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CHAPTER 35

ENGINEERING PROPERTIES

OF COMPOSITES

Keith T. Kedward

INTRODUCTION

Composite materials are simply a combination of two or more different materials

that may provide superior and unique mechanical and physical properties. The most

attractive composite systems effectively combine the most desirable properties of

their constituents and simultaneously suppress the least desirable properties. For

example, a glass-fiber reinforced plastic combines the high strength of thin glass

fibers with the ductility and environmental resistance of an epoxy resin; the inherent

damage susceptibility of the fiber surface is thereby suppressed whereas the low

stiffness and strength of the resin is enhanced.

The opportunity to develop superior products for aerospace, automotive, and

recreational applications has sustained the interest in advanced composites. Currently

composites are being considered on a broader basis, specifically, for applications that

include civil engineering structures such as bridges and freeway pillar reinforcement,

and for biomedical products such as prosthetic devices. The recent trend toward

affordable composite structures with a somewhat decreased emphasis on performance

will have a major impact on the wider exploitation of composites in engineering.

BASIC TYPES OF COMPOSITES

Composites typically comprise a high-strength synthetic fiber embedded within a

protective matrix. The most mature and widely used composite systems are polymer

matrix composites (PMCs), which will provide the major focus for this chapter. Con-

temporary PMCs typically use a ceramic type of reinforcing fiber such as carbon,

Kevlar, or glass in a resin matrix wherein the fibers make up approximately 60 per-

cent of the PMC volume. Metal or ceramic matrices can be substituted for the resin

matrix to provide a higher-temperature capability. These specialized systems are

termed metal matrix composites (MMCs) and ceramic matrix composites (CMCs); a

35.1

35.2

CHAPTER THIRTY-FIVE

TABLE 35.1

Composite Design Comparisons

PMC

CMC

MMC

Specific strength

Generally excellent if

Highest potential for

Moderately high for

and stiffness

exclusively unidirectional

high-temperature

dominantly axial loads and

reinforcement is avoided

applications

intermediate temperatures

Fatigue

Excellent for designs that

Good for high-

Potential concern for other

characteristics

avoid out-of-plane loads

temperature

than dominantly axial

applications loads

Nonlinear

Usually not important

Significant effect after

Can be significant,

effects

for continuous fiber

first matrix and

particularly for

reinforcements

interface cracks have

multidirectional

developed

and off-axis loads

Temperature

Less than 600

Potential for maximum

capability

values between 1000

values up to 1000

and 2000

Degree of

Extreme, particularly

Can develop signifi-

Not usually a major issue

anisotropy

considering out-of-plane

cantly during loading,

where interface effects

properties and conse-

due to matrix and

are negligible

quent coupling effects

interface breakdown

in minimum-gage

configurations

general qualitative comparison of the relative merits of all three categories is sum-

marized in Table 35.1.

SHORT FIBER/PARTICULATE COMPOSITES

The fibrous reinforcing constituent of composites may consist of thin continuous

fibers or relatively short fiber segments, or whiskers. However, reinforcing effective-

ness is realized by using segments of relatively high aspect ratio, which is defined as

the length-to-diameter ratio. Nevertheless, as a reinforcement for PMCs, these short

fiber or whisker systems are structurally less efficient and very susceptible to dam-

age from long-term and/or cyclic loading. On the other hand, the substantially lower

cost and reduced anisotropy on the macroscopic scale render these composite sys-

tems appropriate in structurally less demanding industrial applications.

Randomly oriented short fiber or particulate-reinforced composites tend to

exhibit a much higher dependence on polymer-based matrix properties, as com-

pared to typical continuous fiber reinforced PMCs. Elastic modulus, strength, creep,

and fatigue are most susceptible to the significant limitations of the polymer matrix

constituent and fiber-matrix interface properties. 1

CONTINUOUS FIBER COMPOSITES

Continuous fiber reinforcements are generally required for structural or high-

performance applications. The specific strength (strength-to-density ratio) and spe-

cific stiffness (elastic modulus-to-density ratio) of continuous fiber reinforced PMCs,

for example, can be vastly superior to conventional metal alloys, as illustrated in Fig.

35.1. These types of composite can also be designed to provide other attractive prop-

erties, such as high thermal or electrical conductivity and low coefficient of thermal

expansion (CTE). In addition, depending on how the fibers are oriented or inter-

35.3

ENGINEERING PROPERTIES OF COMPOSITES

SPECIFIC TENSILE MODULUS (cm × 10 6 )

2 4 6 8 10 12 14 16 18 20

GLASS/

EPOXY

KEVLAR/EPOXY

INTERMEDIATE-MODULUS

CARBON/EPOXY

BORON/EPOXY

HIGH-MODULUS

CARBON/EPOXY

TITANIUM

STEEL

ALUMINUM

BERYLLIUM

ULTRAHIGH-

MODULUS GRAPHITE/EPOXY

0 0

MAGNESIUM

10 8 )

(ELASTIC MODULUS-TO-DENSITY RATIO)

SPECIFIC TENSILE MODULUS (in.

FIGURE 35.1

A weight-efficiency comparison.

woven within the matrix, these composites can be tailored to provide the desired

structural properties for a specific structural component. Anisotropy is a term used

to define such a material that can exhibit properties varying with direction. Thus

designing for, and with, anisotropy is a unique aspect of contemporary composites in

that the design engineer must simultaneously design the structure and the material

of construction. Of course, anisotropy brings problems as well as unique opportuni-

ties, as is discussed in a later section. With reference to Fig. 35.1, it should be appre-

ciated that the vertical bars representing the conventional metals signify the

potential variation in specific strength that may be brought about by changes in alloy

constituents and heat treatment. The angled bars for the continuous fiber compos-

ites represent the range of specific properties from the unidirectional, all 0° fiber ori-

entation at the upper end to the pseudo-isotropic laminate with equal proportions of

fibers in the 0°, +45°, −45°, and 90° orientations at the lower end. In the case of the

composites, the variations between the upper or lower ends of the bars are achieved

by tailoring in the form of laminate design.

SPECIAL DESIGN ISSUES AND OPPORTUNITIES

Product design that involves the utilization of composites is most likely to be effective

when the aspects of materials, structures, and dynamics technologies are embraced in

35.4

CHAPTER THIRTY-FIVE

the process of the development of mechanical systems. One illustrative example was

cited in the introductory chapter of this handbook (see Chap. 1), which introduces the

technique of reducing the vibration response of a fan blade by alteration of the natu-

ral frequency. In the design of composite fan blades for aircraft, this approach has been

achieved by tailoring the frequency and the associated mode shape. 2 Such a tailoring

capability can assist the designer in adjusting flexural and torsional vibration and

fatigue responses, as well as the damping characteristics explained later.

A more challenging issue that frequently arises in composite hardware design for

a majority of the more geometrically complex products is the potential impact of the

low secondary or matrix-influenced properties of these strongly nonisotropic mate-

rial forms. The transverse (in-plane) tensile strength of the unidirectional composite

laminate is merely a few percent of the longitudinal tensile strength (as observed

from Tables 35.2 and 35.3). Consequently, it is of no surprise that the through-

thickness or short-transverse tensile strength of a multidirectional laminate is of the

same order, but even lower than the transverse tensile strength of the individual lay-

ers. Thus, the importance of the designer’s awareness of such limitations cannot be

overemphasized. In fact, the large majority of the failures in composite hardware

development testing has arisen due to underestimated or unrecognized out-of-plane

loading effects and interrelated regions of structural joints and attachments. Due to

the many common adverse experiences with delaminations induced by out-of-plane

TABLE 35.2

Properties of Typical Continuous, Fiber-Reinforced Composites and Structural Metals

Unidirectional composite

(60% fiber/40% resin, by volume)

Metals

E-glass/

Kevlar/

carbon/

UHM Gr./

7075-T6

4130

Property

resin

epoxy

aluminum

steel

Elastic

Density, lb/in. 3

0.070 (1.9)

0.047 (1.3)

0.058 (1.6)

0.060 (1.7)

0.100 (2.77)

0.284 (7.86)

(10 3 kg/m 3 )

E L ,10 6 lb/in. 2

(10 3 MPa)

6.5 (45)

11.0 (75.8)

19.5 (134)

40.0 (276)

10.3 (71.0)

30.0 (207)

E T ,10 6 lb/in. 2

(10 3 MPa)

1.8 (12)

1.0 (6.9)

1.5 (10)

1.2 (8.3)

10.3 (71.0)

30.0 (207)

G LT ,10 6 lb/in. 2

(10 3 MPa)

0.7 (4.8)

0.4 (2.8)

0.9 (6.2)

0.65 (4.5)

4.0 (27.6)

12.0 (82.7)

ν LT

0.32

0.33

0.30

0.28

0.30

0.28

Strength

F tu ,10 3 lb/in. 2

(MPa)

180 (1240)

220 (1520)

200 (1380)

100 (689)

79 (545)

100 (689)

F tu ,10 3 lb/in. 2

(MPa)

6 (41)

4.5 (31)

7 (48)

5 (34)

77 (531)

100 (689)

F cu ,10 3 lb/in. 2

(MPa)

120 (827)

45 (310)

170 (1170)

90 (620)

70 (483)

130 (896)

F cu ,10 3 lb/in. 2

(MPa)

20 (138)

70 (483)

130 (896)

F s LT ,10 3 lb/in. 2

(MPa)

8 (55)

4 (28)

10 (69)

9 (62)

47 (324)

60 (414)

35.5

ENGINEERING PROPERTIES OF COMPOSITES

TABLE 35.3

Typical Unidirectional Properties for a Carbon/Epoxy System

Stiffness properties

Strength properties

Thermal properties

E L ,10 6 lb/in. 2

F tu ,10 3 lb/in. 2

20.0

240.0

α L ,

µε

−

0.3

(10 3 MPa)

(138)

(MPa)

(1650)

(µε/K)

(−0.54)

E T ,10 6 lb/in. 2

1.4

F cu ,10 3 lb/in. 2

200.0

α T , µε/°F

17.0

(10 3 MPa)

(9.6)

(MPa)

(1380)

(

µε

/K)

(30.6)

G LT ,10 6 lb/in. 2

F tu ,10 3 lb/in. 2

K L , Btu in./h ft 2

0.8

7.0

40.0

(10 3 MPa)

(5.5)

(MPa)

(48)

(W/m K)

(5.76)

ν LT

0.28

F tu ,10 3 lb/in. 2

20.0

K T , Btu in./h ft 2

°F

4.5

(MPa)

(138)

(W/m K)

(0.65)

F isu

LT ,10 3 lb/in. 2

10.0

(MPa)

(69)

F isu ,10 3 lb/in. 2

ν LT / E L =ν TL / E T

9.0

(MPa)

(62)

load components, this section will be devoted to the identification of the numerous

sources of out-of-plane load development and the candidate approaches to elimi-

nate or minimize their influence.

First, a general overview of many of the common problems created for the engi-

neering designer that are consequences of low-matrix-dominated, elastic, and

strength properties are summarized in Table 35.4. Several of the most common

sources will now be discussed in more detail. Figure 35.2 illustrates these major

sources, which may be broadly categorized as follows:

Category A: Curved sections including curved segments, rings, hollow cylinders,

and spherical vessels that are representative of angle bracket design details,

curved frames, and internally or externally pressurized vessels.

TABLE 35.4 General Overview of Problems Created by the Low Secondary (Matrix-

Dominated) Properties of Advanced Composites

Controlling

property

Problem

F isu

Failure induced by shear in beams under flexural loading.

Premature torsional failures.

Premature crippling failure in compression.*

Failure of adherends in structural bonded joints.*

Failure of laminae due to free-edge effects, e.g., cutouts, ply drops.*

F tu

Failure induced by transverse tensile fracture of curved beams in flexure.

Shock waves during normal impacts.

G LT

Reduction in flexural and torsional stiffness.

Reduction in resonant frequencies of plate and beam members.

Reduction of elastic buckling capability.

Interpretation of experimental stress analysis data.

α T

Distortion at fillets due to high expansion coefficient (through-thickness).

T F tu

Failure due to thermal stresses in thick-walled composite cylinders.

*For these problems, the controlling properties are both F isu and F t T .

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