This document discusses composite materials and their advantages over homogeneous materials. Composites combine the strengths of different materials - composites use strong fibers embedded in a tough matrix material. This allows composites to have superior strength and stiffness compared to metals while maintaining ductility. The document then describes different types of composite systems, including fiber reinforced, laminated, and particulate reinforced composites. It provides examples of both modern and historical composite materials like concrete, bone, wood and more.

1. Composite Materials
• It has always been the hope to produce structural
materials possessing both great ductility and extreme
strength. Strength provides load carrying capability while
ductility prevents sudden and catastrophic failure.
Ductility thereby provides “forgiveness.” Strength and
ductility tend to be incompatible properties of simple,
homogeneous materials, although metals (e.g., mild
steel) have served as a good compromise. However,
increasing demands on technology necessitate stronger,
lighter and tougher structural materials. Composites,
while not a panacea, can be a step toward this material
utopia.

2. I. Composite Systems
A) Mechanism - - Man-made composites are
advantageous mechanically in that they typically
combine the ductility, forgiveness and toughness
of some matrix material with the load carrying
ability of some introduced strong fibers or
particles. Unlike metal alloys, the matrix and
reinforcement do not (essentially) interact
chemically. Structural composites often have
superior strength-to-weight and/or stiffness-to-
weight ratios compared with homogeneous
materials.

3. B) Types of Man-Made Composites
• Fiber (chopped, continuous, whisker)
reinforced matrix - - heterogeneous and
perhaps anisotropic.
• Laminate - - bonded stacked plies of fiber
reinforced material.
• Particulate reinforced materials.
• Woven/fabric materials.

4. C) Man-Made Composites are Often Motivated by
Favorable Response of One or More of the Following
Properties
• Strength
• Stiffness
• Wear resistance
• Weight
• Fatigue life
• Extreme temperature
response
• Thermal insulation or
conduction
• Electrical insulation or
conduction
• Acoustical insulation or
conduction
• Response to nuclear, x-
ray or magnetic radiation
• Chemical response or
inertness to an
environment (corrosion
resistance
• Electromagnetic and
radar insulation or
conduction
• Crack (fracture)
resistance and arrest
• Cost
• Fabrication

5. D) Historical Composites
1. Concrete - - small rocks bonded (cement) in a sand matrix.
Current technology also involves some fiber (and perhaps resin)
introduction. Steel reinforced concrete can approach a fiber
reinforced matrix.
2. Automobile tires - - cord (fibers - - metal, nylon, rayon, Kevlar)
reinforced truncated rubber donut.
3. Teeth - - enamel coated sub-structure
4. Bone1
- - - osteone tubes surrounding blood vessels. Osteones
composed of several plies of collogen fibers. The collogen fibers
of each lamina are parallel and spiral about the axis of the
osteones, the direction of spiral being reversed in each plie.
5. Wood1
- - - cellulose chains embedded in lignin matrix
6. Rock - - - many rock formations consist naturally of layered
deposits of different (mechanical) properties and perhaps different
anisotropy. Other formations may have inclusions of different
properties (heterogeneous).

6. 7. Soil - - - bedding often produces anisotropic material properties
8. Asbestos
9. Clad metal
10. Laminated beams (wood or wood-metal)
11. Plywood
12. Honeycomb structures
13. Paper and wallboard
14. Asphalt mixes2
15. Rigid foams2
- - - aerated polymers (insulations, etc.)
16. Formica and veneer
17. Sintered carbide tools
18. “Shatter-proof” glass
19. Rocket propellant
D) Historical Composites - continued

7. II. Brief History
3000 B.C. - - fiber (straw) reinforced mud bricks for huts
and laminated wood and metal armor
1940 - - glass-polyester radomes
1942 - - - molded fiber- glass boats
1946 - - - U.S. government patented first filament-winding
process
1960 - - - Boeing 727 employed reinforced plastic
components
1952 - - - single crystal whiskers
Since - - - tennis rackets, large-diameter thin-walled pipes,
civilian structural members, golf clubs, automobile and
truck components, skis, fiber-reinforced concrete, epoxy
impregnated concrete, composite pressure vessels,
armor, space vehicles, boats, airplanes, etc., etc., etc.

8. III. Structural Matrix Materials Include:
A) Plastics- thermosetting
• polyesters
• epoxies
• phenolics
• polyimides
– thermoplastic
• nylon (polyamide)
• polyethylene
• polystyrene
• polypropylene (PP)
• PMMA (polymethyl methachylate)
• polycarbonate
• PEEK (polyether- ether- ketone)

9. B) Metals
• steel
• iron
• aluminum
• zinc
• carbon
• copper
• nickel
• silver
• titanium
• magnesium
• alloys and super-alloys
C) Ceramics

10. IV. Reinforcement Materials
A) Glass
B) Nylon, Kevlar, Spectra (polyethylene), etc.
C) Metal (continuous, chopped and whiskers)
• magnesium oxide
• silicon carbides
• silicon nitride
• steel
• titanium
• iron
• molybdenum
• aluminum oxide (alumina,
Al2O3)
• aluminum nitrate
• beryllium oxide
• boron
• graphite
D) Ceramics
E) Particulates

11. V. Fiber-Reinforced Matrix
The loads are carried mostly by the loaded fibers, whereas the
toughness and ductility are provided by the matrix. Since the fibers are
extremely fine in size, and are produced under highly controlled (chemical,
mechanical and metallurgical) conditions, they contain few defects
(whiskers are single crystal). Fibers are extremely strong - - much stronger
than the material in bulk form. Fiber-reinforced materials necessitate
sufficient bonding between fiber and matrix to transfer the load. As most
fiber-reinforced matrices employ essentially parallel fibers, the over-all
material responds anisotropically (orthotropically). The strength and
stiffness parallel to the fibers are much higher than those transverse to the
fibers. Strength and stiffness transverse to the fibers are not much greater
than those of the matrix material. Individual plies (lamina) of fiber-
reinforced matrix are often stacked and bonded to form laminates.
Adjacent plies are typically oriented so their respective fiber directions are
skewed. It is usual to use a single type reinforcement fiber throughout a
ply and perhaps even throughout the various plies of a complete laminate.
When different types of fibers (say some boron and some graphite) are
employed in a single plies, or in adjacent plies - - the composite is referred
to as a hybrid composite.

12. V. Fiber-Reinforced Matrix (cont’d)
Although the strains and deformations of a bonded laminate
may be uniform throughout the various plies, that different laminae
are skewed causes the moduli or stiffness (and compliance) in a
particular direction to change from ply to ply. This can produce
interlaminar stresses (normal and shear).
Most fiber-reinforced composites utilize parallel fibers
(continuous chopped or whisker). If the fibers are randomly
orientated in the plane, plies may be macroscopically assumed to
respond isotropically.
At least conceptually, the orientational dependence of strength
and stiffness of fiber-reinforced composites does permit structural
designs according to the directions of maximum load application.
Nature does this, e.g. bones and skin.

13. VI. Reinforcing — Fiber
Classification
A. Polycrystalline — Al2O3, Zr03, graphite (carbon). These may all be
classified as ceramics. They have high melting temperatures (3000°
to 6500°F), high modulus and low density. Some retain a very high
strength at elevated temperatures while others do not.
B. Multiphase (typically the material of interest is on a substrate or
carrier) - - boron on tungsten (B/W), SiC/B/W & SicIW. These fibers
consist of materials chemically vapor deposited on a substrate of
fine-diameter (.0005”) tungsten wire. In some cases the deposit
reacts with the substrate at elevated temperatures, limiting their
high-temperature use. For this reason boron/tungsten fibers are
limited to about 1500°F. Boron on carbon produces a reinforcement
usable at higher temperatures.
C. Single-crystal ceramics - - Al2O3(sapphire), Al2O3 · Cr2O3 (ruby) and Ti
C. These materials have among the highest high-temperature
strengths (500,000 psi at 2000°F) of any material. They also have
very high moduli.
D. Wiskers (single-crystal materials)

14. VII. Particulate Reinforcement
A. General
From a structural mechanics point of view, two of the most
important particulate reinforced systems are (1) dispersion
hardening and (2) aligned eutectic composites. Other particulate
reinforced composites include
• concrete - - sand and rock bound together by cement and water
• rocket propellant - - aluminum powder dispersed in plastic such as
polyurethane
• metal impregnated plastics
• cermets - - ceramic suspended in metal matrix (grinding wheels)
B. Dispersion Hardening
Plastic deformation and fracture of metal is associated with dislocation mobility.
Strength can be increased by Impeding dislocation motion. The latter can
be done by implanting barriers such as adding a second phase (alloying).
With Solution Hardening, atoms of the second-phase material either
substitute for atoms of the host (matrix) or occupy interstitial positions.

15. VII. Particulate Reinforcement (cont’d)
Precipitation-hardened metals — when a homogeneous solid solution
is quenched from an elevated temp and subsequently aged at a
lower temp to precipitate a second-phase from the supersaturated
solid solution.
Synthetic dispersion-hardened material — Typical second-phase
particles include oxides, nitrides, carbides and borides. The fibers of
a fiber-reinforced composite carry the majority of the load. With
particulate-reinforced metals, the matrix carries the load, the
introduced particles serving chiefly to impede dislocation mobility.
Whereas fiber-reinforced plies are usually orthotropic, particulate
reinforced materials are microscopically heterogeneous but
macroscopically isotropic. In that the reinforcement is not all parallel,
particulate reinforced materials tend to have lower reinforcement
volume fraction content than do fiber-reinforced composites.

16. VII. Particulate Reinforcement (cont’d)
C. Directionally Solidified Eutectic Composites — when a
homogeneous solution of a eutectic composition is cooled to its
eutectic temperature, the solid formed can be considered as a
composite of the two distinct phases. These two phases can be
aligned parallel to each other — to form an orthotropic,
unidirectionally strengthened composite. Directionally solidified
eutectic composites are rod-like, lamillar, platelets, or spiral in
structure. By drawing the molten specimen away slowly, the parallel
structure will be in the direction of drawing.
Advantages of directionally-solidified euthectics include
(1) a one-step operation, and (2) excellent high temperature
performance. A disadvantages of the directionally-solidified eutectic
composites is the fixed volume fraction of the second phase.

17. VII. Particulate Reinforcement (cont’d)
Note:4
Metals are made up of crystals - - an orderly 3-D arrangement of
atoms. Of the three types of primary bonds (covalent, ionic or
metallic), the atoms of metals crystals are joined by metallic bonds.
With metallic bonds, adjacent atoms give up their valence electrons
to form a common electron cloud which engulfs the space occupied
by the atoms. Because of this bonding, metals tend to respond in a
ductile fashion, plus exhibit good electrical and thermal conductivity.
Metals are crystalline, with atoms combining to form regular
crystals, the latter adhering to each other to form the crystal
structure of metals. In contrast to this highly ordered array are
amorphous materials in which molecules tend to be the basic
structural unit. Amorphous materials (glass, etc.) tend to have a
less-ordered structure, and consequently tend to be much less
dense, than metals. Plastics (polymers) and rubber are also
sometimes classified as amorphous materials.

18. VII. Particulate Reinforcement (cont’d)
Plastics may be classified either as thermosetting or thermo
plastic. Thermosetting plastics are those in which the long
monomers (primary covalent bonds) are held together by 3-D cross-
linked primary bonds. Thermosetting plastics are more rigid and do
not soften at elevated temperature. With thermoplastic polymers, the
long chains or monomers are held to each other by weak secondary
(Van de Waals) bonds. Thermoplastics soften upon heating, are
less rigid and tend to be more viscoelastic than thermosetting
plastics.
Ceramics may be partly amorphous and partly crystalline, with
an amorphous framework consisting of silicates.

19. VIII. Polymeric Matrix Materials
A) Epoxy resins - - These are thermosetting plastics,
several varieties of which are commercially available.
They are typically purchased as separate monomer and
hardener (catalyst). Some may necessitate an elevated
curing cycle. Plasticizers may be added to reduce the
stiffness. Epoxies enjoy advantages such as high
strength, relatively low viscosity, low mobility during
curing, and relatively low shrinkage rates during curing.
They also adhere well to many fibers and metals. Epoxy
matrix composites are extensively employed, particularly
for structural and aerospace applications. Applications
include pipes, storage tanks, pressure-vessels,
laminates, electrical conduit, transformer and switch
gear components and aerospace, land and hydrospace
vehicles. Epoxies are frequently reinforced by glass,
boron, Kevlar, graphite or metal fibers, as well as
particulates.

20. B) Polyester Resins - -These are also a
popular thermosetting matrix material for
commercial composite. Like the epoxies,
the polyester resins employ a catalyst to
stimulate polymerization. Elevated curing
temperatures are again typical. A
significant advantage is they are relatively
inexpensive.

21. C. Phenolic Resins - - These materials are much
like a thermoplastic material at room
temperature but change to a thermosetting
material at elevated temperature. They are
cheaper and stronger then epoxy and many
polyester resins. They are not as popular as
composite matrix material as are the epoxy
resins. As with the other plastic matrix materials,
phenolics are relatively poor conductors of heat
and electricity. They are non-magnetic. A
disadvantage is their relatively high shrinkage
during curing.

22. IX. Nomenclature
A. Fiber - - A long single homogeneous (or quasi
homogeneous single or polycrystalline or multiphase)
metal, polymeric, or natural (e.g. wood, animal) unit or
strand typically of small diameter. Fibers may be
continuous (essentially no fiber ends within the
structure) or chopped (finite in length).
B. Filament - - Continuous fibers (strands, ends, tows)
C. Roving - - Parallel (untwisted) gathering of fibers or
filaments
D. Yarn - - When several long fibers (strands, ends, tows,
or filaments) are longitudinally twisted together into a
single strand. Such yarns are often cross-woven to
form a fabric.
E. Wire - - Metal filaments.

23. X. Fiber Reinforcement
A. Glass1,2,5
Glass filaments and fibers are probably the most
popular and oldest fiber reinforcement. The rigid silica
networks are mixed with varying additives to produce
different glasses of different mechanical properties. The
most commonly employed reinforcing glasses are E-
glass (55% Si02, 14% Al2O3, 17% CaO, 8% B2O3 and 5%
MgO), S-glass (65% Si02, 25% Al2O3 and 10% MgO) and
D-glass. S-glass possesses higher tensile strength and
modulus than E-glass. The strength of D-glass is lower
than that of E-or S-glass. While D-glass finds high
electronic application due to its low dielectric constant, E-
and S-glass are the more popular for structural
applications.

24. X. Fiber Reinforcement (glass-cont’d)
Glass fibers are manufactured as continuous
filaments, although they are sometimes subsequently cut
to form short, chopped fibers. The glass filaments are
formed by drawing the molten material rapidly from a
melted batch in a crucible or furnace. After drawing, the
filaments are typically surface treated (to provide
protection and to enhance matrix bonding, etc.) and are
then usually drawn into bundles or rovings. These
rovings may or may not be made into yarns. Individual
glass filaments have diameters from 0.0001 to 0.00075
inches. Physical properties of commercial glass
filaments are contained in Table 1 (Ref. 1).

25. X. Fiber Reinforcement (glass-cont’d)

26. X. Fiber Reinforcement (glass-cont’d)
Glass fibers have relatively high strength but low stiffness. Their major
advantage as a reinforcement material is their relatively low cost - in part due to a
comparatively simple manufacturing process. Since fiber strength is greatly reduced
by surface flaws and defects, extreme care must be taken in handling the fibers.
Much of the commercial glass fiber is produced by Owens-Corning, while
single-and multiple-ply glass-epoxy laminates are commercially available (e.g.,
traditionally Scotchply by 3M Corporation). In addition to forming glass-reinforced
laminates from prepreg tape, glass-reinforced cylinders, pipes and pressure vessels
are fabricated by filament winding on a mandrel. Chopped glass-fiber components
can also be molded.
In many cases, glass filaments of non-circular cross-section are fabricated by
drawing the molten glass through special-shaped dies.
Glass fiber (chopped) and filament (continuous) reinforced epoxy, polymide
and polyester plastics find wide-spread structure applications such as pipes, pressure
vessels and containers, boats, automobile, and aerospace components, skis,
recreational and sporting items, etc., etc., etc.

27. X. Fiber Reinforcement (cont’d)
B. Boron
Boron vacuum deposited onto tungsten (or glass) makes a popular
multiphase reinforcement filament. The specific strength of boron fibers is
comparable to that of glass fibers, while its specific modulus Is four to five
times as great as that of glass. Tungsten precursors of 0.0005” diameter
vacuum deposited to 0.004 in. dia. with boron are typically employed. Table
2 contains some boron fiber properties. Boron fibers can not be bent into
small radii and are hence not typically not available in woven (cloth) form.

28. X. Fiber Reinforcement (boron-cont’d)
The tensile strength of boron fibers is compared in Table 3
below with those of several other reinforcing materials.

29. X. Fiber Reinforcement (boron-cont’d)
Boron reinforced epoxy resin is a popular
composite for aerospace applications, and some
domestic, recreational & sporting equipment,
etc. The material is more expensive than glass
and consequently its use tends to be reserved
for applications necessitating high stiffness and
high performance. The material is employed
mostly in flat laminate form. The plies are
fabricated from resin impregnated fibrous tape.
Continuous fibers are usually employed.

30. X. Fiber Reinforcement (cont’d)
C. Carbon1
Carbon filaments are polycrystalline in structure, while the graphite form tends
to exhibit purer crystallinity. Metallic filaments such as carbon are widely used
reinforcing materials. They are relatively easy to fabricate in fairly large quantities and
bond well to plastics and metals. Such filaments are conventionally manufactured by
drawing the material through a series of successively smaller dies. Carbon fibers are
most advantageously employed in the graphitized condition (as graphite). Persons
might often refer to a material as being a ‘carbon fiber’ composite when in reality it is
a ‘graphite fiber’ composite.
D. Graphite1
Continuous filament graphite fibers are available of high strength and modulus.
They tend to be chemically inert, are of low density and retain their modulus and
stiffness at elevated temperatures. The manufacturing process involves a precursor.
Typical precursors are textile fibers such as rayon or nylon-like organic materials,
e.g., Kevlar, PAN (polyacrylonitril) and pitch (petroleum waste and/or by-products).
During manufacture, the molecular orientation in the fibers is preserved and the
strong carbon-carbon bonds are aligned along the fiber axis. The organic carbon
fibers are heated (pyrolysed) at low temperature (1000°C to 3000°C – often in inert
atmosphere) to produce graphitization, i.e., the somewhat previously amorphous
carbon becomes some-what crystallized. Cross-linking is frequently accomplished by
adding oxidizing agents or other catalysts. The precursor is sometimes mechanically
stretched during pyrolysis to align the graphite basal planes. Like other fibers,
graphite fibers are sometimes coated for protection or to enhance bonding.

31. X. Fiber Reinforcement (graphite-cont’d)
E. Thornel1
Thornel fibers are a rayon based graphite originally manufactured and
marketed by the Union Carbide Corporation. The fibers are available in the
form of a yarn consisting of two ribbons twisted together. Each ply or ribbon
has 720 individual filaments. Some properties of some different Thornel
fibers are listed in Table 4(Ref. 1).

32. X. Fiber Reinforcement (cont’d)
F. Kevlar (formerly PRD-49)
This is an organic fiber manufactured by
DuPont. Its properties are indicated in Table 5.

33. X. Fiber Reinforcement (Kevlar-cont’d)
The mechanical properties of Kevlar -
unidirectionally reinforced epoxy are
given in Table 6.
Y.S. (RER) 1/13/04