Material Selection General

Materials Selection in Engineering

Overview

Factors/Criteria in Material Selection Function Mechanical Properties Failure Modes Manufacturability Cost Environmental Considerations

Decision Making in material selection

Material Selection and Design

Material Selection is Design-led Properties of a new material can suggest the new

product Transistor: High-purity silicon Optical Fiber: High-purity glass

Optical Fiber

Material Selection and Design Need for a new product

can demand the development of a new material Turbine Technology:High-

Temperature Alloys,Ceramics

Space Technology: Lightweight Composites

The solar-powered Pathfinder in flight

Picture: Clark University, Worcester, Massachusetts

Mechanical Design

Deals with function and physical principles Components must

Carry Loads Conduct Heat and Electricity Exposed to Wear and Corrosion Must be Manufactured

Limited by Materials

Engineering Materials

Metals Polymers Elastomers

Six important classes of materials Ceramics Glasses Composites

Successful design exploits and brings out the true potential of materials selected.

The goal is to meet a certain profile of properties

Design-Limiting Material Properties Thermal

Thermal Conductivity Thermal Diffusivity Specific Heat Melting Point Glass Temperature Thermal Expansion

Coefficient Thermal Shock Resistance Creep Resistance

Corrosion/Oxidation Corrosion Rate Parabolic Rate Constant

General: Cost Density

Mechanical Elastic Moduli Strength Toughness Fracture Toughness Damping Capacity Fatigue Endurance Limit

Wear Archard Wear Constant

Menu of Materials Ceramics/Glasses

High Moduli, Hard, Abrasion/Corrosion resistant

Cutting Tools Retain Strength at

High Temperature Brittle Prey to high contact

stresses, low tolerance for cracks

Metals High Moduli Can undergo

Alloying, Heat Treatment

Formed by Deformation Ductile

Yields before fracture Prey to Fatigue, Corrosion

Menu of Materials

Polymers and Elastomers Low Moduli, High Strength

High Elastic Deflection Snap fits

Corrosion Resistant Easy to Shape Minimize Finishing

Operations Temperature Dependent

Properties

Composites High Moduli,

Strength, Lightweight

Can be Tough Optimal performance

at room temperature Expensive Difficult to

Form/Join

Materials Selection Charts Combinations of properties are

important in evaluating usefulness of materials. Strength to Weight Ratio: f/ Stiffness to Weight Ratio: E/

Helpful to plot one property against another

Following charts useful in performance-optimization

Speed of Sound in a solid, v( )Ev = Represented by:

M.F. Ashby. Materials Selection in Mechanical Design. Pp34 1999

Modulus vs. Density Chart

M.F. Ashby. Materials Selection in Mechanical Design. Pp37 1999

Material Indices A method is necessary for translating design

requirements into a prescription for a material Modulus-Density charts

Reveal a method of using lines of constant

to allow selection of materials for minimum weight and deflection-limited design.

Material Index Combination of material properties which characterize

performance in a given application.

3,2,11 =nE n

Material Indices and Performance Combination of material properties which

characterize performance in a given application

Performance of a material:

=

MsticsCharacteriMaterial

GParametersGeometeric

FNeedsFunctional

fp,

,,

,,

)()()( 321 MfGfFfp =

Simplification of Performance

Performance for all F and G is maximized by maximizing f3 (M) f3 (M): Material Index f1 (F) f2(G) : Related to Structural Index

Each combination of function, objective, and constraint leads to a material index.

Example: Calculation of Material Index

Design: cylindrical tie rod Given length, l carries tensile force, F with

minimum mass

Objective Function Mass (m) = Area (A) * Length (l) * Density ( )

Goal: minimize m by varying A Constraint: A must be sufficient to carry

tensile load, F

(failure strength)fAF

Example: Material Index (Continued ) By eliminating A from these equations we obtain

The lightest tie which will carry F safely is that made of the material with the smallest value of

Therefore, the material index can be defined as

A similar calculation for a light, stiff tie leads to the index

( )flFm

f

fM =

EM =

Strength vs. Density Chart

M.F. Ashby. Materials Selection in Mechanical Design. Pp39 1999

Other Materials Selection Charts Modulus-Relative Cost Strength-Relative Cost

Modulus-Strength Specific Modulus-

Specific Strength Fracture Toughness-

Modulus Fracture Toughness-

Strength Loss Coefficient-

Modulus

Facture Toughness-Density

Conductivity-Diffusivity Expansion-Conductivity Expansion-Modulus Strength-Expansion Strength Temperature Wear Rate-Hardness Environmental Attack

Chart

Failure

Can be of many types Wearout Fracture Corrosion

Important to be aware of appropriate repair methods available

Failure mode can be anticipated based on material type

Pipeline Failure

Environmental Attack Chart

M.F. Ashby. Materials Selection in Mechanical Design. Pp62 1999

Manufacturability The material choice must be compatible with

the manufacturing process and configuration

Radius to which a sheet metal is bent depends on ductility

Residual stress due to cooling of a cast part may result in hot tearing

Cast water pump

Factors in Manufacturing Processes

Castability Formability/Workability Machinability Coatability Heat Treatment

Cost Effectiveness and Value Analysis Value:

Extent to which the appropriate performance criteria are satisfied

Cost What has to be paid in order to achieve that level of

value

Material selection in a design must provide most value for the least cost

Cost Effective Materials Selected material must be able to meet the

function and allow production of the product at an acceptable price.

Savings incurred from the quality of a material may outweigh the initial cost in certain applications.

Conversely, an inexpensive, low quality material may meet the needs of a product in other applications.

Cost and Material Selection

Many factors influence the cost of a material Composition Compound Stability Relative Abundance Supply and Demand

Recycling Recycling of packaging material and

certain consumer products is rapidly becoming required by law

Material Selection decisions must include consideration of ease of recovery and recycling

Current Impediments Components made of mixed plastics Use of plating and coating on base material

What Next?

Many factors involved in material selection process

Each must be taken into account before selection can be made

Next Step Selection!

Selection Process

Selection among alternatives can be difficult given Factors and Criteria that must be taken into account

Experience and judgment can be sufficient at times

Formal decision making process can be helpful when there is no one obvious choice of material

Decision Making in Materials Selection

Quantify importance of each desired characteristic Weighting Factor

Quantify ability of a candidate material to satisfy these requirements Rating Factor

Material Indices

Combine Weighting and Rating factors to determine material that offers the best compromise

justice

Final Material Selection

Selection of Material Implementation of Weighting and Rating factors to optimize

the various factors and criteria including Function Manufacturability Cost

Further information can be obtained from the many reference texts available on material selection

Credits

M.F. Ashby, Materials Selection in Mechanical Design. Butterworth-Heinemann. Boston, MA. 1999

G.T. Murray, Handbook of Materials Selection for Engineering Applications. Marcel Deckker, Inc. New York, NY. 1997

E.H. Cornish, Materials and the Designer. Cambridge University Press .New York, NY. 1987

F.A.A. Crane, J.A. Charles, Selection and use of Engineering Materials. Butterworths. Boston, MA. 1984

Materials Selection in EngineeringOverviewMaterial Selection and DesignMaterial Selection and DesignMechanical DesignEngineering MaterialsDesign-Limiting Material PropertiesMenu of MaterialsMenu of MaterialsMaterials Selection ChartsSpeed of Sound in a solid, vModulus vs. Density ChartMaterial IndicesMaterial Indices and PerformanceSimplification of PerformanceExample: Calculation of Material IndexExample: Material Index (Continued )Strength vs. Density ChartOther Materials Selection ChartsFailureEnvironmental Attack ChartManufacturabilityFactors in Manufacturing ProcessesCost Effectiveness and Value AnalysisCost Effective MaterialsCost and Material SelectionRecyclingWhat Next?Selection ProcessDecision Making in Materials SelectionFinal Material SelectionCredits