30079_09b.pdf

9.4.1 Polymer Matrix Composites

There are a large and increasing number of processes for making PMC parts. Many are not very

labor-intensive and can make near-net shape components. For thermoplastic matrices reinforced with

discontinuous fibers, one of the most widely used processes is injection molding. However, as dis-

cussed in Section 9.3, the stiffness and strength of resulting parts are relatively low. This section

focuses on processes for making composites with continuous fibers.

Many PMC processes combine fibers and matrices directly. However, a number use an interme-

diate material called a prepreg, which stands for preimpregnated material, consisting of fibers em-

bedded in a thermoplastic or partially cured thermoset matrix. The most common forms of prepreg

are unidirectional tapes and impregnated tows and fabrics.

Material consolidation is commonly achieved by application of heat and pressure. For thermo-

setting resins, consolidation involves a complex physical-chemical process, which is accelerated by

subjecting the material to elevated temperature. However, some resins undergo cure at room temper-

ature. Another way to cure resins without temperature is by use of electron bombardment. As part

of the consolidation process, uncured laminates are often placed in an evacuated bag, called a vacuum

bag, which applies atmospheric pressure when evacuated. The vacuum-bagged assembly is typically

cured in an oven or autoclave. The latter also applies pressure significantly above the atmospheric

level.

PMC parts are usually shaped by use of molds made from a variety of materials: steel, aluminum,

bulk graphite, and also PMCs reinforced with E-glass and carbon fibers. Sometimes molds with

embedded heaters are used.

The key processes for making PMC parts are filament winding, fiber placement, compression

molding, pultrusion, prepreg lay-up, resin film infusion and resin transfer molding. The latter process

uses a fiber preform which is placed in a mold.

9.4.2 Metal Matrix Composites

An important consideration in selection of manufacturing processes for MMCs is that reinforcements

and matrices can react at elevated temperatures, degrading material properties. To overcome this

problem, reinforcements are often coated with barrier materials. Many of the processes for making

MMCs with continuous fiber reinforcements are very expensive. However, considerable effort has

been devoted to development of relatively inexpensive processes that can make net shape or near-net

shape parts that require little or no machining to achieve their final configuration.

Manufacturing processes for MMCs are based on a variety of approaches for combining constit-

uents and consolidating the resulting material: powder metallurgy, ingot metallurgy, plasma spraying,

chemical vapor deposition, physical vapor deposition, electrochemical plating, diffusion bonding, hot

pressing, remelt casting, pressureless casting, and pressure casting. The last two processes use

preforms.

Some MMCs are made by in situ reaction. For example, a composite consisting of aluminum

reinforced with titanium carbide particles has been made by introducing a gas containing carbon into

a molten alloy containing aluminum and titanium.

9.4.3 Ceramic Matrix Composites

As for MMCs, an important consideration in fabrication of CMCs is that reinforcements and matrices

can react at high temperatures. An additional issue is that ceramics are very difficult to machine, so

that it is desirable to fabricate parts that are close to their final shape. A number of CMC processes

have this feature. In addition, some processes make it possible to fabricate CMC parts that would be

difficult or impossible to create out of monolithic ceramics.

Key processes for CMCs include chemical vapor infiltration (CVI); infiltration of preforms with

slurries, sol-gels, and molten ceramics; in situ chemical reaction; sintering; hot pressing; and hot

isostatic processing. Another process infiltrates preforms with selected polymers that are then py-

rolyzed to form a ceramic material.

9.4.4 Carbon/Carbon Composites

CCCs are primarily made by chemical vapor infiltration (CVI), also called chemical vapor deposition

(CVD), and by infiltration of pitch or various resins. Following infiltration, the material is pyrolyzed,

which removes most non-carbonaceous elements. This process is repeated several times until the

desired material density is achieved.

9.5 APPLICATIONS

Composites are now being used in a large and increasing number of important mechanical engineering

applications. In this section, we discuss some of the more significant current and emerging appli-

cations.

It is generally known that glass fiber-reinforced polymer (GFRP) composites have been used

extensively as engineering materials for decades. The most widely recognized applications are prob-

ably boats, electrical equipment, and automobile and truck body components. It is generally known,

for example, that the Corvette body is made of fiberglass and has been for many years. However,

many materials that are actually composites, but are not recognized as such, also have been used for

a long time in mechanical engineering applications. One example is cermets, which are ceramic

particles bound together with metals; hence the name. These materials fall in the category of metal

matrix composites. Cemented carbides are one type of cermet. What are commonly called "tungsten

carbide" cutting tools and dies are, in most cases, not made of monolithic tungsten carbide, which

is too brittle for many applications. Instead, they are actually MMCs consisting of tungsten carbide

particles embedded in a high-temperature metallic matrix such as cobalt. The composite has a much

higher fracture toughness than monolithic tungsten carbide.

Another example of unrecognized composites are industrial circuit breaker contact pads, made of

silver reinforced with tungsten carbide particles, which impart hardness and wear resistance (Fig.

9.10). The silver provides electrical conductivity. This MMC is a good illustration of an application

for which a new multifunctional material was developed to meet requirements for a combination of

physical and mechanical properties.

In this section, we consider representative examples of composite usage in mechanical engineering

applications, including aerospace and defense; electronic packaging and thermal control; machine

components; internal combustion engines; transportation; process industries, high temperature and

wear, corrosion and oxidation-resistant equipment; offshore and onshore oil exploration and produc-

tion equipment; dimensionally stable components; biomedical applications; sports and leisure equip-

ment; marine structures and miscellaneous applications. Use of composites is now so extensive that

it is impossible to present a complete list. Instead, we have selected applications that, for the most

part, are commercially successful and illustrate the potential for composite materials in various aspects

of mechanical engineering.

9.5.1 Aerospace and Defense

Composites are baseline materials in a wide range of aerospace and defense structural applications,

including military and commercial aircraft, spacecraft, and missiles. They are also used in aircraft

gas turbine engine components, propellers, and helicopter rotors. Aircraft brakes are covered in

another subsection.

PMCs are the workhorse materials for most aerospace and defense applications. Standard modulus

and intermediate modulus carbon fibers are the leading reinforcements, followed by aramid and glass.

Boron fibers are used in some of the original composite aircraft structures and special applications

requiring high compressive strength. For low-temperature airframe and other applications, epoxies

are the key matrix resin. For higher temperatures, bismaleimides, polyimides, and phenolics are

employed. Thermoplastic resins increasingly are finding their way into new applications.

The key properties of composites that have led to their use in aircraft structures are high specific

stiffness and strength and excellent fatigue resistance. For example, composites have largely replaced

Fig. 9.10 Commercial circuit breaker uses tungsten carbide particle-reinforced

silver contact pads.

monolithic aluminum in helicopter rotors because they extend fatigue life by factors of up to six

times those of metallic designs.

The amount of composites used in aircraft structures varies by type of aircraft and the time at

which they were developed. The B-2 "Stealth" Bomber makes extensive use of carbon fiber-

reinforced PMCs (Fig. 9.11).

In general, aircraft that take off and land vertically (VTOL aircraft), such as helicopters and tilt

wing vehicles, use the highest percentage of composites in their structures. For all practical purposes,

most new VTOL aircraft have all-composite structures. The V-22 Osprey uses PMCs reinforced with

carbon, aramid, and glass fibers in the fuselage, wings, empennage (tail section) and rotors (Fig.

9.12).

Use of composites in commercial passenger aircraft is limited by practical manufacturing problems

in making very large structures and by cost. Still, use of composites has increased steadily. For

example, the Boeing 777 has an all-composite empennage.

Fig. 9.11 The B-2 "Stealth" Bomber airframe makes extensive use of carbon fiber-reinforced

polymer matrix composites (courtesy Northrop Grumman).

Fig. 9.12 The V-22 Osprey uses polymer matrix composites in the fuselage, wings, empen-

nage, and rotors (courtesy Boeing).

Thrust-to-weight ratio is an important figure of merit for aircraft gas turbine engines and other

propulsion systems. Because of this, there has been considerable work devoted to the development

of a variety of composite components. Production applications include carbon fiber-reinforced pol-

ymer fan blades, exit guide vanes, and nacelle components; silicon carbide particle-reinforced alu-

minum exit guide vanes; and CMC engine flaps made of silicon carbide reinforced with carbon and

with silicon carbide fibers.

There has been extensive development of MMCs with titanium and titanium aluminide matrices

reinforced with silicon carbide fibers aimed at high-temperature engine and fuselage structures. Com-

posites using intermetallic materials, such as titanium aluminide, are often called intermetallic matrix

composites (IMCs).

The key design requirements for spacecraft structures are high specific stiffness and low thermal

distortion, along with high specific strength for those components that see high loads during launch.

The key reinforcements are high-stiffness PAN- and pitch-based carbon fibers. Figure 9.13 shows

the NASA Upper Atmosphere Research Satellite structure, which is made of high-modulus PAN

carbon/epoxy. For most spacecraft, thermal control is also an important design consideration, due in

large part to the absence of convection as a cooling mechanism in space. Because of this, there is

increasing interest in thermally conductive materials, including PMCs reinforced with ultrahigh-

modulus pitch-based carbon fibers for structural components such as radiators, and for electronic

packaging. MMCs are also being used for thermal control and electronic packaging applications. See

Section 9.5.3 for a more detailed discussion of these applications.

The Space Shuttle Orbiters use boron fiber-reinforced aluminum struts in their center fuselage

sections and CCC nose caps and wing leading edges.

The Hubble Space Telescope high-gain antenna masts, which also function as wave guides, are

made of an MMC consisting of ultrahigh-modulus pitch-based carbon fibers in an aluminum matrix.

Missiles, especially those with solid rocket motors, have used PMCs for many years. In fact,

high-strength glass was originally developed for this application. As for most aerospace applications,

epoxies are the most common matrix resins. Over the years, new fibers with increasingly higher

specific strengths—first aramid, then ultrahigh-strength carbon—have displaced glass in high-

performance applications. However, high-strength glass is still used in a wide variety of related

applications, such as launch tubes for shoulder-fired anti-tank rockets.

Carbon/carbon composites are widely used in rocket nozzle throat inserts.

9.5.2 Machine Components

Composites increasingly are being used in machine components because they reduce mass and ther-

mal distortion and have excellent resistance to corrosion and fatigue.

Fig. 9.13 The Upper Atmosphere Research Satellite structure is composed of lightweight high-

modulus carbon fiber-reinforced epoxy struts, which provide high stiffness and strength and low

coefficient of thermal expansion.

One of the most successful applications has been in rollers and shafts used in machines that

handle rolls of paper, thin plastic film, fiber products, and audio tape. Figure 9.14 shows a chromium-

plated carbon fiber-reinforced epoxy roller used in production of audio tape. The low rotary inertia

of the composite part allows it to start and stop more quickly than the baseline metal design. This

reduces the amount of defective tape resulting from differential slippage between roller and tape.

Rollers as long as 10.7 m (35 ft) and 0.43 m (17 in.) in diameter have been produced. In these

applications, use of carbon fiber-reinforced polymers has resulted in reported mass reductions of 30%

to 60%. This enables some shafts to be handled by one person instead of two (Fig. 9.15). It also

reduces shaft rotary inertia, which, as for the audio machine roller discussed in the previous paragraph,

allows machines to be stopped more quickly without damaging the plastic or paper. The higher critical

speeds of composite shafts also allow them to be operated at higher speeds. In addition, the high

stiffness of composite shafts reduces lateral displacement under load. PMC rollers can be coated with

a variety of materials, including metals and elastomers.

PMCs also have been used in translating parts, such as tubes used to remove plastic parts from

injection molding machines. In another application, use of a carbon fiber-reinforced epoxy robotic

arm in a computer cartridge-retrieval system doubled the cartridge-exchange rate compared to the

original aluminum design.

Specific strength is an important figure of merit for materials used in flywheels. Composites have

received considerable attention for this reason (Fig. 9.16). Another advantage of composites is that

their modes of failure tend to be less catastrophic than for metal designs. The latter, when they fail,

often liberate large pieces of high-velocity, shrapnel-like jagged metal that are dangerous and difficult

to contain.

The high specific stiffness and low coefficient of thermal expansion (CTE) of silicon carbide

particle-reinforced aluminum has led to its use in machine parts for which low vibration, mass, and

thermal distortion are important, such as photolithography stages (Fig. 9.17). The absence of out-

gassing is another advantage of MMC components.

Figure 9.18 shows a developmental actuator housing made of silicon carbide particle-reinforced

aluminum. Properties of interest here are high specific stiffness and yield strength. In addition, com-

pared to monolithic aluminum, the composite offers a closer CTE match to steel than monolithic

aluminum, and better wear resistance.

The excellent hardness, wear resistance, and smooth surface of a silicon carbide whisker-

reinforced alumina CMC resulted in the adoption of this material for use in beverage can-forming

equipment. Here, we find a CMC replacing what is in fact a metal matrix composite; a cemented

carbide or cermet, consisting of tungsten carbide particles in a cobalt binder.

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