Lecture 27 – Choice of materials

Choice of materials

Engineering Materials and Processes

Reference Text Section

Higgins RA & Bolton, 2710. Materials for Engineers and Technicians, 5th ed, Butterworth Heinemann

Ch 27

Reference Text Section

Introduction (Higgins 27)

Engineering Materials and Processes

Bicycle development - frames

Ashby diagram:

Specific strength vs

Specific stiffness

27.2 Selection of materials (Higgins 27.2)

Engineering Materials and Processes

• The ability of the material to withstand service conditions.

• The method(s) by which it will be shaped.

• The overall cost, i.e. the cost of the material(s), with in some cases

the availability of the material, and the cost of the shaping

process(es).

27.3 Service requirements (Higgins 27.3)

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Mechanical properties

Physical properties

Chemical properties

27.3.1 Tensile strength and specific strength

27.3.2 Stiffness, modulus of elasticity and specific modulus

27.3.3 Toughness and impact value

27.3.4 Fatigue resistance

27.3.5 Creep resistance

27.3.6 Refractoriness

27.3.7 Friction and wear resistance

27.3.8 Stability in the environment

27.3.9 Electrical conductivity

27.3.10 Relative costs of important engineering materials

27.4 Choice of shaping process (Higgins 27.4)

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• Malleability

• Ductility

• Strength

• The effects of temperature on the above properties

• Castability

• Machinability

• Can it be heat treatment?

• How can it be joined?

27.4 Choice of shaping process (Higgins 27.4)

Engineering Materials and Processes

27.4.1 Processes

• Number of components required

• Equipment, tooling and labour costs, i.e. the capital costs to set up a

process and then the running costs

• Processing times

• Material costs and availability

• Component form, detail such as holes required, and dimensions

• Dimensional accuracy and surface finish required

27.4 Choice of shaping process (Higgins 27.4)

Engineering Materials and Processes

27.8.2 Changing conditions

Environmental Factors (Additional)

Engineering Materials and Processes

Disposal (bury it or burn it)

Reuse (collect it intact, clean it up and use it again)

Recycle (collect it as scrap and recover the material and use it somehow).

• Aluminium is very energy intensive to produce from ore, but as it is easy to

remelt, it is particularly cost effective to recycle.

• Although thermoplastics can be easily recycled once separated, the bulkiness of

scrap polymer products like drink bottles means that very large volumes have to be

collected, which is rarely economic.

• Even if they can be collected, mixed thermoplastics are difficult to separate and it

is probably more economic to burn the material to produce energy.

• In spite of their higher cost, composites are difficult to recycle because the fibre

and matrix cannot easily be separated, and ceramics cannot effectively be

recycled at all.

Recycling (Additional)

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Ashby Diagram

Recycle Fraction - Cost

Energy cost (Additional)

Engineering Materials and Processes

Ashby Diagram

Energy content - Cost

Selection Summary(Additional)

Engineering Materials and Processes

Overview

Advanced Design and

Technology Third Edition,

Norman, Cubitt, Urry and

Whittaker. Longman 2000

p363

Engineering Materials and Processes

Wikipedia: Welding

Resources.

HI-RESOLUTION ASHBY CHARTS:

http://www-materials.eng.cam.ac.uk/mpsite/interactive_charts/hardcopy/colour/Photoshop300dpi/

DESIGN CASE STUDIES AND TUTORIALhttp://www-materials.eng.cam.ac.uk/mpsite/design.html

Glossary

Engineering Materials and Processes

Specific (density/stress etc)

Mechanical properties

Physical properties

Chemical properties

Service conditions

Duty cycle

Industrial design

Manufacturing energy content

Recycle fraction

Disposal

Re-use

Recycle

Product life-cycle

QUESTIONS: Joining of Materials

Higgins Ch27, Ashby, Norman

1. Define all glossary terms

2. On the specific stiffness - strength chart (Ashby Chart), the bubbles for the metals and alloys

tend to be elongated parallel to the strength axis. By considering the physical origins of

Young's modulus and strength in these materials, explain why this is so. (Norman 2000)

3. Explain why bike frames are made from steel, aluminium alloy, titanium alloy and carbon

fibre composites. Why are carbon fibre composites and titanium generally only found in

performance racing bikes. Discuss the practicality of making a bicycle frame out of a

polymer. (Norman 2000)

4. The dominant material in car bodies is steel, but there is now fierce competition from

aluminium and glass fibre composites. Use the energy content and recycle fraction selection

charts to compare how these materials compete with steel in a life cycle analysis of the car.

(Norman 2000)

5. Briefly the relative advantages and disadvantages of timber, metal and plastic window-

frames. Your answer should refer to one specific named material from each of the three

groups of materials: (i) manufacturing methods employed; (ii) durability and maintenance; (iii)

aesthetic factors. (Norman 2000)

Engineering Materials and Processes

QUESTIONS: Joining of MaterialsHiggins Ch27, Ashby, Norman

6. Research the manufacture of large wind turbine blades. List the required performance

needs, the potential materials and associated production processes. You may need to

research the size, design life, factors influencing wear, storm and other damage, risk factors.

(Norman 2000)

7. Old cars have metal door handles but today they are mostly plastic. Investigate the older

metal door handles and their finish and compare to the modern ones. List the reasons and

outline the pro’s and con’s of such a change. Describe design changes necessary when

switching from a metal to a plastic.

8. Designing and making a one-off product is very different to producing many thousands.

Using, as an example, a project you have made, discuss and sketch the changes which

would be needed to make it viable for mass production. Your answer should compare your

one-off product with a similar mass produced one in terms of: (i) choice of material, (ii)

shaping and forming, (iii) joining and assembly, (iv) applying finishes, and (v) evaluating and

testing. (b) Discuss the disadvantages of mass production of products. (Norman 2000)

9. Explain how a CO2-based penalty/incentive scheme (carbon-tax) would influence material

and process selection for a significant area of industry (e.g. transport, housing,

manufacturing etc). What are the pros and cons of this concept in terms of meeting an

environmental objective? Can you think of a better way to manage environmental resources?

Engineering Materials and Processes