Kutz M. Handbook of materials selection

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BIBLIOGRAPHY 1127

approach to nanophase ceramics materials are the simple processing steps, the

ﬂexibility of solution chemistry, the low-temperature treatments, and small in-

vestment in equipment.

When considering where sol–gel materials are used, most of the applications

are substitutions of a sol–gel material for a ceramic obtainable by other means.

Nevertheless, there are some applications that are unique to sol–gel processing,

such as porous materials for membranes or inﬁltration and organic–inorganic

hybrids.

4 SUMMARY AND PROJECTIONS

In the end, the choice of what method to use depends on many factors. Accord-

ing to the SP

paradigm, performance is the ultimate test of synthesis and proc-

essing. Therefore, choices are made on the basis of how easily the process can

be optimized for chemical properties or physical properties. Other considerations

are the availability of the appropriate raw materials or precursors and the avail-

ability of the processing equipment. The advanced processing schemes in many

cases require high-temperature furnaces or reactive atmospheres. Difﬁculty gain-

ing access to this equipment or conditions may prohibit the use of an advanced

process. Similarly, problems of scale-up in size or number of components may

make a conventional process more straightforward than an advanced process.

Finally, time and cost are factors in the decision whether or not to use an ad-

vanced process. Nevertheless, there continue to be signs of progress in all of the

advanced processes described, with some, such as microwave processing, in

more widespread use than others, such as layered manufacturing. As with any

new technology, change is slow, but competition from other materials is a strong

incentive for advanced processing of ceramics.

Acknowledgments

Some of the ideas for this chapter come from the Materials Science Summer

Institute (MASSI) ‘‘Intelligent Manufacturing of Nanostructured Ceramics,’’

June, 1996, sponsored by Rutgers University, Princeton University, and the Uni-

versity of Pennsylvania, and funded by NSF. My thanks go to the MASSI ‘‘stu-

dents’’ and co-organizer, Rik Riman, for outstanding ideas and discussions.

BIBLIOGRAPHY

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Allen, S. M., and E. L. Thomas, The Structure of Materials, Wiley, New York, 1999.

Bandyopadhyay, A., R. K. Panda, V. F. Janas, M. K. Agarwala, S. C. Danforth, and A. Safari, J. Am.

Ceram. Soc., 80, 1366 (1997).

Barsoum, M. W., Fundamentals of Ceramics, McGraw-Hill, New York, 1997.

Blum, Y. D., K. B. Schwartz, and R. M. Laine, J. Mater. Sci., 24, 1707 (1989).

Brinker, C. J., and G. W. Scherer, Sol–Gel Science, Academic, Boston, 1990.

Brinker, C. J., Y. Lu, A. Sellinger, and H. Fan, Adv. Mater., 11, 579 (1999).

Clark, D. E., and W. H. Sutton, Ann. Rev. Mat. Sci., 26, 299 (1996).

Gelabert, M. C., B. L. Gersten, and R. E. Riman, J. Crystal Growth, 211, 497 (2000).

Gilman, P. S., and J. S. Benjamin, Ann. Rev. Mat. Sci., 13, 279 (1983).

Gleiter, H., Acta Mater., 48, 1 (2000).

Glumac, N. G., G. Skandan, Y. J. Chen, and B. H. Kear, Nanostructured Materials, 12, 253 (1999).

Greil, P., J. Am. Ceram. Soc., 78, 835 (1995).

Hahn, H., Nanostructured Materials, 2, 29 (1993).

1128 ADVANCED CERAMICS PROCESSING

Hahn, H., Nanostructured Materials, 9, 3 (1997).

Holt, J. B., and S. D. Dunmead, Ann. Rev. Mat. Sci., 21, 305 (1991).

Katz, J. D., Ann. Rev. Mat. Sci., 22, 153 (1992).

Klein, L. C. (ed.), Sol–Gel Optics, Processing and Applications, Kluwer Academic, Boston, 1994.

Klein, L. C. (ed.), Sol–Gel Technology for Thin Films, Fibers, Preforms, Electronics and Specialty

Shapes, Noyes Publications, Park Ridge, NJ, 1988.

Lange, F. F., J. Am. Ceram. Soc., 72, 3 (1989).

Lee, S. E., J. M. Xue, D. M. Wan, and J. Wang, Acta Mater., 47, 2633 (1999).

Lewis, J. A., J. Am. Ceram. Soc., 83, 2341 (2000).

McCormick, P. G., and F. H. Froes, JOM, 11, 61 (1998).

Munir, Z. A., Am. Ceram. Soc. Bull., 67, 344 (1988).

Nagelberg, A. S., S. Antolin, and A. W. Urquhart, J. Am. Ceram. Soc. 75, 455 (1992).

Pierre, A. C., Introduction to Sol-Gel Processing, Kluwer Academic, Boston, 1998.

Rahaman, M. N., Ceramic Processing and Sintering, Marcel Dekker, New York, 1995.

Reed, J. S., Principles of Ceramic Processing, Wiley, New York, 1995.

Richerson, D. W., Modern Ceramic Engineering, Marcel Dekker, New York, 1992.

Riman, R. E., M. M. Lencka, L. E. McCandlish, B. L. Gersten, A. Anderko, and S. B. Cho, Rev.

High Pressure Sci. Technol., 7, 1358 (1998).

Schneider, S. J. (ed.), Engineered Materials Handbook, Vol. 4, Ceramics and Glasses, ASM Inter-

national, Metals Park, OH, 1991.

Siegel, R. W., Ann. Rev. Mat. Sci., 21, 559 (1991).

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West, J. K., and L. L. Hench, Principles of Electronic Ceramics, Wiley, New York, 1990.

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PART 7

APPLICATIONS AND USES

HandbookofMaterialsSelection.EditedbyMyerKutz

1131

CHAPTER 36

SPACECRAFT APPLICATIONS OF

ADVANCED COMPOSITE MATERIALS

Kevin R. Uleck

Department of Aerospace Engineering

University of Maryland

College Park, Maryland

Paul J. Biermann

Jack C. Roberts

Bonny M. Hilditch

Applied Physics Laboratory

The Johns Hopkins University

Laurel, Maryland

1 INTRODUCTION 1131

1.1 Advanced Composites 1132

1.2 Unique Beneﬁts of Advanced

Polymer Matrix Composites 1135

1.3 Scope 1136

2 USE OF ADVANCED

FIBER-REINFORCED

COMPOSITES IN SPACECRAFT 1137

2.1 Properties /Characteristics for

Space Applications 1137

2.2 Typical Structures 1142

2.3 Manufacturing 1146

3 EXAMPLE SPACECRAFT

APPLICATIONS 1149

3.1 Primary Spacecraft Bus /Chassis

Structure 1150

3.2 Deployable Structures 1152

3.3 Electronics Enclosures 1153

3.4 Optical Benches and Instrument

Structures 1157

3.5 Antennas, Reﬂectors, and

Mirrors 1159

REFERENCES 1162

1 INTRODUCTION

Many rigorous demands are placed on the design of spacecraft structures. They

must be lightweight to maximize the allowable payload mass while being stiff

enough to maintain precision alignment of instruments under severe orbital en-

vironmental conditions. Launch loads often dictate strength requirements that

are much greater than those needed for space operation. In today’s economic

environment the time and funding available for project completion are reduced,

and more requirements are being placed on the designer. Advanced composite

materials are being used more frequently and in more diverse applications to

assist in meeting these demands, such as those shown in Fig. 1. Effective use

of advanced composites often requires that the analyst, designer, and fabricator

work together to reduce costs, optimize structural performance, and minimize

mass.

HandbookofMaterialsSelection.EditedbyMyerKutz

1132 SPACECRAFT APPLICATIONS OF ADVANCED COMPOSITE MATERIALS

Fig. 1 Common applications of advanced composites on spacecraft structures.

This chapter consists of three sections. Section 1 brieﬂy describes the basic

characteristics and properties of advanced composites. Section 2 focuses on the

speciﬁc properties of advanced composites that are important for spacecraft op-

eration. Examples of typical spacecraft structures constructed in part or entirely

from advanced composite materials are given in Section 3.

1.1 Advanced Composites

A composite material is generally deﬁned as a man-made material composed of

two or more distinct components. The term advanced composites generally refers

to materials consisting of high-stiffness continuous-ﬁber reinforcements within

a comparatively weak matrix. The reinforcing ﬁbers are generally much stronger

and stiffer than the matrix and are the primary drivers for the unidirectional

tensile and compressive stiffness and the strength of the material. The primary

role of the matrix is to provide a load transfer mechanism between ﬁbers. When

ﬁbers and matrices are combined to form a composite material, the result is a

material with properties that are more than simply the sum of the individual

components. Properly selected ﬁber and matrix combinations complement each

other and work together to form materials that have unique and valuable prop-

erties.

Advanced composites consist of several thin plies (lamina) stacked together

to form a laminate. The individual laminae (plies) are orthotropic and trans-

versely isotropic, with the strength and stiffness values in the ﬁber direction

being an order of magnitude or greater than the corresponding values in the

direction transverse to the ﬁbers. Each ply within the laminate can be oriented

at a different angle to create a laminate that meets the structural requirements

desired while using the minimum amount of material. Additionally, plies of

different materials can be combined into one laminate, or individual plies can

be hybridized to obtain the best material combination.

1 INTRODUCTION 1133

The constitutive stress–strain equations for composite laminates are much

different than those for isotropic materials. For laminates the traction (tensile/

compressive) forces per unit width are coupled with the moments per unit width.

Classical laminated plate theory is used to describe the behavior of composite

laminates assuming plane stress conditions (Eqs. 1 and 2):

兩N 兩 ⫽ 兩A 㛳␧ 㛳 ⫹ 兩B 㛳

␬

兩 (1)

iijj ijj

兩M 兩 ⫽ 兩B 㛳␧ 兩 ⫹ 兩D 㛳

␬

兩 (2)

iijij ijj

In this set of equations, i, j ⫽ 1, 2, 3 and N

are the in-plane tensile/compressive

forces per unit width on the laminate, M

are moments per unit on the laminate,

, B

, D

, are the constituent matrices, and ␧ ,

␬

are the reference-plane strain

and laminate curvature. The moments and the forces are coupled through the

bending-coupling stiffness matrix. In its current form, as tensile forces are ap-

plied, the laminate will warp due to the coupling. If the B matrix is identically

equal to zero, the extension and bending equations become decoupled, therefore

physical bending and extension are independent of each other. The laminate can

then be designed (through proper use of ﬁber orientations in the lamina) to take

advantage of this coupling/uncoupling to produce the desired effects. The A, B,

and D matrices relate the lamina properties to the laminate properties by way

of the material properties and laminate thickness. An example is given below

for the calculation of extensional stiffness matrix, A

A ⫽ (Q )[h ⫺ h ] (3)

冘

ij ij k k k

⫺

⫽

where n ⫽ the number of plies (lamina), and h

⫽ the distance from the top of

the kth ply to the laminate reference-plane; is the ply-transformed stiffness

(Q )

matrix, which directly relates to the material properties of the ply in laminate

axes. An example is shown below for the calculation of the transformed matrix:

4224

Q ⫽ Qm⫹ 2(Q ⫹ Q )mn ⫹ Qn (4)

11 11 12 66 22

where m and n are the direction cosines in the transformation and the ply stiff-

ness (Q

) values are represented by:

⫽ E /(1 ⫺

␯␯

) (5)

11 11 12 21

Q ⫽ E /(1 ⫺

␯␯

) (6)

22 22 12 21

Q ⫽ E

␯

/(1 ⫺

␯␯

) (7)

12 11 12 12 21

Q ⫽ G (8)

66 12

where E, G, and

␯

are the familiar elastic modulus, shear modulus, and Poisson

ratio for the different material directions (longitudinal or transverse to the ﬁbers)

in each of the plies. Several texts are available that describe classical laminated

1134 SPACECRAFT APPLICATIONS OF ADVANCED COMPOSITE MATERIALS

plate theory and composite mechanics in detail, such as Mechanics of Composite

Materials.

Reinforcement Fibers

The majority of reinforcement ﬁbers belong to one of four families: carbon

ﬁbers, aramid (aromatic polyimide) ﬁbers, glass ﬁbers, and specialty exotic ﬁ-

bers. Special reinforcement ﬁbers such as boron and alumina ﬁbers are some-

times used for niche applications but are not widely used due to a variety of

reasons such as low availability, high cost, and nonstandard handling require-

ments. In spacecraft applications, carbon ﬁbers are used far more frequently than

aramid or glass because they have the best combination of speciﬁc strength,

speciﬁc stiffness, cost, and handling characteristics. There are, however, some

applications where aramid or glass ﬁbers are better suited, such as those where

thermal isolation, corrosion resistance, or toughness are critical design factors.

Glass ﬁbers were the ﬁrst ﬁbers to be used extensively as reinforcements in

composite materials. They are relatively inexpensive, widely produced, and fre-

quently used today in applications where low cost is a primary requirement,

such as in the automotive and recreation industries. Different grades of glass

ﬁbers are available, the most common being E-glass, which provides good per-

formance at moderate prices. S-glass ﬁbers provide higher strength and stiffness

than E-glass but at a signiﬁcantly higher cost. Although glass ﬁbers have been

used extensively in aerospace applications, they have been replaced by stiffer

and higher strength carbon and aramid ﬁbers in modern high-performance aero-

space applications.

Aramid ﬁbers, such as DuPont’s Kevlar ﬁber, were introduced commercially

in the early 1970s.

Filament wound pressure vessels and lightly loaded second-

ary aircraft structures were early applications for aramid ﬁbers. A unique feature

of these ﬁbers is that they are remarkably damage tolerant. The tensile failure

of aramid ﬁbers is characterized by ﬁbrillation and a large amount of energy

can be absorbed before material failure. This property makes aramid ﬁbers es-

pecially useful in situations where properties of energy absorption or dispersion

are critical, such as micrometeoroid impact protection.

Carbon ﬁbers are the most frequently used reinforcement in high-performance

composite materials today. A broad selection of carbon ﬁbers encompassing a

range of strength, stiffness, thermal, and electrical properties is available. Carbon

ﬁbers are grouped into two families according to the precursor material from

which they are made: (1) synthetic polymer ﬁber precursors and (2) pitch-based

ﬁber precursors. The ﬁrst carbon ﬁbers were produced by pyrolysis of rayon in

1959 by Union Carbide. In the early 1960s, mass-produced ﬁbers appeared,

produced from the pyrolysis of PAN (polyacrylonitrile), a synthetic precursor.

Less expensive carbon ﬁbers made from a pitch base have been produced since

1973.

PAN ﬁbers offer higher strength, stiffness, and improved manufacturing

consistency over pitch-based ﬁbers. Consequently, they are the ﬁbers of choice

for most high-performance composite applications. On the other hand, reﬁne-

ments in the production of pitch-based ﬁbers have produced carbon ﬁbers with

much better thermal and electrical conductivity than is possible in PAN ﬁbers.

Although these advanced pitch-based ﬁbers still do not equal the strength and

stiffness of PAN ﬁbers, they are indispensable for applications that require good

thermal conductivity, such as electrical enclosures.

1 INTRODUCTION 1135

Table 1 Properties of Common Polymer Matrix Materials

Matrix Material

Tensile Strength

(MPa)

Tensile Modulus

(MPa)

Service

Temperature

(⬚C)

Epoxy 1 50 3100 140

Epoxy 2 90 3800 245

Polyester 1 75 3380 90

Cyanate ester 1 80 3600 240

Thermoplastic 100 3900 150

To ease manufacturing and improve the bond between the ﬁber and the poly-

mer matrix, all reinforcement ﬁbers are available with various surface treatments.

These treatments include epoxy sizing for carbon ﬁber, silane coupling agents

for glass ﬁber, and plasma etching for aramid ﬁber surfaces. Reinforcement

manufacturers work closely with their customers to select or develop the appro-

priate surface treatment for optimization of material behavior for a given appli-

cation.

Polymer Matrices

The majority of current advanced ﬁber-reinforced composite materials employ

thermoset or thermoplastic polymers as the matrix material. Table 1 lists some

properties of common polymer matrix materials used in advanced composites.

Thermosetting polymers are characterized by chemical cross-links between

molecular chains. These cross-links are formed during the resin curing process

and are primarily responsible for the mechanical properties of the cured polymer.

Thermoplastic polymers do not have chemical cross-links, and their rigidity de-

pends on the mechanical interaction of highly intertwined molecular chains.

Properties of new high-performance thermoplastics match or exceed many tra-

ditional thermosetting polymer properties, but these are often more difﬁcult to

use because of the very high temperatures and pressures required.

Thermosetting epoxies have been the dominant matrix material for advanced

aerospace applications due to the wide-ranging combination of mechanical prop-

erties, processing and handling characteristics, and variety of formulations avail-

able. Lately, polycyanate thermoset matrix materials are replacing epoxies

because they are much more durable in the space environment (see discussion

in Section 2.1 on environmental durability). Advanced thermoplastics have been

used in some space applications, but their use has been limited due to processing

complexity.

Many aspects must be factored into the polymer matrix selection for a par-

ticular composite material application. The material properties that are important

for composite material applications differ from application to application and

can include service temperature range, environmental durability, toughness,

strength, stiffness, compatibility with ﬁber sizing, availability, processing char-

acteristics, and cost.

1.2 Unique Beneﬁts of Advanced Polymer Matrix Composites

Advanced ﬁber-reinforced composite materials have several key properties that

make them especially useful in spacecraft applications. Their high speciﬁc

1136 SPACECRAFT APPLICATIONS OF ADVANCED COMPOSITE MATERIALS

Table 2 Comparison of Speciﬁc Properties of Metals and Composites

Material Speciﬁc Tensile Strength Speciﬁc Tensile Stiffness

AL 2024-T3 174 26

AL 6061-T6 115 26

Ti-6Al-2Nb-1Ta-1Mo 184 25

T-600 Stainless Steel 127 25

Quasi-isotropic Gr / Ep 258 50

strength and stiffness (strength/density, stiffness/density, examples are given in

Table 2) allow spacecraft structure designs that are lighter than traditional metal

structures. Structural mass may be reduced by more that 50% over comparable

metal structures through the use of advanced composites. Because the total mass

of a spacecraft is limited, the minimization of the structural mass of the space-

craft can extend the spacecraft life by allowing larger fuel loads, or more mass

may be allotted for scientiﬁc payloads.

While simply replacing metal components with quasi-isotropic composite

laminates can result in reduced mass, the exploitation of the directional nature

of advanced continuous ﬁber composite materials results in designs that are

much more structurally efﬁcient. A structure made from isotropic metals is gen-

erally sized according to Von Mises loading principals. This often leads to struc-

tures being dramatically overdesigned in the nonprinciple loading directions.

Composites are much stronger and stiffer in the direction parallel to the ﬁbers

(longitudinal) than the direction transverse to them. Designers can take advan-

tage of this directional strength to produce structures with equal margins of

safety for all loading directions. In this way, the use of anisotropic composite

materials leads to a more efﬁcient, lower mass design.

Another extremely useful trait of advanced composites is that the thermal

expansion properties can be customized. Carbon and aramid ﬁbers have a neg-

ative axial coefﬁcient of thermal expansion (CTE), so composite structures using

carbon ﬁber or aramid ﬁber reinforcements can be designed that have ultralow

thermal expansion. Spacecraft are typically subjected to a wide temperature

range as they move in and out of direct solar radiation. The use of metallic

structures in this environment can lead to substantial dimensional changes that

disturb instrument alignment and induce stresses. The use of composites with

customization of the coefﬁcient of thermal expansion permits the design of pre-

cision structures and assemblies that function uniformly over the whole temper-

ature range.

1.3 Scope

The aim of this chapter is to discuss the beneﬁts of using advanced composite

materials in spacecraft structures and to provide examples of typical applications.

Advanced composites can consist of a wide variety of matrix and reinforcement

materials. Because it is impractical to discuss the full range of possible materials

and their applications in this text, the discussion will be limited to the most

widely used advanced composites, speciﬁcally polymer matrices with long ﬁber

reinforcements. The materials that will be discussed are carbon, glass, aramid

ﬁber reinforcements, and typical polymer matrix materials such as epoxies and

2 USE OF ADVANCED FIBER-REINFORCED COMPOSITES IN SPACECRAFT 1137

cyanate esters. The following paragraphs will brieﬂy mention some of the more

common applications that will not be discussed in detail in this chapter.

Advanced composites have a long history of use in spacecraft launch vehicles

(generally rockets). Various parts of rocket propulsion systems have been made

with composite materials. For example, rocket motor cases that make use of the

directional properties of advanced ﬁber-reinforced composites are stronger and

lighter than metallic motor cases. Also, rocket engine nozzles using ablative

polymer matrices are signiﬁcantly lighter than metal nozzles and offer improved

temperature stability. Other launch vehicle components, such as aerodynamic

shells and fairings, are frequently made from advanced composite materials for

mass reduction.

Heat shields for atmospheric entry constitute a unique application of com-

posite materials. Long ﬁber reinforcements in composite heat shields give them

the ability to maintain form and shape during ablation produced by the heat of

atmospheric entry.

2 USE OF ADVANCED FIBER-REINFORCED COMPOSITES IN

SPACECRAFT

Advanced ﬁber-reinforced composite materials have a long history of spacecraft

use. The primary reason for their use is to reduce the mass of spacecraft struc-

tures and components. Additionally, their unique high speciﬁc strength and stiff-

ness can be utilized to create structures that are impossible with conventional

metal construction. This section addresses the basic characteristics and properties

of advanced composite materials that are important for spacecraft applications.

2.1 Properties/Characteristics for Space Applications

All material properties that can affect component performance must be consid-

ered during design, but the relative importance of each varies for different types

of components. For example, the design of structural truss members must include

criteria such as total mass, thermal stability, environmental durability, and stiff-

ness. On the other hand, electromagnetic shielding and thermal conduction are

critical for the design of electronics enclosures.

Coefﬁcient of Thermal Expansion

Because of the anisotropic nature of advanced composites the coefﬁcient of

thermal expansion (CTE) of composites can be altered to meet a range of design

speciﬁcations. In contrast to most structural materials, carbon and aramid ﬁbers

have a negative axial CTE. Therefore the CTE of composite structures is not a

ﬁxed value as for metals but can be selected by the designer. Realistic carbon

composite structures can and have been produced that have a near-zero or even

negative CTE. Because spacecraft temperatures on orbit can range between ex-

tremes of

⫹120 to ⫺90⬚C as the spacecraft passes in and out of Earth’s shadow,

this near-zero CTE provides stability unmatched by metal structures.

Typical CTE values for high modulus carbon ﬁbers and aramid ﬁbers are

⫺0.9 and ⫺2.7

␮

␧/⬚ C, respectively. Unidirectional polymer matrix composites

made using these ﬁbers then have longitudinal CTEs ranging from

⫺0.7 to ⫺0.4

␮

␧/⬚ C for carbon ﬁber composites, and ⫺4.0 to –2.7

␮

␧/⬚C for aramid ﬁber

composites. For individual lamina, the CTE value transverse to the ﬁbers is