Assignment 809--Principle of Material Selection

MME/06/8201, LOTO OLUWASAYO I.

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Contents 1.0 PRINCIPLE OF MATERIAL SELECTION .................................................................................................. 2

1.1 Material Selection for a New Design .............................................................................................. 2

1.2 Material Substitution for an Existing Design ................................................................................... 3

1.3 Design Limiting Properties ............................................................................................................. 4

1.4 Functions, Objectives, Constraints And Variables ........................................................................... 4

1.5 Process of Material Selection in relation to Design ......................................................................... 5

2.0 PERFORMANCE CHARACTERISITICS OF MATERIALS ............................................................................ 5

2.1 TOOLS IN THE SELECTING PROCEDURE............................................................................................... 8

2.1.1Material Selection Chart........................................................................................................... 8

2.1.2 Material Indices .................................................................................................................... 11

3.0 CASE STUDIES .................................................................................................................................. 12

3.1 SPRINGS ....................................................................................................................................... 12

3.2 Con-Rods For High-Performance Engines ..................................................................................... 19

3.3: THE AUTO COOLING FANS........................................................................................................... 23

3.4: Materials for Flywheels ............................................................................................................... 26

3.5: Brake Disc ................................................................................................................................... 30

3.6: Engine Cylinder ........................................................................................................................... 31

3.7: Automobile Exhaust System ........................................................................................................ 31

3.8: Automobile Chassis- Body in Weight ........................................................................................... 31

3.9: The Dash board ........................................................................................................................... 32

3.10: Automobile Engine block .......................................................................................................... 32

4.0 Conclusion: ...................................................................................................................................... 32

5.0 REFERENCES .................................................................................................................................... 33


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1.0 PRINCIPLE OF MATERIAL SELECTION

The selection of the correct material for a design is a key step in the design process because it is

the crucial decision that links computer calculations and lines on an engineering drawing with

working design.

The enormity of the decision task in selecting the right material is given by the fact that there

are well over 100,000 engineering materials from which the material engineer has to choose.

The importance of material selection in design has increased in recent years as material

engineer are brought to the design process at an earlier stage and the importance given to

manufacturing in the present day product design has reinforced the fact that materials and

manufacturing are closely linked in determining final properties.

Moreover the great activity in material science and research has created a variety of new

materials and focused attention on competition between the six broad classes of materials-

materials, polymers, elastomers, ceramics, glasses and composites. Thus the range of materials

available to engineer is much larger than ever before. This present the opportunity for

innovation in design by utilizing these materials in products that provides greater performance

at lower cost. To achieve this requires a more rational process of material selection.

An incorrect chosen material can lead not only to failure of the part but also to unnecessary

cost. Selecting the best material for a part involves more than selecting a material that has the

properties to provide the unnecessary performance in service; it is also intimately connected

with the processing of the material into the finished product.

Design proceeds from concept design to embodiment (configuration), to detail (parametric

design) and the material process selection then becomes more detailed as the design

progresses through this sequence.

1.1 Material Selection for a New Design

1. Define the functions that design must perform and translate to required material

properties such as stiffness, strength, corrosion resistance, cost and availability of the

material.

2. Define manufacturing requirement such as number of parts required, size, complexity of

part, precision require and tolerance and the overall fabricability of the material.


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3. Do the initial screening process by comparing the needed properties and parameters

with a large material property database to select a few materials that look promising.

Several of the screening properties must be evaluated as it helps to check if the material

can be evaluated further for a particular application.

4. Investigate the candidate materials in more detail particularly in terms of trade-offs in

product performance, cost, fabricability and availability in the grades and sizes needed

for the application. Here material indices and performance indices are employed leading

to selection of a particular material.

5. Develop design data and/or design specification.

1.2 Material Substitution for an Existing Design

1. Characterize the currently used material in terms of performance, manufacturing

requirements and cost.

2. Determine which characteristics must be improved for enhanced product function.

Failure analysis role at this stage.

3. Search for alternative materials /and or manufacturing routes. Use of the idea of

screening properties will be of advantage here.

4. Compile a short list of materials and processing routes and use these to estimate cost of

manufactured parts. A useful tool here is value engineering, a problem solving

methodology that focuses on identifying the key function(s) of a design so that

unnecessary cost can be removed without compromising the quality of the design.

5. Evaluate the results in step (4) and make recommendation for a replacement material.

Determine the critical properties with specification and testing as in step 5 of previous

section.

Generally, this process of material selection can be said to involve four (4) strategies:

(a) Translation (b) Screening (c) Ranking (d) Documentation

Material selection is design-led as properties of a new material can suggest new product.

Example is transistor from high purity silicon or the need for a new product can demand the


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development of new materials e.g. high temperature alloys for turbine technology and light

weight composites for space technology.

1.3 Design Limiting Properties

There are some constraints that translate into limit on the property of the material to be

chosen for a particular application. This means that, to achieve a desired level of performance,

the values of the design-limiting properties must meet certain targets—those that fail to do so

are not suitable. For instance, in the design of power transmission lines electrical resistivity is

design limiting; in the design of a camera lens, it is optical quality and refractive index.

1.4 Functions, Objectives, Constraints And Variables

Any engineering component has one or more functions: to support a load, to contain a

pressure, to transmit heat and so forth. This must be achieved subject to constraints: that

certain dimensions are fixed, that the component must carry the design loads without failure,

the need to insulate against or to conduct heat or electricity that it can function in a certain

range of temperature and in a given environment, and many more. In designing the

component, the designer has one or more objectives: to make it as cheap as possible, perhaps,

or as light, or as safe, or some combination of these. Certain parameters can be adjusted in

order to optimize the objective—the designer is free to vary dimensions that are not

constrained by design requirements and, most importantly, free to choose the material for the

component and the process to shape it. We refer to these as free variables.

A constraint is an essential condition that must be met, usually expressed as a limit on a

material or process attribute. An objective is a quantity for which an extreme value (a

maximum or minimum) is sought, frequently cost, mass or volume, but there are others.

In choosing materials for a super-light sprint bicycle, for example, the objective is to minimize

mass, with an upper limit on cost, thus treating cost as a constraint. But in choosing materials

for a cheap ‘shopping’ bike the two are reversed: now the objective is to minimize cost with a

(possible) upper limit on mass, thus treating it as a constraint. The outcome of the translation

step is a list of the design-limiting properties and the constraints they must meet. The first step


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in relating design requirements to material properties is therefore a clear statement of

function, constraints, objectives and free variables.

Function: What does the component do?

Constraints: What non-negotiable conditions must be met?

Objective: What is to be maximized or minimized?

Free variables: What parameters of the problem is the designer free to change

1.5 Process of Material Selection in relation to Design

A material selection usually involves one or two situations;

a) Selecting a material for a new product or design

b) Evaluation of an alternative material for an existing product or design. Such redesign

effort is usually taken to reduce cost increase, reliability or improve performance. It is

generally not possible to realize the full potential of substituting one material for

another without considering its manufacturer characteristics. In other words, simple

substitution of a new material without changing the design provides optimum utilization

of the material.

2.0 PERFORMANCE CHARACTERISITICS OF MATERIALS

The performance or functional characteristics of a material are expressed chiefly by physical,

mechanical, thermal, electrical, magnetic and optical properties. Material properties are a link

between the basic structure and composition of the material and the service performance of

the component part.

The performance of materials may be found satisfactory within certain limitations or

conditions. However, beyond these conditions, the performance of materials may not be found

satisfactory.


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One can list the major factors affecting the selection of materials as

(i) Component shape

(ii) Dimensional tolerance

(iii) Mechanical properties

(iv) Fabrication requirements

(v) Service requirements

(vi) Cost of the material

(vii) Cost of processing

(viii) Availability of the material.

All these major factors have a complex effect on the selection of materials. The shape and size

of a component has great effect on the choice of the processing unit which ultimately affects

the choice of the material. To make it more clear, we consider an example, let the best possible

production method is selected, under given conditions, it is die casting, obviously, now the

choice of the material becomes limited, i.e. one can only choose materials with lower melting

points, e.g. aluminum, zinc, magnesium and thermoplastics.

There are some materials which can be finished to close tolerance while others cannot.

Obviously, the required dimensional tolerance for finished components will, influence the

choice of materials. To select a suitable material for specific conditions, all mechanical

properties, e.g., hardness, strength, etc. guide us. Method of processing of the material also

affects the properties of a component, e.g., forged components can be stronger than the casted

components. Different types of working processes may also give different types of fiber

structure. However, investment casting can provide precise dimensions at low cost in

comparison to machine operations.

Service requirements are dimensional stability, strength, toughness, heat resistance, corrosion

resistance, fatigue and creep resistance, electrical and thermal conductivity etc. whereas

fabrication requirements are castability, i.e., ease in casting a material, weldability-ease in

welding the material, machinability-ease to machine a material, formability-ease to form a

material, hardenability etc.


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In most of the cases, the cost of raw material accounts about 50 per-cent of the finished cost.

Obviously, the cost of the material is a major factor which influences the choice of the material

or process. We must note that the use of cheaper material will not always reduce the final cost

of the component or product. Use of cheaper material may be associated with higher

processing cost due to large number of operations to be performed and also more scrap. We

can easily see that this sometimes makes the overall cost more than that of expensive raw

material in combination with low processing cost due to lesser number of operations and lesser

scrap.

The type of material affects the detailed aspect of design and hence the choice of material as

well as the process is selected at the early design state, e.g., whether the material is to be

joined by spot welding, screws or rivets must be decided at the design state.

In most of the industries, the processing cost (labour cost) and other costs such as overhead

costs account for about 50% of the production cost. Overhead cost in automatic industries is

much more than the other costs. If one can somehow reduce all such costs, the total

production cost will automatically reduce. In comparison to conventional processes, sometimes

injection moulding process is preferred because the conventional process involves many

intermediate stages and several machining processes. One finds that the cost of production of a

component by rolling or forging operations is twice as compared to the production by powered

metallurgy process because the rolling or forging are followed by several machining operations

for the same finish and tolerances. We may find that sometimes the availability of the material

becomes a governing factor. When the desired material supply is limited, then a costly material

which is available in ample quantity may be chosen.

In the light of above factors, sometimes it may be the case that two or more than two materials

are found suitable for a particular component or job. Sometimes, it may also happen that the

above factors may oppose each other. This shows that there may no exact or true solution and

one has to compromise in the final selection. One can compromise by taking into consideration

the relative merits and demerits, cost of finished component and its life.

Summarizing, we can say that the selection of material is a dynamic process and change in

design may be progressive. Keeping in view, the availability and awareness of latest


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technological developments, one can always change the material from time to time. While

making the selection of material amongst the available large range of different types of

materials the wisest choice should be made keeping in view the above factors to achieve the

efficient utilization of materials.

2.1 TOOLS IN THE SELECTING PROCEDURE

There are some tools that are employed in process of selecting a material which enables us to

easily ascertain the best choice of material to be employed by comparing selected properties

for a set or materials or relating the effect some functional parameters have on the overall cost

of the material. Some of this tools will be discussed below.

2.1.1Material Selection Chart

Is a diagram with one (or a combination) of material properties plotted on each axis. It is a slice

through a multi-dimensional material-property space. Figure 1 illustrates the concept. It shows,

schematically, a chart with Young's modulus and density on the axes. The scales are

logarithmic, and span a range so wide that all materials are included.

FIG 1: Material Selection Chart. The axes are modulus E and density ρ. The logarithmic scales allow performance

indices to be plotted as straight lines. Adapted from CES EDU 2008 version.


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When data for a given material class such as metals are plotted on these axes, it is found that

they occupy a field which can be enclosed in a 'balloon'. Ceramics also occupy a field, and so do

polymers, elastomers, composites, and so on. The fields may overlap, but are nonetheless

distinct.

Almost always, further information can be plotted onto the diagram. The longitudinal wave

speed vl (essentially the speed of sound in the material) is plotted on figure

Because of the logarithmic scales, lines of constant wave speed form a family of parallel lines of

slope 1.

A more comprehensive E-ρ chart is shown in figure 4. Individual materials or sub-classes, like

steels or polypropylenes (PP) appear as little 'bubbles' which define the range of the property.

All of the bubbles for one class of material are enclosed in a balloon: the metals-balloon, the

polymers-balloon, and so on.


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Figure 2: A modulus-density chart illustrating the selection of materials with high values of M1 = E1/2/ρ. Contours

of constant E1/2/ρ appear as a family of lines of slope 2. Materials with M1 greater than a chosen value can be

identified.

A Material Selection Chart with E and ρ as axes can immediately be used to identify the subset

of materials with the greatest value of M1 = E1/2/ρ. Taking logs of both sides of equation ABOVE

gives:

Each value of M1 therefore corresponds to a line on the chart with slope 2. The larger the value

of M1, the higher lies the line. A line corresponding to M1 = 5.5 GPa1/2/(Mg/m3) is shown on

figure 2. The materials which lie above it have exceptionally high values of M1. It can be seen

that woods, engineering composites and some ceramics are the best choices for a light stiff

beam with square cross-section. Why then, are light-weight structures, such as aircraft frames

no longer made of wood? The answer lies in the cross-section shape of the beam, as illustrated

in the following section.

ms-its:C:/Program Files (x86)/CES EduPack 2008/indepth.chm::/html/indepth/materialinfo/reading_fig4.htm


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Another example of a materials selection chart would be a plot of yield strength (failure

strength) σy versus density ρ which would allow selection of materials for light, strong

structures. Charts with axes which are combinations of properties (axes of E / ρ and σy / ρ, for

instance), or which measure relative corrosion resistance or wear resistance can also be helpful.

2.1.2 Material Indices

The design of a mechanical component is specified by three things: the functional requirements

(the need to carry loads, transmit heat, store elastic or thermal energy, etc), the geometry, and

the properties of the material of which it is made, including its cost. The performance of the

element can be described by an equation with the general form

,

where p describes the aspect of performance of the component that is to be optimized: its

mass, or volume, or cost, or life for example; and f( ) means 'a function of'. Optimum design can

be considered to be selection of the material and geometry which maximize (or minimize) p.

The optimization is subject to constraints, some of them imposed by the material properties.

The three groups of parameters in equation above are said to be 'separable' when the equation

can be written

where f1, f2 and f3 are functions. When the groups are separable, the optimum choice of

material becomes independent of the design details. The optimum material is the same for all

geometries G, and all values of the functional requirements F. Then the optimum material can

be identified without solving the complete design problem, or even knowing all the details of F


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and G. This enables enormous simplification: the performance for all F and G is maximized by

maximizing f3(M), which is called the 'performance index'.

Following these principles of material selection, the task of selecting a material may be very

cumbersome, however there is a simplification of the process by the use of already developed

softwares which contain a database of most of the common materials encountered in process

of selecting a material for a component or process. The most common and widely used is the

CES selector developed by Ashby and Granta. It makes the selection process less cumbersome

as different material properties can easily be compared over a wide range of material types.

Depending on the functional requirements, objectives and constraints the best suitable

material can be obtained.

In the remaining aspect of this paper, we will look into some case studies explain the process of

material selection.

3.0 CASE STUDIES

We are going to consider a typical automobile car component. An automobile car has a lot of

component in the design therefore we will select some part for our studies. Generally, any

automobile component design will want to minimize the weight of the car in order to reduce

fuel consumption and invariably gas emissions. These are the basic considerations for design

and there are various standards requirement and organizations that sees to environmental

issues in the design. It should however be noted that each of the component study will be

summarized so that we can discus at least ten of the parts in an automobile design. Hence, it

only gives a basis of what we would expect in designing of such component parts.

3.1 SPRINGS

The best material for a spring is that which can store the greatest elastic potential energy per unit mass

(or volume), without failing. In an automobile we have springs in the shock absorber (helical) and also in

the valves and some other reciprocating part. Springs for vehicle suspensions must resist fatigue (the

selection should then be made with the endurance limit, σe, replacing the modulus of rupture, σMOR).


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Valve springs for the engines must cope with elevated temperatures; here strength-at-temperature is

needed. The performance indices derived below can be used to identify materials which satisfy the

design specification summarized. Note that a generalized form of equation using MOR was used

however parameters can be changed to determine other characteristics.

FIG 3: Springs have many shapes, but all perform the same function: that of storing elastic

energy.

FUNCTION Elastic Spring

OBJECTIVE (a) Maximum stored elastic energy/unit volume

(b) Maximum stored elastic energy/unit mass

CONSTRAINTS No failure by yield, fatigue or fracture (whichever is more restrictive)


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Adequate toughness (Gc > 1 kJ/m2)

Reasonable cost per unit weight (Cm < 100 GBP/kg)

The primary function of a spring is that of storing elastic energy and releasing it again when required. The elastic energy stored per unit volume in a block of material stressed uniformly to a stress σ is:

,

where E is the Young's modulus. It is this that we wish to maximize. The spring will be damaged if the stress σ exceeds the yield stress or failure stress σf. So the constraint is σ ≤ σf. The maximum energy density is therefore:

,

Torsion bars and leaf springs are less efficient than axial springs because some of the material is not fully loaded: the material at the neutral axis, for instance, is not loaded at all. For solid torsion bars

,

and for leaf springs loaded in pure bending the maximum energy storage is

.

But, as these results show, this has no influence on the choice of material. The best material for springs, regardless of the way in which they are loaded, is that with the biggest value of


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.

If mass matters rather than volume, we must divide this by the density ρ (giving energy stored per unit mass), and seek materials with high values of

.

The Selection

The selection of materials for springs of minimum volume is shown in below. Here the modulus

of rupture, σMOR, has been used as the measure of the failure strength σf. The chart shows σMOR

plotted against modulus, E. A family of lines of slope 1/2 link materials with equal values of M1

= σf2/E. Those with the highest values of M1 lie towards the top left. The heavy line is one of the

family. It is positioned at 10 MJ/m3 such that a small subset of materials is left exposed. They

include high-strength steel (spring steel, in fact) lying near the top end of the line, and, at the

other end, rubber. But certain other materials are suggested too: GFRP (now used for truck leaf

springs), titanium alloys (good but expensive), glass fibers (used in galvanometers) and —

among polymers — nylon (children's toys often have nylon springs). The procedure identifies a

candidate from almost every material class: metals, glasses, polymers, elastomers and

composites. A protective stage, limiting the values of the toughness Gc (Gc = KIC2 / E) and the

cost Cm to the those listed in the design requirements, has been added .


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Figure 4: A chart of the modulus of rupture, σMOR, against Young's modulus, E. The diagonal

line shows M1.

Figure 5: A 'protective' chart of the toughness, Gc, against cost per unit weight, Cm. The box

restricts the selection to materials with Gc > 1 kJ/m2 and Cm < 100 GBP/kg.


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MATERIAL

COMMENT

Spring Steel

15 – 25 The traditional choice: easily formed and heat treated.

Ti Alloys 15 – 20 Expensive, corrosion-resistant.

CFRP 15 – 20 Comparable in performance with steel; expensive.

GFRP 10 – 12 Almost as good as CFRP and much cheaper.

Glass fibers 30 – 60 Brittle in tension, but excellent if protected against damage; very low loss factor.

Nylon 1.5 – 2.5 The least good; cheap and easily shaped, but high loss factor.

Rubber 20 – 60 Better than spring steel; but high loss factor.

Materials for efficient springs of low volume

Materials selection for light springs is shown in Figure 4. It is a chart of σMOR/ρ against E/ρ,

where ρ is the density. Lines of slope 1/2 now link materials with equal values of

.

One is shown at the value M2 = 2 kJ/kg. Composites, because of their lower densities, are better

than metals. Elastomers are better still (you can store almost 8 times more elastic energy per

unit weight in a rubber band than in the best spring steel). Elastomeric springs are now widely

used in aerospace because of their low weight and high reliability. Wood — the traditional

material for archery bows, now appears in the list.


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Figure 6: A chart of σMOR/ρ against E/ρ. The diagonal line is a contour of M2.

The mechanical loss coefficient is important in springs which are loaded dynamically: polymers

have high loss factors and therefore dissipate energy when they vibrate; metals, if strongly

hardened, do not. Polymers, because they creep, are unsuitable for springs which carry a

steady load, though they are good for catches and locating-springs which spend most of their

time unstressed. Springs made from unprotected carbon-steel fail rapidly in a chemically

corrosive environment. The least expensive solutions to this problem is to plate or polymer-

coat them to provide a corrosion barrier, but if the coating is damaged, failure can follow. The

more expensive solution is to make the spring from an intrinsically corrosion-resistant material:

stainless steel, copper alloys, nickel and cobalt alloys, titanium alloys, reinforced polymers,

GFRP or CFRP. If high thermal or electrical conduction is required, copper-beryllium alloys are

the best choice. But these are secondary selection-criteria.

MATERIAL

COMMENT

Spring

Steel

2 – 3 Poor, because of high density.

Ti Alloys 2 – 3 Better than steel; corrosion-resistant; expensive.

GFRP 4 – 8 Better than steel; expensive.


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CFRP 3 – 5 Better than steel; less expensive than CFRP.

Glass fibers 10 – 30 Brittle in torsion, but excellent if protected.

Woods 1 – 2 On a weight basis, wood makes good springs.

Nylon 1.5 – 2 As good as steel, but with a high loss factor.

Rubber 20 – 50 Outstanding; 10 times better than steel, but with high loss

factor.

Materials for efficient light springs

In all other examples, we will not go into the detail as this.

3.2 Con-Rods For High-Performance Engines

A connecting rod in a high performance internal combustion engine is a critical component if it

fails, catastrophe follows. Yet to minimize inertial forces and bearing loads, it must weigh as

little as possible. This implies the use of light, strong materials, stressed near their limits. When

cost, not performance, is the design goal, con-rods are made out of cast iron.


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A connecting rod must neither buckle elastically, nor fail by fatigue. The objective is to minimize

its mass, m.

FUNCTION Connecting rod for reciprocating engine

OBJECTIVE Minimize weight

CONSTRAINTS Must not fail by fatigue, or buckling or fast fracture

We seek materials for a connecting rod of minimum weight which will carry a peak load F

without either buckling or failing by fatigue. The Model for the selection of the probable

materials has been developed and plotted using the indices.

.

Maximising this index minimizes the mass of the rod, while meeting the constraint that it must

not buckle.

The fatigue constraint requires that

,

where σe is the endurance limit of the material of which the con-rod is made. Using this to

eliminate A second equation for the mass:

,

containing the material index

.


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This defines a coupling constant which relates the two indices. It depends on the 'structural

loading coefficient' F / 2, specified by the design. The optimum selection for large F / 2

differs from that for small F / 2. Using the coupling equation it is possible to select materials

which simultaneously optimize both constraints.

Materials with high values of both M1 and M2 are identified by creating a chart with these

indices as axes. Materials near the top-right corner are attractive candidates. The optimum

placing of the selection box depends on the value of the loading coefficient F / 2. Two

examples are shown, one isolating the best subset when the loading coefficient is high, the

other when it is low. The candidates include high-strength magnesium, aluminum and titanium

alloys, ultra high-strength steels, and, best of all, CFRP (carbon-fiber reinforced polymers).

FIG 7: A chart with M1 and M2 as axes, constructed from the light alloys branch of the materials tree. The

selection shows materials best for low values of the loading coefficient F / 2


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Figure 8: A chart with M1 and M2 as axes, constructed from the light alloys branch of the materials tree. The

selection shows materials best for high values of the loading coefficient F / 2

MATERIAL COMMENT

Beryllium alloys The ultimate choice for small F / 2

CFRP and GFRP Excellent performance; requires novel design and production methods

Materials for high-performance con-rods, small F/ 2

MATERIAL COMMENT

CFRP and GFRP Excellent performance; requires novel design and production methods

Titanium alloys The ultimate metallic choice for large F/ 2

Magnesium

alloys

Cheaper than titanium; good performance

Aluminum alloys Not the best choice for extreme values of F/ 2 but viable for intermediate

values

Materials for high-performance con-rods, large F / 2


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3.3 THE AUTO COOLING FANS

Automobile engines have a fan which cools the radiator when the forward motion of the car is

insufficient to do the job. Commonly, the fan is driven by a belt from the main drive-shaft of the

engine. The blades of the fan are subjected both to centrifugal forces and to bending moments

caused by sudden acceleration of the motor. Most failures in the fan can be traced to defects

(cracks in fan blades) which propagated under the accelerational loads.

The radius, R, of the fan is determined by other considerations: flow rate of air, and the space

into which it must fit. And the material of which it is made must be cheap. Any automaker who

has survived to the present day has cut costs relentlessly on every component. But safety

comes first. The fan must not fail.

A fan. It must not fail catastrophically during an over speed.

FUNCTION Cooling fan

OBJECTIVE Maximum resistance to propagation of a crack when engine is raced

CONSTRAINTS Radius R specified

Must be cheap and easy to form

The safe rotational velocity ω is maximized by selecting materials with large values of

.


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The material cost of the fan is

where Cm is the cost/kg of the material, ρ its density and V the volume of material in the fan.

The volume is essentially fixed by the radius R, which is a constraint on the selection. Thus the

material cost is minimized by selecting materials with large values of the volume per unit cost:

.

The selection must be balanced against the cost. Low cost fans can be made by die-casting a

metal, or by injection-molding a polymer.

Figure 8: A chart of fracture toughness, KIC, against density, ρ, using the Generic record subset, showing the

index M1.


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Figure 9: A chart of volume/unit cost, M2, against forming method, using the Generic record subset.

The final selection requires judgment. If the fan is viewed as a safety-critical component, we do

not choose cast iron, but instead select steel, aluminum, magnesium or filled polymers.

MATERIAL COMMENT

Cast Iron Cheap and easy to cast

Cast Mg Alloys Can be die-cast to final shape

Cast Al Alloys Can be die-cast to final shape

High density polyethylene (HDPE) Moldable and cheap

Nylons

Rigid PVCs

Acrylobutadienestyrene (ABS), high impact

GFRP (chopped fiber) Lay-up methods too expensive and slow. Press from chopped - fiber molding material.

CFRP (chopped fiber)


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3.4: Materials for Flywheels

Flywheels store energy. The energy can be recovered, either to smooth an intermittent power

supply (as in reciprocating engines) or to drive something (a toy, a dynamo). Steam, gasoline

and diesel engines have cast iron flywheels. There is varying diversity of material for choosing a

flywheel. An efficient flywheel stores as much energy per unit weight as possible, without

failing. Failure (were it to occur) is caused by centrifugal loading. When the centrifugal stress:

reaches the tensile strength (or fatigue strength) the flywheel flies apart. One constraint is that

this should not occur. In this case, and for the flywheel of an automobile engine — we wish to

maximize the energy stored per unit volume at a constant (specified) angular velocity. There is

also a constraint on the outer radius, R, of the flywheel so that it will fit into a confined space.

Figure 9: A flywheel. Its angular velocity (and thus the kinetic energy it can store) is limited by its strength.

FUNCTION Flywheel to store maximum kinetic energy

Flywheel to run at constant (low) velocity

OBJECTIVE Maximize kinetic energy per unit mass Maximize kinetic energy per unit volume

CONSTRAINTS Must not burst. Outer radius, R, fixed.

Adequate toughness (>20 MPa m1/2) Adequate toughness (>20 MPa m1/2)

The Model have been derived and it follows that the best materials for efficient flywheels are

those with high values of the performance index.

It has units of kJ/kg.


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To avoid failure due to fast fracture, the disc must also have adequate toughness. We achieve

this by requiring that

A second selection stage has been performed limiting the selection to materials with KIC > 20

MPa m1/2 and Cm < 100 GBP/ kg. A more elegant way of exploring materials for efficient

flywheels is shown in which combines both stages by using the compound property σe/ρ

Figure 10: A chart of endurance limit, σe , against density, ρ, showing one possible selection

using the Generic record subset with the M1 index.


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A chart of endurance limit, σe, against density, ρ, showing the selection using the Generic

record subset with the constraint M2 > 10 /m3.

The best choices for efficient flywheels are unexpected ones: polymer-matrix composites

reinforced with continuous fibers of carbon or glass; or, less surprisingly, high-strength titanium

alloys. Recent designs use a filament-wound glass-fiber reinforced rotor, and can store around

150 kJ/kg; a 20 kg rotor then stores 3 MJ or 800 kW hours.

The energy density in such a flywheel is considerable; its sudden release in a failure could be

catastrophic. The disk must be surrounded by a burst-shield and precise quality control in

manufacture is essential to avoid out-of-balance forces. This has been achieved in a number of

glass-fiber energy-storage flywheels intended for use in trucks and buses, and as an energy

reservoir for smoothing power flow in wind-power generation.


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Figure 10: A chart of fracture toughness, KIC plotted against material price, Cm, as protection against brittleness

and excessive cost.

The choice for the constant-velocity flywheel is quite different. Lead is good; it is the best

choice for small flywheels. Cast iron is less good, but cheaper. It is the standard material for

larger flywheels and for automotive applications. Gold, platinum and uranium are better than

either of these, but may be thought unsuitable for other reasons.

A chart of specific endurance limit, σe /ρ, against fracture toughness, KIC, showing the selection

for efficient flywheels.


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MATERIAL M1 (kJ/kg)

COMMENT

Composites: CFRP 200 – 350

The best performance — a good choice.

Composites: GFRP 150 – 200

Almost as good as CFRP and considerably cheaper — Excellent choice.

Ti Alloys 100 – 160

The best choice among metals — marginally better than high-strength steels because of lower ρ.

High strength steels

100 – 150

All about equal in performance. Steel and Al alloys are cheaper than Mg and Ti alloys.

High strength Mg alloys

80 – 100

High strength Al-alloys

80 – 100

Materials for efficient flywheels

MATERIAL M1 (kJ/kg)

COMMENT

Lead alloys

3 High density makes these a good (and traditional) selection when performance is velocity-limited, not strength-limited

Cast irons 18

For the remaining part of this paper I will just make a mention of the part, the functional

requirement, objectives, constraints and the alternative materials that can be used without

going into the details.

3.5: Brake Disc

The automobile brake disc will be subjected to high temperature and it is required that it will be

a material that withstand thermal stress, high frictional and abrasive resistance and also of

minimum weight.

Materials that compete for these are Gray cast iron, aluminum alloys, ceramics and composites.

However, cost and formability may inhibit the use of composites, high s.g leading to higher fuel

consumption will inhibit the use of cast iron, ceramics have lower thermal and shock resistance

property. Hence, aluminium with lower strength but having light weight is a choice that can be

manipulated by alloy additions to it.


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3.6: Engine Cylinder

It is required that material to be used for the engine cylinder be of light weight, maximum

resistance to thermal stress and of high temperature property and also corrosion resistant to

the fuel.

Aluminium alloys, steels and carbon alloys have found to be of competing properties but it will

be discovered that steels will be of most suitability.

3.7: Automobile Exhaust System

The exhaust system will convey gases, prevent fume from entering the car. It is therefore

expected that it resist attack to hot and moist gases from the exhaust and be resistant to the

atmosphere attack, water, the mud, road salt. It must also be readily available and of minimum

cost.

Welded low carbon steels with corrosion coating are of the initial design but alternative

materials with better creep and corrosion resistant property have been developed. These are

11%Cr alloys, 17-20% Cr ferritic alloys and Austenitic Cr-Ni alloys.

3.8: Automobile Chassis- Body in Weight

The key technical design strategy for improving vehicle efficiency is by the reduction of vehicle

mass. Steel has been the material choice till recent discoveries and applications of aluminum

alloys and polymeric composites.

The most desirable property is of high impact strength and low weight among other factors.

Competing materials are high strength CFRP, GFRP composites and steel.

Apart from the high strength of the design, considerations must also be made of the cost, easy

formability, ease of repair, and other factors like environmental impact-recyclable, reusable or

not.

It should be noted that this is a generalization; however different grade of materials may be

used depending on the area and the susceptibility to damage. For instance the engine front

cover is more susceptible to damage by impact rather than the door frames.


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3.9: The Dash board

The automobile dashboard is covering for the electrical panels in the automobile system. Hence

it is required that it is resistant to sunlight and UV rays at the same time minimal weight and

prevention from the potentially dangerous heat from the engine compartment.

Polymeric materials have found a lot of application in this area. However, the use of natural

fibre reinforced plastic is finding more application. This is due to the design consideration of

weight saving and environmental impact among other factors.

3.10: Automobile Engine block

Finally, I will look into the choice of material for the engine block. The major design

considerations will include strength, stiffness, thermal conductivity, thermal resistance, fatigue

resistance, density, manufacturability, wear resistance, corrosion resistance and the cost.

Competing materials could include, aluminum alloys, steel alloys, magnesium alloys, titanium

alloys, stainless steels, copper alloys and nickel alloys.

Traditionally, grey cast iron has been used because of its good resistance to wear and low cost

but the use of aluminum and magnesium alloys are finding application in recent years due to

the low thermal expansion of the former and the high fatigue failure and ease of manufacture

of the latter.

4.0 Conclusion:

It is now obvious that material selection process is very important in design of any component

and requires some level of expertise and experience to make the best choice of materials. The

areas covered in this paper especially part one to four are not standard report on the materials

but based on some component factors.


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5.0 REFERENCES

CES EDU Pack 2008 edition

Michael Ashby et al ,Material Engineering and Processing Design, 2007,published by Elsvier.

Journal on: “Material selection in automobile body: Economic opportunity for polymer

composite materials”

Dieter, Overview of Material selection Process.

Proceedings of the World Congress on Engineering 2010: Material Selection Method in Design of

Automobile Brake Disc by M.A Maleque, S.Djuti and M.M Rahman.

India Journal of Engineering and Material Science, Material Selection for Natural Fibre

reinforced polymer composites using analytical hierarchical process.

Assignment 809--Principle of Material Selection

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