Materials Selection for Product Design: A Strategic Approach, Study notes of Design

Download Materials Selection for Product Design: A Strategic Approach and more Study notes Design in PDF only on Docsity! 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 – 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 ‘editions’). 3.2 The design process Figure 3.2 A market need — that of gaining access to wine in corked (corked?) bottles — and three concepts for meeting the need. Devices based on all three of these concepts exist and can be bought. a) Axial traction – pulling via threaded screw b) Shear traction – elastic blades c) Pushed from below – gas pressure via hollow needle Example Figure 3.3 Embodiment sketches for the first concept: direct pull, levered pull, geared pull and spring- assisted pull. Each system is made up of components that perform a sub-function. Detailed design drawings for the lever of embodiment (b) are shown on the right. • 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 • 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 – what does the component do? • Constraints – What non-negotiable target values must be met? • Objective – What is to be maximized or minimized? • Free variables – 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 • Screening: a gate (meet the constraint and you pass through); attributes that candidates must meet are called attribute limits. • Ranking the survivors of the screening step requires criteria of excellence (material indices). Performance is sometimes limited by a single property, sometimes by a combination. • The property or property group that maximizes performance for a given design is called its material index. Examples: best material for a light tie-rod has maximum E/ρ, best for a spring has maximum σy 2/E (as we will see later) • Documentation: final assessment based on factors such as case studies, track record, availability, pricing, environmental impact, etc. Corkscrew lever (Table 3.3) • Function – lever (beam loaded in bending) • Constraints – functional: stiff enough, strong enough, some toughness, corrosion resistant; geometric: length L specified. • Objective – minimize cost • Free variables – choice of materials, choice of cross sectional area. • The design limiting properties are those directly related to the constraints. 3.5 Examples of translation 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? 1. Fracture toughness 2. Hardness 3. Strength 4. 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 – contain and protect a CD • Constraints – optically clear, injection moldable, recyclable, tougher than PS, same dimensions as before. • Objective – minimize cost • Free variables – 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.