Chapter One. Introduction (ABET)

Transcription

1 Chapter One Introduction Introduction Materials selection is an important part of a larger process of creating new solutions to problems. This larger process is called Engineering Design Design of engineering components is limited by the available materials, and new designs are made possible by new materials To see how important is the material selection in the design, consider the definition of engineering used by ABET in the U.S.A Engineering is the profession in which knowledge of the mathematical and natural sciences gained by study, experience, and practice is applied with judgement to develop ways to utilise, economically, the materials and forces of nature for the benefit of mankind (ABET) 1

2 Factors Influencing Materials Selection Issues include Why do we need to select a new Material? 1. When the product is new (never produced before), the decision to select a new material is obvious Properties Physical & Mechanical Manufacturing considerations Castability / formability Machinability / Coatability Heat treatment / Weldability Failure & repairability Cost analysis Availability Materials life & maintenance Environmental issues Ergonomic & safety Recycling 2. With existing products (Material Substitution), selecting a new material is due to the following: Reduce material cost Reduce production cost Accommodate some changes in function Solve some material processing problems Take advantage of new materials or processing methods Incorporate failure analysis recommendations (From M. Ashby and D. Cebon) Alumina Si-Carbide Ceramics, glasses Soda-glass Pyrex Polymer foams Metal foams Foams Ceramic foams Glass foams Steels Cast irons Al-alloys Metals Cu-alloys Ni-alloys Ti-alloys GFRP CFRP Composites KFRP Plywood Woods Natural materials Natural fibres: Hemp, Flax, Cotton PE, PP, PC PA (Nylon) Polymers, elastomers Butyl rubber Neoprene? Ceramics? Polymers Engineering Materials Natural Matrs Which Material? Cu alloys? Metals? Composites Steel Al alloys Cast Alloys Wrought Alloys 2000 series 3000 series 4000 series 5000 series 6000 series 7000 series 2

3 Stress σ Stress σ Mechanical properties Ductile materials Elastic limit, σy Young s modulus, E Strain ε Brittle materials Tensile (fracture) strength, σts Young s modulus, E Strain ε General Weight: Density ρ, Mg/m 3 Expense: Mechanical Stiffness: Strength: Cost/kg C m, $/kg Young s modulus E, GPa Elastic limit σ y, MPa Fracture strength: Tensile strength σ ts, MPa Brittleness: Fracture toughness K ic, MPa.m 1/2 Thermal Basic Material Properties Expansion: Expansion coeff. α, 1/K Conduction: Thermal conductivity λ, W/m.K Electrical Conductor? Insulator? (From M. Ashby and D. Cebon) Thermal expansion Thermal strain ε T 1 l o l Expansion coefficient, α Temperature, T Thermal conduction x Area A Heat flux, Q/A T o (T 1 -T 0)/x Q joules/sec Thermal conductivity, λ From M. Ashby and D. Cebon Stiff Strong Tough Light All OK! Not stiff enough (need bigger E) Not strong enough (need bigger σ y ) Not tough enough (need bigger K ic ) Too heavy (need lower ρ) Why do we need Material Selection? Mechanical products to needs design meet creates functional Function The design process is limited by materials capabilities The type of material affects: Casting Process Extrusion Materials Attributes: Physical Mechanical, Economic, Thermal, Electrical, Environmental Processing method Shape attained (available) Sheet Shape 3-D Source: M. F. Ashby, Materials Selection in Mechanical Design 3

4 The Design Process Design is the process of translating a new idea or a marked need into detailed information from which a product can be made (M.F. Ashby) Engineering design is the process of devising a system, component, or process to meet desired needs - e.g vacuum cleaner The design process consists of several steps, each has certain characteristics and certain tasks to be performed: Recognising a need (problem) Defining the problem Establishing specifications Gathering information The Design Process Types of Design Generating concept solutions Material selection usually occurs at this stage 1. Original Design New idea (from scratch) Analysis and optimisation Preliminary layout of design Detail design Evaluation Presenting results Approval and release for manufacturing 2. Development Design Improvements of existing product Cars, consumer products (mobile phones, cameras etc ) 3. Variant Design Change in scale or dimensions without changing the function e.g; desktop PC to laptop computer 4

5 Mechanical Engineering Materials Engineering Define specification Determine function structure Seek working principles Evaluate and select concepts Increasing complexity Increasing details Develop layout, scale, form Model and analyze assemblies Optimize and functions Evaluate and select layouts Analyze components in details Final choice of material and process Optimize performance and cost Prepare details drawings Source: M. F. Ashby, Materials Selection in Mechanical Design MATERIALS DATA IN THE DESIGN PROCESS (Concept) ALL MATERIALS ROUGH DATA METALS, CERAMICS, POLYMERS, COMPOSITES (Embodiment) SHORT LIST STEEL, ALUMINIUM, TITANIUM, COPPER (Detail) SPECIFIC MATERIALS ACCURATE DATA Cast Al alloy 356 T6 condition At each stage of the design, decisions need to be made about what to make it out of (Materials), and how to make it (Processes and Shapes) Casting, Moulding PROCESSES Powder Methods, Machining Metals, Ceramics MATERIALS Composites, Polymers Flat and Sheet SHAPES Prismatic, 3-D 5

6 1. The clarification of the need involves analysing and clearly stating the problem In this stage, all information and previous knowledge are put together so that a proper decision can be made The designer collects and writes down all: Requirements Constraints The standards to be adhered to The projected date of completion of the design 2. The objective of the conceptual design is to generate possible solutions, methods to solve the problem All ideas are accepted and evaluated according to their merits Information and knowledge include: Engineering science principles Previous experiences Production methods Costs The output of this stage should be two or three solutions that have the greatest chance of achieving the desired objective 3. Embodiment design embodies and evaluate the two or three conceptual solutions selected in greater detail and makes a final choice The output of this stage should be drawings, specifications and compliance to the needs of specifications of the product 4. The detail design stage considers all the large number of small but important details to manufacture the product The work done at this stage must be of very high quality, otherwise delays, higher costs and failure may occur The output of this stage is a set of: Very detailed drawings and Final specifications including: tolerance, precision, joining methods, finishing When selecting materials, current engineering designers choose to meet three general criteria: 1. Property profile Mechanical properties factor Life of component factors 2. Processing profile Physical factors Processing factors Cost / availability 3. Environment Codes, statutory and other factors 6

7 The selection of the material is done with all 3 criteria considered at the same time with the PERFORMANCE & COST as the most important factor 1. Properties profile Material selection based on properties profile is the process of matching the numerical values of the properties of the material to the requirements and constraints 2. Processing profile: Material selection based on processing profile is aimed at identifying the process that will form the material into the desired final shape including joining and finishing at the minimum cost 3. Environmental profile: Relates to the impact of the material (its manufacture, use, reuse and disposal) on the environment. This added constraint increases the cost of the product Designing for the environment is an excellent philosophy because it can be a good marketing tool to environment conscious customers Assessment of the impact of materials on the environment is done by several approaches. The most popular is the life-cycle analysis or LCA (chap. 4.1) Selecting a material is a difficult task due to: 1. Large number of materials available 2. Large number of processes available 3. Complex relationship between selection parameters The problem of material selection is further made difficult by: 1. Insufficient and inaccurate property data 2. Multiple constraints To realise the full potential of a new material the product must be redesigned to exploit both the properties and the manufacturing characteristics of the material. The essence of materials selection process is not that one material compete against another, rather, it is that the processes associated with the production or fabrication of one material compete with the processes associated with the other. 7

8 Several quantitative selection procedures have been developed to analyse the large amount of data in the selection process No single correct solution: Material selection requires 1. Consideration of conflicting advantages & limitations 2. Compromise and trade-off Similar parts which perform similar functions are produced by different manufacturers, from different materials and different manufacturing processes Initial Screening of Materials (new product) Answer questions on the nature of product, its function etc. Need to specify Performance Requirements Outline Materials Performance and Processing Requirements Certain materials will be eliminated but short-list of candidates is obtained 1. Analysis of Materials Performance Requirements Can be divided into five categories: 1. Functional Requirements (Materials Properties) 2. Processability Requirements 3. Cost & Availability 4. Reliability 5. Resistance to Service Conditions (environment) 2. Classification of Materials Requirements Materials Properties can be: Some of the methods used to classify the functional requirements include: 1. Limit on Materials Properties Rigid or go no go requirements (non-negotiable) negotiable) Soft requirements (negotiable) 2. Cost per Unit Property Quantitative or Qualitative Essential (strength, toughness) or Desirable (processability, weight, corrosion, reliability) 1. C = cost 3. Ashby s Method (selection charts) 8

9 3. Development of alternative solutions Search for the material(s) that best meet the requirement selected in the previous stage Selection from alternative solutions requires decisions. Experience and judgement are sufficient to permit a sound decision without a formal process 1. This is true when a candidate material possess some outstanding characteristics or all except one has some serious deficiency 2. When there is no obvious choice, a formal process or procedure can be used. At the end of this stage, the number of candidate materials is narrowed down Materials Selection Chart 4. Evaluation of alternative materials Having narrowed down the number of possible materials to those that satisfy the rigid requirement, the search starts for the material(s) that best meet the soft requirements Several quantitative methods can be used to further narrow the field of possible materials and matching processes to a few optimum candidates. Each candidate material must at least minimally satisfy all the requirements. However, it is also more likely that each material will possess some characteristics that exceed the minimum requirements. Therefore, a formal method for rating how well each material meets the requirements is needed because some requirements are more important than others. One of these methods is the weighted Properties Method Greater weight should be given to the more important requirement. Often, the properties we are comparing cannot be placed on comparable terms. 9

10 Another example. Weight is 4 times as important as strength, strength is 4 times as important as cost, corrosion is 2 /3 the importance of strength, etc Weighting of attributes Need to add, 2/3, 2/4, 2/5, ¾, 3/5, 4/5 Property 1/2 1/3 1/4 1/5 ratio weight Strength Density 2 80 Corrosion 3 40 Colour 4 50 Cost 5 20 Total Need to calculate according to complete, N=5(5-1)/2 = Decisions Selecting a material from among alternatives requires decisions Sometimes experience and judgement are sufficient to make a sound decision without a formal process. This is true when a candidate material possesses outstanding characteristics, or All except one material has serious deficiency 10