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Composite materials �C Introduction
Definition: any combination of two or more different
materials at the macroscopic level.
OR
Two inherently different materials that when
combined together produce a material with
properties that exceed the constituent materials.
? Reinforcement phase (e.g., Fibers)
? Binder phase (e.g., compliant matrix)
Advantages
? High strength and stiffness
? Low weight ratio
? Material can be designed in addition to the structure

Applications
? Straw in clay construction by Egyptians
? Aerospace industry
? Sporting goods
? Automotive
? Construction

Types of Composites
Matrix
phase/Reinforc
ement Phase
Metal Ceramic Polymer
Metal Powder metallurgy
parts �C combining
immiscible metals
Cermets (ceramic-
metal composite)
Brake pads
Ceramic Cermets, TiC, TiCN
Cemented carbides �C
used in tools
Fiber-reinforced
metals
SiC reinforced
Al2O3
Tool materials
Fiberglass
Polymer Kevlar fibers in an
epoxy matrix
Elemental
(Carbon,
Boron, etc.)
Fiber reinforced
metals
Auto parts
aerospace
Rubber with
carbon (tires)
Boron, Carbon
reinforced plastics
MMC��s CMC��s PMC��s
Metal Matrix Composites Ceramic Matrix Comp��s. Polymer Matrix Comp��s

Costs of composite manufacture
Material costs -- higher for composites
? Constituent materials (e.g., fibers and resin)
? Processing costs -- embedding fibers in matrix
? not required for metals Carbon fibers order of magnitude
higher than aluminum
Design costs -- lower for composites
? Can reduce the number of parts in a complex
assembly by designing the material in combination
with the structure
Increased performance must justify higher
material costs

Types of Composite Materials
There are five basic types of composite materials:
Fiber, particle, flake, laminar or layered and filled
composites.

A. Fiber Composites
In fiber composites, the fibers reinforce along the line of
their length. Reinforcement may be mainly 1-D, 2-D or 3-D.
Figure shows the three basic types of fiber orientation.
1-D gives maximum strength in
one direction.
2-D gives strength in two
directions.
Isotropic gives strength equally
in all directions.

Composite strength depends on following factors:
Inherent fiber strength,
Fiber length, Number of
flaws
Fiber shape
The bonding of the
fiber (equally stress
distribution)
Voids
Moisture (coupling
agents)

B. Particle Composites
Particles usually reinforce a composite equally in all directions
(called isotropic). Plastics, cermets and metals are examples of
particles.
Particles used to strengthen a matrix do not do so in the same
way as fibers. For one thing, particles are not directional like
fibers. Spread at random through out a matrix, particles tend to
reinforce in all directions equally.
? Cermets
(1) Oxide�CBased cermets
(e.g. Combination of Al2O3 with Cr)
(2) Carbide�CBased Cermets
(e.g. Tungsten�Ccarbide, titanium�Ccarbide)
? Metal�Cplastic particle composites
(e.g. Aluminum, iron & steel, copper particles)
? Metal�Cin�Cmetal Particle Composites and
Dispersion Hardened Alloys
(e.g. Ceramic�Coxide particles)

C. Flake Composites - 1
Flakes, because of their shape, usually
reinforce in 2-D. Two common flake
materials are glass and mica. (Also
aluminum is used as metal flakes)

C. Flake Composites -2
A flake composite consists of thin, flat flakes
held together by a binder or placed in a
matrix. Almost all flake composite matrixes
are plastic resins. The most important flake
materials are:
1. Aluminum
2. Mica
3. Glass

C. Flake Composites -3
Basically, flakes will provide:
Uniform mechanical properties in the plane of
the flakes
Higher strength
Higher flexural modulus
Higher dielectric strength and heat resistance
Better resistance to penetration by liquids and
vapor
Lower cost

D. Laminar Composites - 1
Laminar composites involve two or more
layers of the same or different materials. The
layers can be arranged in different directions
to give strength where needed. Speedboat
hulls are among the very many products of
this kind.

Like all composites laminar composites
aim at combining constituents to produce
properties that neither constituent alone
would have.
In laminar composites outer metal is not
called a matrix but a face. The inner
metal, even if stronger, is not called a
reinforcement. It is called a base.

We can divide laminar composites into three basic types:
Unreinforced�Clayer composites
(1) All�CMetal
(a) Plated and coated metals (electrogalvanized
steel �C steel plated with zinc)
(b) Clad metals (aluminum�Cclad, copper�Cclad)
(c) Multilayer metal laminates (tungsten, beryllium)
(2) Metal�CNonmetal (metal with plastic, rubber, etc.)
(3) Nonmetal (glass�Cplastic laminates, etc.)
Reinforced�Clayer composites (laminae and laminates)
Combined composites (reinforced�Cplastic laminates
well bonded with steel, aluminum, copper, rubber,
gold, etc.)

A lamina (laminae) is any
arrangement of unidirectional
or woven fibers in a matrix.
Usually this arrangement is
flat, although it may be
curved, as in a shell.
A laminate is a stack of
lamina arranged with their
main reinforcement in at least
two different directions.

E. Filled Composites
There are two types of filled composites. In
one, filler materials are added to a normal
composite result in strengthening the
composite and reducing weight. The second
type of filled composite consists of a skeletal
3-D matrix holding a second material. The
most widely used composites of this kind are
sandwich structures and honeycombs.

F. Combined Composites
It is possible to combine
several different materials
into a single composite. It is
also possible to combine
several different composites
into a single product. A good
example is a modern ski.
(combination of wood as
natural fiber, and layers as
laminar composites)

Forms of Reinforcement Phase
Fibers
? cross-section can be circular, square or hexagonal
? Diameters --> 0.0001�� - 0.005 ��
? Lengths --> L/D ratio
? 100 -- for chopped fiber
? much longer for continuous fiber
Particulate
? small particles that impede dislocation movement
(in metal composites) and strengthens the matrix
? For sizes > 1 mm, strength of particle is involves in
load sharing with matrix
Flakes
? flat platelet form

Fiber Reinforcement
The typical composite consists of a matrix holding
reinforcing materials. The reinforcing materials, the
most important is the fibers, supply the basic
strength of the composite. However, reinforcing
materials can contribute much more than strength.
They can conduct heat or resist chemical corrosion.
They can resist or conduct electricity. They may be
chosen for their stiffness (modulus of elasticity) or for
many other properties.

Types of Fibers
The fibers are divided into two main groups:
Glass fibers: There are many different kinds of glass,
ranging from ordinary bottle glass to high purity quartz
glass. All of these glasses can be made into fibers.
Each offers its own set of properties.
Advanced fibers: These materials offer high strength
and high stiffness at low weight. Boron, silicon, carbide
and graphite fibers are in this category. So are the
aramids, a group of plastic fibers of the polyamide
(nylon) family.

Fibers - Glass
Fiberglass properties vary somewhat according to the type of glass
used. However, glass in general has several well�Cknown
properties that contribute to its great usefulness as a reinforcing
agent:
? Tensile strength
? Chemical resistance
? Moisture resistance
? Thermal properties
? Electrical properties
There are four main types of glass used in fiberglass:
? A�Cglass
? C�Cglass
? E�Cglass
? S�Cglass

Fibers - Glass
Most widely used fiber
Uses: piping, tanks, boats, sporting goods
Advantages
? Low cost
? Corrosion resistance
? Low cost relative to other composites:
Disadvantages
? Relatively low strength
? High elongation
? Moderate strength and weight
Types:
? E-Glass - electrical, cheaper
? S-Glass - high strength

Fibers - Aramid (kevlar, Twaron)
Uses:
? high performance replacement for glass
fiber
Examples
? Armor, protective clothing, industrial,
sporting goods
Advantages:
? higher strength and lighter than glass
? More ductile than carbon

Fibers - Carbon
2nd most widely used fiber
Examples
? aerospace, sporting goods
Advantages
? high stiffness and strength
? Low density
? Intermediate cost
? Properties:
? Standard modulus: 207-240 Gpa
? Intermediate modulus: 240-340 GPa
? High modulus: 340-960 GPa
? Diameter: 5-8 microns, smaller than human hair
? Fibers grouped into tows or yarns of 2-12k fibers

Fibers -- Carbon (2)
Types of carbon fiber
? vary in strength with processing
? Trade-off between strength and modulus
Intermediate modulus
? PAN (Polyacrylonitrile)
? fiber precursor heated and stretched to align structure
and remove non-carbon material
High modulus
? made from petroleum pitch precursor at lower
cost
? much lower strength

Fibers - Others
Boron
? High stiffness, very high cost
? Large diameter - 200 microns
? Good compressive strength
Polyethylene - trade name: Spectra fiber
? Textile industry
? High strength
? Extremely light weight
? Low range of temperature usage

Fibers -- Others (2)
Ceramic Fibers (and matrices)
? Very high temperature applications (e.g.
engine components)
? Silicon carbide fiber - in whisker form.
? Ceramic matrix so temperature resistance
is not compromised
? Infrequent use

Fiber Material Properties
Steel: density (Fe) = 7.87 g/cc; TS=0.380 GPa; Modulus=207 GPa
Al: density=2.71 g/cc; TS=0.035 GPa; Modulus=69 GPa

Matrix Materials
Functions of the matrix
? Transmit force between fibers
? arrest cracks from spreading between fibers
? do not carry most of the load
? hold fibers in proper orientation
? protect fibers from environment
? mechanical forces can cause cracks that allow
environment to affect fibers
Demands on matrix
? Interlaminar shear strength
? Toughness
? Moisture/environmental resistance
? Temperature properties
? Cost

Matrices - Polymeric
Thermosets
? cure by chemical reaction
? Irreversible
? Examples
? Polyester, vinylester
? Most common, lower cost, solvent resistance
? Epoxy resins
? Superior performance, relatively costly

Polyester
Polyesters have good mechanical properties, electrical
properties and chemical resistance. Polyesters are
amenable to multiple fabrication techniques and are low
cost.
Vinyl Esters
Vinyl Esters are similar to polyester in performance.
Vinyl esters have increased resistance to corrosive
environments as well as a high degree of moisture
resistance.
Matrices - Thermosets

Epoxy
Epoxies have improved strength and stiffness properties
over polyesters. Epoxies offer excellent corrosion
resistance and resistance to solvents and alkalis. Cure
cycles are usually longer than polyesters, however no
by-products are produced.
Flexibility and improved performance is also achieved
by the utilization of additives and fillers.
Matrices - Thermosets

Matrices - Thermoplastics
Formed by heating to elevated temperature
at which softening occurs
? Reversible reaction
? Can be reformed and/or repaired - not common
? Limited in temperature range to 150C
Examples
? Polypropylene
? with nylon or glass
? can be injected-- inexpensive
? Soften layers of combined fiber and resin and
place in a mold -- higher costs

Matrices - Others
Metal Matrix Composites - higher
temperature
? e.g., Aluminum with boron or carbon fibers
Ceramic matrix materials - very high
temperature
? Fiber is used to add toughness, not
necessarily higher in strength and stiffness

Important Note
Composite properties are less than
that of the fiber because of dilution
by the matrix and the need to orient
fibers in different directions.

MANUFACTURING PROCESSES
OF COMPOSITES
Composite materials have succeeded remarkably in their
relatively short history. But for continued growth,
especially in structural uses, certain obstacles must be
overcome. A major one is the tendency of designers to
rely on traditional materials such as steel and aluminum
unless composites can be produced at lower cost.
Cost concerns have led to several changes in the
composites industry. There is a general movement
toward the use of less expensive fibers. For example,
graphite and aramid fibers have largely supplanted the
more costly boron in advanced�Cfiber composites. As
important as savings on materials may be, the real key
to cutting composite costs lies in the area of processing.

The processing of fiber reinforced laminates can be
divided into two main steps:
? Lay�Cup
? Curing
Curing is the drying and hardening (or polymerization) of
the resin matrix of a finished composite. This may be
done unaided or by applying heat and/or pressure.
Lay�Cup basically is the process of arranging fiber�C
reinforced layers (laminae) in a laminate and shaping
the laminate to make the part desired. (The term lay�Cup
is also used to refer to the laminate itself before curing.)
Unless prepregs are used, lay�Cup includes the actual
creation of laminae by applying resins to fiber
reinforcements.

Laminate lay�Cup operations fall into three main
groups:
A. Winding and laying operations
B. Molding operations
C. Continuous lamination
Continuous lamination is relatively unimportant
compared with quality parameters as not good as wrt
other two processes. In this process, layers of fabric or
mat are passed through a resin dip and brought
together between cellophane covering sheets.
Laminate thickness and resin content are controlled by
squeegee rolls. The lay�Cup is passed through a heat
zone to cure the resin.

A. Winding Operation
The most important operation in this category is filament
winding. Fibers are passed through liquid resin, and then
wound onto a mandrel. After lay�Cup is completed, the
composite is cured on the mandrel. The mandrel is then
removed by melting, dissolving, breaking�Cout or some
other method.

B. Molding Operations
Molding operations are used in making a large number of
common composite products. There are two types of
processes:
A. Open�Cmold
(1) Hand lay�Cup
(2) Spray�Cup
(3) Vacuum�Cbag molding
(4) Pressure�Cbag molding
(5) Thermal expansion molding
(6) Autoclave molding
(7) Centrifugal casting
(8) Continuous pultrusion and pulforming.

1. Hand Lay-up
Hand lay�Cup, or contact molding, is the oldest and
simplest way of making fiberglass�Cresin composites.
Applications are standard wind turbine blades, boats,
etc.)

2. Spray-up
In Spray�Cup process, chopped fibers and resins are
sprayed simultaneously into or onto the mold. Applications
are lightly loaded structural panels, e.g. caravan bodies,
truck fairings, bathtubes, small boats, etc.

3. Vacuum-Bag Molding
The vacuum�Cbag process was developed for making
a variety of components, including relatively large
parts with complex shapes. Applications are large
cruising boats, racecar components, etc.

4. Pressure-Bag Molding
Pressure�Cbag process is virtually a mirror image of
vacuum�Cbag molding. Applications are sonar domes,
antenna housings, aircraft fairings, etc.

5. Thermal Expansion Molding
In Thermal Expansion Molding process, prepreg layers
are wrapped around rubber blocks, and then placed in
a metal mold. As the entire assembly is heated, the
rubber expands more than the metal, putting pressure
on the laminate. Complex shapes can be made
reducing the need for later joining and fastening
operations.

6. Autoclave Molding
Autoclave molding is similar to both vacuum�Cbag and
pressure�Cbag molding. Applications are lighter, faster
and more agile fighter aircraft, motor sport vehicles.

Continuous pultrusion
is the composite
counterpart of metal
extrusion. Complex
parts can be made.
7. Centrifugal Casting
Centrifugal Casting is used to form round objects such as
pipes.
8. Continuous Pultrusion and Pulforming

Pulforming is similar to pultrusion in many ways.
However, pultrusion is capable only of making straight
products that have the same volume all along their
lengths. Pulformed products, on the other hand, can be
either straight or curved, with changing shapes and
volumes. A typical pulformed product is a curved
reinforced plastic car spring. (shown in figure.)

B. Closed�Cmold
(1) Matched�Cdie molding: As the name
suggests, a matched�Cdie mold consists
of closely matched male and female
dies (shown in figure). Applications are
spacecraft parts, toys, etc.
(2) Injection molding: The injection
process begins with a thermosetting
(or sometimes thermoplastic) material
outside the mold. The plastic may
contain reinforcements or not. It is first
softened by heating and/or mechanical
working with an extrusion�Ctype screw.
It is then forced, under high pressure
from a ram or screw, into the cool
mold. Applications are auto parts,
vanes, engine cowling defrosters and
aircraft radomes.

Material Forms and Manufacturing
Objectives of material production
? assemble fibers
? impregnate resin
? shape product
? cure resin

Sheet Molding Compound (SMC)
? Chopped glass fiber added to polyester
resin mixture
?Question: Is SMC isotropic or anisotropic?

Manufacturing - Filament Winding
Highly automated
? low manufacturing
costs if high
throughput
? e.g., Glass fiber pipe,
sailboard masts

Prepregs
Prepreg and prepreg layup
? ��prepreg�� - partially cured mixture of fiber
and resin
? Unidirectional prepreg tape with paper backing
? wound on spools
? Cut and stacked
? Curing conditions
? Typical temperature and pressure in autoclave
is 120-200C, 100 psi

Manufacturing - Layups
compression
molding
vacuum bagging

Material Forms
Textile forms
? Braiding or weaving
?Tubular braided form
? can be flattened and cut for non-tubular
products

Fabric Structures
Woven: Series of Interlaced yarns at 90�� to each other
Knit: Series of Interlooped Yarns
Braided: Series of Intertwined, Spiral Yarns
Nonwoven: Oriented fibers either mechanically,
chemically, or thermally bonded

Woven Fabrics
Basic woven fabrics consists of two
systems of yarns interlaced at right
angles to create a single layer with
isotropic or biaxial properties.

Physical Properties
Construction (ends & picks)
Weight
Thickness
Weave Type

Braiding
A braid consists of two sets of yarns, which are helically
intertwined.
The resulting structure is oriented to the longitudinal axis
of the braid.
This structure is imparted with a high level of
conformability, relative low cost and ease of manufacture.

Triaxial Yarns
A system of longitudinal yarns can be introduced
which are held in place by the braiding yarns
These yarns will add dimensional stability, improve
tensile properties, stiffness and compressive strength.
Yarns can also be added to the core of the braid to
form a solid braid.

Fabric effects on material properties

Resin transfer molding (RTM)
Dry-fiber preform placed in a closed
mold, resin injected into mold, then
cured

Material Forms
Pultrusion
? Fiber and matrix are pulled through a
die, like extrusion of metals --
assembles fibers, impregnates the
resin, shapes the product, and cures
the resin in one step.
? Example. Fishing rods

Manufacturing
Tube rolling - tubular products
? Examples
? fishing rods
? golf clubs
? oars
? Prepreg tape typically used wrapped in 2
directions or spiral wrapped

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