ME 215 – Engineering Materials I

Dr. A. Tolga Bozdanawww.gantep.edu.tr/~bozdana

Mechanical EngineeringUniversity of Gaziantep

Chapter 2

Design Engineering and Material Selection

1

Introduction Design of new products and development of the existing ones is

the essential purpose of engineering. In the course of designingany machine element or component, an engineer has to considermany requirements.

Selection of the material from which a part is to be produced hasalways been a predominant factor in the overall performance ofa design because of its influence on other factors.

Therefore, it is not usually possible to make the final decision ongeometry and dimensions of a part until the material is selected.

2

Fundamental Aspects of Design Designer must be careful in making the right distinction between demands

that are truly related to material properties and certain design features.

For example, strength of a part depends upon strength of the material andgeometrical parameters. This does not mean that high strength materialsare needed to achieve the required strength of the part. Instead, designercan prefer a weaker material but larger dimensions as long as there are nospace or weight restrictions. When such restrictions are tight, then strengthof material itself becomes considerably important.

In addition, material selection has a great influence that selection of a totallydifferent material would result in a new design approach. Inevitably, chosingthe most suitable material is possible by good storehouse knowledgeconcerning the material properties. The most decisive factor for making aproper selection is the experience, which no book could provide.

3

Design Procedure 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

4

Design Procedure Every design effort is aimed at satisfying existent or potential “need”.

From analysis of the need, designer determines essential and desirablefeatures of the design, namely “functional requirements”.

Furthermore, a design must be in compliance with certain inevitable“design limitations”: Manufacturing, Money, Maintenance (“3M rule”)

Arising from nature of design, sometimes functional requirementsand sometimes design limitations dictate the properties to be desiredin the material for the design work at hand.

As it is impossible to satisfy all requirements to the same degree, theyare arranged in the order of importance to identify areas of comprimise.

5

Functional Requirements They concern mechanical properties of material (e.g. strength, stiffness,

resilience, toughness, hardness, etc.) and physical properties (such ascoefficient of expansion, thermal and electrical conductivity, and so on).

Production requirements are logically the first to be considered. Designermust consider functional merits of the material as well as its ability to bemanufactured (i.e. machined, shaped, formed, cast, welded, and so on).

Economic requirements are based on the final product cost composed ofraw material cost and production costs with overheads. The product costshould be as high as the customers can pay for it.

Finally, maintenance requirements (i.e. whether replacement or repair isrequired) depend upon size of the part, extent of possible damage, facilitiesof the customers, and the acceptable level of costs.

6

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 andspecifications, where the production group may give useful hints.

Material selection depends upon such factors: the functional demands,how many parts will be produced, which materials can be used, andwhat properties are related for that design.

The production requirements can be gathered in the following groups:1. Machinability2. Formability3. Castability4. Suitability for Compacting5. Weldability6. Heat Treatability7. Adaptability to Special Processes8. Adaptability to Forms of Protection

7

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, drilling, boringare 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 usedjointly to describe “machinability”. Quantitatively speaking, a highlymachinable material is the one that allows the maximum amount of chipremoval with the minimum tool wear, yielding a high surface quality.

The above factors vary not only from one material to another, but also fromone machining process to another.

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Production Requirements – Formability Forming processes (like rolling, forging, stamping, pressing, drawing)

provide special advantage of enabling the desired shape to be obtainedwith ease, without machining the surfaces that are not mating. Hence, thisis a great advantage over chip removal processes.

Another advantage of such processes is that, unlike casting process, mostengineering materials are amenable to forming. However, the main problemis that they are costly processes.

During forming operations, the material is subjected to considerable degreeof deformation affecting its mechanical properties. This can be beneficial ordetrimental depending upon type of the material as well as type and extentof the forming process.

9

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 canalso be cast. However, in practice, few metals are truly amenable to casting.

Casting has a special advantage to produce parts with sophisticated shapeespecially in large numbers, which usually cannot be possible by the otherprocesses (e.g. the carburetor of a car).

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 the mold.As a metal shrinks upon freezing, the molten metal must be constantly fedinto mold during solidification to compensate the shrinkage, otherwisespongy metal is obtained. Hence, designer must decide type of the materialand type of the casting process together.

10

Production Requirements – Suitability for Compacting This is required when the part is to be produced by powder metallurgy.

The metal powder is compacted in a die to the desired form, and thensintered to fuse the powder particles together.

Most metals and alloys can be used in this process, but only few of themare economically justified. This process is the best way to produce partsfrom brittle and very hard metals.

Although the intricate forms with desirable mechanical properties can beproduced, availability of the required metal in powder form and high capitalinvestment 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), butalso used for maintenance and repairs (i.e. fixing broken or worn parts).

“Weldability” does not mean ability of metals to be welded, it representsthe relative ability of metals (usually steels) to be welded without cracking.

Production of especially large parts by welding is regarded as an alternativeto casting when they are needed in few numbers. However, the successfulproduction of complicated shapes by welding demands a special designapproach (like in case of casting) as well as a careful planning of stress-relief treatments and operations.

Two special welding techniques (electron beam welding and laser beamwelding utilizing beams to generate heat of fusion) have made it possible toweld hardenable and heat treated steels.

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Production Requirements – Heat Treatability Heat treatment causes structural changes in metals to improve essential

mechanical properties, change grain size and relieve residual stresses.

“Hardenability (depth of hardening)” is desirable material property if the aimof heat treatment is to increase strength and/or hardness. It is dependent uponmaterial’s rate of hardening.

Some ferrous and nonferrous alloys can be hardened by age (precipitation)hardening. The alloy is heated to certain temperature at which it exists ashomogeneous solid-solution phase, then cooled rapidly (quenched). Finally, it isheld at room temperature (natural aging) or above the room temperature(artificial aging) to allow precipitation of solid- solution.

Heat treatment is also used to alter surface properties of ferrous alloys. Rapidheating of surface by induction/flame followed by quenching (induction/flamehardening) produces a hardened surface while the interior of material is softer.

In other thermal surface treatments (e.g. carburizing, cyaniding, nitriding,carbonitriding, chromizing), a substance diffuses into the heated metal surface.

13

Production Requirements – Adaptability to Special Processes

Many intricate and special parts are produced by chipless manufacturingprocesses.

In chemical milling, material is removed by etching reaction of chemicalsolutions with metal. It can also be used on plastics and glass.

Electrochemical machining (ECM) employs electroplating process, wherethe tool (with inverse shape of part) is cathode and the workpiece is anode.

Electrodischarge machining (EDM) cuts metal by action of high-energyelectric 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 costs andslow production speeds make them suitable only when parts to be producedare of special nature and are few in numbers.

14

Production Requirements – Adaptability to Forms of Protection

In many cases, material properties could not meet some functional demands,especially arising from environmental conditions. As high quality materials forthis 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.

Coatings may be classified under four main groups:1. Organic coatings: resins, pigments, lacquers, varnishes, paints, dispersion

coatings, emulsion coatings, hot-melt coatings, plastic powder coatings2. Metallic coatings: electroplates, chemical-deposition and sprayed-metal

coatings, hot-dip coatings, diffusion coatings, vapour-deposited coatings3. Conversion coatings: phosphate, chromate, and chemical oxide coatings4. Ceramic coatings: vitreous (glass-like), porcelain, and ceramic coatings

15

Economic Requirements 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 andprocessing), labour costs, and capital costs.

The foremost economic factor is availability. Candidate materials in a designproject must be available in market. Expensive delays will be incurred dueto supply difficulties. Market search is a must before final material selection.

Actual cost of raw material is cost of material used in part plus cost of scrapmaterial. Adjustment of dimensions (whenever possible) to available stocksizes is a regular design procedure to reduce scrap and production time.

Production costs depend on number of operations, amount of skilled labor,time in each operation. In most cases, surface finish is important for part’sperformance and apperance. Thus, secondary finishing operations may beneeded in such cases.

16

Maintenance Requirements Maintenance covers activities that are necessary but not directly concerned

with operation or use (such as cleaning, lubrication, adjustment, overhauland repair of damaged/worn equipment).

In principle, durability is considered in design as a user requirement. It isannoying that a recently purchased product does not work any longer,which causes inconvenience for customer, heavy repair bills, or scapping ofproduct. So, the complaints about service life and cost must be minimized.

How often and at what cost are inevitable questions to be answeredduring the design stage; requiring a firm decision on whether replacementor repair, or both will be envisaged. When frequent replacements areenvisaged, part must be cheap so that it is more logical than repair. If repairis envisaged, the material must lend itself to acceptable forms of repair.

Non-stick frying pans and self cleaning ovens are recent examples of“how use of a new material facilitates maintenance”. Plastic surfaces notonly improve apperance, but also facilitate “cleaning” problems.

17

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 lossof many lives as in airplane crashes. A conceptual understanding of failureis necessary to utilize the material properties safely and economically.

In most design problems, primary concern should be reducing the possibilityof a premature failure in service. Service life ranges from seconds (in caseof space applications) to many years (in case of bridges).

Possible failure types during service are excessive deformation, fracture,inordinate wear, and deterioration. In practice, it is impossible to predictfailure mode of part under severe service conditions. Some failures happensoon after the part is in service, which are covered by “factor of safety”.

Time dependent failures are difficult or even impossible to avoid by applyingfactor of safety. In such cases, parts are withdrawn from service and testedfor reliability. Such specific data are not found in general reference books.

18

Failure – Excessive Deformation “Gross-scale yielding” and “buckling” are the types of this failure. Little

elastic deformation of an element in precision machines may cause problemwhile plastic deformation of an element in a building may be feasible.

Excessive deformation may also be responsible for the critical vibration of apart under dynamic load, which does not only disturbs the function but alsoleads to the complete destruction of part.

Failure by excessive deformation can be “immediate” or “time-dependent”(like “creep” which is significant in high-temp. applications).

In many cases, the failure criterion is based on material’s strength althougha failure by excessive deformation is implied. This is due to the fact that“stress approach” is more universal for covering the fracture aspect.

It must be remembered that when a failure by inordinate elastic deformationis of issue, design approach must always be based on deformation analysissince the results shall be compatible with functional requirements.

19

Failure – Fracture When analyzing fracture failure modes, preceding deformation is important.

If failure occurs following a large deformation, such fractures are called“ductile fracture”, which is very not common in engineering applications.In contrast, a fracture with no/little prior deformation is “brittle fracture”.

Many materials fail by fracture in three ways: sudden brittle fracture,fatigue (progressive) fracture, time-dependent (creep) fracture.

Brittle fracture is not only experienced by brittle materials. Higher rate orsudden application of load and presence of a complex stress may causeductile-to-brittle transition (embrittlement) of a material.

Fatigue failure (the most common failure in many applications) is a highlylocalized microscopic phenomenon. It occurs in parts that are subjected torepeated (cyclic) stresses even if they are below the yield point of material.

Creep failure (stress rupture) occurs when a material is loaded at highertemperatures for a long time. In polymeric materials, it can occur even atnormal temperatures and under relatively low stresses.

20

Failure – Wear Wear results from the action of abrasive or other forces on surface of a part.

It is manifested by a loss of surface material (either in regular or irregularform) which causes change in the part dimensions.

Wear is a complex subject due to many variables involved in the process.Lubrication, condition of surface and type of material with which the part isin contact are the most effective factors.

There is no quantitative test or criterion of wear. Thus, design evaluationsare based on past experience more than anything else.

21

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

Most common examples are caused by the reaction of environment (suchas “corrosion” and “oxidation”) in which materials operate.

No material is completely resistant to liquid or gas. “Liquid/gas absorption”may cause “embrittlement”, a special problem in nuclear applicationsbecause of danger of nuclear substances.

Speaking of nuclear applications, material properties are significantlyaltered by “irradiation”. In some cases, the effects of irradiation can bebeneficial as it causes an increase in yield strength.

“Fungus or other growths” cause deterioration in strength and/or othermaterial properties (noticable in wood and some plastics), or loss ofefficiency of the whole system (some sea bacteria on a ship body).

22

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 answers the need where the requirements areslightly exceeded by capabilities of the design. “Under-designing” tends tofail in certain ways whereas “over-designing” is not only economicallypointless but also unapplicable or useless.

Fundamental factors related to failure or shortening of service life are listedbelow (the failure may be due to any or combination of them):

Problematic design Improper selection of material Heat treatment Fabrication Improper machining and assembly

23

Material Selection The first step in material selection process is to reduce the number of

candidate materials to manageable number. Past experiences, investigationof materials currently used for similar designs, existing standards, codes orlegal requirements help to narrow the selection list.

“Design philosophy” has important role in screening material alternatives.It determines the general trend of design varying in different industries,countries, and companies. For instance, due to foreign currency issues,relying on domestically produced materials can be a design philosophy.

It is difficult to define design philosophy. For instance, the design philosophyapplied for the products in car industry may be similar. However, aircraft orspace 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.

24

Material Selection “Measures of value”, that are highly dependent on design philosophy, are

standards by which the merits of a material can be weighed. Its properestablishment provides a clever and economical material selection.

In an engineering design, the benefits are often based on intangible factors.The most universal method for measures of value is in monetary termssuch as comparison of the product price with its rivals.

However, the designer must know that incorrect comparison leads to biasedresults and misleading benefit analysis. For instance, it is not correct to lookat only the cost per unit weight of raw material without considering howmuch material is actually required to produce a certain part. The example inthe next page illustrates this problem.

Material Selection – An ExampleA 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 to15 kg/mm2, while Material B with raw material cost of 150 TL/kg canbe stressed up to 25 kg/mm2. Both materials have the same density of7.8 10-6 kg/mm3. Which material must be preferred based on cost?

/

/

Area Load Strength

Part A kg kg mm mm

Part B kg kg mm mm

2 2

2 2

600 15 40

600 25 24

1

.

0.24

Volume Area Length

Part A cm cm cm

Part B cm cm cm

2 3

2 3

0 4 50 20

50 12

2

7.8 /

7.8 / .

Weight Volume Density

Part A cm g cm g

Part B cm g cm g

3 3

3 3

20 156

12 93 6

3

. 100 / .

. 150 / .

Cost Weight Material Cost

Part A kg TL kg TL

Part B kg TL kg TL

0 156 15 600 0936 14 04

4

Material B should be preferred (although it is more expensive per unit weight)5

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Performance Rating Method The designer must ask these questions: “What should be the desirable

properties of the candidate materials?” and “How important is each andevery of these desirable features?”

Hence, a more difficult exercise is to determine “the relative order” and“degree of importance” for the desirable properties. The process startswith drawing a matrix of comparisons to compare such properties in pairs.For this purpose, the properties must be listed and given a code number.

1 2 3 4 512345

Suppose that thereare five properties:(1) raw material cost(2) wear resistance(3) castability(4) machinability(5) heat conductivity

For this purpose,a square matrix isdrawn. Note thatthe property list isnot in the order ofimportance.

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Performance Rating Method

All pairs of attributes are compared for relativeimportance in the form of column-row such as:1-2, 1-3, 1-4, 1-5, 2-1, 2-3, and so on

The following marks 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 marks in each column are summed sothat the order of importance of properties can be obtained. From the table,property 1 has the first ranking with 6X.

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Performance Rating Method 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).

Here, raw material cost is only 6/5 times more important than castability,but it may not be the exact mathematical equivalent of actual importance.So, “level of desirability” must also be defined by assigning certainnumerical values that provide a scale for comparison.

For instance, such scale may be devised: (5) most desirable, (4) highlydesirable, (3) desirable, (2) slightly desirable, (1) least desirable. However,a set of numbers 10, 8, 5, 3, 1 is usually employed (as below) due to itsclose approximation to a linear scale:

Order of Importance Merit Ranking Measure of Value1 (6X) Raw material cost 102 (5X) Castability 83 (4X) Wear resistance 84 (3X) Machinability 55 (2X) Heat conductivity 3

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Performance Rating Method Standing of a certain material among candidate materials is determined

according to its “performance rating (R)”. To calculate this, the designerhas to devise a grading system for “performance factor”.

The scale for performance factor is also optional. It can be from the poorestto the best (e.g. 0 to 5, 0 to 10, or even 0 to 1).

Performance rating (Rm) of a candidatematerial is calculated by the measure ofvalue (Ci) and performance factor (Gim):

N

ii

N

iimim CGCR

11

After the performance rating for all candidate materials is determined,designer can specify the highest ranking material for the design. Obviously,a systematic and objective selection is provided by the above method onthe condition that values of C & G are established in an unbiased manner.

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Performance Rating Method

Property ListLevel 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 Resistance 4 8 2 3 16 24

Machinability 3 5 3 3 15 15

Heat Conductivity 2 3 4 3 12 9

Σ = 34 Σ = 115 Σ = 130

Finally, the material having the highest ranking is chosen:

RA = 115 / 34 = 3,38

RB = 130 / 34 = 3,82 ()

The comparison table for two materials (A & B):

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Checklist for a Systematic Material Selection1. Check the existing standards, codes or legal requirements.2. Make a functional analysis and determine the functional requirements.3. Perform a failure analysis and determine in how many ways the designed

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 possessthe essential properties. “Backward” method of selection would be to lookfor materials which possess the essential properties.

8. Assign a performance factor to each pertinent properties of candidatematerials to see how closely it meets the desirable material properties.

9. Use the equation of perfomance rating (Rm) for each material.10. The material with the highest performance rating is the optimum material.