The outcome of the steps so far is a ranked short-list of candidates that meet the constraints and that maximise or minimise the criterion of excellence, whichever is required. You could just choose the top-ranked candidate, but what hidden weaknesses might it have? What is its reputation? Has it a good track record? To proceed further we seek a detailed profile of each: its documentation (Figure 3.6, bottom).
What form does documentation take? Typically, it is descriptive, graphical or pictorial: case studies of previous use of the material, details of its corrosion behaviour in particular environments, of its availability and pricing, warnings of its environmental impact or toxicity, or sensitivity in some of its properties to the way it is processed. Such information is found in handbooks, suppliers’ data sheets, CD-based data sources and high-quality websites. Documentation helps narrow the short-list to a final choice, allowing a definitive match to be made between design requirements and material and process attributes.
Why are all these steps necessary? Without screening and ranking, the candidate pool is enormous and the volume of documentation is overwhelming. Dipping into it, hoping to stumble on a good material, gets you nowhere. But once a small number of potential candidates have been identified by the screening-ranking steps, detailed documentation can be sought for these few alone, and the task becomes viable.
3.5 Examples of translation
The following examples illustrate the translation step for a number of problems, starting with the lever for the corkscrew.
E x a m p l e 3 . 1 A c o r k s c r e w l e v e r
Figure 3.3 shows the lever for one of the corkscrews in the design case study. In use it is loaded in bending. It must carry the bending moment without deflecting to an awkward degree, which means a high modulus, E. It must not bend permanently (though some cheap corkscrews do), which means a high yield strength, σy. And it must not snap off altogether, which means it must have adequate fracture toughness, K1c. Finally, it must not corrode in wine or water. The length of the lever is specified, but the cross-section is not—we are free to choose a section that is sufficient to bear the use-loads. Given all these, the lever should be as cheap as possible.
Formulate the translation.
Answer
Table 3.3 lists the translation.
Table 3.3
Translation for the corkscrew lever
Function • Lever (beam loaded in bending)
Constraints • Stiff enough
• Strong enough
• Some toughness
• Resist corrosion in wine and water
Functional constraints
• Length L specified A geometric constraint
Objective • Minimise cost
Free variable • Choice of material
• Choice of cross-section area
The design-limiting properties are those directly relating to the constraints: modulus , strength , fracture toughness and corrosion resistance.
E x a m p l e 3 . 2 R e d e s i g n o f a C D c a s e
Music lovers will affirm that CDs—the best of them—are divine. But the cases they come in are the work of the devil (Figure 3.7). They are called ‘jewel’ cases for reasons of their optical clarity, but in performance they are far from jewels. They are usually made of polystyrene, PS, chosen for its low cost and water-clear transparency;
and they are made by injection moulding and that, too, is cheap if you are making millions. Polystyrene can, at least in principle, be recycled. But PS jewel cases crack easily, jam shut, the hinges break, and the corners of the case are hard and sharp enough to inflict terminal damage on a CD. They badly need redesign. Decide on the features you think really matter, and formulate constraints, objective and free variables for the redesign of a CD case.
FIGURE 3.7 A polystyrene CD case. It is cheap, but it is brittle and cracks easily.
Answer
The way to tackle redesign is to seek a replacement material that retains the good properties of the old one, but without the bad. Thus we seek a material that is optically transparent to allow the label to be read, is able to be injection moulded because this is the most economic way to make large numbers, and is recyclable. But it must be tougher than polystyrene. Of the materials that meet these constraints, we want the cheapest. Table 3.4 summarises the translation.
Table 3.4
Translation for the redesigned CD case
Function • Contain and protect a CD Constraints • Optically clear
• Able to be injection moulded
• Recyclable
• Tougher than polystyrene
Functional constraints
• Dimensions identical with PS case A geometric constraint Objective • Minimise cost
Free variable • Choice of material
Potential design-limiting properties are optical transparency, fracture toughness (must be better than PS) and the ability to be injection moulded and recycled.
E x a m p l e 3 . 3 H e a t s i n k s f o r m i c r o c h i p s
A microchip may only consume milliwatts, but this power is dissipated in a tiny volume, making the power- density high. As chips shrink and clock speeds grow, overheating becomes a problem. The chips of today’s PCs require forced cooling to limit temperatures to 85 °C. Multiple-chip modules (MCMs) pack as many as 130 chips on to a single substrate, and they get even hotter—up to 180 °C. Heating is kept under control by attaching the chips to a heat sink (Figure 3.8), taking pains to ensure good thermal contact between chip and sink. The heat sink is a critical component, limiting further miniaturisation of the electronics. How can its performance be maximised?
FIGURE 3.8 A heat sink. It must conduct heat well, but be electrically insulating.
To prevent electrical coupling and stray capacitance between chip and heat sink, the heat sink must be a good electrical insulator. If it is to work with one surface at 180 °C, it must have a maximum service temperature (the temperature at which it can operate continuously without damage) that is at least as great as 180 °C. These define the constraints. To drain heat away from the chip as fast as possible, it must also have the highest possible thermal conductivity, , defining the objective. Formulate the translation.
Answer
The translation step is summarised in Table 3.5, where we assume that all dimensions are constrained by other aspects of the design.
Table 3.5
Translation for the heat sink
Function • Heat sink
Constraints • Material must be good electrical insulator
• Maximum operating temperature > 200 °C Functional constraints
• All dimensions are specified Geometric constraints
Objective • Maximise thermal conductivity
Free variable • Choice of material
The design-limiting properties, clearly, are maximum service temperature , electrical resistivity and thermal conductivity .
E x a m p l e 3 . 4 H F t r a n s f o r m e r c o r e s
An electrical transformer uses electromagnetic induction to convert one AC voltage to another (Figure 3.9). To minimise energy loss the material must be a soft magnet—one that is easy to magnetise and de-magnetise (Chapter 14). And to avoid eddy current losses at high frequencies it must also be an electrical insulator. The constraints of ‘soft magnetic material’ and ‘electrical insulator’ are very restrictive—they will screen out all but a small number of candidates. If the transformer is for an everyday product, the objective would be to minimise the cost. Formulate the translation.
FIGURE 3.9 A transformer. The core must be a soft magnetic material, and if this is a high-frequency transformer, it must be an electrical insulator.
Answer
Table 3.6 lists the translation.
Table 3.6
Translation for the transformer core
Function • HF transformer core
Constraints • Soft magnetic material
• Electrical insulator Functional constraints
• All dimensions are specified Geometric constraints
Objective • Minimise cost
Free variable • Choice of material
These translations are the first step in selection. In them we have identified the constraints; they will be used for screening. We have also identified the objective; it will be used for ranking. We will return to all four of these examples in later chapters when we know how to screen and rank.
3.6 Summary and conclusions
The starting point of a design is a market need captured in a set of design requirements. Concepts for a product that meet the need are devised. If initial estimates and exploration of alternatives suggest that the concept is viable, the design proceeds to the embodiment stage: working principles are selected, size and layout are decided and initial estimates of performance and cost are made. If the outcome is successful, the designer proceeds to the detailed design stage:
optimisation of performance, full analysis of critical components and preparation of detailed production drawings (usually as a CAD file), showing dimensions, specifying precision and identifying material and manufacturing path. But design is not a linear process, as Figure 3.1 might suggest. Some routes lead to a dead end, requiring reiteration of earlier steps. And, frequently, the task is one of redesign, requiring that constraints be rethought and objectives realigned.
The selection of material and process runs parallel to this set of stages. Initially the search space for both is wide, encompassing all possible candidates. As the design requirements are formulated in increasing detail, constraints emerge that both must meet, and one or more objectives is formulated. The constraints narrow the search space and the objective(s) allow ranking of those that remain. Identifying the constraints, the objectives and the free variables (the process we called ‘translation’) is the first step in selection. This chapter ended with examples of translation when the task was that of choosing a material; the exercises suggest more. When the task is the choice of process, a similar translation is needed; we return to this in Chapter 18. The other steps—screening, ranking and documentation—are discussed in the chapters that follow.
3.7 Further reading
1. Ashby MF. Materials Selection in Mechanical Design. 4th ed. Oxford, UK: Butterworth Heinemann; 2011; ISBN 978- 1-85617-663-7.
(An advanced text developing material selection methods in detail.)
2. Cross N. Engineering Design Methods. 3rd ed. Chichester, UK: Wiley; 2000; ISBN 0-471-87250-3.
(A durable text describing the design process, with emphasis on developing and evaluating alternative solutions.) 3. Pahl G, Beitz W, Feldhusen J, Grote, K. H. Engineering Design: A Systematic Approach. Translated by K. Wallace and
L. Blessing 3rd ed. London, UK: The Design Council, and Berlin, Germany: Springer Verlag; 2007; ISBN 978-1-84- 628318-5.
(The Bible—or perhaps more exactly the Old Testament—of the technical design field, developing formal methods in the rigorous German tradition.)
4. Ullman DG. The Mechanical Design Process. New York, USA: McGraw-Hill; 1992; ISBN 0-07-065739-4.
(An American view of design, developing ways in which an initially ill-defined problem is tackled in a series of steps, much in the way suggested by Figure 3.1.)
5. Ulrich KT. Design: Creation of Artefacts in Society. Philadelphia, USA: University of Pennsylvania Press; 2011; ISBN 978-0-9836487-0-3.
(An excellent short introduction to the kind of structured reasoning that lies behind good product design. The text is available on http://www.ulrichbook.org/)
6. Ulrich KT, Eppinger SD. Product Design and Development. 4th ed. New York, USA: McGraw Hill; 2008; ISBN 978- 007-125947-7.
(A readable, comprehensible text on product design, as taught at MIT. Many helpful examples but almost no mention of materials.)
3.8 Exercises
Exercise
E3.1 What are the steps in developing an original design?
Exercise E3.2
What is meant by an objective and what by a constraint in the requirements for a design? How do they differ?
Exercise
E3.3 Describe and illustrate the ‘translation’ step of the material selection strategy.
Exercise E3.4
Translation (1). The teeth of a scoop for a digger truck, pictured in Example 1.1 of Chapter 1, must cut earth, scoop stones, crunch rock, often in the presence of fresh or salt water and worse. Translate these requirements into a prescription of Function, Constraints, Objectives and Free variables.
Exercise
E3.5 Translation (2). A material for an energy-efficient saucepan, pictured in Example 1.2 of Chapter 1, must transmit and spread the heat well, resist corrosion by foods, and withstand the mechanical and thermal loads expected in normal use. The product itself must be competitive in a crowded market. Translate these requirements into a prescription of Function, Constraints, Objectives and Free variables.
Exercise
E3.6 Translation (3). A material for eyeglass lenses, pictured in Example 1.3 of Chapter 1, must have optical-quality transparency. The lens may be moulded or ground with precision to the required prescription. It must resist sweat and be sufficiently scratch- resistant to cope with normal handling. The mass-market end of the eyeglass business is very competitive so price is an issue.
Translate these requirements into a prescription of Function, Constraints, Objectives and Free variables.
Exercise
E3.7 Bikes come in many forms, each aimed at a particular sector of the market:
• Sprint bikes.
• Touring bikes.
• Mountain bikes.
• Shopping bikes.
• Children’s bikes.
• Folding bikes.
Use your judgment to identify the primary objective and the constraints that must be met for each of these.
Exercise
E3.8 A material is required for the windings of an electric air furnace capable of operating at temperatures up to 1000 °C. Think out what attributes a material must have if it is to be made into windings and function properly in a furnace. List the function and the constraints; set the objective to ‘minimise material price’ and the free variables to ‘choice of material’.
Exercise
E3.9 A material is required to manufacture office scissors. Paper is an abrasive material, and scissors sometimes encounter hard obstacles like staples. List function and constraints; set the objective to ‘minimise material price’ and the free variables to
‘choice of material’.
Exercise E3.10
A material is required for a heat exchanger to extract heat from geo-thermally heated saline water at 120 °C (and thus under pressure). List function and constraints; set the objective to ‘minimise material price’ and the free variables to ‘choice of material’.
Exercise
E3.11 A material is required for a disposable fork for a fast-food chain. List the objective and the constraints that you would see as important in this application.
Exercise
E3.12 Formulate the constraints and objective you would associate with the choice of material to make the forks of a racing bicycle.
Exercise
E3.13 Cheap coat hangers used to be made of wood, but now only expensive ones use this material. Most coat hangers are now metal or plastic, and both differ in shape from the wooden ones, and from each other. Examine wood, metal and plastic coat hangers, comparing the designs, and comment on the ways in which the choice of material has influenced them.
Exercise
E3.14 Cyclists carry water in bottles that slot into bottle holders on their bicycles. Examine metal and plastic bottle holders, comparing the designs, and comment on the ways in which the choice of material has influenced them.
3.9 Exploring design using CES
Exercise
E3.15 A company wishes to enhance its image by replacing oil-based plastics in its products with polymers based on natural materials.
Use the ‘Search’ facility in CES to find biopolymers. List the materials you find.
Exercise
E3.16 A maker of garden furniture is concerned that the competition is stealing part of his market with furniture made by RTM, a term with which he is unfamiliar. Use the ‘Search’ facility in CES to find out what RTM is, and whether it is used to make things like garden furniture.
Exercise
E3.17 Use the ‘Search’ facility in CES Level 2 to find materials for furnace windings.
Exercise E3.18
Use the ‘Search’ facility in CES Level 2 to find materials for scissors and knife blades.
Exercise
E3.19 Use the ‘Search’ facility in CES Level 2 to find materials for heat exchangers.
Exercise
E3.20 Use the ‘Search’ facility in CES Level 2 to find materials for flooring.
Exercise
E3.21 Eyeglass lenses require a material with optical-quality transparency that can be moulded with precision and will resist sweat. A high refractive index (greater than 1.5, say) allows thinner, lighter lenses. Open CES Edu Materials Level 2, choose the data subset Materials with Durability properties, and open a Limit stage. Open Optical properties and select Optical quality and set a lower limit of 1.5 for the refractive index. Open Processability and put a lower limit of 5 (the best possible) on Mouldability.
Open Durability: water and aqueous environments and click on Excellent in Fresh water and in Salt water (sweat). Finally, click Apply. What is the selected short list? (It appears in the lower left of the CES user-interface.)