30079_09a.pdf

CHAPTER 9

COMPOSITE MATERIALS AND

MECHANICAL DESIGN

Carl Zweben

Lockheed Martin Missiles and Space—Valley Forge Operations

King of Prussia, Pennsylvania

9.1 INTRODUCTION

131

9.5.3

Electronic Packaging and

Thermal Control

9.1.1 Classes and Characteristics

of Composite Materials

168

132

9.5.4

Internal Combustion

Engines

9.1.2 Comparative Properties of

Composite Materials 133

9.1.3 Manufacturing Considerations 136

168

9.5.5

Transportation

170

9.5.6

Process Industries, High-

Temperature Applications,

and Wear-, Corrosion-,

and Oxidation-Resistant

Equipment

9.2 REINFORCEMENTS AND

MATRIX MATERIALS

136

176

9.2.1 Reinforcements

137

9.2.2 Matrix Materials

139

9.5.7

Offshore and Onshore Oil

Exploration and Production

Equipment

178

9.3 PROPERTIES OF COMPOSITE

MATERIALS

9.5.8

Dimensionally Stable

Devices

143

9.3.1 Mechanical Properties of

Composite Materials

178

144

9.5.9

Biomedical Applications

179

9.3.2 Physical Properties of

Composite Materials

9.5.10 Sports and Leisure

Equipment

153

180

9.5.11 Marine Structures

182

9.4 PROCESSES

161

9.5.12 Miscellaneous Applications

182

9.4.1 Polymer Matrix Composites

163

9.4.2 Metal Matrix Composites

163

9.6 DESIGNANDANALYSIS

184

9.4.3 Ceramic Matrix Composites

163

9.6.1 Polymer Matrix Composites

185

9.4.4 Carbon/Carbon Composites

163

9.6.2 Metal Matrix Composites

187

9.6.3 Ceramic Matrix Composites

187

9.6.4 Carbon/Carbon Composites

187

9.5 APPLICATIONS

163

9.5.1 Aerospace and Defense

164

9.5.2 Machine Components

166

9.1 INTRODUCTION

The development of composite materials and related design and manufacturing technologies is one

of the most important advances in the history of materials. Composites are multifunctional materials

having unprecedented mechanical and physical properties that can be tailored to meet the require-

ments of a particular application. Many composites also exhibit great resistance to high-temperature

corrosion and oxidation and wear. These unique characteristics provide the mechanical engineer with

design opportunities not possible with conventional monolithic (unreinforced) materials. Composites

technology also makes possible the use of an entire class of solid materials, ceramics, in applications

for which monolithic versions are unsuited because of their great strength scatter and poor resistance

to mechanical and thermal shock. Further, many manufacturing processes for composites are well

adapted to the fabrication of large, complex structures, which allows consolidation of parts, reducing

manufacturing costs.

Mechanical Engineers' Handbook, 2nd ed., Edited by Myer Kutz.

Composites are important materials that are now used widely, not only in the aerospace industry,

but also in a large and increasing number of commercial mechanical engineering applications, such

as internal combustion engines; machine components; thermal control and electronic packaging; au-

tomobile, train, and aircraft structures and mechanical components, such as brakes, drive shafts,

flywheels, tanks, and pressure vessels; dimensionally stable components; process industries equipment

requiring resistance to high-temperature corrosion, oxidation, and wear; offshore and onshore oil

exploration and production; marine structures; sports and leisure equipment; and biomedical devices.

It should be noted that biological structural materials occurring in nature are typically some type

of composite. Common examples are wood, bamboo, bone, teeth, and shell. Further, use of artificial

composite materials is not new. Straw-reinforced mud bricks were employed in biblical times. Using

modern terminology, discussed later, this material would be classified as an organic fiber-reinforced

ceramic matrix composite.

In this chapter, we consider the properties of reinforcements and matrix materials (Section 9.2),

properties of composites (Section 9.3), how they are made (Section 9.4), their use in mechanical

engineering applications (Section 9.5), and special design considerations for composites (Section 9.6).

9.1.1 Classes and Characteristics of Composite Materials

There is no universally accepted definition of a composite material. For the purpose of this work, we

consider a composite to be a material consisting of two or more distinct phases, bonded together. 1

Solid materials can be divided into four categories: polymers, metals, ceramics, and carbon, which

we consider as a separate class because of its unique characteristics. We find both reinforcements

and matrix materials in all four categories. This gives us the ability to create a limitless number of

new material systems with unique properties that cannot be obtained with any single monolithic

material. Table 9.1 shows the types of material combinations now in use.

Composites are usually classified by the type of material used for the matrix. The four primary

categories of composites are polymer matrix composites (PMCs), metal matrix composites (MMCs),

ceramic matrix composites (CMCs), and carbon/carbon composites (CCCs). At this time, PMCs are

the most widely used class of composites. However, there are important applications of the other

types, which are indicative of their great potential in mechanical engineering applications.

Figure 9.1 shows the main types of reinforcements used in composite materials: aligned contin-

uous fibers, discontinuous fibers, whiskers (elongated single crystals), particles, and numerous forms

of fibrous architectures produced by textile technology, such as fabrics and braids. Increasingly,

designers are using hybrid composites that combine different types of reinforcements to achieve more

efficiency and to reduce cost.

A common way to represent fiber-reinforced composites is to show the fiber and matrix separated

by a slash. For example, carbon fiber-reinforced epoxy is typically written "carbon/epoxy," or,

"C/Ep." We represent particle reinforcements by enclosing them in parentheses followed by "p";

thus, silicon carbide (SiC) particle-reinforced aluminum appears as "(SiC)p/Al."

Composites are strongly heterogeneous materials; that is, the properties of a composite vary

considerably from point to point in the material, depending on which material phase the point is

located in. Monolithic ceramics and metallic alloys are usually considered to be homogeneous ma-

terials, to a first approximation.

Many artificial composites, especially those reinforced with fibers, are anisotropic, which means

their properties vary with direction (the properties of isotropic materials are the same in every direc-

tion). This is a characteristic they share with a widely used natural fibrous composite, wood. As for

wood, when structures made from artificial fibrous composites are required to carry load in more

than one direction, they are used in laminated form.

Many fiber-reinforced composites, especially PMCs, MMCs, and CCCs, do not display plastic

behavior as metals do, which makes them more sensitive to stress concentrations. However, the

absence of plastic deformation does not mean that composites are brittle materials like monolithic

ceramics. The heterogeneous nature of composites results in complex failure mechanisms that im-

part toughness. Fiber-reinforced materials have been found to produce durable, reliable structural

components in countless applications. The unique characteristics of composite materials, especially

anisotropy, require the use of special design methods, which are discussed in Section 9.6.

Table 9.1 Types of Composite Materials

Matrix

Reinforcement

Polymer

Metal

Ceramic

Carbon

Polymer

Metal

Ceramic

Carbon

Fig. 9.1 Reinforcement forms.

9.1.2 Comparative Properties of Composite Materials

There are a large and increasing number of materials that fall in each of the four types of composites,

making generalization difficult. However, as a class of materials, composites tend to have the follow-

ing characteristics: high strength; high modulus; low density; excellent resistance to fatigue, creep,

creep rupture, corrosion, and wear; and low coefficient of thermal expansion (CTE). As for monolithic

materials, each of the four classes of composites has its own particular attributes. For example, CMCs

tend to have particularly good resistance to corrosion, oxidation, and wear, along with high-

temperature capability.

F 7 Or applications in which both mechanical properties and low weight are important, useful figures

of merit are specific strength (strength divided by specific gravity or density) and specific stiffness

(stiffness divided by specific gravity or density). Figure 9.2 presents specific stiffness and specific

tensile strength of conventional structural metals (steel, titanium, aluminum, magnesium, and beryl-

lium), two engineering ceramics (silicon nitride and alumina), and selected composite materials. The

composites are PMCs reinforced with selected continuous fibers—carbon, aramid, E-glass, and

boron—and an MMC, aluminum containing silicon carbide particles. Also shown is beryl-

lium-aluminum, which can be considered a type of metal matrix composite, rather than an alloy,

because the mutual solubility of the constituents at room temperature is low.

The carbon fibers represented in Figure 9.2 are made from several types of precursor materials:

polyacrilonitrile (PAN), petroleum pitch, and coal tar pitch. Characteristics of the two types of pitch-

based fibers tend to be similar but very different from those made from PAN. Several types of carbon

fibers are represented: standard-modulus (SM) PAN, ultrahigh-strength (UHS) PAN, ultrahigh-

modulus (UHM) PAN, and ultrahigh-modulus (UHM) pitch. These fibers are discussed in Section

9.2. It should be noted that there are dozens of different kinds of commercial carbon fibers, and new

ones are continually being developed.

Because the properties of fiber-reinforced composites depend strongly on fiber orientation, fiber-

reinforced polymers are represented by lines. The upper end corresponds to the axial properties of a

unidirectional laminate, in which all the fibers are aligned in one direction. The lower end represents

a quasi-isotropic laminate having equal stiffness and approximately equal strength characteristics in

all directions in the plane of the fibers.

As Figure 9.2 shows, composites offer order-of-magnitude improvements over metals in both

specific strength and stiffness. It has been observed that order-of-magnitude improvements in key

properties typically produce revolutionary effects in a technology. Consequently, it is not surprising

that composites are having such a dramatic influence in engineering applications.

In addition to their exceptional static strength properties, fiber-reinforced polymers also have

excellent resistance to fatigue loading. Figure 9.3 shows how the number of cycles to failure (N)

varies with maximum stress (S) for aluminum and selected unidirectional PMCs subjected to tension-

tension fatigue. The ratio of minimum stress to maximum stress (R) is 0.1. The composites consist

of epoxy matrices reinforced with key fibers: aramid, boron, SM carbon, high-strength (HS) glass,

and E-glass. Because of their excellent fatigue resistance, composites have largely replaced metals

Specific Modulus (MPa)

Fig. 9.2 Specific tensile strength (tensile strength divided by density) as a function of

specific modulus (modulus divided by density) of composite materials and monolithic

metals and ceramics.

in fatigue-critical aerospace applications, such as helicopter rotor blades. Composites also are being

used in commercial fatigue-critical applications, such as automobile springs (see Section 9.5).

The outstanding mechanical properties of composite materials have been a key reason for their

extensive use in structures. However, composites also have important physical properties, especially

low, tailorable CTE and high-thermal conductivity, that are key reasons for their selection in an

increasing number of applications.

Many composites, such as PMCs reinforced with carbon and aramid fibers, and silicon carbide

particle-reinforced aluminum, have low CTEs, which are advantageous in applications requiring di-

mensional stability. By appropriate selection of reinforcements and matrix materials, it is possible to

produce composites with near-zero CTEs.

Coefficient of thermal expansion tailorability provides a way to minimize thermal stresses and

distortions that often arise when dissimilar materials are joined. For example, Figure 9.4 shows how

the CTE of silicon carbide particle-reinforced aluminum varies with particle content. By varying the

Number of Cycles to Failure, K

Fig. 9.3 Number of cycles to failure as a function of maximum stress for aluminum and

unidirectional polymer matrix composites subjected to tension-tension fatigue with a stress

ratio, R = 0.1 (from Ref. 2).

amount of reinforcement, it is possible to match the CTEs of a variety of key engineering materials,

such as steel, titanium, and alumina (aluminum oxide).

The ability to tailor CTE is particularly important in applications such as electronic packaging,

where thermal stresses can cause failure of ceramic substrates, semiconductors, and solder joints.

Another unique and increasingly important property of some composites is their exceptionally

high-thermal conductivity. This is leading to increasing use of composites in applications for which

heat dissipation is a key design consideration. In addition, the low densities of composites make them

Particle Volume Content (%)

Fig. 9.4 Variation of coefficient of thermal expansion with particle volume fraction for silicon

carbide particle-reinforced aluminum (from Ref. 3).

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