MATERIALS SELECTION
Chapter Two
Methods of Materials Selection
Faculty of Mechanical Engineering
Ali Ourdjini, UTM - 2005
MATERIALS SELECTION
• Some of the more common and analytical methods of materials
selection are:
1. Cost versus Performance
2. Weighted Property Indices
3. Value Analysis
4. Failure Analysis
5. Benefit-Cost Analysis
Faculty of Mechanical Engineering
Ali Ourdjini, UTM - 2005
MATERIALS SELECTION
1. Cost vs Performance
• Because COST is so important in selecting materials, it is logical to
consider cost at the start of the material selection process
• Usually, a target cost is set to eliminate the materials that are very
expensive
• The final choice is a trade-off between COST and PERFORMANCE
• Overall, cost is the most important criterion in selecting a material
Faculty of Mechanical Engineering
Ali Ourdjini, UTM - 2005
MATERIALS SELECTION
• Cost is a most useful parameter when it can be related to a critical
material property that controls the performance of the design
• Such a cost vs performance index can be used for optimising the
selection of a material
• However, the cost of a material expressed in $$$ / kg may not always be
the most valid criterion
• It depends on the material function: whether it is used as a load bearing
or just as space filling
• It is also very important to emphasise that there are many ways to
compute costs
Faculty of Mechanical Engineering
Ali Ourdjini, UTM - 2005
MATERIALS SELECTION
Total life-cycle cost is the
most appropriate cost to
consider. This cost consists
of:
Total Cost
Min. Cost
$$
A: Manufacturing Costs
The initial material costs +
manufacturing costs +
operation costs +
maintenance costs
B: Other Costs
Consideration of factors
Yield Strength (MPa)
beyond just the initial
materials cost leads to
relations such as shown in
Figure 2: Relations between cost factors and a
Figure 2
material property
Faculty of Mechanical Engineering
Ali Ourdjini, UTM - 2005
MATERIALS SELECTION
Cost per unit property method
• This method is suitable for initial screening in situation where one
property stands out as the most critical service requirement.
• In this case, it is possible to estimate how much various materials to
provide this requirement will cost
• Cost / unit tensile ($$/ MPa) strength is usually one of the most
important criteria
Faculty of Mechanical Engineering
Ali Ourdjini, UTM - 2005
MATERIALS SELECTION
• By introducing the density of the material and the market price, the cost
of buying 1 MPa of strength, C, can be calculated:
C =
P: material price / unit weight
ρ: density
σ: tensile strength
Faculty of Mechanical Engineering
Pρ
σ
Ali Ourdjini, UTM - 2005
MATERIALS SELECTION
• Materials with lower cost/ unit strength are preferable.
• Since manufacturing costs are a significant factor in evaluating
materials, it can be considered in the cost /unit property analysis by
considering P as the cost of material + manufacturing and finishing
costs
Limitations of this method
• It considers only one property as the most critical and ignoring other
properties
Faculty of Mechanical Engineering
Ali Ourdjini, UTM - 2005
MATERIALS SELECTION
• Since comparison of materials is a fundamental part of material
selection
• A basis material can be selected and the other candidate materials
compared against it
• The relative cost / unit property, RC, is then given by:
Pi ρ i σ b
×
×
RC =
Pb ρ b σ i
i: candidate material, b: basis material
• If RC < 1:
material
the candidate material is less expensive than the basis
Faculty of Mechanical Engineering
Ali Ourdjini, UTM - 2005
MATERIALS SELECTION
Relationships to determine the relative cost per unit property for strength
and stiffness are given in Table 2
Faculty of Mechanical Engineering
Ali Ourdjini, UTM - 2005
MATERIALS SELECTION
2. Weighted Property Method
•
In most applications, the selected material should satisfy more than one
functional requirement
•
In this method each material requirement (or property) is assigned a
certain weight (which depends on its importance to the performance of
the design)
•
This method attempts to:
1.
Quantify how important each desired requirement is by determining
a weighting factor (α)
2.
Quantify how well a candidate material satisfies each requirement
by determining a scaling factor (β)
Faculty of Mechanical Engineering
Ali Ourdjini, UTM - 2005
MATERIALS SELECTION
Weighted Properties Method
Convert properties of
different materials into
scaled properties
Find weighting factors
of properties of
candidate materials
Find the Performance
Index (γ)
γ = ∑α β
Faculty of Mechanical Engineering
Ali Ourdjini, UTM - 2005
MATERIALS SELECTION
2.1. Ranking of Attributes
• Attributes are characteristics that can be described to distinguish one
item from another
• Some attributes are more important that others. Determining the
relative importance of the various properties assigned to these
attributes is therefore necessary if this method is to be used
• 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
Faculty of Mechanical Engineering
Ali Ourdjini, UTM - 2005
MATERIALS SELECTION
2.2. Weighting factors
• It is desirable to quantify the relative importance of the attributes
• One attribute may be very much more important than another, while
others may be quite similar in importance
• The relative importance is shown by using a point scale that does not
exceed 100 points
e.g; if strength is 4 times as important as cost, it will be represented
by an 80 / 20 division
Faculty of Mechanical Engineering
Ali Ourdjini, UTM - 2005
MATERIALS SELECTION
Weight 4 times as important as strength, strength is 4 times as important
is 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
60
50
80
1.0
0.14
4.0
0.58
0.66
0.10
1.0
0.14
.25
0.04
6.91
1.00
Strength
1
20
Density
2
80
Corrosion
3
Colour
4
Cost
5
Total
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40
50
20
Ali Ourdjini, UTM - 2005
MATERIALS SELECTION
• We can also use the Digital Logic Method
Faculty of Mechanical Engineering
Ali Ourdjini, UTM - 2005
MATERIALS SELECTION
• The number of attributes that should be listed vary between 5 - 10
• This method combine properties with different units. This limitation is
overcome by the use of a “scaling factor”
• The relative merit of each property of the candidate material may be
incorporated by assigning the value of 100 (%) to the best material in
that property category
Faculty of Mechanical Engineering
Ali Ourdjini, UTM - 2005
MATERIALS SELECTION
• For a given property, the scaling factor (β) for a given candidate
material is:
•
For properties that should have maximum values (strength, toughness…)
numerical value of property
β=
×100
max value in the list
•
For properties that should have low values (density, cost corrosion …)
lowest value in the list
β=
× 100
numerical value of property
Faculty of Mechanical Engineering
Ali Ourdjini, UTM - 2005
MATERIALS SELECTION
• The best material may either have the largest value of the given
property or the smallest
• For example;
High Strength is given 100
Low density or low corrosion rate are given 100
2.3. Performance Index
• The material performance index (γ) is calculated:
γ = ∑α β
• The material with the highest γ is considered to be the best
Faculty of Mechanical Engineering
Ali Ourdjini, UTM - 2005