Materials Selection Methods 1
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Transcript of Materials Selection Methods 1
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MATERIALS SELECTION
Ali Ourdjini, UTM - 2005Faculty of Mechanical Engineering
Chapter Two
MMethods of ethods of MMaterials aterials SSelectionelection
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MATERIALS SELECTION
Ali Ourdjini, UTM - 2005Faculty of Mechanical Engineering
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
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MATERIALS SELECTION
Ali Ourdjini, UTM - 2005Faculty of Mechanical Engineering
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
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MATERIALS SELECTION
Ali Ourdjini, UTM - 2005Faculty of Mechanical Engineering
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
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MATERIALS SELECTION
Ali Ourdjini, UTM - 2005Faculty of Mechanical Engineering
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)
$$$$
Min. Cost
A: Manufacturing Costs
B: Other Costs
Total Cost
Figure 2: Relations between cost factors and a
material property
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MATERIALS SELECTION
Ali Ourdjini, UTM - 2005Faculty of Mechanical Engineering
Cost per unit property methodCost per unit property method
This method is suitable for initial screening in situation whereThis method is suitable for initial screening in situation where one one
property stands out as the most critical service requirement.property stands out as the most critical service requirement.
In this case, it is possible to estimate how much various materiIn this case, it is possible to estimate how much various materials to als to provide this requirement will costprovide this requirement will cost
Cost / unit tensile ($$/ Cost / unit tensile ($$/ MPaMPa) strength is usually one of the most ) strength is usually one of the most important criteriaimportant criteria
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MATERIALS SELECTION
Ali Ourdjini, UTM - 2005Faculty of Mechanical Engineering
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 =
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MATERIALS SELECTION
Ali Ourdjini, UTM - 2005Faculty of Mechanical Engineering
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
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MATERIALS SELECTION
Ali Ourdjini, UTM - 2005Faculty of Mechanical Engineering
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
=
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MATERIALS SELECTION
Ali Ourdjini, UTM - 2005Faculty of Mechanical Engineering
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
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MATERIALS SELECTION
Ali Ourdjini, UTM - 2005Faculty of Mechanical Engineering
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 ())
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MATERIALS SELECTION
Ali Ourdjini, UTM - 2005Faculty of Mechanical Engineering
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 ()
=
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MATERIALS SELECTION
Ali Ourdjini, UTM - 2005Faculty of Mechanical Engineering
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
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MATERIALS SELECTION
Ali Ourdjini, UTM - 2005Faculty of Mechanical Engineering
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
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MATERIALS SELECTION
Ali Ourdjini, UTM - 2005Faculty of Mechanical Engineering
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
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MATERIALS SELECTION
Ali Ourdjini, UTM - 2005Faculty of Mechanical Engineering
We can also use the We can also use the Digital Logic MethodDigital Logic Method
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MATERIALS SELECTION
Ali Ourdjini, UTM - 2005Faculty of Mechanical Engineering
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
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MATERIALS SELECTION
Ali Ourdjini, UTM - 2005Faculty of Mechanical Engineering
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
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MATERIALS SELECTION
Ali Ourdjini, UTM - 2005Faculty of Mechanical Engineering
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
=