materials
I.
Lecture outline
A.
B.
C.
D.
Introduction to materials
Solids
1.
Form
2.
Bonding
3.
Hooke’s Law….stress, strain
4.
Elasticity
Material Strength
Strength testing
How does processing influence structure?
Why is this important????
This will influence material properties….and ultimately performance
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some of the things made possible/impacted by materials science…….
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what is structure?
what is the basis of structure??
a little chemistry is required at this point…….
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II.
some chemistry
A.
__________________________
protons,
neutrons & electrons  atom
1. what are atoms?
smallest subunit of an element
2.
3.
4.
5.
6.
________
protons & neutrons  nucleus
_____
electron “cloud”
# of protons determines identity
# electrons = # protons (neutral)
_______
Electrons arranged in shells
7. Electrons are the basis of materials properties
atom = stadium
nucleus = housefly on center
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II.
some chemistry
A.
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 >1 element
11. in a chemical reaction, atoms are neither created nor destroyed – only change
partners to produce new substances
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II.
some chemistry
A.
atoms
12. Can we see them?
Yes
electron microscopy or scanning probe microscopy
XeononNiNi
Xe
Au surface
surface
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II.
some chemistry
A.
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
science’s quest for simplicity…..
various combinations of the 100 elements make up all matter on earth
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materials
III.
what holds the atoms in a crystal/ceramic/polmyer/elastomer
___________________
together?........primary
bonds
A.
Covalent
bonding
________________
1.
Two or more atoms share electrons
2.
Strong and rigid
3.
Found in organics and sometimes ceramics
4.
Strongly directional
5.
E.g.methane CH4
6.
C has 4 valence electrons; H has 1
7.
Elemental solids e.g. diamond
8.
Can be strong (diamond)
9.
Can be weak (Bi)
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III.
what holds the atoms in a crystal/ceramic/polmyer/elastomer
together?........ primary bonds
___________________
ionic bonding
B.
1.
Metal and non-metal
2.
Metal gives up valence electron(s) to non-metal
3.
Result is all atoms have a stable configuration…also an electrical charge
4.
E.g. Na+Cl-
5.
metal becomes +ly charged (cation); non-metal becomes –ly charges (anion)
6.
Electrostatic attraction
7.
Omnidirectional
8.
Close-packed
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materials
III.
what holds the atoms in a crystal/ceramic/polmyer/elastomer
together?........primary bonds
C.
metallic bonding
___________________
1.
Hold metals and alloys together
2.
Enables dense packing of atoms – reason why metals are heavy
3.
Valence electrons (1, 2 or at most 3) not bound to a particular atom
4.
Free to drift throughout the entire material –”sea of electrons”
5.
Nonvalence electrons + atomic nuclei = ion core (net + charge)
6.
Good conductors of electrons & heat
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III.
___________________
what holds molecules together?........secondary
bonds
2ndry bonds are physical bonds and are weaker than what we’ve just
talked about
A. ___________________
Hydrogen bonds
1.
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
2.
Weak
3.
Due to charge distribution on molecule
4.
Often seen in organic compounds
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III.
what holds molecules together?........secondary bonds
___________________
Van
der Waals forces
B.
1.
Again, interactions are much weaker (~10kJ/mol) as compared to chemical
bonds (100kJ/mol)
2.
Forces arising from surface differences across molecules
3.
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
materials
Takes a fictional superhero to bring
nanotech to the under 5’s
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IV.
structure
A.
What do I mean by structure?
1.
Structure is related to the arrangement of a materials components
a.
This could be on any length scale
b.
c.
2.
Diamond
Atomic, nano-, micro-, macroAll of these length scales matter
Types of carbon (literally just carbon)
Graphite
C60 - Fullerene
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Carbon nanotubes
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V.
properties
A.
A material trait in terms of the kind and magnitude of response to an
imposed stimulus
1.
e.g. sample subjected to force will experience deformation
2.
A polished metal surface will reflect light
B.
Categories of properties
1.
Mechanical, electrical, thermal, magnetic, optical & deteriorative
2.
Each has a characteristic stimulus provoking a response
C.
D.
mechanical properties relate deformation to an applied load or force
mechanical properties include elastic modulus, strength
E.
Electrical properties (conductivity) respond to an electric
field
______
what causes differences in properties of materials???
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Many properties of a material are consequence of
1.
Identity of atoms that comprise them
2.
Spatial arrangement of those atoms
3.
Interactions between atoms
atomic structure and bonding are important
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Material properties
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Same material – aluminum oxide. Depending on structure (which is
influenced by processing) materials are transparent, translucent, opaque
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VI.
Solid materials
A.
Classification
1.
Crystals
a.
b.
2.
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
a.
b.
c.
d.
3.
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
a.
b.
c.
d.
4.
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
a.
b.
long-chain polymers which fold or coil e.g. artificial rubber
Totally elastic due to cross-linking
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VI.
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
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VII. mechanical properties
Let’s think about spaghetti
spaghetti crop
A.
B.
How easy is to break it by pulling (tension)?
Is thicker spaghetti easier or harder to break by pulling?
C.
Theory says that force needed increases with cross sectional area
D.
E.
How easily will it buckle if you compress the ends?
Depends on force, material strength, length and thickness of spag
1.
A longer piece buckles easier than a shorter piece
2.
Thinner piece buckles easier than a thicker piece
F.
G.
How easily will it bend if you push perpendicular?
Is it tension, compression?
H.
Deflection depends on force, material strength, length of span, area of
spaghetti
1.
Larger force, larger deflection
2.
For a given force, longer pieces bend easier
3.
For a given force, thin pieces bend easier
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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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VII. mechanical properties
A.
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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VII. mechanical properties
A.
first need to define stress and strain
2. strain is related to the response of the material to the applied
force
a. strain = ε = change in length over original length Δl/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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VII. mechanical properties
D.
End up with a stress-strain curve
1.
provides huge amount of information about material properties
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VII. mechanical properties
D.
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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VII. mechanical properties
E.
____?____
Hooke’s
Law and Young’s modulus, E
1. stress () and strain (ε) are proportional under certain conditions (low
stress)
_?__ Law
a.  = ε E Hooke’s
el
b. E - Young’s modulus, modulus of elasticity, stiffness, resistance to
elastic deformation (GPa or psi)
c. physical meaning of E being large?
Material range of E
Metal
45 – 400 GPa
Ceramics
60 – 500 GPa
Polymers
0.01 – 4 GPa
Spaghetti
4.8 GPa
higher E implies greater stiffness
_____?_____
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VII. mechanical properties
F.
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)
Or differences in E are due
to differences in bonding!
E
dF
dr
In other works microscopic
(bonding) determines
macroscopic (E)
ro
Also as T increases, E
generally decreases
___?___
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VII. mechanical properties
G. Young’s 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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properties of materials
VII. mechanical properties
H.
tensile strength (TS)
1. 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
__?__is concentrated at M
3. stress
4. fracture ultimately occurs at F
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2. mechanicalmaterials
properties of materials
VII. mechanical properties
I.
ductility and elongation
1. ductility is the degree of plastic deformation at (prior to) failure
__?__
2. low or no ductility – brittle
3. ductility is quantified as % elongation, %EL
(i) % EL   l f  lo  100%
 l 
 o 
lf = length at fracture
l0 = initial length
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VIII. Material strength
A.
Tensile strength
1.
How hard does something need to be pulled to break material bonds
2.
Some examples:
a.
b.
c.
B.
Steel piano wire = 450,000 psi
Aluminum = 10,000 psi
Concrete = 600 psi
Compression strength
1.
Materials fail in compression in many ways depending on geometry, support
a.
b.
c.
C.
Buckling – hollow cylinders e.g. tin can
Bending – long rod or panel
Shattering – heavily loaded glass
Yield strength
1.
D.
Load required to cross line from elastic to plastic deformation
Ultimate tensile strength
1.
Maximum possible load without failure
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IX. Material testing
A.
Tensile strength… most common method
1. apply stress uniaxially along sample
2. continually increase force on ends
3.
4.
5.
6.
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
extensometer
steel 1018 stress-strain
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specimen
2. mechanicalmaterials
properties of 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
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IX. Material testing
B.
Euler buckling load, Pc
1.
P
load (MLT-2)
2.
I
moment of inertia (L4)
3.
E
Young’s modulus (ML-1T2)
4.
L
length (L)
5.
4 variables, 3 primitive dimensions = 1 dimensionless group
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IX. Material testing
What if the material is very brittle….can we do a tensile test?
Tensile tests can’t easily be done on ceramics/brittle material because
A.
Difficult to prepare and test samples with required geometry
B.
C.
Difficult to grip brittle materials without fracturing them
Ceramics fail very quickly (0.1% strain)
Transverse bending test is more usually employed
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IX. Material testing
C.
bending
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IX. Material testing
C.
Bending
1.
At point of loading, top surface is in compression and bottom surface is in
tension
2.
Stress is computed from specimen thickness, the bending moment, and the
moment of inertia of cross-section
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IX. Material testing
minimizing moments of inertia to increase rates of rotation
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IX. Material testing
D.
Compressive strength
what’s going to happen a beam (spaghetti) under compression?
1.
a.
b.
c.
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
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