Post on 18-Jul-2016
description
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C H A P T E R 1
M A T E R I A L S
A N D E N G I N E E R I N G
1.1 MATERIALS AND ENGINEERING 1.1.1 Propert ies 1.1.2 Structure 1.1.3 Process ing 1.1.4 Per formance
1.2 CLASSIFICATION OF MATERIALS
1.3 MATERIALS SELECTION AND DESIGN
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1.1 MATERIALS AND ENGINEERING
1.1.1 Properties
• Most material problems in engineering involve selecting or
designing materials with the appropriate properties for an
application.
There are two broad classes of properties:
• Mechanical properties (Fig. 1.1-1): hardness, strength, stiffness,
ductility, toughness, fatigue / creep / wear behaviour.
• Physical properties: electrical, magnetic, optical, thermal,
chemical behaviour, density (weight).
• Material properties may be affected by extreme
environments (e.g. very low/high temperatures (Fig. 1.1-2)
and pressures, corrosive atmospheres, radiation, etc.) or
degrade over time in normal service (corrosion and wear).
• Material properties must always be considered in the
context of the service environment.
• Properties desired in service may be different from
properties desired during manufacturing.
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Fig. 1.1-1 Some mechanical properties.
Fig. 1.1-2 Effects of temperature on properties.
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1.1.2 Structure
• It is important to realize that the structure of a material has
a profound influence on its properties.
Structure may be considered at 3 levels, depending on its
scale (Fig. 1.1-3):
• Atomic scale structure refers to the types of atoms/
elements (i.e. the composition), the arrangement of
electrons within individual atoms, interaction between
atoms (i.e. atomic bonding), and the way atoms/molecules
pack together (i.e. crystal structure in metals/ceramics)
• Microscopic scale structure, also known as
microstructure, refers to the arrangement of large groups
of atoms. Microstructure may be seen using a microscope.
By altering the microstructure, the same material may be
made to exhibit considerably different properties.
• Macroscopic scale structure, or macrostructure refers to
features that may be seen under low magnification or with
the naked eye, such as pores, voids, cracks, surface finish,
as well as shape and size.
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Fig. 1.1-3 Length scale of structures in metals and the properties which they determine. Each interval on the length scale is a factor of 1000.
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1.1.3 Processing
• Processing/synthesis methods (Fig. 1.1-4) and conditions
greatly affect material structures, and hence, properties.
• By selecting different processing routes, often involving
temperature, the microstructure of a material may be
tailored to yield diverse properties for varied applications.
Example: the same steel may be made soft and tough, or hard
and brittle, simply by cooling it very slowly or very quickly
from high temperatures (Fig. 1.1-5).
• The development of new processing techniques expand
the usefulness of traditional materials. Example: carbon has
limited application as graphite or diamond until recently,
when new processes now produce carbon fibres, carbon-
carbon composites and carbon nanotubes (Fig. 1.1-5 & Table 1.1-1).
• New synthesis methods also create new classes of materials
for new or existing applications. Example: the ability to
control the diffusion of dopant atoms into silicon gave rise to
microelectronics; or the chemical synthesis of synthetic
polymers (plastics), and composites, which have extensively
replaced metal, wood and other natural materials.
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Fig. 1.1-4 Common processes.
Fig. 1.1-5 Cooling the same steel at different rates will produce
different microstructures and hardness.
(c)
4 µm
Har
dnes
s (B
HN
)
Cooling Rate (°C/s)
100
200
300
400
500
600
(d)
30 µm
(b)
30 µm
(a)
30 µm
0.01 0.1 1 10 100 1000
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(a) Diamond
(c) Fullerene C60
(b) Graphite
(d) Carbon nanotube
Fig. 1.1-6 Different structures of carbon: (a) diamond, (b) graphite,
(c) fullerene C60, and (d) carbon nanotube.
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1.1.4 Performance
• The engineering performance of a material depends on its
properties, which is determined by the structure, which in
turn, may be controlled by the processing method (Fig. 1.1-7).
• To achieve successful and widespread adoption, the
material must also perform its given task in an economical,
environmentally and socially acceptable manner.
Fig. 1.1-7 Interrelationships between structure, properties, processing and performance.
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1.2 CLASSIFICATION OF MATERIALS
Fig. 1.2-1 Classification of materials
according to properties and structure.
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1.3 MATERIALS SELECTION AND DESIGN
• Material selection cannot be separated from the choice of
manufacturing process required to make the end product.
Fig 1.3-1 The interaction between design requirements, material, shape and process. The selection of material and process run
parallel to all stages in design.
Fig 1.3-2 The strategy for material selection: translation, screening, ranking and documentation. All
these steps may be implemented in software, alloying large populations
of materials to be investigated.
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Fig 1.3-3 Graphical tool for materials selection.
Fig 1.3-4 Strategy for selecting materials in environmentally-responsible design.
Density, ρ (Mg/m3)0.01
Str
engt
h, σ
y or
σel
(MP
a)
0.01100.1 1
0.1
1
10
100
1000
10 000
PPPE
Woods,
T
Foams
Polymers and elastomers
MetalsCeramics
Composites
Natural materials
Lead alloys
Tungsten alloys
SteelsTi alloys
Mg alloys
CFRP
GFRP
Al alloys
Rigid polymer foams
Flexible polymer foams
Ni alloys
Copper alloys
Zinc alloys
PAPEEK
PMMAPC
PET
Cork
Woods, ll
Butyl rubber
Silicone elastomers
Concrete
Tungstencarbide
Al2O3SiC
Si3N4Strength–Density
MFA, 07
Metals and polymers: yield strengthCeramics and glasses: MORElastomers: tensile tear strengthComposites: tensile failure