TP14_Materials_Selection.pdf

Design for Manufacture and Assembly

Materials Selection

RevVersion 99_00 1

Materials Selection Property Maps

Reference

MF Ashby, Materials Selection in Mechanical

Design 3rd Edition, Elsevier, 2005

Design for Manufacture and Assembly

Materials Selection

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Market Need

Concept

Detail

Embodiment

Product

All Materials

Low precision data

Subset of Materials

Higher Precision

One Material

Highest Precision

All Processes

Low Resolution

Subset of Processes

Higher Resolution

One Process

Highest Resolution

Design Materials Processes

Design for Manufacture and Assembly

Materials Selection

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Comparison of Properties of Materials

Property Metals Ceramics Polymers

Density, gcm-3 2 - 22 2-19 1-2

Melting Point low[ Ga 29.8 C]

High [ W, 3410 C]

High

[ up to 4000 C]

Low

Hardness Medium High Low

Machinability Good Poor Good

Youngs Modulus 15 400 GPa 150 450 GPa 0.001 10 GPa

Tensile Strength up to 2500 MPa up to 400 MPa up to 140 MPa

Compressive Strength

up to 2500 MPa up to 5000 MPa up to 350 MPa

Design for Manufacture and Assembly

Materials Selection

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Comparison of Properties of Materials (contd.)

Property Metals Ceramics Polymers

High Temperature Creep Resistance

Poor - Medium Excellent

Thermal Expansion

Medium to High Low to Medium Very High

Thermal Conductivity

Medium to High Medium to Low at high temperature

Very Low

Thermal Shock Resistance

Good Poor

Electrical Characteristics

Conductors Insulators Insulators

[some conductors]

Chemical Resistance

Low to Meduim Excellent Good

Oxidation Resistance

Generally Poor Oxides-excellent Carbides, Nitrides

- good

..

Design for Manufacture and Assembly

Materials Selection

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Material Property Maps

Design for Manufacture and Assembly

Materials Selection

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Example Maps

Strength Density

Modulus Density

Fracture Toughness - Density

Strength Relative Cost

Modulus Relative Cost

rod steel mild of kgper Cost

material theof kgper Cost CR Relative Cost

Design for Manufacture and Assembly

Materials Selection

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Modulus Density

Map

Ref:MF Ashby, Materials Selection in Mechanical Design , Pergammon Press, 1992

Design for Manufacture and Assembly

Materials Selection

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Strength-Density

Map

Ref:MF Ashby, Materials Selection in Mechanical Design , Pergammon Press, 1992

Design for Manufacture and Assembly

Materials Selection

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Fracture Toughness

- Density Map

Ref:MF Ashby, Materials Selection in Mechanical Design , Pergammon Press, 1992

Design for Manufacture and Assembly

Materials Selection

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Modulus - Relative

Cost Map

Ref:MF Ashby, Materials Selection in Mechanical Design , Pergammon Press, 1992

Design for Manufacture and Assembly

Materials Selection

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Strength Relative

Cost Map

Ref:MF Ashby, Materials Selection in Mechanical Design , Pergammon Press, 1992

Design for Manufacture and Assembly

Materials Selection

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Relationships Between Failure Modes and Material Properties

Failure Mode Material Property

Gross Yielding Yield Strength, Shear Yield Strength

BucklingCompressive Yield Strength, Modulus of

Elasticity

Creep Creep rate

Brittle Fracture Impact energy, Transition temperature, K1C

Fatigue, Low cycle Fatigue Properties, Ductility

Fatigue, High cycle Fatigue Properties

Contact Fatigue Compressive Yield Strength

Design for Manufacture and Assembly

Materials Selection

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Relationships Between Failure Modes and Material Properties

Failure Mode Material Property

FrettingCompressive Yield Strength, Electrochemical

potential

Corrosion Electrochemical potential

Stress-Corrosion

Cracking

Ultimate Tensile Strength, K1SSC ,

Electrochemical potential

Galvanic Corrosion Electrochemical potential

Hydrogen

EmbrittlementUltimate Tensile Strength

Wear Hardness

Thermal Fatigue Creep rate, Coefficient of Thermal Expansion

Corrosion Fatigue Fatigue Properties, Electrochemical potential

Design for Manufacture and Assembly

Materials Selection

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Market Need

Concept

Detail

Embodiment

Product

All Materials

Low precision data

Subset of Materials

Higher Precision

One Material

Highest Precision

All Processes

Low Resolution

Subset of Processes

Higher Resolution

One Process

Highest Resolution

Design Materials Processes

Design for Manufacture and Assembly

Materials Selection

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Selection Methodologies-Concept Design

Make use of Materials Desirability factors or Performance

Indices

The aim is to find a material which maximises the

performance index

Design for Manufacture and Assembly

Materials Selection

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Beam in Bending

b

d

L

Design for Manufacture and Assembly

Materials Selection

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Example Materials Desirability Factor

Consider a beam in bending. Irrespective of precise nature of

loading the maximum stress, max, in the beam will be given by

(1)

where I = second moment of area

M = bending moment

b = depth

d = width

3

121

21

max21

maxmax

MM

bd

d

I

d

Design for Manufacture and Assembly

Materials Selection

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Assuming that we are comparing different materials on the basis

that length, width and loading are fixed but beam depth is a design

variable then

(2)

where b1= constant

b

12d

But the mass, m, of the beam is given by

(3)

where r = material density

bdLm r

Design for Manufacture and Assembly

Materials Selection

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Substituting for d from 2 into 3 gives

4

where b2 is the same constant for all materials

is a materials selection criterion but is more usually expressed as

which is the Materials Desirability Factor where and r are yield

strength and density values for the materials being compared

21

2

rbm

r

12

r

12

Design for Manufacture and Assembly

Materials Selection

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If we want a beam of minimum mass but maximum

strength we will look for materials with high values of the

Desirability Factor

Desirability Factors for other loading situations

Design for Manufacture and Assembly

Materials Selection

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Example Desirability Factors

Design for Manufacture and Assembly

Materials Selection

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Example Desirability Factors

Design for Manufacture and Assembly

Materials Selection

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Example Desirability Factors

Ref: MF Ashby, Materials Science and Technology, 1989, Vol5, , 517-526

Design for Manufacture and Assembly

Materials Selection

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Use of Desirability Factors with Property Maps

Strength-Density Map Consider a Beam in Bending, Lightest beam to carry a given load is one where the condition below is a Maximum

Lines of Slope = 2 can be drawn on the map and used to identify materials which are optimal for the loading condition

constant,log2 log

or

constant,

11

21

CC

CC

r

r

Design for Manufacture and Assembly

Materials Selection

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Materials Selection

A Simple but relevant example

Design for Manufacture and Assembly

Materials Selection

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Ashbys Approach

Primary Constraints

Any design imposes any non-negotiable constraints on the material(s) of which it is MADE

E.g., Temperature Components which carry load at 300cC cannot be made

from Polymers

Primary Constraints take the form

P > Pcrit or P < Pcrit

where P = Property set by the design

Primary Constraints appear as vertical or horizontal lines on a property chart

Design for Manufacture and Assembly

Materials Selection

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Primary Constraints

Design for Manufacture and Assembly

Materials Selection

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Performance Maximising Criteria 1

Stiff Lightweight Design

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Materials Selection

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Performance Maximising Criteria 2

Stiff Lightweight Design

Design for Manufacture and Assembly

Materials Selection

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Bicycle Forks

Fork = Tube which should resist bending

Desirability factor to maximise is

2/3/r

Design for Manufacture and Assembly

Materials Selection

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Bicycles Forks

Strength is not the only criterion

Cost

Manufacturability

Resistance to fracture

Stiffness

Are also important

Design for Manufacture and Assembly

Materials Selection

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Other Criteria Strength vs Relative cost

CCR

f

r

3/2

Design for Manufacture and Assembly

Materials Selection

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Fork Materials

Material Comments

Aluminium Alloy (7075) Popular choice for Racing and

Mountain Bikes

Titanium alloy Expensive no advantage over Al

alloys

Magnesium alloy Difficult to shape

Steel (Reynolds 531) Standard for cheaper bicycles

CRFP (with some glass or kevlar) Outstanding but expensive, used on

racing and up-market mountain

bikes

Wood Material properties good but cannot

be easily shaped, bio-degradeable!