Material Selection 1-5

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Faculty of Engineering

Design & Prod. Eng. Dept.

Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Material

Selection

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Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Ain Shams University

Faculty of Engineering

Design & Prod. Eng. Dept.

Lecture 1 [Introduction to Material Selection in Mechanical Design]

Lecture 1: Introduction to Material Selection

in Mechanical Design

The Design Process

COMPETITIVE DESIGN of new products is the key capability that companies must master to remain in business. It requires more than good engineering, it is fraught with risks and opportunities, and it requires effective judgment about technology, the market, and time.

The concept and configuration development process:

Activities occur throughout product development

The process starts with identifying the customer population for the product and developing a representation of the feature demands of this group. Based on this representation, a functional architecture is established for the new product, defining what it must do. The next step is to identify competitive products and analyze how they perform as they do. This competitive benchmarking is then used to create a customer-driven specification for the product, through a process known as quality function deployment. From this specification, different technologies and components can be systematically explored and selected through functional models. With a preliminary concept selected, the functional model can be refined into a physically based parametric model that can be optimized to establish geometric and physical targets. This model may then be detailed and established as the alpha prototype of a new product.

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Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Ain Shams University

Faculty of Engineering

Design & Prod. Eng. Dept.

Customer Needs & Problem Definition:

In the early 1980s, Sony offered an improved magnetic videotape recording technology, the Betamax system. Although it offered better magnetic media performance, it did not satisfy customers, who rather were more concerned with low cost, large selection of entertainment, and standardization.

In 1996, both Ford and Toyota launched new family sedans. Three years earlier, each had torn apart and thoroughly analyzed each other's cars. Ford decided to increase the options in its Taurus, matching Toyota's earlier Camry, while Toyota decided to decrease the options in its Camry, matching Ford's earlier Taurus.

Note how the design depends on the viewpoint of the individual who defines the problem

As Proposed by Project Sponsor As Specified in the Project Request As designed by the senior designer

As producer by manufacturing As installed at the user’s site What the user wanted

Task Clarification

Conceptual and configuration design of products begins and ends with customers, emphasizing quality processes and artifacts throughout. We thus initiate the conceptual design process with task clarification: understanding the design task and mission, questioning the design efforts and organization, and investigating the business and technological market. Task clarification sets the foundation for solving a design task, where the foundation is continually revisited to find weak points and to seek structural integrity of a design team approach. It occurs not only at the beginning of the process, but throughout.

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Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Ain Shams University

Faculty of Engineering

Design & Prod. Eng. Dept.

Mission Statement and Technical Questioning

A mission statement and technical clarification of the task are important first steps in the conceptual design process. They are intended to:

• Focus design efforts • Define goals

• Define timelines for task completion

• Provide guidelines for the design process, to prevent conflicts within the design team and concurrent engineering organization

The first step in task clarification is usually to gather additional information. The following questions need to be answered, not once but continually through the life cycle of the design process:

• What is the problem really about?

• What implicit expectations and desires are involved?

• Are the stated customer needs, functional requirements, and constraints truly appropriate?

• What avenues are open for creative design?

• What avenues are limited or not open for creative design? Are there limitations on scope? • What characteristics/properties must the product have?

• What characteristics/properties must the product not have? • What aspects of the design task can and should be quantified?

• Do any biases exist in the chosen task statement or terminology? Has the design task been posed at the appropriate level of abstraction?

• What are the technical and technological conflicts inherent in the design task?

For further information about the design process, review

ASM Handbook, Volume 20, Materials Selection and Design

Relation of Materials Selection to Design:

• An incorrectly chosen material can lead not only to failure of the part but also to unnecessary cost.

• Selecting the best material for a part involves more than selecting a material that has the properties to provide the necessary performance in service; it is also intimately connected with the processing of the material into the finished part.

• A poorly chosen material can add to manufacturing cost and unnecessarily increase the cost of the part.

• Also, the properties of the material can be changed by processing (beneficially or detrimentally), and that may affect the service performance of the part.

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Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Ain Shams University

Faculty of Engineering

Design & Prod. Eng. Dept.

The Place of Materials Selection in the Design Process

Materials selection should contribute to every part of the whole design process. This is because it is hardly possible to proceed very far with a genuinely innovative design without taking account of all the materials and manufacturing methods that are available for use. Any technical system consists of assemblies and components, put together in a way that performs a function. It can be described and analyzed in more than one way based on the ideas of systems analysis-thinks of the flows of information, energy and materials into and out of the system. The system transforms inputs into outputs.

Analysis of a technical system

Component 1.1 Assembly [1] Component 1.2 Component 4

The figure illustrates the analysis of a technical system as a breakdown of the system into assemblies and components.

Each component is made of a material

,

“different

components of different materials”.

Material selection is at the component level. Some components are standard,

common to many designs: a wood screw, for instance; but even among

standards there is a choice of material (the screw could be of brass, or mild

steel, or stainless steel). Some are specific, unique to the design: then the

designer must select the material, the shape, and the processing route.

Component Assembly [2]

Technical

System

_Component Component Component Assembly [3] Component Component

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Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Ain Shams University

Faculty of Engineering

Design & Prod. Eng. Dept.

The Design Flowchart

Design is an iterative process. The starting point is a market need or an idea; the end point is a product that fills the need or embodies the idea. A set of stages lie between these points: the stages of conceptual design, embodiment design and detailed design, leading to a set of specifications the production information, which define how the product should be made.

Design flow chart

The design flow chart shows how design tools and materials selection enter the procedure.

Information about materials is needed at each stage, but at very different levels of breadth and precision.

At the conceptual design stage all options are open: the designer considers the alternative

working principles or schemes for the functions which make up the system, the ways in which sub functions are separated or combined, and the implications of each scheme for performance and cost.

Embodiment design takes a function structure and seeks to analyze its operation at an

approximate level, sizing the components. And selecting materials, which will perform properly in the ranges of stress, temperature and environment suggested by the analysis. The embodiment stage ends with a feasible layout that is passed to the detailed design stage.

At the detailed design stage, specifications for each component are drawn up. Critical

components may be subjected to precise mechanical or thermal analysis using finite element methods. Optimization methods are applied to components and groups of components to maximize performance; materials are chosen the production route is analyzed and the design is costed. The stage ends with detailed production specifications.

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Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Ain Shams University

Faculty of Engineering

Design & Prod. Eng. Dept.

Function, Material, Shape and Process Interactions

Function, material, shape and process interact:

• Function dictates the choice of material.

• The shape is chosen to perform the function using the material.

• Process is influenced by material properties: by formability, machinability,

weldability, heat-treatability and so on.

• Process obviously interacts with shape. The process determines the shape,

the size, the precision and of course the cost.

• The interactions are two-way.

• Specification of shape restricts the choice of material, so does specification

of process.

• The more sophisticated the design, the tighter the specifications and the

greater the interactions.

The figure shows the central problem of material selection in mechanical design, which is the interaction between function, material, process and shape.

MATERIAL PROCESS SHAPE FUNCTION Transmit loads, heat, contain pressure, store energy, etc.

Interaction of function, material, process and shape

The interaction between function, material, shape and process lie

at the heart of the Design process.

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Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Ain Shams University

Faculty of Engineering

Design & Prod. Eng. Dept.

Motivations for Material Selection

Forces for Change: [1] Market Competition & Cost Reduction

The creation of a completely new product should commence with a clearly defined objective, derived from market research in the case of a component for sale, and associated cost accountancy and with a time scale which should allow an optimum choice to be made. For such a venture to be successful a program for market entry in relation to the cost of development and getting into production has to be fulfilled.

However, markets will change, new competitors will arise and to some extent known competitors may change their approach also. A new venture in an engineering product will always be something of a gamble.

However, for the maximum chance of success, the choice of materials will be a key decision in terms of 'value for money' in service and the impact on the market. Also, since the choice may well control the method of fabrication, it will influence the whole production line specification involving a very large capital investment, which cannot always accommodate a subsequent change of material.

The design process must continually operate even in an established manufacturing operation. The figure below illustrates the product lifetime.

Here we see that each product offered in the market place has a life-cycle. Research and development (R&D) enables its introduction to be effected, prior to the period of growth during which the product finds acceptance.

After a while, it becomes mature, either through built-in obsolescence or as a result of new developments; by this time the far-seeing company will have replacement products already in the R&D stage.

Inevitably (and this may occupies a period of months or of decades), the product will go into market decline. Decisions must be made as to whether any of the design features can be retained to produce a new revitalized product, or whether the operation has to be closed down to make way for an entirely new family of products.

Technical decision

Concept Market screening Design feasibility

pro-production Production Modification to broaden product family Cost reduction Phase-out Obsolescence Cut-off point Return on investment

Introduction Growth Maturity Decline

Types of corporate decision Capital investment Recruitment of new employees Change of price Expansion of production New market strategies Changes in product design Extend market to overseas Reduce the product price Time Profit Research & development Sales volume

The life cycle of a product

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Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Ain Shams University

Faculty of Engineering

Design & Prod. Eng. Dept.

Forces for Change: [2] The Design Status of the Product

The terms dynamic and static are used to describe the type of change in the product design. Dynamic

product

is a product where design changes are innovative, the concept is likely to change, and Static Product is a product where design changes are incremental or non-existent, the concept is unlikely to change.

Factors that create or retain a

STATIC plateau

Improving environment for the existing design

Factors that cause a product to become DYNAMIC

Commodities and resources Government action or legislation Customers not willing to

change Changing environment User familiarity Commodities and resources Stable technology

Customers willing to change Conformance standards

Technical advancement Stable or decreasing number of producers

No conformance standards Few large producers Many small producers

(increasing)

Product available for a long time

No infrastructure Existing infrastructure

Balance diagram of the macro factors that change / maintain a product status.

Factors that create or retain a

STATIC plateau

Insufficient design resources Poor market research

Restricted design

Product interfaces with existing design

Rationalization or commonality

of parts

Assembling component made

by others

Using experience in design Factors that cause a product to

become DYNAMIC

More process design than product design

Management committed to deign

Management not committed to deign

Changing PDS Stable effective PDS

Process design small Restricted PDS Adequate time for design Limited Design time Wide effective market research Limitation

Companies seeking new concepts

Automation CAD

Flexible machinery subcontract, manufacture

Purchasing new machinery

(dedicated)

Balance diagram of the micro factors that change / maintain a product status. 8

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Ain Shams University

Faculty of Engineering

Design & Prod. Eng. Dept.

Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Lecture 1 [Introduction to Material Selection in Mechanical Design] 9

Forces for Change: [3] The Science-Push: Curiosity-driven Research

Curiosity is the life-blood of innovative engineering. Technically advanced countries sustain the flow of new ideas by supporting research in three kinds of organization: universities, government laboratories and industrial research laboratories.

Some of the scientists and engineers working in these institutions are encouraged to pursue ideas, which may have no immediate economic objective, but which can evolve into the materials and manufacturing methods of the next decade. Numerous now-commercial materials started in this way.

Forces for Change: [4] Energy and Environment: Green Design

There is a growing interest in reducing and reversing the environmental damage. This requires processes, which are less toxic and products, which are easier to recycle, lighter, and less energy-intensive; and this must be achieved without compromising product quality. New technologies must be developed which can allow productivity without cost to the environment.

Concern about environmental friendliness must be injected into the design process, taking a life-cycle view of the product, which includes manufacture, distribution, use and final disposal.

All materials contain energy. Energy is used to mine, refine, and shape metals; it is consumed in the firing of ceramics and cement; and it is intrinsic to oil-based polymers and Elastomers.

When we use a material, we are using energy, and energy carries with it an environmental penalty: CO2, oxides of nitrogen, sulphur compound, dust, and waste heat. The energy content is only one of the ways in which the production of materials pollutes, but it is the one, which is easier to quantify than most others are.

Forces for Change: [5] The Pressure to Recycle and Reuse:

Discarded materials damage the environment; they are a form of pollution. Materials removed from the manufacturing cycle must be replaced by drawing on a natural resource. And materials contain energy, lost when they are dumped.

Recycling is obviously desirable. But in a market economy it will happen only if there is profit to be made. To allow this we have to look first, at where recycling works well and where it does not.

Primary scrap-the turnings, trimmings and tailings, which are a by-product of manufacture-has high value: it is virtually all recycled. That is because it is uncontaminated and because it is not dispersed.

Secondary scrap has been through a consumption cycle-a newspaper, a beer can, or an automobile; the other materials to which it is joined; by corrosion products; by ink and paint contaminate it; and it is dispersed. It is worth little or nothing or less than nothing meaning that the cost of collection is greater than the value of scraps itself.

Newsprint and bottles are common examples: in a free market it is not economic to recycle either of these. Recycling does take place, but it relies on social conscience and good will, encouraged by publicity. It is precarious for just those reasons.

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Ain Shams University

Faculty of Engineering

Design & Prod. Eng. Dept.

Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Main Situations for Material Selection:

The decision-making process of materials selection may be initiated for a variety of reasons and several situations.

The three main situations are:

1 The introduction of a new product, component or plant, which is being produced or built for the first time by the organization concerned.

2 A desire for the improvement of an existing product, or a recognition of over design where economy can be effected, which may be considered as an evolutionary change. 3 A problem situation, due for example to the failure of components leading to rejection

by customers, failure of supplies, or failure of in-house manufacturing plant, necessitating a change in material use.

This is the area where the metallurgist must be employed, for investigating a failure, and on determination of the cause, suggesting a change of design or of the material

employed.

Materials Selection Objectives:

The

selected material should be: 1 Readily available.

2 Can be formed into the desired shape with the required dimensional tolerances. 3 After getting the shape, will perform the designed functions of the product.

4 Will continue performing the functions satisfactorily for the required lifetime of the product. 5 Can be disposed of, or recycled, in the way, which is environmentally acceptable.

Note that:

• The selected material should achieve these objectives at a cost, which permit the product to be offered at a price that attracts customers and gives a profitable return to the manufacturer.

• Among the material selection many objectives, there is a main objective, which is failure

prevention.

Material Failure Modes

The different material failure modes are listed in following table as classified by Collinos, Each failure mode has:

• a failure mechanism • material selection

guide lines

• material selection rules

to prevent the failure mode from taking place. 1. Elastic deformation 2. Yielding 3. Brinelling 4. Ductile failure 5. Brittle fracture 6. Fatigue a. High-cycle fatigue h. Low-cycle fatigue c. Thermal fatigue d. Surface fatigue e. Impact fatigue f. Corrosion fatigue g. Fretting fatigue 9. Impact a. Impact fracture b. Impact deformation c. Impact wear d. Impact fretting e. Impact fatigue 8. Corrosion

a. Direct chemical attack b. Galvanic corrosion c. Crevice corrosion d. Pitting corrosion e. Intergranular corrosion f. Selective leaching g. Erosion-corrosion h. Cavitation i. Hydrogen damage j. Biological corrosion k. Stress corrosion 9. Wear a. Adhesive wear b. Abrasive wear c. Corrosive wear d. Surface fatigue wear e. Deformation wear f. Impact wear g. Fretting wear 10. Fretting a. Fretting fatigue b. Fretting wear c. Fretting corrosion 11. Galling and seizure 12. Scoring

13. Creep

14. Stress rupture 15. Thermal shock 16. Thermal relaxation 17. Combined creep and fatigue 18. Buckling 19. Creep buckling 20. Oxidation 21. Radiation damage 22. Bonding failure 23. Delamination 24. Erosion

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Ain Shams University

Faculty of Engineering

Design & Prod. Eng. Dept.

Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Lecture 1 [Introduction to Material Selection in Mechanical Design] 11 Investigations about the frequency of failure causes in some engineering industries indicate that the main cause for failure is improper material selection.

Origin % Improper material selection 38

Fabrication defects 15

Faulty heat treatments 15

Mechanical design fault 11

Unforeseen operating conditions 8 Inadequate environment control 6 Improper or lack of inspection and quality control 5

Frequency of Causes of Failure

in Some Engineering Industries Investigations: Material mix-up 2 Origin % Corrosion 29 Fatigue 25 Brittle fracture 16 Overload 11 High temperature corrosion 7

Stress corrosion / corrosion fatigue / hydrogen embrittlement

6

Creep 3

Frequency of Failure Modes in

Some Engineering Industries Investigations.

Wear, abrasion, and erosion 3

F

ailure experience matrix

Collins suggested a failure experience matrix,

which is an attempt to place failure analysis on a firm analytical basis by classifying each failure with respect to failure mode, the elemental function that the component provided, and the corrective action that should be taken recurrence of the failure. Thus the failure experience matrix is a three dimensional assemblage of information cells. Corrective action is defined as any measure or steps taken to return failed component or system to satisfactory performance.

Three dimensional experience matrix assemblage of information cells

• Elemental Mechanical Function • Failure Mode

• Corrective Action

Dieter stated that if there ware a computerized database that encompassed a national

inventory of failures, it would have a great use in engineering design. An engineer who needed to design a critical component would enter the matrix with elemental mechanical function and learn about failure modes that likely to occur as well as the corrective actions most likely to avoid failure.

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Ain Shams University

Faculty of Engineering

Design & Prod. Eng. Dept.

Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Lecture 1 [Introduction to Material Selection in Mechanical Design] 12 Some of elemental mechanical functions and the corrective actions of failure experience matrix in a study on 500 failed parts from U.S. army helicopters

Elemental mechanical functions:

1. Supporting 36. Permanent fastening 71. Force sensing 2. Attaching 37. Pressure increasing 72. Spacing

3. Motion constraining 38. Streamlining 73. Temporary supporting 4. Force transmitting 39. Motion reducing 74. Gas switching

5. Sealing 40. Filtering 75. Electrical transforming 6. Friction reducing 41. Lighting 76. Power absorbing 7. Protective covering 42. Pumping 77. Information attaching 8. Liquid constraining 43. Gas transferring 78. Sound absorbing 9. Pivoting 44. Aero. force transmitting 79. Constraining 10. Torque transmitting 45. Motion transmitting 80. Flexible coupling 11. Pressure supporting 46. Signal transmitting 81. Removable coupling 12. Oscillatory sliding 47. Motion damping 82. Damping

13. Shielding 48. Force distributing 83. Electrical distributing 14. Sliding 49. Reinforcing 84. Load distributing 15. Energy transforming 50. Pressure sensing 85. Gas guiding 16. Removable fastening 51. Information transmitting 86. Pressure indicating 17. Limiting 52. Coupling 87. Electrical insulating 18. Electrical conduction 53. Displacement indicating 88. Sound insulating 19. Contaminant constraining 54. Clutching 89. Temporary latching 20. Linking 55. Fastening 90. Force limiting 21. Continuous rolling 56. Information indicating 91. Force maintaining

22. Liquid transferring 57. Position indicating 92. Variable position maintenance 23. Force amplifying 58. Movable lighting 93. Liquid pumping

24. Power transmitting 59. Partitioning 94. Electrical reducing 25. Covering 60. Position restoring 95. Rolling

26. Oscillatory rolling 61. Flexible spacing 96. Position sensing 27. Energy absorbing 62. Electrical amplifying 97. Energy storing 28. Light transmitting 63. Adjustable attaching 98. Liquid storing 29. Viewing 64. Shape constraining 99. Flexible supporting 30. Energy dissipating 65. Deflecting 100. Switching

31. Guiding 66. Disconnecting 101.Pressure to torque transmitting 32. Latching 67. Electrical limiting 102. Electrical transmitting

33 electrical switching 68. Motion limiting 103. Flexible motion transmitting 34. Stabilizing 69. Pressure limiting 104. Flexible torque transmitting 35. Gas constraining 70. Sensing 105. Torque limiting

Corrective actions for failure-experience matrix:

Direct replacement Changed vendor Improved instructions to user Change Of material Changed dimensions Design change to improve part Supplement part Improved quality control Changed mechanism of operation Added adhesive Changed lubricant type Improved run-in procedure

Provided drain Improved lubrication Changed manufacturing procedure Added sealant Applied surface coating Changed mode of attachment Repositioned part Applied surface treatment Changed method of lubrication Repaired part More easily replaceable part Added or changed locking feature Reinforced part Changed to correct part Revised procurement specification Eliminated part Made part interchangeable Provided for proper inspection Strengthened part Changed loading on part Changed electrical characteristics Adjusted part Relaxed replacement criteria

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Ain Shams University

Faculty of Engineering

Design & Prod. Eng. Dept.

Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Review question:

• What is the meaning of Task Clarification & Mission Statement?

• Explain how the Information about materials is needed at each design stage.

• Discuss the different forces for change, which motivate the material selection process. • Discuss the interaction between Function, Material, Shape and Process.

• Explain the Main Situations for Material Selection. • What are the main Materials Selection Objectives? • What is the meaning of Failure-experience matrix?

Text Book:

M. F. Ashby, (1992), Materials Selection in Mechanical Design, Pergamon Press. References:

J.A. Charles, FAA Crane, (1989), Selection and Use of Engineering Materials, Butterworths

Heinemann.

E.H. Cornish, (1987) Materials and The Designer, Cambridge University Press Bill Hollins, and Stuart Pugh, (1990), Successful Product Design, Butterworths. J. A. Collins, (1981) Failure of Materials in Mechanical Design, Wiley-Inter-science.

George Dieter, (1983) Engineering Design, A Materials and Processing Approach,

McGraw-Hill.

ASMMetals Handbook, (1999), Volume 20, Materials Selection and Design, American

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Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Faculty of Engineering

Design & Prod. Eng. Dept.

Lecture 2 [Engineering Materials & Their Properties]

Lecture 2:

Engineering Materials & Their Properties

Classes of Engineering Materials: Metals

• They have relatively high elastic moduli. • They can be made strong by alloying,

mechanical working, and heat treatment. • They show good ductility. This allows

them to be formed by deformation processes.

• They typically yield before fracturing. • They are prone to fatigue failure.

• Relative to other material classes they are not very resistant to corrosion.

Ceramics and Glasses:

• They have too high elastic moduli, but unlike metals they are brittle. Because ceramics have no ductility, they have a low tolerance to stress concentrations or for high contact stresses.

• Their strength in compression is about 15 times larger than their strength in tension. Brittle materials always show a wide scatter in strength.

• They are stiff hard and abrasion resistant, hence their use in bearing and cutting tools.

• They retain their strength to high temperatures.

• They are resistant to corrosion.

Polymers & Elastomers:

• They have low elastic moduli, about 50 times less than those of metals. However, some polymers can be very strong – nearly as strong as metals. As a

consequence, the elastic deflections can be large.

• Polymers creep even at room temperature. Very few polymers having useful strength above 250C.

• When specific properties, e.g. strength per unit mss, are important, then some

polymers are as good as metals. • They are easy to shape.

• Polymers are corrosion resistant. • They have a low coefficient of friction.

Composites:

• They combine attractive properties of other classes of materials while avoiding some of their drawbacks.

• They are light, stiff and strong, and they can also be tough.

• Most currently available composites have polymer matrices – epoxy or polyester, usually enforced by fibers of glass, graphite, or Kevlar. They cannot be used above 250C because of the polymer matrices.

• Composite components are expensive, and manufacturing processes are not well developed. They are also difficult to join.

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Faculty of Engineering

Design & Prod. Eng. Dept.

Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Lecture 2 [Engineering Materials & Their Properties] 2

Material classes, generic members, and abbreviated names:

Class Members Short name

Engineering alloys

(The metals and alloys of engineering)

Aluminium alloys Copper alloys Lead alloys Magnesium alloys Nickel alloys Steels Tin alloys Titanium alloys Zinc alloys Al alloys Cu alloys Lead alloys Mg alloys Ni alloys Steels Tin alloys Ti alloys Zn alloys Engineering polymers

(The thermoplastics and thermosets of engineering) Epoxies Melamines Polycarbonate Polyesters

Polyethylene, high density Polyethylene, low density Poly formaldehyde Poly methyl metha crylate Polypropylene

Poly tetra fluor ethylene Polyvinyl chloride EP MEL PC PEST HDPE LDPE PF PMMA PP. PTFE PNC Engineering ceramics

(Fine ceramics capable of load bearing application) Alumina Diamond Sialons Silicon Carbide Silicon nitride Zirconia Al2O3 C Sialons SiC Si3N4 ZrO2 Engineering composites

(The composites of engineering practice)

A distinction is drawn between the properties of a ply – UNIPLY – and of a laminate – LAMINATES

Carbon fiber reinforced polymer Glass fiber reinforced polymer Kevlar fiber reinforced polymer

CFRP GFRP KFRP

Porous ceramics

(Traditional ceramics, cement, rocks, & minerals) Brick Cement Common rocks Concrete Porcelain Pottery Brick Cement Rocks Concrete Pcln Pot Glasses

(Ordinary silicate glass) Borosilicate glass Soda glass Silica

B-glass Na-glass SiO2

Woods

Separate envelopes describe properties parallel to the grain and normal to it, and wood products)

Ash Balsa Fir Oak Pine Wood products Ash Balsa Fir Oak Pine Wood products Elastomers

(Natural and artificial rubbers) Natural rubber Hard butyl rubber Polyurethane Silicone rubber Soft butyl rubber

Rubber Hard butyl PU Silicone Soft butyl Polymer foams

(Foamed polymers of engineering) Cork Polyester Polystyrene Polyurethane Cork PEST PS PU Note that abbreviated names as used in material selection charts developed by M.F. Ashby.

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Faculty of Engineering

Design & Prod. Eng. Dept.

Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

[2] Material Properties:

• Each material has a set of attributes (properties).

• The designer seeks a specific combination of these attributes (a property profile). • The material name is the identifier for a particular property profile.

• The properties themselves are standard, density, strength, toughness, etc. Design Limiting Material Properties

Class

Property Symbol

Units

General Relative

Cost

_C

_R

_---

Density

_ρ

_Mg/m

3

Mechanical Elastic

Modulus

_{E, G, K}

_GPa

Strength (yield / ultimate / fracture)

_σ

f

MPa

Toughness

_G

_c

_KJ/m

2

Fracture

Toughness

_K

_IC

_MPa

_m

1/2

Damping

Capacity

_η

_---

Fatigue

Ratio

_{f ---}

Thermal Thermal

Conductivity

_λ

_{W/m K}

Thermal

Diffusivity

_a

_m

2

_/s

Specific

Heat

_C

_P

_{J/Kg K}

Melting

Point

_T

_m

_K

Glass

Temperature

_T

_g

_K

Thermal Expansion Coefficient

_α

_K

-1

Thermal Shock resistance

_ΔT

_K

Creep

Resistance

_---

Wear

Archard Wear Constant

_K

_A

_MPa

-1

Corrosion /

Corrosion Rate

_---

Oxidation Parabolic

rate

constant

_K

_P

_m

2

_/s

Elastic Modulus Shear Modulus Bulk Modulus

E= 3G/(1+G/3K) G= E/2(1+ν) K= E/3(1-2ν) ν =1/3

σ

f

= K

IC

/√ (πC) K

IC

the resistance to the propagation f a crack.

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Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Faculty of Engineering

Design & Prod. Eng. Dept.

Material Mg/m3

Density,

ρ

Mass per unit volume, Mg/m

3

Iron, Steels Titanium alloys Aluminium alloys Magnesium alloys Polycarbonate 7.8 4.5 2.7 1.7 1.2 Material E, GPa ν

Stiffness, Elastic MODULUS, E

Slope of the liner elastic part of the

stress-strain curve, GN/m

2

= GPa

Poisson’s ratio,

ν

ν = ε

lateral

/

ε

axial Iron, Steels Titanium alloys Aluminium alloys Magnesium alloys Polycarbonate Rubbers Silicon SiC 200 116 70 43 2.6 0.01-0.1 160 410 0.27 0.34 0.33 0.35 0.4 0.49 0.22 0.3 For isotropic materials:

E Young’s Modulus

ν

Poisson’s ratio G= E/2(1+ν) Shear Modulus

K= E/3(1-2ν) Bulk Modulus Typically

ν ≈ 1/3, G ≈ 3/8 E K ≈ E

Elastomers are exceptional:

ν ≈ 1/2, G ≈ 1/3 E K>>E

Strength,

σ

f, MN/m2 = MPa.

Strength requires careful definition and usually defined differently for different materials and mode of loading.

Material

_σ

_y,_MPa

Metals

σ

f is identified with the 0.2% offset yield strength

σ

y.

It is the stress level the application of which has caused dislocations to move large distances through the crystals of the metal, so that upon unloading from this stress level there is a measurable permanent plastic strain of 0.2%.

σ

y in compression ≈ σy in tension Steels Titanium alloys Aluminium alloys Magnesium alloys 200-2000 800-1200 200-500 100-200 4

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Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Faculty of Engineering

Design & Prod. Eng. Dept.

Ceramics & Glasses

Strength for ceramics and glasses depends strongly on the mode of loading. In tension, strength means the fracture strength,

σ

_ft. In compression it means the crushing strength

σ

_fC, which is much larger, typically

σ

fC

in compression ≈ 15 σ

ft

in tension

Modulus of Rupture, MOR – MPa

If the material is difficult to grip, as is the case with ceramics, its strength can be measured in bending.

The Modulus or Rupture, MOR, is the maximum surface stress in a bent beam at the instant of failure.

In ceramics MOR ≈ 1.3 σ

_ft

in tension

Polymers:

σ

f is identified as the stress

σ

y at which

the stress strain curve has become markedly non-linear- typically a strain of 1%. Yield mechanisms: shear yielding, crazing.

Material

_σ

_y,_MPa Polymers are a little stronger ≈ 20% in compression than

in tension.

σ

y in compression ≈ 1.2 σy in tension Polycarbonate PMMA 100 80

Composites:

The strength of a composite is typically defined by a set deviation e.g. 0.5% from linear elastic behaviour.

The strength of long fibre composites is approximately 30% lower in compression than in tension, because in compression the fibres buckle.

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Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Faculty of Engineering

Design & Prod. Eng. Dept.

Ultimate tensile strength,

σ

_u- MPa

This defined as the maximum engineering stress that can be achieved in an un-notched round bar of the material loaded in tension. For brittle solids – ceramics, glasses and brittle polymers it is the same as

σ

_f in tension. For metals, ductile polymers and most composites it is larger than

σ

_f, by factor of between 1.1and 3. In metals

σu

is higher than

σy

because of work hardening.

Hardness, H – MPa:

The hardness of material is a crude measure of its strength. It is measured by pressing a point diamond or hardened steel ball into the surface of the material. It is defined as the indenter force divided by the projected area of the indent.

H ≈ 3 σ

f

Resilience, R- J/m3

This measure the maximum elastic strain energy per unit volume stored in a material. It is the area under the elastic part of the stress strain curve.

R = ½

σ

f

ε

f

R =

σ

f2

/ 2E

Materials with large values of R are suitable for good springs

Fracture Toughness, KIC- MPa √m

The fracture toughness of a material is a measure of the resistance of the material to failure by parting of the solid into two or more pieces by the propagation of a macro crack.

Where;

KIC is the critical stress intensity factor,

material property, and 2c= crack length.

K

IC

= σ√πc

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Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Faculty of Engineering

Design & Prod. Eng. Dept.

Material

_K

_IC_{MPa √m}

Fracture Criterion:

K

I

< K

IC

No Fracture

K

I

>= K

IC

Fracture

Rule of thumb:

Avoid materials with fracture toughness less than 15 MPa √m Most metals have values of

K

IC in the range 20 – 100 MPa √m

Engineering ceramics have values of

K

IC - 1 – 5 MPa √m

Therefore, engineers view them with great suspicion.

Steels Titanium alloys Aluminium alloys Epoxies Polystyrene Polycarbonate PMMA PETP Soda-Lime Glass Al2O3 Si3N4 SiC Al2O3, 15% ZrO2 50-200 20-75 20-40 0.3-0.5 0.5 2.5-3.8 1.2-1.7 3.5-6.0 0.7 3.0-5.0 4.0-5.0 3.5 10.0

Loss coefficient The loss coefficient η, measures the fractional energy dissipated in a stress-strain cycle.

D=

ΔU/U

specific damping capacity

η = D / 2π

η = ΔU/ 2π

The loss coefficient

Thermal ConductivityThermal

conductivity λ measures the flux of heat driven by a temperature gradient dT/dX.

q=

λ (dT / dX)

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Faculty of Engineering

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Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Linear thermal ExpansionThe

linear-thermal expansion coefficient a measures the change in length, per unit length, when the sample is heated.

α = (1/L) (dδ/dT)

T

m, melting temperature

T

g, glass temperature, is a property of non-crystalline solids, which do not have a sharp melting

point; it characterizes the transition from true solid to a very viscous liquid.

T

max is the maximum service temperature, at which the material can be used reasonably without

oxidation, chemical change or excessive creep becoming a problem.

T

s is the softening temperature, which is needed to make the material flow easily for forming and

shaping.

The thermal shock resistance is the maximum temperature difference through which a material can

be quenched suddenly, without damage.

The thermal shock resistance and creep resistance are important for high temperature design.

CreepCreep is the slow time dependent

deformation, which occurs when materials are loaded above 1/3

T

m or 2/3

T

g. it is

characterized by a set of creep constants:

n, creep exponent (dimensionless) Q, activation energy (KJ/mole) A, kinetic factor (s-1)

σo, reference stress (MPa)

The strain rate

ε

o

ε

o

_{= A [}

_{σ /σ}

o

]

n

* exp

–[Q/RT]

Wear & Corrosion:

Wear, oxidation and corrosion are harder to quantify, partly because they are surface, not bulk, phenomena, and partly because they involve interactions between two materials, not just the property of one.

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Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Faculty of Engineering

Design & Prod. Eng. Dept.

WearWear is the loss of material from

surfaces when they slide. The wear

resistance is measured by the Archard wear constant KA (m2/MN or MPa-1)

W/A = KA P

Where;

W, wear rate (volume of weight lost per

unit distance slid)

A, area of the surface. P, normal pressure.

Data of KA is available, but it must be interpreted as the property of the sliding couple, not of just one member of it.

Corrosion

Lecture 2 [Engineering Materials & Their Properties] 9 Corrosion is the surface reaction

of the material with gases or liquids. Sometimes a simple rate equation can be used but normally the process is too complicated to allow this.

Dry corrosion, oxidation behavior is

characterized by the parabolic rate constant for oxidation KP (m2/s).

Wet corrosion is much more complicated, and cannot be captured by rate equations, it is more useful to catalogue corrosion resistance by a simple scale such as A (very good) to E (very bad).

Summary

There are six important classes of materials for mechanical design: Metals, polymers, ceramics,

glass, and composites.

Within a class there is certain common ground:

• Ceramics as a class are hard, brittle, and corrosion resistant. • Metals as a class are ductile, tough, and electrical conductors. • Polymers as a class are light, easily shaped, and electrical insulators. This is makes the classification of materials into classes useful.

Importance of material properties versus material classes:

• Each material has some attributes, its properties, e.g. density, modulus, strength, toughness, thermal conductivity, etc.

• A designer does not seek a particular material, but a specific combination of these attributes: a property-profile.

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Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Faculty of Engineering

Design & Prod. Eng. Dept.

Lecture 3: The Performance Maximizing Indices

Material Selection has 4 basic steps:

1. Translation of design requirements into a material specification

2. Screening out of materials that fail constraints

3. Ranking by ability to meet objectives; material indices

4. Search for supporting information for promising candidates

Note that: the task is explained in the following three lectures as follows; Step 1 Lecture 3 Performance maximizing indices

Step 2 Lecture 4 Material selection charts

Step 3 & 4 Lecture 5 Formalization of material selection

Analysis of design requirements:

The analysis of design requirements and development of performance index steps are:

• Identify function, constraints, objective and free variables, (list simple constraints for limit-stage). • Write down equation for objective -- the “performance equation”.

• If it contains a free variable other than material identify the constraint that limits it. • Use this to eliminate the free variable in performance equation.

• Read off the combination of material properties that maximise performance.

The concept is illustrated in more details in the next page.

1

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Faculty of Engineering

Design & Prod. Eng. Dept.

Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Lecture 3 [Performance Maximizing Indices] 2

Performance Maximizing Indices:

Three things specify the design of a structural element, the functional requirements, the geometry, and the properties of the material of which it is made. The performance of the element is described by an equation of the form:

P= f (F, G, M)

Where:

F

is “functional requirements”,

G

is “geometric parameters”, and

M

is “material properties”.

P

describes some aspect of performance of the components: its mass, or volume, or cost, or life for example.

Optimum design is the selection of the material and geometry, which maximize or minimize P, according to its desirability. The three groups of parameters can be separable, P can be written as follow P= f1 (F)* f2 (G)* f3 (M), Where f1, f2, and f3 are functions.

When the groups are separable, the optimum choice of material becomes independent of the details of the design; it is the same for all the details of F and G. This enables enormous simplification; the performance for all F and G is maximized by maximizing f3 (M), which is called the performance index. Experience shows that he groups are usually separable.

Procedure for driving a Performance Index:

1

Identify the attribute to be maximized or minimized (weight, cost, stiffness, strength, etc.).

2

Develop an equation for this attribute in terms of functional requirements, the geometry and the material properties (the objective function).

3

Identify the free (unspecified) variables.

4

Identify the constraint; rank them in order of importance.

5

Develop equation for the constraints (no yield, no fracture, no buckling, max heat capacity, cost below target, etc.).

6

Substitute for the free variables from the constraints into the objective function.

7

Group the variables into three groups: functional requirements, F, geometry, G, and material properties, M, thus: ATTRIBUTE< f (F, G, M)

8

Read the performance index, expressed as a quantity M to be maximized.

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Faculty of Engineering

Design & Prod. Eng. Dept.

Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Lecture 3 [Performance Maximizing Indices] 3

Example 1: Performance Index for a Light Strong Tie

A material is required for a solid cylindrical tie rod of length L, to carry a tensile force F with safety factor Sf; it is to be of minimum mass.

The mass is:

Where A is the cross sectional area, ρ is the density

m= A L ρ

To carry the tensile load F

F/A = σ

f

/ S

f

Eliminating A between the two equations. • The first bracket contains the functional

requirement that is the specified load is safely supported.

• The second bracket contains the specified geometry (the length of the tie).

• The last bracket contains the material properties.

m= (S

_f

F ) (L) (ρ / σ

_f

)

The lightest rod, which will safely carry the load F without failing is that with the largest value of the performance index:

M = [σ

f

/ ρ]

Example 2: Performance Index for a Light Stiff Column

A material is required for a solid cylindrical column of length L, to carry a compressive force F with safety factor Sf; it is to be of minimum mass.

The mass is:

Where A is the cross sectional area, ρ is the density

m= A L ρ

The column will buckle elastically when the Euler load, Fcrit, is exceeded.

The design is safe if:

n is a constant that depends on the ends constraints.

F<= (F

_crit

/ S

_f

) = (nπ

2

E I /L

2

)

=(n π

2

E/

S

_f

L

2

) (πr

4

/4)

Eliminating A from the two equations.

The three brackets form.

m= 2 [S

f

F]

1/2

[L

4

/ nπ ]

1/2

[ρ/E

1/2

]

The best materials for a light column are those with large values of the performance index:

M = [E

1/2

/ρ]

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Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Faculty of Engineering

Design & Prod. Eng. Dept.

4

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Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Faculty of Engineering

Design & Prod. Eng. Dept.

5

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Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Faculty of Engineering

Design & Prod. Eng. Dept.

This simplified material selection chart explains the use of selection guidelines of the previous three examples for Screening out of materials that fail the selection constraints.

Attachments:

Performance maximizing Property Groups table in 2 pages as carried out by M. Ashby.

6

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Faculty of Engineering

Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Design & Prod. Eng. Dept.

Lecture 4 [Material Selection Charts]

Lecture 4: Material Selection Charts

Material Selection has 4 basic steps:

1. Translation of design requirements into a material specification

2. Screening out of materials that fail constraints

3. Ranking by ability to meet objectives; material indices

4. Search for supporting information for promising candidates

Material Selection Charts:

The use of graphical relationship approach from the data is ideally engineer-friendly and particularly effective in the initial sorting stages of a selection procedure.

Ashby has described such a graphical approach for materials selection in conceptual design, i.e. the first stages of design, choosing from the vast range of engineering materials, an initial subset on which design calculations can be based.

In this approach the data for the mechanical and thermal properties of all materials are presented as a set of Materials Selection Charts. The axes are chosen to display the common performance-limiting properties: modulus, strength, toughness, density, and thermal conductivity wear rate etc. The, logarithmic scales allow performance-limiting combinations of to be examined and compared.

List of material selection charts proposed by Ashby: 1. Young’s' Modulus v Density

2. Strength v Density

3. Fracture Toughness v Density 4. Young's Modulus v Strength

5. Specific Modulus v Specific Strength 6. Fracture Toughness v Young's Modulus 7. Fracture Toughness v Strength

8. Loss Coefficient v Young's Modulus 9. Thermal Conductivity v Thermal Diffusivity

10. Thermal Expansion Coefficient v Thermal Conductivity 11. Thermal Expansion Coefficient v Young's Modulus 12. Normalized Strength v Thermal Expansion Coefficient 13. Strength v Temperature

14. Young's Modulus v Relative Cost 15. Strength v Relative Cost

16. Wear Rate v Hardness

17. Young's Modulus v Energy Content 18. Strength v Energy Content

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Faculty of Engineering

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Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Lecture 4 [Material Selection Charts] 2

Using Material Selection Charts

There are three main things to think about when choosing materials

(in order of importance):

1.Will they meet the performance requirements?

2.Will they be easy to process?

3.Do they have the right 'aesthetic' properties?

So that leaves us with performance requirements...

Most products need to satisfy some performance targets, which we

determine by considering the design specification. e.g. they must be cheap,

or stiff, or strong, or light, or perhaps all of these things...

Each of these performance requirements will influence which materials we

should choose - if our product needs to be light we wouldn't choose lead and

if it was to be stiff we wouldn't choose rubber!

So what we need is data for lots of material properties and for lots of

materials. This information normally comes as tables of data and it can be a

time-consuming process to sort through them. And what if we have 2

requirements - e.g. our material must be light and stiff - how can we

trade-off these 2 needs?

The answer to both these problems is to use material selection charts.

Here is a materials selection chart for 2 common properties: Young's

modulus (which describes how stiff a material is) and density.

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Faculty of Engineering

Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Design & Prod. Eng. Dept.

On these charts, materials of each class (e.g. metals, polymers) form

'clusters' or 'bubbles' that are marked by the shaded regions. We can see

immediately that:

• Metals are the heaviest materials,

• Foams are the lightest materials,

• Ceramics are the stiffest materials.

Selection charts are really useful is in showing the trade-off between 2

properties, because the charts plot combinations of properties. For instance if

we want a light and stiff material we need to choose materials near the top

left corner of the chart - so composites look good.

3

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Faculty of Engineering

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Dr. Ahmed Farid A. G. Youssef

Design & Prod. Eng. Dept.

Consider a design problem where the specification is for a component that is

both light and stiff (e.g. the frame of a racing bicycle).

What can we conclude?

• The values of Young's modulus for polymers are low, so most

polymers are unlikely to be useful for stiffness-limited designs.

• Some metals, ceramics and woods could be considered – but

composites appear best of all.

• Note that the values for Young's modulus cover a huge range and we

have therefore used a logarithmic scale.

4

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Faculty of Engineering

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Dr. Ahmed Farid A. G. Youssef

Design & Prod. Eng. Dept.

It is unlikely that only 2 material properties matter, so what other properties

are important? Let's consider strength and cost - these properties are plotted

as another chart.

What can we conclude?

• The strength of ceramics is only sufficient for loading in compression -

they would not be strong enough in tension, including loading in

bending.

• Woods may not be strong enough, and composites might be too

expensive.

• Metals appear to give good overall performance

5

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Faculty of Engineering

Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Design & Prod. Eng. Dept.

Selection charts can also be used to select between members of a given class

by populating it with the main materials. For instance, we can do this for

metals in the stiffness-density chart.

What can we conclude?

Some metals look very good for light, stiff components - e.g. magnesium,

aluminum, titanium, while others are clearly eliminated - e.g. lead.

Steels have rather a high density, but are also very stiff. Given their high

strength and relatively low cost, they are likely to compete with the other

metals.

6

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Faculty of Engineering

Design & Prod. Eng. Dept.

Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Lecture 4 [Material Selection Charts] 7

Summary:

By considering 2 (or more) charts, the properties needed to

satisfy the main design requirements can be quickly assessed.

The charts can be used to identify the best classes of

materials, and then to look in more detail within these

classes.

There are many other factors still to be considered,

particularly manufacturing methods.

The selection made from the charts should be left quite broad

to keep enough options open.

A good way to approach the problem is to use the charts to

eliminate materials, which will definitely not be good

enough, rather than to try and identify the single best

material too soon in the design process.

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Faculty of Engineering

Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Design & Prod. Eng. Dept.

Example: Materials for Lightweight Table Legs

Solved by Cambridge material Engineering Selector software CES Courtesy M. F. Ashby

The selection methodology used in CES Materials can be encapsulated by developing a case study. Here, we will use the design of a simple table to illustrate the development of some selection criteria; we will apply them and plot them on some selection stages by using CES.

The Design Problem

Luigi Tavolino, furniture designer, conceives of a lightweight table of daring simplicity: a flat sheet of toughened glass supported on slender, cylindrical legs. The legs must be solid (to make them thin) and as light as possible (to make the table easier to move). They must support the tabletop and whatever is placed on it without buckling. What materials could one recommend?

Design Requirements

We must first identify the Function, Objective and Constraints of our problem. FUNCTION Column (support compressive loads)

OBJECTIVE Minimize mass

CONSTRAINTS Must not buckle

The Model

Figure 1 - A lightweight table with slender cylindrical legs

The performance-maximizing index

M

1

= [E

1/2

/ρ]

8

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Faculty of Engineering

Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Design & Prod. Eng. Dept.

The Selection

We can now plot the material properties of our Performance Index using the CES software. In order to identify which materials maximize the performance index, we need to plot a line representing it on the graph. We use logarithmic axes on the graph and note that a simple performance index typically has the form:

M = P1/P2n

Taking logs of this equation gives: log P1 = n log P2 + log M

So, if P1 and P2 are plotted on logarithmic scales, the equation describes a line of slope n on the plot, with its position determined by the value of M. We are seeking to maximize the value of M, so our selection is optimised by moving the line to the highest value of M, which still leaves a viable subset of materials exposed above the line.

For the table, we are seeking the subset of materials which have high values of E1/2 / ρ, so we plot a line of slope 2 on our graph.

Figure 2 shows the appropriate chart: Young's Modulus plotted against the density. The guideline is displaced upwards (retaining the slope) until a reasonably small subset of materials is isolated above it; it is shown in the position M1 = 6 GPa1/2/(Mg/m3). Materials above this line have higher values of M1. They are identified on the figure.

The thinnest legs is that made of the material with the largest value of M2 =E

Figure 2 Materials for light slender legs

9

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Faculty of Engineering

Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Design & Prod. Eng. Dept.

Woods meet the criteria and so do composites such as CFRP. Certain of the engineering ceramics also meet the stated design goals. However, ceramics, we know, are brittle – they lack fracture toughness. Table legs are exposed to abuse - they get knocked and kicked; common sense suggests that an additional constraint is required - that of adequate fracture toughness. A Selection stage that takes this into account is shown in Figure 3.

Figure 3 A Protective Selection Stage to eliminate brittle and expensive materials

Results

Material M1 (GPa ½ m3/Mg) M2 (GPa) Comment

Woods 5-8 4-20 Outstanding M1, Poor M2, Cheap,

traditional, reliable.

CFRP 4-8 30-200 Outstanding M1, and M2, but

expensive.

GFRP 3.5-5.5 20-90 Much cheaper than CFRP, but not so

good.

Ceramics 4-8 150-1000 Outstanding M1, and M2, eliminated by

brittleness.

So, woods and CFRP make good materials for table legs - although the cost of CFRP may cause Snr Tavolino to reconsider his design. If (improbably) the goal were to design a light slender-legged table for use at high temperatures, then ceramics would have to be reconsidered. The brittleness problem can be designed around by protecting the legs or by pre-stressing them in compression.

Review the material selection charts at:

M. F. Ashby, (1992), Materials Selection in Mechanical Design, Pergamon Press. Chapter 5.

10

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Summary of Lecture Notes

Dr. Ahmed Farid A. G. Youssef

Faculty of Engineering

Design & Prod. Eng. Dept.

Lecture 5 [Formalization of Material Selection]

Lecture 5: Formalization of Material Selection

Material Selection has 4 basic steps:

1. Translation of design requirements into a material specification

2. Screening out of materials that fail constraints

3. Ranking by ability to meet objectives; material indices

4. Search for supporting information for promising candidates

Formalization of Material Selection:

Formalization of material selection is the third step after defining one or two material groups by applying a selection criterion (performance index) and a corresponding selection chart, as previously explained.

The aim here is to define the optimum material name within the proposed material group.