Assessment 2
26 August 2020
COURSE:
ENGINEERING
TIME:
2.0 HRS
SUBJECT:
STRENGTH OF MATERIALS 2B (SLRBCB2/SLR 2B21)
MARKS:
100
This paper consists of 2 pages
Examiners:
Dr D. M. Madyira
Prof R. F. Laubscher
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This is an open book assessment

Make sure you clearly shown your working
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No answers in pencil will be accepted
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Answer all the questions in English
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Start a new question on a new page for clarity
---------------------------------------------------------------------------------------------------------------------------------QUESTION 1 [50]
a.)
In the assembly shown in Figure Q1(a), the copper bar on the left has a diameter of 30 mm
and the brass bar on the right one of 60 mm. The bars are stress-free at 25°C. Find the
stresses and the change in length of the bars, when the temperature rises to 100°C if the
supports are unyielding. For copper, E = 120 GPa and α= 18 × 10 -6/oC, and for brass E =
90 GPa and α= 20 × 10-6/oC.
[25]
Figure Q1 (a): Compound bar subjected to thermal changes
b.)
A compressive load P is transmitted through a rigid plate to three magnesium-alloy
bars that are identical except that initially the middle bar is slightly shorter than the
other bars (see Figure Q1(b)). The dimensions and properties of the assembly are
as follows: length L =1.0 m, cross-sectional area of each bar A = 3000mm2, modulus
of elasticity E = 45 GPa, and the gap s =1.0 mm.
[25]
1.
Calculate the load P1 required to close the gap.
2.
Calculate the downward displacement d of the rigid plate when P = 400 kN.
3.
Calculate the total strain energy U of the three bars when P = 400 kN.
Figure Q1 (b): Compound columns
QUESTION 2 [50]
a.)
A an alloy steel shaft has an outside diameter of 100 mm. A central hole of 60 mm diameter
is bored in part of its length as shown in Figure Q2(a). Compute the shearing stress in the
hollow section is the stress in the solid section is 200 MPa. What is the overall angle of twist
of the shaft.
[25]
Figure Q2(a): Combined hollow and solid shaft
b.)
Figure Q2(b) shows a shaft carrying three pulleys that rotates at 1750 rpm. Pulley A
delivers 15 kW to a mating pulley that drives a fan. Pulley C delivers 20 kW to a different
mating pulley that drives a conveyor. All power comes into the shaft through pulley B.
Considering only torsion, compute the shearing stress in each part of the shaft. Ignore
stress concentrations.
[25]
Figure Q2: Power transmitting shaft
THE END!!