Evaluation Methods for Material Selection with Case Study

Evaluation Methods for Material Selection
1
Assignment No. 1
On
‘Evaluation Methods for Material Selection- Case Study Approach’
Submitted by
DESHPANDE ADITYA NARESH
Roll No: S-42, GR No: 142130
(Fourth year B. Tech)
Under the Guidance of
Prof. M. R. Khodke Sir
Department of Mechanical Engineering
Vishwakarma Institute of Technology,Pune
Pune-411037
August 2016

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Introduction to Materials:
Materials are probably more deep-seated in our culture than most of us realize.
Transportation, housing, clothing, communication, recreation, and food production— virtually
every segment of our everyday lives is influenced to one degree or another by materials.
Historically, the development and advancement of societies have been intimately tied to the
members’ ability to produce and manipulate materials to fill their needs. In fact, early
civilizations have been designated by the level of their materials development (i.e., Stone Age,
Bronze Age).
The earliest humans had access to only a very limited number of materials, those that
occur naturally: stone, wood, clay, skins, and so on. With time they discovered techniques for
producing materials that had properties superior to those of the natural ones; these new
materials included pottery and various metals. Furthermore, it was discovered that the
properties of a material could be altered by heat treatments and by the addition of other
substances. At this point, materials utilization was totally a selection process, that is, deciding
from a given, rather limited set of materials the one that was best suited for an application by
virtue of its characteristics. It was not until relatively recent times that scientists came to
understand the relationships between the structural elements of materials and their properties.
This knowledge, acquired in the past 60 years or so, has empowered them to fashion, to a large
degree, the characteristics of materials. Thus, tens of thousands of different materials have
evolved with rather specialized characteristics that meet the needs of our modern and complex
society; these include metals, plastics, glasses, and fibres.
The development of many technologies that make our existence so comfortable has
been intimately associated with the accessibility of suitable materials. An advancement in the
understanding of a material type is often the forerunner to the stepwise progression of a
technology. For example, automobiles would not have been possible without the availability
of inexpensive steel or some other comparable substitute. In our contemporary era,
sophisticated electronic devices rely on components that are made from what are called
semiconducting materials.
There are, it is said, more than 50,000 materials available to the engineer. In designing
a structure or device, how is the engineer to choose from this vast menu the material which
best suits the purpose? Mistakes can cause disasters. During World War II, one class of welded
merchant ship suffered heavy losses, not by enemy attack, but by breaking in half at sea: the

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fracture toughness of the steel - and, particularly, of the welds was too low. More recently,
three Comet aircraft were lost before it was realised that the design called for a fatigue strength
that - given the design of the window frames – was greater than that possessed by the material.
We can take example of aerospace field. The total weight has fallen down almost by 60% from
past 50 years. Also field of nanotechnology, composites, etc. have given boost to the new
developed world.
Material Selection meaning:
Many times, a materials problem is one of selecting the right material from themany
thousands that are available. There are several criteria on which the finaldecision is normally
based. First of all, the in-service conditions must be characterized,for these will dictate the
properties required of the material. On only rareoccasions does a material possess the
maximum or ideal combination of properties.Thus, it may be necessary to trade off one
characteristic for another. The classicexample involves strength and ductility; normally, a
material having a high strengthwill have only a limited ductility. In such cases a reasonable
compromise betweentwo or more properties may be necessary.
A second selection consideration is any deterioration of material propertiesthat may
occur during service operation. For example, significant reductions inmechanical strength may
result from exposure to elevated temperatures or corrosiveenvironments.Finally, probably the
overriding consideration is that of economics: What willthe finished product cost? A material
may be found that has the ideal set ofproperties but is prohibitively expensive. Here again,
some compromise is inevitable.The cost of a finished piece also includes any expense incurred
during fabricationto produce the desired shape.The more familiar an engineer or scientist is
with the various characteristicsand structure–property relationships, as well as processing
techniques of materials,the more proficient and confident he or she will be to make judicious
materialschoices based on these criteria.
Importance of material selection for any purpose:
Material selection is one of the vital step in design of any machine element. Following
diagram shows its place in stepwise procedure for design of any component or system.

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Figure 1: Basic procedure for design of machine element
Figure 2: Basic factors for materials election
As seen from above diagram, the material selection is important process. Six basic factors
which are considered in selecting the material are as follows.
1
• Specify functions of element
2
• Determine forces acting on element
3
• Select suitable material for element
4
• Determine failure mode of element
5
• Find geometric dimensions of element
6
• Modify dimensions for manufacturing and assembly
7
• Proper working drawing of element
1
• Specify functions of element
2
• Determine forces acting on element
3
• Select suitable material for element
4
• Determine failure mode of element
5
• Find geometric dimensions of element
6
• Modify dimensions for manufacturing and assembly
7
• Proper working drawing of element

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The basic question is how do we go about selecting a material for a given part? This may seem
like a very complicated process until we realize than we are often restrained by choices we
have already made. For example, if different parts have to interact then material choice
becomes limited. When we talk about choosing materials for a component, we take into account
many different factors. These factors can be broken down into the following areas.
Now clearly these issues are inter-linked in some fashion. For example, cost is a direct result
of how difficult a material is to obtain and to machine. And the effect of the environment on
the material is clearly related to the material properties. So if we really want to use a novel or
unusual material, the choice must be made early in the design process. Then we can do the
detailed design work using the correct material properties. Consider the example of wooden
airplanes and metal-framed airplanes. If we were to design an airplane of either material we
will have to make the choice early. The end designs are quite different. So, the material choice
can radically alter the final design. But the possibility also exists that it may not.
Analysis of Material Performance Requirements:
The material performance requirements can be divided into five broad categories: functional
requirements, processability requirements, cost, reliability, and resistance to service conditions.
Functional Requirements
Functional requirements are directly related to the required characteristics of the part or the
product. For example, if the part carries a uniaxial tensile load, the yield strength of a candidate
material can be directly related to the load-carrying capacity of the product. However, some
characteristics of the part or product may not have simple correspondence with measurable
material properties, as in the case of thermal shock resistance, wear resistance, reliability, etc.
Under these conditions, the evaluation process can be quite complex and may depend upon
predictions based on simulated service tests or upon the most closely related mechanical,
physical, or chemical properties. For example, thermal shock resistance can be related to the
thermal expansion coefficient, thermal conductivity, modulus of elasticity, ductility, and tensile
strength. On the other hand, resistance to stress–corrosion cracking can be related to tensile
strength and electrochemical potential.
Manufacturing Processability Requirements

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The processability of a material is a measure of its ability to be worked and shaped into a
finished part. With reference to a specific manufacturing method, processability can be defined
as castability, weldability, machinability, etc. Ductility and hardenability can be relevant to
processability if the material is to be deformed or hardened by heat treatment, respectively. The
closeness of the stock form to the required product form can be taken as a measure of
processability in some cases. It is important to remember that processing operations will almost
always affect the material properties so that processability considerations are closely related to
functional requirements.
Cost
Cost is usually an important factor in evaluating materials, because in many applications there
is a cost limit for a given component. When the cost limit is exceeded, the design may have to
be changed to allow for the use of a less expensive material or process. In some cases, a
relatively more expensive material may eventually yield a less expensive component than a
low-priced material that is more expensive to process.
Reliability Requirements
Reliability of a material can be defined as the probability that it will perform the intended
function for the expected life without failure. Material reliability is difficult to measure,
because it is not only dependent upon the material’s inherent properties, but it is also greatly
affected by its production and processing history. Generally, new and nonstandard materials
will tend to have lower reliability than established, standard materials. Despite difficulties of
evaluating reliability, it is often an important selection factor that must be taken into account.
Failure analysis techniques are usually used to predict the different ways in which a product
can fail and can be considered as a systematic approach to reliability evaluation. The causes of
failure of a part in service can usually be traced back to defects in materials and processing,
faulty design, unexpected service conditions, or misuse of the product.
Resistance to Service Conditions
The environment in which the product or part will operate plays an important role in
determining the material performance requirements. Corrosive environments, as well as high
or low temperatures, can adversely affect the performance of most materials in service.
Whenever more than one material is involved in an application, compatibility becomes a
selection consideration. In a thermal environment, for example, the coefficients of thermal
expansion of all the materials involved may have to be similar in order to avoid thermal
stresses. In wet environments, materials that will be in electrical contact should be chosen

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carefully to avoid galvanic corrosion. In applications where relative movement exists between
different parts, wear resistance of the materials involved should be considered. The design
should provide access for lubrication; otherwise self-lubricating materials have to be used.
More Common Analytical Methods of Materials Selection:
1. Cost versus Performance method
2. Cost per unit quality method
3. Weighted Property Indices method
4. Benefit -Cost Analysis method
1. Cost versus Performance method
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 Rs. / 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.
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 3.
Figure 3:Relations between cost factors and a material property
So we have to consider for optimum cost the yield strength i.e. material from such diagram, as
sown the minimum cost is at lowest of all three which gives required yield strength of material.

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2. 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 (Rs/ MPa) strength is usually
one of the most important criteria. By introducing the density of the material and the market
price, the costof buying 1 MPa of strength, C, can be calculated
𝑐𝑜𝑠𝑡 =
𝑃𝜌
𝜎
Here, P: material price / unit weight
𝜌: Density
𝜎: Tensile strength
Materials with lower cost/ unit strength are preferable.
Since manufacturing costs are a significant factor in evaluatingmaterials, it can be considered
in the cost /unit property analysis byconsidering P as the cost of material + manufacturing and
finishingcosts
Limitations of this method
It considers only one property as the most critical and ignoring otherproperties so we can use
other approach since comparison of materials is a fundamental part of material selection. A
basis material can be selected and the other candidate materialscompared against it. The
relative cost / unit property, RC, is then given by:
Where, i: candidate material, b: basis material
If RC < 1: candidate material is less expensive than the basismaterial
3. Weighted Property Method:
In most applications, the selected material should satisfy more than onefunctional requirement.
In this method each material requirement (or property) is assigned acertain weight (which
depends on its importance to the performance ofthe design)
This method attempts to:
1. Quantify how important each desired requirement is by determininga weighting factor (α)
2. Quantify how well a candidate material satisfies each requirementby determining a scaling
factor (β)
2.1 Ranking of Attributes
Attributes are characteristics that can be described to distinguish oneitem from another

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Some attributes are more important than others. Determining therelative importance of the
various properties assigned to theseattributes is therefore necessary if this method is to be used.
There are two steps for ranking attributes:
1. Rank in order of importance with no consideration of how importantone attribute is to
another
2. Weight is assigned to the importance of each attribute
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, whileothers may be quite similar in importance. The
relative importance is shown by using a point scale that does notexceed 100 points.
e.g.; if strength is 4 times as important as cost, it will be representedby an 80 / 20
divisionWeight 4 times as important as strength, strength is 4 times as importantis as cost,
corrosion is 2 /3 the importance of strength, etc.
The number of attributes that should be listed vary between 5 to10. This method combine
properties with different units. This limitation isovercome by the use of a “scaling factor”
The relative merit of each property of the candidate material may beincorporated by assigning
the value of 100 (%) to the best material inthat property category. For a given property, the
scaling factor (β) for a given candidatematerial is:
For properties that should have maximum values (strength, toughness…)
For properties that should have low values (density, cost corrosion …)
3. Cost–Benefit Analysis:
The cost–benefit analysis is more suitable for the detailed analysis involved in making the final
material substitution decision. Because new materials are usually more complex and often
require closer control and even new technologies for their processing, components made from
such materials are more expensive. This means that for materials substitution to be
economically feasible, the economic gain as a result of improved performance should be more
than the additional cost incurred as a result of substitution.
For this analysis it is convenient to divide the cost of materials substitution into the following:

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 Cost Differences in Direct Material and Labour. New materials often have better
performance but are more expensive. When smaller amounts of the new material are used
to make the product, the increase in direct material cost may not be as great as it would
appear at first. Cost of labour may not be an important factor in substitution if the new
materials do not require new processing techniques and assembly procedures. If, however,
new processes are needed, new cycle times may result and the difference in productivity has
to be carefully assessed.
 Cost of Redesign and Testing. Using new materials usually involves design changes and
testing of components to ensure that their performance meets the requirements. The cost of
redesign and testing can be considerable in the case of critical components.
 Cost of New Tools and Equipment. Changing materials can have considerable effect on life
and cost of tools, and it may influence the heat treatment and finishing processes. This can
be a source of cost saving if the new material does not require the same complex treatment
or finishing processes used for the original material. The cost of equipment needed to
process new materials can be considerable if the new materials require new production
facilities, as in the case of replacing metals with plastics.
Based on the above analysis, the total cost of substituting a new material, n, in place of an
original material, in a given part is
Where,
Pn, Po = price per unit mass of new and original materials used in part
Mn, Mo = mass of new and original materials used in part
ƒ = capital recovery factor, can be taken as 15% in absence of information
Ct = cost of transition from original to new materials
N = total number of new parts produced
Tn, To = tooling cost per part for new and original materials
Ln, Lo = labour cost per part using new and old materials
The gains as a result of improved performance can be estimated based on the expected
improved performance of the component, which can be related to the increase in performance

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index of the new material compared with the currently used material. Such increases include
the saving gained as a result of weight reduction or increased service life of the component:
Where,
γn, γo= performance indices of new and original materials, respectively
A = benefit of improved performance of component expressed in $ per unit increase in material
performance index.

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CASE STUDY
MATERIAL SELECTION FOR TABLE LEGS FOR DOMESTIC USE
Here, we want to select material for table legs. And following data shows the highlights of
project. We want to minimize cross-section and mass without buckling and toughness and cost
are factors.
Figure: 4 Case study problem: table
VARIOUS HIGHLIGHTS OF PROJECT:
• Length specified
• Must not buckle
• Must not fracture
• Material
• Minimize
mass
• Maximize
slenderness
• Support
compressive
loads
Function Objective
Constraints
Free
variables

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STEP 1 FINDING LOADS AND SLENDERNESS
Equation used for calculating critical load is
The 𝑃𝑐𝑟𝑖𝑡 =
𝜋2
𝐸 𝐼
l2
Where, l= length of beam
E= Material Young’s Modulus
I= moment of inertia of cross section
Use constraints to eliminate free variable ‘r’
𝑚 >
4𝑃
𝜋
l2
{
𝜌
𝐸1/2} where m is mass of component
𝑟 > {
4𝑃
𝜋3}^1/4 l1/ 2
{
1
𝐸1/4} where r is radius of beam of leg.
Here,
Minimize mass by maximizing M1, 𝑀1 =
𝐸
1
2
𝜌
and,
Maximize slenderness by maximizing M2, M2= E
STEP 2 MATERIAL ELIMINATON PROCESS
Materials considered are wood, steel, composites, foams, ceramics, polymers. Here now the
following materials are eliminated as per the mass and stiffness consideration.
Metals become too heavy and costly. Now as we can see, polymers foams, metals are
eliminated. This is done on strength and mass consideration. Polymers are not stiff enough.
Figure 5 Screening process- Material Elimination

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STEP 3 MATERIAL SCREENING PROCESS
So we come with Possibilities of Ceramics, wood, composites only out of 5.Again here the
other properties like brittleness matters of ceramic comes in picture. These are vulnerable
material and shown by pink colour in above figure. And the Composites are too expensive.
Figure 6 Screening process- Material Elimination 2
STEP 4 MATERIAL SCREENING PROCESS FINAL SELECTION
And Final choice is wood, and shown by green colour.
Figure 7 Screening process- Final Selection

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Here, we have used Weighted Property Indices method to suit the problem specification with
goal to select the proper material for legs of chair for domestic use.
Property Rank 1/2 1/3 1/4 1/5 Weight
Strength 1 20 60 50 80 0.30
Density 2 80 - - - 0.5
Cost 3 - - - 20 0.20
Total 1.00
Figure:8 Analysis of Properties according to Weighted Property Method
CONCLUSION:
This case study shows the potential of Weighted Property Indices method. Here we found
weighted property and by cost and density and energy absorption data we come to know that
the use of wooden material shows great mass reduction with increased strength the cost also
reduced to about 30%. The case study serves as a beneficial example for material selection of
modern automobiles.
REFERENCES
1. Material Selection in Mechanical Design, Michael F. Ashby, Butterworth-Heinemann
2. Materials and Process Selection for Engineering Design: Mahmoud Farag
3. Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third
Edition, Wiley publication, USA.
4. Design Optimization Case Study: Car Structures, Mark Carruth, Universityof Cambridge.
5. Design Of Machine Elements, Bhandari V. B., Mc-Graw Hill Publications, 2013

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