A07 - Introduction to Composite Materials
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Introduction to Composite Materials
References

AERO 304 notes and Introduction to Aerospace Structural Analysis ,
Allen and Haisler

Principles of Composite Material Mechanics , R.F. Gibson,
McGraw-Hill, 1994

Mechanics of Composite Materials , Robert M. Jones, McGraw-
Hill, 1975

Introduction to Composite Materials, S.W. Tsai and H.T.
Hahn, Technomic Publishing Co., 1980
Introduction and Terminology
Structural materials can be divided into 4 basic categories:

Metals

Polymers

Ceramics

Composites

A07 - Introduction to Composite Materials
Composites , which consist of two or more separate materials combined in a macroscopic structural unit, are made from various combinations of the other three materials.
The relative importance of the four basic materials in a historical context has been presented by Ashby (Technology of the 1990s:
Advanced Materials and Predictive Design, M.F. Ashby,
Philosophical Transactions of the Royal Society of London , A322,
393-407, 1987) and is shown schematically below (figure taken from Gibson):
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Mankind has used composites since early time; for example, strawreinforced clay bricks used by Israelites (the book of Exodus in the
Old Testament ), plant fiber-reinforced pottery, etc. They knew from daily use that fiber reinforcement of a material is very effective because many materials (but not all) are much stronger and stiffer in fiber form than they are in bulk form.
For example, Griffith found that as glass rods and fibers got thinner, they got stronger. He found that that for very small diameters the fiber strength approached the theoretical cohesive strength between adjacent layers of atoms, whereas for large diameters the fiber strength dropped to near the strength of bulk glass.
Fibers allow one to obtain the maximum tensile strength and stiffness of a material, but there are disadvantages. Fibers alone cannot support longitudinal compressive loads and their transverse mechanical properties are generally not as good as the

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5 corresponding longitudinal (fiber direction) properties. Thus, there is often the need to place fibers in different directions depending upon the particular loading application.
Types of Fiber-Reinforced Composites
One generally finds four types of fiber-reinforced composites as shown below (from Gibson). They differ in how the fibers are utilized to make the composite (orientation and length of fibers).

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Continuous fiber composites are generally "laid-up" in plies (or laminae ) with each ply having fibers oriented in the same direction. A layer of fibers all oriented in the same direction is imbedded in a homogeneous material (called the matrix ) to make a single ply or laminae. For example, glass-epoxy has a layer of glass fibers running more-or-less parallel within an epoxy resin matrix material.
Individual plies can be stacked or layered and bonded together with individual ply fiber directions being selected so as to tailor the lay-up (or laminate ) to have desired overall structural characteristics of the laminate. Under loading, the potential for delamination (or separation of the laminae) is a major problem because the interlaminar strength is matrix dominated (i.e., if the matrix is weak, ply delamination can occur).
Woven fiber composites are similar to ordinary cloth used in the textile industry. The woven fiber may be 2-D (fibers interwoven in

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2 directions) or 3-D (fibers interwoven in 3 directions). Woven fiber composites do not generally have distinct laminae and are not nearly as susceptible to delamination; however, strength and stiffness are sacrificed due to the fact that the fibers are not as straight (because of the weaving) as in the continuous fiber laminate.
Chopped fiber composites have fibers that are relatively short and have a random orientation and distribution of the fibers. Chopped fiber composites generally have mechanical properties that are considerable poorer than those of continuous fiber composites.
However they are cheaper to manufacture and are used in highvolume applications.
Hybrid composites generally consist of mixed chopped and continuous fibers; or mixed fiber types such as glass/graphite.

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Sandwich composites are also common. They consist of high strength composite facing sheets (which may be any of the four fiber composites discussed above) bonded to a lightweight foam or honeycomb core (from Gibson).

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Sandwich structures have extremely high flexural stiffness-toweight ratios and are widely used in aerospace structures. The design flexibility offered by these and other composite configurations is obviously quite attractive to designers, and the potential now exist to design not only the structure, but also the structural material itself.
Almost all of the fiber-reinforced composite types discussed above can be utilized in complex curved geometries although the manufacturing process may be much more costly and difficult. For example, wound fiber-reinforced pressure vessels are common and are manufactured by winding either individual fiber filaments on a mandrel (having the shape of the vessel) or individual plies are wound on the mandrel. Curved composite material panels on aircraft wings, fuselage and nacelles are common.
All of the composite types have various manufacturing processes required to bond individual plies. Common glass-epoxy

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11 composites must be cured using a high temperature and vacuum
(or pressure) process designed not only to bond individual plies but also to minimize residual laminae and interlaminar stresses.
Fiber Materials
Glass fibers consist primarily of silica (silicon dioxide) and metallic-oxide-modifying elements are generally produced by mechanical drawing of molten glass through a small orifice. Eglass accounts for most of the glass fiber production and is the most widely used reinforcement for composites. The second most popular glass fiber, S-glass, has roughly 30 percent greater tensile strength and 20 percent greater modulus of elasticity than E-glass but is not as widely used because of its higher cost.
Graphite or carbon fibers are the most widely used advanced fiber, and graphite/epoxy or carbon/epoxy composites are now used routinely in aerospace structures. The actual fibers are usually produced by subjecting organic precursor fibers such as

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12 polyacrylonitrile (PAN) or rayon to a sequence of heat treatments, so that the precursor is converted to carbon by pyrolysis. Graphite fibers are typically subjected to higher heat treatments than are carbon fibers. Carbon fibers are typically 90-95% carbon, whereas graphite fibers are at least 99% carbon.
Aramid polymer fibers, produced primarily by E.I. duPont deNemours & Company under the tradename "Kevlar

," were originally developed for use in radial tires. The density of Kevlar is about half that of glass and its specific strength is among the highest of currently available fibers. Kevlar also has excellent toughness, ductility, and impact resistance; unlike brittle glass or graphite fibers.
Boron fibers are actually composites consisting of a boron coating on a substrate of tungsten or carbon. The diameter of boron fibers is among the largest of all the advanced fibers, typically 0.002-
0.008 in. Boron fibers have much higher strength and stiffness

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13 than graphite, but they also have higher density. Boron/epoxy and boron/aluminum composites are widely used in aerospace structures, but high cost prevents more widespread use.
Silicon carbide (SiC) fibers are used primarily in high-temperature metal and ceramic matrix composites because of their excellent oxidation resistance and high-temperature strength retention. SiC whisker-reinforced metals are increasingly being used as alternative to un-reinforced metals and continuous fiber-reinforced metals. SiC whiskers are quite small, typically 8-20

in. diameter and about 0.0012 in. long so that standard metal-forming processes such as extrusion, rolling and forging can be easily used.
The list of fibers goes on … On the following pages are a)
Selected properties of fibers and bulk metals, b) Specific strength vs. specific modulus for various fibers and c) Specific strength vs. specific modulus (stiffness) for various composites (from Gibson).
Specific value is the value of the property divided by its density.

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Matrix and Filler Materials
Polymers, metals and ceramics are all used as matrix materials in composites. The matrix
 holds the fibers together in a structural unit,
 protects them from external damage,
 transfers and distributes the applied loads to the fibers, and
 in many cases, contributes some needed property such as ductility, toughness, or electrical insulation.
Because the matrix must transfer load to the fibers, a strong interface bond between the fiber and matrix is extremely important; either through a mechanical or chemical bond between fibers and matrix. Fibers and matrix must obviously be chemically compatible to prevent undesirable reactions at the interface; this is especially important at high temperature where chemical reactions can be accelerated.

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Service temperature is quite often a controlling factor in consideration of a matrix material. Listed in order of increasing temperature capability, we have:
Polymers are the most widely used matrix materials. They may be either thermosets (e.g., epoxy, polyester, phenolics) or thermoplastics (e.g., polyimide (PI), polyetheretherketone (PEEK), polyphenylene sulfide (PPS)). Upon curing, thermosets form a highly cross-linked, three-dimensional molecular network which does not melt at high temperature. Thermoplastics, however, are based on polymer chains that do not cross-link. As a result, thermoplastics will soften and melt at high temperature, then harden again upon cooling.
Epoxies and polyesters are also widely used. High grade epoxies are typically cured at about 350F and are generally not used at temperatures about 300

F. The advanced thermoplastics (PEEK,
PI and PPS) have melting temperatures in the range of 600-700

F.

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For higher temperatures, metal, ceramic or carbon matrix materials are required.
Lightweight metals such as aluminum, titanium and magnesium and their alloys such titanium aluminide and nickel aluminide may be used as matrix materials. For some of these, operating temperature can be extended to about 2,250

F. Advantages of metal matrices include higher strength, stiffness and ductility
(compared to polymers) but at the expense of higher density.
Ceramic matrix materials such as silicon carbide and silicon nitride can be use at temperatures up to 3,000

F. Hoever, ceramics have poor tensile strength are are quite brittle.
Carbon fiber/carbon matrix composites can be used at temperatures approaching 5,000

F, but the cost is such that they are only used in a few critical aerospace applications.

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Filler materials are often used as a third component of a composite, and are typically mixed with the matrix material during fabrication.
Fillers do not typically enhance mechanical properties but are used to alter or improve some other characteristic of the composite.
Examples include: hollow glass microspheres are used to reduce weight, clay or mica particles are used to reduce cost, carbon black particles are used for protection against ultraviolet radiation, and alumina trihydrate is used for flame and smoke suppression.