Askeland D.R., Fulay P.P. Essentials of Materials Science & Engineering

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Fiber Properties In most ﬁber-reinforced composites, the ﬁbers are strong, sti¤, and

lightweight. If the composite is to be used at elevated temperatures, the ﬁber should also

have a high melting temperature. Thus the speciﬁc strength and speciﬁc modulus of the

ﬁber are important characteristics:

Speciﬁc strength ¼

ð17-11Þ

Speciﬁc modulus ¼

ð17-12Þ

where TS is the tensile strength, r is the density, and E is the modulus of elasticity.

Properties of typical ﬁbers are shown in Table 17-2 and Figure 17-14. Note in

Table 17-2, the density is in g/cm

. The highest speciﬁc modulus is usually found in

materials having a low atomic number and covalent bonding, such as carbon and

boron. These two elements also have a high strength and melting temperature.

Figure 17-13

A three-dimensional weave for

fiber-reinforced composites.

Specific strength (3 10

cm)

Specific modulus (3 10

cm)

2.5 5 7.5 10 12.5 15 17.5

Figure 17-14

Comparison of the specific strength

and specific modulus of fibers

versus metals and polymers.

C HA P T E R 1 7 Composites: Teamwork and Synergy in Materials560

Aramid ﬁbers, of which Kevlar

is the best known example, are aromatic poly-

amide polymers strengthened by a backbone containing benzene rings (Figure 17-15)

and are examples of liquid-crystalline polymers in that the polymer chains are rod-

like and very sti¤. Specially prepared polyethylene ﬁbers are also available. Both the

aramid and polyethylene ﬁbers have excellent strength and sti¤ness but are limited to

low-temperature use. Because of their lower density, polyethylene ﬁbers have superior

speciﬁc strength and speciﬁc modulus.

TABLE 17-2 9 Properties of selected reinforcing materials*

Tensile Modulus of Melting Specific Specific

Density Strength Elasticity Temperature Modulus Strength

Material (g/cm

) (MPa) (D10

MPa) (

C) (D10

cm) (D10

cm)

Polymers:

Kevlar

1.44 4480 124 500 86.8 31.3

Nylon 1.14 83 3.5 249 2.5 7.3

Polyethylene 0.97 21–48 0.3–0.7 147 17.8 34.3

Metals:

Be composites 1.83 276–345 303 1277 193.8 7.0

Boron 2.36 3450 379 2030 161.8 11.8

W 19.40 4000 407 3410 21.3 2.0

Glass:

E-glass 2.55 3450 72.4 <1725 28.5 14.0

S-glass 2.50 4480 86.9 <1725 35.0 18.0

Carbon:

HS (high strength) 1.75 5650 276 3700 158.8 32.5

HM (high modulus) 1.90 1860 531 3700 280.0 9.8

Ceramics:

3.95 2070 379 2015 97.0 5.3

C 2.36 2280 483 2450 206.0 9.8

SiC 3.00 3930 483 2700 118.3 13.3

ZrO

4.84 2070 345 2677 71.5 4.3

Whiskers:

3.96 20700 428 1982 108.5 52.5

Cr 7.20 8900 241 1890 33.5 12.3

Graphite 1.66 20700 703 3700 425.0 125.5

SiC 3.18 20700 483 2700 152.0 65.5

3.18 13800 379 119.5 43.8

Figure 17-15

The structure of

Kevlar

. The fibers are

joined by secondary

bonds between oxygen

and hydrogen atoms on

adjoining chains.

17-4 Chara cteristics of Fiber-Reinforced Composites 561

Ceramic ﬁbers and whiskers, including alumina, glass, and silicon carbide, are strong

and sti¤. Glass ﬁbers, which are the most commonly used, include pure silica, S-glass

(25% Al

, 100% MgO, balance SiO

), and E-glass (18% CaO, 15% Al

, balance

SiO

). Although they are considerably denser than the polymer ﬁbers, the ceramics can

be used at much higher temperatures. Beryllium and tungsten, although metallically

bonded, have a high modulus that makes them attractive ﬁber materials for certain ap-

plications. The following example discusses issues related to designing with composites.

EXAMPLE 17-9

Design of an Aerospace Composite

We are now using a 7075-T6 aluminum alloy (modulus of elasticity of

69  10

MPa to make a 227 kg panel on a commercial aircraft. Experience

has shown that each kg reduction in weight on the aircraft reduces the fuel

consumption by 4200 litres each year. Design a material for the panel that will

reduce weight, yet maintain the same speciﬁc modulus, and will be economical

over a 10-year lifetime of the aircraft.

SOLUTION

There are many possib le materials that might be used to provide a weight

savings. As an example, let’s consider using a boron ﬁber-reinforced Al-Li al-

loy in the T6 condition. Both the boron ﬁber and the lithium alloying addition

increase the modulus of elasticity; the boron and the Al-Li alloy also have

densities less than that of typical aluminum alloys.

The speciﬁc modulus of the current 7075-T6 alloy is:

Speciﬁc modulus ¼

ð69  10

MPaÞ

2:7



¼ 2:61  10

The density of the boron ﬁbers is approximately 2.36 g/cm

and that of a

typical Al-Li alloy is approximately 2.5 g/cm

. If we use 0.6 volume fraction

boron ﬁbers in the composite, then the density, modulus of elasticity, and spe-

ciﬁc modulus of the composite are:

¼ð0:6Þð2:36Þþð0:4Þð2:5Þ¼2:416 g=cm

¼ð0:6Þð379  10

MPaÞþð0:4Þð76  10

MPaÞ¼258  10

MPa

Speciﬁc modulus ¼

258  10

MPa

2:416 g=cm

¼ 10:9  10

If the speciﬁc modulus is the only factor inﬂuencing the design of the

component, the thickness of the part might be reduced by 75%, giving a com-

ponent weight of 56.8 kg rather than 227 kg. The weight savings would then be

170.2 kg, or (4200 litres/kg)(170.2 kg) ¼ 714,840 litres per year. At about $.50

per litre, about $357,000 in fuel savings could be realized each year, or $3.57

million over the 10-year aircraft lifetime.

C HA P T E R 1 7 Composites: Teamwork and Synergy in Materials562

This is certainly an optimistic comparison, since strength or fabrication

factors may not permit the part to be made as thin as suggested. In addition,

the high cost of boron ﬁbers (over $660/kg) and higher manufacturing costs

of the composite compared with those of 7075 aluminum would reduce cost

savings. As mentioned before, Boeing has used 50% carbon-ﬁber-reinforced

plastic in its latest 787 Dreamliner airplane to achieve 20% increase in fuel

e‰ciency.

Matrix Properties The matrix supports the ﬁbers and keeps them in the proper position,

transfers the load to the strong ﬁbers, protects the ﬁbers from damage during manu-

facture and use of the composite, and prevents cracks in the ﬁber from propagating

throughout the entire composite. The matrix usually provides the major control over

electrical properties, chemical behavior, and elevated-temperature use of the composite.

Polymer matrices are particularly common. Most polymer materials—both ther-

moplastics and thermosets—are available in short glass ﬁber-reinforced grades. These

composites are formed into useful shapes by the processes described in Chapter 16.

Sheet-molding compounds (SMCs) and bulk-molding compounds (BMCs) are typical

of this type of composite. Thermosetting aromatic polyimides are used for somewhat

higher temperature applications.

Metal-matrix composites include aluminum, magnesium, copper, nickel, and inter-

metallic compound alloys reinforced with ceramic and metal ﬁbers. A variety of aero-

space and automotive applications are satisﬁed by the MMCs. The metal matrix

permits the composite to operate at high temperatures, but producing the composite is

often more di‰cult and expensive than producing the polymer-matrix materials.

The ceramic-matrix composites (CMCs) have good properties at elevated temper-

atures and are lighter in weight than the high-temperature metal-matrix composites. In

a later section, we discuss how to develop toughness in CMCs.

Bonding and Failure Particularly in polymer and metal-matrix composites, good

bonding must be obtained between the various constituents. The ﬁbers must be ﬁrmly

bonded to the matrix material if the load is to be properly transmitted from the matrix

to the ﬁbers. In addition, the ﬁbers may pull out of the matrix during loading, reducing

the strength and fracture resistance of the composite if bonding is poor. Figure 17-16 on

the next page illustrates poor bonding of carbon ﬁbers in a copper matrix. In some

cases, special coatings may be used to improve bonding. Glass ﬁbers are coated with a

silane coupling or ‘‘keying’’ agent (called sizing) to improve bonding and moisture re-

sistance in ﬁberglass composites. Carbon ﬁbers are similarly coated with an organic

material to improve bonding. Boron ﬁbers have been coated with silicon carbide or

boron nitride to improve bonding with an aluminum matrix; in fact, these ﬁbers have

been called Borsic ﬁbers to reﬂect the presence of the silicon carbide (SiC) coating.

Another property that must be considered when combining ﬁbers into a matrix is

the similarity between the coe‰cients of thermal expansion for the two materials. If the

ﬁber expands and contracts at a rate much di¤erent from that of the matrix, ﬁbers may

break or bonding can be disrupted, causing premature failure.

In many composites, individual plies or layers of fabric are joined. Bondi ng

between these layers must also be good or another problem—delamination—may occur.

Delamination has been suspected to be a cause in some accidents involving airplanes

using composite-based structures. The layers may tear apart under load and cause fail-

ure. Using composites with a three-dimensional weave will help prevent delamination.

17-4 Chara cteristics of Fiber-Reinforced Composites 563

17-5 Manufacturing Fibers and Composites

Producing a ﬁber-reinforced composite involves several steps, including producing the

ﬁbers, arranging the ﬁbers into bundles or fabrics, and introducing the ﬁbers into the

matrix.

Making the Fiber Metallic ﬁbers, glass ﬁbers, and many polymer ﬁbers (including

nylon, aramid, and polyacrylonitrile) can be formed by drawing processes, as described

in Chapter 8 (wire drawing of metal) and Chapter 16 (using the spinnerette for polymer

ﬁbers).

Boron, carbon, and ceramics are too brittle and reactive to be worked by conven-

tional drawing processes. Boron ﬁber is produced by chemical vapor deposition (CVD)

[Figure 17-17(a)]. A very ﬁne, heated tungsten ﬁlament is used as a substrate, passing

through a seal into a heated chamber. Vaporized boron compounds such as BCl

are

introduced into the chamber, decompose, and permit boron to precipitate onto the

tungsten wire (Figure 17-18). SiC ﬁbers are made in a similar manner, with carbon

ﬁbers as the substrate for the vapor deposition of silicon carbide.

Figure 17-16 Scanning electron micrograph of the fracture surface of a silver-copper alloy

reinforced with carbon fibers. Poor bonding causes much of the fracture surface to follow the

interface between the metal matrix and the carbon tows (3000). (From Metals Handbook,

American Society for Metals, Vol. 9, 9th Ed., 1985.)

C HA P T E R 1 7 Composites: Teamwork and Synergy in Materials564

Carbon ﬁbers are made by carbonizing, or pyrolizing, an organic ﬁlament, which is

more easily drawn or spun into thin, continuous lengths [Figure 17-17(b)]. The organic

ﬁlament, known as a precursor, is often rayon (a cellulosic polymer), polyacrylonitrile

(PAN), or pitch (various aromatic organic compounds). High temperatures decompose

the organic polymer, driving o¤ all of the elements but carbon. As the carbonizing

temperature increases from 1000



C to 3000



C, the tensile strength decreases while the

modulus of elasticity increases (Figure 17-19 on the next page). Drawing the carbon

ﬁlaments at critical times during carbonizing may produce desirable preferred ori-

entations in the ﬁnal carbon ﬁlament.

Whiskers are single crystals with aspect ratios of 20 to 1000. Because the whiskers

contain no mobile dislocations, slip cannot occur and they have exceptionally high

strengths. Because of the complex processing required to produce whiskers, their cost

may be quite high.

Figure 17-17 Methods for producing (a) boron and (b) carbon fibers.

Figure 17-18 Photomicrographs of two fiber-reinforced composites: (a) In Borsic fiber-

reinforced aluminum, the fibers are composed of a thick layer of boron deposited on a small-

diameter tungsten filament (1000). (From Metals Handbook, American Society for Metals,

Vol. 9, 9th Ed., 1985.) (b) In this microstructure of a ceramic-fiber–ceramic-matrix composite,

silicon carbide fibers are used to reinforce a silicon nitride matrix. The SiC fiber is vapor-

deposited on a small carbon precursor filament (125). (Courtesy of Dr. R.T. Bhatt, NASA

Lewis Research Center.)

17-5 Manufacturing Fibers and Composites 565

Arranging the Fibers Exceptio nally ﬁne ﬁlaments are bundled together as rovings,

yarns, or tows. In yarns, as many as 10,000 ﬁlaments are twisted together to produce

the ﬁber. A tow contains a few hundred to more than 100,000 untwisted ﬁlaments

(Figure 17-20). Rovings are untwisted bundles of ﬁlaments, yarns, or tows.

Often, ﬁbers are chopped into short lengths of 1 cm or less. These ﬁbers, also called

staples, are easily incorporated into the matrix and are typical of the sheet-molding and

bulk-molding compounds for polymer-matrix composites. The ﬁbers often are present

in the composite in a random orientation.

Long or continuous ﬁbers for polymer-matrix composites can be processed into

mats or fabrics. Mats contain non-woven, randomly oriented ﬁbers loosely held

together by a polymer resin. The ﬁbers can also be woven, braided, or knitted into two-

dimensional or three-dimensional fabrics. The fabrics are then impregnated with a pol-

ymer resin. The resins at this point in the processing have not yet been completely

polymerized; these mats or fabrics are called prepregs.

When unidirectionally aligned ﬁbers are to be introduced into a polymer matrix,

tapes may be produced. Individual ﬁbers can be unwound from spools onto a mandrel,

which determines the spacing of the individual ﬁbers, and prepregged with a polymer

resin. These tapes, only one ﬁber diameter thick, may be up to 120 cm wide. Figure 17-21

illustrates that tapes can also be produced by covering the ﬁbers with upper and lower

layers of metal foil that are then joined by di¤usion bonding.

Figure 17-19

The effect of heat-treatment

temperature on the strength and

modulus of elasticity of carbon

fibers.

Figure 17-20

A scanning electron micrograph of a carbon tow

containing many individual carbon filaments

(200).

C HA P T E R 1 7 Composites: Teamwork and Synergy in Materials566

Producing the Composite A variety of methods for producing composite parts are

used, depending on the application and materials. Short ﬁber-reinforced composites are

normally formed by mixing the ﬁbers with a liquid or plastic matrix, then using rela-

tively conventional techniques such as injection molding for polymer-base composites

or casting for metal-matrix composites. Polymer matrix composites can also be pro-

duced by a spray-up method, in which short ﬁbers mixed with a resin are sprayed

against a form and cured.

Special techniques, however, have been devised for producing composites using

continuous ﬁbers, either in unidirectionally aligned, mat, or fabric form (Figure 17-22).

In hand lay-up techniques, the tapes, mats, or fabrics are placed against a form, satu-

rated with a polymer resin, rolled to assure good contact and freedom from porosity,

and ﬁnally cured. Fiberglass car and truck bodies might be made in this manner, which

is generally slow and labor intensive.

Tapes and fabrics can also be placed in a die and formed by bag molding. High-

pressure gases or a vacuum are introduced to force the individual plies together so that

good bonding is achieved during curing. Large polymer matrix components for the

skins of military aircraft have been produced by these techniques. In matched die

Figure 17-21

Production of fiber tapes by

encasing fibers between metal cover

sheets by diffusion bonding.

Figure 17-22 Producing composite shapes in dies by (a) hand lay-up, (b) pressure bag

molding, and (c) matched die molding.

17-5 Manufacturing Fibers and Composites 567

molding, short ﬁbers or mats are placed into a two-part die; when the die is closed, the

composite shape is formed.

Filament wind ing is used to produce products such as pressure tanks and rocket

motor castings (Figure 17-23). Fibers are wrapped around a form or mandre l to grad-

ually build up a hollow shape that may be even several feet in thick ness. The ﬁlament

can be dipped in the polymer-matrix resin prior to winding, or the resin can be im-

pregnated around the ﬁber during or after winding. Curing completes the production of

the composite part.

Pultrusion is used to form a simple-shaped product with a constant cross section,

such as round, rectangular, pipe, plate, or sheet shapes (Figure 17-24). Fibers or mats

are drawn from spools, passed through a polymer resin bath for impregnation, and

gathered together to produce a particular shape before entering a heated die for curing.

Curing of the resin is accomplished almost immediately, so a continuous product is

produced. The pultruded stock can subsequently be formed into somewhat more com-

plicated shapes, such as ﬁshing poles, golf club shafts, and ski poles.

Metal-matrix composites with continuous ﬁbers are more di‰cult to produce than

are the polymer-matrix composites. Casting processes that force liquid around the ﬁbers

using capillary rise, pressure casting, vacuum inﬁltration, or continuous casting are

used. Various solid-state compaction processes can also be used.

17-6 Fiber-Reinforced Systems and Applications

Before completing our discussion of ﬁber-reinforced composites, let’s look at the

behavior and applications of some of the most common of these materials. Figure 17-25

compares the speciﬁc modulus and speciﬁc strength of several composites with those of

Figure 17-23

Producing composite shapes by filament winding.

Figure 17-24 Producing composite shapes by pultrusion.

C HA P T E R 1 7 Composites: Teamwork and Synergy in Materials568

metals and polymers. Note that the values in this ﬁgure are lower than those in Fig-

ure 17-14, since we are now looking at the composite, not just the ﬁber.

Advanced Composites The term advanced composites is often used when the compo-

site is intended to provide service in very critical applications, as in the aerospace

industry (Table 17-3). The advanced composites normally are polymer–matrix compo-

sites reinforced with high-strength polymer, metal, or ceramic ﬁbers. Carbon ﬁbers are

used extensively where particularly good sti¤ness is required; aramid—and, to an even

greater extent, polyethylene—ﬁbers are better suited to high-strength applications in

which toughness and damage resistance are more important. Unfortunately, the poly-

mer ﬁbers lose their strength at relatively low temperatures, as do all of the polymer

matrices (Figure 17-26 on the next page).

The advanced composites are also frequently used for sporting goods. Tennis

rackets, golf clubs, skis, ski poles, and ﬁshing poles often contain carbon or aramid ﬁ-

bers because the higher sti¤ness provides better performance. In the case of golf clubs,

carbon ﬁbers allow less weight in the shaft and therefore more weight in the head.

Fabric reinforced with polyethylene ﬁbers is used for lightweight sail s for racing yachts.

A unique application for aramid ﬁber composites is armor. Tough Kevlar

com-

posites provide better ballistic protection than other materials, making them suitable

for lightweight, ﬂexible bulletproof clothing.

Specific strength (3 10

cm)

Specific modulus (3 10

cm)

2.5

2.5 5 7.5 10

Figure 17-25 A comparison of the specific modulus and specific strength of several composite

materials with those of metals and polymers.

TABLE 17-3 9 Examples of fiber-reinforced materials and applications

Material Applications

Borsic aluminum Fan blades in engines, other aircraft and aerospace applications

Kevlar

-epoxy and

Kevlar

-polyester

Aircraft, aerospace applications (including space shuttle), boat hulls, sporting

goods (including tennis rackets, golf club shafts, fishing rods), flak jackets

Graphite-polymer Aerospace and automotive applications, sporting goods

Glass-polymer Lightweight automotive applications, water and marine applications, corrosion-

resistant applications, sporting goods equipment, aircraft and aerospace

components

17-6 Fiber-Reinforced Systems and Applications 569

Askeland D.R., Fulay P.P. Essentials of Materials Science &amp; Engineering

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Askeland D.R., Fulay P.P. Essentials of Materials Science & Engineering