Past Exam papers Sample 1

Question 1

Explain why ceramics do not find wide application in industry but are useful for furnace linings

Answer 1

Ceramics do not find wide application in industry due to their brittle nature. However,
they are useful for furnace linings because of their low thermal conductivity and thermal
stability.

Question 2

Define composites.

Answer 2

Composites are materials produced through mixture of two or more distinctly
different materials to achieve final properties that are superior to the properties of
individual components

Question 3

Explain how fibre composites derive their improved properties such as strength and
stiffness.

Answer 3

The improved properties in strength and stiffness are achieved by incorporating
strong, stiff but brittle fibres into a softer more ductile matrix. The matrix material
transmits the load to the fibres while the fibres carry most of the applied load.

Question 4

Explain the functions of the matrix in a fibre composite.

Answer 4

The matrix is used to bind together the fibres and to protect their surfaces from damage
or chemical attack. The matrix separates the individual fibres and prevents brittle
cracks from spreading across the composite.

The matrix can also be considered as a medium to transfer and distribute the load to
the fibres. The bond between the fibre and the matrix must be strong enough to
prevent interfacial separation or fibre pull-out under axial loads.

Question 5

List three factors that influence the strength of fibre composites.

Answer 5

The strength of fibre composites is determined by (Any 3):
* Strength of the fibres,
* Orientation of the fibres with respect to the applied load,
* Continuity of the fibres,
* Properties of the matrix,
* Strength and nature of the bond between the fibres and the matrix.

Question 6

Explain how ultrasound is generated and give two examples of the crystals used to
generate ultrasound.

Answer 6

Ultrasound is generated by means of piezo-electric materials that
convert electrical energy to mechanical energy and vice-versa.
Two examples of crystals: Quartz crystals, Barium titanate or Lithium
Sulphate.

Question 7

State the two techniques of ultrasound testing and briefly describe two differences.

Answer 7

Pulse-echo Technique / Through Transmission Technique
(Any 2)
1. Uses only one probe / uses two probes
2. The same probe acts as both the transmitter and receiver / one probe act as
transmitter and the other as receiver
3. Measurement of thickness of plate is possible / cannot measure thickness of plate

Question 8

List four advantages of dye penetrant testing for the detection of surface discontinuities.

Answer 8

(Any 4)
1. Suitable on all homogeneous
2. Not affected by defect orientation
3. Minimum skill required to perform
4. Defects must be open to surface detects
5. May be applied without sophisticated equipment

Question 9

What are the two basic groups of the penetrant process? Briefly describe their main
differences

Answer 9

Penetrants processes are usually separated into two basic groups, visible and
fluorescent.
Visible penetrants are those which contain a very bright dye, usually red, that is
viewed under bright white light.
Fluorescent penetrants contain a dye which fluoresces (glows) brilliantly under
filtered ultra-violet light (often called black light).

Question 10

What are the two types of magnetic particles that are commonly used in the
magnetic particle test?

Answer 10

There are two types of magnetic particles for use in the magnetic inspection, the wet
particles and dry particles.

Question 11

magnetic particle testing can result in residual magnetism in the specimen.
Give two reasons where residual magnetism is undesirable

Answer 11

Any 2
(a) Adverse effects on instruments, eg. aircraft compass.
(b) Interference with future machining operations by causing chips to adhere to
cutting tools.
(c) Adherence of magnetic particles to moving parts thereby accelerating wear.
(d) Induction of stray voltages in adjacent circuits.

Question 12

Corrosion can occur under direct chemical or electrolytic-chemical attack. Briefly
discuss their differences with regard to
(a) electron movement, and
(b) normal temperature in which they occur.

Answer 12

Direct chemical attack
● electrons travel through a metal for a very short distance.
● usually occurs at higher temperature.
Electro-chemical attack
● electrons travel through some measurable distance.
● usually occurs at any temperature.

Question 13

What are the two basic conditions for electrolytic corrosion to occur?

Answer 13

The existence of a difference in electrode potential between the anode and the cathode
● The presence of an electrolyte in contact with the metal.

Question 14

Name two ways in which stress corrosion can be overcome.

Answer 14

● Remove the stress through annealing
● Redesign the area of high stress concentration
● Remove / neutralise the chemical corroding medium.

Question 15

What type of corrosion would occur if steel comes in contact with copper in the presence
of an electrolyte? How is this corrosion affected by the ratio of surface area of steel to
that of copper?

Answer 15

Galvanic corrosion
If the ratio of surface area of steel to that of copper is large,
the corrosion will be widespread and not very deep,
OR
If the ratio of surface area of steel to that of copper is small, the corrosion will be
concentrated and greatly accelerated.

Question 16

Explain how a galvanised (zinc-coated) steel sheet is protected from corrosion.

Answer 16

The zinc coating is electronegative with respect to the steel sheet.
Even if the zinc coating is damaged, zinc will still act as a sacrificial anode.
It will corrode preferentially, and thus protecting the underlying metal.

Question 17

State three common methods of preventing corrosion. Give one practical example for
each method.

Answer 17

(Any 3 @ 2 marks for each method with an example)
(a) Correct choice of material
e.g. use materials that are corrosion resistant for a particular environment.
(b) Design to minimise the effect of corroding agents
e.g. a tank is designed in such a way that no liquid is left after draining.
(c) Cathodic protection
e.g. galvanised steel, or underground pipe is wired to Zn or Mg ingot.
(d) Use of protective coatings
e.g. electroplating, painting, cladding or inorganic coating.
(e) Environmental control
e.g. lower the temperature, decrease fluid velocity, remove oxygen from
liquids, reduce ion concentrations or add inhibitors to electrolytes.

Question 18

Heat treatment of steel comprises heating, holding and quenching stages. Explain why
it is important to control the holding time and temperature.

Answer 18

• The work piece must be held long enough to ensure uniformity of temperature
  throughout its entire volume.
• Temperature control during holding stage is important as the temperature must be
  higher than upper critical temperature to ensure a fully austenitic structure.
• However, the temperature must not be excessively high otherwise grain growth
  occurs.

Question 19

Briefly describe the martempering and austempering processes

Answer 19

In martempering process, the workpiece is first quenched in a molten salt bath, or
molten metal bath, at temperature above the Ms.
The workpiece is removed from the molten bath and air-cooled to form martensite
before any transformation from austenite to bainite takes place.
In austempering, after quenching at temperature above Ms, the workpiece is held
at the temperature for sufficient time to allow transformation from austenite to
bainite takes place.

Question 20

State two reasons why it is highly undesirable to have retained austenite in a hardened
steel

Answer 20

Retained austenite in a hardened steel is highly undesirable because: (Any 2)
● it is much softer than martensite thus will reduce the overall hardness of steel
● spontaneous cracking may occur later, when the austenite transforms to martensite
● volume changes which occur when austenite transforms to martensite will lead to
dimension instability of the component.

Question 21

List the two main categories of surface heat treatment processes. State the main
difference between the two process.

Answer 21

Thermochemical processes and selective surface hardening processes.
(1 mark each)

In the thermochemical processes, the surface chemistry of the steel is modified by the
introduction of carbon and/or nitrogen and occasionally other elements.
In the selective surface hardening process, there is no change in the surface composition

Question 22

State two advantages of nitriding as compared to carburizing

Answer 22

The advantages of nitriding as compared to carburising are: (Any 2)
* Since the steel is treated in the ferritic state and requires no quenching, cracking
and distortion are eliminated.
* A very hard case is obtained
* Resistance to corrosion is good
* The process is clean and simple to operate