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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.