EXAM COVER SHEET
Student name:
Student ID
Campus:
Associate Degree of Applied Engineering
(Renewable Energy Technologies)
Subject number:
ENMAT101A
Subject name:
Engineering Materials and Processes
Semester
1, 2013
Time allowed 1:50 hours plus 10 minutes reading time
General instructions
Marks
Write your answers using black or blue pen
Total marks: 25
Write your name and campus at the top of
each page
All questions must be attempted.
NO liquid paper (whiteout) can be used – if
you make a mistake, just cross out your
attempt.
Marks allocated for each question are shown
throughout the examination paper.
Examination aids permitted as indicated
Standard
dictionaries
Bilingual
dictionaries
Technical
dictionaries
Programmable
calculators
Nonprogrammable
calculators
No
No
No
No
Yes
Other examination aids permitted



Writing implements (pens, pencils, erasers, highlighters)
Ruler
Reference information included at end
ENMAT101A Engineering Materials and ProcessesI
Semester 1, 2013
Question 1 (6 marks total)
(a) Complete the following table. Give the approximate (rounded off) atomic mass
unit values for the three particles that make up an atom. (2 marks)
Particle Name
Proton
Neutron
Electron
Atomic Mass Units
1
1
0
Charge
+1
0
-1
(b) Explain the main difference between a nuclear reaction and a chemical reaction
with reference to the simple atomic model (Bohr model). (3 marks)
Nuclear reaction changes nucleus but chemical reaction only effects electrons – and only the
outer shell at that.
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(c) Draw the Bohr model representation (in 2d) of an atom of Lithium and one of Fluorine.
Show the correct number of each particle as from part (a). Illustrated what happens when
the two atoms combine to form Lithium Fluoride. Include labels (3 marks)
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(d) Describe whether Lithium Fluoride would conduct electricity in the solid state and/or
when dissolved in water. Explain.
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Lithium Fluoride forms a relatively simple cubic type of crystal structure that has no free
electrons – so it will not conduct electricity. When dissolved in water it is split into
Lithium(+) ions and Fluorine(-) ions. Being charged atoms (ions) their movement is
effectively an electric current – so is conductive.
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Question 2 (6 marks total)
(a) Explain why an optical microscope cannot see atoms.
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The wavelength of visible light is much larger than the an atom, so not capable of this
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resolution.
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(b) Describe / illustrate a method of “seeing” atoms to form images of an atomic lattice.
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STM = Scanning Tunnelling Microscope: “Feels” shape of electron clouds associated with
atoms by scanning probe tip over surface and measuring minute current.
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X-ray diffraction: Crystal lattice, identification
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(c) What are Van der Waal’s forces? Illustrate and explain with reference to the table of
properties for Alkanes (see Reference Section at the end)
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Small (compared to ionic bonding) electrical charge imbalances in molecule begin to
attract when in very close proximity. For alkanes, longer molecules have more VDW
force, hence more attraction > stiffer > higher viscosity and higher MP & BP.
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(d) Illustrate the atomic structures of a pure metal, and compare to a simple thermoplastic.
Metal Lattice
Thermoplastic
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Metal Lattice of positive ions with a sea of electrons. Ions attracted to sea but repulsed by
adjacent ions – hence a packing arrangement.
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Thermoplastic has carbon chains that mix randomly held by tangling and VanDWaals
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Question 3 (6 marks total)
Give definitions for the following (1 marks each)
a) Ductility: Ability to remain deformed after load removed (plasticity). Ductility is tensile plasticity,
______
measured as % elongation = elongation/original length
b) Hardness: Ability of a material to resist indentation or abrasion.
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c) Elasticity: Ability of a material to be deformed and then return to its original size after removing
____________
the load.
d) Stiffness: Resistance to deformation under stress.
______________________________
e) Toughness: Energy absorbed in breaking the material
______
f) Stress: Intensity of force in a solid: Measured as Force per unit area
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g) Strain: Proportion of deformation. Strain = elongation/original length
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h) Yield strength: The stress at which plastic (permanent) deformation begins.
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i) Modulus of elasticity: Axial stiffness, measured as Axial Stress/Axial Strain.Units Pa
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j) Modulus of rigidity: Shear stiffness, measured as Shear Stress/Shearl Strain.Units Pa
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k) Creep: Gradual deformation under stress, usually at elevated temperature
____________
l) Fatigue strength: The (true) stress at which a material will maintain a certain number of full cycles
of alternating stress
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m) Stress concentration: A geometric feature or flaw that increases the nominal stress at a certain
point on the part.
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n) Resilience: Energy absorbed elastically. Area under force/extension graph up to yield point.
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o) Coefficient of thermal expansion: Thermal strain per change by 1K
______
Question 4 (6 marks total)
Sketch a set of STRESS/STRAIN curves on the same axes. Label the UTS, YS and/or elastic
limit. Use values from Reference Section.
(a) Mild steel: Use a dotted line to represent true stress
(b) Grade 8.8 bolt. Indicate toughness
(c) Grey cast iron:
(d) PVC
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Question 5 (6 marks total)
(a) Explain the mechanism of failure for the two specimens shown below with reference to
slip in a metallic crystal lattice.
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(b) Describe the two main ways the yield strength of a metal can be increased. Explain in
terms of both the microstructure and the bulk mechanical properties. Give example
materials in each case.
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Question 6 (6 marks total)
This high performance shaft is made
from hardened alloy steel (as listed in
the Reference Properties).
In the application the stress was never
higher than 30% of the yield strength.
It had been running for some months
before the shaft fractured suddenly. It
was designed to last many years, if not
indefinitely.
(a) What type of fracture is this?
(b) Sketch a generalised S/N curve for steel using the rule of thumb that the endurance limit is
approximately half the ultimate strength. Include an S/N curve for aluminium for
comparison. Label both curves.
(c) Explain why this shaft does not comply with the S/N curve for this material. Use
appropriate terminology.
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(d) What is the difference between fatigue strength and endurance limit?.
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(e) Explain and illustrate how shot peening works and give an example of where it would be
used.
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a. What could happen if shot-peening is over-done?
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b. What parameter/s can be changed to alter the depth of treatment?
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(f) Illustrate three design changes that improve the fatigue resistance of a high strength bolt.
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Question 7 (6 marks total)
Give definitions for the following (1 marks each)
a) Dendritic Structure
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b) BCC
____________
c) FCC
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d) Allotropy
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e) Recrystallisation
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f) Amorphous
____________
g) Explain the difference between melting point and recrystallisation temperature.
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h) What sort of grain-structure problem occurs if heat treatment was done at
excessive temperature and/or for too long?
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Question 8 (6 marks total)
(a) Describe how this grain structure formed on this
aluminium ingot. (Sort the grain structures into 3
groups and explain why there is a cone-shaped
hole at the top)
(b) What is the main cause of porosity in a casting? Describe how this can be
prevented/reduced by the design of the product.
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(c) Describe how porosity can be prevented/reduced by the arrangment of a low
pressure casting process.
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(d) List advantages/disadvantages of each metal casting process listed in the table:
Include melting point, accuracy, setup costs, production costs, design limitations
like complex geometry and size.
PROCESS
Advantages
Disadvantages
Typical metal
Sand Casting
Investment
Gravity die
High pressure
die
Centrifugal
Lost Foam
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Question 9 (6 marks total)
Explain the mechanism of work hardening (at the grain microstructure level) with reference to the
Stress/Strain curve below. Follow the process from points 1 to 7.
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Question 10 (6 marks total)
(a) What specific material property distinguishes hot-working from cold-working
processes?
(b) Compare forming processes. Include melting point, accuracy, setup costs,
production costs, design limitations, strength, size.
PROCESS
Advantages
Disadvantages
Typical metal
Hot rolling
Extrusion
Cold Rolling
Forging
Powder
Metallurgy
Machining
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Question 11 (6 marks total)
(a) Explain why alloys are usually more useful in engineering than pure metals.
(b) Describe two diffusion processes and explain how they have the effect of
increasing the strength of a ductile metallic lattice. Give an example of one of
these.
(c) The tin/lead phase diagram (Reference Section): What is the word used to
describe the 61.9% Sn mixture and what is special about it?
(d) The tin/lead phase diagram (Reference Section): Compare 62/38 solder with
50/50 solder. Which one is more likely for small electrical soldering, and which
one for plumbing work where solder cools more gradually?
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Question 12 (6 marks total)
(a) The above samples were all Carbon steel and cooled slowly. Describe the grain types
and give an estimate of their carbon content.
A.
B.
C.
D.
(b) What is the main difference in the process of normalising of a forging vs
annealing of a casting?
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(c) Describe the cooling of each of the 4 samples above. Plot the cooling process and
label important points to include in your descriptions.
A.
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B.
C.
D.
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REFERENCE SECTION
3
Alkane
Formula
Boiling point [°C]
Melting point
[°C]
Methane
CH4
-162
-182
gas
Ethane
C2H6
-89
-183
gas
Propane
C3H8
-42
-188
gas
Butane
C4H10
0
-138
gas
Pentane
C5H12
36
-130
0.626 (liquid)
Hexane
C6H14
69
-95
0.659 (liquid)
Heptane
C7H16
98
-91
0.684 (liquid)
Octane
C8H18
126
-57
0.703 (liquid)
Nonane
C9H20
151
-54
0.718 (liquid)
Decane
C10H22
174
-30
0.730 (liquid)
Undecane
C11H24
196
-26
0.740 (liquid)
Dodecane
C12H26
216
-10
0.749 (liquid)
Icosane
C20H42
343
37
solid
Triacontane
C30H62
450
66
solid
Tetracontane
C40H82
525
82
solid
Pentacontane
C50H102
575
91
solid
Hexacontane
C60H122
625
100
solid
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Density [g·cm ] (at
20 °C)
20
Tin / lead phase diagram
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