Lecture outline

1
materials

• Lecture outline
• Introduction to materials
• Solids
• Form
• Bonding
• Hookes Law.stress, strain
• Elasticity
• Material Strength
• Strength testing

How does processing influence structure? Why is
this important???? This will influence material
properties.and ultimately performance
2
materials
some of the things made possible/impacted by
materials science.
3
what is structure? what is the basis of
structure?? a little chemistry is required at
this point.
4
business

• II. some chemistry
• A. protons, neutrons electrons ? atom
• what are atoms?
• smallest subunit of an element
• 2. protons neutrons ? nucleus
• 3. electron cloud
• of protons determines identity
• electrons protons (neutral)
• Electrons arranged in shells
• Electrons are the basis of materials properties

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atom stadium nucleus housefly on center
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business

• II. some chemistry
• atoms
• 8. All atoms of a given element are identical
• 9. Atoms of different elements have different
  masses
• 10. a compound is a specific combination of atoms
  of gt1 element
• 11. in a chemical reaction, atoms are neither
  created nor destroyed only change partners to
  produce new substances

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business

• II. some chemistry
• atoms
• 12. Can we see them?
• Yes
• electron microscopy or scanning probe microscopy

Xe on Ni
Xe on Ni
Au surface
Au surface
http//www.almaden.ibm.com/vis/stm/gallery.html
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business

• II. some chemistry
• atoms
• 13. What can they do?
• a. form bonds with other similar atoms
  elemental substances (molecules, metals, network
  solids)
• b. form bonds with atoms of other elements to
  make compounds

sciences quest for simplicity.. various
combinations of the 100 elements make up all
matter on earth
http//www.almaden.ibm.com/vis/stm/gallery.html
8
materials

• III. what holds the atoms in a crystal/ceramic/pol
  myer/elastomer together?........primary bonds
• Covalent bonding
• Two or more atoms share electrons
• Strong and rigid
• Found in organics and sometimes ceramics
• Strongly directional
• E.g.methane CH4
• C has 4 valence electrons H has 1
• Elemental solids e.g. diamond
• Can be strong (diamond)
• Can be weak (Bi)

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materials

• III. what holds the atoms in a crystal/ceramic/pol
  myer/elastomer together?........ primary bonds
• ionic bonding
• Metal and non-metal
• Metal gives up valence electron(s) to non-metal
• Result is all atoms have a stable
  configurationalso an electrical charge
• E.g. NaCl-
• metal becomes ly charged (cation) non-metal
  becomes ly charges (anion)
• Electrostatic attraction
• Omnidirectional
• Close-packed

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10
materials

• III. what holds the atoms in a crystal/ceramic/pol
  myer/elastomer together?........primary bonds
• C. metallic bonding
• Hold metals and alloys together
• Enables dense packing of atoms reason why
  metals are heavy
• Valence electrons (1, 2 or at most 3) not bound
  to a particular atom
• Free to drift throughout the entire material
  sea of electrons
• Nonvalence electrons atomic nuclei ion core
  (net charge)
• Good conductors of electrons heat

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11
materials

• III. what holds molecules together?........seconda
  ry bonds
• 2ndry bonds are physical bonds and are weaker
  than what weve just talked about
• A. Hydrogen bonds
• Intermolecular attraction in which a H atom
  bonded to a small, electronegative atom (N, O or
  F)is attracted to lone pair of electrons on
  another N, O or F
• Weak
• Due to charge distribution on molecule
• Often seen in organic compounds

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12
materials

• III. what holds molecules together?........seconda
  ry bonds
• B. Van der Waals forces
• Again, interactions are much weaker (10kJ/mol)
  as compared to chemical bonds (100kJ/mol)
• Forces arising from surface differences across
  molecules
• Gecko feet microscopic branched elastic hairs
  on toes which take advantage of these
  atomic-scale attractive forces to grip and
  support heavy loads

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Autumn et al. PNAS 2002, 99, 12252
13
materials

Takes a fictional superhero to bring nanotech to
the under 5s
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Higher potential energy
Energy difference
Lower potential energy
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materials

• IV. structure
• What do I mean by structure?
• Structure is related to the arrangement of a
  materials components
• This could be on any length scale
• Atomic, nano-, micro-, macro-
• All of these length scales matter
• Types of carbon (literally just carbon)

Carbon nanotubes
Diamond
Graphite
C60 - Fullerene
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materials

• V. properties
• A material trait in terms of the kind and
  magnitude of response to an imposed stimulus
• e.g. sample subjected to force will experience
  deformation
• A polished metal surface will reflect light
• Categories of properties
• Mechanical, electrical, thermal, magnetic,
  optical deteriorative
• Each has a characteristic stimulus provoking a
  response
• mechanical properties relate deformation to an
  applied load or force
• mechanical properties include elastic modulus,
  strength
• Electrical properties (conductivity) respond to
  an electric field
• what causes differences in properties of
  materials???

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materials

• Many properties of a material are consequence of
• Identity of atoms that comprise them
• Spatial arrangement of those atoms
• Interactions between atoms
• atomic structure and bonding are important

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materials

• Material properties

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materials
Same material aluminum oxide. Depending on
structure (which is influenced by processing)
materials are transparent, translucent, opaque
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materials

• VI. Solid materials
• Classification
• Crystals
• molecules attracted to one another try to cohere
  in a systematic way, minimizing volume (dense
  materials)
• stiff yet ductile (capable of large amounts of
  deformation without fracture)
• Glasses/ceramics
• materials whose high viscosity at liquid/solid
  point prevents crystallization amorphous
• E.g. porcelain, SiO2, glass, cement
• Stiff, strong, hard BUT very brittle and
  susceptible to fracture
• insulators
• Polymers
• materials built up of long chains of simple
  molecular structures plastics and living things
• Low densities
• Extremely ductile, pliable can be formed into
  complex shapes
• Soften/decompose at high T
• Elastomers
• long-chain polymers which fold or coil e.g.
  artificial rubber
• Totally elastic due to cross-linking

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materials

• solid materials
• Elastomers

Elastic deformation Partial uncoiling,
straightening elongation
Unstressed Amorphous Twisted, kinked, coiled
Removal of stress..spring back
silly putty smash
silly putty pull
22
materials

• VII. mechanical properties
• Lets think about spaghetti
• How easy is to break it by pulling (tension)?
• Is thicker spaghetti easier or harder to break by
  pulling?
• Theory says that force needed increases with
  cross sectional area
• How easily will it buckle if you compress the
  ends?
• Depends on force, material strength, length and
  thickness of spag
• A longer piece buckles easier than a shorter
  piece
• Thinner piece buckles easier than a thicker piece
  
• How easily will it bend if you push
  perpendicular?
• Is it tension, compression?
• Deflection depends on force, material strength,
  length of span, area of spaghetti
• Larger force, larger deflection
• For a given force, longer pieces bend easier
• For a given force, thin pieces bend easier

spaghetti crop
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materials

• VII. mechanical properties
• How do engineers figure in the picture?
• 2 concepts stress and strain
• structural engineers determine stress/strain
  distributions in objects subjected to
  well-defined loads (beams in bridges)
• materials/metallurgical engineers produce
  materials that will have the desired mechanical
  properties

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materials

• VII. mechanical properties
• first need to define stress and strain
• 1. stress is related to the force or load
  applied to a material
• a. stress ? force/original area
• b. from figure ? F/A0 (units?)
• F newton kg m / s2
• ? F/A0 N/m2
• pascal N/m2
• MPa 106 Pa, GPa 109 Pa
• from figure ? F/A0 Pa or F/A0 x 10-6
  MPa

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materials

• VII. mechanical properties
• first need to define stress and strain
• strain is related to the response of the material
  to the applied force
• a. strain e change in length over original
  length ?l/l0
• b. strain is unitless but m/m (or in/in) may be
  used
• strain can be expressed as a
• c. 2 types elastic plastic
  strain/deformation,
• (i) elastic strain exists only while stress is
  applied
• elasticity
• (ii) plastic strain does not disappear upon
  removal of
• stress plasticity

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materials

• VII. mechanical properties
• C. compression stress/strain test
• 1. force is now in the opposite direction
• compressive force taken to be negative ? negative
  stress
• since l0 gt li, ? negative strain
• tensile tests are easier to perform
• very little additional information from
  compressive tests
• compressive tests more useful if
• (i) materials behavior under large and permanent
  (plastic) strain is needed
• (ii) material is brittle

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27
materials

• mechanical properties
• End up with a stress-strain curve
• 1. provides huge amount of information about
  material properties

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materials

• mechanical properties
• End up with a stress-strain curve
• 2. Initial part of curve is especially
  interesting..

Yield strength
Yield strength Load required to go
from elastic-plastic deformation
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materials

• mechanical properties
• E. Hookes Law and Youngs modulus, E
• 1. stress (?) and strain (e) are proportional
  under certain conditions (low stress)
• a. ? eelE Hookes Law
• b. E - Youngs modulus, modulus of elasticity,
  stiffness, resistance to elastic deformation (GPa
  or psi)
• c. physical meaning of E being large?
• higher E implies greater stiffness

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Material range of E Metal 45 400
GPa Ceramics 60 500 GPa Polymers 0.01 4
GPa Spaghetti 4.8 GPa
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30
materials

• mechanical properties
• microscopic description of elastic deformation
• 1. strain manifests as small changes in
  interatomic spacing of bonds
• 2. E is a measure of resistance to separation
  of adjacent atoms/ions/ molecules (i.e. it is
  related to bonding forces)

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Or differences in E are due to differences in
bonding! In other works microscopic (bonding)
determines macroscopic (E) Also as T increases,
E generally decreases
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31
materials

• mechanical properties
• G. Youngs modulus, E for different materials
• 1. Values of E for ceramics are similar to
  metals for polymers E is lower
• Why?
• 5. As temperature increases, E diminishes

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2. mechanical properties of materials
materials

• mechanical properties
• H. tensile strength (TS)
• maximum load / initial area
• a. TS is the stress value at the maximum of
  the s-s curve, point M
• b. corresponds to maximum stress sustainable
  by a structure in tension
• c. if this stress is maintained, fracture will
  result
• d. All deformation so far is uniform
  throughout speciman
• 2. at point M, neck formation occurs
• 3. stress is concentrated at M
• 4. fracture ultimately occurs at F

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youtube.com
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2. mechanical properties of materials
materials

• mechanical properties
• I. ductility and elongation
• ductility is the degree of plastic deformation at
  (prior to) failure
• low or no ductility brittle
• ductility is quantified as elongation, EL
• (i)
• lf length at fracture
• l0 initial length

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youtube.com
34
materials

• VIII. Material strength
• Tensile strength
• How hard does something need to be pulled to
  break material bonds
• Some examples
• Steel piano wire 450,000 psi
• Aluminum 10,000 psi
• Concrete 600 psi
• Compression strength
• Materials fail in compression in many ways
  depending on geometry, support
• Buckling hollow cylinders e.g. tin can
• Bending long rod or panel
• Shattering heavily loaded glass
• Yield strength
• Load required to cross line from elastic to
  plastic deformation
• Ultimate tensile strength
• Maximum possible load without failure

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materials

• IX. Material testing
• A. Tensile strength most common method
• 1. apply stress uniaxially along sample
• continually increase force on ends
• perform test until fracture (sample breaks)
• measure force vs. sample elongation
• tensile testing machine elongates specimen at a
  constant rate
• applied load and resulting elongations are
  continuously and simultaneously measured

steel 1018 stress-strain
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2. mechanical properties of materials
materials

• IX. Material testing
• Aside..
• 1. in stress-strain plots it appears that
  stress is decreasing between
• M and F
• 2. it is not decreasing.any ideas what is
  happening?
• 3. cross-sectional area is decreasing in the
  necking region
• 4. results in a reduction in the load-bearing
  capacity of specimen

youtube.com
37
materials

• IX. Material testing
• B. Euler buckling load, Pc
• P load (MLT-2)
• I moment of inertia (L4)
• E Youngs modulus (ML-1T2)
• L length (L)
• 4 variables, 3 primitive dimensions 1
  dimensionless group

38
materials

• Material testing
• What if the material is very brittle.can we do a
  tensile test?
• Tensile tests cant easily be done on
  ceramics/brittle material because
• Difficult to prepare and test samples with
  required geometry
• Difficult to grip brittle materials without
  fracturing them
• Ceramics fail very quickly (0.1 strain)
• Transverse bending test is more usually employed

39
materials

• Material testing
• C. bending

40
materials

• Material testing
• Bending
• At point of loading, top surface is in
  compression and bottom surface is in tension
• Stress is computed from specimen thickness, the
  bending moment, and the moment of inertia of
  cross-section

41
materials

• Material testing

minimizing moments of inertia to increase rates
of rotation
42
materials

• Material testing
• Compressive strength
• whats going to happen a beam (spaghetti) under
  compression?
• Will fail by crushing or buckling, depending on
  material and L/d
• Crushing atomic bonds begin to fail, inducing
  increased local stresses, which causes more bonds
  to fail
• Buckling complicated as there are many modes