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2002 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 2/e

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

ch 9 (1).pdf

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