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Chapter 3 Strategic thinking: matching material to design
- Develop a design-led approach to materials selection.
- Briefly review the design process. A market need leads to design requirements and product specifications. Choice of materials and processes evolves in parallel.
- Selection strategy: translation , screening , ranking , documentation. 3.1 Introduction and synopsis Lecture notes version 13 Sep 2011
- Starting point: market need or new idea End point: full product specification
- Critical to have a precise need statement expressed as a set of design requirements.
- Stages: conceptual design (all options are considered) embodiment ( concretisering , most promising concepts are analyzed approximately โ preliminary sizing, selection, performance, and cost) detailed design (detailed specifications, analysis, optimization, final choices, detailed product specification)
- Design can be original (new idea or working principle, often stimulated by new materials with improved properties) or redesign (improving an existing product) driven by failures, poor relative performance, low profit, new โeditionsโ). 3.2 The design process
Figure 3.1 The design flow chart, showing how material and process selection enter. Information about materials is needed at each stage, but at very different levels of breadth and precision. The broken lines suggest the iterative nature of original design and the path followed in redesign. start end
Figure 3.2 A market need โ that of gaining access to wine in corked ( corked? ) bottles โ and three concepts for meeting the need. Devices based on all three of these concepts exist and can be bought. a) Axial traction โ pulling via threaded screw b) Shear traction โ elastic blades c) Pushed from below โ gas pressure via hollow needle Example
- Materials selection enters each stage of the design: approximate in the concept stage, more detailed in the embodiment, and most precise in the detailed design stage.
- We narrow the materials search space by screening out unsuitable choices, ranking the remainder, and selecting the most promising.
- Material selection must be linked to process and final form or shape. Process selection runs parallel to material selection. Process choice is influenced by material, shape, and cost.
- The interaction between material, shape, and process is the core of materials selection. 3.3 Material and process information for design
Figure 3.4 The narrowing of material search space as design constraints are applied.
- Translation : converting design requirements into a prescription for selecting materials. Identify the constraints that must be met and the objectives the design must fulfill (filters).
- Screening : eliminating materials that cannot meet the constraints.
- Ranking : ordering by ability to meet a criterion of excellence (e.g., cost).
- Documentation : final assessment based on factors such as case studies, availability, pricing, environmental impact, etc.
- Process selection follows a parallel route. 3.4 The strategy: translation, screening, ranking, and documentation
- Translation : identify the key functions, constraints, objectives, and free variables of the design problem.
- Function โ what does the component do?
- Constraints โ What non-negotiable target values must be met?
- Objective โ What is to be maximized or minimized?
- Free variables โ What parameters of the problem is the designer free to change?
- Common constraints: target values for stiffness, strength, fracture toughness, thermal conductivity, electrical resistivity, magnetic remanence, optical transparency, cost, mass.
- Common objectives: minimize cost, mass, volume, environmental impact, heat loss; maximize energy storage, heat flow. Table 3.1 Function, constraints, objectives and free variables Table 3.2 Common constraints and objectives
Figure 3.6 The strategy applied to materials. The same strategy is later adapted to select processes. There are four steps: translation, screening, ranking and supporting information. All can be implemented in software, allowing large populations of materials to be investigated.
Figure 3.7 A bicycle. The forks are loaded in bending. Minimize cost Maximize stiffness Maximize strength Minimize mass Importance depends on bicycle type
PI question E 1.9. The cases in which most CDs are sold have an irritating way of cracking and breaking. Which design-limiting property has been neglected in selecting the material of which they are made?
- Fracture toughness
- Hardness
- Strength
- Stiffness
Redesign of a CD case (Table 3.4)
- Current technology: polystyrene (PS); cheap, clear, injection moldable, in principle recyclable; but: crack easily, broken hinges, sharp edges.
- Challenge: keep good qualities and improve the fracture toughness.
- Function โ contain and protect a CD
- Constraints โ optically clear, injection moldable, recyclable, tougher than PS, same dimensions as before.
- Objective โ minimize cost
- Free variables โ choice of material. 3.5 Examples of translation Figure 3.8 A polystyrene CD case. It is cheap, but it is brittle and cracks easily.
HF transformer cores (Table 3.6)
- Uses electromagnetic induction to convert one AC voltage to another. Must be a soft magnet to minimize energy loss and an electrical insulator to avoid eddy current losses at high frequencies.
- Function โ HF transformer core
- Constraints โ soft magnet, electrical insulator, dimensions specified
- Objective โ minimize cost
- Free variables โ choice of material 3.5 Examples of translation Figure 3.10 A transformer. The core must be a soft magnetic material, and if this is a high- frequency transformer, it must be an electrical insulator.
E 3.6. A material is required for the windings of an electric air-furnace capable of temperatures up to 1000 ยบC. 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, the constraints, the most probable objective and the free variables. Exercise Answer
