Material Science and Engineering 2 Tutorial Questions and Solutions PDF

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Material Science and Engineering 2 Tutorial Questions and Solutions...

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Materials Science and Engineering 2 School of Engineering, University of Edinburgh Tutorial 1 1. Consider Ashby’s materials selection charts (Ashby 1989; & Materials Selection in Mechanical Design, Ashby, 2005, Elsevier) You can see from the charts that the materials in a particular class cluster together to form groups that have similar properties. (a) What are the main classes of engineering materials? (b) Why do different material classes (e.g. metals) show similar properties? (c) How can materials selection charts be used? Hint: read the Ashby paper 2. Density (a) Define density & state units (b) How can you determine the density of an irregular shaped metallic object? (c) Imagine concrete (concrete = aggregate + cement). Estimate how large a piece of concrete you need to get a reliable measurement of density? Explain your reasoning. 3. “On knowing material property values” You are expected to know approximate values of density. How can you know these without mindlessly memorizing the values for all materials? Write down your approach, and use it to create a list of values for typical materials (e.g. the materials used in the density experiment in the first lecture on properties). 4. Consider Ashby’s materials selection charts What relationships, or patterns, can you see/discover between density and other engineering material properties? 5. Sketch a stress-strain curve for steel, (typically steel is ductile). (a) Use Ashby 1989 to find data, and estimate data you cannot find. Write down the data you use and state whether it is from the paper, or estimated. (b) Sketch the curve and label the features (e.g. axes, properties, regions of elastic and plastic deformation). (c) On your sketch draw sketches to show how the shape of the sample alters at selected points in the test (you choose which values of strain make sense). (d) What is the length of a specimen “just” before it fails; and “just” after it fails? (They are different) – explain why. Hint: see Ashby Engineering Materials 1. 6. Poisson’s ratio (a) Define Poisson’s ratio (in lecture notes & Ashby Engineering Materials 1). (b) Give typical values of Poisson’s ratio for metals and ceramics, and elastomers. (c) Describe Poisson’s ratio by drawing sketches. Label your sketches. (d) Draw sketches to show the behaviour of materials when loaded elastically in tension for Poisson’s ratio of 0, 0.3, 0.5 and 1. (e) Explain how Poisson’s ratio is used in engineering (i.e. why is it useful to know?) Deeper questions These are intended to make you think more deeply and apply your knowledge of materials 7. Ceramics, in contrast to metals and polymers, are not typically tested in tension tests to determine their mechanical properties. Begin to think about why, note your ideas.

See Engineering Materials 1 (this will be covered in section 4 of the course) 8. Consider mechanical and thermal properties. Give applications in chemical, civil and mechanical engineering where one mechanical and one thermal property is important and explain why, (attempt this for all three engineering subjects whatever discipline you’re studying). An example: in chemical engineering strength is important in design of pressure vessels. 9. How can you identify materials? The different classes of materials are generally straightforward to identify (e.g. a coke can, a brick, a wine glass, a squash ball…). But how can you tell what type of metal, ceramic, polymer? This question can be answered on many levels from trivial to sophisticated. Being able to identify different materials (and understanding why they are used) is great skill to develop. [I’ll show you an example in the lectures – where a sophisticated technique showed us not to trust everything]. 10. Rank, and give approximate values for, the thermal conductivity of: snow, ice, polyethylene, wood and brass. - Use thermal conductivity values from Ashby 1989. - The thermal conductivity for ice, measured at –20 deg C, is 2.4 W/mK [Physics of ice, Petrenko & Whitworth 1999]. Using this data explain why a brass ski would slide less well than a polymer or wood ski. What other material properties are important for skis? Experiment Question (can be done before tutorial, or in the tutorial if you bring items with you) 11. Consider the tensile behaviour of for polyethylene (the material that’s used for supermarket plastic bags). Cut strips from a bag and do a tensile test yourself. Things you may want to consider: does it matter what direction you cut the strips from the bag (e.g. vertical, horizontal or at an angle)? If yes – what happens, and can you suggest why? What effect does straining the material at different rates have (i.e. if you pull it fast or slow)? Do you notice any other effects? A similar test can be made with the polymer that connects a 4 pack of beer together; in some ways testing this material is easier because it is already in the correct shape for a simple test.

JRB 21 Jan 2015

Materials)Science)and)Engineering)2) School)of)Engineering,)University)of)Edinburgh) ) Tutorial)2)

1.#Consider#a#steel#paper#clip.## (a)#What#are#the#property#requirements#for#a#paperclip?# (b)#Sketch#and#label#microstructure,#add#a#scale#bar#to#you#sketch#[we#have#not# covered#this#explicitly#in#lectures;#however#we#have#covered#this#subject# sufficiently#to#do#this#question].# (c)#Sketch#and#label#the#unit#cell#of#the#crystal#structure,#add#approximate# dimensions#to#your#sketch.# # 2.#Again,#consider#a#steel#paper#clip.## (a)#Bend#it.#What#happens?#Explain#why#by#considering#the#processes#that#occur# at#an#atomic#level.# (b)#Bend#it#back.#What#happens?#Again#explain#why.# (c)#Bend#it#repeatedly.#What#happens?#(We’ve#not#covered#this#in#the#course#yet,# but#you#may#be#able#to#work#out#why#this#happens.#Think#about#why#this# behaviour#is#important#in#engineering)# # 3.#Sketch#an#edge#dislocation#and#discuss#why#dislocations#play#an#important#role# in#the#deformation#of#metals.# # 4.#(a)#What#mechanical#properties#are#improved#with#finer#grain#sizes#in#alloys?# #(b)#Use#sketches#and#a#few#words#to#explain#how#a#finer#grain#size#can#be# produced#in#metals#by#deformation#followed#by#heat#treatment.## # 5.#Describe#four#strengthening#mechanisms#in#metallic#alloys.#Explain#the# mechanisms#with#the#aid#of#sketches.#Give#specific#examples#of#engineering# alloys#for#each#mechanism.# # 6.#Draw#sketches#to#show#the#atomic#arrangement#of## (a)#a#substitutional#solid#solution,#and## (b)#an#interstitial#solid#solution,# and#give#an#example#of#each.# # 7.#(a)#Sketch#the#microstructures#of#slowVcooled#iron#and#steels#with#0,#0.4,#0.8# and#1.4%#carbon#and#explain#how#the#structure#develops#on#cooling.# (b)#What#is#the#maximum#solubility#of#carbon#in#(i)#ferrite;#(ii)#austenite?# # 8.#(a)#At#what#temperature#and#composition#is#the#eutectic#in#the#FeVC#system?# (b)#Cast#irons#(typically#FeV2V4#wt%#C)#have#higher#carbon#content#than#most# steels.#Why#is#cast#iron#easier#to#cast#than#low#carbon#steel?# # 9.#How#do#the#properties#of#steel#change#if#you#fast#cool#it?#Explain#why.# # 10.#What#properties#do#the#wingskin#materials#for#jet#aircraft#need?#How#do# these#alter#when#the#aircraft#speed#increases#to#supersonic#flying#speeds?# #

Materials Science and Engineering 2 School of Engineering, University of Edinburgh Preparation for tutorial 3 click on these links Challenger shuttle disaster: https://www.youtube.com/watch?v=j4JOjcDFtBE Feynman - in plain English https://www.youtube.com/watch?v=6Rwcbsn19c0 - it’s basic, and a profound example about why thinking about engineering and materials is important Please check out these videos too: http://www.doitpoms.ac.uk/tlplib/glass-transition/demos.php Tutorial 3 1. (a) Sketch binary phase diagrams for two component (i.e. two metals) systems that exhibit (i) complete solid-solubility (ii) partial solid-solubility and (iii) zero solid-solubility. (b) For a system that exhibits partial solid-solubility (e.g. Pb-Sn), select a composition that has two different phase regions in the solid state, make sketches of the microstructure for all different phase regions. Annotate your sketches. 2. There are different classifications of polymers. Sketch the polymer chain structure of (a) thermoplastic polymer (b) thermosetting polymer (c) an elastomer 3. Sketch stress strain curves for (a) thermoplastic polymer (b) thermosetting polymer (c) an elastomer Explain the differences in behaviour. 4. Use an annotated sketch to explain why ‘blu-tac’ can be used to stick paper to walls (i.e. why is ‘blu-tac’ sticky)? 5. How does the stress strain behaviour of: an elastomer change with temperature? Use a sketch to illustrate your answer (i.e. sketch a stress strain curve). Explain the importance of the glass transition temperature for elastomers; and give an example where this is significant in an engineering application.

6. In view of your answers to Q4 and Q5 explain why the concept of glass transition temperature is important for tyres used in cold climates. Is a higher or lower glass transition temperature desirable for use in cold conditions, and explain why? Experiment Question 7. Watch the videos of a polymer balls bouncing at different temperatures, and silly putty deforming at different strain rates (hit with a hammer, dropped and allowed to “rest” on a surface) on Doitpoms: http://www.doitpoms.ac.uk/tlplib/glass-transition/demos.php Explain the behaviour of the balls at the different temperatures, by using annotated sketches. How does the behaviour of the silly putty change with different strain rates? And why? What are the similarities between the behaviour of elastomers (or silly putty) at different strain rates, and the bouncing balls at different temperatures?

Question for discussion 8. Consider the examples of applications of polymers and ceramics given in the lectures (see lecture slides). For the given applications is it possible to substitute polymers or metals for ceramics (or ceramics or metals for polymers)? In cases where you answer “yes” what are the implications (e.g. for design, performance, cost…)?

JRB March 2015

Materials Science and Engineering 2 School of Engineering, University of Edinburgh Tutorial 4 1. State three applications for composites and explain why they are used. 2. Composites are an important class of engineering materials. (i) Sketch a stress strain curve for a continuous fibre composite; on the same sketch add lines to show how it relates to the stress-strain behaviour of the fibres and the matrix. (ii) Draw a sketch to illustrate the propagation of a crack through a continuous fibre composite (e.g. a glass fibre reinforced epoxy). Add labels to explain the mechanisms of crack propagation. 3. What methods are available to protect metals from electrochemical corrosion (describe at least three)? 4. Draw annotated sketches to describe the formation of the oxide layer in: elevated temperature oxidation of a nickel superalloy (highly resistant to oxidation) and steel (which oxidises readily). 5. Explain why aircraft turbine blades need good creep resistance. Use annotated sketch(s) to describe features of turbine blades that improve their creep performance. 6. Give two examples of fatigue failures in engineering. What are the typical features of the fracture surfaces? Consider, and state, the locations in engineering structures and components where fatigue fractures may begin, with this in mind, what steps can be taken in engineering design to decrease the possibility of fatigue failures? 7. Why are ceramics usually much stronger in compression than in tension? (use annotated sketches in your answer). 8. Modulus-of-rupture tests were carried out on samples of silicon carbide using the three-point bend test geometry shown in Fig. 17.2 (in Engineering Materials 2, see below). The samples were 100 mm long and had a 10 mm by 10 mm square cross section. The median value of the modulus of rupture was 400 MPa. Tensile tests were also carried out using samples of identical material and dimensions, but loaded in tension along their lengths. The median value of the tensile strength was only 230 MPa. Account in a qualitative way for the difference between the two measures of strength.

9. Sketch a graph to illustrate the probability of failure in terms of stress for (i) chalk sticks, (ii) alumina samples (alumina is a “technical” or “engineering” ceramic), and (iii) steel samples. Comment on the different distributions of behaviour for the three materials. How is this behaviour captured by the Weibull modulus? How can the Weibull modulus be used in design? 10. Consider an Al alloy karabiner and a nylon climbing rope. What differences would you expect in their mechanical behaviour to deteriorate if they were tested: (i) in 20 years time, having been stored in a dry store cupboard (ii) in 20 years time, having been in the sun (iii) in 20 years time, having been in the sun and a seawater environment - the purpose of this question is to encourage you to think about the effects of time and environment on degradation of the mechanical properties of metallic alloys and polymers EXPERIMENT QUESTION 11. In particular materials stresses can be visualised quite easily. For example if polyethylene sheet is placed between crossed polarising sheets, and pulled in tension different coloured fringes appear when the material is stressed. It is also possible to examine the influence of stress concentrations and their geometry (e.g. sharp crack or circular hole) that are in the material. These materials will be available in the tutorial to experiment.

JRB 3/2015

Materials Science and Engineering 2 School of Engineering, University of Edinburgh Tutorial 1 1. Consider Ashby’s materials selection charts (Ashby 1989; & Materials Selection in Mechanical Design, Ashby, 2005, Elsevier) You can see from the charts that the materials in a particular class cluster together to form groups that have similar properties. (a) What are the main classes of engineering materials? Metals, polymers, ceramics & glasses To make further classifications we can include: Composites Engineering composites are a mixture of two materials e.g. polymer and glass; section 4D of course). Polymers can be further divided into “polymers” (i.e. bulk polymers), and elastomers (section 4B of course) On the Ashby charts “foams” [of any material] often appear as a further classification of materials. (b) Why do different material classes (e.g. metals) show similar properties? essentially because the bonding is similar; read [Ashby 1989], p1292, section 4, paragraph 2. (c) How can materials selection charts be used? Hint: read the Ashby paper This is answered well in [Ashby 1989], p1292, section 4. 2. Density (a) Define density & state units • ratio of mass/volume (usually denoted by the symbol ρ) ρ = m/v units: kgm-3 (b) How can you determine the density of an irregular shaped metallic object? see Hall 2009 p17 (c) Imagine concrete (concrete = aggregate + cement). Estimate how large a piece of concrete you need to get a reliable measurement of density? Explain your reasoning. My estimate – a piece about the size of a tennis ball (and larger is fine too; but not so large it is difficult to handle). My reasoning: estimating the largest aggregate particles to be about 10 mm in diameter it means we will have about 10 of them across the diameter of a tennis ball sized sphere. As a rule of thumb when you want to measure properties of a “composite” material it is good to have at least 10 ”structural features” in a line across a cross-section. For example if the largest aggregate is about 10 mm the minimum size for your test specimen would be about 100 mm diameter. But this is a rule of thumb – and you should think about what it is you are measuring and what it is you want to find out.

3. “On knowing material property values” You are expected to know approximate values of density. How can you know these without mindlessly memorizing the values for all materials? Write down your approach, and use it to create a list of values for typical materials (e.g. the materials used in the density experiment in the first lecture on properties). 1. does the material float/sink in water. 2. float => density < density of water (1000 kgm-3); sink => the density is higher than water 3. Materials density < 1000 kgm-3 tend to be composed of light elements (e.g. polymers); or they contain pores or are foamed (e.g. squash ball; this is also the case for most woods/timbers) 4. Metals and ceramics are denser than water (unless they contain much porosity). 5. In general (though there are exceptions) metals have higher densities than ceramics as they comprise elements with higher average atomic number. 6. Within material classes e.g. metals. This becomes a little trickier, and may involve some memorising, however your experience/intuition appears to be good (and will get better with more experience): most of you when given a chunk (or karabiner) of steel or of aluminium alloy can distinguish between the two materials (steel is about 3 times higher density than Al). The materials used in the density experiment were: nickel superalloy, steel, aluminium alloy, zirconia hip joint, concrete, porcelain ceramic, glass, piece of washing up sponge foam, rubber squash ball (complete and cut in half), section of winter car tyre, polyethylene milk container, plywood, carbon fibre-epoxy composite.

4. Consider Ashby’s materials selection charts What relationships, or patterns, can you see/discover between density and other engineering material properties? This question follows on from Q3; knowing density of different materials, and understanding the bonding/structure of the materials allows you to relate the density to make inspiredestimates/predictions of other properties.

as density increases Young’s modulus tends to increase as density increases strength tends to increase as Young’s modulus [or density] increases linear expansion coefficient tends to decrease It’s also useful to look for exceptions to the trends or patterns too for example • e.g. how do strength and fracture toughness relate to each other? •

Composites (course section 4D) are combinations of two different material classes – and they tend to occupy parameter space (i.e. different regions on the Ashby map) that differs from that of the constituent materials (this gives more possibilities for engineering design).

5. Sketch a stress-strain curve for steel, (typically steel is ductile). see Ashby Engineering Materials 1, pp121-123. (a) Use Ashby 1989 to find data, and estimate data you cannot find. Write down the data you use and state whether it is from the paper, or estimated. Young’s modulus is c. 200 GPa; yield stress is between 200 and 1000 MPa (I’ll choose 200 MPa) There are no plots in Ashby 1989 that give [plastic] elongation to failure – so you have to estimate this. For metals this is typically a few % to c. 50% (I’ll chose 30%). If you have no idea of typical values (though we have covered them in lectures) you could begin to develop your basic knowledge by googling/reading text books. The most brittle metals will exhibit negligible plastic elongation before failure; while superplastic alloys can have values of several hundred % - however these are extremes: few % to c. 50% are more typical. (b) Sketch the curve and label the features (e.g. axes, properties, regions of elastic and plastic deformation). (c) On your sketch draw sketches to show how the shape of the sample alters at selected points in the test (you choose which values of strain make sense). (d) What is the length of a specimen “just” before it fails; and “just” after it fails? (They are different) – explain why.

6. Poisson’s ratio (a) Define Poisson’s ratio (in lecture notes & Ashby Engineering Materials 1). (b) Give typical values of Poisson’s ratio for metals and ceramics, and elastomers. (c) Describe Poisson’s ratio by drawing sketches. Label your sketches. (d) Draw sketches to show the behaviour of materials when loaded elastically in tension for Poisson’s ratio of 0, 0.3, 0.5 and 1. (e) Explain how Poisson’s ratio is used in engineering (i.e. why is it useful to know?)

Deeper questions These are intended to ...