. Ofcourse carbon oxidizesand so would have to be protected if exposed to hotair. In contrast, therefore, a good deal of interestcenters on the use of refractory oxide glasses such ascordierite or a material of almost amorphous siliconnitride or silicon carbide for use at high temperature,containing fibers again of silicon nitride or siliconcarbide. These will restrain cracking and give anengineering material usable for thousands of hours attemperatures in excess of ll00C. As yet these are notcommercially available but aircraft engine manufac-turers are making and testing pieces of ceramics,sometimes unreinforced, for aerospace applications;but the main thrust of research is towards reinforcedmaterials.

Ceramic materials are of widespread abundanceand are all essentially cheap. If they can be fashionedby simple chemical means, such as reacting acids withalkalis, they are classed as phosphate-bonded ma-terials. These again have great potential for replacing,say, sheet steel in normal household use, again pro-vided cracking can be restrained. Fibers will do thisand, if the material is not exposed to high temperature,glass fibers or even textile fibers may be employed.

4. Arrangement of the EncyclopediaWith this introduction the reader will now hopefullybe able to understand the appearance of the varioustopics within this Encyclopedia. It is arranged, ofcourse, with the articles (all by world experts) inalphabetical order to facilitate ease of access by the


reader. The content of the book follows a patternperhaps not apparent without explanation.

It is the existence of manufactured strong stiff fiberswhich has given rise to the utility of composite ma-terials. Fibers, not necessarily very strong and stiff, areused alone for many .purposes: in textiles, hence inclothing and apparel; in industrial belting in gaskets;in automobile tires; and in many other uses. Nowa-days both man-made and natural fibers are used. Thenewer, strongest and stiffest artificial fibers have ex-tended many of the conventional uses of textiles. Theuses of fibers in textiles are reviewed in the articleFibers and Textiles: An Overview.

5. FibersThe very strong and stiff fibers comprise the following,each with a separate section in this Encyclopedia:asbestos (a naturally occurring fiber), boron and car-bon of varying degrees of graphitization, made inprinciple from a number of starting materials but inpractice mainly two: PAN (polyacrylonitrile) andpitch. Inorganic glasses based on silica yield manyfibers. Those made in the form of continuous filamentsare most important for composite materials. Manylinear polymers such as polyparabenzamide or simplepolyethylene can be processed to form very stiff andstrong fibers. The ways of doing so are very varied.They are all described in High-Modulus High-StrengthOrganic Fibers. Fibers of Al20 3 are important forhigh-temperature use in filtration and other applica-tions because of their chemical inertness. These,and fibers based on silicon carbide and nitride,again made via various routes, are described inOxide Inorganic Fibers, Silicon Carbide Fibers andSilicon Nitride Fibers, respectively. Whiskers ortiny (,...... 1 urn diameter) fibers are known of mostmaterials. They are often very strong and stiff andpossibly will have uses as agents producing tough-ness, stiffness or wear resistance when introducedinto other materials. Their genesis is described inWhiskers.

Historically, the first materials designed speci-fically by chemists to hold fibers in a compositematerial were the organic thermosetting resins. Themodern varieties of these resins are described inThermosetting Resin Matrices.

6. Examples of Composite MaterialsMany natural products such as wood, shells andmanufactured derivatives such as paper and plywoodare of course composites, and the understanding of thephysical properties of these materials both illuminatesour understanding of the properties of wholly syn-thetic composites and, more frequently, is itself en-hanced from our knowledge of the behavior of themodern composite materials. The sections in thisEncyclopedia on the principal composite materialsystems describe both. Hence there are sections on

xxv


natural composite materials, on the breaking strengthof woods, on the production and properties of paperand paperboard, on plywood, and on molded fiberproducts (where articles are molded using essentiallyonly water as the dispersing agent, which is sub-sequently removed) and on wood-polymer com-posites. To these must be added a description ofcomposites based on natural fibers other than wood,such as sisal, sunhemp, coir and jute. The incorpor-ation of these into matrices such as polyesters isdescribed in Natural-Fiber- Based Composites.

The principal composite systems in use commer-cially are of course described. Glass-reinforced plastic(GRP) is by far the largest in commercial volume. Thefabrication and properties of glass-reinforced plasticsbased on thermosetting resins are described in Glass-Reinforced Plastics: Thermosetting Resins, and theproperties of GRP based on the thermoplastic resinsin Glass-Reinforced Plastics: Thermoplastic Resins.Carbon-fiber-reinforced plastic (CFRP) has beendeveloped rapidly within the last ten years or so; basedinitially on thermosetting resins, they have recentlyemployed thermoplastics. The methods of fabricationand some of the properties are described in Carbon-Fiber-Reinforced Plastics. The newer thermoplasticpolymers such as poly(ether ether ketone), polysulfoneor the modified polyamides have advantages over thethermosetting resins: they can withstand higher strainbefore failure, they have lower moisture absorptionand they can be shaped when heated and repairedwithout requiring a lengthy and intricate cycle of cure.They also have, in effect, an infinite storage life at roomtemperature. The newer thermoplastic polymers are,however, expensive. Their properties when containingglass and the stiffer fibers are described in High-Performance Composites with Thermoplastic Matrices.

The automobile tire is made of a composite materialbased on an elastomeric matrix. I ts properties arebetter appreciated than analytically understood. Ananalysis of its performance in terms usually used in thedescription of modern composites is given in Auto-mobile Tires.

There is no reason why a fibrous composite needcontain fibers of only one chemical type. Hybrids arepossible containing more than one type of fiber andthe advantage of these is described in HybridFiber-Resin Composites. Nor is it necessary that fiberand matrix be chemically different: carbon-carboncomposites, in which strong stiff carbon fibers areincorporated into a matrix of quasiamorphous carbonproduced by pyrolysis, chemical vapor deposition orby other means, have a number of important uses andgreat potential for use at higher temperatures (seeCarbon-Carbon Composities). It is the potential ofceramics to displace metals for use at high tempera-tures (> 1000"C), because of the ceramics' greateroxidation resistance and lower density, that primarilycauses interest in the use of these materials in com-posites containing fibers. Ceramic materials used

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alone are brittle and hence produce fragilecomponents. One way, perhaps the only way, toproduce very high fracture toughness in them is toincorporate fibers. Fiber-reinforced ceramics possessgreat potential but have few uses at present. However,the incorporation of fibers to increase toughness hasbeen commercially realized (see Fiber-Reinforced Ce-ments) . In order to provide adequate toughness,the fibers must be spatially oriented in threedimensions. The modern three-dimensionalarrangements of fibers are described in the articleThree-Dimensional Fabrics for Composites.

Under some conditions metals such as aluminum ormagnesium may be better vehicles for carrying fibers,and hence for making a useful fiber composite, thanpolymer matrices. These metals have been reinforcedwith a large number of fibers because as a matrix theyshow advantages over all thermosetting resins andsome thermoplastic resins; they have high thermalconductivity, little hydrothermal degradation andpossess dimensional stability, and they are not suscep-tible to radiation damage or low-temperature brittle-ness. Their higher melting point can be an advantage.The article Metal-Matrix Composites describes theseand also the use of fibers in order to ameliorate theproperties of a particular metal, e.g., lead or copper.Some metals, e.g., tungsten, are extremely stiff (tung-sten has a Young's modulus of 350 G Pa) and possessvery high melting points besides being well known asfibers. If they can be protected from oxidation, theyoffer the possibility of being used as a reinforcement ofanother metal, e.g., nickel-base alloy, or of a ceramicfor high-temperature use.

The idea of making a composite for engineering usehas arisen in recent years from the desire to utilize verystiff and strong fibers. The fibers cannot be used alone,so they require a matrix. The need to manufacture thetwo component materials separately could be avoidediffibers were made in situ in a matrix. Here ingenuity isat a premium since the resulting composite, if it hasadequate properties, is likely to be more cheaplyproduced than if each component is made separately.It may well also be that the two components in thefinal product are so chemically related that the stab-ility of each in the presence of the other is ensured. InSitu Composites: Fabrication describes the processesused with metals and inorganic materials; processessuch as plastic deformation, eutectic solidification,precipitation and the recently discovered method ofgreat promise, melt oxidation, which can form ceramicfibers or particles either alone or within a metalmatrix.

The corresponding processes in organic-polymersystems are based on copolymerisation and extrusion,and the drawing of multicomponent mixtures contain-ing polymers of different melting points are describedin In Situ Polymer Composites and Polymer-PolymerComposites.

Although the great majority of composites referred

to are advanced composites, the increasing use of stifffibers and the interest in these enhances interest inparticulate composites where a stiff and thermallyresistant phase of nonfibrous form is used in thecomposite. Some of these composites have beenknown for years and have found use as cutting tools,wear resistant parts, etc. They are described in Particu-late Composites.

7. Properties of CompositesThe science of the physical properties of compositematerials relates the properties of the composite tothose of the individual constituents and of the interfacebetween them. Most of the simple physical propertiessuch as density, elastic modulus and thermal ex-pansion coefficient can be calculated and, noting theremarks in Sect 1.1, these calculations and their com-parison with experiment are dealt with in FibrousComposites: Thermomechanical Properties essentiallyfor unidirectional fibrous composites. The elasticproperties of laminates which consist of thin layers(lamellae) stacked upon one another, the fibers beingparallel within each layer, is an important topic fordesign and details of how these calculations can bemade appear in Laminates: Elastic Properties. Anotherform of arrangement of nonparallel fibers is that of thewoven fabric and the elastic properties of these aredealt with in Woven-Fabric Composites.Properties.

An engineer may wish to choose a particulararrangement of fibers of given properties within agiven matrix in order to attain or to approach aparticular set of properties within the composite. Forthe simpler properties, where there is confidence in theprediction of a property from consideration of theproperties of the constituents, such as for elasticmodulus or for thermal expansion coefficient, at-tainable combinations of properties can be displayedgraphically, leading to the concept of a structure-performance map, described in Structure-PerformanceMaps.

Properties such as strength and toughness of com-posite materials are not as well understood as thesimpler elastic properties because in many cases themodes of failure under a given system of external loadare not predictable in advance. An added complicationis the fact that the breaking strength of individualfibers within a population shows a spread of values.This variation is described generally by what are calledWeibull statistics. Since the breaking strength of acomposite will depend not only on the highest break-ing strength of the individual fibers but also on howthe individual breaks are arranged in space within thecomposite, the accurate prediction of the breakingstrength requires a complicated statistical theory evenwhen time-dependent effects are ignored. In the articleStrength of Composites: Statistical Theories, a simpli-fied account of the breaking strenght of unidirectionalcomposities is given. Despite the complications of


arriving at an exact prediction of the strength, ad-vanced composites are used nonetheless and theirstrength measured. Because of the anisotropy of elas-ticity and strength, these quantities must be measuredin special ways. Methods of achieving reliablemeasurements of strength and what simple rules thereare for variation of strength with orientation, fiberlength, and so on are dealt with authoritatively inStrength of Composites.

I n engineering practice, the static strength of acomposite in the presence of notches and stress con-centrations due to geometric form is at least as import-ant as that of simple test pieces used in the laboratory;a very short account of some important features isgiven in Failure of Composites: Stress Concentrations,Cracks and Notches. In practice, composite materialsmust be joined to one another and to other structuralmembers. The peculiar methods of joining employedare dealt with in Joining of Composites. Here everyeffort is made to minimize stress-concentrating effectssuch as abrupt changes of elastic moduli and ofgeometrical section or form.

The ability of a material to remain serviceable whencontaining cracks, or when cracks arise during service,depends upon its toughness. It is by no means clearthat measures of toughness or crack resistance, such asthe concept of a critical stress intensity factor derivedfrom linearly elastic mechanics, are applicable tocomposite materials, and some aspects of the problemare dealt with in Failure ofComposites.Stress Concen-trations, Cracks and Notches. Some progress can bemade in elucidating the primary mechanisms respon-sible for the energy required to break a compositematerial. These are the sliding friction between fibersand matrix if broken fibers pull past one another, thedeformation within the fiber and matrix, and theformation of subsiduary cracks. These energy-dissi-pating mechanisms depend on the variables of volumefraction, diameter of fibers, etc., and so representativediagrams indicating the relative importance of thesecan be constructed, as in the article Toughness ofFibrous Composites.

This description of the mechanical properties ofcomposite materials has so far made no mention oftime-dependent properties; in particular, strength hasbeen described almost as a static property. Thestrength under oscillating stress is described in FatigueofComposites, and the variation of deformation of thecomposite with time is described in Creep of Com-posites. Both are aspects of a general investigativeanalysis of the failure of composite materials which hasbeen called damage mechanics. In Fatigue of Com-posites, it is shown that emphasis on the strain range towhich a given load subjects a composite can simplifygreatly the interpretation of the results. During cyclictesting of a composite material, cracks in one of theconstituents occur usually within the matrix. The sameoccurs in a static test where it is called multiplefracture. It is necessary then to understand the physi-

xxvii


cal properties of a material containing a large numberof cracks.

The creep deformation, though not the creep rup-ture, of aligned fibrous composites with continuousfibers stressed parallel to these cracks is, on the otherhand, relatively well understood and can be modelledin terms of the constitutive equations of deformationfor the fibers and matrix deforming in parallel. Dis-continuous-fiber composites can also be dealt with tosome extent and hence deformation normal to thefibers at least partly understood.

The long-term stability of composite materialsunder load depends on chemical factors, oxidativestability of the two components, and so on, particu-larly at high temperatures. For most composites basedon resin matrices, water absorption at room andslightly elevated temperatures is of great importance.Epoxy resins, for example, absorb water and this alterstheir glass transition temperature. If the fibers aresusceptible to water attack, as glass is, the ingress ofwater leads to attack on the reinforcing fibers (seeLong-Term Degradation of Polymer-Matrix Com-posites.

Before composite materials enter service, and whilethey are in engineering service, their properties mustbe evaluated in a nondestructive manner. In the articleNondestructive Evaluation of Composites, this is dealtwith from firsthand experience. The principal objectiveof nondestructive evaluation is to provide assuranceon the quality and structural integrity of a particularcomponent. This can be achieved directly by using anondestructive evaluation technique (NDE) or in-directly by monitoring or controlling the fabricationprocess. The latter is really only an extension of theusual procedures of process control necessary toachieve a consistent product. But NOE can also assistin optimizing fabrication procedure by, for example,monitoring the local state of cure. Since compositematerials are essentially arrays of fibers assembled intoplace, the possibility arises of using the fibers them-selves as monitors of their performance in service oralternatively doing this by incorporation of othertypes of sensors. The monitoring of fracture within aglass fiber or of the failure of resin adhesion to thesurface of a fiber of glass is quite possible using visiblelight, and other fibers may be interrogated using otherwavelengths of radiation.

These ideas raise intriguing possibilities of moni-toring performances in situ by NOE so that thematerial itself becomes perceptive and says "how itfeels." These ideas may be of great importance for theuse of advanced materials generally, not just forcomposites (Kelly 1988). They have developedgreatly recently and are described in detail in thearticle Smart Composite Materials Systems.

A section is of course included on nonmechanicalproperties. In Nonmechanical Properties of Com-posites, composite structures such as multilayercapacitors, piezoelectric transducers, varistors

XXVIll

and sensors of a variety of types are dealt with.Composites have become such an exciting part of

materials science and engineering that other struc-tures, many of which have only recently beenrecognized as composites, are now being describedin the same terms as are composites (see Nano-composites) .

8. Applications: Use of Composite Materialsin Engineering

It is already apparent that the applications of com-posite materials, even if mention is not made of thosebased on naturally occurring composites such aswood, are very wide, ranging from a simple glassreinforced plastic (GRP) tank or boat hull to a sophis-ticated aeroplane wing aeroelastically designed, or toanother primary aircraft structure such as a helicopterrotor blade.

In Applications of Composites:An Overview, appli-cations of GRP in the building industry, in marineapplications (boats and minehunters), in building,transport and the electrical industry are covered. Inmany of these applications it has been corrosionresistance which has determined the success in substi-tuting for a traditional material. It is mainly in leisuregoods, medical materials and in aerospace, whereperformance requirements overcome the consider-ations of cost, that the newer advanced composites aregaining ground. Consideration of manufacturingmethods (see Manufacturing Methods for Composites:An Overview) will often dominate which composite isto be used, because an important principle of com-posite materials is that almost any desired combina-tion of physical properties can, within certain limits, beobtained.

The manufacturing methods depend, of course,upon the feedstock, and again the glass-reinforced-plastics industry has developed most of the methodsemployed. Glass is available in many forms, asrovings, chopped fibers or fabrics. The fibers may belaid up and then impregnated with resin, or guns maybe used to spray liquid resin or fibers onto a suitablyshaped core. Alternatively, preimpregnated arrays offibers (prepregs) are used which in their simplest formconsist of parallel tows, rovings or aligned individualfibers spread out to produce a uniform distribution offibers within the thickness of the sheet, which isimpregnated with resin to produce a material ofcontrolled volume fraction. The resin is partiallycured (B-staged) to a slightly tacky condition. Some-times this must be refrigerated until used. Sheet, doughand bulk molding compounds are variants of this,generally using discontinuous fibers.

All manufacturing methods aim to avoid the entrap-ment of air or the formation of voids since theserepresent gross defects. Articles may be made by handor spray placement, by press molding using heatedmatched male/female tools under pressures of

3-7 MPa, or by vacuum molding using a flexiblemembrane to obviate the need for a press. If higherdensity and lower void content are required, moldingis done in an autoclave using pressures of, say,1-2 MPa at an elevated temperature, "" lOOC orso.

Alternatively, fibers may be enclosed in a mold andthe resin injected, the resin being precatalyzed so thatit cures. In reaction injection molding, two fast-reacting components (usually based on urethanes) ofinitial low viscosity are pumped into the mold. Thismethod gives low cycle times (1-2 minutes).

In the process of pultrusion, continuous fibers arepassed through a bath of resin and the impregnatedfibers passed through a heated die so that the resincures. This process is good for the production of rod,sheet, tube and bar forms. The most accurate position-ing of fibers necessary for some high-performace appli-cations is attained by filament winding, in which fibersimpregnated with resin are wrapped onto a formermandrel which is withdrawn after the resin is cured.Modern filament winding machinery can producecomplicated nonaxisymmetric shapes as a result of theapplication of robotics. The process is admirablysuited to computer control and this will become morewidespread in the future.

The extrusion process is described in Fiber-Re-inforced Polymer Systems: Extrusion. This is essentiallya form of polymer processing involving the incorpor-ation of discontinuous reinforcing fibers such aschopped glass-fiber or natural cellulose.

All of these more conventional methods are usedwith high-performance composites but the increasinginterest, and some use, of higher melting point thermo-plastic matrices (such as poly(ether ether ketone))containing carbon fibers, is leading to the developmentof forming methods in the solid state by pressing,drawing and extrusion comparable to conventionalmetal-forming methods.

The application of the new composites in a varietyof industrial sectors and with diverse applications arealso described in this Encyclopedia. (See Aircraft andAerospace Applications of Composites; Artificial Bone;Automotive Components: Fabrication; Composite Armor;Dental Composites; Friction and Wear Applications ofComposites; Helicopter Applications of Composites;Solid Fiber Composites as Biomedical Materials).

The introduction into use and the commercialprospects of all materials are greatly affected by thepossibilities for extensive recycling and so an article


on this topic has been included (see Recycling ofPolymer-Matrix Composites).

BibliographyBunsell A R (ed.) 1988 Fibre Reinforcements for Compo-

site Materials. Elsevier, AmsterdamBunsell A R, Kelly A, Massiah A (eds.) 1993 Develop-

ments in the science and technology of compositematerials. Proc. 6th European Conf. Composite Ma-terials. Woodhead, Cambridge

Chou T W 1992 Microstructural Design of Fiber Compo-sites. Cambridge University Press, Cambridge

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Clyne T W, Withers P J 1993 Introduction to Metal MatrixComposites. Cambridge University Press, Cambridge

Cogswell F N 1992 Thermoplastic Aromatic PolymerComposites. Butterworth-Heinemann, Oxford

Dhingra A K, Lauterbach H G 1986Fibers, Engineering. In:MarIe H F, Bikales N M, Overberger C G, Menges G 1986Encyclopedia of Polymer Science and Engineering, Vol. 6,2nd ed. Wiley, New York, pp. 756-802

Gerstle F P Jr 1986Composites In: Marie H F, Bikales N M,Overberger C G, Menges G 1986Encyclopedia of PolymerScience and Engineering, Vol.3,2nd edn. Wiley,New York,pp. 776-820

Hale D K 1976 The physical properties of composite ma-terials. J. Mater. Sci. 11: 2105-41

Hannant D J 1978Fiber Cements and Concretes. Wiley, NewYork

Hashin Z, Shtrikman S 1962 A variational approach to thetheory of the effective magnetic permeability of multiphasematerials. J. Appl. Phys. 33: 3125-31

Hull 0 1981 An Introduction to Composite Materials.Cambridge University Press, Cambridge

Kelly A 1988 Advanced new materials; substitution viaenhanced mechanical properties. In: Advancing with Com-posites, International Conference on Composite Materials.CUEN, Naples, pp. 15-23

Kelly A, Macmillan N H 1986 Strong Solids. 3rd edn.Clarendon, Oxford

Kelly A, Rabotnov Y N (eds.) 1983 Handbook ofComposites(4 Volumes). North Holland, Amsterdam

Maddock B J 1969Superconductive composites. Composites1: 104-11

Naslain R, Lamon J, Doumeingts D (eds.) 1993 Hightemperature ceramic matrix composites. Proc. 6thEuropean Conf. Composite Materials. Woodhead,Cambridge, UK

Suchtelen J van 1972Product properties: A new applicationof composite materials. Phillips Res. Rept. 27: 28-37

Suchtelen J van 1980 Non-structural applications of com-posite materials. Ann. Chim. Fr. 5: 139-53

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