Selection Process Overview

• At concept (preliminary) level

– Analyze material requirements (from PDS)

• Conditions of service and operating environment

• Translate conditions to material properties

– Screen for candidate materials

• Compare req’d properties with a materials database to select candidate materials

• During embodiment phase

– Analysis of candidate materials in terms of tradeoffs in performance, cost, manufacturability

Material Selection and Design

• Selecting an appropriate material is a critical part of almost all engineering designs.

• A methodology for selecting materials and processes which is driven by design, uses as inputs the functional requirements of the design.

• There are many factors to consider

– Mechanical Properties:• Strength, stiffness, ductility, fracture toughness , fatigue, creep…

– Physical Properties:• Density, melting point, thermal/electrical conductivity…

– Other• Cost, corrosion, formability, etc

Material Selection

Basic Methods

Translation:

express design requirements as constraints and objectives.

Screening:

eliminate materials that cannot do the job.

Ranking:

find materials that best do the work using performance indices.

Research Supporting Info:

handbooks, expert systems, web, etc.

Translation (express design requirements as constraints and objectives)

Function: What does the component do?

Objective: What essentials conditions must be met?

Constraints: What is to be maximized or minimized?

Free Variables: Identify which design variables are free?

• Which can be modified?

• Which are desirable?

Screening(Eliminate materials that cannot do the job)

• Need an effective way to evaluate a large range of material properties and classes.

• Methods to evaluate materials

– Material Bar Charts

– Material Property Charts

(for example, density vs. Young’s Modulus)

Material Bar Charts

Metals Polymers Ceramics Hybrids

PEEK

PP

PTFE

WC

Alumina

Glass

CFRP

GFRP

Fibreboard

Steel

Copper

Lead

Zinc

Aluminum

Material Selection Charts

Screening Example:Heat Sink for Power Electronics

Retain materials with:

1. Max service temp > 200 C

2. “Good insulator”, or R > 1019 mohm.cm

3. “Good T-conductor” or T-conduct. > 100 W/m.K

• Function:

• Heat sink

•Constraints:

• operate at 200 C

• be electrical insulator

• conduct heat well

• Free variable:

• Choice of material

Therefore…

2000C

Screening using Bar Charts (max service temperature >200oC)

Metals Polymers Ceramics

Composites

PEEK

PP

PTFE

WC

Alumina

Glass

CFRP

GFRP

Fibreboard

Steel

Copper

Lead

Zinc

Aluminium

Screening using Property Charts

R=1019

λ=100

Screening using Material Property Charts (>100W/m.K, e>1019m.cm)

Ranking(find the materials that do the job best)

• What if multiple materials remain after screening?

• Which one is best?

• What if there are multiple material parameters for evaluation?

Use the Material Performance Index

Start with Translation

Rank on Objectives because objectives define performance metrics

Example: Tie Rod

• Function: Support a tensile load

• Objective: Minimize mass

• Constraints: Required length (L)

Load carrying capability w/o failure

• Free Variables: Cross-sectional area

Material

Write design requirements as equations.

• Objective

m = A*L*

• Constraint

F/A < sy

Eliminate free variable

m ≥ FL (/sy)

therefore, minimize weight by maximizing sy /

Using the Performance Index

• Method

1) Identify function, constraints, objective and free variables (as before)

2) Write equation for objective (the Performance Equation)

3) Define combination of material properties that maximize performance (the Material Index)

4) Use these for ranking

The Performance Equation

P ,properties

Material

G ,parameters

Geometric,

F ts,requiremen

FunctionalP

or P= f (F,G,M)

Use constraint to eliminate free variable

Performance equation for Tie Rod example

m ≥ FL (/sy)

Example: material for stiff, light beam

• Function:Support a bending load

• ObjectiveMinimize mass

• ConstraintsLength

Carry load F with minimal deflection

• Free VariablesCross-sectional area

Material

F

Deflection, dBeam has length = L and area = A

Objective:

m = AL

Constraint:

3L

CEIFS

d

Beam Example con’t

• Objective:

m = AL

• Constraint:

• Solve for “m” substitute and rearrange.

3L

CEIFS

d

2/12/1

2/52/14

d

C

LFm

Material Indices

Minimize weight by minimizing

or by maximizing

2/1E

2/1E

Material Index Calculation Process Flow

Tie Rod

Beam

Shaft

Column

Function

Stiffness specified

Fatigue limit

Geometry specified

Constraints

Minimize cost

Minimize weight

Maximum energy

Storage

Minimum

eco-impact

ObjectiveStrength specified

Could be mechanical,

thermal, electrical…

(Each combination has a characterizing material index)

2/1EM

Index

Material Index Examples

• An objective defines a performance metric

For example mass or resistance

• Equation for performance metric contains material properties

Sometimes a single property*

Sometimes a combination*

*either is a material index

Material Index Examples

• Material Indices for a Beam

Loading Stiffness Limited

Strength Limited

Tension E/ sf /

Bending E1/2/ sf 2/3/

Torsion G1/2/ sf 2/3/

Objective:

minimize mass

Performance Metric:

mass

Maximize indices

Optimized Selection using Material Indices and Property Charts

Example:

Tension load, strength limited

Maximize: M = s/

In log space:

log s = log + log M

This is a set of lines with slope =1

Materials above the line are candidates

Source

• Material Selection in Mechanical Design, Michael F. Ashby, Butterworth-Heinemann