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A. K. M. B. Rashid Professor, Department of MME

BUET, Dhaka

MME445: Lecture 19

Materials Selection – The Basis

Learning Objectives

Knowledge &

Understanding Understanding the design process and the role of material on it

Skills & Abilities Ability to translate resign requirements into constraints on material

properties

Values &

Attitudes

Appreciation of design-led decision-making and systematic

selection strategy

Resources • M F Ashby, Materials Selection in Mechanical Design, 4th Ed., Ch. 05

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Outline of this lecture

Introduction

The selection strategy

translation - screening - ranking – documentation

Introduction and synopsis

• A material has attributes: density, strength, cost, resistance to corrosion, …

• A design demands a certain profile of these: a low density, a high strength, a modest

cost, good resistance to sea water, …

In this lecture, we set out the basic procedure for

selection, establishing the link between material

and function.

The task of selection is that of

1. identifying the desired attribute profile

2. comparing this profile with those of real engineering materials to find the best match

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The first step in the selection process is translation

examination of the design requirements and express them as constraints

that they impose on material choice and objectives to meet

The immensely wide choice is narrowed first

by screening out the materials that cannot meet the constraints

Further narrowing is achieved by ranking the survivor candidates

using the objectives by their ability to maximize performance

The material property charts can be used with these criteria

constraints and objectives can be plotted on them, isolating the subset

of materials that are the best choice for the design

The whole procedure can be implemented using software

as a design tool, allowing computer-aided selection

the procedure is fast and makes for lateral (“what if...?”) thinking

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Product specification

Concept

Embodiment

Detail

Market need

Problem statement

Review of the Design Process

Material data needs

Data for material family (metals, ceramics, polymers..)

Data for material class (Steel, Al-alloy, Ni-alloy…..)

Data for single material (Al-2040, Al-6061, Al-7075…..)

design flow chart

Concepts Need

Embodiments

Direct pull Levered pull Spring-assisted pull Geared pull

Need – Concept – Embodiment

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Detail Embodiment

How are those choices made?

Embodiment in detail

Design requirements:

expressed as

Constraints and

Objectives

Data:

Material attributes

Process attributes

Documentation

Final selection

Comparison engine

Screening

Ranking

Documentation

Density

Price

Modulus

Strength

Durability

Process compatibility

More…….

Able to be molded

Water and UV resistant

Stiff enough

Strong enough

As cheap as possible

(As light as possible)

The Selection Strategy: Materials

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The selection strategy

1. Translation: use design equations, including materials related equation

to maximise or minimise objective function to develop an

expression which consists of materials properties, functional

properties and design variables for the component

2. Screening: set minimum or maximum values on properties which all

candidates must meet and eliminate materials that cannot

3. Rank: use the materials selection charts to narrow the choices

down to a few candidates that do the job best

4. Documentation: Detailed information of the top-ranked candidates

to select one material

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Translation – Defining boundary conditions

(a) Function: what does the component do?

(b) Constraints: dictated by design

• What specific requirements must be met (hard constraints) e.g., stiffness, strength, dimensions, thermal conductivity …

• Are there other constraints that are desirable, but not compulsory, to fulfill (soft constraints)

e.g., cost, finish, colour …..

(c) Objective: what is to be minimised or maximised e.g., mass, dimension, cost, or a combination of these (some of which may be in conflict)

(d) Free variable(s): what is the designer free to change e.g., material, dimension, colour

1. Define design requirements

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(a) List the constraints of the problem e.g., no buckling, high stiffness, no yielding, fracture toughness

(b) Develop constraint equation(s) this must be satisfied in the design

2. Developing constraint equation(s)

(a) Develop equation(s) for the design objective in terms of functional requirements, geometry and materials properties

(b) One such equations is a objective equation which indicates

the quantity that must be maximised or minimised e.g., mass: m = ρ A L

3. Developing objective function(s)

which are related to the design objectives e.g., area

4. Defining and isolating free variables

from the objective equation(s) into the constraint equation(s)

5. Substituting free variables

by grouping the variables into three groups:

(1) functional requirements (F)

(2) geometry (G)

(3) materials (M)

minimise: P ≤ f1(F), f2(G), f3(M)

maximise: P ≥ f1(F), f2(G), f3(M)

6. Developing performance metric, P

using materials index (M) with the help of materials selection charts

7. Maximising / minimising performance metric, P

At this point in time, all possible materials are candidates to fulfill the needs of the application

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Example of Translation

Identifying Constraints

Identifying Objectives

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Screening – Applying attribute limits

All designs have non-negotiable limits which all candidate materials

must meet in order to be considered

e.g. electrical resistivity, transparency, yield strength, etc.

Screening allows us to eliminate materials that do not meet

these requirements (a.k.a. attribute limits)

E ≥ 10 GPa (stiff)

r < 3000 kg/m3 (light)

KIC ≥ 15 MPa·m1⁄2 (tough)

ρelect ≤ 105 Ω·m (good conductor)

An unbiased selection requires that all materials be considered

candidates until shown to be otherwise

E > 10 GPa

Search Area

Modulus – Density Chart showing lower limit for modulus and upper limit for density

r < 3000 kg/m3

Attribute Limits

E ≥ 10 GPa (stiff)

r < 3000 kg/m3 (light)

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One should not be too hasty in applying attribute limits

• it may be possible to engineer a route around them

• a component that gets too hot can be cooled ;

one that corrodes can be coated with a protective film

Many designers apply attribute limits for fracture toughness, K1C, and ductility, εf,

insisting on materials with, as rules of thumb

K1C > 15 MPa.m1/2 and εf > 2%

in order to guarantee adequate tolerance to stress concentrations

By doing this they eliminate materials that the more innovative designer is

able to use to good purpose

• the limits just cited for K1C and εf eliminate all polymers and all ceramics,

a rash step too early in the design

At this beginning stage, it is wise to keep as many options open as possible

Ranking – Materials indices

Materials indices and the materials selection charts are used to select

a smaller number of candidates whose performance is optimised

with respect to the application

performance is sometimes limited by a single property,

sometimes by a combination of them

Materials indices are specific functions, the criteria of excellence,

derived from design equations that involve only material properties

this can be used with materials selection charts to form an objective function

Example:

• Thermal insulation – minimise l (l = thermal conductivity)

• strong, light tie rod in tension – maximise σYS / ρ

Attribute limits do not help with ordering the candidates that remain.

To do this we need optimization criteria.

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Tie rod M = E/r

Beam M = E1/2/r

Panel M = E1/3/r

Material Index, M, for minimizing mass

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Mp = E1/3/r Mb = E1/2/r Mt = E/r

guidelines for minimum mass

design

Modulus – Density Chart showing three material indices for stiff, lightweight design

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0.1

M = E1/3/r (GPa)1/3 / (kg/m3)

Increasing value of index

E1/3/r

Search Area

Modulus – Density Chart showing a grid of lines for the material index M = E1/3/r

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Selected Area

A selection based on the index M > 2 (GPa)1/3/(kg/m3) and the property limit E > 50 GPa

Index, M = E1/3/r

> 2 (GPa)1/3/(kg/m3)

Modulus, E > 50 GPa

Selected Area

A selection based on the index M > 2 (GPa)1/3/(kg/m3) and the property limit E > 50 GPa

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Index, M = E1/3/r

> 2 (GPa)1/3/(kg/m3)

Modulus, E > 50 GPa

Selected Area

A selection based on the index M > 2 (GPa)1/3/(kg/m3) and the property limit E > 50 GPa

The materials contained in the search area become the candidate for the next stage of the selection process.

To summarize:

• screening isolates candidates that are capable of doing the job

• ranking identifies those among them that can do the job best

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Documentation

From the ranked short list of candidate materials,

one material that is the most suitable is chosen

To proceed further, we seek a detailed profile of each candidate:

its documentation

• Such information is found in handbooks, suppliers’ data sheets,

case studies of use, and failure analyses

Why not just choose the top-ranked candidate?

But what do we know about this candidate?

• Does it be shaped, joined, or finished easily?

• What are its strengths and weaknesses?

• Does it have a good reputation?

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Local conditions

The final choice between competing candidates will often depend on

the local conditions:

• in-house expertise or equipment

• the availability of local suppliers, and so forth

A systematic procedure cannot help here;

the decision must instead be based on local knowledge.

This does not mean that the result of the systematic procedure

is irrelevant.

It is always important to know which material is best,

even if for local reasons you decide not to use it.

The selection procedure summary of 4 steps

1. Translation and deriving the index

• Identify the material attributes that are constrained by the design,

• decide what you will use as a criterion of excellence (to be minimized or maximized)

• substitute for any free variables using one of the constraints, and

• read off the combination of material properties that optimize the criterion of excellence

2. Screening: Applying attribute limits

• Any design imposes certain non-negotiable demands (“constraints”) on the material of which it is made

• Translate these constrains into attribute limits and plot them as horizontal or vertical lines on material

selection charts

3. Ranking: Indices on charts

• Seek those materials, from the subset of materials that meet the property limits, that maximize

performance and rank them

4. Documentation

• Explore the characters that cannot be expressed as simple attribute limits in depth of the shortlisted and

ranked candidate materials

• Many of these relate to the behavior of the material in a given environment or to aspects of the ways in which

the material can be shaped, joined, or finished

• Such information can be found in handbooks, manufacturers’ data sheets and computer-based sources.

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Next Class

MME445: Lecture 20

Materials selection: The material index