Post on 19-Jan-2016
PHF110: Basic Physics and Materials
Lecture 1b: Materials’ Properties
27th November 2015
Human Time Periods
Na
no
Ag
e?G
rap
hen
e?
???
?
Sili
con
Ag
eP
last
ics
Ag
e
Stone age
Mid
dle
ages
Indu
stria
l Rev
Ma
chin
e A
ge
Ren
aiss
ance
Bro
nze
age
600
0-2
500
BC
130
0-1
000
BC
Iron
age
2.5
mill
ion
BC
Figurines28,000 BC
FlintPottery
14,000 BC Bronze sword
Iron toolsPlastic
artefacts Electronics
Materials Science
Properties
Processing
Microstructure
Fac
ts
•Melting temperature•Ductility•Hardness•……
Pro
cess
es
•Casting•Injection moulding
•Forging •Rolling•……
Fea
ture
s
•Grain structure•Chemical distribution
•Phases•Particles•……
Materials science – an (everyday) example
End plugBarrel
Cartridge
Lid
Ink
Point(?)Insert
TipBall
Question: How many different materials?
A More complex example…
Question: How many different materials?Answer: Many – all with tailored properties
Materials Science
The understanding of properties is extremely important for correct materials selection and use
ProcessingMicrostructure
Pro
cess
es
•Casting•Injection moulding
•Forging •Rolling•……
Fea
ture
s
•Grain structure•Chemical distribution
•Phases•Particles•……
Properties
Intended Learning Outcomes
Define a few common properties of materials Describe some materials’ properties
– note as we go along Recommend basic materials properties
requirements for simple applications
Materials grouping
3 Objects, 3 materials, 1 function You instinctively know which is which. What properties do they have?
Metal Ceramic Polymer
Some Properties
Melting / freezing / boiling point Strength Elasticity / plasticity Ductility Hardness Toughness Electrical and thermal conductivity There are MANY more to learn about!
Strength (General)
OED: “ability to sustain the application of force without breaking or yielding”
ED: “…refers to a level of stress at which there is a significant change in the state of the material, e.g., yielding or rupture”
However, it’s not quite as simple as that… What is yielding and rupture? What is stress?
Yielding
The point at which a material no longer exhibits linear-elastic behaviour
Elastic means reversible!
2. Small load
F
d
bonds stretch
1. Initial 3. Unload
return to initial
Plastic means permanent!
Plastic Deformation (Metals)
1. Initial 2. Small load 3. Unload
planes
still
sheared
F
d elastic + plastic
bonds
stretch
& planes
shear
d plastic
Plastic Deformation (Polymers)
Deformation stages Crystallites separate (reversible) Rotation of crystallites (irreversible) Separation of segments Orientation of segments and
amorphous
Load and displacement vs stress and strain
= 1 unit of load
100 apples
Small CSA Large CSA
= 100 apples
Load and displacement vs stress and strain
Displacement, δ
Orig
inal
leng
th,
L o
Stress vs strain
Allow us to relate load to load bearing area And displacement to overall length of the piece
Calculating them is very simple:
original area before loading
s =Ft
A o2m
NArea, Ao
Ft
Ft
lo
δ2
= δlo m
m
δ2
Stress and Strain
We can measure the amount of stress required to cause a strain – Stress –strain curve
This curve can tell us quite a lot of information
Stress – Strain Curve (metals)
Plastic Deformation (Polymers)
Deformation stages Crystallites separate (reversible) Rotation of crystallites (irreversible) Separation of segments Orientation of segments and
amorphous
Stress – Strain Curve (polymers)
Stiffness OED: “the force required to produce unit
deflection or displacement of an object” ED: “For a structure, stiffness is the ratio of
the force divided by the displacement. When discussing materials, substitutes for force, and substitutes for displacement”
NOTE: stiffness is altered by the structure We describe material stiffness by defining a
modulus
stressstrain
The Young Modulus – the measure of stiffness
Symbol E E= stress / strain = σ / ε Units? Some common values? Many uses! For example; medical
Ductility
A measure of a material’s ability to undergo plastic deformation before fracture After a tensile test a ductile material will have
formed a neck – check your polymer sample Useful for
Wire drawing Can making – deep drawing Rolling
NOTE: Not the same as malleability
Hardness
A measure of a materials ability to resist deformation by indentation or abrasion (scratching)
Scratching - Moh’s scale Indentation
Moh’s scale – scratch resistance
Qualitative measure of hardness Quantitative methods discussed in Week 3
1 2 3 4 5 6 7 8 9 10
Talc Diamond
Glass
Steel
Aluminium
Indentation Methods
A known mass is applied to a material through a hard indenter for a known time Deformation of the material is measured
Gives a quantitative measure of hardness
toughness
A measure of the amount of energy a material absorbs as it fractures
Can be affected by speed of test (impact) Specimen geometry important – Notch
sensitivity
Can be ascertained (low speed load application) from stress-strain curve
Electrical conductivity
The ease with which an electrical current may flow within the material
Electricity is conducted by electron transport within the material
Controlled by the bond types Metals – metallic bonding = good conductor Ceramics – ionic bonding = poor conductor Polymers – covalent bonding = poor conductor
Comparison of some (general) properties
Material class
Electrical conductivity
Hardness Toughness Tensile strength
Ductility Melting temp
Metal Good Medium High High Medium High
Ceramic Poor High Low Low Low V.High
Polymer Poor Low Low Low Very high Low
VERY generalised table Missing properties include:
Cost, formability, availability, recyclability, chemical resistance, density and many many more!
Materials selection
Screwdriver is made of 2 materials Steel and Perspex What properties do they have which make them
suitable?
Consider important properties for these items…
Safety goggles? Pen (think of the different parts)? Fizzy drinks bottle? Touch screen of a smart phone?
What properties do they require and why?
Intended Learning Outcomes
Define a few common properties of materials Describe some materials’ properties Recommend basic materials properties
requirements for simple applications