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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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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?

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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?

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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

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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

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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

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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?

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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

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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 Young’s 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

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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: doesn’t 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

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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 can’t 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.

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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.

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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

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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 < σf 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

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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 = E½

r

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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 can’t 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.

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MME131: Lecture 32

Non-Destructive Testing

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