MME131: Lecture 30
Composite Materials
A. K. M. B. Rashid
Professor, Department of MME
BUET, Dhaka
Topics to Discuss …..
What are composites?
Why do we make composite material?
Common terminologies
Classifications of composite materials
Benefits of composites
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What are composite materials ?
 Composites are artificially materials containing of two or more
physically distinct phases, separated by a distinct interface
THE MATRIX
(aluminium)
INTERFACE
(allows transfer of
stress from the matrix
to the dispersed
phase)
REINFORCEMENT
(tungsten fibre)
tungsten fibre reinforced
aluminium composite
Some examples
of composite
materials
(a) plywood is a laminar
composite of layers of
wood veneer
(b) fiberglass is a fiberreinforced composite
containing stiff, strong
glass fibers in a softer
polymer matrix (175)
(c) concrete is a particulate
composite containing
coarse sand or gravel in a
cement matrix (reduced to
50%).
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Natural Composites:
wood and bamboo, shells, bones, muscles
Natural fibres:
silk, wool, cotton, jute
Abalone shell:
CaCO3 + 3% organic material
>3000 times stronger than calcite
Wood:
cellulose-filaments in
a matrix of lignin and
hemicellulose
Why do we make composites ?
 The combination of phases produces properties that are different
from those of its constituents
 Offset the poor qualities of one phase with the good qualities of another
 The primary needs for making composites:
☐ light weight
☐ greater strength and stiffness
☐ better corrosion resistance
☐ higher operating temperatures
☐ higher impact and wear resistance
☐ higher reliability and affordability
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“The best of both worlds”
Metals
Pros
 electrically, thermally conductive
 good strength and ductility
 high toughness
 magnetic
Pros
 electrically, thermally insulating
 wear and corrosion resistant
 high strength and stiffness
 creep resistant
 low density
Cons
 dense
 low creep resistance
 low/moderate corrosion resistance
Composites
Ceramics
Cons
 difficult to form/machine
 very low toughness
Pros
 very ductile
 easy to form
 corrosion resistant
 high strength-to-weight ratio
Polymers
Cons
 low stiffness & strength
 poor high temperature properties
Common terminologies
The matrix
 Continuous phase, or the bulk material, the property of which is
generally reinforced
 Made from metals, polymers or ceramics
 Some ductility of the matrix and high bonding strength between
matrix and reinforcements are desirable
 Functions of matrix




Binds the reinforcements together
Mechanically supporting the reinforcements
Transfer the applied load to the reinforcements
Protect the reinforcements from surface damage due to abrasion or
chemical attacks
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 Metal matrix
moderately stiff and strong
moderately hard, wear and abrasion resistance
moderately creep and fatigue resistance
Aim – to make much stiffer, stronger and wear, creep and fatigue resistant
Common matrices: Al, Cu, Ti, Ni
Example: SiC reinforced Al
 Ceramic matrix
hard and brittle
Aim – to make tougher and more reliable
Common matrices: glass, cement, Al2O3, ZrO2, TiO2
Example: ZrO2 toughened Al2O3, Ag toughened Al2O3 , steel reinforced concrete
 Polymer matrix
weaker and have low melting point
Aim – to make more stronger and temperature resistant
Common matrices: epoxy, polyester, polyurethane, rubber
Example: GFRP, CFRP
The Reinforcing Material
 The dispersed phase in the matrix
 Made from metals, polymers or ceramics
 Can be in the form of particles, fibres or various other geometries
 Functions of reinforcing material: to enhance matrix properties
 Particle reinforcement
Silver, Cobalt; Silica, Carbon black, Rocks, Alumina, Talc, SiC, Si3N4, Glass beads
 Fibre reinforcement
Boron, Steel, Tungsten, Chromium; Carbon, Alumina, SiC, Glass, Kevlar
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Classification of composites
Based on Matrix Phase
Metal matrix
composites
Matrix: Moderately strong, stiff,
wear resistant and fatigue
resistant
Aim: To significantly improve
above properties
Example: SiC reinforced Al,
Precipitation hardened Al, etc.
Ceramic matrix
composites
Matrix: Hard and brittle
Aim: To make tougher and
more reliable
Example: Ag reinforced Al2O3 ,
ZrO2 reinforced TiO2 , steel
reinforced concrete
Polymer matrix
composites
Matrix: Weaker and have low
melting point
Aim: To make stronger and
more temperature resistant
Example: GFRP, CFRP
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Based on Dispersed Phase
Fibrous
composites
 continuous vs. discontinuous
 aligned vs. randomly oriented
 GFRP
 CFRP
Structural
composites
Particulate
composites
 Large particle vs.
dispersion strengthened
 WC particle
reinforced Co
 sandwich structure vs.
honeycomb structure
 Polymer core
sandwiched by Al faces
Fibre materials for reinforcement
Whiskers
 thin single crystals - large length to diameter ratio
 high crystal perfection – extremely strong, strongest known
 very expensive
 example: graphite, SiN, SiC
Fibers
 polycrystalline or amorphous
 generally polymers or ceramics
 example: Al2O3 , Aramid, E-glass, Boron
Wires
 metal – steel, Mo, W
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Structural composites
 Properties of structural composites depends upon the geometrical
design of the reinforcement.
(a) Laminar composite structure – conventional
(b) Sandwich structure
(c) Honeycomb sandwich structure
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Rule of Mixture for Fibre Reinforcement
Composite stress:
s c = s f Vf + s m Vm
Composite strain:
ec = ef = em
Hook’s law:
sc
Ec
sf
sm
Ef
Em
Composite strength:
s c = s f V f + s m Vm
Composite stiffness:
Ec = EfVf + EmVm
Problem
A continuous and aligned glass fibre-reinforced composite consists of 40 vol.%
glass fibres having a modulus elasticity of 69 GPa and 60 vol.% polyester resin
that, when hardened, displays a modulus of 3.4 GPa.
(a) Compute the modulus of elasticity of this composite in the longitudinal
direction.
(b) If the cross-sectional area is 250 mm2 and a stress of 50 MPa is applied in
the longitudinal direction, compute the magnitude of the load carried by each
of the fibre and matrix phases.
(c) Determine the strain that is sustained by each phase when the stress in part
b is applied.
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Answer:
Given data:
EC = Ef Vf + Em Vm
Ef = 69 GPa
Em = 3.4 GPa
Vf = 0.40
Vm = 0.60
= (69 GPa).(0.40) + (3.4 GPa).(0.60)
= 30 GPa
(a)
Manipulating Hooks’ law for longitudinal directions, one may find
the ratio of forces on the fibres and the matrix
Ff
Ef Vf
=
Fm
Em Vm
Again, forces on the composite
FC = sC AC
(69 GPa).(0.40)
= (3.4 GPa).(0.60)
 Ff = 13.5 Fm
[1]
Given data:
= (50 MPa).(250 mm2)
= 12500 N
 FC = Ff + Fm = 12500 N
Using these two equations, one may find
Ff = 11640 N and
Fm = 860 N
For an unit length of composite
Am = V m AC
= (0.6).(250 mm2)
= 150 mm2
and Af = 100 mm2
sC = 50 MPa
AC = 250 mm2
[2]
(b)
sf = Ff / Af
= (11640 N) / (100 mm2)
= 116.40 MPa
sm = Fm / Am
= (860 N) / (150 mm2)
= 5.73 MPa
Then individual strain in each phase
ef = sf / Ef
= (116.40 MPa) / (69 GPa)
= 1.69x10-3 (c)
em = sm / Em
= (5.73 MPa) / (3.4 GPa)
= 1.69x10-3 (c)
Thus, as they should be, strains for both fibre and matrix
phases are identical
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Particle materials for reinforcement
 Particles used can be ranging in size from microscopic (dispersionstrengthened composites) to macroscopic (large-particle composites)
 Dispersion strengthening


Similar to precipitation hardening
Strengthening occurs in atomic/molecular level by making it harder for
dislocation to move
 Large-particle strengthening
 Harder and stiffer reinforcing particles tend to restrain movement of the
matrix phase in the vicinity of each particle
 Particles may be of any shape – ranging from irregular to spherical, plate-like
to needle-like.
 The distribution of particles in the composite matrix is random, and therefore
strength and other properties of the composite material are usually isotropic
 Particulate strengthening is much less efficient than fibre-reinforcing
SiC reinforced Al casting
(compliant)
(ductile)
(stiffer)
(brittle, hard)
Large-particle composites
Dispersion-strengthened composites
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Example
A cemented carbide cutting tool used for machining contains 75 wt% WC,
15 wt% TiC, 5 wt% TaC, and 5 wt% Co. Estimate the density of the
composite.
SOLUTION
First, we must convert the weight percentages to volume fractions. The
densities of the components of the composite are:
From the rule of mixtures, the density of the composite is
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Next Class
Lecture 34
Materials Selection
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