AMME 2302/5302 – Materials 1. Semester 2 2010 Examination Paper

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University of Sydney
Faculty of Engineering and Information Technologies

School of Aerospace, Mechanical and Mechatronic Engineering


November 2010 TIME ALLOWED: TWO HOURS

Answer all 5 questions. Marks for each question and parts of a question are given in
brackets.

Question 1 (50 Marks)

Please circle the correct answer for each of the following multiple-choice questions (2 marks
each)

1) The atomic planes with the highest planar density in body-centered cubic, face-
centered cubic and hexagonal close-packed metals are
A. {111}, {111} and {1000}, respectively
B. {110}, {111} and (0001), respectively
C. {110}, {100} and (0001), respectively
D. {100}, {100} and (0001), respectively

2) The {123} family in a tetragonal crystal system in which a = b ≠ c includes, but not
limited to, the following planes:
A. (123), (213) and (321)
B. ( 312 ), ( 231 ) and ( 321 )
C. ( 312 ), ( 321 ) and (132)
D. (312), ( 132 ) and ( 213 )

3) Which of the following represents a family of directions in a crystal lattice?
A. [111]
B. {111}
C. <111>
D. (111)

4) Which of the following technique/s can be used to study the surface morphology of
materials?
A. Scanning electron microscopy
B. Transmission electron microscopy
C. Atom probe tomography
D. All of the above three techniques

5) X-ray diffraction is used for the determination of crystal structures because
A. X-rays can penetrate thick specimens
AMME 2302/5302 – Materials 1. Semester 2 2010 Examination Paper

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B. X-rays can have wave lengths comparable to or shorter than the spacings
between atomic planes
C. X-rays are invisible
D. it is easy to focus X-ray

6) Point defects in crystalline metallic materials do not include
A. vacancy
B. self-interstitial
C. cation interstitial
D. substitutional impurity atom

7) A metal with a face-centered cubic (FCC) structure is usually more ductile than a
metal with an hexagonal close-packed (HCP) structure because
A. there are more dislocation slip systems in an FCC structure than in an HCP
structure
B. an FCC structure is denser than an HCP structure
C. an FCC structure has a higher atomic packing factor than an HCP structure
D. none of the above

8) Metals cannot be strengthened by
A. reducing grain sizes
B. alloying
C. cold work
D. annealing

9) Cold work normally increases
A. dislocation density
B. elastic modulus
C. grain size
D. ductility

10) The fabrication techniques of metals and alloys do not include
A. sintering
B. welding
C. casting
D. extrusion

11) Which of the following statements is not correct?
A. The diamond has a face-centered cubic lattice structure
B. The diamond can be treated as two identical face-centered cubic structures
with displacement of <111>/4 between the two structures
C. The diamond is a stable structure
D. The diamond is the hardest material available in the nature

AMME 2302/5302 – Materials 1. Semester 2 2010 Examination Paper

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12) For a ceramic compound, the following characteristic/s of the component ions
determine the crystal structure:
A. the magnitude of the electrical charge on each ion
B. the relative sizes of the cations and anions
C. both A and B
D. neither A nor B

13) In general, which of the following steel microstructure offers the highest strength?
A. Martensite
B. Tempered martensite
C. Pearlite
D. Spheroidite

14) In general, which of the following steel microstructure offers the highest ductility?
A. Martensite
B. Tempered martensite
C. Pearlite
D. Bainite

15) The phase/s in tempered martensite in a steel is/are
A. body-centered cubic Ferrite
B. monoclinic Fe3C
C. body-centered tetragonal martensite
D. both A and B

16) In a tensile test on a low carbon steel, necking starts on the engineering stress versus
engineering strain curve at:
A. lower yield stress
B. upper yield stress
C. maximum tensile stress
D. just before final fracture

17) If the surface crack causing fracture in a brittle material is made twice as deep,
assuming the fracture toughness is unchanged, the fracture strength will
A. decrease by a factor of 2
B. decrease by a factor of 2
C. decrease by a factor of 4
D. no change

18) Tick the method that improves fatigue resistance of materials:
A. imposition of stress concentrators
B. tensile residual stresses on surfaces
C. roughening the surfaces
D. case hardening
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19) In many metals, creep occurs when the homologous temperature exceeds 0.4. Which
of the following statements is correct?
A. steel and lead will both creep at room temperature
B. steel will creep but lead will not creep at room temperature
C. lead will creep but steel will not creep at room temperature
D. steel and lead will both not creep at room temperature

20) The Liberty suffered brittle fracture during WWII because:
A. ship hull and deck were poorly welded
B. the DBTT of the ship steel plates is higher than the ocean temperature
C. the DBTT of the ship steel plates is lower than the ocean temperature
D. none of the above

21) A hardness test conducted on a particular grade of steel gives a Brinell Hardness
Number (HB) of 300, what is its approximate tensile strength?
A. 300 MPa
B. 600 MPa
C. 1000 MPa
D. 2000 MPa

22) Glass has the following characteristics:
A. it is a tough material with high ductility
B. it experiences large thermal expansions (contractions) with changes in
temperature
C. it is a brittle material with a low toughness value
D. it has a crystalline structure

23) Increasing the degree of crystallinity of a polymer
A. increases its strength, decreases modulus, and decreases ductility
B. decreases its strength, decreases modulus, and decreases ductility
C. increases its strength, increases modulus, and increases ductility
D. increases its strength, increases modulus, and decreases ductility

24) Which of the following statements is true?
A. Thermoplastic polymers have a cross-linked and network structure
B. Thermosetting polymers have a cross-linked and network structure
C. Thermoplastics once hardened cannot be softened upon heating
D. Thermosets softens when heated and hardens when cooled

25) A continuous and aligned glass fiber/epoxy resin composite has 60 vol% of glass
fibers with a Young’s modulus of 70 GPa and 40 vol% of epoxy resin with a Young’s
modulus of 3 GPa. What is the composite elastic modulus in the longitudinal
direction?
AMME 2302/5302 – Materials 1. Semester 2 2010 Examination Paper

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A. 43.2 GPa
B. 29.8 GPa
C. 36.5 GPa
D. 49.9 GPa

Question 2 (10 Marks)

A unit cell for the rock salt crystal structure is shown below.
(1) Please determine the coordination number/s for both cations and anions. (3 Marks)
(2) Prove that the minimum cation-to-anion radius ratio for this type of structures is 0.414.
[Hint: assume that anions and cations are just touching
along cube edges and across face diagonals]. (7 Marks)










Question 3 (20 marks)

(1) (5 marks)
Part of the Cu-Ni equilibrium phase diagram is shown below. Prove that the weight fractions
of the liquid phase and the solid α phase at temperature TB can be expressed by:

L
L
LL
L
L CC
CC
SR
RW
CC
CC
SR
S
MM
MW −
−=+=−
−=+=+= ααα
α
α
00 and

Please do not directly apply the lever rule to
this question.







AMME 2302/5302 – Materials 1. Semester 2 2010 Examination Paper

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(2) (10 Marks)
A portion of Fe-C equilibrium phase diagram is shown below.

(i) For the composition of 2wt.% carbon and at the temperature of 800ºC:
• What are the equilibrium phases present (2 Marks)?
• What is the equilibrium composition of each of these phases in weight percent carbon
(2 Marks)?
• What are their relative proportions in weight percent (2 Marks)?

(ii) For the same composition of 2wt.% carbon and at the temperature of immediately below
727ºC:
• Calculate the weight percentage of proeutectoid and eutectoid (pearlite) phases (2
Marks)
• Calculate the weight percentage of ferrite and cementite (Fe3C) (2 Marks)













AMME 2302/5302 – Materials 1. Semester 2 2010 Examination Paper

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(3) (5 Marks)
The following figure shows the continuous cooling transformation diagram for a 0.35 % C
iron-carbon alloy superimposed by
five continuous cooling curves
(marked by (a), (b), (c), (d) and (e),
respectively). What are the final
microstructures for each of the five
continuous cooling conditions?


















Question 4 (10 Marks)

(a) What is the physical importance of the critical transfer length of a fiber in a composite
material? (2 marks)
(b) Explain why in brittle materials, such as ceramics and glasses, (a) there are significant
scatter in fracture strength, and (b) why the fracture strength decreases with increasing
specimen size. (4 marks)
(c) Engineering stress or true stress: which is more meaningful and why? Derive and show
how engineering stress is related to true stress. Under what condition is this equation
valid and why? (4 Marks)


Question 5 (10 Marks)

A thick high strength aluminum plate contains an edge crack of length (a) equal to 10 mm,
which runs perpendicular to the applied stress. For this aluminum, the plain strain fracture
toughness (KIc) is 35 MPam1/2, and the yield stress (σy) is 400 MPa. If the applied stress is
increased steadily, will the plate fail by brittle fracture or by general yielding? If the edge
crack length (a) is changed to 1 mm, what is your answer? [Hint: The fracture toughness
equation is: KIc = σcY(πa)1/2, where σc is fracture stress and Y=1.12] (10 marks)