A manufacturing process is a process that changes the shape or properties of materials' Hence, mater

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Chapter 3 Engineering Material

• A manufacturing process is a process that changes
  the shape or properties of materials. Hence,
  materials are the foundation of manufacturing
• In this chapter, we will study the basics of
  materials structure, physical and mechanical
  properties, surface, wear and friction, and etc.
• The roadmap ahead
• An outline of engineering materials
• An outline of the behavior and manufacturing
  properties of materials

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• This chapter corresponds to Chapter 1, 2 and 3 in
  the textbook
• Learning objectives
• Understand structure of metals
• How atoms are arranged in a metal
• Types of imperfections that exist in crystal
  structures and their effects
• How grains and grain boundaries are developed
• Mechanisms of strain hardening and anisotropy
• Understand important mechanical properties of
  materials
• Types of tests for determine the mechanical
  behavior of materials
• Elastic and plastic features of stress-strain
  curves and their significance

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• Understand important mechanical properties of
  materials
• Relationships between stress and strain and their
  significance, as influenced by temperature and
  deformation rate
• Characteristics of hardness, fatigue, creep,
  impact, an residual stresses, and their role in
  materials processing
• Why and how materials fail when subjected to
  external forces.
• Understand physical properties of materials
• Thermal, electrical, magnetic, and optical
  properties
• Corrosion and its importance in the service life
  of components
• How a combination of physical properties effects
  the processing of materials

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• The atomic structure of materials
• Materials are made of elements
• The atomic structure of the elements
• The periodic table of elements at Los Alamos
  National Laboratory http//pearl1.lanl.gov/period
  ic/default.htm

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e
e
e
e
e

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e
e
e
Neon
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• The bonds between atoms and molecules
• Primary bonds atom-to-atom bonding
• Secondary bonds molecules attract each other

Atom attract
Inter-molecular attract
Temporary attract
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• The structure of engineering materials
• Crystalline most solids
• Non-crystalline most liquids and gases
• Crystalline structures
• A typical crystalline structure

A practical BCC material
Body-Centered Cubic (BCC) unit cell
The model of BCC
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• The types of crystalline structures
• BCC Body Centered Cubic is stable and hence, is
  hard
• FCC Face Centered Cubic is easy to slide and
  hence, is soft
• HCP Hexagonal Close-Packed is very stable
• Materials may change their structure under
  different temperature (e.g., water)

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• Crystalline structures of common metals
• BCC Iron (Fe), Tungsten (W),
• FCC Aluminum (Al), Copper (Cu), Gold (Au), ..
• HCP Magnesium (Mg), Titanium (Ti),
• Crystalline structures may be imperfect

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• Crystalline structure deformation
• Crystalline structure may deform under stress
• Types of deformation
• Elastic deformation the lattice structure titles
  resulting temporary change of shape
• Plastic deformation the lattice structure
  changes resulting in permanent change of shape

Elastic deformation
Plastic deformation
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• Non-crystalline (Amorphous) structure
• A comparison
• Crystalline structure regular, repeating and
  densely packed
• Non-crystalline structure random and loosely
  packed
• Although many non-crystalline materials are
  liquid and gas, there are solid non-crystalline
  materials such as glass, some plastics and rubber

Crystalline
Non-crystalline
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• Non-crystalline structure (continue)
• Non-crystalline structures may mix to crystalline
  structures within one material
• Materials may change its structure under
  different temperature

Melting temperature
Glass-transition temperature
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• Grains and grain boundaries
• Individual crystals are called grains.
• Materials are made of many randomly oriented
  crystals

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• Grains and grain boundaries (continue)
• Grain size effects the materials properties
• Large grain ? low strength, low hardness, low
  ductility and rough surface
• Grain size
• The formula
• N 2n-1
• where, n is the ASTM grain size number and N is
  the number of grains per square inch at a
  magnification of 100 (0.0645 mm2).
• Examples
• n - 3, N 1 grains/mm2, 0.7 grains / mm3,
• n 0, N 8 grains/mm2, 16 grains / mm3,
• n 3, N 64 grains/mm2, 360 grains / mm3,
• Grain boundary has a more complicated effect

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• Structures under plastic deformation
• If a materials undergoes a plastic deformation,
  it will become anisotropic

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• Structures under plastic deformation (continue)
• The effect of the temperature recovery,
  recrystallization and grain growth

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• Structures of engineering materials
• Metals
• Crystalline structure BCC, FCC or HCP
• Primary bonding (metallic bonding)
• Polymers
• Mostly non-crystalline structures
• Large molecules with secondary bonding
  (inter-molecular bonding)
• Ceramics
• Either crystalline or non-crystalline structures
• Primary bonding (ionic or covalent or both) and
  secondary bonding (atom attraction force)

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• The structure determines the property

Modeling the structure is extremely difficult if
not impossible
A piece of metal
Crystal structures
Grain structures
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• Material properties
• Mechanical properties
• Stress-strain
• Hardness
• Fatigue and Creep
• Fluid property
• Viscosity
• Physical properties
• Volumetric property
• Thermal property
• Mass diffusion
• Electronic property
• Electrochemical property

Quantitative measures of material
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• Mechanical property 1 stress-strain
• Types of stress
• Tensile stress stretch
• Compression stress squeeze
• Shear stress tear apart
• Stress testing

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• Stress calculation
• The formula
• Note
• se engineering stress, PSI or MPa
• F applied force, lb or N
• A0 original area of the specimen, in2 or mm2
• Strain calculation
• The formula
• Note it has no unit

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• Reduction of area
• Typical strain-stress graph

Stress (se)
Strain (e)
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• The process of stress-strain testing

Stress (se)
Strain (e)
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• The relationship between the stress and strain in
  the elastic deformation zone
• The specimen will return to original shape after
  the force is removed
• The formula (the Hookes law)
• se Ee
• where, E modulus of elasticity, or Youngs
  module
• The relationship between the stress and strain in
  the plastic deformation zone
• The specimen will not return to the original
  shape after the force is removed
• Necking is when localized material deformation
  occurs.
• It will be detailed later.

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• An example
• The experiment setup
• The testing data on an aluminum alloy specimen

Yield stress 22 ksi Tensile stress 35
ksi Youngs module 7x104 MPa
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• The stress and strain properties of selected
  engineering materials
• Material E (MPa) Y (MPa) UTS (MPa)
• Al and alloys 69 x 103 175 350
• Case iron 138 x 103 275 275
• Copper alloys 16 x 103 205 410
• Steel (low C) 209 x 103 175 300
• Steel (high C) 209 x 103 400 600
• Titanium 117 x 103 800 900
• Concrete 48 x 103
• Silicon carbide 448 x 103
• Diamond 1035 x 103
• Polyethylene 7.0 x 103
• Nylon 3.0 x 103

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• Other important measures
• Total elongation
• Total area reduction
• The specific (per volume) work to fracture the
  material

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• True strain-stress
• The problem of engineering strain-stress
• True stress
• True strain
• The difference to the stress-strain
• the plastic deformation is more
• clearly shown

e
Plastic deformation
Y
Elastic deformation
s
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• True stress-strain (continue)
• The correlation to the engineering stress-strain
• ? ln(1 e)
• s se(1 e)

Engineering strain
Engineering stress
s
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• Strain hardening
• From the figure, it is seen that after exceeding
  the tensile strength, the material will require
  less force to deform
• In practice, however, we know that the larger the
  deformation, the larger the force. This is called
  strain hardening
• The interpretation lays on
• the strain hardening the
• size of the material has
• changed. In fact, if the size
• does not change, then the
• required force will continue
• to increase

e
Plastic deformation
Elastic deformation
s
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• The flow curve equation (applicable to the
  plastic region)
• s Ken
• where, K is the strength coefficient or flow
  strength (MPa) and is equal to the true stress at
  a true strain of unity, and n is the strain
  hardening exponent and is equal to the true
  strain at the onset of necking.
• Another form
• logs logK nloge
• Two important formulas
• ? n
• n a/b

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• An example
• A0 0.056 in2, Af 0.016 in2, l0 2 in.
• Other data and computation in Excel
• Note
• True stress s P/A
• True strain e ln(l / l0) up to necking
• At fracture ef ln(A0 / Af)
• Up to necking l l0 Dl
• Also, A0l0 Al
• Fit the model
• s Ken
• K is the true stress at a true strain of unity
  from the figure, it is found that K 180,000 lb
• n is equal to the true strain at the onset of
  necking from the figure, it is found that n
  0.36

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• An example (continue)
• The graph

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• Application examples
• Example 1 strain hardening and stamping
  operation Larger forces are needed after
  initial metal deformation
• Example 2 the large the n, the more difficult it
  is to break (necking). For instance, steel (n
  0.4) is more difficult to break than the aluminum
  (n 0.15)
• The types of stress-strain relationship

Perfect elastic
Perfect plastic
Elastic and strain hardening
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• Compression test
• How to test the compression stress-strain
• The formula
• A comparison to tensile stress-strain much more
  load is required in the plastic region because
• The size increases
• The friction increases (barreling effect)

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• Illustration of the barreling effect

Friction prevents the material to move
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• A typical compression curve
• The elastic deformation zone is about the same
• The plastic deformation requires more force
• The engineering compression stress-strain and
  true compression stress-strain are almost the
  same
• Question when does a specimen fail?

e
Plastic deformation
Y
Elastic deformation
s
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• Shearing (Torsion)
• Shearing is to apply stresses in opposite
  directions of a specimen
• The shear stress and strain
• where, t shear stress (MPa), F applied force
  (N), A area over which the force is applied
  (mm2), g shear strain (no unit), d deflection
  of the element, and b orthogonal distance over
  which deflection occurs (mm).

F
F
?
A
b
F
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• Shearing test setup
• Stress and strain
• Typical shear curves
• The relationship in the
• elastic region
• t Gg

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• Bending and testing of brittle materials
• The setup
• The transverse rupture strength
• where, TRS transverse rupture strength (MPa),
  F applied force (N), L length of the specimen
  between supports (mm) and b and t are the
  dimensions of the cross section (mm).

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• Mechanical property 2 Hardness
• Definition of hardness the resistance to
  permanent indentation
• Hardness tests

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• Brinell test
• Use a carbide ball of 10 mm diameter to press the
  surface of a specimen
• The applied force is 500, 1,500 or 3,000 kg.
• The formula to compute the HB value
• An empirical relationship with the ultimate
  tensile stress for steel
• UTS (N / mm2) 3.5 HB (N / mm2)

Indentation must be fully developed in the test
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• Rockwell test
• Use a cone-shaped indenter to press the specimen
• The applied force is first 10 kg (minor force)
  and then 150 kg (major force)
• The additional depth of indentation is the
  hardness
• The Rockwell scales
• Scale Symbol Indenter Load Specimen
• A HRA Cone 60 carbide
• B HRB (1/16) ball 100 aluminum
• C HRC Cone 150 steel

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• Vickers test
• Use a pyramid-shaped indenter made of diamond to
  press the specimen
• The formula
• The relationship of different hardness scales
• The hardness of various materials check
  www.matweb.com

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• A list of commonly used material
• hardness

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• The effect of the temperature
• The strength decreases when the temperature
  increases
• The ductility increases when the temperature
  increases
• The hardness decreases when the temperature
  increase

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• Material property 3 Fatigue and Creep
• Fatigue material strength decreases under
  constant loading
• Creep material elongates under constant loading

Fatigue examples
Creep examples
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• Material property 4 Fluid property
• Viscosity
• Definition the resistance to flow
• Measuring the viscosity

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• Viscosity values of selected fluids
• Material viscosity ? (N-s /m2 or Pas)
• Water at 20 oC 0.001
• Water at 100 oC 0.0003
• Mercury at 20 oC 0.0016
• Machine oil at 20 oC 0.1
• Pancake syrup at 20 oC 50
• Polymer at 151 oC 115
• Polymer at 205 oC 55
• Polymer at 260 oC 28
• Glass at 540 oC 1012
• Glass at 815 oC 105
• Glass at 1095 oC 103
• Glass at 1370 oC 15

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• Viscoelastic
• Viscoelastic viscosity at elastic state
• Owing to the effect of viscosity, the material
  (such as polymer) may not return to its original
  shape after the elastic deformation immediately.
  Instead, it returns to its original shape
  gradually.
• An example bread dough
• The relationship between strain and stress of a
  viscoelastic material
• s(t) f(t)e

f(t)
t (temperature)
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• Physical property 1 volumetric and melting
  properties
• Density (?) the weight per unit volume in
  (g/cm3)
• Thermal expansion coefficient (a) the change in
  length per degree of temperature increase in
  (oC-1)
• Melting point the temperature at which the
  material changes from solid to liquid
• The properties of typical materials

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• Physical Property 2 thermal properties
• Specific heat (C) the quantity of heat energy
  required to increase the temperature of a unit
  mass of the material by 1 degree, in (Cal/g-oC).
• Thermal conductivity (k) the capability to
  transfer heat, in (J/sec-mm-oC).
• An example computing the required amount of heat
  to melt 1,000 g of steel (W)
• The formula
• H CW(T2 T1)
• Hence,
• (0.11)(1000)(1600 20) 173800 Cal
• 0.2022 KW-hour

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It is temperature dependent!
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• Physical Property 3 electrical properties
• Resistivity
• How to compute the resistance
• where, L is the length, A is the area, and ? is
  the resistivity of the material.
• Resistivity is a measure of conductivity
• The electric conductivity
• The formula
• Note that the unit is (?-m)-1.

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• The resistivity of selected materials
• Material Resistivity ((?-m)-1)
• Conductors 10-6 10-8
• Steel 17.0 10-8
• Aluminum 2.8 10-8
• Copper 1.7 10-8
• Silver 1.6 10-8
• Semiconductors 101 105
• Silicon 1.0 103
• Insulators 1012 1015
• Rubber 1.0 1012
• polyethylene 100 1012

Note that the resistivity is also a function of
temperature
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• Find the material information you need at
  http//www.matweb.com/
• This chapter represents the basic concepts in
  engineering materials. In the next chapter, we
  will focus on specific types of materials, namely
• Metals (and heat treatment)
• Plastics, and
• Ceramics