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A. K. M. B. Rashid Professor, Department of MME

BUET, Dhaka

MME131: Lecture 30

Composite Materials

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)

REINFORCEMENT

(tungsten fibre)

INTERFACE

(allows transfer of stress from the matrix to the dispersed phase)

tungsten fibre reinforced aluminium composite

(a) plywood is a laminar

composite of layers of

wood veneer

(b) fiberglass is a fiber-

reinforced 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%).

Some examples of composite materials

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Abalone shell: CaCO3 + 3% organic material

>3000 times stronger than calcite

Natural Composites: wood and bamboo, shells, bones, muscles

Natural fibres: silk, wool, cotton, jute

Wood:

cellulose-filaments in

a matrix of lignin and

hemicellulose

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 ☐ higher operating temperatures

☐ greater strength and stiffness ☐ higher impact and wear resistance

☐ better corrosion resistance ☐ higher reliability and affordability

Why do we make composites ?

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Pros electrically, thermally conductive good strength and ductility high toughness magnetic

Cons dense low creep resistance low/moderate corrosion resistance

Pros electrically, thermally insulating wear and corrosion resistant high strength and stiffness creep resistant low density

Pros very ductile easy to form corrosion resistant high strength-to-weight ratio

Cons difficult to form/machine very low toughness

Cons low stiffness & strength poor high temperature properties

Metals

Ceramics Polymers

Composites

“The best of both worlds”

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

The matrix

Common terminologies

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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 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

The Reinforcing Material

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Based on Matrix Phase

Ceramic matrix composites

Metal matrix composites

Polymer matrix composites

Matrix: Hard and brittle

Aim: To make tougher and

more reliable

Example: Ag reinforced Al2O3 ,

ZrO2 reinforced TiO2 , steel

reinforced concrete

Matrix: Moderately strong, stiff,

wear resistant and fatigue

resistant

Aim: To significantly improve

above properties

Example: SiC reinforced Al,

Precipitation hardened Al, etc.

Matrix: Weaker and have low

melting point

Aim: To make stronger and

more temperature resistant

Example: GFRP, CFRP

Classification of composites

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Particulate composites

Fibrous composites

Structural composites

sandwich structure vs. honeycomb structure

Large particle vs. dispersion strengthened

continuous vs. discontinuous

aligned vs. randomly oriented

WC particle reinforced Co

GFRP

CFRP

Polymer core sandwiched by Al faces

Based on Dispersed Phase

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

Fibre materials for reinforcement

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Properties of structural composites depends upon the geometrical

design of the reinforcement.

(a) Laminar composite structure – conventional

(b) Sandwich structure

(c) Honeycomb sandwich structure

Structural composites

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Composite stress:

Composite strain:

Hook’s law:

sc = sfVf + smVm

ec = ef = em

sc

Ec

sf

Ef

sm

Em

Composite strength:

Composite stiffness:

sc = sfVf + smVm

Ec = EfVf + EmVm

Rule of Mixture for Fibre Reinforcement

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:

EC = Ef Vf + Em Vm

= (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

Fm Ef Vf

Em Vm =

=

Ff = 13.5 Fm [1]

Again, forces on the composite

FC = sC AC

= (50 MPa).(250 mm2)

= 12500 N

FC = Ff + Fm = 12500 N [2]

Using these two equations, one may find

Ff = 11640 N and Fm = 860 N (b)

(69 GPa).(0.40)

(3.4 GPa).(0.60)

Given data: Ef = 69 GPa

Em = 3.4 GPa

Vf = 0.40

Vm = 0.60

Given data: sC = 50 MPa

AC = 250 mm2

For an unit length of composite

Am = Vm AC = (0.6).(250 mm2)

= 150 mm2

and Af = 100 mm2

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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Particles used can be ranging in size from microscopic (dispersion-

strengthened composites) to macroscopic (large-particle composites)

Particle materials for reinforcement

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

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

SiC reinforced Al casting

(ductile)

(brittle, hard)

(compliant)

(stiffer)

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