UNIVERSITY OF NAIROBI

FINAL YEAR PROJECT

PROJECT NO: MFO/03/2012

TITLE: DEVELOPMENT OF A MATERIALS SELECTION PROCESS IN

ENGINEERING DESIGN AND MANUFACTURING

A final year project submitted in partial fulfillment of the requirements for the award of the degree of Bachelor of Science in Mechanical Engineering

WRITTEN BY:

MOGAKA DAVIDSON ONCHANA F18/1735/2007

&

MOMANYI GODFREY MARAMBE F18/1869/2007

SUPERVISED BY: PROF. M. F. ODUORI

May 2012

DEPARTMENT OF MECHANICAL AND

MANUFACTURING ENGINEERING

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DECLARATION

We certify that the information presented in this report, except where indicated and acknowledged, is our original effort and has not been presented before to the best of our knowledge. MOGAKA DAVIDSON ONCHANA F18/1735/2007 Signature Date... MOMANYI GODFREY MARAMBE F18/1869/2007 Signature Date... This project has been submitted with the approval of the supervisor Project supervisor: Prof. ODUORI, M. F. Signature Date of Submission

Copyright 2012 Mogaka & Marambe

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DEDICATION

To my Dad, Mom (R.I.P), and brother Paul for your continued support. God bless you all.

Mogaka Davidson

To my parents, brothers and sisters who have accorded me with endless support during

my undergraduate studies, to my girlfriend Assumpter with love, to my lecturers in the

department of mechanical and manufacturing engineering for the knowledge they have

imparted in me in my undergraduate studies and from whom I have learnt so much.

Marambe Godfrey

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TABLE OF CONTENTS

Declaration...i

Dedicationii

Contents...iii

Acknowledgementsv

Abstractvi

Objectives.........vii

Chapter One ....................................................................................................................................... 1

1.1 Introduction .................................................................................................................................... 1

1.2 Standards And Codes .................................................................................................................. 2

Chapter Two ...................................................................................................................................... 3

Review Of Literature On Design, Materials Selection And Manufacturing Processes ......... 3

2.1 Introduction .................................................................................................................................... 3

2.2 Overview Of The Engineering Design Process .................................................................. 4

2.3 Material Selection ......................................................................................................................... 6

2.3.1 Materials Selection Process ............................................................................................. 7

2.3.2 Factors Influencing Materials Selection ...................................................................... 9

Chapter Three ................................................................................................................................. 17

Literature Review On Engineering Materials, Their Properties And Categories ................ 17

3.1 Introduction .................................................................................................................................. 17

3.2 Material Properties .................................................................................................................... 17

3.3 Categories Of Engineering Materials................................................................................... 18

3.3.1 Metallic Materials .............................................................................................................. 19

3.3.2 Non-Metallic Materials .................................................................................................... 24

Chapter Four ................................................................................................................................... 27

4.1 Introduction .................................................................................................................................. 27

4.1.1 Rank Order: Pair Wise Comparison Charts.............................................................. 27

4.1.2 Relative Order: Analytic Hierarchy Process (AHP) .............................................. 28

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4.2 Information Processing ............................................................................................................ 29

Chapter Five .................................................................................................................................... 31

Case Study: Selection Of A Material For A Reverted Two Stage Compound Gear Train... 31

5.1 Introduction .................................................................................................................................. 31

5.1.1 Classification Of Gears...................................................................................................... 31

5.1.2 Gearing Terminology ........................................................................................................ 34

5.1.3 Design Considerations For A Gear Train .................................................................. 35

5.1.4 Modes Of Gear Failure ...................................................................................................... 36

5.2 Reverted Compound Gear Train Design ............................................................................ 36

5.2.1 The Design Constraints .................................................................................................... 38

Chapter Six ....................................................................................................................................... 46

6.1 Material Ranking Indices ......................................................................................................... 46

6.2 Support Information ................................................................................................................. 48

6.3 Materials Selection System ..................................................................................................... 48

6.3.1 Database Structure ............................................................................................................ 49

6.3.2 Material Selection System .............................................................................................. 51

Chapter Seven: Closure ............................................................................................................... 59

7.1 Discussion ...................................................................................................................................... 59

7.2 Conclusion ..................................................................................................................................... 61

7.3 Recommendations...................................................................................................................... 61

7.4 References and Appendices .................................................................................................... 62

References ............................................................................................................................ 62

Appendices ........................................................................................................................... 63

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ACKNOWLEDGEMENTS

A project such as this could not have been accomplished without the assistance of a

large number of individuals. First and foremost we would like to sincerely thank Prof.

F.M. Oduori our project supervisor, and a senior lecturer at the Department of

Mechanical and Manufacturing Engineering - University of Nairobi; for the continued

guidance and support he gave us through relevant literature material and helpful

information to undertake this project.

We are grateful to Mr. Enoch Kimanzi for criticizing our work and providing us with

relevant information without which it would be difficult to accomplish our work. We

would also like to extend our hand of appreciation to Prof. S. Mutuli, chairman -

Department of Mechanical and Manufacturing Engineering for his efforts to ensure good

working conditions as well as support through departmental facilities especially the

departmental library.

We would like to acknowledge the staff of East African Foundry Ltd and Kensmetal Ltd

for providing us with meaningful information on engineering materials.

Finally, we would like to thank our families for their support and encouragement.

God bless.

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ABSTRACT

The selection of proper materials is fundamental to engineering design. Engineering

materials are many hence a formalized selection process is required to select a reliable

material for a product. The objective of this project was to develop an online material

selection process based on principles of decision theory and implement it as an

information processing routine on a computer system. A case study was undertaken

that involved selection of a material in the design of a reverted two stage compound gear

train. Selection was done in two stages: screening followed by ranking. The first stage

reduces the large material database to a small candidate list which are locally available

and meets the critical property limits such as strength. The second stage involves

ranking the candidate materials using indices formulated from availability, cost and

machinability. Supporting information is then sought and used to narrow down the

ranked materials to a final choice allowing a definite match to be made between design

requirements and material attributes. This material selection system helps the designer

perform the rigorous process of material selection for the gear train at fast speeds thus

saving time and money during design.

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OBJECTIVES

To develop an online materials selection process that will be based on the principles of decision theory.

To develop a knowledge intensive methodology for screening and ranking

engineering materials.

To implement the materials selection process so developed as an information processing routine on a computer system.

To document and evaluate the materials selection process so developed by

means of a case study (A Reverted two stage compound gear train)

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1 CHAPTER ONE

1.1 INTRODUCTION The selection of a material for machine part or structural member is one of the most

important decisions the engineering designer has to make. Poor material choice can

lead to failure of a part or system or to unnecessary cost. The process of materials

selection is difficult one and typically involves multiple conflicting material

characteristics as well as large number of constraints.

A good material selection process considers the limiting factors for a particular design

exercise which include material properties, material processing, material cost and

material availability. Through systematic and optimizing approach, one can list all the

limiting factors associated with the design e.g. strength, hardness, cost and availability.

Weighting measure can be used to prioritize on what materials are more important than

others after which all available materials are listed and ranked.

For ranking purposes, indices like cost and availability of the various materials are

computed. After this task a list of the materials meeting the limiting requirements is

produced in which the materials are ranked from the one with the highest composite

index to the one with the least. In this case, the material with the highest composite

index based on cost and availability is taken as the best for the application. Materials for

other engineering applications can be selected in the same way.

With the advent of the internet, utilization of an online material selection process is a

major advancement in the selection of a material for a particular product. The process

gives accurate information at fast speeds thus saving time and money during design.

The computer can play a major role in storing information (database) on materials

properties. In addition a computer code is created using PHP (recursive acronym for

hypertext preprocessor) in which the information in the database can be accessed and

retrieved. Thus entering the machine part specifications in the program, the computer

searches for the qualified materials in the database and displays them to the user.

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1.2 STANDARDS AND CODES A standard is set of specifications for parts, materials, or processes intended to achieve

uniformity, efficiency, and a specified quality. One of the most important purposes of a

standard is to place a limit on the number of items in the specifications so as to provide

a reasonable inventory of tooling, sizes, shapes, and varieties.

A code is set of specifications for the analysis, design, manufacture and construction of

something. The purpose of a code is to achieve a specified degree of safety, efficiency,

and performance or quality. However, its important to observe that safety codes do not

imply absolute safety1.

This project and the case study identify materials to Unified Numbering System (UNS)

standards. An ideal case of choice of standards should be based on such factors as the

location where the product is applicable and acceptability of the standard under the

applicable design/construction code. In this case therefore, Kenyan Standards (KS)

would have been preferred.

In order to provide a consistent basis for basic specifications of the materials, only UNS

standards for the materials have been used. In some cases where the materials

common name is available, then the materials common name is given.

1Shigleys Mechanical Engineering Design

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2 CHAPTER TWO

REVIEW OF LITERATURE ON DESIGN, MATERIALS SELECTION

AND MANUFACTURING PROCESSES

2.1 INTRODUCTION To design is either to formulate a plan for the satisfaction of a specified need or to solve

a problem. If the plan results in the creation of something having a physical reality, then

the product must be functional, safe, reliable, usable, manufacturable and marketable.

Design establishes and defines solutions to, and pertinent structures, for problems not

solved before, or new solutions to problems which have previously been solved in a

different way (Dieter, George., 1983).

Design is an innovative highly iterative, and a decision making process. Decisions

sometimes have to be made with too little information, occasionally with just the right

amount of information, or with an excess of partially contradictory information. These

decisions are made tentatively, with the right reserved to adjust as more becomes

known.

A designers personal resources of creativeness, communicative ability, and problem

solving skill are intertwined with knowledge of technology and first principles.

Engineering tools (such as mathematics, statistics, computers, graphics and languages)

are combined to produce a plan that, carried out, produces a product that is functional

safe, reliable, competitive, usable, manufacturable, and marketable, regardless of who

builds it or who uses it.

The selection of proper materials is a key step in the design process because is a crucial

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

real or working design. The enormity of this decision process can be appreciated when

its realized that there are over forty thousand metallic alloys and probably close to that

number of non-metallic engineering materials, currently in use (Ashby, M., 1999)

Improper selection of a material, may lead not only to failure of the material 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 necessary service performance; the

processing of the material into a finished part also has a key role to play. This is because

the properties of the part may be altered by processing resulting to a change in the

service performance of the part.

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2.2 OVERVIEW OF THE ENGINEERING DESIGN PROCESS There is no particular step categorization or step nomenclature universally accepted,

but generally the complete design process is outlined in below:

Iteration

Fig. 2.1: Phases in Engineering Design Process (Adapted from: Madara Ogot & Gul Kremer,

Engineering Design: A Practical Guide).

It must be emphasized that engineering design is an iterative process requiring the

repetition of most steps based on what is learned at a later stage. The primary iterations

occur between the conceptual design and preliminary design steps.

2.2.1 Recognition of need The need for a product typically arises from these three distinct scenarios.

The need to design a new product or process that will solve a particular problem or need where none exists.

The need to redesign: to design a product or process that improves on an existing one. Improvements include lower cost, highier efficiency, lower

pollution and better ergonomics.

The need for technology-push product or process: to design a new product or process and generate need for it. For example , a company develops a new

technology and then seeks a market to apply it.

Problem definition

Recognition of need

Conceptual design

Preliminary design

Detailed design

Production

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2.2.2 Problem definition Its a crucial part in the design process and includes;

Condensed formal problem statement clearly stating objective of the design process.

Listing of technical and non-technical design constraints. Breakdown of the problem into smaller manageable sub-problems. Compilation and ranking of customer needs. What exactly does the customer

expect in final product or process?

Definition of criteria to be used to evaluate the design.e.g testing of prototypes developed in preliminary design step.

2.2.3 Conceptual design A concept is a very preliminary description of the form, required principles and

technology for the solution. This stage is divided into two phases: external and internal

searches. An internal search entails the design team developing several concepts from

which the best suited to the need is selected. The stage is creative, inventive and most

difficult in engineering design process. The external search includes performing

literature searches, looking at previous patents, talking with experts, and benchmarking

similar product. The conclusion of this stage results in the generation and selection of

few promising concepts that warrant further development.

2.2.4 Preliminary design and evaluation Feasible concepts are further developed by evaluating; leading to selection of one

concept. Selection is based on all design criteria specified during problem definition, as

well as cost estimates. System and component design requirement that will dictate the

detailed design specifications are established. During this stage, working prototypes

(where appropriate) are constructed and evaluated. Based on test results, parts of the

design or the entire design may need to be redone (iteration).

2.2.5 Detailed design This stage of the design processes develops part geometry, technical drawings, and

tolerances. During this stage:

All hitherto undefined system specifications and design requirements are defined such as operating parameters, test requirements, design life, material

requirements, and reliability requirements.

Detailed manufacturing drawings are produced. Detailed assembly drawings are generated. Testing is performed to evaluate components, validate computer models and the

design itself. Evaluation ensures all the conclusions reached during preliminary

testing stage are accurate. If errors are found or if components do not meet

anticipated design requirements, a redesign is initiated (Iteration).

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2.2.6 Production Prior to production, production process planning is carried out. This involves

Design drawings and specifications interpretation. Production processes and machines selection. Stock material selection. Determination of production sequence of operations. Determination of processing time.

The implementation involves successful testing of prototypes after which the final

solution is developed and preceded with full production.

2.2.7 Design Reviews Design review (DR) is system that involves gathering and evaluating objective

knowledge about the product design quality and the concrete plans for making it a

reality , suggesting improvements at each point, confirming that the process is ready to

proceed to the next phase.-JUSE2 Design Review Committee, 1976 (Ichida, 1996).

Design review ensures design meets all requirements, and product quality is within cost

and time constraints. DRs should include:

Collecting and compiling relevant information. Defining quality target. Evaluating product and process designs and supporting operations. Proposing improvements. Defining subsequent actions and confirm readiness for the next stage.

2.3 MATERIAL SELECTION Selecting materials usually begins in the preliminary design stage. The problem of

material selection usually involves the selection of materials for a new product or new

design, and re-evaluation of an existing product or design to reduce cost, increase

reliability and improve performance. In selecting the appropriate material one must

consider:

1. Material properties which affects the part perfomance. 2. Material processing which affects manufacturing costs and therefore the final

part cost.

3. Material cost. 4. Availability. Is the material available in desired quantity and time frame? 5. Regulatory properties; Code acceptance and repairability

The relative importance of the above factors depends on the applications. For example

in military and aerospace applications, pushing the materials properties to the limits

takes precedence over cost. For consumer products, lowering cost typically plays the

2Japanese union of scientists and engineers.

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leading role. For an engineering project on a tight schedule, material availability is

important.

2.3.1 MATERIALS SELECTION PROCESS Usually, a problem of material selection involves either selection of materials for new

product or design; or re-evaluation of an existing design/product to increase reliability,

reduce cost and improve performance. Materials selection process, being a problem

solving process, is achieved through the following steps:

1. Determination of required critical properties from the design operating conditions and enviroment. Material selection occurs at every step of design

process. At conceptual stage a wider spectrum of materials should be considered

to inspire more innovative designs. In the material screening process , material

properties considered will depend on possible failure modes likely to be

encountered during service, as well as other desired characteristics. By

establishing all the possible failure modes for each particular component and

matching them with the associated material properties, a list of material

properties for the screening process can be established. Table 2.1 below lists

some of common failure modes and associated influencing material properties.

Table 2.1: Adapted from Engineering Design by Madara Ogot., Gul Kremer

KEY

US-Ultimate strength E- Modulus of elasticity

YS-Yield strength CR-Creep rate

CS-Compressive yield strength HD-Hardness

SS-Shear yield strength CE-Coeffient of expansion

FP-Fatigue properties

2. Screening of large material database for candidate materials that meet the critical material properties is determined in steps. These critical properties

can be divided into three groups

a) Non-discriminating parameters are those that must be met if material is to be used at all. Examples include availability and corrosion resistance.

Mode of failure US YS CS SS FP E CR HD CE

Fatigue(High cycle) Fatigue(Low cycle) Yielding Buckling Wear Thermal fatigue Creep Gross deformation

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b) Go/no-go parameters. These are minimum or maximum property values which candidate materials must meet. Excess or under values of these fixed

parameters dont make up for other deficiencies in other properties.

Examples include cost and strength.

c) Discriminating parameters. These are minimum or maximum property values which candidate materials must meet, and where any excess or under values

can make up for other deficiencies in other areas. Includes cost, density and

strength.

Depending on material application, a characteristic that is considered a

go/no-go parameter for one application may be considered discriminating or

non-discriminating parameter in another. For example in aerospace

applications cost is a discriminating parameter, whereas in consumer

products, cost is a go/no-go parameter.

3. Selecting the final material based on a trade-off of discriminating parameters. This is done using desicion tools such as pairwise comparison

charts(ranking method), analytic hierarchy process (AHP) and decision matrices.

These tools will be discussed later in the decision making section.

All materials

Non-discriminating parameters

go/no-go parameters

discriminating parameters

Final material

Fig. 2.2: Three general steps in material selection

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2.3.2 FACTORS INFLUENCING MATERIALS SELECTION There are several important factors that need to be considered during material

selection. These are commercial properties (Cost and Availability), material

properties(mechanical , physical, and environmental resistance), material processing,

and regulatory properties.

2.3.2.1 MATERIAL PROPERTIES

2.3.2.1.1 Mechanical properties Mechanical properties of materials are those related to its ability to withstand external

mechanical forces such as tensile forces, compression forces, twisting, bending, and

sudden impact.

A. Strength Strength is a measure of how a material withstands a heavy load without breaking.

Material strength information is used in engineering design in order to prevent the

failure of a product component by rapture. Following are parameters of strength:

Elastic Limit: This is the force required to produce permanent deformation. Yield Point: This refers to the level of the load at which strain continues at a

constant stress.

Yield Strength: The amount of tensile force required to just cause a well-defined permanent deformation in a material.

Ultimate Tensile Strength (UTS): This is the maximum strength of a material and corresponds to the maximum load stress a structural member can withstand

before fracture.

Compressive Strength: This is the ability of a material to resist a gradually applied compressive load.

Yield strength and tensile strength are the most significant values in many engineering

applications. Appreciable permanent deformation occurs before the stress reaches the

UTS value. Therefore, to guard against permanent deformation in engineering

components, information on elastic limit of the candidate materials should be used in

design. For ductile materials, yield point information should be used instead of elastic

limit value. For this project, yield strength and compressive strength of materials have

been used as screening properties in the materials selection process.

B. Rigidity This is the resistance of a material to deflection under a bending force. Its specified by

the elastic modulus of a material .Modulus of elasticity is the ratio of the applied stress

to the corresponding strain in the elastic limit of a material. The higher the value of the

elastic modulus the more rigid the material is.

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C. Resistance to fatigue: Fatigue is defined as the progressive and localized structural damage of a material

under cyclic loading. Thus, fatigue strength, expressed in terms of the fatigue limit or

endurance limit of a material means the stress below which a material will not fail in

fatigue. This value is used in design of parts subjected to repeated alternating stresses

over an extended period of time. Since the strength of a material under cyclic loading is

less than the strength of the same material under static loading, resistance to fatigue

forms the basis for the design of components that are subjected to cyclic loading.

D. Ductility Ductility is a measure of the degree of plastic deformation that has been sustained at

fracture. A material that experiences very little or no plastic deformation upon fracture

is termed brittle.

E. Resilience Resilience is the capacity of a material to absorb energy when it is deformed elastically

and then, upon unloading, to have this energy recovered. The associated property is the

modulus of resilience, which is the strain energy per unit volume required to stress a

material from an unloaded state up to the point of yielding.

F. Toughness Toughness is a mechanical term that is used in several contexts; basically, it is a

measure of the ability of a material to absorb energy up to fracture. Specimen geometry

as well as the manner of load application are important in toughness determinations. A

related property is fracture toughness which is indicative of a materials resistance to

fracture when a crack is present

G. Hardness Another mechanical property that may be important to consider is hardness, which is a

measure of a materials resistance to localized plastic deformation (e.g., a small dent or a

scratch. Quantitative hardness techniques have been developed over the years in which

a small indenter is forced into the surface of a material to be tested, under controlled

conditions of load and rate of application. The depth or size of the resulting indentation

is measured, which in turn is related to a hardness number; the softer the material, the

larger and deeper the indentation, and the lower the hardness index number.

H. Damping capacity The damping capacity of a material is defined as energy dissipated as heat by a unit

volume of the material during a completely reversed cycle of stress. It is related to

internal friction in the material and depends on maximum stress. The critical value

suggested for engineering design is the value at the endurance limit. High damping

capacity is desirable in most machine parts to reduce accumulation of harmful resonant

stresses, vibration, and to decrease noise in machine tools.

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I. Friction Surface friction is an energy dissipative process which takes place with relative

tangential displacement of contacting solids in zones of real contact between them,

formed by the action of an external load. It is that component of the load which resists

lateral (tangential) motion of solid surfaces, fluid layers or material elements in contact.

Friction is described by a ratio of friction force to normal load, termed the coefficient of

friction, . This value depends not only on the surface finish but also on the contacting

materials. It thus occurs that, in the process of engineering design, decisions must

always be made as to which materials and what processes can be used according to the

friction requirements of the component.

J. Formability Formability can be defined as the relative ease with which a metal can be shaped

through plastic deformation while avoiding machining operations. Usually, shaping of

the component is achieved by stretching it using mechanical force. Formability

determines the amount the material in question can be stretched or drawn without

necking and failing3 . Forming limit is thus defined as the extent to which the metal can

be stretched before failure occurs.

2.3.2.1.2 Mechanical failure modes

A. Fracture

Fracture refers to the local separation of an object or material into two or more pieces

under the action of stress. Fracture toughness is a property which describes the ability

of a material containing a crack to resist fracture, and is one of the most important

properties of any material for virtually all design applications.

B. Fatigue Fatigue is the progressive and localized structural damage that occurs when a material

is subjected to cyclic loading. It occurs when a material is subjected to repeated loading

and unloading. If the loads are above a certain threshold, microscopic cracks will begin

to form at the surface. Eventually a crack will reach a critical size, and the structure will

suddenly fracture.

C. Wear Wear is erosion or sideways displacement of material from its "derivative" and original

position on a solid surface performed by the action of another surface. It is related to

interactions between surfaces and more specifically the removal and deformation of

material on a surface as a result of mechanical action of the opposite surface.

D. Creep Creep is a slow or progressive deformation of a material with time under constant

3 Ashby, M.F., 1999

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stress. It is triggered via thermal activation and is more severe in materials that are

subjected to heat for long periods near the melting point.

E. Corrosion Corrosion is the disintegration of an engineering material into its constituent atoms due

to chemical reactions with its surroundings. It involves electrochemical oxidation of

metals in reaction with an oxidant such as oxygen. A well-known example of

electrochemical corrosion is formation of an oxide iron due to oxidation of the iron

atoms in solid solution.Effects of corrosion are magnified by stress concentration and

cyclic loading.

F. Hydrogen embrittlement Hydrogen embrittlement is the process by which various metals, most importantly high-

strength steels, become brittle and fracture following exposure to hydrogen. It results

from unintentional introduction of hydrogen into susceptible metals during forming or

finishing operations

2.3.2.1.3 Physical properties

A. Density Density is commonly defined as mass per unit volume. It is the weight of a material per

unit volume and is measured by weighing it in air and in a fluid of known density.

Different engineering applications demand different density requirements from

materials. Low density materials may be preferred in some applications like in aircraft

components (fuel economy). On the contrary, weight is found to be advantageous in

some cases such as while making foundations and flywheels.

B. Electrical properties Typical electrical properties include;

Resistivity which is the measure a materials ability to resist the flow of electricity. The higher its value, the higher the resistance of the material.

Resistivity changes with temperature.

Dielectric strength. Materials can be categorised in terms their electrical properties as conductors, semiconductors or insulators. For an insulator, the

dielectric strength is the voltage required to break down the insulation ( i.e.,

allow electrical conduction ) through a unit thickness of the material.

C. Thermal properties Typical thermal properties include

Thermal conductivity- Measure of the rate at which heat can be conducted through a material. Its measured with the coefficient of thermal conductivity, k.

13

The higher the coefficient, the better the thermal conductivity. For cases where

thermal insulation is required, materials with low thermal conductivity are used.

Specific heat- is the amount of thermal energy required to increase a unit mass of a materials temperature by 1 degree.

Coefficient of thermal expansion- it gives a measure of an objects change in length per degree change in temperature.

2.3.2.2 MANUFACTURING PROCESS The manufacturing process influences amount of material wasted, surface defects of the

product, cost and to some extent material properties of finished products. The material

manufacturing process selected is determined largely by its cost and properties of the

material to be used. Typical material processes considered during material selection

process are:

A. Machining Machining operations can be classified as the ones in which material is removed in chip

form by means of a cutting tool or an abrasive wheel or block. Some of the machining

operations include: turning, grinding, drilling, boring, reaming, milling, planing, shaping

and broaching.

The designer differentiates the machining processes mainly on the basis of the cost to

achieve a certain shape, accuracy and surface finish. The processes are usually costly

and produce scrap and should therefore be avoided if possible. The designer will specify

abrasive methods when he/she seeks high accuracy and surface finish or when the

material is too hard for other cutting tools.

B. Casting Metal casting is the process by which a metal or metal alloy is poured into a mould and

hardened in the shape of the mold cavity. The casting process involves:

Melting the metal. Pouring the molten metal into the mold. Allowing the metal to cooland solidify. Removing the finished part from the mold.

This manufacturing process allows the creation of complex parts and can be used to

make small or large parts. In addition, it is well suited for mass production. The types of

casting processes available are sand casting, pressure die casting, investment casting

and ingot casting. The choice of any of these processes depend mainly on the material,

size, tolerances involved and more importantly, on the number of pieces to be produced.

C. Forging Forging involves plastic deformation of material between two dies to achieve the

desired configuration. Depending upon complexity, forging is carried out as open die

forging and closed die forging. In open die forging, the metal is compressed by repeated

blows using a mechanical hammer and its shape is manipulated manually. In closed die

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forging, the desired configuration is obtained by squeezing the work piece between two

shaped and closed dies. On squeezing the die cavity gets completely filled and excess

material comes out around the periphery of the die as flash which is later trimmed. Both

open and closed die forging processes are carried in hot as well as in cold state. In

forging, favorable grain orientation of metal is obtained.

D. Rolling Rolling is the most extensively used metal forming process. The material to be rolled is

drawn by means of friction into the two revolving roll gap. The compressive forces

applied by the rolls reduce the thickness of the material or changes its cross sectional

area. The geometry of the product depends on the contour of the roll gap. Roll materials

are cast iron cast steel and forged steel because of high strength and wear resistance. In

rolling the crystals get elongated in the rolling direction. In cold rolling, the crystal more

or less retains the elongated shape but in hot rolling they start reforming after coming

out from the deformation zone.

E. Extrusion In extrusion, the material is compressed in a chamber and the deformed material is

forced to flow through a die. The die opening corresponds to the cross section of the

required product. It is basically a hot working process; however, for softer materials

cold extrusion is also performed. In direct extrusion metal flows in the same direction as

that of the ram. Because of the relative motion between the heated billet and the

chamber walls, friction is severe and is reduced by using a lubricant. In indirect

extrusion, the metal flows in the opposite direction of the ram. It is more efficient since

it reduces friction losses considerably.

F. Drawing Large quantities of wires, rods, tubes and other sections are produced by drawing

process which is basically a cold working process. In this process the material is pulled

through a die in order to reduce it to the desired shape and size. In a typical wire

drawing operation, one end of the wire is reduced and passed through the opening of

the die, gripped and pulled to reduce its diameter. By successive drawing operation

through dies of reducing diameter the wire can be reduced to a very small diameter.

Annealing before each drawing operation permits large area reduction.

G. Case Hardening The purpose of case hardening is to produce a hard outer surface on a specimen of low

carbon steel while at the same time retaining the ductility and toughness in the core.

This is done by increasing the carbon content at the surface by using solid, liquid, or

gaseous carburizing materials. The process consists of introducing the part to be

carburized into the carburizing material for a stated time, and temperature depending

upon the depth of case desired and the composition of the part. The part may then be

quenched directly from the carburization temperature and tempered, or in some cases

it must undergo a double heat treatment in order to ensure that both the core and the

15

case are in proper condition. Some of the more useful case-hardening processes are

pack carburizing, gas carburizing, nitriding, cyaniding, induction hardening, and flame

hardening.

H. Powder Metallurgy The powder metallurgy process is a quantityproduction process that uses powders

from a single metal, several metals, or a mixture of metals and non-metals. Essentially it

consists of mechanically mixing the powders, compacting them in dies at high pressures

and heating the compacted part at a temperature less than the melting point of the

major ingredient. Waste material and machining operations are reduced significantly.

However, the cost of materials and dies are high. Parts commonly made by this process

are: Oil impregnated bearings, incandescent lamp filaments, cemented carbide tips for

tools and permanent magnet.

I. Plastic Injection Molding Plastic injection is the most common process for manufacturing plastic products. It

involves:

Heating a polymer to a molten state. Forcing the molten polymer to flow into a mold. Cooling and removing the molded part.

This process is suitable for large scale production. In such production scale, the

expenditure on tooling cost is high, and therefore its important that the designer

consults the manufacturer at an early stage in design.

2.3.2.3 COMMERCIAL PROPERTIES (COST AND AVAILABILITY) These properties involve aspects of both direct cost of materials and availability of

materials. This is because availability of a material greatly determines its cost. A

material is selected bearing in mind the cost of manufacture using available methods.

Other costs include:

The cost of labour required to produce the finished product from that material. Cost of indirect materials (processing chemicals and cleaning materials). Cost of services incurred (electric power, gas, air, water, coal, and fuel). Tool replacement cost. Depreciation of plant and machinery.

2.3.2.4 REGULATORY PROPERTIES

A. Code Acceptance Professional Engineering oganisations provide performance oriented codes, standards

and evaluation procedures by which a product can be tested and evaluated for

compliance.This helps provide a uniform and widely recognised basis for acceptance of

new products.After the new product has been tested to indicate conformance to the

16

code, a technical report is issued describing the new system, the information and the

tests submitted , and the recommended usage.

B. Reparability This is the ability of the of the damaged or failed equipment, machine or system to be

restored to acceptable operating condition within a specified time. This property should

be taken into account to avoid losses that would be suffered if replacement was to be

done for whole component or equipment. The spare parts should be available and

affordable.

17

3 CHAPTER THREE

LITERATURE REVIEW ON ENGINEERING MATERIALS, THEIR

PROPERTIES AND CATEGORIES

3.1 INTRODUCTION Many at times, a materials problem is really one of selecting that material which has the

right combination of characteristics for a specific application. This necessitates that the

engineering designers have some familiarity with the general characteristics of a wide

variety of materials.

Engineering materials are classified on the basis of their chemical, physical and

mechanical properties. They include metallic materials (metals and their alloys) and

non-metallic materials (polymers, ceramics and composites).

3.2 MATERIAL PROPERTIES A material property is the measured magnitude of its response to a standard test

performed according to a standard procedure in a given environment. An

understanding of material properties and behavior puts a designer in a position to

choose a proper material for a given product. Material properties are usually formalized

through specifications namely,

Performance specifications which delineate the basic functional requirements of the product and sets out the basic parameters from which the design can be

developed.

Product specifications which define conditions under which components of the designs are purchased or manufactured.

3.2.1 MECHANICAL PROPERTIES Mechanical properties of materials are those related to its ability to withstand external

mechanical forces such as tensile forces, compression forces, twisting (torque), bending

and sudden impact. They include; strength and rigidity, resistance to fatigue, resilience

and toughness, hardness, ductility, damping capacity, friction, and formability. These

were discussed in detail in chapter 2.

3.2.1.1 RELATIONSHIP BETWEEN FAILURE MODES AND MECHANICAL PROPERTIES

In most modes of failure two or more mechanical properties interact to control the

material behavior. In addition, the service conditions met by the material in general use

are more complex than the test conditions under which the material properties are

usually measured.

The service condition may consist of a complex superposition of environments such as

fluctuating stress (fatigue) at high temperature (creep) in a highly oxidizing atmosphere

18

(corrosion). Specialized simulation tests are developed to screen materials for

complex service conditions.

3.3 CATEGORIES OF ENGINEERING MATERIALS Engineering materials can be classified into two major categories: Metallic materials

and Non- metallic material. These are further subdivided into various classes as

illustrated in the diagram below:

Note: HSLA High Strength Low Alloy

Fig. 3.2: Categories of engineering materials

ENGINEERING MATERIALS

Metal & Alloys Non-Metals

Aluminium

Non Ferrous Composites Ceramics Polymers

Ferrous

Copper Titanium Nickel Cast irons Steels

Alloy steels

High alloy Stainless HSLA

Austenitic Ferritic Martensitic

Carbon steels

High Carbon Low Carbon

Medium Carbon

Magnesium

Duplex

19

3.3.1 METALLIC MATERIALS These consist of metals and metal alloys. In this category we have ferrous and non

ferrous metals. They have vast application due to their good electrical and thermal

conductivity. They can be classified into ferrous and non ferrous metals.

3.3.1.1 FERROUS ALLOYS Ferrous alloys contain iron as the prime constituent. Their widespread use is accounted

for by three factors:

Iron-containing compounds exist in abundant quantities within the earths crust Metallic iron and steel alloys may be produced using relatively economical

extraction, refining, alloying, and fabrication techniques.

Ferrous alloys are extremely versatile, in that they may be tailored to have a wide range of mechanical and physical properties. The principal disadvantage of

many ferrous alloys is their susceptibility to corrosion.

3.3.1.1.1 STEELS Steels are ironcarbon alloys that may contain appreciable concentrations of other

alloying elements. The mechanical properties are sensitive to the content of carbon,

which is normally less than 1.0 wt%. Steels are classified according to carbon

concentration, namely, into low, medium, and high carbon types.

A. Low-Carbon Steels Have carbon content of less than 0.25 wt%. Microstructures consist of ferrite and

pearlite constituents. As a consequence, these alloys are relatively soft and weak, but

have outstanding ductility and toughness; in addition, they are machinable, weldable. A

sub group of low-carbon alloys are the high-strength, low-alloy (HSLA) steels. They

contain other alloying elements such as copper, vanadium, nickel, and molybdenum in

combined concentrations as high as 10 wt%, and possess higher strengths.

B. Medium-Carbon Steels Have carbon concentrations between about 0.25 and 0.60 wt%. These alloys may be

heat treated by austenitizing, quenching, and then tempering to improve their

mechanical properties. They are most often utilized in the tempered condition, having

microstructures of tempered martensite. The plain medium-carbon steels have low

hardenabilities and can be successfully heat treated only in very thin sections and with

very rapid quenching rates.

C. High-Carbon Steels Have carbon contents between 0.60 and 1.4 wt%. They are the hardest, strongest, and

yet least ductile of the carbon steels. Used in a hardened and tempered condition and, as

such, are especially wear resistant and capable of holding a sharp cutting edge. The tool

and die steels are high-carbon alloys, usually containing chromium, vanadium, tungsten,

20

and molybdenum. These alloying elements combine with carbon to form very hard and

wear-resistant carbide compounds.

3.3.1.1.2 STAINLESS STEELS The stainless steels are highly resistant to corrosion. Their predominant alloying

element is Chromium with a concentration of at least 11 wt%. Corrosion resistance may

also be enhanced by nickel and molybdenum additions. They are divided into three

classes:

Ferritic steels: contain 12-27% chromium. Martensitic steels: contain 12% chromium and no nickel. Austensitic steels: contain 18% chromium and 8% nickel

3.3.1.1.3 CAST IRONS Generically, cast irons are a class of ferrous alloys with carbon content above 2.14 wt %.

However, most cast irons contain between 3.0 and 4.5 wt% C and, other alloying

elements. They are easily melted and amenable to casting. Cast irons are grouped into:

A. Gray cast Iron The carbon content varies between 2.5 - 4.0 wt %, with Silicon content varying between

1.0 - 3.0 wt%. The graphite exists in the form of flakes (similar to corn flakes), which

are normally surrounded by ferrite or pearlite matrix. Its weak and brittle in tension as

a consequence of its microstructure; the tips of the graphite flakes are sharp and

pointed, and may serve as points of stress concentration when an external tensile stress

is applied. Strength and ductility are much higher under compressive loads. They are

very effective in damping vibration energy.

B. Ductile (or Nodular) Iron It is formed by adding a small amount of magnesium and/or cerium to the gray iron

before casting. Graphite forms as nodules or sphere-like particles instead of flakes. The

matrix phase surrounding these particles is either pearlite or ferrite, depending on heat

treatment. It is normally pearlite for a cast piece. However, heat treatments for several

hours at about 700 0C will yield a ferrite matrix .Castings are stronger and much more

ductile than gray cast iron. Ductile cast iron has mechanical characteristics approaching

those of steel.

C. White cast Iron and Malleable cast Iron White cast iron contains low-silicon (less than 1.0 wt% Si) and undergoes rapid cooling

rates. Carbon exists as cementite instead of graphite. It is extremely hard but also very

brittle, to the point of being virtually unmachinable. White iron is used as an

intermediary in the production of malleable iron. Heating white iron at temperatures

between 800- 9000C for a prolonged time period and in a neutral atmosphere (to

prevent oxidation) causes a decomposition of the cementite, forming graphite, which

21

exists in the form of clusters or rosettes surrounded by a ferrite or pearlite matrix,

depending on cooling rate. The microstructure of malleable iron is similar to that for

nodular iron hence its relatively high strength and appreciable ductility or malleability.

3.3.1.2 NON-FERROUS ALLOYS Steel and other ferrous alloys are consumed in exceedingly large quantities because

they have such a wide range of mechanical properties, may be fabricated with relative

ease, and are economical to produce. However, they have some distinct limitations,

chiefly:

Relatively high density, Comparatively low electrical conductivity, and An inherent susceptibility to corrosion in some common environments.

Thus, for many applications it is advantageous or even necessary to utilize other alloys

having more suitable property combinations. Alloy systems are classified either

according to the base metal or according to some specific characteristic that a group of

alloys share.

3.3.1.2.1 COPPER AND ITS ALLOYS Copper and copper-based alloys, possessing a desirable combination of physical

properties, have been utilized in quite a variety of applications since antiquity.

A. Copper Unalloyed copper has excellent thermal and electrical conductivity. It is soft, ductile, and

has an almost unlimited capacity to be cold worked. It is highly resistant to corrosion in

diverse environments including the ambient atmosphere, seawater, and some industrial

chemicals. The mechanical and corrosion-resistance properties of copper may be

improved by alloying. Most copper alloys cannot be hardened or strengthened by heat-

treating procedures; consequently, cold working and/or solid-solution alloying must be

utilized to improve these mechanical properties.

B. Copper alloys Brasses

Zinc is the predominant alloying element. brasses are relatively soft, ductile, and

easily cold worked. Brass alloys having higher zinc content contain both and phases

at room temperature. The phase has an ordered body centred cubic (BCC) crystal

structure and is harder and stronger than phase; consequently, + alloys are

generally hot worked. Some of the common brasses are yellow, naval, and cartridge

brass, muntz metal, and gilding metal.

Bronze

The bronzes are alloys of copper and several other elements, including tin, aluminum,

silicon, and nickel. These alloys are somewhat stronger than the brasses, yet they still

22

have a high degree of corrosion resistance. Generally they are utilized when, in addition

to corrosion resistance, good tensile properties are required.

Beryllium coppers

They possess a remarkable combination of properties: tensile strengths as high as 1400

MPa, excellent electrical and corrosion properties, and wear resistance when properly

lubricated; they may be cast, hot worked, or cold worked. High strengths are attained by

precipitation-hardening heat treatments. These alloys are costly because of the

beryllium additions, which range between 1.0 and 2.5 wt%. Applications include jet

aircraft landing gear bearings and bushings, springs, and surgical and dental

instruments.

3.3.1.2.2 ALUMINIUM AND ITS ALLOYS Aluminium and its alloys are characterized by a relatively low density (2700Kg/m3),

high ductility, high electrical- thermal conductivities, and a resistance to corrosion.

Since aluminium has a face centred cubic (FCC) crystal structure, its ductility is retained

even at very low temperatures. The chief limitation of aluminium is its low melting

temperature, which restricts the maximum temperature at which it can be used.

Principal alloying elements include copper, magnesium, silicon, manganese, and zinc.

Aluminium alloys are classified as either cast or wrought. Some of the more common

applications of aluminum alloys include aircraft structural parts, beverage cans, bus

bodies, and automotive parts (engine blocks, pistons, and manifolds).Recent attention

has been given to alloys of aluminum and other low-density metals (e.g. Mg and Ti) as

engineering materials for transportation, to effect reductions in fuel consumption. An

important characteristic of these materials is specific strength, which is quantified by

the tensile strengthspecific gravity ratio.

A generation of new aluminum-lithium alloys has been developed recently for use by

the aircraft and aerospace industries. These materials have relatively low densities

(between 25002600 Kg/m3), high specific moduli (elastic modulus specific gravity

ratios), and excellent fatigue and low-temperature toughness properties.

3.3.1.2.3 MAGNESIUM AND ITS ALLOYS The most outstanding characteristic of magnesium is its density (1700 Kg/m3); hence

its alloys are used where light weight is an important consideration (e.g. in aircraft

components).It is relatively soft, and has a low elastic modulus. At room temperature

magnesium and its alloys are difficult to deform. Consequently, most fabrication is by

casting or hot working. It has a moderately low melting temperature. Chemically,

magnesium alloys are relatively unstable and especially susceptible to corrosion in

marine environments. On the other hand, corrosion or oxidation resistance is

reasonably good in the normal atmosphere (due to impurities). Fine magnesium

powder ignites easily when heated in air; consequently, care should be exercised when

handling it in this state.

23

These alloys are also classified as either cast or wrought, and some of them are heat

treatable. Aluminum, zinc, manganese, and some of the rare earths are the major

alloying elements. These alloys are used in aircraft and missile applications.

3.3.1.2.4 TITANIUM AND ITS ALLOYS Titanium and its alloys are relatively new engineering materials that possess an

extraordinary combination of properties. The pure metal has a relatively low density

(4500 Kg/m3), a high melting point [16680C], and an elastic modulus of 107 GPa.

Titanium alloys are extremely strong, with room temperature tensile strengths as high

as 1400 MPa. Furthermore, the alloys are highly ductile, easily forged and machined.

The major limitation of titanium is its chemical reactivity with other materials at

elevated temperatures. This property has necessitated the development of

nonconventional refining, melting, and casting techniques; consequently, titanium

alloys are quite expensive. In spite of this high temperature reactivity, the corrosion

resistance of titanium alloys at normal temperatures is unusually high; they are

virtually immune to air, marine, and a variety of industrial environments

They are commonly utilized in airplane structures, space vehicles, surgical implants,

and in the petroleum and chemical industries.

3.3.1.2.5 THE SUPER ALLOYS The super-alloys have superlative combinations of properties. Most are used in aircraft

turbine components, which must withstand exposure to severely oxidizing

environments and high temperatures for reasonable time periods. Mechanical integrity

under these conditions is critical; in this regard, density is an important consideration

because centrifugal stresses are diminished in rotating members when the density is

reduced. These materials are classified according to the predominant metal in the alloy,

which may be cobalt, nickel, or iron. Other alloying elements include the refractory

metals (Nb, Mo, W, and Ta), chromium, and titanium. In addition to turbine applications,

these alloys are utilized in nuclear reactors and petrochemical equipment.

3.3.1.2.6 MISCELLANEOUS ALLOYS NONFERROUS The discussion above covers the vast majority of non-ferrous alloys; however, a number

of others are found in a variety of engineering applications. These include:

Nickel and its alloys are highly resistant to corrosion in many environments, especially

those that are basic (alkaline). Nickel is often coated or plated on some metals that are

susceptible to corrosion as a protective measure. Monel, a nickel based alloy containing

approximately 65 wt% Ni and 28 wt% Cu (the balance iron), has very high strength and

is extremely corrosion resistant; it is used in pumps, valves, and other components that

are in contact with some acid and petroleum solutions.

24

Lead, tin, and their alloys find some use as engineering materials. Both are

mechanically soft and weak, have low melting temperatures, are quite resistant to many

corrosion environments, and have re-crystallization temperatures below room

temperature. Many common solders are leadtin alloys, which have low melting

temperatures. Applications for lead and its alloys include x-ray shields and storage

batteries. Tin is used as a very thin coating on the inside of plain carbon steel cans (tin

cans) that are used for food containers; this coating inhibits chemical reactions between

the steel and the food products.

Zinc is a relatively soft metal having a low melting temperature and a re-crystallization

temperature. Chemically, it is reactive in a number of common environments and,

therefore, susceptible to corrosion. Galvanized steel is just plain carbon steel that has

been coated with a thin zinc layer; the zinc preferentially corrodes and protects the

steel .Typical applications of galvanized steel are familiar (sheet metal, fences, screen,

screws, etc.). Common applications of zinc alloys include padlocks, automotive parts

(door handles and grilles), and office equipment.

Zirconium and its alloys are ductile and have other mechanical characteristics that are

comparable to those of titanium alloys and the austenitic stainless steels. However, the

primary asset of these alloys is their resistance to corrosion in a host of corrosive media,

including superheated water. Furthermore, zirconium is transparent to thermal

neutrons, so that its alloys have been used as cladding for uranium fuel in water-cooled

nuclear reactors.

3.3.2 NON-METALLIC MATERIALS

These are the materials that do not exhibit metallic characteristics in their properties.

Examples are composites, ceramics, rubbers, plastics and polymers.

3.3.2.1 POLYMERS These are compounds of high molecular weight derived by the addition of smaller

molecules (monomers) or by the condensation of smaller molecules with the

elimination of water, alcohol and other solvents. There are many different polymeric

materials that are familiar to us and find a wide variety of applications.

3.3.2.1.1 Plastics They have a wide variety of combinations of properties. Some plastics are very rigid and

brittle; others are flexible, exhibiting both elastic and plastic deformations when

stressed, and sometimes experiencing considerable deformation before fracture. Plastic

materials may be either thermoplastic or thermosetting.

A. Thermoplastics These are also known as thermo softening plastics. They have very weak Van Der Waals

forces. They are polymers that liquefy on heating and when cooled, they form a very

25

glassy state. They are easily molded and extruded into films, fibers and packaging

materials. E.g. Polyvinylchloride, polyethylene

B. Thermosetting plastics These are polymers that cure irreversibly. Once cooled and hardened, they return to

their shapes but cannot return to their original form. The curing is by heating or

through a chemical reaction. They can be used for automobile parts, aircraft parts and

tyres. Example are vulcanized rubber and epoxy resins.

3.3.2.1.2 Elastomers They have a cross linked structure with a looser mesh than thermosets. Thus they have

the ability to be deformed to quite large deformations, and then elastically spring back

to their original form. Their moduli of elasticity are quite small. They are to produce

automobile tyres. Example is Natural poly-isoprene (natural rubber)

3.3.2.1.3 Fibers Fibers are capable of being drawn into long filaments (100: 1 length-to-diameter ratio).

Fiber polymers are utilized in the textile industry, being woven or knit into cloth or

fabric. While in use, fibers may be subjected to a variety of mechanical deformations:

stretching, twisting, shearing, and abrasion. Consequently, they must have a high tensile

strength (over a relatively wide temperature range) and a high modulus of elasticity, as

well as abrasion resistance.

3.3.2.2 CERAMICS These are inorganic non-metallic materials made up of two or more elements bonded

together. They can be dense or light in weight but with excellent strength and hardness

properties. Typical properties of ceramics include:

Ceramics are brittle, wear resistant, hard and oxidation-resistant. They are very strong in compression but very weak in tension due to presence of

minute cracks.

They are also widely applicable in positions involving chemicals because they are inert.

Ceramics are hard and strong. Ceramics are divided into four sections of application, namely:-

Structural application ceramics e.g. bricks, roof and floor tiles. Refractory applications: These are the ceramics used as kiln linings and gas fire

radiant.

Technical engineering applications: These include fire ceramics used in space shuttle programmers.

Whiteware applications ceramics: Become white after the high-temperature firing. E.g. porcelain, pottery, tableware, china, and plumbing fixtures (sanitary

ware).

26

3.3.2.3 COMPOSITES These are engineering materials made from two or more materials with significantly

different chemical and physical properties and these materials remain separate or

distinct on the microscopic level within a finished structure. The constituent material is

either a matrix or reinforcement.

The matrix, usually a polymer matrix, surrounds and supports the reinforcement by

maintaining their relative positions. The reinforcement; usually fibers, metals, ceramics

and polymers impart their mechanical and physical properties to enhance the matrix

properties. Composites have special properties like:-

Fire resistance. Light weight. Chemical and weathering resistance. Good electrical properties. High strength to weight ratio.

Composites fail by: Shock, impact and repeated cyclic loading causing separation of the

layers (de-lamination). Some composites are brittle and have little reserve strength

beyond initial onset of failure while others have reserve energy absorbing capacity past

the onset of damage. In comparison with other materials, composites have poor bearing

strength.

27

4 CHAPTER FOUR REVIEW OF LITERATURE ON DECISION MAKING AND

INFORMATION PROCESSING

4.1 INTRODUCTION Engineering design is inherently a decision making process where choices are

constantly being made between alternatives, such as selection of concepts, components,

or the rating of client needs. The tools used in solving these problems depend largely on

the type of data available (deterministic, probabilistic, or uncertain)4.

Numerous methods have been developed to help design teams make the correct choices

by using structured approaches. The two widely used tools include:

i. Rank order: Pairwise comparison charts (PCCs) ii. Analytic hierarchy process(AHP)

4.1.1 RANK ORDER: PAIR WISE COMPARISON CHARTS Dym and Little (2003) proposed using of PCCs based on the premise that it is easier to

differentiate between pairs of alternatives e.g. A is better than B or A is similar to B.

PCCs use a matrix structure to compare each alternative individually with every other

(Pair wise comparison). The results from the comparison are summed to obtain an

overall rank order.

PCCs can be generated using the following steps:

1) In a table, the n items to be compared are listed as row and column headings in an nn matrix. An additional column is added at the end of the matrix to record

the total score for each item.

Table 4.1- Structure of PCCs; Adapted from: Madara Ogot & Gul Kremer, Engineering

Design: A Practical Guide.

Comparison criteria

Evaluated A B C D E F Total A -1 -1 -1 -1 -1 -5 B 1 -1 1 -1 0 0 C 1 1 1 1 1 5 D 1 -1 -1 0 -1 -2 E 1 1 -1 0 -1 0 F 1 0 -1 1 1 2

4Taha, A., 2008

28

Key

A-Size E-Availability

B-Weight F-Manufacturability

C-Strength

D-Cost

2) The first row is compared individualy to all other column items. Scores of 1, 0, and -1 are assigned if the row item is better, similar, or worse, respectively than

the column item.

3) The row scores are totalled, yielding the overall score of thr first alternative. 4) Steps 2 and 3 are repeated for all alternatives. 5) The ranking order for alternatives is compiled. The higher the overall score, the

higher the alternatives rank. From the table above, strength (C) is ranked

highest.

4.1.2 RELATIVE ORDER: ANALYTIC HIERARCHY PROCESS (AHP) It is used when a relative score is required for a set of qualitative alternatives.AHP

determines by how much each alternative is better (or worse) than the others. It is

based on the fundamental scale which captures individual preferences with respect to

qualitative or quantitative attributes.

Example

Consider two choices of materials all of which meet the basic properties desired for a

particular product. To select one material, the designer specifies three main criteria: its

availability, cost, and the manufacturing process. Giving a weight of approximately 45%

to availability, 35% to cost and 20% to manufacturing process, the designer uses a

systematic analysis to rank these two materials. The table below ranks the three criteria

for the two materials:

Table 4.2: Criteria ranking for the three materials

Criterion Manufacturing process (20%)

Cost (35%)

Availability (45%)

Composite weights

Index Estimates

Material A

0.63 0.42 0.33 0.4215

Material B

0.37 0.58 0.67 0.5785

The problem involves a single hierarchy (level) with three criteria (manufacturing

process, cost, and availability) and two decision alternatives (Material A and material

B). The ranking of each material is based on computing the following composite

weights:

29

Material A=0.2*0.63+0.35*0.42+0.45*0.33=0.4215

Material B=0.2*0.37+0.35*0.58+0.45*0.67=0.5785

Material B has the highest composite weight, and is therefore the best material choice

for the application.

4.2 INFORMATION PROCESSING

4.2.1 Hypertext Pre-Processor (PHP) PHP is a server-side scripting language. A server- side scripting language allows the

user to embed little programs (scripts) into the HTML of a web page. When executed,

such scripts allow the user to control what will actually appear in the browser window

with more flexibility than is possible using straight HTML. Although to some extent PHP

is similar to JavaScript, the key difference between the two is that JavaScript is

interpreted by the web browser once the web page that contains the script has been

downloaded whereas PHP is interpreted by the web server before the page is sent to the

browser. Once interpreted, the results of the script replace the PHP code in the Web

page so that all the browser sees is a standard HTML file. The script is processed

entirely by the server, hence the designation: server-side scripting language.

Some of the merits of PHP include:

Access to server-side resources. Interpretation of scripts by the Web server thus eliminating browser

compatibility issues.

Reduced load on client. PHP syntax is similar to that of C, C++, Java, or any other C-derived language.

4.2.2 Definitions The following definitions are commonly associated with PHP:

a) Constant:- This is an identifier (name) for a simple value and does not change in the execution of the script. Constant identifiers are always in upper case.

b) Expression:- This is anything that has value. The basic forms of expressions are constants and variables.

c) Operator:- An operator is anything that you feed with one or more values (or expressions) which yields another value. Examples of operators are unary

operators operating on one value and ternary operator which select between

two expressions depending on a third one.

d) Open database connectivity (ODBC) is an application programming interface (API) that allows one to connect to data source.

e) A web server is a computer program that delivers (serves) content, such as web pages, using the Hypertext Transfer Protocol (HTTP) over the World Wide

Web (WWW). The primary function of a web server is to deliver web pages

which are basically HTML documents to clients (i.e. a web browser). A full

implementation of HTTP also includes a way of receiving content from clients.

This feature is used for submitting web forms, including uploading of files. A

30

client initiates communication by making a request for a specific resource using

HTTP and the server responds with the content of that resource, or an error

message if unable to do so.

f) Mysql:- Mysql is a relational database management system (RDBMS) that runs as a server providing multi-user access to a number of databases. Many web

applications use mysql as a database component. PhpMyadmin, a free web

based protocol widely installed by web hosts worldwide, can connect to

local/remote MySQL servers to manage databases, tables, column structure and

individual data records. MySQL can be built and installed manually from source

code, though it is more commonly installed from binary package - unless

customizations are required.

g) Data base:- This is a collection of data typically describing activities of one or more related organizations.

h) Data base management system (DBMS):- Software designed to assist in maintaining and utilizing large collections of data.

i) Variables in PHP are represented by a dollar sign followed by the name of the variable. It is possible to access Microsoft server from PHP on a windows

machine by simply using ODBC support and the correct ODBC drive. Variables in

PHP are always assigned by value i.e. when you assign an expression to a

variable, the entire value of the original expression is copied into the destination

variable.PHP also allows for the assignment of value to a variable by reference.

This means that the new variable simply references to the original variable.

Changes in the new variable affects the original variable and vice versa.

4.2.3 How php Works When a client visits a page on a database driven website, the clients web browser

requests for the web page using a standard URL. The web server software (Apache)

recognizes that the requested file is a PHP script and so the server interprets the file

using its PHP plug-in before responding to the page request. Certain PHP commands

connect to the MySQL database and requests the content that belongs to the web page.

The MySQL database responds by sending the requested content to the PHP script. The

PHP script stores the content into one or more PHP variables and then uses the echo

function to output the content as part of the web page. The web server sends the HTML

to the web browser as it would for a plain HTML file except that instead of coming

directly from an HTML file, the page is the output provided by the PHP plug-in.

31

5 CHAPTER FIVE

CASE STUDY: SELECTION OF A MATERIAL FOR A REVERTED

TWO STAGE COMPOUND GEAR TRAIN

5.1 INTR0DUCTION Gears are machine elements that transmit motion by means of successively engaging

teeth. This form of transmission is possible because of the rigidity of the material from

which the gear wheels are made. From kinematical point of view, gear wheels may be

assumed to be completely rigid, such that there is no deformation whatsoever when the

gear wheel is subjected to force. Thus the kind of transmission of motion that occurs in

gear drives is known as a positive drive in which there should be no loss of motion at all.

This is as opposed to belt drives, for instance, in which loss of motion may occur due to

creep, slip or both creep and slip of the belt relative to the pulleys.

5.1.1 CLASSIFICATION OF GEARS Gears are generally classified according to the orientation of the teeth; as follows:

Spur gears: The teeth are lengthwise parallel to the axis of rotation of the gear wheel. The overall form of the gear wheel is actually cylindrical.

Fig. 5.1: Spur gears

Helical gears: Similar to spur gears except that the teeth of a helical gear are cut at an angle (known as the helix angle) to the axis. Helical gears are made in

both right and left hand configurations.

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Fig. 5.2: Helical gears

Bevel gears:The teeth lie upon a cone rather than a right cylinder. Variants of the bevel gears are the straight bevel, spiral bevel and the hypoid gears.

Fig. 5.3: Straight bevel

Fig. 5.4: Spiral bevel

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Fig. 5.5: Hypoid bevel

Worm and worm wheel: A worm is a type of gear with one or more cylindrical threads or starts (that resemble screw threads) and a face that is usually wider

than its diameter. A worm wheel, on the other hand, is a helical gear that meshes

with the worm.

Fig. 5.5: Worm and wheel gears

Gear drives have a number of advantages compared to the other mechanical power

transmission devices such as belt drives and chain drives. The major advantages are the

following:

They provide a constant speed ratio. They dont exhibit chordal action, as in chain drives. They are more compact as compared to belt and chain drives. The range of speeds and loads with which gear drives may be used is far broader

than with belt and chain drives.

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5.1.2 GEARING TERMINOLOGY

Fig. 5.6: Illustration of gearing terminology; Adapted from Boston-Gear

The following defined terms are generally applicable to gears:

Pitch circle is an imaginary circle that corresponds to the circumference of the friction

wheel that corresponds to the gear. The pitch circle of meshing gears roll on each other

without slipping.

Pitch circle diameter (D) is the diameter of the pitch circle of a gear or pinion. Addendum (a) is the radial distance from the pitch circle to the top of the tooth. Dedendum (d) is the radial distance from the pitch circle to the bottom of the tooth space.

Outside diameter (D) is the diameter of the addendum circle. Thus 2 Root diameter (D) is the diameter of the root circle.5 Thus 2 Whole depth (h) is the total height of the tooth or the total depth of the tooth space. Thus Working depth (h) is the distance that a tooth that projects into the mating tooth space. Thus 2 5 The root circle is also known as the dedendum circle

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Clearance (c) is the distance between the top of the tooth and the bottom of the mating tooth space. Thus; Circular pitch (p) is the distance, along the pitch circle, from a point on one tooth to a corresponding point on adjacent tooth. Therefore /(z is the number of teeth). Module (m) is the ratio of the pitch circle diameter of a gear wheel to the number of teeth on the gear wheel. Thus /. It therefore follows that and that the circular pitch and the module are really measures of the same quantity, to different

scales.

Pressure angle or tooth shape () is the angle at which the pressure from the tooth

of one gear is passed on to the tooth of another gear. Spur gears come in two pressure

angles: 14 !"#and 20. Diametral pitch (P) is the ratio of the number of teeth on a gear wheel to the pitch circle diameter of the gear wheel. Thus % / 1/ Backlash (B) of a pair of meshing teeth is the amount by which the width of a tooth

space exceeds the thickness of a mating tooth on the pitch circle. A small amount of

backlash is usually desirable, or necessary. But if it is excessive the gears will rattle

under light loads or when running idle.

Face width (b) is the lengthwise width of the teeth in the direction parallel to the axis of

rotation of the gear wheel

Gear ratio (G) is the mathematical ratio of a pair of spur gears determined by dividing

the number of teeth on the larger gear with the number of teeth on the pinion.

5.1.3 DESIGN CONSIDERATIONS FOR A GEAR TRAIN Prior to the design of a gear train, the following data is usually required:

The power to be transmitted The speed of the driving gear The speed of the driven gear or the gear ratio The centre distance

Also the following requirements must be met in design of a gear train:

The gear teeth should have sufficient strength so that they will not fail under static loading or dynamic loading during normal running conditions.

The gear teeth should have wear characteristics so that their life is satisfactory. The use of space and material should be economical. The alignment of the gears and deflections of the shafts must be considered

because of their effect on the performance of gears.

The lubrication of the gears must be satisfactory.

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5.1.4 MODES OF GEAR FAILURE Gear failure occurs as a result of a material having or lacking particular attributes

closely related to its mechanical properties. The following are the various modes of

gear failure common in practice, and possible remedies:

Bending failure: Every gear tooth acts as a cantilever. If the total dynamic load acting on the gear tooth is greater than its beam strength, failure due to bending

will occur i.e. the tooth will break. To avoid such failure, the module and face

width is adjusted such that the beam strength is greater than the dynamic load.

Pitting: Its surface fatigue failure which occurs due to many repetition of Hertz 6contact stresses. The failure occurs when the surface contact stresses are

higher than the endurance of the material. It starts with formation of pits which

continue to grow resulting in the rupture of the tooth. To avoid pitting, the

dynamic load must be less than the wear strength of the gear tooth.

Scoring: Excessive heat is generated when there is an excessive surface pressure, high speed, or failure of lubrication system. This causes a stick- slip

phenomenon in which shearing and welding takes place rapidly. To avoid

scoring, proper design of parameters such as speed, pressure and proper flow of

the lubricant should be carried out.

Abrasive wear: