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COMPOSITE MATERIALS
Technology and Classification of Composite Materials
Metal Matrix Composites
Ceramic Matrix Composites Polymer Matrix Composites
Guide to Processing Composite Materials
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Composite Material Defined
A materials system composed of two or more physically
distinct phases whose combination produces
aggregate properties that are different from those of
its constituents
Examples:
Cemented carbides (WC with Co binder)
Plastic molding compounds containing fillers
Rubber mixed with carbon black
Wood (a natural composite as distinguished from
a synthesized composite)
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Why Composites are Important
Composites can be very strong and stiff, yet very lightin weight, so ratios of strength-to-weight andstiffness-to-weight are several times greater than
steel or aluminum Fatigue properties are generally better than for
common engineering metals
Toughness is often greater too
Composites can be designed that do not corrode likesteel
Possible to achieve combinations of properties notattainable with metals, ceramics, or polymers alone
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Disadvantages and Limitations of
Composite Materials Properties of many important composites are
anisotropic - the properties differ depending on the
direction in which they are measured this may be
an advantage or a disadvantage
Many of the polymer-based composites are subject to
attack by chemicals or solvents, just as the polymers
themselves are susceptible to attack
Composite materials are generally expensive
Manufacturing methods for shaping composite
materials are often slow and costly
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One Possible Classification of
Composite Materials1. Traditional composites composite materials that
occur in nature or have been produced by
civilizations for many years
Examples: wood, concrete, asphalt
2. Synthetic composites - modern material systems
normally associated with the manufacturing
industries, in which the components are first
produced separately and then combined in acontrolled way to achieve the desired structure,
properties, and part geometry
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Components in a Composite Material
Nearly all composite materials consist of two
phases:
1. Primary phase - forms the matrixwithin which the
secondary phase is imbedded
2. Secondary phase - imbedded phase sometimes
referred to as a reinforcing agent, because it
usually serves to strengthen the composite
The reinforcing phase may be in the form of
fibers, particles, or various other geometries
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Our Classification Scheme for
Composite Materials1. Metal Matrix Composites (MMCs) - mixtures of
ceramics and metals, such as cemented carbidesand other cermets
2. Ceramic Matrix Composites (CMCs) - Al2O3 and SiCimbedded with fibers to improve properties,especially in high temperature applications
The least common composite matrix
3. Polymer Matrix Composites (PMCs) - thermosettingresins are widely used in PMCs
Examples: epoxy and polyester with fiberreinforcement, and phenolic with powders
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Functions of the Matrix Material
(Primary Phase) Provides the bulk form of the part or product made of
the composite material
Holds the imbedded phase in place, usually
enclosing and often concealing it
When a load is applied, the matrix shares the load
with the secondary phase, in some cases deforming
so that the stress is essentially born by the
reinforcing agent
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The Reinforcing Phase
(Secondary Phase) Function is to reinforce the primary phase
Imbedded phase is most commonly one of thefollowing shapes:
Fibers
Particles
Flakes
In addition, the secondary phase can take the form of
an infiltrated phase in a skeletal or porous matrix Example: a powder metallurgy part infiltrated with
polymer
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Figure 9.1 - Possible physical shapes of imbedded phases in
composite materials: (a) fiber, (b) particle, and (c) flake
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Fibers
Filaments of reinforcing material, usually circular incross-section
Diameters range from less than 0.0025 mm to about
0.13 mm, depending on material Filaments provide greatest opportunity for strength
enhancement of composites
The filament form of most materials is significantly
stronger than the bulk form As diameter is reduced, the material becomes
oriented in the fiber axis direction and probabilityof defects in the structure decreases significantly
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Continuous vs. Discontinuous Fibers
Continuous fibers - very long; in theory, they offer a
continuous path by which a load can be carried by
the composite part
Discontinuous fibers (chopped sections of continuous
fibers) - short lengths (L/D = roughly 100)
Important type of discontinuous fiber are
whiskers - hair-like single crystals with diameters
down to about 0.001 mm (0.00004 in.) with veryhigh strength
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Fiber Orientation Three Cases
One-dimensionalreinforcement, in which maximum
strength and stiffness are obtained in the direction of
the fiber
Planarreinforcement, in some cases in the form of a
two-dimensional woven fabric
Random or three-dimensional in which the composite
material tends to possess isotropic properties
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Figure 9.3 - Fiber orientation in composite materials:
(a) one-dimensional, continuous fibers; (b) planar, continuous fibers in
the form of a woven fabric; and (c) random, discontinuous fibers
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Materials for Fibers
Fiber materials in fiber-reinforced composites:
Glass most widely used filament
Carbon high elastic modulus Boron very high elastic modulus
Polymers - Kevlar
Ceramics SiC and Al2O3
Metals - steel
The most important commercial use of fibers is in
polymer composites
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Particles and Flakes
A second common shape of imbedded phase is
particulate, ranging in size from microscopic to
macroscopic
Flakes are basically two-dimensional particles - small
flat platelets
The distribution of particles in the composite matrix is
random, and therefore strength and other properties
of the composite material are usually isotropic
Strengthening mechanism depends on particle size
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The Interface
There is always an interface between constituentphases in a composite material
For the composite to operate effectively, the phasesmust bond where they join at the interface
Figure 9.4 - Interfaces between phases in a composite material:
(a) direct bonding between primary and secondary phases
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Interphase
In some cases, a third ingredient must be added toachieve bonding of primary and secondary phases
Called an interphase, this third ingredient can be
thought of as an adhesive
Figure 9.4 - Interfaces between phases: (b) addition of a third
ingredient to bond the primary phases and form an interphase
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Figure 9.4 - Interfaces and interphases between phases in acomposite material: (c) formation of an interphase by solution of
the primary and secondary phases at their boundary
Another InterphaseInterphase consisting of a solution of primary and
secondary phases
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Properties of Composite Materials
In selecting a composite material, an optimum
combination of properties is usually sought, rather
than one particular property
Example: fuselage and wings of an aircraft mustbe lightweight and be strong, stiff, and tough
Several fiber-reinforced polymers possess this
combination of properties
Example: natural rubber alone is relatively weakAdding significant amounts of carbon black to
NR increases its strength dramatically
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Properties are Determined by
Three Factors:1. The materials used as component phases in the
composite
2. The geometric shapes of the constituents and
resulting structure of the composite system
3. The manner in which the phases interact with one
another
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Figure 9.5 - (a) Model of a fiber-reinforced composite materialshowing direction in which elastic modulus is being estimated bythe rule of mixtures (b) Stress-strain relationships for thecomposite material and its constituents. The fiber is stiff butbrittle, while the matrix (commonly a polymer) is soft but ductile.
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Figure 9.6 - Variation in elastic modulus and tensile strength as afunction of direction of measurement relative to longitudinal axis
of carbon fiber-reinforced epoxy composite
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Fibers Illustrate Importance of
Geometric Shape Most materials have tensile strengths several times
greater as fibers than in bulk
By imbedding the fibers in a polymer matrix, a
composite material is obtained that avoids the
problems of fibers but utilizes their strengths
The matrix provides the bulk shape to protect the
fiber surfaces and resist buckling
When a load is applied, the low-strength matrix
deforms and distributes the stress to the
high-strength fibers
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Other Composite Structures
Laminar composite structure conventional
Sandwich structure
Honeycomb sandwich structure
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Laminar Composite StructureTwo or more layers bonded together in an integral piece
Example:plywoodin which layers are the samewood, but grains are oriented differently to increaseoverall strength of the laminated piece
Figure 9.7 - Laminar compositestructures: (a) conventional laminar
structure
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Sandwich Structure Foam Core
Consists of a relatively thick core of low density foambonded on both faces to thin sheets of a different
material
Figure 9.7 - Laminar
composite structures: (b)sandwich structure using foam
core
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Sandwich Structure Honeycomb Core An alternative to foam core
Either foam or honeycomb achieves highstrength-to-weight and stiffness-to-weight ratios
Figure 9.7 - Laminar
composite structures: (c)
sandwich structure using
honeycomb core
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Other Laminar Composite Structures
Automotive tires - consists of multiple layers bonded
together
FRPs - multi-layered fiber-reinforced plastic panels
for aircraft, automobile body panels, boat hulls
Printed circuit boards - layers of reinforced plastic
and copper for electrical conductivity and insulation
Snow skis - composite structures consisting of layers
of metals, particle board, and phenolic plastic Windshield glass - two layers of glass on either side
of a sheet of tough plastic
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Metal Matrix Composites (MMCs)
A metalmatrix reinforced by a second phase
Reinforcing phases:
1. Particles of ceramic (these MMCs are commonlycalled cermets)
2. Fibers of various materials: other metals,
ceramics, carbon, and boron
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Cermets
MMC with ceramiccontained in a metallic matrix
The ceramic often dominates the mixture,
sometimes up to 96% by volume
Bonding can be enhanced by slight solubility
between phases at elevated temperatures used in
processing
Cermets can be subdivided into
1. Cemented carbides most common
2. Oxide-based cermets less common
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Cemented Carbides
One or more carbide compounds bonded in a metallic
matrix
The term cermetis not used for all of these materials,
even though it is technically correct
Common cemented carbides are based on tungsten
carbide (WC), titanium carbide (TiC), and chromium
carbide (Cr3C2)
Tantalum carbide (TaC) and others are less common
Metallic binders: usually cobalt (Co) or nickel (Ni)
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Figure 9.8 - Photomicrograph (about 1500X) of cemented carbide
with 85% WC and 15% Co (photo courtesy of Kennametal Inc.)
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Figure 9.9 - Typical plot of hardness and transverse rupture
strength as a function of cobalt content
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Applications of Cemented Carbides
Tungsten carbide cermets (Co binder) - cutting tools
are most common; other: wire drawing dies, rock
drilling bits and other mining tools, dies for powder
metallurgy, indenters for hardness testers
Titanium carbide cermets (Ni binder) - high
temperature applications such as gas-turbine nozzle
vanes, valve seats, thermocouple protection tubes,
torch tips, cutting tools for steels Chromium carbides cermets (Ni binder) - gage
blocks, valve liners, spray nozzles, bearing seal rings
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Ceramic Matrix Composites (CMCs)
A ceramicprimary phase imbedded with a secondary
phase, which usually consists of fibers
Attractive properties of ceramics: high stiffness,
hardness, hot hardness, and compressive strength;
and relatively low density
Weaknesses of ceramics: low toughness and bulk
tensile strength, susceptibility to thermal cracking
CMCs represent an attempt to retain the desirable
properties of ceramics while compensating for their
weaknesses
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Polymer Matrix Composites (PMCs)
Apolymerprimary phase in which a secondary phase is
imbedded as fibers, particles, or flakes
Commercially, PMCs are more important than MMCs
or CMCs
Examples: most plastic molding compounds, rubber
reinforced with carbon black, and fiber-reinforced
polymers (FRPs)
FRPs are most closely identified with the term
composite
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Fiber-Reinforced Polymers (FRPs)
A PMC consisting of apolymer matriximbedded with
high-strength fibers
Polymer matrix materials:
Usually a thermosetting(TS) plastic such as
unsaturated polyester or epoxy
Can also be thermoplastic(TP), such as nylons
(polyamides), polycarbonate, polystyrene, and
polyvinylchloride
Fiber reinforcement is widely used in rubber
products such as tires and conveyor belts
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Fibers in PMCs
Various forms: discontinuous (chopped), continuous,
or woven as a fabric
Principal fiber materials in FRPs are glass, carbon,
and Kevlar 49
Less common fibers include boron, SiC, and Al2O3,
and steel
Glass (in particular E-glass) is the most common fiber
material in today's FRPs; its use to reinforce plastics
dates from around 1920
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Common FRP Structure
Most widely used form of FRP is a laminar structure,
made by stacking and bonding thin layers of fiber and
polymer until desired thickness is obtained
By varying fiber orientation among layers, a specifiedlevel of anisotropy in properties can be achieved in
the laminate
Applications: parts of thin cross-section, such as
aircraft wing and fuselage sections, automobile andtruck body panels, and boat hulls
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FRP Properties
High strength-to-weight and modulus-to-weight ratios
Low specific gravity - a typical FRP weighs only
about 1/5 as much as steel; yet, strength and
modulus are comparable in fiber direction
Good fatigue strength
Good corrosion resistance, although polymers are
soluble in various chemicals
Low thermal expansion - for many FRPs, leading togood dimensional stability
Significant anisotropy in properties
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FRP Applications
Aerospace much of the structural weight of todaysairplanes and helicopters consist of advanced FRPs
Automotive somebody panels for cars and truck
cabs Continued use of low-carbon sheet steel in cars is
evidence of its low cost and ease of processing
Sports and recreation
Fiberglass reinforced plastic has been used forboat hulls since the 1940s
Fishing rods, tennis rackets, golf club shafts,helmets, skis, bows and arrows.
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Figure 9.11 - Composite materials in the Boeing 757
(courtesy of Boeing Commercial Airplane Group)
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Other Polymer Matrix Composites
In addition to FRPs, other PMCs contain particles,flakes, and short fibers as the secondary phase
Called fillers when used in molding compounds
Two categories:
1. Reinforcing fillers used to strengthen orotherwise improve mechanical properties
Examples: wood flour in phenolic and aminoresins; and carbon black in rubber
2. Extenders used to increase bulk and reducecost per unit weight, but little or no effect onmechanical properties
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Guide to Processing Composite Materials
The two phases are typically produced separately
before being combined into the composite part
Processing techniques to fabricate MMC and CMC
components are similar to those used for powderedmetals and ceramics
Molding processes are commonly used for PMCs
with particles and chopped fibers
Specialized processes have been developed forFRPs