materials selection methods 1

MATERIALS SELECTION

Ali Ourdjini, UTM - 2005Faculty of Mechanical Engineering

Chapter Two

MMethods of ethods of MMaterials aterials SSelectionelection

MATERIALS SELECTION


Some of the more common and analytical methods of materials Some of the more common and analytical methods of materials

selection are:selection are:

1.1. Cost versus PerformanceCost versus Performance

2.2. Weighted Property IndicesWeighted Property Indices

3.3. Value AnalysisValue Analysis

4.4. Failure AnalysisFailure Analysis

5.5. BenefitBenefit--Cost AnalysisCost Analysis

MATERIALS SELECTION


1. Cost 1. Cost vsvs PerformancePerformance

Because Because COSTCOST is so important in selecting materials, it is logical to is so important in selecting materials, it is logical to

consider cost at the start of the material selection processconsider cost at the start of the material selection process

Usually, a target cost is set to eliminate the materials that arUsually, a target cost is set to eliminate the materials that are very e very

expensiveexpensive

The final choice is a tradeThe final choice is a trade--off between off between COSTCOST and and PERFORMANCEPERFORMANCE

Overall, cost is the most important criterion in selecting a matOverall, cost is the most important criterion in selecting a materialerial

MATERIALS SELECTION


Cost is a most useful parameter when it can be related to a critCost is a most useful parameter when it can be related to a critical ical material property that controls the performance of the designmaterial property that controls the performance of the design

Such a Such a costcost vsvs performanceperformance index can be used for optimising the index can be used for optimising the selection of a materialselection of a material

However, the cost of a material expressed in $$$ / kg may not alHowever, the cost of a material expressed in $$$ / kg may not always be ways be the most valid criterionthe most valid criterion

It depends on the material function: whether it is used as a It depends on the material function: whether it is used as a load bearingload bearingor just as or just as space fillingspace filling

It is also very important to emphasise that there are many ways It is also very important to emphasise that there are many ways to to compute costscompute costs

MATERIALS SELECTION


Total life-cycle cost is the most appropriate cost to consider. This cost consists of:

The initial material costs + manufacturing costs + operation costs + maintenance costs

Consideration of factors beyond just the initial materials cost leads to relations such as shown in Figure 2

Yield Strength (MPa)

$$$$

Min. Cost

A: Manufacturing Costs

B: Other Costs

Total Cost

Figure 2: Relations between cost factors and a

material property

MATERIALS SELECTION


Cost per unit property methodCost per unit property method

This method is suitable for initial screening in situation whereThis method is suitable for initial screening in situation where one one

property stands out as the most critical service requirement.property stands out as the most critical service requirement.

In this case, it is possible to estimate how much various materiIn this case, it is possible to estimate how much various materials to als to provide this requirement will costprovide this requirement will cost

Cost / unit tensile ($$/ Cost / unit tensile ($$/ MPaMPa) strength is usually one of the most ) strength is usually one of the most important criteriaimportant criteria

MATERIALS SELECTION


By introducing the density of the material and the market price,By introducing the density of the material and the market price, the cost the cost

of buying 1 of buying 1 MPaMPa of strength, C, can be calculated:of strength, C, can be calculated:

P: material price / unit weightP: material price / unit weight

: density: density

: tensile strength: tensile strength

PC =

MATERIALS SELECTION


Materials with lower cost/ unit strength are preferable. Materials with lower cost/ unit strength are preferable.

Since manufacturing costs are a significant factor in evaluatingSince manufacturing costs are a significant factor in evaluatingmaterials, it can be considered in the cost /unit property analymaterials, it can be considered in the cost /unit property analysis by sis by considering P as the cost of material + manufacturing and finishconsidering P as the cost of material + manufacturing and finishing ing

costscosts

Limitations of this method

It considers It considers only one propertyonly one property as the most critical and ignoring other as the most critical and ignoring other propertiesproperties

MATERIALS SELECTION


Since comparison of materials is a fundamental part of material Since comparison of materials is a fundamental part of material selectionselection

A basis material can be selected and the other candidate materiaA basis material can be selected and the other candidate materials ls compared against itcompared against it

The relative cost / unit property, RC, is then given by:The relative cost / unit property, RC, is then given by:

i: candidate material, b: basis materiali: candidate material, b: basis material

If RC < 1:If RC < 1: the candidate material is less expensive than the basis the candidate material is less expensive than the basis materialmaterial

i

b

b

i

b

i

P

PRC

=

MATERIALS SELECTION


Relationships to determine the relative cost per unit property fRelationships to determine the relative cost per unit property for strength or strength and stiffness are given in Table 2and stiffness are given in Table 2

MATERIALS SELECTION


2. Weighted Property Method2. Weighted Property Method

In most applications, the selected material should satisfy In most applications, the selected material should satisfy more than one more than one functional requirementfunctional requirement

In this method each material requirement (or property) is assignIn this method each material requirement (or property) is assigned a ed a certain weight (which depends on its importance to the performancertain weight (which depends on its importance to the performance of ce of

the design)the design)

This method attempts to:This method attempts to:

1.1. Quantify how important each desired requirement is by determininQuantify how important each desired requirement is by determining g

a a weighting factor (weighting factor ())

2.2. Quantify how well a candidate material satisfies each requiremenQuantify how well a candidate material satisfies each requirement t

by determining a by determining a scaling factor (scaling factor ())

MATERIALS SELECTION


Weighted Properties MethodWeighted Properties Method

Find weighting factors of properties of

candidate materials

Convert properties of different materials into scaled properties

Find the Performance Index ()

=

MATERIALS SELECTION


2.1. Ranking of Attributes 2.1. Ranking of Attributes

Attributes are characteristics that can be described to distinguAttributes are characteristics that can be described to distinguish one ish one

item from anotheritem from another

Some attributes are more important that others. Determining the Some attributes are more important that others. Determining the relative importance of the various properties assigned to these relative importance of the various properties assigned to these

attributes is therefore necessary if this method is to be usedattributes is therefore necessary if this method is to be used

There are two steps for ranking attributes:There are two steps for ranking attributes:

rank in order of importance with no consideration of how important one attribute is to another

a weight is assigned to the importance of each attribute

MATERIALS SELECTION


2.2. Weighting factors2.2. Weighting factors

It is desirable to quantify the relative importance of the attriIt is desirable to quantify the relative importance of the attributesbutes

One attribute may be very much more important than another, whilOne attribute may be very much more important than another, while e others may be quite similar in importanceothers may be quite similar in importance

The relative importance is shown by using a point scale that The relative importance is shown by using a point scale that does not does not

exceed 100 pointsexceed 100 points

e.g; if strength is 4 times as important as cost, it will be repe.g; if strength is 4 times as important as cost, it will be represented resented by an 80 / 20 divisionby an 80 / 20 division

MATERIALS SELECTION


Weight 4 times as important as strength, strength is 4 times as Weight 4 times as important as strength, strength is 4 times as important important is as cost, corrosion is 2 /3 the importance of strength, etcis as cost, corrosion is 2 /3 the importance of strength, etc

Weighting of attributes

Property 1/2 1/3 1/4 1/5 ratio weight

Strength 1 20 60 50 80 1.0 0.14

Density 2 80 4.0 0.58

Corrosion 3 40 0.66 0.10

Colour 4 50 1.0 0.14

Cost 5 20 .25 0.04

Total 6.91 1.00

MATERIALS SELECTION


We can also use the We can also use the Digital Logic MethodDigital Logic Method

MATERIALS SELECTION


The number of attributes that should be listed vary between 5 The number of attributes that should be listed vary between 5 -- 1010

This method combine properties with different units. This limitaThis method combine properties with different units. This limitation is tion is overcome by the use of a overcome by the use of a scaling factorscaling factor

The relative merit of each property of the candidate material maThe relative merit of each property of the candidate material may be y be incorporated by assigning the value of 100 (%) to the best materincorporated by assigning the value of 100 (%) to the best material in ial in that property categorythat property category

MATERIALS SELECTION


For a given property, the scaling factor (For a given property, the scaling factor () for a given candidate ) for a given candidate material is:material is:

For properties that should have maximum values (strength, toughnFor properties that should have maximum values (strength, toughnessess))

For properties that should have low values (density, cost corrosion )

100max

=listtheinvalue

propertyofvaluenumerical

100=propertyofvaluenumerical

listtheinvaluelowest

MATERIALS SELECTION


The best material may either have the largest value of the givenThe best material may either have the largest value of the given

property or the smallestproperty or the smallest

For example;For example;

High Strength is given 100High Strength is given 100

Low density or low corrosion rate are given 100Low density or low corrosion rate are given 100

2.3. Performance Index2.3. Performance Index

The material performance index (The material performance index () is calculated:) is calculated:

The material with the highest The material with the highest is considered to be the bestis considered to be the best

=