Materials

Q2. The sketch graph shows the tensile stress–tensile strain graphs up to their breaking stress, for four materials, A, B, C and D.

(a) State what is meant by tensile stress and tensile strain.

Tensile stress is the force exerted per/divided by crosssectional area.

Tensile strain is extension per/divided by original length.

[2 marks]

(b) Identify a property of material A using evidence from the graph to support your choice.

Any one property will do:

The material is brittle. This is shown on graph by little or no of plastic behaviour. The line is a straight line right up to breaking stress.

OR

The material has high Young modulus (or the material is stiff). This is shown on graph by a steeper gradient than the lines for the other materials.

[2 marks]

(c) A cylindrical specimen of material A under test has a diameter of 1.5 × 10^–4 m and a breaking stress of 1.3 GPa.

Calculate the tensile force acting on the specimen at its breaking point.

cross sectional area = πr² = π(d/2)² = πd²/4

cross sectional area = A = π × (1.5 × 10^-4)²/4

A = 1.77 × 10^-8 m²

breaking stress = 1.3 x 10⁹ = F/A

1.3 x 10⁹ = F /1.77 × 10^-8

F = 1.3 x 10⁹ x 1.77 × 10^-8

tensile force = 23 N

[3 marks]

(d) Discuss which of the four materials shown on the graph is most suitable for each of the following applications:

the cable supporting a lift in a tall building

a rope or cable attached to a person doing a bungee jump.

For each application, you should discuss the reason for your choice and why you rejected the other materials.

[6 marks]

This type of question is marked in two ways:

- the examiner looks for relevant points

- but also for a logical sequence and clear expression.

It is not just a 'tick fest' - but a marker does look to see how many relevant points you have made, then considers how you have strung them together to put your answer into a 'level'

Note that the quality of your spelling and grammar is important at this level!

 

Lift Cable Choice

Material B is the most suitable material for the lift cable as it has a high breaking stress

Material A should not chosen because it fails without warning

Material C/D not chosen because they have a lower breaking stress than A.

Material D not chosen as larger increase in strain for a given increase in stress (stretches too much!)

 

Bungee Jump Cable

Material D is the most suitable as for a given stress produces a large strain meaning a large extension

Material D is chosen because D can store a large amount of energy before failure

Not A ,B or C because high Young Modulus so they would make a sudden stop - resulting in large forces

Not A as brittle and therefore limited strain and sudden failure

Not C because requires a large strain before it exhibits plastic behaviour

Not C because if behaves plastically will not return to original length

Level 6, 5, 4, 3:

Correct materials selected for each application (B for lift and D for bungee).

One reason for choices given for each application and explanation why at least one other material would be rejected for each application.

Level 4 , 3:

Correct material selected for one application (B for lift and D for bungee).

One reason for choices given application.

Level 2:

No correct material selected but at least two properties necessary for an application given.

Level 1:

No correct material selected but at least one property necessary for an application given.

Level 0:

No correct material selected and no properties necessary for an application given.

(Total 13 marks)