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Spring Examinations 2008 / 2009
Exam Code(s) 3BM1, 3BG Exam(s) 3 rd^ Mechanical Engineering 3 rd^ Biomedical Engineering
Module Code(s) ME Module(s) Metals and Metal Processing
Paper No. I
External Examiner(s) Professor N. O’Dowd Internal Examiner(s) Professor Sean Leen *Dr. M. Bruzzi
Instructions: This paper contains FIVE questions.
Attempt THREE questions. All questions carry equal marks.
Duration 2 hours
No. of Pages 5 – including cover page Department(s) Mechanical & Biomedical Engineering
Requirements Graph Paper
1.(a) Figure 1(a) shows a stress-strain curve for a 316 stainless steel:
0
50
100
150
200
250
300
350
400
450
500
0 2 4 6 8 10 12 14 16 % Strain
Stress (MPa)
Figure 1(a)
Determine the following, as accurately as you can: (i) Yield Stress (1) (ii) Yield Strain (1) (iii) Young’s Modulus (1) (iv) Strength (1) (v) Ductility (1)
(b) Plot, using graph paper, the true stress – true strain curve for the 316 stainless steel properties shown in Figure 1(a), using at least 8 points on the curve. Discuss the relevance of this curve and outline under what circumstances it is valid. (7)
(c) Define the relationship between stress and strain (constitutive behaviour) for the cases of (a) uniaxial tension (b) uniform shear loading (c) uniform hydrostatic compression. (6)
(d) A metal sample, with a Young’s modulus of 50 GPa and a Poisson’s ratio of 0.3, is subjected to an elastic compressive force, F, such that the strain is 0.1%. The sample is clamped such that it experiences no deformation in the y- direction, as shown in Figure 1(b). Using Mohr’s strain circle, determine the maximum shear stress. In your analysis assume uniform linear elastic deformation in the sample. (7)
Figure 1(b)
F y^ F x
4.(a) Imperfections in crystals can be categorised as: o Point defects o Line defects o Planar defects
Discuss each of these categories outlining the different types of defects associated with each category. (6)
(b) The following diagram plots the resolved shear stress, τR, against the shear deformation, γ, for a single crystal of iron. Describe how dislocation motion contributes to the plastic deformation mechanisms indicated by the different stages on Figure 4 below. (6)
Figure 4
Discuss the relevance of the curve shown in Figure 4 to the stress-strain curve of a polycrystalline steel. (2)
(c) Alloys are commonly used as materials for engineering applications. Describe what is meant by ‘coring’ for a binary alloy. (4)
(d) Microstructural investigation has indicated that coring has occurred due to the rapid cooling of a binary alloy of aluminium and copper. To refine the cored alloy, an annealing process is carried out. Determine the approximate time at 500ºC that will produce the same diffusion result (in terms of concentration of aluminium and copper at a given point) as a 10 hour heat treatment at 600ºC.
(The diffusion coefficients of copper in aluminium at 500 and 600ºC are 4.8x10-14^ and 5.3x10-13^ m^2 /s, respectively). (7)
5.(a) The iron-carbon phase equilibrium diagram is shown in Figure 5 below.
Figure 5
Describe the following reactions and identify, with the aid of a sketch, the constituents in each reaction: (i) Eutectic reaction (2) (ii) Peritectic reaction (2) (iii) Eutectoid reaction (2) (iv) Solvus reaction (2)
(b) For a 99.6 wt% Fe – 0.4 wt% C alloy, at a temperature just below 1011K, determine the following: (i) The fractions of total ferrite () and cementite (C) phases (4) (ii) The fractions of the total proeutectoid ferrite and pearlite (4) (iii) The fraction of eutectoid ferrite (4)
(c) Discuss the effect of cooling rate on the microstructure and mechanical properties of a plain carbon steel. (5)
0.76%C
2.14%C
4.03%C
0.022%C 6.7%C
