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PHF110: Basic Physics and Materials Lecture 1b: Materials’ Properties 27 th November 2015

Transcript of Preparation… Need ceramic mug Alu tin Plastic cup ELASTIC BAND SPRING.

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PHF110: Basic Physics and Materials

Lecture 1b: Materials’ Properties

27th November 2015

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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

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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•……

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Materials science – an (everyday) example

End plugBarrel

Cartridge

Lid

Ink

Point(?)Insert

TipBall

Question: How many different materials?

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A More complex example…

Question: How many different materials?Answer: Many – all with tailored properties

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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

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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

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Materials grouping

3 Objects, 3 materials, 1 function You instinctively know which is which. What properties do they have?

Metal Ceramic Polymer

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Some Properties

Melting / freezing / boiling point Strength Elasticity / plasticity Ductility Hardness Toughness Electrical and thermal conductivity There are MANY more to learn about!

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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?

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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

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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

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Plastic Deformation (Polymers)

Deformation stages Crystallites separate (reversible) Rotation of crystallites (irreversible) Separation of segments Orientation of segments and

amorphous

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Load and displacement vs stress and strain

= 1 unit of load

100 apples

Small CSA Large CSA

= 100 apples

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Load and displacement vs stress and strain

Displacement, δ

Orig

inal

leng

th,

L o

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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

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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

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Stress – Strain Curve (metals)

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Plastic Deformation (Polymers)

Deformation stages Crystallites separate (reversible) Rotation of crystallites (irreversible) Separation of segments Orientation of segments and

amorphous

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Stress – Strain Curve (polymers)

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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

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The Young Modulus – the measure of stiffness

Symbol E E= stress / strain = σ / ε Units? Some common values? Many uses! For example; medical

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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

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Hardness

A measure of a materials ability to resist deformation by indentation or abrasion (scratching)

Scratching - Moh’s scale Indentation

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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

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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

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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

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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

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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!

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Materials selection

Screwdriver is made of 2 materials Steel and Perspex What properties do they have which make them

suitable?

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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?

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Intended Learning Outcomes

Define a few common properties of materials Describe some materials’ properties Recommend basic materials properties

requirements for simple applications