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Introduction

8-2

Every material is composite at one or the other

level.

A composite material is a material system, a

mixture or combination of two or more micro or

macroconstituents that differ in form andcomposition and do not form a solution.

Properties of composite materials can be superior

to its individual components.

Examples: Fiber reinforced plastics, concrete,

asphalt, wood etc.

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L a r g e -p a r t i c l e

D i s p e r s i o n -s t r e n g t h e n e d

P a r t i c l e - r e i n f o r c e d

C o n t i n u o u s( a l i g n e d )

A l i g n e d R a n d o m l y

o r i e n t e d

D i s c o n t i n u o u s( s h o r t )

F i b e r - r e i n f o r c e d

L a m i n a t e s S a n d w i c hp a n e l s

S t r u c t u r a l

C o m p o s i t e s

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Terminology/Classification

Composites:

-- Multiphase material w/significantproportions of each phase.

Dispersed phase:-- Purpose: enhance matrix properties.

MMC: increase y, TS, creep resist.

CMC: increase Kc

PMC: increase E, y, TS, creep resist.

-- Classification: Particle, fiber, structural

Matrix:-- The continuous phase

-- Purpose is to:

- transfer stress to other phases- protect phases from environment

-- Classification: MMC, CMC, PMC

metal ceramic polymer

woven

fibers

crosssectionview

0.5mm

0.5mm

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Examples:Adapted from Fig.

10.19, Callister 7e.

(Fig. 10.19 is

copyright United

States Steel

Corporation, 1971.)

- Spheroidite

steel

matrix:ferrite ()

(ductile)

particles:cementite(Fe3C)

(brittle)

60m

Adapted from Fig.

16.4, Callister 7e.

(Fig. 16.4 is courtesy

Carboloy Systems,

Department, General

Electric Company.)

- WC/Co

cemented

carbide

matrix:cobalt(ductile)

particles:WC(brittle,hard)Vm:

10-15 vol%! 600m

Adapted from Fig.

16.5, Callister 7e.

(Fig. 16.5 is courtesy

Goodyear Tire and

Rubber Company.)

- Automobile

tires

matrix:rubber(compliant)

particles:C(stiffer)

0.75m

Particle-reinforced Fiber-reinforced Structural

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Concrete gravel + sand + cement- Why sand andgravel? Sand packs into gravel voids

Reinforced concrete - Reinforce with steel rerod or remesh- increases strength - even if cement matrix is cracked

Pre-stressed concrete - remesh under tension during setting ofconcrete. Tension release puts concrete under compressive force

- Concrete much stronger under compression.

- Applied tension must exceed compressive force

threadedrod

nut

Post tensioning tighten nuts to put under tension

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Elastic modulus, Ec, of composites:-- two approaches.

Application to other properties:

-- Electrical conductivity, e: Replace Ein equations with e.

-- Thermal conductivity, k: Replace Ein equations with k.

lower limit:

1Ec

= VmEm

+ VpEp

c m m

upper limit:

E = V E + VpEp

rule of mixtures

Data:

Cu matrix

w/tungstenparticles

0 20 40 60 80 100

150

200

250

300

350

vol% tungsten

E(GPa)

(Cu) (W)

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Fiber Materials Whiskers - Thin single crystals - large length to diameter ratio

graphite, SiN, SiC

high crystal perfection extremely strong, strongest known

very expensive Fibers

polycrystalline or amorphous

generally polymers or ceramics

Ex: Al2O3 , Aramid, E-glass, Boron, UHMWPE

Wires

Metal steel, Mo, W

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alignedcontinuous

aligned randomdiscontinuous

Fiber Alignment

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Aligned Continuous fibers Examples:

-- Metal: '(Ni3Al)-(Mo)by eutectic solidification.

matrix: (Mo) (ductile)

fibers: (Ni3Al) (brittle)

2m

-- Ceramic: Glass w/SiC fibersformed by glass slurry

Eglass = 76 GPa; ESiC = 400 GPa.

(a)

(b)

fracturesurface

From F.L. Matthews and R.L.

Rawlings, Composite Materials;Engineering and Science, Reprint

ed., CRC Press, Boca Raton, FL,

2000. (a) Fig. 4.22, p. 145 (photo by

J. Davies); (b) Fig. 11.20, p. 349

(micrograph by H.S. Kim, P.S.

Rodgers, and R.D. Rawlings). Used

with permission of CRC

Press, Boca Raton, FL.

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Discontinuous, random 2D fibers Example: Carbon-Carbon

-- process: fiber/pitch, then

burn out at up to 2500C.

-- uses: disk brakes, gas

turbine exhaust flaps, nose

cones.

Other variations:-- Discontinuous, random 3D

-- Discontinuous, 1D

(b)

fibers liein plane

view onto plane

C fibers:very stiffvery strong

C matrix:less stiffless strong

(a)

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Critical fiber length for effective stiffening & strengthening:

Ex: For fiberglass, fiber length > 15 mm needed

c

fd

>15lengthfiber

fiber diameter

shear strength of

fiber-matrix interface

fiber strength in tension

Why? Longer fibers carry stress more efficiently!Shorter, thicker fiber:

c

fd

15lengthfiber

Better fiber efficiency

(x) (x)

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Composite Strength: Longitudinal Loading

Continuous fibers - Estimate fiber-reinforcedcomposite strength for long continuous fibers in a

matrix

Longitudinal deformation

c = mVm+ fVf but c= m= f

volume fraction isostrain Ece= Em Vm + EfVf longitudinal (extensional)

modulus

mm

ff

m

f

VE

VE

F

F= f = fiber

m = matrix

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Composite Strength: Transverse Loading

In transverse loading the fibers carry less of the

load - isostress

c= m= f = c= mVm+ fVf

f

f

m

m

ctE

V

E

V

E+=

1transverse modulus

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Equation for Elastic Modulus of Lamellar Composite

Isostrain condition: Stress on composite causes uniform

strain on all composite layers.Pc = Pf + Pm

= P/A

cAc = fAf + mAm

Since length of layers are equal,

cV

c=

fV

f+

mV

mWhere V

c, V

fand V

mare volume

fractions (Vc =1)

Since strains c = f = m,

Ec = EfVf + EmVm

m

mm

f

ff

c

c VV

+=

Pc = Load on composite

Pf = Load on fibers

Pm = load on matrix

Rule of mixture of binary composites

Figure 11.14

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Loads on Fiber and Matrix Regions

Since = E and f = m

Pc = Pf + Pm

From above two equations, load on each offiber and

mm

ff

mm

ff

mmm

fff

mm

ff

m

f

VE

VE

AE

AE

AE

AE

A

A

P

P====

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Isostress Condition

Stress on the composite structure produces an equal stress

condition on all the layers.c = f + m

c = f + m

Assuming no change in area

and assuming unit length of the composite

c = fVf + mVm But

Therefore

m

m

f

f

c

c

EEE

=== ,,

m

m

f

f

c E

V

E

V

E

+=

Figure 11.15

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Elastic Modulus for Isostress Condition

We know that

Dividing by

m

m

f

f

c E

V

E

V

E

+=

fmmf

mf

c

fm

fm

mf

mf

c

m

m

f

f

c

EVEV

EEE

EE

EV

EE

EV

E

E

V

E

V

E

+

=

+=

+=

1

1

Higher modulus values are

obtained with isostrain

loading for equal volume of

fibers

Figure 11.16

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Estimate ofEcand TSfor discontinuous fibers:

-- valid when

-- Elastic modulus in fiber direction:

-- TSin fiber direction:

efficiency factor:-- aligned 1D: K= 1 (aligned )

-- aligned 1D: K= 0 (aligned )

-- random 2D: K= 3/8 (2D isotropy)

-- random 3D: K= 1/5 (3D isotropy)

(aligned 1D)

c

fd

>15lengthfiber

(TS)c= (TS)mVm + (TS)fVf

Ec= EmVm + KEfVf

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Stacked and bonded fiber-reinforced sheets

-- stacking sequence: e.g., 0/90

-- benefit: balanced, in-plane stiffness

Particle-reinforced Fiber-reinforced Structural

Sandwich panels-- low density, honeycomb core

-- benefit: small weight, large bending stiffness

honeycombadhesive layer

face sheet

Adapted from Fig. 16.18,

Callister 7e. (Fig. 16.18 is

from Engineered Materials

Handbook, Vol. 1, Composites, ASM International, Materials Park, OH, 1987.)

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Polymer Matrix Composite (PMC)

Glass fiber reinforced plastic composite materials

have high strength-weight ratio, good dimensional

stability, good temperature and corrosion

resistance and low cost.

E Glass : 52-56% SiO2, + 12-16% Al2O3,16-25% CaO + 8-13% B2O3

Tensile strength = 3.44 GPa, E = 72.3 GPa

S Glass : Used for military and aerospace

application.

65% SiO2 + 25% Al2O3 + 10% MgO

Tensile strength = 4.48 GPa, E = 85.4 GPa

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Produced by drawing monofilaments from a

furnace and gathering them to form a strand.

Strands are held together with resinous binder.

Properties: Density

and strength are lowerthan carbon and aramid

fibers.

Higher elongation.

Low cost and hence

commonly used.

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Fiber Reinforced-Plastic Composite Materials

Fiberglass-reinforced polyester resins: Higher the wt% of glass, stronger the reinforcedplastic is.

Nonparallel alignment of glass fibers reducesstrength.

Carbon fiber reinforced epoxy resins: Carbon fiber contributes to rigidity and strength

while epoxy matrix contributes to impact strength.

Polyimides, polyphenylene sulfides are also used.

Exceptional fatigue properties. Carbon fiber epoxy material is laminated to meet

strength requirements.

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Properties of Fiber Reinforced Plastics

Fiberglass polyester

(Carbon fibers and epoxy)

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Aramid Fibers for Reinforcing Plastic Resins

Aramid = aromatic polyamide fibers.

Trade name is Kevlar

Kevlar 29:- Low density, high strength, and used for ropes and

cables.

Kevlar 49:- Low density, high strength and modulus and used for

aerospace and auto applications.

Hydrogen bonds bond fiber together.

Used where resistance to fatigue, high

strength and light weight is important.

Table 11.1

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Fatigue Characteristics of Fiber Reinforced Plastics

Lamination

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Ceramic-Matrix Composites (CMCs)

Continuous fiber reinforced CMCs:

SiC fibers are woven into mat and SiC is impregnatedinto fibrous mat by chemical vapor deposition.

SiC fibers can be encapsulated by a glass ceramic.

Used in heat exchanger tube and thermal protection

system. Discontinuous and particulate reinforced CMCs:

Fracture toughness is significantly increased.

Fabricated by common process such as hot isolatic

pressing.

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Portland Cement

Production: Lime (CaO), Silica (SiO2), alumina(Al2O3)

and iron oxide (Fe2O3) are raw materials.

Raw materials are crushed, ground and proportional for

desired composition and blended.

Mixture is fed into rotary kiln and heated to 1400-1650

0

Cand then cooled andpulverized.

Chemical Composition:

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Types of Portland Cement

Types of Portland cement differ by composition.

TypeI: Used when high sulfate attack from soil and

water, and high temperature are absent.

Examples: Sidewalks, buildings, bridges.

TypeII: Used in case of moderate sulfate attack as in case

of drainage.

Type III: Early strength type for quick use.

TypeIV: Low heat of hydration type and used when rate

and heat generated must be minimized.

Type V: Used for heavy sulfate attack as in case of

groundwater.

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Hardening of Portland Cement

Tricalcium silicate and dicalcium silicate constitute 75% of

portland cement. Hydration reactions:

2C3S + H2O C3S2.3H2O + 3Ca(OH)2

2C2S + 4H2O C3S2.3H2O + Ca(OH)2

Tricalcium silicate hydrate

C3S is responsible for early strength.

Most of compressive strength is

developed in 28 days.

Strengthening might continue

for years

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Water Aggregate and Air

Drinking and Non-Drinking water can be used.

Non-drinking water should be tested forlevel ofimpurities.

Aggregates make up 60-80% of concrete volume.

Fine aggregates are ofsand particles and coarse

aggregates are rocks.

Air entraining agents are sometimes added.

They increase resistance to freezing and thawing and

improved workability.

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Compressive Strength

Compressive strength is higher than tensile strength and

depends up on settled time. High water content reduces compressive strength.

Air entrainment improves workability and hence water

content can be reduced.

Air

Bubbles

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Toughening Mechanisms in CMCs

Toughening is due to fibers interfering with crack

propagation.

Crack deflection: Up on encountering

reinforcement, crack is deflected making

propagation more meandering. Crack bridging: Fibers bridge the crack and help

to keep the cracks together.

Fiber pullout: Friction caused by pulling out the

fiber from matrix results in higher toughness.

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Metal Matrix Composites (MMCs)

Continuous fiber reinforced MMCs: Continuous fibers are

reinforced in metal matrix used in aerospace, autoindustry and sports equipments.

Example:- Aluminum alloy Boron fiber composite

Boron fiber is made by depositingboron vaporon tungstensubstrate.

Boron fibers are hotpressed between aluminum foils.

Tensile strength of Al6061 increases from 310 to 1417GPa and E

increases from 69 to 231 GPa

Tungsten filament

Boron

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Discontinuous fiber and particulate reinforced MMCs

Particulate reinforced MMCs: Irregular shaped alumina

and silicon carbide particulate are used. Particulate is mixed into molten aluminum and cast into ingots or

billets.

Al 6061 + 20% SiC

Discontinuous fiber reinforced MMcs: Needle like SiCwhiskers (1-3 micron diameter, 20-200 micron in length)

are mixed with metal powder.

Mixture is consolidated by hot pressing

and then forged or extruded.

Tensile strength of Al 6061 increases to

480 MPa and E increases to 115 GPa

Tensile strength increased to 496 MPa

E increased to 103 GPa

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Composite Benefits

CMCs: Increased toughness

fiber-reinf

un-reinf

particle-reinfForce

Bend displacement

PMCs: Increased E/

E(GPa)

G=3E/8K=E

Density, [mg/m3].1 .3 1 3 10 30

.01

.1

1

10

102

103

metal/metal alloys

polymers

PMCs

ceramics

Adapted from T.G. Nieh, "Creep rupture of

a silicon-carbide reinforced aluminum

composite", Metall. Trans. A Vol. 15(1), pp.

139-146, 1984. Used with permission.

MMCs:Increased

creep

resistance

20 30 50 100 20010-10

10-8

10-6

10-4

6061 Al

6061 Alw/SiCwhiskers

(MPa)

ss (s-1)

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Open Mold Process for Fiber Reinforced Plastics

Hand lay-up process:

Gel coat is applied to open mold. Fiberglass reinforcement is

placed in the mold.

Base resin mixed

with catalysts is

applied by pouring

brushing or spraying.

Spray-up process: Continuous

strand roving is fed by chopper

and spray gun and choppedroving and catalyst resin is

deposited in the mold.

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Vacuum Bag-Autoclave and Filament WindingVacuum Bag-Autoclave and Filament Winding

Vacuum bag-autoclave process:

Long thin sheet or prepeg carbon-fiber epoxy material is laid onthe table.

The sheet is cut and laminate is constructed.

Laminate is put in vacuum bag to remove entrapped air andcured in autoclave.

Filament winding: Fiber reinforcement is fed

through resin bath and

wound around suitable

mandrel.

Mandrel is cured and mold part is stripped from mandrel.

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Closed Mold Process

Compression and injection molding:

Same as in polymers except that the fiber reinforcement is mixedwith resin.

Sheet molding compound process:

Highly automated continuous molding process.

Continuous strand fiberglass

roving is chopped and deposited

on a layer ofresin-filler paste.

Another layer of paste is

deposited on first layer.

Sandwich is compacted

and rolled into rolls.

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Sheet Molding

The rolled up sheet is stored in a maturation room for 1-4

days. The sheets are cut into proper size and pressed in hot mold

(1490C) to form final product.

Efficient, quick, good quality and uniformity.

Continuous protrusion: Continuous strand fibers areimpregnated in resin bath, fed into heated die and drawn.

Used to produce

beams, channels,

and pipes.