Deck 14- Looking inside materials 2

Question 1

Draw a diagram to show what happens to a material with a crack which is under tensile stress.

Answer 1

• Crack deflects tensile tress, stress concetrated below crack.

Question 2

Draw diagrams and explain why metals are tough.

Answer 2

• Metals resist cracking because they are ductile.
• Under stress cracks are broadened and blunted, they do not propagate.
• Metals tough because they are ductile.

Question 3

How do you calculate fracture energy?

Answer 3

Fracture energy= Total energy used to fracture/ Specimen cross sectional area.

Question 4

How do you calculate tensile strength?

Answer 4

Tensile strength = breaking force/specimen cross-sectional area.

Question 5

What does a large fracture energy tell you?

Answer 5

It tells you the material is tough.

Question 6

What does the large tensile strength tell you?

Answer 6

It tells you the material is strong.

Question 7

Explain why glass is strong but not tough.

Answer 7

Glass has quite a high tensile strength but a low tougness: you have to pull hard to break it but it is brittle and snaps withought much energy.

Question 8

Explain the two materials that make carbon fibre- reinforced plastic.

Answer 8

• Carbon fibres are made from heating filaments of polymer polyacrylonitrile. Their young modulus is in the 200-500GPa range. The problem is the fibres are brittle.
• The way to keep this composite stiff yet tough is to put the carbon fibres into an epoxy resin matrix. The epoxy resin stricks strongly to the carbon fibers so that they can take up stress. The combination of the two gives a large young modulus.

Question 9

Explain why the combination of the two materials which make carbon fibre reinforced plastic make each other stronger.

Answer 9

A single fiber is still brittle and snaps if a crack propagates across it, but now the fracture damage is limited to one fibre. The epoxy resin is tough, and when under stress it shares the extra stress among many nearby fibres, so they are not as likely to break.

Question 10

Explain why reinforced concrete is strong.

Answer 10

The concrete can be used to bear loads in tension(due to the steel in it) as well as compression.

Question 11

Draw a diagram and annotate it to show how fibres composites stop cracks propagating.

Answer 11

• fibre reinforced materials use a matrix to share stress among many strong fibres, the matrix also protects the fibers from cracking.

Question 12

Why is gold used for fillings?

Answer 12

Gold is used for fillings because it does not corrode or tarnish in the damp environment of the mouth. Fillings have to be able to withstand crushing forces when your jaws close on your food- up to 400N. So the gold is usually an alloy of gold and palladium.

Question 13

Give an example of a ductile metal.

Answer 13

Gold

Question 14

What is ductility?

Answer 14

A typical property of metals that makes them different from ceramics or polymers.

Question 15

What is the problem with dislocations in metals?

Answer 15

Because dislocations in metals may exhibit as little as one thousandth the strength of that they would otherwise have.

Question 16

What is meant when a metal is said to be work hardened?

Answer 16

When many dislocations in a metal are created(though bending it repeatedly) they can get entangled and stop each other from moving. This makes a metal less ductile.

Question 17

Draw a diagram and annotate it to describe how alloying a pure metal makes it less ductile.

Answer 17

• This disrupts the regularity of the metals crystal structure and makes it harder for atoms to slide past one another.