Indian Journal of Fibre & Textile Research Vol. 26, September 2001 , pp. 327-340

Review Article

Fibre-reinforced composites: Their fabrication, properties and applications

VB Guptaa

Department of Textile Technology, Indian Institute of Technology, Hauz Khas, New Delhi I IO 016, India

Received 19 March 2001; accepted 7 June 2001

An attempt has been made to describe the wide range of composites based on the combination of reinforcing fibres like carbon, aramids, high-modulus polyethylene, ceramic, boron and glass with matrices like thermosetting resins, thermo­plastic polymers, soft metals, ceramics and carbon. Starting with the definition and classification of composites, the advan­tages of the fibrous shape and of an optimum adhesion between the fibre and the matrix are considered. Thi s is followed by an account of the properties of the constituents of the composites, viz. the fibres and the matrices. The composite fabrication processes are then described. Finally, the properties and major applications of the composites are briefly discussed.

Keywords: Composites, Laminates, Matrices, Polymers, Reinforcing fibres

1 Introduction

1.1 Definition

A composite may be defined' ·2 as a bi-phase or a multi-phase material which is made by combining two or more materials differing in composition or form which remain bonded together but retain their identity and properties. To illustrate the concept of such a composite, Kelly3 cited the example of a bimetallic strip which is used in devices such as thermostats. The strip might be made of a flat piece of brass and a similar piece of iron. If the two pieces are separate and heated simultaneously, the brass would expand more than the iron but both will expand independently and retain their linear shapes. If the two pieces are welded together and the composite is heated, the greater extension of the brass forces the iron to bend and the bending of the iron forces the brass to bend. Thus from the concept of composites, which is implicit in this example, it may be concluded that a two-component composite would show the following features3: (i) while either material alone would be useless for the application, the combination has an entirely new property, and (ii) the two components act toge'ther to equalize the di fferent strains; this combined action is the most important in the design of the composite ' product. It is noteworthy that both these requirements are met by composites, provided there is reasonable

"Present address: A-9/39A. Kalkaji Extension, Gomti Apartments, New Delhi 1 IO 019, India Phone: 6094622; E-mail: vbgupta @hotmail.com

adhesion between the two phases. It is worth pointing out that galvanised steel, which is steel sheet coated with a layer of zinc, and similar combinations of materials will not come under the definition given for a composite material.

1.2 Natural Composites

The two composites ingeniously created by nature, viz. wood and bone, which have served as a benchmark for the designers and manufactures of composites, come within the broad definition given in the previous section. Wood is a composite of oriented cellulosic fibrils embedded in a stiff, glassy amorphous matrix, viz. lignin, which binds the reinforcement. Bone is a composite composed of collagen, which is present in the form of a fibrous structure of strong but soft protein, embedded in apatite, a hard and brittle mineraI3

.4.

1.3 Classification of Composites

Depending on the shape and type of the two or more phases in the composite, the composites may be classified in several different ways, e.g. in terms of the shape of the reinforcing phase or the type of the matrix phase, etc. Broadly speakjng, the reinforcing phase may be particulate, platelet or fibrous in shape. The particulate reinforced products may have large particles as in concrete in which sand or larger aggregates are held in place by a hydrated cement matrix or smaller particles as those of carbon in a matrix of rubber. Steel or metal alloys may also be

328 INDIAN J. FIBRE TEXT. RES., SEPTEMBER 2001

considered as particulate-reinforced composites. The platelet reinforcement is genera lly achieved by usi ng naturally occurring mineral s like talc (principally magnesium silicate) and mica (aluminium si licate) in muscovite form, which are 1 0-1 000 ).Am across and I­S ).Am thick, in an appropriate matrix. The particulate and platelet shaped materials generally act as fillers rather than as a reinforcement for the matrix unless significant chemical interaction occurs between the filler surface and the polymer in the composite, as for example, in the case of carbon black in rubber or in the case of calcium carbonate in polyamides or in cases where such interaction is brought about by the use of a coupl ing agent. In the absence of such chemical interaction, the fillers act only as extenders and not as reinforcements and though the sti ffness may be enhanced, the strength is generally reduced .

1.4 Advantages of the Fibrous Shape

The fibrou s shape offers a number of advantages as a reinforcement over the particulate and platelet shapes. This is best illustrated taking the example of glass fibre which has been in existence for over 3000 years but has been used as a reinforcement for a polymer (unsaturated polyester resin) only in the 1930's. The question that must be addressed is: Why glass fibres, which have excellent strength and quite a high stiffness, high temperature resistance and low thermal expansion and are quite inexpensive and easi ly available, have been so little used by themselves as load-bearing structural materialss.6? The work of Griffith in England in the 1920's showed that a material's ability to retain its strength in the presence of cracks is determined by the work of fracture of the material, i.e. the energy required to break its.6. Since on fracture, two new surfaces of the material are created, this is equal to the surface energy of these two fracture surfaces. For glass, it is small compared to that for other materials (Table I). However, the flexible polymers like polyethylene are more like metals in their resistance to cracks though they will not take as much stress as metals. In metals and polymers, the atoms or molecules can slide over one-another at the leading edge of a crack and, therefore, a crack cannot penetrate the internal surface of a metal or polymer as it can the structure of a glass. Since it is very difficult to produce and retain a material without any flaws, the materials like glass cannot be used for load-bearing applications on their own. The question that arises is: How can these strong and stiff but brittle materials be best utilised for load

Table J - Fracture surface energy of some material s

Material

Dural Copper Steel (h igh strength) Cast iron Polystyrene Epoxy Pol yester resin Graphite Glass

Fracture surface energy, J/m2

140.000 50,000 50,000 4,000 1.000 400 220

50- 100 4

bearing applications? The behaviour of a composite of strong tungsten fibres in a soft copper matrix2

provides a clue. When a tensile load is appl ied in the direction of the tungsten fibres, the principle of combined action, described earlier for a bimetallic strip, cOmes into operation and hence the strains in the fibre and the matri x have to be equal. The tungsten fibres , which are strong but brittle, carry very high load and therefore break while the soft copper matri x, which is weak and ductile, carries a much lower load and does not break (Fig. l). The propagation of the crack through the brittle reinforcing material is hindered by the softness of the matrix. Two other features prevent cracks from running through the composite: (i) reinforcing fibres that break do not all lie in one plane, and (ii) since the adhesion between the fibre and the matrix is not too high , the cracks are deflected and rendered harmless. This suggests that the best way to utilize glass for load-bearing applications is (a) to prepare the composites in which the glass is in the form of thin fibres so that any cracks present cannot find a continuous path through the material and (b) to bind the fibres in a matrix.

1.5 Characteristics of the Matrix

The matrix must be relatively soft so that it (i) does not damage the fibres by scratching, (ii) acts as a stress-transmitting medium, (iii) is plastic and adhesive so that it holds the fibres well, and (iv) deflects and controls the cracks in the composite. These requirements are met by a polymer or a soft metal like aluminium or copper. Such a matrix is soft (weak in shear) and does not scratch the fibres. It also contributes to the cohesion of the product by keeping the fibres in the desired location and orientation. Further, it protects them from abrasion and from environmental damage due to elevated temperature and humidity . This example also highlights the

GUPTA: FIBRE-REINFORCED COMPOSITES 329

./

' .

Fig. I-Effect of matrix in blunting cracks is shown in a composite of tungsten fibres in a copper matrix . The tungsten fibres, which are the clearer bars, broke when the material was pulled in a direction parallel to the fibre ; the copper did not. The dark flow lines in the copper show that the matrix has retarded the cracking by the process of shear. The movement of the crack was from left to right. (Reproduced with Prof. A. Kelly 's permission).

importance of optimum adhesion between the reinforcing phase and the matrix.

1.6 Adhesion between the Fibre and the Matrix

If the adhesion between the two phases is non­existent or very poor, the stress transfer is poor and the full advantage of the reinforcement is not obtained. If the adhesion is prefect between the two phases, the energy absorbing capacity may not be exploited. Since the adhesion is not usually automatically present, the use of appropriate adhesion promoters or coupling agents is often made, The presence of the fibre affects the surrounding matrix and the matrix extending to about 0.1 Ilm around the fibre of 10 Ilm diameter will have different characteristics than the matrix far from the fibre. It is in this region that stress transfer takes place and this interphase is sometimes referred to as the third phase in a bi-phase composite. It is worth adding at this stage that even if the reinforcing fibre is not as notch­sensitive as glass, unlike the conventional textile fibres with relatively low stiffness, high breaking extension and low notch sensitivity, the fibre bundle (yarn) of the reinforcing fibre with high stiffness and low breaking extension cannot be twisted to allow the stress to be transmitted between individual fibres by sliding friction when the yarn is stretched. The stress transfer function for such fibres is, therefore, generally performed by the matrix in which they are embedded.

1.7 Fibrous Materials

It is interesting to note that since pre-historic times,

the fibrous shape has been used by mankind for reinforcement. For example, the use of chopped straw in bricks or of plant fibres in pottery, besides reinforcing the clay after it had hardened, also reduced the cracking of the clay during hardening. It is also noteworthy that almost all textile and industrial fibres have been used as reinforcements in one way or the other. For example, in cycle tyres, cotton is used to reinforce rubber while in heavy duty tyres, nylon 66, glass fibres, Kevlar type high performance fibres or steel fibres , are used as reinforcements. In this article, the emphasis will be on high performance fibres like carbon fibres, wholly aromatic polyamide fibres, ultra high-molecular weight high-density polyethylene fibres and ceramic fibres of silicone carbide and aluminium oxide and boron in addition to glass fibres . Metallic wires will also be briefly considered.

Besides fibres, which are long slender filaments with a length of at least several millimeters, the whiskers, which are like discontinuous fibres and are single crystals produced by filamentary growth of unbranched hair-like crystals5 with a diameter not exceeding 10 Ilm and with length of a mm or more, are also used in composite fabrication .

The fibrous reinforcement is used in composites in various forms. These include bundles of continuous filaments as yarns, cords, roving or tows or fabric forms as tapes, woven fabrics, non-woven fabrics or continuous filaments chopped into discontinuous fibres or whiskers.

330 INDIAN J. FIBRE TEXT. RES., SEPTEMBER 200 1

1.8 Matrix Materials The principal functions which the matrix IS

expected to perform have already been referred to earlier. It is essential that for designing and fabricating a composite product based on a specific reinforcing fibre, the choice of the matrix should be made considering both the phases as parts of a total system. Since the composite has to meet the requirements of the end application, which are well defined, the matrix must be able to meet these requirements. Matrices based on polymers are ex tensively used and will receive more attention in thi s article. Matrix materi als based on metals and ceramics are still in their formative stages of development. The polymeric matrices can be thermosetting or thermoplastic . The thermosetting resins are more common and in them solidification from the liquid phase takes pl ace by the action of an irreversible chemical crosslinking reaction, generally in the presence of heat and pressure. The thermoplast ics are incorporated in the system by melting and then solidifying by cooling, the physical reaction being reversible in nature. The principal thermosetting resins used as matrices are epoxies and polyesters and the main thermoplastics are nylons , polypropylene, etc. In general, thermoplastics have low creep resistance and low thermal stability compared to thermosetting resins. Metallic (aluminium and titanium) matrices with low specific gravities , ceramic (silicon carbide and aluminium oxide) matrices and carbon are primarily considered for high temperature applications (300°-500°C) .

2 Properties of the Constituents of Composites

2.1 Desirable Properties of a Composite A composite material must have a combination of

properties and these include good mechanical properties, low density , high temperature and chemical resistance, appropriate electrical properties, vibration damping characteristics, etc. Good mechanical properties like high stiffness, strength and toughness combined with low density are generally required in all composite products. Generally, a wide spectrum of properties is available in composites to choose from . Broadly speaking, for aircraft and aerospace applications, viz. for primary and secondary aircraft structures, the mechanical properties have to be outstanding, often with high temperature resistance, and cost may not be the primary consideration. On the other hand, in large

scale application areas like sports goods, land and sea transportation, machinery parts, building and construction industry and other industries where the property requirements may not be as severe, the cost considerations are important and also the ability to be mass produced rapidly .

When a two-phase composite with well-aligned fibres is stressed along the fibre axis, a force is set up at the fibre interface. If the fibre is long enough and the interface has enough cohesion, this transmitted force is borne by the fibre and when the force exceeds the breaking stress of the fibre, the latter breaks . In such a case, the fibre has contributed fully to the mechanical properties of the composite. Because the stiffness and strength of the matrix are generally much less than those of the reinforcing fibre, the latter carries bulk of the load and control s the axial stiffness and strength of the composite, whil e the transverse or torsional properti es, such as compressi ve and torsional moduli and strengths, are large ly controlled by the matrix. The toughness ari ses mainly from interaction between the fibre and the matrix, with the adhesion between the two phases and the nature of the interphase pl ay ing a stellar role in controlling the property. In thi s context, it is noteworthy that in wood there is no sharp boundary between the crystalline cellulose core and the amorphous matrix (lignin) at which steep stress gradients could occur7

. In case of composites based on thermoplastic res ins, transcrystallization may occur around the fibres while in composites based on thermosetting resins, a crosslink density gradient may occur.

The densities of both the constituents, viz. the fibre and the matrix , must be low to have a composite material that is light.

2.2 Properties of Reinforcing Fibres

The merit of the fibrous shape over the particulate or platelet shape was demonstrated earlier taking glass fibre as an example and the superiority of the fibrous shape in deflecting or blunting the crack was established. This is of particular significance for g lass fibres which have very low fractu re surface energy and their strength is therefore considerably reduced in the presence of flaws. However, it needs to be emphasised that glass has a molecular architecture of a three-dimensional network and is thus isotropic and amorphous even in the fibrous state . A number of important reinforcing fibres have a sheet like two­dimensional (carbon fibres) or a line like one

GUPTA: FIBRE-REINFORCED COMPOSITES 331

dimensional (high performance polyethylene fibres) architecture and are highly anisotropic and crystalline materials and score over glass fibres both in terms of mechanical properties and density. In these fibres the advantage is taken of the reinforcing potential offered by molecular orientation in the direction of the fibre aXIs.

Stiffness is a very important property, particularly in structural materials, though its importance might not be as intuitively obvious as that of strength. The flapping of a flag even in mild breeze is indicative of the low modulus of the fabric of which the fl ag is made. An aeroplane wing with inadequate modulus will show very high deflection and sway very apparently. For the same reason, a space shuttle can be rolling violently because of atmospheric pressure when plunging into the atmosphere8

. Also in engineering structures, the deflections of the different materials under stress must be compatible. Amongst solids used in engineering structures, the stiffness can vary by a factor of about 200,000 and thus material se lection has to be done with care.

2.3 Structural Basis of High Stiffness and Strength

Amongst the various bonds present in solids the covalent bond is the strongest while the van der Waal s type bond is the weakest; between these extremes come the ionic, metallic and hydrogen bonds. Covalently bonded materials are stiffest when they are composed of small atoms linked by short bonds so that there are a large number of bonds per unit volume. Diamond, which is a three dimensional , covalently bonded material , is one of the stiffest materials with a stiffness of 1160 GPa in the [110] direction. It was pointed out by Frank9 that in the [110] direction, diamond is composed of fully aligned zig-zag chains of carbon just like those in polyethylene, utilizing half the neighbour to neighbour bonds in the crystal while the other half of the bonds are at right angles to this direction, contributing nothing to the Young's modulus in this direction, just as the bonds between carbon and hydrogen in fully aligned polyethylene contribute nothing to its longitudinal Young's modulus. The cross-sectional area in diamond is 0 .0488 nm2 which is four times smaller than in polyethylene (0.182 nm\ Hence from this analogy, Frank argued that we could expect a modulus of 285 GPa for fully aligned polyethylene, well above that of steel. 'Kevlar' fibre wi th a very rigid backbone has a cross-sectional area

of 0.205 nm2 and its theoretical modulus is 194 GPa, less than that of polyethylene, a highly flexible polymeric chain. Materials with strongly polarised ionic bonds have high stiffness, particularly if they contain small ions with high charge.

A simple estimate of the theoretical cleavage stress

((Jmax) of an infinite perfect solid leads to the following expression:

... (1)

where E is the Young's modulus; Y I the surface energy per unit area of the fracture surface; and ao, the equilibrium separation of the atomic planes. Eq. (I) reveals that besides a high Young's modulus , which arises from strong chemical and physical bonds, a large surface energy and small separation between atoms are also necessary for high-strength. These three properties are inter-related and covalent bonded and metallic crystals yield the highest value of

(Jmax' The requirement of small atoms ensures that lighter elements are present and directional bonding implies non-close packed crystal structures. Further, a high elastic modulus implies large binding energy, which, in turn , implies a high melting point and a low coefficient of thermal expansion. Thus, the strongest solids will possess high e lastic moduli, low densities, high melting points and small thermal expansion coefficients.

Ceramic fibres with an ionic character but with predominantly covalent bonding have very high Young's modulus and strength. Compared to metals which contain heavy atoms packed relatively closely because of the non-directional bonding and therefore have high density, ceramics have lower density because they contain relatively lighter atoms. This gives ceramic fibres an advantage over metallic fibres . Polymers and other organic materials have the lowest density because they are composed predominantly of light atoms, mainly carbon and hydrogen, and tend to form linear or two- or three-dimensional networks that pack relatively poorly.

In their sensitivity to a flaw , ceramics, though better than glass, are inferior to metals. In metals, atoms are able to shift positions relatively easily in the vicinity of a flaw and the flaw does not increase in size or propagate. In a typical ceramic, there are strong, highly directional covalent bonds that form a

332 INDIAN J. FIBRE TEXT. RES., SEPTEMBER 2001

network which does not facilitate rearrangement of atoms in the vicinity of a flaw.

The properties of some of the leading reinforcing fibres are shown in Table 2. It is noteworthy that the spectrum of available properties is very large. Further, within each fibre type a number of grades are available.

2.4 Matrix Materials and their Properties

We now consider the matrices used for manufacturing fibre-reinforced composites. The three important matrix materials are polymers, metals and ceramics. While some aspects relating to matrices have been briefly considered earlier, some more specific information on them will now be presented.

Polymers constitute the most important matrix material and are used in more than 95% of the composite products in use today. Amongst the thermosetting resins, organic compounds like general

purpose polyester resins, epoxy resins, vinyl ester resins, phenolics, silicones and urethanes are noteworthy matrix materials \0. The range of thermoplastics is perhaps more impressive and includes polyether ether ketone (PEEK), polyethy­lene, polypropylene, polyethylene terephthalate, polyamides, polyvinyl chloride, polyimides and polycarbonate. In thermosetting polymers, the liquid resins are converted into hard brittle solids by chemical crosslinking which leads to the formation of a tightly bound three-dimensional network of polymer chains. The low viscosity of the thermosetting resins allows ready impregnation of the resin into the fibres . On the other hand, when thermoplastics form the matrix, the composite products can be produced using high speed processes. However, because of the low strength and modulus of polymers in general and thermoplastics in particular, the loads are not transferred very efficiently from one fibre to the other.

Table 2-Properties of some reinforcing fibres

Fibre Specific Young' s Tensile Extension-to- Specific Specific gravity modulus (E) strength break Young' s tensile

(pl GPa ( O"b) % modulus strength GPa (£/p) (O"~p)

GPa GPa

Polymeric fibres Aramids

Kevla?M 49 1.45 125 3.5 2.2 86.21 2.41 Kevlar™ 149 1.47 185 3.4 1.2 125.85 2.31

Aromatic polyester Vectran™ 1.40 70 2.6 3.0 50.00 1.86

Ordered polymer PBO· 1.58 360 5.7 1.8 227.85 3.61

UHMWHDPEb spectra™ 1000 0.97 170 3.0 2.2 175.26 3.51 Inorganic fibres

E-glass 2.58 75 3.5 3.0 29. 10 1.36 Boron 2.55 415 3.5 0.9 162.75 1.37 SiC (CVD)C 3.0 400 3.5 0.44 133.33 1.17 SiC whisker 3.18 480 21.0 4.4 150.94 6.60 Alumina (AI20 3) 3.7 370 1.7 0.5 100.00 0.46 Asbestos 2.2 170 3.5 77.27 1.59

Carbon fibres Pitch-based E-75 2.10 516 3.1 0.56 245.71 1.48 PAN-based HMS-40 1.81 392 3.43 0.87 216.57 1.90

Metallic fibres Tungsten 19.3 360 5.5 18.65 0.28 Stainless steel 18-8 8.0 198 1.0 24.75 0.13

Beryllium 1.85 300 1.8 162.16 0.97

• Poly (p-phenylene benzobisoxazole) b Ultra high-molecular weight high-density polyethylene C Chemical vapour deposition

GUPTA: FIBRE-REINFORCED COMPOS ITES 333

When discontinuous fibres form the reinforcing phase, the fibre length must be as long as possible and the shear modulus large so that load transfer is achieved and the fibre contributes its share to the mechanical properties of the composi te . Polymer resins exploit the properties of continuous filaments very effectively.

Metal matrices have higher temperature resistance than polymeric matrices and are prefen'ed for service temperatures above ISOaC. Because of higher modu lus and strength , they transmit the loads between the fibres better than in the case with polymer matrices and are thus the right matrix material for whi sker-based composite products. The metals that have been extensively used are aluminium, magnesium and metals with special electrical properties like copper and lead. Titanium and nickel attract interest because of their great strength .

Ceramics are strongly bonded materials with ionic and covalent character, which results in high strength and hardness. However, a consequence of thi s is that there is little dislocation movement and the materi al is

brittle. Consequently , the strengths of polycrystalline ceramics are much lower than the theoretical values because of flaws in the material.

In addition to these three classes of matri x material s, carbon is also lIsed as a matrix, mainly with carbon fibre as the reinforcement. The carbon matri x is derived from a pitch, a resin or a carbonaceous gas. Depending on the carbonization/graphiti zati on temperature, the resulting carbon matrix can range from being amorphous to being graphi tic II . Since the carbon matrix develops around the carbon fibre during carbon-carbon composite fabrication, the properties of the carbon matrix alone are not unique, nor are they readi ly avai lable. The propert ies of some matri x materials belonging to the three important classes, viz. polymeric, metallic and ceram iC, are summarised in Table 3.

3 Composite Fabrication Processes

3.1 Some General Features

Composite fabri cation processes in volve a

Table 3-Properties of some matrix material s

Matrix material

Polymeric Thermoset

Epoxy Polyes ter Polyim ide

Thermoplastic Pol ypropy lene Nylon 6.6 Pol ycarbonate PEEK

Metallic Alum inium

Copper

Nickel

Ceramic Silicon nitride

Aluminium oxide

Silicon carbide

' Yield strengt h

Specific grav ity (p)

1.2 1.35 1.46

0.9 1.1 4 1.10 1.30

2.7

8.9

8.9

3.2

3.9

3.2

bYalues in parentheses represent maximum strength

Young's modulus (£)

GPa

4.5 3.5

1.2 2.0 2.3

70

120

2 10

310

380

420

( o m"x)

Tensi le strength (Ob)

GPa

0.08 0.06 0. 12

0.03" 0.07" o.oe 0.09"

0.04" (0.2)b 0.06' (O.4)b 0.07" (O.4)b

0.4 1

0.28

0.3 1

Ex tension-to­break

%

3 2

>300 60 75

Specific Young's modulus

(£/p) GPa

3.75 2.59

1.33 1.75 2.09

25.93

13.48

23 .60

96.86

97.44

13 1.25

Specific tensil e

strength (oJp) GPa

0.07 0.04 0.08

0.03 0.06 0.05 0.07

0.0 1

0.007

0.008

0.31

0.072

0.10

334 INDIAN 1. FIBRE TEXT. RES. , SEPTEMBER 2001

combination of the two or more phases of the composite, viz. the reinforcing fibre and the matrix, usually under heat and pressure, ensuring that the fibres are properly wetted out and laid within the matrix appropriately. The reinforcing fibres are available as continuous filaments or in short lengths. Continuous filaments are available in various forms ranging from monofilaments to multifilament bundles and from undirectional ribbons to woven and non­woven fabrics. Short fibres are generally used in the form of chopped strands. The principal matrix materials are thermosetting resins, which are usually in the form of viscous fluids, or thermoplastics generally available as solid granules. The other matrices are metals and ceramics.

With such a wide spectrum of starting materials, a whole range of processes may be used to fabricate composite products. Though the main emphasis in this article is on composites based on polymeric matrices, a brief summary of the fabrication techniques used for producing composites based on metal, ceramic and carbon matrices is also given.

3.2 Polymer Matrix Composites (PMCs)

PMCs have established themselves as engineering and structural materials and represent a high growth rate area.

The reinforcing fibres are available as filaments, yarns, cords, rovings, tows, woven tapes, woven and knitted two-dimensional and three-dimensional fabrics and single and multi-layer non-woven fabrics . The compos-ite component manufacturing processes either use these as the starting materials or intermediate products which are manufactured mainly by preparatory processes, one of the most important being the fabrication of oriented pre-pregs, which are thin sheets around I mm thick , using continuous rovings, mat or woven fabric and a resin or thermoplastic. The method that is commonly used for pre-preg manufacture is based on the use of epoxy resins which have an advantage over unsaturated polyesters in that they can be parti all y cured or B­staged. The pre-preg may be produced by laying, for example, the tows and the res in between release film s or wax paper which are pressed and ro lled and then partially cured. The final product is made by removing the release film and then either consolidating the stacked up layers of pre-preg sheets by pressure, achi eving the fin al cure under heat and pressure or by winding the pre-preg tape on a winding mandrel in a fil ament winding machine. Most of the

laminated structures are made like this. Thermoplastic pre-pregs are made by hot melt-impregnation with the help of an extruder which is used mainly for semi­crystalline thermoplastics such as PEEK and poly­phenylene sulfide (PPS).

The second preparatory route for intermediate products is mainly designed for unsaturated polyester resins (which cannot be B-staged like epoxies) and therefore cannot be converted to pre-preg sheets. Intermediate products in this case are known as sheet moulding compound (SMC) or dough moulding compound (DMC) . SMC is prepared by mixing resin containing chemical thickening agents and particulate fillers like calcium carbonate containing 15-40% volume fraction of chopped fibres to form a sheet in which the fibres are predominantly parallel to the long axis of the sheet. DMC is made in a similar way but there is more filler and less fibres with the latter having a three-dimensional orientation. The dough is converted into a strong rigid material by hot-pressing, during which alignment of fibres occurs, and curing.

The third preparatory route is for making short glass fibre-reinforced thermoplastics (SGFRTP) which, unlike thermosetting resins , are not cross­linked. They have high molecular weight and are either amorphous or semi-crystalline. In amorphous polymers, entanglements act as physical crosslinks while in semi-crystalline polymers besides entanglements, the crystallites also act as physical crosslinks. The fibre may be used in the form of roving which is coated with the polymer in an extruder using a cable-coating die and after the molten polymer coating has solidified on cooling, the thin extruded rod is chopped into long fibre granules containing 20-40% by wei ght of the fibre . Short fibre granules are produced by first dry- mixing chopped strands of glass fibre (3-6 mm long) with the thermopl as tic granules and then feeding them into an extruder to produce an extrudate wh ich is chopped to give small fibre granules. These glass fibre-containing granules are sold to the processor fo r being converted to useful products by suitable mou lding techniques like injection moulding.

A number of methods are used for prod ucing composite products and will be considered under two broad categories, depending on whether the starting materi al is a fibrou s mass in diffe ren t forms, e.g. roving, tow, tape or fabrics (ca tegory T) , or an intermedi ate product, e.g. pre-pregs, moulding compounds, SFRTP granules (category IT).

GUPTA: FIBRE-REINFORCED COMPOSITES 335

3.2.1 Category I Methods

Hand lay-up: This is the simplest method in which the fibres in the form of non-woven mats are laid onto a mould by hand and the resin is applied by spraying or brushing on, the deposited layers being densified with the help of rollers. The most common fibre used is glass fibre and the resin polyester or epoxy thermo­setting resins. Accelerators and catalysts are fre­quently used. Curing may be done at room tempera­ture or at high temperature. Fishing boats, protective helmets, tanks, doors of buses, helicopter blades, etc. are made by this method.

Pultrusion: Strands of glass, carbon, aramid or other fibres are first impregnated in a resin bath and the resin-impregnated strands are drawn through a die which shapes the extrudate and controls the resin content. The product is then drawn through a curing chamber. Continuous lengths are obtained which can be cut into desired lengths by a saw. I or T-beams, sheets, hollow pipes, etc. can be made by this tech­nique which is an adaptation of the well - established extrusion technique.

Filament winding: This is another versatile tech­nique in which continuous tow or roving of glass, aramid or carbon fibres is first passed through a resin impregnation bath (generally a thermosetting resin) and then with the help of a traverse mechanism wound over a rotating or stationary mandrel of appro­priate design. The winding is discontinued after the desired thickness is achieved and the product may then be cured at room temperature or at a higher tem­perature. Large cylindrical pipes, spherical vessels and bottles for chemical storage, etc. are thus made. Pre-impregnated tapes may also be used as the start­ing material and in this case, winding will be dry.

Resin injection: Fibres in cloth form are placed in the tool, which is then closed. The resin is injected at low pressure into the cavity and flows through the fibres to fill the mould space.

Centrifugal casting: A mixture of fibres in the form of chopped strand and catalysed resin is introduced into a rotating mould, which is like a hollow mandrel, and allowed to cure in situ. Fibre reinforced-plastic (FRP) products of large size can be made by this technique.

Reinforced reaction injection moulding (RRIM): RRIM is an extension of reaction injection moulding (RIM) in which short fibres are added to one or both of the liquid components which are pumped at high speeds and pressures into a mixing head and then into a mould where the components react and polymerize

rapidly. Most RRIM applications are in the automo­ti ve industry, e.g. manufacture of car bonnets.

3.2.2 Category II Methods Bag moulding: B-staged pre-preg plies are laid up

in the desired fibre orientation and in the desired se­quence on a mould plate and the assembly is placed inside an autoclave where a combination of external pressure, vacuum and heat is applied to consolidate and densify the plies as they cure into a solid lami­nate. The vacuum removes air and volatiles, while the pressure consolidates individual layers into a lami­nate. This kind of laminate construction is mostly done by hand and widely used for makjng aircraft and aerospace components.

Compression moulding: Compression moulding is used for transforming SMCs and DMCs into finished products in matched moulds. Parts of complex ge­ometry like road wheels, bumpers and leaf springs are produced in short periods of time.

Injection moulding: Reciprocating-screw injection machines are used to plasticise long fibre and short fibre granules and feed the rruxure of fibres in molten thermoplastic between the two halves of the mould where the component is formed on solidification by cooling.

Extrusion: Single or double screw extruders can be used to produce continuous extrudates using long fi­bre and short fibre granules. Typical products would be rods, tubes, T or I-sections, etc.

3.3 Metal Matrix Composites (MMCs) Metal matrices are sometimes reinforced with high

modulus fibres which are also light. The fibres used in metal matrix composite systems are either metallic or non-oxide cerarruc (SiC, C, Boron) or oxide ceramic (Ah03, glass). The reinforcing fibres, particularly with ionic bonding, have lower surface energies than do the common metals. This mean that the equilibrium contact angle between liquid metals and such bodies is rather high and therefore arrays of whiskers or fibres of a cerarruc are not easily infiltrated by molten metals to make a composite. To overcome this, fibres can be covered by a thin film of metal of much higher melting point than the metal matrix. An additional factor is the possibility of chemical interaction between the fibre and the matrix, particularly at elevated temperatures. Fibre coating may again be helpful here, e.g. when alumjnium is reinforced with boron fibres, the fibre is coated with boron carbide or with silicon carbide.

336 INDIAN 1. FIBRE TEXT. RES .. SEfYfEMBER 2001

The fabrication of MMCs may be achieved in different ways. Some of the methods are briefly described below: • Solid state fabrication can be achieved by a number of techniques, e.g.

(i) Alternate layers of properly spaced boron fibres and aluminium foils or boron fibres and titanium foils are stacked and a combination of heat and pressure in vacuum causes the matrix to flow around the fibres. (ii ) Powder metallurgy techniques can be used with discontinuous fibres or whiskers. Matrix metal pow­der and fibres are mixed and then pressed to consoli­date. This may be followed by sintering to densify the product. (iii) Co-extrusion or drawing can be used to incorpo­rate continuous ductile wires or filaments in a ductile metallic matrix. (iv) Plasma spray or chemical or physical vapour deposition of matrix material on properly laid up fi­bres followed by vacuum hot pressing to consolidate is also used to form components.

• Liquid state fabrication involves the infiltration of continuous fibres by the metal matrix in the liquid form under atmospheric or inert gas pressure or under vacuum. In the case of discontinuous fibres, proper mixing by stirring with the molten metal is done before casting. • Squeeze casting, also called liquid metal forging, is preferred for aluminium alloys that are difficult to cast by conventional methods. A porous fibre pre­form is first prepared, say of discontinuous alumina (Saffil) fibres, and inserted into the die located on the bed of a hydraulic press. Molten metal (aluminium) is poured into the pre-heated die. At a pressure of 70-100 MPa, the molten aluminium penetrates the fibre preform and bonds the fibres , solidification of the matrix being completed under pressure, followed by ejection of the component.

• In-situ or eutectic composites are fabricated by controlled undirectional solidification of a eutectic alloy with the other phase in fibrous form distributed in the matrix. Slow freezing rates are used.

3.4 Ceramic Matrix Composites (CMCs)

More recently , CMCs, in which carbon, alumina, silicon carbide and metallic fibres are incorporated in glass, glass-ceramic and oxide ceramic matrices, are gai ning importance.

Amongst the techniques used for incorporation of fibres in a ceramic material , slurry infiltration with

hot pressing is perhaps the most common technique. In this, a fibre is drawn through a slurry of ceramic particles. The particles coat the fibre and the impregnated yarn is wound onto a mandrel to form undirectional tapes which can be dried, cut, laminated and hot pressed to produce the ceramic composites. Hot pressing is the most common and perhaps one of the most expensive techniques used to consolidate CMCs and the products so made are of high quality . However, the impregnation of the slurries into 3-dimensional preforms is limited by the filtering action of the preforms and thus poses limitations on the thickness of the specimen that can be impregnated.

Other techniques used to produce CMCs include sol-gel , polymer pyrolysis, melt infiltration and in-situ chemical reaction 12.

3.5 Carbon Matrix Composites The reinforcing fibre first passes through a resin

which is later converted to carbon matrix through heat treatment. The temperatures of treatment may be up to 2800°C.

4 Properties of Composites

4.1 PMCs

The typical mechanical properties of some PMC laminae containing unidirectional continuous fila­ments are shown in Table 4 and for some PMCs containing fibrous forms other than undirectional filaments are shown in Table 5. Following important conclusions can be drawn from an examination of these tables: • The mechanical properties and the coefficient of thermal expansion of the aligned laminae are very anisotropic. This can be attributed to the anisotropic properties of the reinforcing fibre; the matrix generally being isotropic. The mechanical properties and thermal expansion parallel to the fibres are predominantly controlled by the fibre properties while the properties perpendicular to the fibre axis show the predominance of matrix properties. Laminates of polymer matrix composites made by stacking of appropriately oriented plies show anisotropic characteristics, the degree of anisotropy being related to the stacking scheme. • The mechanical properties of the reinforcing fibre in the fibre direction are clearly reflected in the mechanical properties of the aligned ply in the fibre direction with carbon and boron fibres giving the ma"imum reinforcement followed by Kevlar 49 and glass fibres.

GUPTA: FIBRE-REINFORCED COMPOSITES 337

Table 4-Some properties of PMC laminae containing unidirectional continuous filaments as reinforcement (fibre content given in volume fraction, Va

Composite Specific Young's Tensile Elongation- Specific Specific gravity modulus (E) strength to-break Young 's tensile

(p) GPa ( crb) % modulus strength GPa (E/p) (crJp)

GPa Gpa Epoxy resin

Carbon fibre (Vr 0.60)

0° 1.62 220 1.4 0.8 135 0.865 90° 1.62 7 0.038 0.6

Kevlar 49 fibre (Vr O.6O)

0° 1.38 76 1.38 55.07 1.00 90° 1.38 5.6 0.030 4.06 0.022

Boron fibre (Vr O.6O)

0° 2.10 2 15 104 102.38 0.67 90° 2.10 24.2 0.063 11 .52 0.03

E-glass fibre (Vr O.6O)

0° 2.00 40 0.780 20.00 0.39 90° 2.00 10 0.D28 5.00 0.014

Polyester resin Glass fibre (Vr O.6O)

0° 1.93 38 0.750 1.8 19.69 0.39 90° 1.93 10 0.022 0.2 5.18 0.01

Ther moplastic (PEEK) Carbon fibre (Vr O.6O)

0° 1.56 123 2.2a 1.78 78.85 1.41

aFlexural strength

Table 5-Some properties of PMC laminae containing fibrous forms other than unidirectional filaments

Composite laminae Specific Young's Tensile Elongation-to- Specific Specific gravity (p) modulus (E) strength (crb) break Young's tensile

GPa GPa % modulus strength (E/p) (crJp) GPa GPa

Discontinuous fibr e E-glass fibre-ny lon 66 1.47 14 0.207 2.2 9.5 0.141 (Yo I. fraction 0.25) ( I. 14)a (2)" (0.07) a (60) a (1.8) a (0.061) a

Chopped strand mat E-glass fibre epoxy resin lAO 7 0.10 5.0 0.07 (Yol. fraction 0.20)

Planar Rendom fibres E-glass fibre-polyester resin 1.55 8.5 0.11 2 5.5 0.071 (Yol. fraction 0.20)

Bidirectional cloth E-glass-epoxy resin (Yol. fraction 0.35)

0° 1.7 16.5 0.28 9.71 0.016 90° I.7 16.5 0.28 9.71 0.016

aYalues in parentheses are for matrix on ly

~38 INDIAN J. FIBRE TEXT. RES., SEPTEMBER 2001

• The matrix also affects the properties of the composite, e.g. a composite based on carbon fibre and epoxy resin shows much lower elongation-to-break than the one based on carbon fibre and PEEK. • In PMCs other than unidirectional laminae, the reinforcement from random fibres or fabrics or chopped strand mat or discontinuous fibres is considerably less than that from continuous fibres. For example, when polyester resin is reinforced by 0.2 volume fraction of aligned discontinuous glass fibres, the modulus parallel to the fibre direction is 17.4 GPa while the perpendicular modulus is 5.4 G.Pa. For the same volume fraction, the planar random fibres give a modulus of 8.5 GPa.

4.2 MMCs

The desirability of high strength for structural applications is obvious. In compression or flexural loading of beams as in a plane, rocket or truck, the weight and stiffness of a member are also very important. The problem was considered in the previous section where it was shown that it is advantageous to reinforce metals with fibres which have high modulus and which are also light.

Broadly speaking, the metal matrices have been reinforced with metallic non-oxide ceramic, oxide ceramic and carbon fibres. Generally, MMCs consist of a ductile matrix reinforced with strong brittle fibres with a low density of critical flaws . Continuous fibres strongly bonded to the matrix for effective load transfer are often used. The crack deflecting role of the ductile matrix requires maximum inter-fibre spacing for a given volume fraction. The copper­tungsten composite shown earlier in Fig. I illustrates this . The properties of two representative MMCs are shown in Table 6.

4.3 CMCs The development of composites based on ceramic

matrices has received a considerable boost in the recent past, particularly with ceramic fibres as the reinforcement, a result of better understanding of how the toughness of the composite can be augmented by appropriate consideration of the following four factors: matrix microcracking at interfaces, load transfer from matrix to reinforcing fibre where matrix fails, crack deflection and matrix pull out. The most effective toughening is that produced by continuous fibre reinforced system where the interfacial bond is relatively weak. The combination provides materials of high toughness operating at temperatures of 1800°C. The properties of some CMCs are shown in Table 7.

4.4 Carbon-Matrix Composites

Carbon fibre reinforced carbon matrix composites have low density and very high resistance to thermal shock. These characteristics combined with their high strength, toughness, and thermal conductivity and almost zero thermal expansion make them ideal materials for reentry thermal protection of space shuttle and the spacecrafts, rocket nozzles and aircraft brakes.

The mechanical properties of three carbon-carbon compsoites l2

.13 are shown in Table 8.

4.5 General

Various aspects of the properties of fibre­reinforced composites have been considered by a number of other authors I4

.17

•

5 Applications of Composites

5.1 PMCs

PMCs constitute bu lk of the composi te applications

Table ~Representati ve properties of some MMCs

Composite and direction Specific gravity Young' s Maximum Specific Specific (p) modulus (E) stress ( ama,) Young 's maximum stress

GPa GPa modulus (E/p) ( at!p) GPa GPa

Boron fibres in aluminium (Vol. fraction 0.5)

0° 2.65 210 1.5 79.25 0.57 90° 2.65 150 0.14 56.60 0.053

Silicon carbide fibres in aluminium (Vol.-fraction 0.5)

0° 2.84 310 0.25 109. 15 0.09 900 2.84 0.105 0.04

GUPTA: FIBRE-REINFORCED COMPOSITES 339

Table 7-Representative properties of some CMCs

Composite

Carbon fibre reinforced glass matrix (unidirectional, Vr 0.5) SiC fibres in lithium alumino silicate (LAS) (unidirectional, Vr 0.5)

0° 90°

aValues in parentheses are for matrix only bValue in KJm,2 eValue in MPa min

Sp. gravity Young's modulus (E)

(p) GPa

193 (60)"

2.5 2.5

Flexural strength Work of fracture GPa

0.70 5.0b

(O.IO)a (O.OO4)a

0.6 17e

0.38 lOe

Table 8-Mechanical properties of three carbon-carbon composites with pitch-based (A), PAN-based (B) and rayon-based (C) carbon fibres and of petroleum coke graphite (G)

[Heat treatment temperature for all samples, 3000°C]

Property A

Specific gravity 1.68 Young's modulus, GPa 13.5 Vickers hardnessa, MPa 135 Bending strength, MPa 65 .7 Tensile strength, Mpa

RT 35.7 800°C 43.4 I 600°C 42.0 2400°C 62.7

Fracture toughness, MPa min RT 2.96 800°C 2.82 I 600°C 4.64 2400°C 5.30

aLoad, 5 kg

and may be considered under the following four heads: • Products with moderate properties for general use where large numbers are to be produced at an affordable cost and high productivity . Glass fibres dominate this area with polyester and epoxy resins as the two matrices. Glass fibre reinforced polymers are extensively used to produce appliance casings, covers and fans in automobiles, sporting goods, window and door frames , fishing boats, helmets, etc. Glass fibre reinforced polyester resin is used for making process and effluent pipes, tanks and vessels and also structural shapes by pultrusion. Tennis racket handles, pole-vaulting pole and rowing oars are made from carbon fibre-epoxy system. • Products similar to those in above category but for specialised uses where performance is more important than cost, e,g. racing boats and racing yatchs for which the reinforcing fibre would be Kevlar rather than glass fibre. Similarly, for golf clubs and tennis

B

1.77 26.3 163

96.9

55.4 65.4 50.4 83 .0

3.44 3.58 6.75 12.9

C

1.57 17.0

68 88 102 III

4.0 5.5 6.1 7 .0

G

1.76 10.5 172 39.6

28 30 37 44

0 .8 0 .8 1.0 1.9

rackets in the competition category , boron-epoxy composites have been used. • Various components for small planes, helicopters and large civilian planes, where a combination of high performance and lightness may be of prime importance, Kevlar fibre and S-2 glass fibre and Kevlar fibre are used respectively for aircraft doors, fairings and radomes and for floorings and storage bins in civilian aircraft. Kevlar fibre is extensively used along with carbon fibres for load bearing components of small planes and helicopters. A number of components in the tail section and other parts of large civilian aircraft are made from carbon fibre-reinforced epoxy resin . Machinery items such as turbine, compressor and windmill blades, fly wheel and moving parts in textile machinery are made from carbon fibre composites. • Military aircraft and aerospace component make the most severe demand on performance while productivity and cost are secondary. These include

340 INDIAN 1. FIBRE TEXT. RES., SEPTEMBER 2001

rocket engine cases, tail section of military aircraft, cargo bay doors and booster rocket casings in the space shuttles. Carbon fibre reinforced resins are generally used in these applications.

5.2 MMCs

Compared to PMCs, the MMCs have higher temperature capability, greater thermal and electrical conductivity, greater transverse tensile strength, greater shear and compressive strength and no problem of flammability and moisture sorption.

Most of the current applications of MMCs are in the automotive and space-related areas. One of the first MMCs to be developed was boron fibre reinforced aluminium matrix with high elastic modulus, high tensile strength and compressive strength and low density, designed for use as tubular struss members in space shuttle orbiter. The two composite systems that have been used as turbine blades are SiC-coated boron fibre-reinforced aluminium matrix and carbon fibre-reinforced magnesium or magnesium alloy matrix . Aluminium matrix reinforced with SiC whiskers has enhanced stiffness and wear resistance with higher use temperature and is useful for automobile components like pistons which are superior and cheaper than conventional pistons. Glass fibre reinforced lead provides a superior material for battery plates. Tungsten fibre reinforced copper matrix is an example of excellent combination of strength, stiffness and thermal and electrical conductivity and is the right material for liners for cryogenic rocket engines. Nickel fibre reinforced stainless steel is used for making rocket engines.

5.3 CMCs

One of the most promising applications for CMCs is in the area of cutting tools where hot pressed SiC fibre-reinforced alumina matrix is replacing the metallic carbide cutting tools. Another area is in heat engines where components requiring resistance to aggressive environments can be made from CMCs.

5.4 Carbon-Carbon Composite

Carbon-carbon composites find use as aircraft brakes, protective tiles for re-entry vehicles, rocket motor nozzles, hip replacement, biomedical implant, tools and dies, engines pistons and heat pipes.

6 Conclusions A wide range of fibre-reinforced composites has

been described. The advantages of the fibrous shape for reinforcement and of optimum adhesion between the fibre and the matrix to give a light, stiff, strong and tough composite have been given. The desirable properties of the reinforcing fibre, matrix and composite for various applications are described and the selection of the right fibre and matrix and suitable fabrication processes for various applications are also described.

Acknowledgement The author is indebted to Prof. A. Kelly of the

University of Cambridge, England, for the photograph reproduced in Fig. 1

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