ECMM160 CONTINUE 1
UNIVERSITY OF EXETER
COLLEGE OF ENGINEERING, MATHEMATICS
AND PHYSICAL SCIENCES
Engineering Materials and the Environment
Module Convenor: Prof Oana Ghita
Duration: TWO HOURS + 30min UPLOAD TIME
Answer ALL questions in Section A (50%) and any TWO of the three questions
in Section B (50%).
This is an OPEN book examination
ECMM160 CONTINUE 2
Answer ALL of the questions in this section.
Question 1 (12 marks)
Figure Q1 is a diagram of a fully open cellular structure comprising rigid cylinders and
(a) Sketch a diagram of this structure in its partially closed form, indicating with arrows
the loading direction.
(b) Describe in words, what is happening to both the beams and the cylinders during
this deformation process.
(c) From what you can see in the diagram, what is the likely Poisson’s ratio of the
structure in Fig. Q1 and why?
(d) If you wanted this type of structure to have a negative Poisson’s ratio both in
tension as well as in compression what would you have to do to it?
(e) Give an example of a possible use for the structure represented by Fig. Q1 and
from what material it might be made in this application.
ECMM160 CONTINUE 3
Question 2 (38 marks)
In an attempt to find a solution to in-house waste produced during manufacturing
PA12/Cf composites (20% Cf by volume), company A decides to recycle the scrap
material by milling the composite to powder for use in the Powder Bed Fusion (known
as Laser Sintering) process.
(a) Propose four tests you would carry out to identify whether the powder has the
right flow characteristics for the process. Describe the tests and explain the
reason for selection.
(b) The company A claims that the milled PA12/Cf powder obtained from scrap
PA12/Cf is significantly better for the powder bed fusion process than the dry
blend of PA12 and Cf, commercially available. To support their statement the
company provided Scanning Electron Microscopy (SEM) images of the powder
and of sintered films of the two types of powders (see Figure Q2a – milled
PA12/Cf and Figure Q2b – dry blend of PA12 and Cf). Give your view on
whether the statement is correct based on the evidence provided (Figure Q2a
and Q2b), explaining why.
Figure Q2a Milled PA12/Cf (i) powder (ii) laser sintered
Figure Q2b Dry Blend (i) powder (ii) laser sintered
(c) Give two examples of applications which would benefit from using one of the
two materials in additive manufacturing.
ECMM160 CONTINUE 4
(d) Company A identified another in house source of scrap of PA12/Cf but of a
different volume fraction (40% Cf). Describe what differences you expect to
see in the milled powder by comparison with the first material milled which has
a lower volume fraction of Cf.
Answer any TWO out of the three questions in this section.
Question 3 (25 marks)
Figure Q3 is a high speed camera image of an auxetic blast curtain undergoing a blast
test. The curtain is being viewed from the side. The vertical black area in the middle
of the picture is the side view of the wall that the window is embedded in. The blast
occurred to the right of the picture. The curtain is to be seen extended to the left. Glass
fragments are visible to the right of the wall.
(a) Describe the sequence of events that took place from the initiation of the
explosion to the image in Figure Q3.
(b) Describe the sequence of events that took place from the time of the image in
Fig. Q3 until the process completed with the curtain and glass fragments
coming to rest.
ECMM160 CONTINUE 5
(c) The auxetic blast curtain is woven from a Helical Auxetic Yarn. Describe, with
a diagram, what is a Helical Auxetic Yarn.
(d) Explain, with diagrams, why a Helical Auxetic Yarn might provide better
performance in a blast curtain than a conventional yarn.
(e) Explain, preferably with a diagram, why it is important that adjacent strands of
auxetic yarn should line-up (register) in a particular way in any auxetic fabric.
(f) Explain why it is particularly important that adjacent strands of auxetic yarn
should line-up (register) in the specific case of a blast curtain.
(g) Give an example of a material that might be used to make each separate
component of a Helical Auxetic Yarn for a blast curtain.
(h) Describe a different example of the application of an auxetic fabric.
Question 4 (25 marks)
You are provided with a new material for trial and testing in the powder bed fusion
process. The thermal profile (recorded using Differential Scanning Calorimetry- DSC)
of the new powder material is shown in Figure Q4.
ECMM160 CONTINUE 6
(a) Describe Figure Q4. (comment on the thermal transitions)
(b) How do you envisage the powder to behave in a process based on data
provided in Figure Q4? (comment on the sintering window, shrinkage, warping,
Question 5 (25 marks)
Figure Q5 presents the relationship between density and laser power of Hastelloy X
at different layer thicknesses, at a scan speed of 100 mm/s. Hastelloy X powder is a
nickel based super-alloy with 17%~20% high iron content, with the particle size
ranging from 15 to 53 m and average particle size of 42.4 m.
(a) Discuss the results plotted in Figure Q5 and the trends you notice.
(b) Knowing that the samples were sintered at a scan speed of 100 mm/s, scan
spacing of 180 m, for a layer thickness of 50 m calculate the volumetric
energy density applied at laser powers of 200 W, 300 W and 600 W. Explain
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