AS Physics Unit 11 Materials – Further Reading

KS5 AS PHYSICS AQA 2450Mr D Powell

Mr Powell 2009Index

How do materials behave?

This image show a broken section taken from the edge of the tennis racquet.

You should be able to see the carbon fibres and how many have been pulled out of the surface. In between the fibres it is possible to see some of the fractured epoxy resin.

This image shows the cell-like structure of the material which enables it to be compressed by small stresses.

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How do materials behave?

This image shows a classic brittle failure in a metal.

You can see small flat cleavage facets which will reflect the light, giving the surface a shiny appearance.

The size of the facets can be taken as approximately equal to the grain size of the material.

This image shows a classic ductile failure in a metal, with voids around particles being evident.

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Looking inside glasses

cracks

human scale

fracture strength

flow

stiffnessthermal conduction

semiconductionoptical

Si Si

1 m

1 mm

1 m

1 nm

1 pm

random atomic packing

atoms

Source: MF Ashby and HR Shercliff, Cambridge University Engineering Department.

cell wall fibre composite

human scale

Looking inside woods

thermal

strengthstiffness

strengthstiffnessthermal

strengthstiffnessthermal

1 m

1 mm

1 m

1 nm

1 pm

cellular structure

cellulose fibres

Source: MF Ashby and HR Shercliff, Cambridge University Engineering Department.

cellulose molecules

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human scale

Looking inside metals and ceramics

stiffnesselectricalopticalthermal

yield strength(metals)

yield strength(metals)

fracturestrength(ceramics)

XXXX

crack

1 m

1 mm

1 m

1 nm

1 pm

precipitates

arrays of dislocations

alloying element

dislocation

atoms and electrons

Source: MF Ashby and HR Shercliff, Cambridge University Engineering Department.

human scale

Looking inside polymers

electrical

stiffnessthermaloptical

flow

strength

XXX X

H

C

H

H

C H

C

H

H

C

H

H

C

H

H

C

C

H

H

C

H

H

C

H

H

C

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C

C

C

1 m

1 mm

1 m

1 nm

1 pm

craze

tangled molecules

molecules

atoms

Source: MF Ashby and HR Shercliff, Cambridge University Engineering Department.

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Defects...Cracks and stress

Material is bent,upper surfacestretchedlower surfacecompressed.

h

Crack deflectstensile stress.Stressconcentratedbelow crack.

with a crack

Contours ofequal tensilestress. Largeststress nearsurface.

no cracks

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Under tensile stress, cracks propagate through materials.

very small areaat tip of crack:stress verylarge here

bond between atomsat tip highly stressed bond breaks

next bond is stressed

Shaping and slippingAtoms in gold are in a regular array: a crystal lattice. To shape the metal, onelayer must be made to slide over another.

To slip, layer ofatoms mustmove as a whole

Layer hasmoved oneatomic spacing

Atoms canmove oneby one

All atoms move:layer moves

Dislocationreachesedge ofcrystal

atommoves

dislocationmoves

One atommoves:dislocationmoves

in both a layer has slipped by one atomic spacing

dislocation

Mr Powell 2009Index

ductile metal flows, crack blunted

Stopping cracks propagating

Metals resist cracking because they are ductile. Cracks are broadened and blunted. They do not propagate.

Metals are tough because they are ductile

Metals

stress stress stress stress

high stress stress reduced

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Fibre-reinforced materials use a matrix to share stress amongst many strong fibres. The matrix also protects the fibresfrom cracks forming.

Fibre-reinforcement

Fibre-reinforced materials are tough because cracks can't propagate through the soft matrix

one fibre breaks, stress takenup by other fibres

strong fibre

stress stress stress stress

soft matrixsticks to fibres

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dislocation free to move: slip occurs easily

Pure crystal Alloy

alloy atom pins dislocation: slip is more difficult

dislocation pinneddislocation

Alloys are generally less ductile than pure metals

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Explaining stiffness and elasticity

Metals

++ + + + + +

+ + + + + + +

+ + + + + + +

+ + + + + + +

a metal is an array ofpositive ions bondedby negative electron'glue'

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stretching has to pull bonds apart

Elastic extensibility ~ 0.1% Young modulus

~1011 — 1012 Pa

Stretching a metal stretches bonds — but not much.

gaps open up a little

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Explaining stiffness and elasticity

Polythene

polythene is along flexiblechain moleculewhich folds up

bondrotates

stretching can rotate some bonds,making the folded chain longer

Young modulus

~108 — 109 Pa

chains arefolded

bondrotates

Stretching polythene rotates bonds

Elastic extensibility ~ 1%

Mr Powell 2009Index

Explaining stiffness and elasticity

Stiffer polymers

Young modulus

~1010 Pa

Young modulus

~109 — 1010 Pa

Polystyrene hasbenzene ringssticking out sideways.They make chainrotations difficult.

Bakelite hasmassively cross-linked chains. Thecross-links stopthe chains fromunfolding.

Polystyrene Bakelite – a thermoset

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Explaining plasticity

polythene strip 10 mm 100 mm thin crystalline strip ‘pulled outof’ wider region

Polythene is semi-crystalline. Think of polytheneas like cooked spaghetti. In amorphous regionsthe chains fold randomly. In crystalline regionsthe chains line up.

When stretched plastically, the chains slip pasteach other. More of the material has lined-upchains. More of it is crystalline.

Plastic extensibility > 100%

Polythene

‘neck’

crystalline amorphous new crystallineregion

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Rubber

Rubber stretches and contracts by chains uncoiling and coiling up again. Rubber is elastic, not plastic.

In stretched rubber, the chain bonds rotate, and chainsfollow straighter paths between cross-links. When let go,the chains fold up again and the rubber contracts.

In unstretched rubber, chains meander randomlybetween sulphur cross-links.

sulphur cross-linkssulphur cross-links

Elastic extensibility > 100%

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Ceramics versus metals

Ceramics have rigid structures

Covalent structures

example: silica (also diamond, carborundum)

Covalent bonds share electrons between neighbouratoms. These bonds are directional: they lock atomsin place, like scaffolding.

The bonds are strong: silica is stiff

Atoms are linked in a rigid giant structure

oxygen atom

joins to others

silicon atom

The atoms cannot slip: silica is hard and brittle

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Ionic structures

example: common salt

chlorine ion

sodium ion

Ionic bonds pass electrons from one atom to another.Because like charges repel and unlike charges attract,the charged ions hold each other in place.

The bonds are strong: salt crystals are stiff

The ions cannot slip: salt crystals are hard and brittle

Ions are linked in a rigid giant structure

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+–

– –

– –

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Metals have non-directional bonds

Metallic structures

example: gold

Atoms in metals are ionised. The free electronsmove between the ions. The negative charge of theelectrons 'glues' the ions together. But the ions caneasily change places.

The bonds are strong: metals are stiff

The ions can slip: metals are ductile and tough

Ions are held together, but can move

negativeelectron 'glue'

gold ion

++ + + + + +

+ + + + + + +

+ + + + + + +

+ + + + + + +

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Summary

Ceramics covalent or ionic

Metals

strong, rigid scaffolding,stiff, hard, brittle

mobile strong electron glue,stiff, ductile, tough