UNIT II 0.1 INTRO TO DESIGN AND SELECTION OF MATERIALS.pdf

7/24/2019 UNIT II 0.1 INTRO TO DESIGN AND SELECTION OF MATERIALS.pdf

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ME 215 Engineering Materials I

Dr. Ouzhan YILMAZ

Assistant Professor

Mechanical Engineering

University of Gaziantep

Chapter 2 Design Engineering and

Selection of Materials


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1

Introduction

Design of new products and development of the existing ones is

the essential purpose of engineering.

For the designing of a machine element or a component, an

engineer has to consider many requirements.

The selection of the material from which a part is to be produced

has always been a predominant factor in the overall performance

of a design because of its influence on other factors.

Hence, it is not usually possible to make

the final decision on the geometry and

dimensions of a part until the material is

selected.


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Fundamental Aspects of the Design Procedure

Making the right distinction between demands that are truly appropriate to

material properties and certain design features.

For example, strength of a part depends upon the

strength of material and geometrical parameters.

This does not mean that high strength materials

are needed to achieve the required strength of the

part.

Instead, designer can prefer a weaker material but

larger dimensions as long as there are no space

or weight restrictions. When such restrictions are

tight, then strength of material itself becomes

considerably important. Do not understimate me!


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In addition, material selection has such a

great influence;- the selection of a totally different material

would result in a new design approach.

- Inevitably, chosing the most suitable

material for a part is possible by goodstorehouse knowledge concerning the

material properties.

- The most decisive factor for making a

proper selection is the experience, whichno book could provide.


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A simple flow diagram of design thinking for material selection:

NEED FUNCTIONAL REQUIREMENTS

Order of Importance

Level of Satisfaction

Failure Criteria

DESIGN LIMITATIONS

Production Requirements

Economic Requirements

Maintenance Requirements

PROBLEM DEFINITION

Material Selection

MATERIAL ALTERNATIVES

Storehouse Knowledge

Experience

FINAL CHOICE


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Every design effort is aimed at satisfying an existent orpotential need.

From the analysis of the need, a designer

determines essential and desirable

features of the design that are expressed in

the form of functional requirements.

As it is impossible for a design to satisfy

all of the requirements to the same

degree, they are then arranged in theorder of importance in order to identify

the areas of comprimise.


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Furthermore, a design has to be in

compliance with certain inevitabledesign limitations that are grouped

as: Manufacturing (production), Money

(economic) and Maintenance

requirements (i.e. 3M rule).

Depending upon the nature of design, it is

sometimes the functional requirements

and sometimes the design limitationsthat dictate the properties to be desired in

the metarial for the design work at hand.


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Functional requirements concern the

mechanical properties of material (such

as strength, stiffness, resilience,

toughness, dimensional stability,

hardness, etc.) and the physical

properties (e.g. coefficient of linear

expansion, thermal and electrical

conductivity, and so on).

Production requirements are logically

the first to be considered. Hence, the

designer must consider functional

merits of the material as well as its

ability to be machined, shaped, formed,

cast, welded, and so on.


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Economic requirements are based

on the final product cost composed

of raw material cost and production

costs with overheads. The cost of

any product should be as high as the

customers can pay for it.

Finally, maintenance requirements

(i.e. whether replacement or repair is

required) depend upon size of the

part, extent of possible damage,

facilities of the customers and the

acceptable level of costs.


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Production Requirements

A design is realized only after it is produced. Hence, the designer must be

aware of the fact that production is carried out according to drawings and

specifications, where the production group may give useful hints.

Material selection depends upon the functional demands, how many parts

will be produced, which materials can be used, and what properties are

related for that design.

The production requirements can be gathered in the following groups:

1. Machinability

2. Formability

3.Castability

4. Suitability for Compacting

5. Weldability

6. Heat Treatability

7.Adaptability to Special Processes

8.Adaptability to Forms of Protection


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Production Requirements Machinability

Machining is shaping a part by removing the unwanted material in the form

of chips to achieve the desired shape. Turning, milling, shaping, drilling and

boring are the familiar examples of chip removal processes. In addition;

grinding, honing and lapping remove the material with abrasives.

Speed of chip removal, tool life and quality of machined surfaces are used

jointly to describe the machinability. Quantitatively speaking, a highly

machinable material is the one that allows the maximum amount of chip

removal with the minimum tool wear, yielding a high surface quality.

The above factors vary not only from one material to another, but also from

one machining process to another.


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Production Requirements Formability

Forming processes (e.g. rolling, forging, cold heading, stamping, pressing,

and drawing) provide special advantage of enabling the desired shape to

be obtained with ease, without machining the surfaces that are not mating.

This is a great advantage over chip removal processes.

Another advantage of such processes is that, unlike casting process, most

engineering materials are amenable to forming. However, the main problem

is that they are costly processes.

During forming operations, the material is subjected to considerable degree

of deformation affecting its mechanical properties. This can be beneficial or

detrimental depending upon the material as well as type and extent of theforming process.


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Production Requirements Castability

Casting is used to produce finished parts as well as intermediate forms

requiring further operations. In theory, any material that can be melted can

also be cast.

Casting has a special advantage to produce parts with sophisticated shape

especially in large numbers, which usually cannot be possible by the other

processes (e.g. the carburetor of a car).


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Production Requirements Castability

The main difficulty is that the process is quite dependent on the design.

Shape of casting must enable the molten metal to fill all cavities in themould. Also, as a metal shrinks upon freezing, the molten metal must be

constantly fed during solidification into mould to compensate the shrinkage,

otherwise a spongy metal is obtained. Hence, the designer must decide the

material and the type of casting process together.


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Production Requirements Suitability for Compacting

This is required when the part will be produced by powder metallurgy.

The metal powder is compacted in a die to the desired form, and then

sintered to fuse the powder particles together.

Most metals and alloys can be used in this process, but only few of them

are economically justified. This process is the best way to produce parts

from brittle and very hard metals.

Although intricate forms with desirable mechanical properties could be

produced, the availability of required metal in the powder form and capital

investment are the main limitations.


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Production Requirements Weldability

Welding process is not only used to

produce large and complex parts by

welding the simpler parts together(like

frames of certain machine tools), but

also used for maintenance and repairs.

Weldability does not mean the ability

to be welded, but represents the

relative ability of metals (usually

steels) to be welded without cracking

or error-free welding.


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Production Requirements Weldability

Production of especially large parts by

welding is regarded as an alternative to

casting when they are needed in few

numbers. However, the successful production

of complicated shapes by welding demands a

special design approach (like in case of

casting) as well as a careful planning ofstress-relief treatments and operations.

Two recent welding techniques (electron and

laser beam welding utilizing beams to

generate heat of fusion) have made it possible

to weld hardenable and heat treated steels,

which were not be welded before.


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Production Requirements Heat Treatability

Heat treatment (causing structural changes in metal) is used to improve essential

mechanical properties, change grain size and relieve residual stresses.

Hardenability (depth of hardening) is a desirable material property if the aim

of heat treatment is to increase strength and/or hardness. It is dependent upon

materials rate of hardening.

Some ferrous and nonferrous alloys can be hardened by age (precipitation)

hardening. The alloy is heated to a temp. where it exists as a homogeneous

solid-solution phase, then it is cooled rapidly (quenched). Finally, it is held at

room temp. (natural aging) or above the room temp. (artificial aging) to allow

precipitation of solid- solution.


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Production Requirements Heat Treatability

Heat treatment is also used to alter surface properties of ferrous alloys. Rapid

heating of surface by induction/flame followed by quenching (induction/flame

hardening) produces a hardened surface while the interior of material is softer.

In other thermal surface treatments (calorizing, carburizing, cyaniding, nitriding,

carbonitriding, chromizing, etc.), a substance diffuses into heated metal surface.


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Production RequirementsAdaptability to Special Processes

Many intricate and special parts are produced by chipless manufacturing

processes. In chemical milling, material is removed by the etching reaction

of chemical solutions with metal. It can also be used on plastics and glass.

Electrochemical machining (ECM) employs electroplating process, where

the tool (with inverse shape of part) is cathode and the workpiece is anode.

Electrodischarge machining (EDM) cuts metal by action of high-energy

electric sparks or electrical discharges.

Laser beam cutting is a recently developed cutting process using laser.

These processes are not fast methods of production. High capital cost andslow production speed make them suitable only when parts to be produced

are of special nature and are few in numbers.


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Production RequirementsAdaptability to Forms of Protection

In many cases, material properties could not meet some functional demands,

especially arising from environmental conditions. As high quality materials for this

purpose are too expensive, designer may use finishes and coatings.

Such finishes and coatings are employed in order to:

protect the base material against hostile environmental conditions

give functional properties that are not attainable within base material

improve the appearance of product by colour, polish or decoration

The finishes or coatings may be classified under four (4) main groups:

1. Organic coatings: resins, pigments, lacquers, varnishes, paints, dispersion

coatings, emulsion coatings, hot-melt coatings, plastic powder coatings

2. Metallic coatings: electroplates, chemical-deposition and sprayed-metal

coatings, hot-dip coatings, diffusion coatings, vapour-deposited coatings

3. Conversion coatings: phosphate, chromate, and chemical oxide coatings

4. Ceramic coatings: vitreous (glass-like), porcelain, and ceramic coatings


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Metallic Coating

Plastic Powder Coating

Ceramic CoatingChromate coating


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Design requirements concerning the cost are simple: keep them as low as

possible without impairing the essential design features.

Cost of a design comprises production costs (built up from material and

processing), labour costs and capital costs.

The foremost economic factor is

availability.

- Candidate materials in a design

project must be available in market.

Expensive delays will be incurred

due to supply difficulties.

- A market research is a must

before final material selection.


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Actual cost of raw material is cost of material used in part plus cost of scrap

material.Adjustment of dimensions (whenever possible) to available stock

sizes is a regular design procedure to reduce scrap and production time.

Production costs depend on;

- number of operations,

- amount of skilled labour,

- time in each operation.

In most cases, surface finish is

important for parts performance and

apperance.

- Thus, secondary finishing

operations may also be needed in

such cases.


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Maintenance covers activities that are necessary but not directly concerned

with operation or use (such as cleaning, lubrication, adjustment, overhaul

and repair of damaged/worn equipment).

In principle, durability (long life) is considered in

design as a user requirement. It is annoying that

a recently purchased product does not work anylonger, which causes inconvenience for customer,

heavy repair bills, or scapping of the product. So,

the complaints about service life and cost must

be minimized.


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How often and at what cost are

inevitable questions to be

answered during the design stage;requiring a firm decision on whether

replacement or repair, or both will

be predicted. When frequent

replacements are predicted, part

must be cheap so that it is more

worthwhile than repair. If repair is

predicted, the material must lend

itself to acceptable forms of repair.

Non-stick frying pans and self cleaning ovens are recent examples indicating

that how use of a new material facilitates maintenance.

Plastic surfaces not only improve apperance, but also facilitate the

cleaning problems.


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Failure

Failure happens when a design is no longer able to satisfy any of functional

requirements. Failures not only cause costly damage, but may lead to loss

of many lives as in airplane crashes.

A conceptual understanding of failure is necessary to utilize the material

properties safely and economically.

In most design problems, primary concern

should be reducing the possibility of a

premature failure in service.

The service life can be in seconds (in case

of space applications) or many years (in

case of bridges).


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Failure

Possible failure types during service are excessive

deformation, fracture, inordinate wearand

deterioration.

- In practice, it is impossible to predict failure mode of

a part under severe service conditions.

- Some failures happen soon after the element is in

service, which are covered by a factor of safety.

Time dependent failures are difficult or even

impossible to avoid by applying factor of

safety.

- In such cases, parts are withdrawn from

service and tested for reliability. Such

specific data are not found in general

reference books.


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f


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Failure - Excessive Deformation

In many cases, the failure

criterion is based on

materials strength

although a failure by

excessive deformation is

implied. This is due to the

fact that the stressapproach is more

universal covering the

fracture aspect as well.

It must be remembered that when a failure by inordinate elastic deformation

is of issue, design approach must always be based on deformation analysis

since the results shall be compatible with functional requirements.

F il F


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Failure - Fracture

When analysing the fracture failure modes, the preceding deformation is of

importance. If failure occurs following a large deformation, such fractures are

called ductile fracture (which is not common in engineering applications). Incontrast, a fracture with no or very little prior deformation is brittle fracture.

Many materials fail by fracture in three ways: sudden brittle fracture,

fatigue (progressive) fracture and time dependent (creep) fracture.

F il F t


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Failure - Fracture

Brittle fracture is not only

experienced by brittle materials.

Higher rate or sudden

application of load and

presence of a complex stress

may cause ductile to brittle

transition (embrittlement) of amaterial.

Fatigue failure (the most common failure in applications) is a highly localized

microscopic phenomenon. It occurs in parts that are subjected to repeatedstresses even if they are below the yield point of material.

Creep failure (stress rupture) occurs when a material is loaded at higher

temperatures for a long time. In polymeric materials, it can occur even at

normal temperatures and under relatively low stresses.

F il W


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Failure - Wear

Wear is result of the action of abrasive or other forces on the surface of a

machine part. It is manifested by a loss of surface material (either in regular

or irregular form) which causes change in the part dimensions.

Wear is a complex subject due to many variables involved in the process

where lubrication, condition of surface and type of material with which part

is in contact are the most effective factors.

There is no a quantitative test or criterion of wear. Thus, design evaluations

are based on past experience more than anything else.

F il D t i ti


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Failure - Deterioration

Deterioration (loss of original properties) may occur in certain applications.

Most common examples are caused by the environment in which materials

operate. Reaction of a chemical environment (corrosion and oxidation) arethe most common examples.

No material is completely resistant to liquid or gas. Liquid or gas absorption

may cause embrittlement which is a special problem in nuclear applications

because of danger of nuclear substances.

Speaking of nuclear applications, material properties are significantly altered

by irradiation. In some cases, the effects of irradiation can be beneficial as

it causes increase in yield strength.

Fungus or other growths cause deterioration of strength or other material

properties (noticable in wood and some plastics), or loss of efficiency of the

whole system (some sea bacteria on a ship body).

P F il A l i


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Proper Failure Analysis

Proper application of failure analysis provides a valuable checklist to design

problems and material limitations.

A good design is the one that just answers the need where the requirements

are slightly exceeded by the capabilities of the design. Under-designing

tends to fail in some way whereas over-designing is not only economically

pointless but also unapplicable or useless.

Fundamental factors related to failure or shortening of service life are listed

below (the failure may be due to any or combination of them):

1. Problematic design

2. Improper selection of material3. Heat treatment methods

4. Fabrication techniques

5. Improper machining and assembly methods

M t i l S l ti P


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Material Selection Process

- reduce the number of candidate materials to a manageable number.

Past experiences, investigation of materials currently used for similar

designs, existing standards, codes or legal requirements (if any) help to

narrow the selection list.

Design philosophy plays important role in screening material alternatives.

It determines general trend of design varying in different industries, countries

and companies.

It is difficult to define design philosophy. For instance, the design philosophy

applied for the products in car industry may be similar. However, aircraft or

space industry needs specific design philosophy requiring certain criteria:

strength must be combined with lightness accuracy and design efficiency are more important than cost

life in operating hours is relatively limited

frequent and careful maintenance must be ensured

wide extremes of service conditions must be taken into account

M t i l S l ti P


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Material Selection Process

Measures of value (highly dependent on design philosophy) are standards

by which the merits of a material can be weighed. Its proper establishment

provides a clever and economical material selection.

In an engineering design, benefits are often based on intangible factors.

The most universal method for measures of value is in monetary terms

such as comparison of the product price with its rivals.

However, the designer must know that incorrect comparison leads to biased

results and misleading benefit analysis.

For instance, it is not correct to look at only the cost per unit weight of rawmaterial without considering how much material is actually required to

produce a certain part. The example in the next page illustrates this problem.

Material Selection An E ample


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Material SelectionAn Example

A cylindrical part of 500 mm long will carry an axial load of 600 kg.

Material A with raw material cost of 100 TL/kg can be stressed up to

15 kg/mm2, while Material B with raw material cost of 150 TL/kg can

be stressed upto 25 kg/mm2. Both materials have the same density of

7.8 10-6 kg/mm3. Which material must be preferred based on cost?

The cross-sectional area of the parts using materials A and B:

Part A: 600/15 = 40 mm2 & Part B: 600/25 = 24 mm2

Corresponding material costs:

Part A: (40

500

7.8

10

-6

)

100 = 15.60 TLPart B: (24 500 7.8 10-6) 150 = 14.04 TL

Although material B is more expensive per unit weight, the part will be

cheaper if it is produced from this material.

Performance Rating Method


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The designer asks simple questions: What should be the desirable

properties of the candidate materials and how important is each

and every of these desirable features?A more difficult exercise is

to determine the relative orderand degree of importance.

The process starts with drawing a matrix of comparisons in order to

compare the desirable properties in pairs. For this purpose, desirable

material properties must be listed and given a code number.

1 2 3 4 5

1

23

4

5

Suppose that there are five desirable

properties: (1) raw material cost, (2) wear

resistance, (3) castability, (4) machinability,(5) heat conductivity. A square matrix is

then drawn up with the attributes (note that

the listing is not in the order of importance).



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All pairs of attributes in the matrix are

compared for relative importance in the

form of column-row such as 1-2, 1-3, 1-4,

1-5, 2-1, 2-3, and so on.

Marks below are put in the matrix:

XX : 1 is more important than 2.

X : no decision is made in favour of 1 or 3.

: 2 is less important than 1.

1 2 3 4 5

1 - X X -

2 XX XX - -

3 X - X X

4 X XX X X

5 XX XX X X

6X 4X 5X 3X 2X

After all comparisons are made, the number of marks in each column

are added. This way, the order of importance of properties can beobtained. From the table, property 1 has the first ranking with 6X.

In order to weigh the merits, the designer must also devise a value

scale for each property (i.e. measures of value must be established).



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Raw material cost is only 6/5 times more important than castability, but

it may not be the exact mathematical equivalent of actual importance.

Therefore, level of desirability must also be defined by assigning

certain numerical values that provide a scale for comparison.

The following scale may be devised: (5) most desirable, (4) highly

desirable, (3) desirable, (2) slightly desirable, (1) least desirable.However, a set consisting of the numbers 10, 8, 5, 3 and 1 is usually

employed as below due to its close approximation to a linear scale:

Order of Importance Merit Ranking Measure of Value

1 (6X) Raw material cost 10

2 (5X) Castability 8

3 (4X) Wear resistance 8

4 (3X) Machinability 5

5 (2X) Heat conductivity 3



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Standing of a certain material among candidate materials is determined

according to its performance rating (R). In order to calculate this,

the designer has to devise a grading system for performance factor.

The performance factor scale is also optional. It can be from poorest to

best (e.g. 0 to 5, 0 to 10, or even 0 to 1).

Performance rating (Rm) of a candidate

material is calculated by the measure of

value (Ci ) and performance factor (Gim):

N

i

i

N

i

imim CGCR

11

When the performance rating for all candidate materials is determined,the designer can specify the highest ranking material for the design.

Obviously, a systematic and objective selection is provided by above

method on the condition that the values of C and G are established in

an unbiased manner.



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Level of

Importance

Measure

of Value

(Ci)

Performance Factor (Gi) Ranking (CiGi)

A B A B

Raw Material Cost 6 10 4 5 40 50

Castability 5 8 4 4 32 32

Wear 4 8 2 3 16 24

Machinability 3 5 3 3 15 15

Heat Treatibility 2 3 4 3 12 9

= 34 = 115 = 130

Finally, the material having the highest ranking is chosen:RA = 115 / 34 = 3,38

RB = 130 / 34 = 3,82 ()

Checklist for a Systematic Material Selection


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Checklist for a Systematic Material Selection

1. Check existing standards, codes orlegal requirements.

2. Make a functional analysis and determine the functional requirements.

3. Make a failure analysis and determine in how many ways the designed

machine part can fail to fulfil these functions.

4. Determine the essential parameters.

5. Establish the measures of value and the performance factors.

6. Analyse similar designs and determine the list of materials that can be

used, paying attention to the design philosophy.

7. Screen all candidate materials and discard those which do not possess

the essential properties. A backward method of selection would be to

look for materials which possess the essential properties.8. Assign a performance factorto each pertinent properties of candidate

materials to see how closely it meets the desirable material properties.

9. Perform the equation of perfomance rating (Rm) for each material.

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UNIT II 0.1 INTRO TO DESIGN AND SELECTION OF MATERIALS.pdf