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Why we need different material?
The main reason for the choice of material is the financial constraints one has to see, besides technical factors for instance, strength, shear strength, tensile strength. During the last decades many new materials and material types have been developed. At present era, there is about 1 lac engineering materials that exist. Upon research, many existing materials have successfully improved their properties. This material selection is a part of several design criteria. For example, previously cast iron in cylinder heads, where as now cast aluminum alloys are also used so that the better material properties formed as a result of combination of two different materials which help the product to have long life.
Design process:
It is vital to consider every possible alternative during the design process as design decisions will determine the feasibility and cost of the manufacturing processes and the final product. It is difficult process to determine feasible combinations of material and manufacturing processes during concept design since the requirements and product characteristics are not known completely before hand.
Methods of material selection:
First best material method is used in industrially developing countries (like Pakistan) where designing is rarely done while industries emphasize merely upon production. Another method which is used Common material method. It is assumed that the component which is to be designed will work same in different products.
The above mentioned methods are non- systematic design processes are not preferable due to the wastage of both time and money. This is why efficient designers prefer systematic designing process.
Systematic designing process:
Material selection can be made during any stage of the life cycle of a product but usually it is done when the component is first designed or when it is redesigned. The reason is simply that the introduction of a new material in general requires the modification of the component geometry. If failures
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of the part take place it may be necessary to change the material. Many times even small geometrical changes make a new material the optimum one.
The Ashby approach is design-led. It starts by asking 'What is the function of the component in the design?', 'What objectives need to be optimized?', and 'What constraints must be satisfied?'
For instance, a car body panel (function) needs to be as light as possible (objective) for a specified stiffness and cost (constraint). Other constraints on the design might be acceptable resistance to mechanical impact and to contact with various environments.
The advantage of this approach is that it is systematic and unbiased in its focus on product objectives and it leads to optimization or innovation of the product. We will be using this approach in our case study.
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Factors that affect design process:
There are no hard and fast rules that how many factors should be considered as key factors. It depends from case to case.
For example: If you are designing an industrial robotic arm you will be bound to account for the max shear and normal stresses, the density of the material that is to be considered for use and correspondingly the total weight of the arm, the tensile strength as well the factors for the working of internal components involved such as the motors used to move the arm ,their maximum and minimum angular frequencies, maximum and minimum voltages that can be applied to motor, the sensors and controllers etc.
Aesthetic appearance of the product is also a factor which helps to gain more value of the product in market.
Case study:
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(For CNG cylinder of car)
We now consider case study of material selection for a CNG cylinder, we shall keep the dimensions constant for each material which we shall consider. We shall select material for frame only.
Assumptions:
We assume that this case study is for a thin walled cylinder and there is no torsion or shear stresses.
The procedure of our methodology:
1) Compare Aluminum alloys among themselves and picking the most suitable material in Al alloys
2) Compare Steel and carbon alloys among themselves and picking the most suitable material in Fe and C alloys
3) Compare above two dominant representatives, the most suitable will be chosen.
The factors that we will be taking in account will be:
1) Weight or density2) Yield strength3) Tensile strength4) Specific heat5) Thermal expansion6) Thermal conductivity
Comparison Among Steels:
Iron/ steel Density(Kg/m^3)
Tensile strength(Mpa)
Yield strength(Mpa)
Specific heat
thermal expansion*10^-6(20 C)
Thermal conductivity
SAE 4027 7872 515 325 477 12.6 44.6
SAE 4340 7872 745 472 477 12.6 44.6
SAE 8620 7872 536 357 477 12.2 46.6
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SAE 1010 7872 325 305 448 12.2 51.9
SAE 1045 7872 625 310 486 11.7 50.9
SAE 1070 7872 640 495 490 12.1 51.2
ASTM A536 6640 496 345 506 11.6 32.3
ASTM 40 7200 276 827 490 9 53.3
Comparison of properties of different steels using graphs:
Weight or density:
SAE 4027
SAE 4340
SAE 8620
SAE 1010
SAE 1045
SAE 1070
ASTM A536
ASTM 40
60006200640066006800700072007400760078008000
Density
Density
Weight considerations are important because too much weight will affect the performance of the car as we are dealing with case study of a CNG cylinder for a car. As dimensions are kept constant initially,so keeping that in view, it is clear from the above chart that ASTM A536 and ASTM 40 may be sensible choices while considering the density as the major factor.
Yield strength:
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SAE 4027
SAE 4340
SAE 8620
SAE 1010
SAE 1045
SAE 1070
ASTM A536
ASTM 40
0
100
200
300
400
500
600
700
800
900
Yield strentgh
Yield strentgh
Yield strength is an important criterion for the mechanical stability of a material. ASTM 40 has the greatest yield strength of 827 MPa while other steels will mostly deform at about 300-400 MPa and SAE 1070 at 495 MPa, Note that it is the minimum value and it is for the case of compressive stresses.
Tensile strength:
SAE 4
027
SAE 4
340
SAE 8
620
SAE 1
010
SAE 1
045
SAE 1
070
ASTM A536
ASTM 40
0100200300400500600700800
Tensile strength
Tensile strength
Tensile strength determines to what extent a material is ductile. The graph shows that ASTM 40 has the least resistance to normal stresses and
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it is the most ductile among all. So, it should not be selected for the situations where normal stresses are very large.
Specific heat:
Thermal properties related with the material for a CNG cylinder are also crucial.
Specific heat determines how much heat the material would absorb. Specific heat is directly proportional with its tendency to absorb heat. We shall be interested in choosing that material with lowest specific heat. The lowest is SAE1010 and the highest is ASTM A536. ASTM 40 and SAE 1070 are sharing the second spot for the comparison. So, if one wishes to keep in view specific heat as the main factor, he would naturally go for SAE1010.
SAE 4027
SAE 4340
SAE 8620
SAE 1010
SAE 1045
SAE 1070
ASTM A536
ASTM 40
400
420
440
460
480
500
520
specific heat
specific heat
Thermal expansion:
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SAE 4027
SAE 4340
SAE 8620
SAE 1010
SAE 1045
SAE 1070
ASTM A536
ASTM 40
0
2
4
6
8
10
12
14
Thermal expansion
Thermal expansion
If the temperature inside the cylinder varies largely, significant thermal expansion may occur which is known as thermal stress.The graph shows that ASTM 40 has the maximum resistant to thermal stresses as its co-efficient of thermal expansion is the lowest
Thermal conductivity:
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SAE 4027
SAE 4340
SAE 8620
SAE 1010
SAE 1045
SAE 1070
ASTM A536
ASTM 40
0
10
20
30
40
50
60
Thermal conductivity
Thermal conductivity
The thermal conductivity of a material is a quantitative measure of its ability to conduct heat. The cylinder under the application of high pressure (as the CNG name specifies Compressed Natural Gas), if temperature difference between surrounding and cylinder is high then the heat transfer will take place which cannot be ignored. Keeping temperature difference constant, the most efficient material will be the one having the least thermal conductivity. That material would allow the minimum heat transfer to take place which will avoid surplus expansion due to high temperature and hence pressure of the gas.
ASTM 40 has the largest value for thermal conductivity among the all which is its main disadvantage while ASTM A536 has the lowest.
Comparison of Al-Alloys:
Aluminum is a light weight metal but have low strength that is why it is used in combination with other materials.
Below is the tabular data of some of Al-alloys:
Name of Alloy
Density(Kg/m^3)
Tensile strength(Mpa)
Yield strength(Mpa)
Specific heat
thermal expansion*10^-6(20 C)
Thermal conductivity
Al-Mg 5050 2690 221 55 900 23.8 193
Al-Zn 7005 2780 195 80 875 23.6 166
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Al-Mn 3003 2730 110 41 893 23.2 193
Al-Cu 2011 2830 379 296 880 22.9 151
Al-Cu 2014 2800 186 97 880 23 193
Al-Cu 2017 2790 179 69 880 23.6 193
Density:
Al-Zn 7005
Al-Mn 3003
Al-Cu 2011
Al-Cu 2014
Al-Cu 2017
2680
2700
2720
2740
2760
2780
2800
2820
2840
density
density
Al-Cu 2011 has the largest value for density making it unsuitable for weight considerations. Al-Mn3003 has the least density
Yeild Strength:
The graph shows that Al-Cu (especially Al-Cu 2011) has a better and greater tendency to bear normal stresses than any of the other alloys of Aluminum.
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Al-Mg 5050
Al-Zn 7005
Al-Mn 3003
Al-Cu 2011
Al-Cu 2014
Al-Cu 2017
0
50
100
150
200
250
300
350
yield strength
yield strength
Tensile strength:
Al-Mg 5050
Al-Zn 7005
Al-Mn 3003
Al-Cu 2011
Al-Cu 2014
Al-Cu 2017
0
50
100
150
200
250
300
350
400
Tensile strength
Tensile strength
Al-Cu 2011 has the greatest tensile strength and Al-Mn 3003 has the least value.
Specific heat:
All Al-Cu alloys have same value for specific heat but yet Al-Zn 7005 has the least specific heat, making it a bit competent.
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Al-Mg 5050
Al-Zn 7005
Al-Mn 3003
Al-Cu 2011
Al-Cu 2014
Al-Cu 2017
860865870875880885890895900905
specific heat
specific heat
Thermal Expansion:
Al-Mg 5050
Al-Zn 7005
Al-Mn 3003
Al-Cu 2011
Al-Cu 2014
Al-Cu 2017
22.4
22.6
22.8
23
23.2
23.4
23.6
23.8
24
Thermal expansion
Thermal expansion
Al-Cu 2011 has the least value for the thermal expansion and this thing indicates that it is the most resistant to thermal stresses.
Thermal conductivity:
AL-Cu 2011 has the least conductivity among the all while Al-Mg5050 ,Al-MN 3003 and ,Al-Cu 2014 and Al-Cu 2017 have same value that 193.
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Al-Mg 5050
Al-Zn 7005
Al-Mn 3003
Al-Cu 2011
Al-Cu 2014
Al-Cu 2017
0
50
100
150
200
250
thermal conductivity
thermal conductivity
Semi-final decision:
From the above mentioned detailed comparison carried out, we conclude that among the Al alloys Al-Cu 2011 is the most appropriate and among the steels ASTM A536 and ASTM 40 are suitable.
Density of Al-Cu 2011 is greatest among Al alloys which constraints it. ASTM 40 has the greatest thermal conductivity among the steels, while ASTM A536 has the greatest specific heat value. Now we need to compare these three.
Al-Cu 2011 ASTM 40 ASTM A5360
1000
2000
3000
4000
5000
6000
7000
8000
Density
DEnsity
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Al-Cu 2011 ASTM 40 ASTM A5360
100
200
300
400
500
600
Tensile strength
Tensile strength
Al-Cu 2011 ASTM 40 ASTM A5360
100200300400500600700800900
1000
Specific heat
Specific heat
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Al-Cu 2011 ASTM 40 ASTM A5360
100
200
300
400
500
600
700
800
900
Yield strength
Yield strength
Al-Cu 2011 ASTM 40 ASTM A5360
20
40
60
80
100
120
140
160
Thermal conductivity
Thermal conductivity
Tabular results of graphs:
Al-Cu 2011 ASTM 40 ASTM A536density low highest mediumTensile Strength medium low highestSpecific heat highest low mediumYield strength low highest mediumThermal conductivity highest medium low
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So, Al-Cu 2011 is ruled out straight away in the semi-final comparison. We are left with ASTM 40 and ASTM A536 which are tough competitors. We shall find out that which one of these two is more appropriate.
Final decision:
Property ASTM 40 ASTM A536
Corrosion resistance
Low Low
Weldibility Low Low
Machinability Good Good
Castability High Good
Wear resistance Low Low
ASTM A536 is more suitable for the CNG cylinder. Although, it has relatively a lower castability than ASTM 40 and also it has higher yield strength, specific heat and co-efficient of thermal expansion.
Despite all these facts and properties it is lighter, has a higher tensile strength which is very important because we are going to assume that this case study was for a thin walled cylinder and there are no torsion or shear stresses.
Difference in the values of co-efficient of thermal expansion and specific heat is very small, so it does not matter much. Rather the thermal conductivities do matter and that is significantly low for ASTM A536. Castability of ASTM 40 is high but that of ASTM A536is good rather than low.
Result:
So we chose ASTM A536 for the application in CNG cylinder of car.
