Material Selection 1-5

Ain Shams University Faculty of Engineering Design & Prod. Eng. Dept.

Material & Process Selection Summary of Lecture Notes Dr. Ahmed Farid A. G. Youssef

Material Selection Charts



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

1

Material & Process Selection Summary of Lecture Notes

Ain Shams University Faculty of Engineering

Dr. Ahmed Farid A. G. Youssef

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.

Lecture 1 [Introduction to Material Selection in Mechanical Design]

2



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.


3





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. Assembly [1]

Component 1.1 Component 1.2

Component

Technical System

Assembly [2]

Component Component Component

Assembly [3]

Component Component Component

Analysis of a technical system 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.


4




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

5





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.

FUNCTION Transmit loads, heat, contain pressure, store energy, etc.

MATERIAL

SHAPE

PROCESS

Interaction of function, material, process and shape

The interaction between function, material, shape and process lie at the heart of the Design process.


6




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 proproduction

Production Modification to broaden product family Cost reduction

Phase-out

Obsolescence Cut-off point

Introduction

Growth

Maturity

Decline

Sales volume Return on investment Profit

Research & development

Time 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

The life cycle of a product Lecture 1 [Introduction to Material Selection in Mechanical Design]

7



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 cause a product to become DYNAMIC Government action or legislation

Factors that create or retain a STATIC plateau Improving environment for the existing design Commodities and resources

Changing environment

Customers not change User familiarity

willing

to

Commodities and resources

Stable technology

Customers willing to change

Conformance standards

Technical advancement No conformance standards

Stable or decreasing number of producers Few large producers

Many small producers (increasing) No infrastructure

Product available for a long time Existing infrastructure

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

Factors that cause a product to become DYNAMIC Management committed to deign Changing PDS Process design small Adequate time for design Wide effective market research Companies seeking new concepts Flexible machinery subcontract, manufacture

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 More process design than product design Management not committed to deign Stable effective PDS Restricted PDS Limited Design time Limitation Automation CAD Purchasing (dedicated)

new

machinery

Balance diagram of the micro factors that change / maintain a product status. Lecture 1 [Introduction to Material Selection in Mechanical Design]

8



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 manufacturehas 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.


9




Design & Prod. Eng. Dept. 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, 1. Elastic deformation 8. Corrosion 10. Fretting Each failure mode has: • a failure mechanism • material selection guide lines • material selection rules to prevent the failure mode from taking place.

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

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


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

10



Investigations about the frequency of failure causes in some engineering industries indicate that the main cause for failure is improper material selection. Frequency of Causes of Failure in Some Engineering Industries Investigations:

Origin Improper material selection Fabrication defects Faulty heat treatments Mechanical design fault Unforeseen operating conditions Inadequate environment control Improper or lack of inspection and quality control Material mix-up

% 38 15 15 11 8 6 5 2

Frequency of Failure Modes in Some Engineering Industries Investigations.

Origin Corrosion Fatigue Brittle fracture Overload High temperature corrosion Stress corrosion / corrosion fatigue / hydrogen embrittlement Creep Wear, abrasion, and erosion

% 29 25 16 11 7 6 3 3

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


11



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 2. Attaching 3. Motion constraining 4. Force transmitting 5. Sealing 6. Friction reducing 7. Protective covering 8. Liquid constraining 9. Pivoting 10. Torque transmitting 11. Pressure supporting 12. Oscillatory sliding 13. Shielding 14. Sliding 15. Energy transforming 16. Removable fastening 17. Limiting 18. Electrical conduction 19. Contaminant constraining 20. Linking 21. Continuous rolling 22. Liquid transferring 23. Force amplifying 24. Power transmitting 25. Covering 26. Oscillatory rolling 27. Energy absorbing 28. Light transmitting 29. Viewing 30. Energy dissipating 31. Guiding 32. Latching 33 electrical switching 34. Stabilizing 35. Gas constraining

36. Permanent fastening 37. Pressure increasing 38. Streamlining 39. Motion reducing 40. Filtering 41. Lighting 42. Pumping 43. Gas transferring 44. Aero. force transmitting 45. Motion transmitting 46. Signal transmitting 47. Motion damping 48. Force distributing 49. Reinforcing 50. Pressure sensing 51. Information transmitting 52. Coupling 53. Displacement indicating 54. Clutching 55. Fastening 56. Information indicating 57. Position indicating 58. Movable lighting 59. Partitioning 60. Position restoring 61. Flexible spacing 62. Electrical amplifying 63. Adjustable attaching 64. Shape constraining 65. Deflecting 66. Disconnecting 67. Electrical limiting 68. Motion limiting 69. Pressure limiting 70. Sensing

71. Force sensing 72. Spacing 73. Temporary supporting 74. Gas switching 75. Electrical transforming 76. Power absorbing 77. Information attaching 78. Sound absorbing 79. Constraining 80. Flexible coupling 81. Removable coupling 82. Damping 83. Electrical distributing 84. Load distributing 85. Gas guiding 86. Pressure indicating 87. Electrical insulating 88. Sound insulating 89. Temporary latching 90. Force limiting 91. Force maintaining 92. Variable position maintenance 93. Liquid pumping 94. Electrical reducing 95. Rolling 96. Position sensing 97. Energy storing 98. Liquid storing 99. Flexible supporting 100. Switching 101.Pressure to torque transmitting 102. Electrical transmitting 103. Flexible motion transmitting 104. Flexible torque transmitting 105. Torque limiting

Corrective actions for failure-experience matrix: Direct replacement Change Of material Supplement part Added adhesive Provided drain Added sealant Repositioned part Repaired part Reinforced part Eliminated part Strengthened part Adjusted part

Changed vendor Changed dimensions Improved quality control Changed lubricant type Improved lubrication Applied surface coating Applied surface treatment More easily replaceable part Changed to correct part Made part interchangeable Changed loading on part Relaxed replacement criteria

Improved instructions to user Design change to improve part Changed mechanism of operation Improved run-in procedure Changed manufacturing procedure Changed mode of attachment Changed method of lubrication Added or changed locking feature Revised procurement specification Provided for proper inspection Changed electrical characteristics


12



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, McGrawHill. ASM Metals Handbook, (1999), Volume 20, Materials Selection and Design, American Society for Metals, Metals Park, Ohio, USA.


13



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.

Lecture 2 [Engineering Materials & Their Properties]

1



Material classes, generic members, and abbreviated names: Class Members Engineering alloys (The metals and alloys of engineering)

Engineering polymers (The thermoplastics and thermosets of engineering)

Engineering ceramics (Fine ceramics capable of load bearing application)

Engineering composites (The composites of engineering practice) A distinction is drawn between the properties of a ply – UNIPLY – and of a laminate – LAMINATES Porous ceramics (Traditional ceramics, cement, rocks, & minerals)

Glasses (Ordinary silicate glass) Woods Separate envelopes describe properties parallel to the grain and normal to it, and wood products)

Elastomers (Natural and artificial rubbers)

Polymer foams (Foamed polymers of engineering)

Short name

Aluminium alloys Copper alloys Lead alloys Magnesium alloys Nickel alloys Steels Tin alloys Titanium alloys Zinc alloys Epoxies Melamines Polycarbonate Polyesters Polyethylene, high density Polyethylene, low density Poly formaldehyde Poly methyl metha crylate Polypropylene Poly tetra fluor ethylene Polyvinyl chloride Alumina Diamond Sialons Silicon Carbide Silicon nitride Zirconia Carbon fiber reinforced polymer Glass fiber reinforced polymer Kevlar fiber reinforced polymer

Al alloys Cu alloys Lead alloys Mg alloys Ni alloys Steels Tin alloys Ti alloys Zn alloys EP MEL PC PEST HDPE LDPE PF PMMA PP. PTFE PNC Al2O3 C Sialons SiC Si3N4 ZrO2 CFRP GFRP KFRP

Brick Cement Common rocks Concrete Porcelain Pottery Borosilicate glass Soda glass Silica Ash Balsa Fir Oak Pine Wood products Natural rubber Hard butyl rubber Polyurethane Silicone rubber Soft butyl rubber Cork Polyester Polystyrene Polyurethane

Brick Cement Rocks Concrete Pcln Pot B-glass Na-glass SiO2 Ash Balsa Fir Oak Pine Wood products Rubber Hard butyl PU Silicone Soft butyl Cork PEST PS PU

Note that abbreviated names as used in material selection charts developed by M.F. Ashby. Lecture 2 [Engineering Materials & Their Properties]

2



[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

General

Relative Cost Density

Mechanical

Elastic Modulus Strength (yield / ultimate / fracture) Toughness Fracture Toughness Damping Capacity Fatigue Ratio

Thermal

Thermal Conductivity Thermal Diffusivity Specific Heat Melting Point Glass Temperature Thermal Expansion Coefficient Thermal Shock resistance Creep Resistance

Wear

Archard Wear Constant

Corrosion / Oxidation

Corrosion Rate Parabolic rate constant

Elastic Modulus E= 3G/(1+G/3K)

σf = KIC /√ (πC)

Shear Modulus G= E/2(1+ν)

CR ρ

Units

--Mg/m 3

E, G, K GPa MPa σf Gc KJ/m 2 KIC MPa m 1/2 -----η f -----λ a CP Tm Tg α ΔT -----

W/m K m 2/s J/Kg K K K K-1 K ------

KA ----KP

MPa -1 -----m 2/s

Bulk Modulus K= E/3(1-2ν)

ν =1/3

KIC the resistance to the propagation f a crack.


3

Ain Shams University Faculty of Engineering Design & Prod. Eng. Dept. Density, ρ

Material & Process Selection Summary of Lecture Notes Dr. Ahmed Farid A. G. Youssef Mg/m3 7.8 4.5 2.7 1.7 1.2

Material

Mass per unit volume, Mg/m

3

Stiffness, Elastic MODULUS, E Slope of the liner elastic part of the stress-strain curve, GN/m2 = GPa Poisson’s ratio, ν

ν = ε lateral / εaxial

Iron, Steels Titanium alloys Aluminium alloys Magnesium alloys Polycarbonate Material Iron, Steels Titanium alloys Aluminium alloys Magnesium alloys Polycarbonate Rubbers Silicon SiC

E, GPa 200 116 70 43 2.6 0.01-0.1 160 410

ν 0.27 0.34 0.33 0.35 0.4 0.49 0.22 0.3

For isotropic materials: E

ν

Young’s Modulus

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, σf, MN/m2 = MPa.

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

Metals

σf is identified with the 0.2% offset yield strength σy. 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%.

Material Steels Titanium alloys Aluminium alloys Magnesium alloys

σy, MPa 200-2000 800-1200 200-500 100-200

σy in compression ≈ σy in tension


4



Ceramics & Glasses Strength for ceramics and glasses depends strongly on the mode of loading. In tension, strength means t the fracture strength, σf . In compression it means C

the crushing strength σf , which is much larger, typically

σfC in compression ≈ 15 σft 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 σft in tension

Polymers:

σf is identified as the stress σy at which the stress strain curve has become markedly non-linear- typically a strain of 1%. Yield mechanisms: shear yielding, crazing.

Polymers are a little stronger ≈ 20% in compression than in tension.

σy in compression ≈ 1.2 σy in tension

Material Polycarbonate PMMA

σy, MPa 80 100

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. Lecture 2 [Engineering Materials & Their Properties]

5



Ultimate tensile strength, σu - 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

σf in tension. For metals, ductile polymers and most composites it is larger than σf, by factor of between 1.1and 3. In metals σu is higher than σy because of work hardening.

as

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 σf 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 σf εf R = σf2 / 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.

KIC = σ√πc


6



Fracture Criterion:

KI < KIC KI >= KIC

No Fracture Fracture

Rule of thumb: Avoid materials with fracture toughness less than 15 MPa √m Most metals have values of KIC in the range 20 – 100 MPa √m Engineering ceramics have values of KIC - 1 – 5 MPa √m Therefore, engineers view them with great suspicion.

Material Steels Titanium alloys Aluminium alloys Epoxies Polystyrene Polycarbonate PMMA PETP Soda-Lime Glass Al2O3 Si3N4 SiC Al2O3, 15% ZrO2

KIC MPa √m 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 The loss coefficient η = ΔU/ 2π

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

q= λ (dT / dX)


7



Linear thermal ExpansionThe linearthermal 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) o The strain rate ε

εo = A [σ /σo ]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.


8



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. CorrosionCorrosion 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. • The material name is merely the identifier for a particular property-profile


9



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.

Lecture 3 [Performance Maximizing Indices]

1



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 2 3 4 5 6 7 8 9

Identify the attribute to be maximized or minimized (weight, cost, stiffness, strength, etc.). Develop an equation for this attribute in terms of functional requirements, the geometry and the material properties (the objective function). Identify the free (unspecified) variables. Identify the constraint; rank them in order of importance. Develop equation for the constraints (no yield, no fracture, no buckling, max heat capacity, cost below target, etc.). Substitute for the free variables from the constraints into the objective function. Group the variables into three groups: functional requirements, F, geometry, G, and material properties, M, thus: ATTRIBUTE< f (F, G, M) Read the performance index, expressed as a quantity M to be maximized. Note that a full solution is not necessary in order t o identify the material property group.


2



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:

m= A L ρ

Where A is the cross sectional area, ρ is the density To carry the tensile load F

F/A = σ f / S f

Eliminating A between the two equations.

m= (Sf F ) (L) (ρ / σ f )

•

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.

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 The column will buckle elastically when the Euler load, Fcrit, is exceeded. The design is safe if:

m= A L ρ F<= (F crit/ Sf) = (nπ2E I /L2) =(n π2 E/ Sf L2 ) (πr4/4)

n is a constant that depends on the ends constraints. Eliminating A from the two equations. The three brackets form. The best materials for a light column are those with large values of the performance index:

m= 2 [Sf F] 1/2 [L4/ nπ ] 1/2 [ρ/E 1/2 ] M = [E1/2/ρ]

The examples are explained with the aid of sketches in the following two pages.


3




4




5



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. Lecture 3 [Performance Maximizing Indices]

6



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 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

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

Lecture 4 [Material Selection Charts]

1



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 tradeoff 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.


2



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



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



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 Lecture 4 [Material Selection Charts]

5



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



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.


7



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

M1 = [E1/2/ρ]


8



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 Lecture 4 [Material Selection Charts]

9



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 Woods

M1 (GPa m3/Mg) 5-8

M2 (GPa) 4-20

CFRP

4-8

30-200

GFRP

3.5-5.5

20-90

Ceramics

4-8

150-1000

½

Comment Outstanding M1, Poor M2, Cheap, traditional, reliable. Outstanding M1, and M2, but expensive. Much cheaper than CFRP, but not so good. 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



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 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

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.

Lecture 5 [Formalization of Material Selection]

1



Formalization of Material Selection Procedure Illustrative Example of Material Selection Formalization By: J.A. Charles & FAA Crane In selecting a material for a given application the materials engineer is faced with an almost endless number of possibilities. If the choice is to be made economy of time and effort, but also with the assurance that no possibilities are overlooked, some systemization of procedures is essential. The basis for materials selection is a ‘shopping list’ of design requirements and the selection procedure should be as numerate as possible. However the extent to which this can be achieved varies from one design requirement to another A useful reduction in the initial number of candidate materials can be obtained by establishing the outset of upper and lower bounds for the various design requirements. On the basis that every design requirement must be present to an acceptable degree, the costs will increase if the design requirements present to a greater extent than is strictly necessary. The following table illustrates the concept of material selection procedure in detailed six steps example. 1st. Step is drawing up a table summarizing the merits and demerits of the contenders so as to permit early elimination of unsuitable materials. Considerable knowledge and experience are required to reject a material at this stage, because materials properties can be varied widely during manufacture and processing, and so also can the costs. A material would not necessarily be rejected because it was unsatisfactory in respect of a single secondary design requirement, or even a primary one, if there were scope for ameliorating the disadvantage during design and manufacture. Whether or not over-provision of some property is cause for rejection depends upon the effect on cost. Excessive cost is always a cause for rejection but cost is also a function of processing. Clearly, any version of a basically expensive material, such as titanium, will be costly but whereas steels are mostly cheap, they become expensive when highly alloyed or manufactured to tight tolerances or compositional limits. It is likely that any class of material, which passes this initial stage of selection, will produce three or more competing variants of the same material to be considered at a later stage. 1st. Step Materials M1 M2 M3 M4 M5 M6

U = under provision, O= over provision, E = excessive, a= acceptable Design requirements Primary Secondary Cost Decision DR1 DR2 DR3 DR4 DR5 a O a a a E Reject a a a O a a U a a a a a Reject a O a a O a a a a a a E Reject a a a U a a

2nd. Step is refining the table by replacing the simple go/no-go criteria of satisfactory and unsatisfactory by varying degrees of merit. For properties that are not reliably quantifiable, more-orless vague terms such as poor, fair, excellent, etc. are best abandoned in favour of numerical ratings of, 1 to 5 in ascending order of merit. 2nd. Step

Material M1 M2 M3 M4 M5 M6

Heat resistance 4 2 5 1 4 3

Rigidity 3 3 4 1 5 2


Resistance to stress cracking 3 4 1 4 1 5

Mouldability 3 3 1 3 3 5

Overall rating (Max. =20) 13/20=0.65 12/20=0.60 11/20=0.55 09/20=0.45 13/20=0.65 15/20=0.75

2



3rd. Step is obtaining an overall numerical rating, when individual merits ratings are totalled. Clearly, in the example shown in table, that overall superiority of the material M6 is derived from its maximum ratings in respect of stress cracking and mouldability, but what if heat resistance and rigidity were the properties more urgently required? This might be so, since although the life of the component could be determined by its resistance to stress cracking, but its resistance to heat and its rigidity could determine whether or not it could do the job at all. (Mouldability is important mainly through its influence on costs.). The relative importance of the various properties therefore depends upon the nature of the application and this can only be assessed in the mind of the designer. He can exercise his judgment in this respect by assigning weighting factors to the various properties. 3rd. step Material

M1 M2 M3 M4 M5 M6

Heat resistance X5 20 10 25 5 20 15

Rigidity X5 15 15 20 5 25 10

Resistance to stress cracking X2 6 8 2 8 2 10

Mouldability X3 9 9 3 9 9 15


4th. Step is assigning weighting factors to the various properties. The choice now moves to M5, see the table, which means that a short-life material has been preferred to a long-life material. This emphasizes that weighting factors must be used cautiously, since by their use small changes in heavily weighted properties can mask the effects of large changes in more lightly weighted properties. Materials selection is more effective, when it can be carried out in terms of precisely defined quantitative property parameters. The example shown in table is dealing with materials, which might be considered for use in an airplane wing. The values of cost/ton given are illustrative only, and must not be taken as definitive. Price instability will bring change of magnitude and possibly even relationships. 4th. Step Material Aluminum alloy 1 Aluminum alloy 2 Titanium alloy Stainless steel

YS (MPa) 350 550 880 900

KIc (MPa m1/2) 45 25 60 100

ρ (ton/m3) 2.7 2.7 4.5 7.8

E (GPa) 70 70 110 200

Cost ($/ton) 590 700 5500 500

5th. Step is assigning the performance maximizing indices. The data cannot be used in raw form, firstly, because the significance of the individual properties varies from one part of the structure to another, and secondly, because the units are variegated. The first point can be dealt with by combining units appropriately in the performance maximizing indices; the second by expressing the data in each column as proportions of the largest figure appearing in that column. In the shown example in table, the results of this calculation are not highly informative since it could have been anticipated that the high price of titanium would force it to the bottom of the list. This, together with the high density of the steel, leaves the aluminum alloys as the main contenders. If, however, the airplane were to be a military supersonic aircraft it might be appropriate to downgrade the importance of cost by the use of suitable weighting factors. It is then necessary to include data for resistance to elevated temperature since this is an important requirement for high-speed flight. Lecture 5 [Formalization of Material Selection]

3

Ain Shams University Faculty of Engineering Design & Prod. Eng. Dept. 5th. Step Material

YS/ρ Abs. Rel. =A 130 0.64 204 1.00 193 0.96 115 0.56

Aluminum alloy 1 Aluminum alloy 2 Titanium alloy Stainless steel

Material & Process Selection Summary of Lecture Notes Dr. Ahmed Farid A. G. Youssef [KIC/YS]2 Abs. Rel. =B 16.5 1.00 2.1 0.13 4.6 0.27 12.3 0.75

E1/3/ρ Abs. Rel. =C 1.50 1.0 1.50 1.0 1.06 0.71 0.75 0.50

Cost Abs. Rel. =D 590 0.11 700 0.13 5500 1.00 500 0.09

Overall rating A+B+C+(1-D) 4 0.88 0.75 0.49 0.68

6th. Step is calculating overall ratings by using weighting factors. In the shown example weighting factors of 10 for the mechanical properties, 20 for temperature resistance and unity for cost were used. The overall ratings obtained thereby correlate to some extent with experience since stainless steels and titanium alloys have been used in prototype aircraft. 6th. Step Material

Al-alloy 1 Al-alloy 2 Ti-alloy St-St.

[KIC/YS]2

E1/3/ρ

Abs

Rel. =A

Abs

Rel. =B

Abs

Rel. =C

Temp. Limit (C) Abs Rel. =D

130 204 193 115

0.64 1.00 0.96 0.56

16.5 2.1 4.6 12.3

1.00 0.13 0.27 0.75

1.50 1.50 1.06 0.75

1.0 1.0 0.71 0.50

150 150 300 400

YS/ρ

0.38 0.38 0.75 1.00

Cost Abs

Rel. =E

590 700 5500 500

0.11 0.13 1.00 0.09

Overall rating 10A+10B+10C+ 20D+(1-E) 51 0.68 0.58 0.67 0.76

It is instructive to observe the magnitude of the weighting factors required for achieving such correlation. Clearly, however, the results are distorted by the fact that some properties are overprovided for. For example, the highest speed for a supersonic aircraft is Mach 3, which corresponds to a saturation temperature of (200 °C). This figure should therefore be used as the basis for the relative temperatureresistance column so that stainless steel does not benefit from a degree of temperature-resistance, which cannot be used. Again, the first aluminium alloy benefits excessively from its high toughness. Now, toughness and strength are conceptually different in that increasing strength is always beneficial because it allows progressively less material to be used, whereas toughness need be provided only in sufficient quantity to allow satisfactory service at a given level of stress. It may be adequate, therefore, to account for toughness only as a lower bound and not incorporate it into the overall rating. On the other hand, since for any material toughness and strength are inversely related, there exists the possibility of mutual optimisation. This detailed formalization, which have been suggested and adopted in many references, indicates that the selection process is mainly two tasks. The design engineer task is to define the design requirements and their weighting factors, and the materials engineer task is to define the optimum material. The work in the present thesis is concerning with carrying out the second task. The example is collected in one page table (next page) for easier review Remember that:

Step 4: Search for supporting information for promising candidates One step is still needed, which is the last step, which can be carried out by collecting supporting information (structured and unstructured data) about the proposed material to ensure the suitability of the selection by considering the other fine details.


4



Here is the example collected in one page table for easier review: Illustrative Example of Material Selection Formalization By: J.A. Charles & FAA Crane 1st. Step

U = under provision, O= over provision, E = excessive, a= acceptable Design requirements Primary Secondary Cost Decision DR1 DR2 DR3 DR4 DR5 a O a a a E Reject a a a O a a U a a a a a Reject a O a a O a a a a a a E Reject a a a U a a

Materials M1 M2 M3 M4 M5 M6 2nd. Step Material

Heat resistance 4 2 5 1 4 3

M1 M2 M3 M4 M5 M6

Rigidity

Resistance to stress cracking 3 4 1 4 1 5

3 3 4 1 5 2

Mouldability 3 3 1 3 3 5


3rd. step Material

Heat resistance X5 20 10 25 5 20 15

M1 M2 M3 M4 M5 M6

Rigidity

Resistance to stress cracking X2 6 8 2 8 2 10

X5 15 15 20 5 25 10

Mouldability X3 9 9 3 9 9 15


4th. Step Material

YS (MPa) 350 550 880 900

Aluminum alloy 1 Aluminum alloy 2 Titanium alloy Stainless steel 5th. Step Material

Aluminum alloy 1 Aluminum alloy 2 Titanium alloy Stainless steel 6th. Step Material YS/ρ

Al-alloy 1 Al-alloy 2 Ti-alloy St-St.

YS/ρ Abs. Rel. =A 130 0.64 204 1.00 193 0.96 115 0.56

KIc (MPa m1/2) 45 25 60 100

[KIC/YS]2 Abs. Rel. =B 16.5 1.00 2.1 0.13 4.6 0.27 12.3 0.75

[KIC/YS]2

ρ (ton/m3) 2.7 2.7 4.5 7.8

E1/3/ρ Abs. Rel. =C 1.50 1.0 1.50 1.0 1.06 0.71 0.75 0.50

E1/3/ρ

Abs

Rel. =A

Abs

Rel. =B

Abs

Rel. =C

Temp. Limit (C) Abs Rel. =D

130 204 193 115

0.64 1.00 0.96 0.56

16.5 2.1 4.6 12.3

1.00 0.13 0.27 0.75

1.50 1.50 1.06 0.75

1.0 1.0 0.71 0.50

150 150 300 400


E (GPa) 70 70 110 200

Cost Abs. Rel. =D 590 0.11 700 0.13 5500 1.00 500 0.09

0.38 0.38 0.75 1.00

Cost Abs

Rel. =E

590 700 5500 500

0.11 0.13 1.00 0.09

Cost ($/ton) 590 700 5500 500 Overall rating A+B+C+(1-D) 4 0.88 0.75 0.49 0.68 Overall rating 10A+10B+10C+ 20D+(1-E) 51 0.68 0.58 0.67 0.76

5



Material Selection Software List: The following list contains some of the most common material selection software. The prices vary widely. Four price groups are given: free, modest (less than $1000), expensive (between $1000 and $10,000) and very expensive (more than $10,000). CMS: Cambridge Materials Selector (l992) Cambridge University Engineering Department, Cambridge, UK (Tel: 0223 334755; Fax: 0223 332797). All materials, PC formats. Allows successive application of up to six selection stages. (Modest-price). Mat.DB (1990) (replacing METSEL 2) Materials Database; ASM International, Metals Park, Ohio 44073, USA. (Tel: 216 338 515 I; Fax: 216 338 4634). PC formats. Databases of property and processing (of metals and some polymers) are now available; more are in preparation. Selection based on user-defined target values. Expensive, and cumbersome. PERITUS Matsel Systems Ltd., 6th Floor, Cunard Building, Water St, Liverpool L3 IEG, UK (Tel: 051 227 5080; Fax: 051 236 1934). PC formats. A database for metals, polymers and ceramics, aimed at materials and process selection. Selection based on requesting "high", "medium” or "low” values (or given properties rather than numerical values; a display shows the match between candidate materials and the target profile. Typical uses are given. (Expensive), (An educational version, Peritus-ED, is more modestly priced) PLASCAMS 220: Plastics Materials Selector (1998) RAPRA Technology Ltd., Shrewsbury, Shrewsbury SY4 4NR, UK. PC formats Polymers only. Mechanical and processing properties of polymers, thermoplastics and Thermosets. Easy to use for data retrieval, with much useful information. Selection procedure cumbersome and not design-related. Modest initial Price plus annual maintenance fee. Updated regularly. DataPLAS: Plastics Information System (1990) Modern Plastics, 43rd Floor, 1221 Avenue of the Americas, New York, NY 10020, USA (Tel: 1 800 845 5056; Fax: 212 512 6111). PC formats. Properties, processing and Producer information for 1000 high-performance thermoplastics available from US suppliers. Updated regularly, (Expensive). CAMPUS, Computer Aided Material Pre-selection by Uniform Standards (1988) Hoechst Aktiengesellschaft, Verkauf Kunststoffe, D-6320 Frankfurt am Main 80, Germany. PC formats. A collection of four databases of Hoechst, BASF, and Bayer and Huels thermoplastic polymers, containing information on modulus, strength, viscosity and thermal properties. Regularly updated, but limited in scope. Free. EPOS, Engineering Plastics on Screen (1989) ICI Engineering, Plastics Sales Office, PO Box 90, Wilton, Middlesborough, Cleveland TS6 8JE, UK (Tel: 0642 454144 or 0707 337852). PC formats. The software lists general and electrical properties of ICI polymer products, with a search facility. Updated periodically. Free. MATUS: Materials User Service Engineering Information Company Ltd., 15/17 Ingate Place, London SWS 3NS UK. An on-line data bank of UK material suppliers, trade names and properties for metals polymers and ceramics, using data from suppliers' catalogues and data sheets. A stand-alone system that can be customized to the user’s needs is now available. Expensive. M-VISION (1990) PDA Engineering, 2975 Redhill Avenue, Costa Mesa, CA 92626, USA (Tel: 714 540 8900; Fax: 714 979 2990). Requires a workstation. An ambitious image and database, with flexible selection procedures. Data for aerospace alloys and composites. Very expensive. IMAMAT: Institute of Metals and Materials, Australia, PO Box 19, Parkville 3052, Vic, Australia (Tel: 03 347 2544; Fax: 03 348 1208). Price and functionality not known. THERM: Thermal Properties of Materials; Rob Bailey, Lawrence Livermore Laboratory, Materials Laboratory, PO Box 808, Livermore, Ca 94550, USA (Tel: 415 422 8512). Very simple but useful PC-based compilation of thermal data for materials: specific heat, thermal conductivity. Density and melting point. Free. STRAIN: Plastic Properties of Materials; Rob Bailey, Lawrence Livermore Laboratory, Materials Laboratory, PO Box 808, Livermore, Ca 94550, USA (Tel: 415 422 8512). Very simple but useful PC-based compilation of room-temperature mechanical properties of ductile materials. Free. CopperSelect: Computerized System for Selecting Copper Alloys: Copper Development Association Inc, Greenwich Office, Park No 2, Box 1840, Greenwich CT 06836, USA (Tel: 203 625 8210; Fax: 203 625 0174). PC formats. A database of properties and processing information for wrought and cast copper alloys. Free. DESIGN DATA-CAST IRON: BCIRA, the Cast Metals Technology Center, Alvechurch, Birmingham B48 7QB, UK (Tel: 0527 66414; Fax: 0527 585070). PC system, which retrieves the physical and mechanical properties of ductile, gray and malleable, cast irons. Modest price. PM Selector: Structural Powder Metallurgy Materials Selector (1990) MPR Publishing Services Ltd., Old Bank Buildings, Bellstone, Shrewsbury SY1 1HU, UK (Tel: 0743 64675; Fax: 0743 62968). A PC-based selector for powder metallurgical materials for structural use. Modest price. UNSearch: Unified Metals and Alloys Composition Search; ASTM, 1916 Race Street, Philadelphia, PA 19103, USA. PC system which retrieves information about composition, US designation and specification of common metals and alloys. Modest price. CUTDATA: Machining Data System; Metcut Research Associates Inc, Manufacturing Technology Division, 11240 Cornell Park Drive, Cincinnati, Ohio 45242 USA (Tel: 513 489 6688). A PC-based system, which guides the choice of machining conditions: tool materials, geometry, feed rates, cutting speeds, and so forth. Modest price. SteCal: Steel Heat-Treatment Calculations; ASM International, Metals Park, Ohio 44073, USA. (Tel: 216 338 5151; Fax: 216 338 4634). PC formats software, which computes the properties resulting from defined heat treatments of low-alloy steels, using the composition as input. Modest price. STEELMASTER: Schwing UK Ltd., Summerton Road, Oldbury, Warley, West Midlands B69 2KL, UK (Tel: 021 511 1203). PC formats. A database of compositions, properties, trade names and heat treatment procedures for steels. Expensive. ELBASE: Metal Finishing/Surface Treatment Technology (1992) Metal Finishing Information Services Ltd., PO Box 70, Stevenage, Herts SG1 4DF, UK (Tel: 0438 745115). PC formats. Comprehensive information on published data related to surface treatment technology. Regularly updated. Modest price. EASel: Engineering Adhesives Selector Program (1986) The Design Center Bookshop, Haymarket, London SW1Y 4SU, UK. PC and Apple format. A knowledge-based program to select industrial adhesives for joining surfaces. Modest price. PAL: Permabond Adhesives Locator (1990) Permabond, Woodside Road, Eastleigh, Hants SOS 4EX, UK (Tel: 0703 629628; Fax: 0703 629629). A knowledge-based, PC system for adhesive selection among Permabond adhesives. An impressive example of an expert system that works. Modest price. MDP: Network: MDP, 2540 Olentangy River Road, PO Box 02224, Columbus, Ohio, USA (Tel: 614 447 3706). An on-line network of eight linked data sources. Numeric data, plus references abstracts and keywords of publications relating to materials and their uses.

References: J.A. Charles, FAA Crane, (1989), Selection and Use of Engineering Materials, Butter-worths Heinemann. M. F. Ashby, (1992), Materials Selection in Mechanical Design, Pergamon Press.


6

Material Selection 1-5

Recommend Documents