UNIVERSITY OF CAPE TOWN
DEPARTMENT OF MECHANICAL ENGINEERING
JUNE 2009 EXAMINATION
MEC 2042F: MATERIALS SCIENCE IN ENGINEERING
_____________________________________________________________________
INTERNAL EXAMINER :
PROFESSOR RD KNUTSEN
EXTERNAL EXAMINER
:
Dr M Topić
_____________________________________________________________________
DURATION
:
MARKS
100
:
THREE [3] HOURS
INSTRUCTIONS:
(1)
Answer ALL questions.
(2)
Use neat diagrams with clear labels and concise explanations where appropriate.
(3)
Numbers in the right-hand margins indicate the relative weighting of the questions.
(4)
Pay particular attention to this weighting when constructing your answers.
Useful Information
K1C = Y σ (πa)½
Note: The geometrical factor, Y, is equal to 1 for simple geometries.
Ecl=EfVf + EmVm
Ect=EfEm/( EfVm + EmVf)
σ TS = V f σ ff + (1 − V f )σ ym
i
Question 1
Below are three sets of simple materials requirements that could have been extracted from
different product design specifications. For each in turn describe your material choice, giving
reasons why your choice is appropriate. Include in your answers the reasons why you may
have excluded certain materials types. In all cases, try to be as specific as possible in your
material choice.
(a) High electrical conductivity, low density (within the constraints of the other
performance criteria), reasonable resistance to corrosion, able to be cast from liquid.
[5]
(b) Electrically non-conducting, easily cast into complex shapes, must withstand
deformation by up to 50% elongation for long periods but quickly recover to original
dimensions.
[5]
(c) Thermally non-conducting, strong at temperatures in excess of 600K, optically
transparent.
[5]
Question 2
Provide concise answers to the following:
(a) How, in general, do thermosets differ from thermoplastics in terms of structure,
properties and processing? What are the requirements for thermoset formation?
[5]
(b) How do fibre length and orientation distributions affect the tensile elastic moduli of
composites?
[5]
Question 3
You are provided with (i) samples of low carbon steel and (ii) samples of concrete for
mechanical property determination. What mechanical properties would you measure for each
material ((i) and (ii)) and explain how the properties would be used in design with reference to
typical examples of engineering applications of these materials.
[8]
Question 4
A structural member from an overhead hoisting device (a crane) has suffered brittle fracture
under normal working loads. The member is manufactured from medium carbon steel and the
original design specifies a yield strength of 350MPa and minimum elongation of 15%.
However, the measured properties of the failed part indicate that the yield strength is
1080MPa and the elongation at fracture is only 3%. By taking into account the effect that heat
treatment can have on altering the mechanical properties of medium carbon steel, give an
account of why you think that the incorrect heat treatment was performed on the structural
member before it went into service. Emphasise the microstructural changes that occur during
heat treatment and how they influence the mechanical properties.
[15]
………/Question 5
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Question 5
(a) Several kilometres of steel cable are being used to support the glass roof structure of
the new 2010 soccer stadium in Green Point (close to the sea). You are asked to assist
the construction engineers with the choice of steel type for the cables. Answer the
following:
(i) What factors need to be taken into account in making the optimum steel selection?
[3]
(ii) Give two possible example solutions for the steel cable selection and discuss the
advantages and disadvantages of each choice.
[4]
(b) The plain carbon steel reinforcement bar in concrete is known to corrode over time,
which leads to degradation of the concrete structure. Describe sound engineering
approaches that can be employed to avoid or minimise the incidence of corrosion of
the reinforcement bar.
[3]
Question 6
A manufacturer of fishing hooks, which will be used for big game fishing, approaches you for
assistance. He has obtained the specifications for a particular steel, but he has no idea as to
how the specifications translate into properties applicable to the service requirements of the
hooks. The specifications are given as elastic modulus, yield strength, ultimate tensile
strength and maximum plastic elongation. Explain the definition of each of these mechanical
property terms and describe the meaning of each property in relation to the performance of the
fishing hooks.
[12]
Question 7
(a) A high strength steel rod has a yield strength of 1460MPa and a K1c value of 98
MPa.m½. Calculate the size of a surface crack that will lead to fast fracture at an
applied stress of ½σy. [Assume that the geometrical factor for the rod is 1]. [2]
(b) A polymer bar’s dimensions are 25mm x 50mm x 375mm. The polymer has a
modulus of elasticity of 4.1GPa. What tensile force is required to stretch the bar
elastically to 381mm?
[2]
(c) A parallel-aligned fibre composite rod (diameter = 10mm) has 40 vol% glass fibres in
an epoxy matrix. The fibres are aligned along the length of the rod and the rod is
loaded in uniaxial tension. Calculate the following:
(i)
The elastic modulus of the composite rod.
[2]
(ii) The load at which the fibres would start to break in the composite if the fibres
have a fracture strain of 1%. Assume that the matrix does not yield before fibre
fracture.
[2]
The properties of the different components are as follows:
Material
Epoxy
Glass
Elastic modulus
3.1 x 103 MPa
72 GPa
Tensile strength
69 MPa
3.5 GPa
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………/Question 8
Question 8
A structural rod support on the cableway system on Table Mountain is to withstand a
maximum applied tensile force of 20 000kg. To assure sufficient safety, the maximum
allowable stress on the rod is limited to half the tensile yield stress of the material. The rod
must be at least 3.75m long but must not deform elastically by more than 6.25mm when the
maximum force is applied. Further requirements that must also be met include: (i) The rod
must not undergo fast fracture if it contains surface cracks less than 7mm long, (ii) The
strength of the rod must not be altered by exposure to temperatures up to 200°C, (iii) the
weight of the support rod must be minimised, (iv) the rod must be corrosion resistant for at
least 5 years, and (v) the diameter of rod must not exceed 45mm.
Design an appropriate rod (determine the minimum diameter) using the optimum
material chosen from the table below. You should show clearly, by calculation and logical
argument, how you derived the design and material selection, and in particular, you should
indicate why you eliminated materials.
[22]
Material
Elastic
modulus
(GPa)
Density
(g/cm3)
Fracture Toughness
K1C, (MPa(m)½)
Yield Strength
(MPa)
Al-Cu alloy
68
2.8
24.0
450
Ti-6%Al-4%V
110
4.4
17.0
890
Low alloy steel
210
7.8
50.0
1628
Zinc- galvanised
mild steel
210
7.8
55.0
300
Brass
103
8.2
52.0
250
Work-hardened
aluminium
70
2.8
35.0
315
Stainless steel
190
7.6
33.0
220
-oo0oo-
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