1ENSC 1004 Engineering Materials
Final Revision
W12 – L24 Revision Part I
W12 – L25 Revision Part II
Associate Professor Ali Karrech
Room: ENCM 1.40
Phone: 6488 3523, email: ali.karrech@uwa.edu.au
Materials were prepared by Professor Hong Yang
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T1: Engineering materials in three basic types
• Basic classifications dictated by intrinsic bonding types
• General physical, mechanical and deteriorative properties resulted from bonding nature
• Typical mechanical behaviour of three basic types
• Typical deteriorative behaviour of three basic types
• Typical applications of three basic types, more specifically into engineering alloys
• Life cycle of materials – sustainable utilization and environmental/societal impacts
T2: Mechanical deformation and failure of materials
• Foundational knowledge: terminology, concept, tensile deformation behaviour, tensile
mechanical properties, bonding nature and crystal structure basis. (assume established)
• Failure by plastic yielding – design by yield strength
• Failure by brittle fracture with existing cracks – design by fracture toughness
• Failure by fatigue under cyclic loading – design by fatigue life
Key topic areas in Chapters 4-7:
2
T3: Thermal properties
• Heat capacity
• Thermal expansion
• Thermal conductivity
• Thermal stresses
3There are many applications and working environments where we need to
consider or make use of the thermal properties of materials. The most common
thermal properties considered in engineering application include the following:
• Heat capacity
• Thermal conductivity
• Melting temperature
• Thermal expansion coefficient
How Do Materials Respond to Heat?
- Thermal properties of materials
Heat Capacity is a measure of a material’s ability to store thermal energy (heat). It may be
measured by measuring the amount of heat absorbed by a material when it is heated or the
amount of heat released when it is cooled.
Thermal Properties of Materials
(1) Heat capacity
4
• Quantitatively, specific heat capacity is defined as the amount of heat required to raise the
temperature by one degree (K or °) per one mole (or kg) of a substance:
=

∆
where
C: heat capacity, / ∙
: heat,
∆: temperature change,
n: quantity of the substance, mole
• Heat capacity can be measured under two different common conditions:
: heat capacity at constant pressure
: heat capacity at constant volume
In engineering practice, Specific Heat (c) is also commonly used to describe the heat capacity
of a material, which is the heat capacity per unit mass, expressed in / ∙ .
5Thermal energy is stored in matters (solids, liquids and gases) in the form of atomic vibration.
As temperature increases, the average energy of atomic vibration increases.
• Temperature is a measure of the average atomic vibration.
• The energy required to reach a temperature is measured by heat capacity.
Atomic vibrations are in the form of lattice
waves or phonons.
Thermal Properties of Materials
(1) Heat capacity – how do materials store heat
(/ ∙ )
at 25°
• Polymers
Polypropylene
Polyethylene
Polystyrene
Teflon
1925
1850
1170
1050
• Metals
Aluminum
Steel
Tungsten
Gold
900
486
138
128
• Ceramics
Magnesia (MgO)
Alumina (Al2O3)
Glass
940
775
840
Attention:
• Most of the thermal energy
assimilated by solid materials is
associated with increasing
vibrational energy of the atoms.
• Thus, a stronger interatomic
bonding requires more thermal
energy to raise the temperature.
• The specific heats are generally
higher for polymers (covalent
bonding) than for ceramics (ionic
+ covalent bonding) and metals
(metallic bonding).
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Thermal Properties of Materials
(1) Heat capacity – specific heat data
in
c
re
a
s
in
g


https://www.khanacademy.org/science/physics/thermodynamics/specific-heat-and-
heat-transfer/a/what-is-thermal-conductivity
Thermal conductivity measures the ability of a material
to conduct heat through its body. Heat is conducted
from a high temperature location to a low temperature
location. The heat flux () conducted is determined by
the temperature gradient (


) within the body, as
expressed by the Fourier’s Law:
= −


: heat flux, /(2 ∙ )
: thermal conductivity, /( ∙ ∙ ) or /( ∙ )


: temperature gradient, /
For a steady state heat conduction, the total
amount of heat (Q) transported through a
thermal conductor of thermal conductivity ,
cross-sectional area A, and over a period of time
t is:
= = −
∆

Thermal Properties of Materials
(2) Thermal conductivity
7
16
Thermal energy transfer mechanisms:
• Metals: motion of free electrons
• Ceramics: vibration of atoms
• Polymers: vibration/rotation of chain
molecules
http://wattsupwiththat.com/2015/02/08/thanks-to-the-ipcc-the-public-doesnt-know-
water-vapor-is-most-important-greenhouse-gas/
The knowledge of thermal conductivity
helps us to develop more energy efficient
materials and environmentally friendly
houses
in
c
re
a
s
in
g

• Polymers
Polypropylene 0.12
Polyethylene 0.46-0.50
Polystyrene 0.13
Teflon 0.25
• Ceramics
Magnesia (MgO) 38
Alumina (Al2O3) 39
Soda-lime glass 1.7
Silica (cryst. SiO2) 1.4
• Metals
Aluminum 247
Steel 52
Tungsten 178
Gold 315
(W/mK)
8
Thermal Properties of Materials
(2) Thermal conductivity data
Most solid materials expand when heated and contract
when cooled, for example:
The automatic tensioning system to compensate for
thermal expansion and sagging of railway powerlines in
hot season.
The railway tracks deform as a result of thermal expansion
in summer.
http://forums.mylargescale.com/11-public-forum/19643-you-thought-
we-had-problems-thermal-expansion.html
http://www.railway-
technology.com/features/featuretension-and-strain-
on-overheated-trains/featuretension-and-strain-on-
overheated-trains-3.html
Thermal Properties of Materials
(3) Thermal expansion
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There are exceptions, for example,
• ZrW2O8 expands on cooling!
• Water expands on cooling between 4~0 C.
• ‘Invar’ is a special type of alloys that show no
volume change (invariable) when heated or
cooled! Swiss physicist Charles Édouard
Guillaume who discovered Invar in 1896
received the Nobel Prize in Physics in 1920.
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Typically, the E-r curve is asymmetric, thus
increasing temperature,
• increases atomic vibration amplitude
• increases mean interatomic separation
• results in thermal expansion
Thermal Properties of Materials
(3) Thermal expansion – atomic perspective
The interatomic potential energy (U) versus interatomic separation curve of materials:
If the E-r curve were symmetric, then
increasing temperature,
• increases atomic vibration amplitude
• does not change interatomic separation
• does not result in thermal expansion
Considering a cuboid volume of a
material heated from 1 to 2, the linear
CTE is calculated as:
2 − 1
1
= 2 − 1
or
∆

= ∆
• Thermal expansion is a change of volume with temperature. It can be measured
either as the volume change, or as an isotropic linear dimensional change.
• The linear dimensional change with temperature is more often used to indicate the
thermal expansion of a material. It is called the linear Coefficient of Thermal
Expansion (CTE), .
• For most materials, is found to be a constant within certain temperature ranges.
Thermal Properties of Materials
(3) Thermal expansion
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• is a material property
• It has the unit ° −1
• For many metals, it is usually of
the order of 10-6 ~ 10-5 ° −1
x1
x2
Polypropylene 145-180
Polyethylene 106-198
Polystyrene 90-150
Teflon 126-216
• Polymers
• Ceramics
Magnesia (MgO) 13.5
Alumina (Al2O3) 7.6
Soda-lime glass 9.0
Silica (cryst. SiO2) 0.4
• Metals
Aluminum 23.6
Steel 12.0
Tungsten 4.5
Gold 14.2
(10-6/ºC)
at 25°
in
c
re
a
s
in
g


In general, the greater is the interatomic
bonding energy, the smaller is of the
solid:
• Metals (metallic bonding) have a
moderate CTE.
• Ceramics (ionic + covalent bonding)
have smaller than metals.
• Polymers generally have the highest
due to weak secondary bonds! For some
highly crosslinked polymers (almost
entirely covalent bonding), the can be
as low as that of Al.
https://commons.wikimedia.org/wiki/File:Nylon-
3D-h_bond.png
The Van der
Waals bonds of
polymers (Nylon)
are weak,
rendering their
high coefficients
of thermal
expansion
Thermal Properties of Materials
(3) Thermal expansion – linear CTE,
12
13
Thermal Properties of Materials
(3) Thermal expansion – linear CTE,
Material × 10−6 °
−1 Material × 10−6 °
−1
Summary
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The thermal properties of materials include:
Heat capacity (, ) and Specific Heat (, ):
• the ability of a material to absorb heat in order to raise its temperature by 1 or 1 °C
• polymers have the highest heat capacity values
Linear coefficient of thermal expansion ():
• the ability of a material to expand (or contract) after a 1°C temperature change.
• polymers have the largest values
• if a material is prevented from expansion/contraction, it will experience thermal stress
Thermal conductivity ():
• the ability of a material to transfer heat
• metals have the largest values