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Lec 31, Page 1/18 MME131: Lecture 31 Materials Selection Prof. A.K.M.B. Rashid Department of MME BUET, Dhaka Topics to discuss … The design process General problems in materials selection How do we select materials? Materials selection charts Steps in materials selection Setting up the problem Boundary conditions Performance and materials indices Case studies

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lecture 31 MME 131 course of AKM bazlur rashid sir

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  • Lec 31, Page 1/18

    MME131: Lecture 31

    Materials Selection

    Prof. A.K.M.B. Rashid Department of MME BUET, Dhaka

    Topics to discuss The design process

    General problems in materials selection

    How do we select materials? Materials selection charts

    Steps in materials selection Setting up the problem Boundary conditions Performance and materials indices

    Case studies

  • Lec 31, Page 2/18

    Introduction

    The selection of materials is an extremely important part of engineering design.

    In many technological fields, the design of engineering components and structures is limited by the available

    materials.

    The 20th Century has been a period of unprecedented evolution of materials. This will

    continue in the 21st Century.

    New materials enable new designs.

    Design is....

    ...the process of translating a new idea or a market need into detailed information from which

    a product can be manufactured.

    invention Design innovation

    What is a design?

  • Lec 31, Page 3/18

    Market pull vs. Technology push

    e.g., cellular phone, high capacity hard drives

    e.g., Teflon, amorphous metals, quasicrystals

    Mechanical Design

    Industrial Design

    Original Design new idea or working principle

    e.g. CD vs Tape

    Types of design

    Adaptive or Development Design takes existing product and seeks an incremental advance

    in performance through a refinement in working principle

    e.g. beverage cans, automobiles,

    Variant Design change in scale/dimension without change of function

    e.g. desktop to laptop computer

    Question: Microsoft Windows 2007 What type of design is this?

  • Lec 31, Page 4/18

    Design problems are open ended - no single correct answer

    Design is an iterative process

    Products are technical systems composed of assemblies and components

    Vocabulary of design

    Must formulate a need statement:

    a device is requested to perform task x

    solution is neutral

    Design and production of Boeing 767

  • Lec 31, Page 5/18

    Market Assessment

    Specification

    Concept Design

    Detail Design

    Manufacture

    Sell

    THE

    DESIGN

    CORE

    The design flow chart

    Concept

    Embodiment

    Detail

    Product Specification

    Market Need

    Define specification Determine function structure Seek working principles Evaluate and select concepts

    Develop layout, scale, form Model and analyse assemblies Optimise the functions Evaluate and select layout

    Analyse components in detail Select processing route Optimise performance and cost Prepare detailed drawing

    Iterate

  • Lec 31, Page 6/18

    Concept

    Embodiment

    Detail

    Product Specification

    Market Need

    Design Tools

    Function modeling

    Viability studies

    Geometric analysis

    Simulation

    Optimisation methods

    Cost modeling

    Component modeling

    Finite element analysis

    Materials Selection

    All materials (low precision)

    Subset of materials (high precision)

    One material (best available data)

    Design Tools and Materials Data

    Engineering design uses the methods and tools of engineers and materials scientists Different tools are required at different stages of the design process

    Metals, ceramics, glasses

    MATERIALS polymers

    composites... Casting , moulding

    PROCESSES powder methods,

    machining...

    Flat and dished sheet

    SHAPES prismatic

    3-D

    At each materials selection stage, decisions need to be made about what (material) is to be used to make the product, and how to make it (process and shape)

    General problems in materials selection

  • Lec 31, Page 7/18

    The Materials kingdom

    MATERIALS DATA IN THE DESIGN PROCESS

    ALL MATERIALS SHORT LIST SPECIFIC MATERIALS

    Rough Data Accurate dataExample:

    Metals/Ceramics/Polymers/Composites

    Example:Steel/Titanium/

    Aluminium

    Example:Aluminium Alloy 2024

    Heat Treatment T6 or T4

    Method #1: Data browsing Find material data by reference books, software and/or internet

    How do we select materials?

  • Lec 31, Page 8/18

    Method #2: Property bar chart

    Plot property data as bar charts showing range of properties for a given material

    Method #3: Materials selection chart

    For designs requiring the optimization of two (or more) attributes, plot one property versus the another

    Ashby Charts

  • Lec 31, Page 9/18

    Can be qualitative (or, yes/no) material must be transparent

    Often quantitative material must be able to operate above 1000oC material must be less dense than water material must have a Youngs modulus > 100 GPa

    Can use Property Charts to find subset of

    materials that satisfy the screening criteria

    Steps in materials selection First Step: Screening

    By applying primary constraints

    Screen materials to reduce the number of candidates a subset is formed

    Second Step: Comparison of materials

    How can we compare two material properties ? Are we going to compare apples with oranges ?!?!?

    Problem: How do we quantitatively compare materials within the active subset ?

    which maximise the performance

    Need More Information!

    A more formal approach, employing performance indices and constraints (with a dose of common sense!), is required

  • Lec 31, Page 10/18

    Setting up the problem

    Consider the need statement e.g., device is required to allow access to juice in a corked bottle

    Concept stage

    Solution neutral: doesnt specify how to do it

    Sets the playing field: what are the basic constraints (only considering juice bottles, only considering corked bottles, only considering bottles,.)

    Develop concepts based on need statement (e.g. screw, shear, pressure,.)

    Once we have chosen a single concept, to develop further, we need to re-examine in terms of

    What does the component do?

    What do we want to maximize/minimize?

    What are our constraints?

    Function

    Objectives

    Constraints

  • Lec 31, Page 11/18

    Boundary conditions of materials selection

    Function: support a load, contain a pressure, transmit heat,

    Objective: to make it as cheap as possible, to make it as light as possible, to make it as strong as possible,

    Constraints: length is fixed, component must carry a certain load, component must operate above a certain temperature,

    Free Variables: materials choice (always for us!), cross sectional area is free, cross sectional shape is free,

    Hard Constraints (non negotiable): component MUST meet the specification

    often mechanical in nature; e.g. beam must support the weight of a 300 lb person.

    If a design cant meet the soft constraints, no one dies !!

    Soft Constraints (negotiable): Component MAY meet the specification

    often aesthetic or cost related; e.g. beam should cost less than $50.

  • Lec 31, Page 12/18

    Performance and Materials Indices

    The Material Index (M) is a combination of material properties which characterizes the performance of a material in a given application

    Structural members perform a function We want to know what materials optimize the performance of the member

    When M maximises, performance is also maximised

    Design of structural element specified by:

    Functional Requirements, F

    Geometric Parameters, G

    Material Properties, M

    Thus, the performance index, P, of a structural element can be written mathematically as

    P = f (F, G, M) eq.(1)

    Aspects of the performance of a component can be described by its functional requirements, geometry and material properties.

  • Lec 31, Page 13/18

    Optimum choice of material is often independent of geometry and functional requirements.

    So, we can select optimum materials independent of the details of F and G.

    If F, G and M are independent to one another then we can re-write eq.(1) as:

    P = f1(F) f2(G) f3(M) eq.(2)

    Thus, the equation for performance index P becomes,

    P = f (M) eq.(3)

    Step 0: Initial screening

    Step 1: Identify function, constraints, objective and free variable(s)

    Step 2: Write down equation for objective performance equation

    Step 3: If performance equation contains a free variable identify the constraint that limits it

    Step 4: Use this constraint to eliminate the free variable in the performance equation

    Step 5: Read off the combination of properties that maximize performance

    The 5-Step Program to Material Selection

  • Lec 31, Page 14/18

    Case Study 1: Deriving Materials Index for a Light, Strong Tie Rod

    Strong structural member of minimum mass loaded in tension

    1. Function: Tie rod

    2. Objective: Minimise mass

    m = (AL)r eq.(1)

    4. Free variable: Material choice Cross-sectional area (A)

    3. Constraints: Length (L) is specified Must not fail under load F Must be tough

    Constraint on area:

    F/A < f eq.(2)

    Combine eq.(1)

    & eq.(2) to give

    m = FL r sf

    M = sf r

    Best material minimum m, maximum M

    Use of Ashby charts to select material:

    Charts plotted on log-log scale.

    Materials falling on this line give equal performance

    Rearranging the materials index as:

    M = (sf /r)

    log M = log sf log r

    log sf = log r + log M

  • Lec 31, Page 15/18

    Case Study 2: Deriving Materials Index for a Stiff, Light Panel

    2

    2

    L

    EInFbuckling

    4/4/ 24 ArI

    rALm

    24

    2

    2

    22

    44 r

    L

    Emn

    L

    EAnFbuckling

    En

    LFm

    r

    4

    2

    f1(F) f2(G) f3(M)

    Search Region

    Case Study 3: Deriving Materials Index for a light, stiff column (circular)

    So, to minimize mass m,

    maximise M = E

    r

  • Lec 31, Page 16/18

    How do we make materials selection in a rational manner?

    Design requirements are translated into a prescription for selecting a material by analysing (i) the function of the component, (ii) the constraints must meet, and (iii) the objective of the design.

    Simple constraints are applied as limits on material attributes, screening out materials that cant do the job.

    Constraints that limit objectives must be combined with the objective to identify a material index.

    The objective is best displayed on a material-property chart, allowing optimised selection

    Summary Design is iterative.

    Starts with the market pull (or, technological push).

    First describe market need with need statement.

    Concept Stage: ideas stage, test ideas.

    Embodiment Stage: working principles selected, size and layout decided on estimates of cost and dimensions.

    Detailed Design Stage: full analysis of all components, production methods.

    Materials Selection enters at all stages. At the end an optimum selection must be made.

  • Lec 31, Page 17/18

  • Lec 31, Page 18/18

    MME131: Lecture 32

    Non-Destructive Testing

    Prof. A.K.M.B. Rashid Department of MME BUET, Dhaka