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Answer script

Peperiksaan Semester Pertama

Sidang Akademik 2013/2014 (Januari 2014)

ENT 145 – Materials Engineering

Question 1

(a) Identify the components and its specific materials used in a computer.

(4 Marks / Markah) Answer:

1. The keyboard, monitor, and tower housing –polymers (ABS, high impact polystyrene, blends)

2. Tower casing – metal (aluminum alloy)

3. Cable, cord covers – polymers (polyethylene, Teflon, PVC, etc.)

4. Chip materials – metals, ceramics, electronic materials (silicon, silicon dioxide, copper, gold,

silver, etc…)

5. Monitor (cathode-ray tube type) - Polymers and metals (Glass, steel, copper, PVC, rubber)

(b) With the aid of sketches, explain the differences between ionic, covalent and

metallic bonding. Give an example of chemical compound for each primary

bonding.

(6 Marks / Markah) Answer:

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(c) A small silver spoon contains 92.5 wt. % silver and 7.5 wt. % copper. The spoon

has a mass of 100 grams. Given Avogadro’s number, NA = 6.022x1023

atoms/mol.

(i) Determine the number of copper and silver atoms in the spoon.

(8 Marks / Markah) Answer:

𝜌 = 𝑛𝐴

𝑉𝐶𝑁𝐴

𝑛 = 𝜌𝑉𝐶𝑁𝐴

𝐴

Since 𝑚 = 𝜌𝑉 , so,

number of atom, n is;

𝑛 = 𝑚𝑁𝐴

𝐴

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(ii) Explain the importance of adding copper to the silver spoon.

(2 Marks / Markah) Answer:

Copper is added to silver to make the metal stronger and more durable.

Question 2 [Soalan 2]

(a) Briefly explain conditions of atomic motion in diffusion.

(2 Marks / Markah) Answer:

1) There must be empty adjacent site.

2) Must have sufficient energy to break bonds with its neighbour atoms and then cause

some lattice distortion during the displacement.

(b) Discuss the concept of non-steady state as it applies to diffusion. Give an example

in your answer.

(4 Marks / Markah) Answer:

(c) A FCC iron-carbon alloy initially contains 0.2 wt. % C is carburized at an

elevated temperature and in an atmosphere where the surface carbon

concentration is at 1.0 wt. %.

(i) If after 49.5 hour the concentration of carbon is 0.35 wt. % at a position

4.0 mm below the surface of the alloy, determine the temperature at which

the treatment was carried out. Refer Table A1 and Table A2 in Appendix.

(10 Marks / Markah) Answer:

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(ii) If the diffusion treatment is to be applied to an iron gear, propose a

diffusion mechanism. Justify your answer.

(4 Marks / Markah) Answer:

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Question 3

(a) Discuss the differences between brittle and ductile metals with respect to its

tensile stress-strain behaviour.

(4 Marks / Markah) Answer:

(b) A cylindrical specimen of hypothetical metal alloy has a diameter of 8.0 mm. A

tensile force of 1000 N produces an elastic reduction in diameter of 2.8 x 10-4

mm.

Compute the modulus of elasticity for this alloy, given that the Poisson’s ratio is

0.30.

(6 Marks / Markah) Answer:

Ductile Metal experience plastic

deformation upon fracture

Brittle Metal very little or no

plastic deformation

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(c) Table 1 shows a list of materials and their mechanical properties. Each of the

material will be tested as a cylindrical rod specimen with 100 mm long and

having a diameter of 10 mm. If the tensile load is 27.5 kN, answer the following

questions.

(i) From Table 1, choose the material(s) that will not experience plastic

deformation. Justify your choice(s).

(6 Marks / Markah) Answer:

(ii) By referring answer in (i), select the material(s) that will not experience a

diameter reduction of more than 7.5 x 10-3

mm.

(4 Marks / Markah) Answer:

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Table 1 [Jadual 1]

Material Modulus of

Elasticity (GPa)

Yield Strength

(MPa) Poisson’s Ratio

Aluminum alloy 70 200 0.33

Brass alloy 101 300 0.34

Steel alloy 207 400 0.30

Titanium alloy 107 650 0.34

Question 4

(a) Distinguish between ductile and brittle failures. Give an example for your answer.

(4 Marks / Markah) Answer:

Ductile Fracture:

– Accompanied by

significant plastic

deformation

Brittle Fracture:

– Little or no plastic

deformation

– Catastrophic

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(b) Figure 1 shows the circumferential stress, σ, (also called hoop stress) in a

pressurized cylindrical vessel is calculated by the equation, 𝜎 = 𝑃𝑟/𝑡, where P is

the internal pressure, r is the radius of the vessel, and t is thickness. The vessel has

91.44 cm diameter, 6.35 mm thickness and an internal pressure of 34.5 MPa.

Assume the crack occurred at the center of the vessel and Y=1.0. For the fracture

toughness KIC and yield strength σY values of each vessel materials, refer Table

A3 in Appendix.

(i) Compute the critical crack length (2a) if the vessel is made of Al 7178-T651.

(8 Marks / Markah) Answer:

(ii) If the material of alloy steel (17-7pH) to be used, compare the difference of

the critical crack length (2a) with your answer in (i). Justify your answer.

(6 Marks / Markah) Answer:

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(iii) The vessel will fracture catastrophically if the crack lengths above the

calculated value as shown in your answer in (i) and (ii), propose preventive

measures need to be taken to increase the resistance to fatigue failure.

(2 Marks / Markah) Answer:

Measures that may be taken to increase the fatigue resistance of the vessel:-

Polish the surface to remove stress amplification sites.

Reduce the number of internal defects (pores, etc.) by means of altering processing

and fabrication techniques.

Modify the design to eliminate notches and sudden contour changes.

Harden the outer surface of the structure by case hardening (carburizing, nitriding)

or shot peening.

Figure 1

Question 5

(a) Discuss the difference between pearlite, spheroidite and martensite with respect to

microstructures and mechanical properties.

(6 Marks / Markah) Answer:

Microstructure Mechanical Properties

Pearlite Alpha ferrite + cementite/ layer

which alternate with one another

Less ductile than spherodite. Harder

than spherodite.

Spherodite Alphaferrite + cementite / sphere-

shaped particles

extremely ductile. softest and weakest.

Martensite Alphaferrite + cementite / needle

shaped grains

Extremely hard but brittle

(b) The complete isothermal transformation diagram of 0.76 wt% C steel alloy is

shown in Figure 2.

(i) Develop the heat treatment process to produce a microstructure of 100 %

martensite structure.

(5 Marks / Markah) Answer:

r

P

2a

crack

σ

σ

t

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(ii) Develop the heat treatment process for producing a microstructure of 50%

bainite and 50% martensite structure.

(5 Marks / Markah) Answer:

(iii) Alloy in question (i) is tested for bending fatigue test. It has experienced a

fracture which occurs in brittle manner. Propose a suitable solution to

improve mechanical properties of the alloy. Justify your answer.

(4 Marks / Markah) Answer:

Ductility of martensite may be enhanced by heat treatment process known as tempering.

Process of heating a martensitic steel to temperature between 250-650 oC for 1 hour and

slowly cool to room temperature.

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Figure 2 [Gambarajah 2]

-ooOoo-

T (oC)

t (h)

Pre-heat Heating at 250-650

oC

for 1 hour Slow cooling at Tamb