Chapter-07
Selection of Feasible Alternative Materials for Manual Compost Maker
using Weighted Average Method
7.1 Introduction
Material selection is a step in the process of designing any physical object. In the context of product
design, the main goal of material selection is to minimize cost while meeting product performance
goals. It is a fundamental step for the proper and efficient functioning of our product and also for
valuing its environmental impact.
Material is needed to execute any physical design, thought or concept. Not every material is
suitable for every product. It defers from product to product and the situation the product will be
used. So, finding suitable material is very important because at the end of the day it’ll make the
difference from an average product to a quality product. That’s why we need to undergo a material
selection process to find out the perfect material for our product.
From the perspective of engineering design, it is a multiple attribute decision-making problem. To
ensure a simple, systematic and logical methods to make right decisions for material selection, we
use weighted average method which considers the objective weights of importance of the attributes
as well as the subjective preferences of the decisions maker to decide the integrated weights of
importance of the attributes.
7.2 Structural parts of Manual Compost Maker
1.
2.
3.
4.
5.
6.
7.
Hopper
Hand Wheel
Chopping Plate
Shaft
Base
Tray
Bin
7.3 Grouping of Components
Based on our assumption of using same material, components had been assigned in three different
group.
Group -1: Hand Wheel, Shaft, Base
Group -2: Hopper, Chopping Plate, Tray
Group -3: Bin
7.4.1 Material selection for Hand wheel, Base and Shaft
1 2 3 4 5 6 7 8 9
Cost
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8
1 1 1 0 0 0 1
Availability
0
Compressive
Strength
Modulus of
Elasticity
Rust
Resistance
4
0.143
3
0.107
1
0.0365
3
0.107
4
0.143
4
0.143
0
5
0.179
0 1
4
0.143
28
∑α=1
1 1 0 1 0 0
0
0
0
1
Hardness
0 0 0 1 0
0
1
1
1
Machinability
1 1 0 0
1
0
1
Mass Density
1
1
0
0
0 0 1
0
0
1
1
1
1
0 1
1
1
1
0
Relative
Emphasis
coefficien
t
Number of positive decision, N =n (n-1)/2=8(8-1)/2=28
Selection
Criteria
Positive
decision
Table 7.1: determination of relative importance of goals using digital logic method
Total number of positive decisions
Table 7.2 Calculation of the Performance Index:
Selection
Criteria
Weighting
Factor(α)
Cost
Availability
Mild steel
Stainless steel
Aluminum
Scaled
Property(β)
Weighted
Score(αβ)
Scaled
Property(β)
Weighted
Score(αβ)
Scaled
Property(β)
Weighted
Score(αβ)
0.143
100
14.3
60
8.58
100
14.3
0.107
100
10.7
75
8.03
75
8.03
0.0365
100
3.5
41.70
1.46
68.68
2.40
0.107
69.08
7.4
100
10.7
22.70
2.43
0.143
100
14.3
75
10.72
50
7.15
Hardness
0.143
70.27
10.05
100
14.3
1.51
0.22
Machinability
0.179
100
17.9
50
8.95
75
13.43
Mass Density
0.143
34.65
4.95
33.88
4.84
100
14.3
Compressive
Strength
Modulus of
Elasticity
Rust
Resistance
Material
performance
index, γ
∑αβ=83.1
∑αβ=67.58
∑αβ=62.26
Result
Material Performance Index (γ) is greater for Mild steel (83.1). So, we will use mild steel for Hand
wheel, Base and Shaft.
Table 7.3: Material properties of Mild steel, Stainless steel and Aluminum
Property
Mild steel
Stainless steel
Aluminum
Density
7820
8000
2710
Compressive
Strength
407.7×106
170×106
280×106
Modulus of elasticity
210
304
69
Hardness
130
185
2.8
Table 7.4: Numerical Values for Cost, Availability, Corrosion resistant and machinability
Very high
5
High
4
Medium
3
Low
2
Poor
1
Selection criteria
Mild steel
Stainless steel
Aluminum
Cost
3
5
3
Availability
4
3
3
Rust Resistance
4
3
2
Machinability
4
2
3
Formula Used:
1. For goals: Availability Compressive Strength Modulus of Elasticity Rust Resistance
Hardness MachinabilityNumerical value of property
Scaled Property, β =
× 100%
Maximum value in list
2. For goals: Cost & Mass density Minimum value in list
Scaled Property, β =
× 100%
Numerical value of property
7.4.2 Material selection for Hopper, Chopping plate, Tray
Table 7.5: determination of relative importance of goals using digital logic method
1
2 3
4
5
6
Cost
1
0 0
0
1
0
Availability
0
Shear
Modulus
Density
Wear
Resistance
Hardness
Rust
Resistance
7
0
1
0 0 1
0
0 1
0
1
0
1
2
0.095
3
0.142
3
0.142
5
0.238
0
1
0.047
1
5
0.238
21
∑α=1
0
1
1
0
0.095
1
1
1
2
8 9 10 11 12 13 14 15 16 17 18 19 20 21
1
1
Relative
Emphasis
coefficient
Selection
Criteria
Positive
decision
Number of positive decision, N =n (n-1)/2=7(7-1)/2=21
1
0
0
Total number of positive decisions
0 0
1 0
1
1
0
1
1
Table 7.6 Calculation of Performance index for Hopper, Chopping plate, Tray
Mild steel
Stainless steel
Alloy steel
Scaled
Scaled
Scaled
Weighted
Weighted
Weighted
Property(β
Property(β
Property(β) Score(αβ)
Score(αβ)
Score(αβ)
)
)
100
9.5
75
7.125
60
5.7
Selection
Criteria
Weighting
Factor(α)
Cost
0.095
Availability
0.095
100
9.5
100
9.5
50
4.75
Shear Modulus
0.142
87.5
12.425
32.5
4.615
100
14.2
Density
0.142
98
13.916
100
14.2
95
13.49
Wear Resistance
0.238
60
14.28
100
23.8
40
9.52
Hardness
0.047
50.71
2.383
100
4.7
45.71
2.148
Rust Resistance
0.238
60
14.28
100
23.8
80
19.04
Material
performance
index, γ
∑αβ=
76.28
∑αβ=
87.74
∑αβ=
68.84
Result
Material Performance Index (γ) is greater for Stainless steel (87.74). So, we will use mild steel
for Hopper, Chopping plate, Tray.
Table 7.7: Material properties of mild steel, stainless steel and alloy steel
Property
Mild steel
Stainless steel
Alloy steel
Shear Modulus
70
74
80
Density
7820
7690
8020
Hardness
71
140
64
Table 7.8: numerical values for cost, availability, wear resistance, rust resistance
Very high
5
High
4
Medium
3
Low
2
Poor
1
Selection criteria
Mild steel
Stainless steel
Alloy Steel
Cost
5
4
5
Availability
4
4
2
Wear resistance
3
5
2
5
4
Rust resistance
3
Formula Used:
1. For goals: Availability Compressive Strength Modulus of Elasticity Rust Resistance
Hardness Machinability-
Scaled Property, β =
Numerical value of property
Maximum value in list
× 100%
2. For goals: Cost & Mass density Scaled Property, β =
Minimum value in list
Numerical value of property
× 100%
7.5 Conclusion
Applying weighted average method between those alternatives we found the decision for
performing material for our Manual Compost Maker in different group. We always have
considered all criteria and required properties of the materials while applying weighted average
method that gave us a clear vision and helped us to make a decision to choose one between of
those candidates. So, we can say weighted average method was reliable and effective in our case
and quite easy. Now we can move to the next section of product design where suitable
manufacturing criterion will be selected by implying weighted average method for different parts
and processes.