Section A

Question 1

a) Is the creep mechanism Bulk Diffusion highly dependent with stress? Explain why help yourself with drawings.

b) Is the creep mechanism Bulk Diffusion highly dependent on grain size? Explain why help yourself with drawings.

c) Which mechanism of creep can be prevented using a single crystal part? Why?

d) Which mechanism of creep can be accentuated with alloying elements? Why?

Question 2

a. Draw the T-T-T diagram for plain carbon steel of eutectoid composition and show on the diagram the critical cooling curve, the transformation lines, the phases, the axis.

b. Use your diagram to explain the processes of quenching and tempering of the steel.

c. Explain the change of structure with martensitic transformation.

d. Discuss the implications of hardenability with the dimensions of the part to be manufactured.

e. In case the hardenability of the material is not good enough, discuss some strategies to increase the transformation without changing the geometry of the part.

Section B

Question 3

Springs for trucks. In vehicle suspension design it is desirable to minimize the mass of all components. You have been asked to select a material and dimensions for a light spring to replace the steel leaf-spring of an existing truck suspension. The existing leaf-spring is a beam, shown schematically in the figure. The new spring must have the same length L and stiffness S as theexisting one, and must deflect through a maximum safe displacement δ_max without failure. The width b and thickness t are free variables.

Figure E9

Derive a material index for the selection of a material for this application. Note that this is a problem with two free variables and there are two constraints. Use the two constraints to fix free variables. The table catalogues the requirements.

You will need to translate the problem first

You will need the equation for the mid-point deflection of an elastic beam of length L loaded in three-point bending by a central load F:

δ = 1/48 .(FL³)/EI

and that for the deflection at which failure occurs
δ_max = 1/6 . (σ_f L²)/tE

where I is the second moment of area; for a beam of rectangular section and E and σ_f are the modulus and failure stress of the material of the beam.

Question 4

It is found that the force F will inject a given weight of a thermosetting polymer into intricate mould in 30s at 177 degrees Celsius and in 81.5s at 157 degrees Celsius. If the viscosity of the polymer follow an Arrhenius law, with a rate of process proportional to exp?(-Q/RT)m²s^-1, calculate how long the process will take at 207 degrees Celsius.

Discuss the possible behaviour of this material in relation to creep and compare it with the possible behaviour of a thermoplastic taking into consideration the main polymer structure characteristic of both. Compare the equation to model the injection with the equation to model creep.

Question 5

In order to design a high performance connecting rod for a racing car, you need to select the suitable material for this application. The engine is a modified engine with the stroke and bore diameter fixed. Previous experience has demonstrated that in case of failure of high performance connecting rods, they are due to either fatigue or buckling. The engineer leader wants the engine as light as possible. The connecting rod length is L=0.1 m and has to resist F=20000 N in compression.

Figure

In order to translate the problem you need to answer these questions:

a) What is the function of the part?

b) What are the constraints of the part?

c) What is the objective?

d) What are the free variables? Discuss the implication of the free variables for the optimal choice.

e) By neglecting the bearing houses calculate the mass of the as a function of material properties to minimise the likelihood of fatigue failure.

f) What is the performance material index for fatigue?

g) By neglecting the bearing houses calculate the mass of the rod as a function of material properties to minimise the likelihood of buckling.

h) What is the performance material index for buckling?

i) Discuss the implication of two performance index for the selection of one material.

Materials properties that are available for ranking materials are: density (ρ); Young modulus (E); fatigue strength at 10⁷ cycles (σ_e);

The buckling load is given by:

F_Buckling = (π²EI)/L²