Engineering Materials Vol II (microstructures_ processing_ design) 2nd ed. - M. Ashby_ D. Jones (1999) WW Part 12 ppsx

At the same time, a tentative component design is drawn up, able to fill thefunction which must be carefully defined at the start; and an approximate stressanalysis is carried out to ass

Design with materials 289

Table 27.1. Design-limiting properties of materials

Metals Stiff (E ≈ 100 GPa) Yield (pure, sy ≈ 1 MPa) → alloy High E, K IC Ductile (ef ≈ 20%) – formable Hardness (H ≈ 3sy ) → alloy

Low sy Tough (K IC > 50 MPa m 1/2 ) Fatigue strength (se = 1 – 2sy)

High MP (T m ≈ 1000°C) Corrosion resistance → coatings T-shock (DT > 500°C)

Ceramics Stiff (E ≈ 200 GPa) Very low toughness (K IC ≈ 2 MPa m 1/2 ) High E, s y Very high yield, hardness (sy > 3 GPa) T-shock (DT ≈ 200°C)

Low K IC High MP (Tm ≈ 2000°C) Formability → powder methods

Corrosion resistant Moderate density Polymers Ductile and formable Low stiffness (E ≈ 2 GPa)

Adequate sy , IC Corrosion resistant Yield (sy = 2–100 MPa)

Toughness often low (1 MPa m 1/2 )

High E, s y , IC Strong (sy ≈ 200 MPa) Cost

but cost Tough (K IC > 20 MPa m 1/2 ) Creep (polymer matrices)

Fatigue resistant Corrosion resistant Low density

At and near room temperature, metals have well-defined, almost constant, moduliand yield strengths (in contrast to polymers, which do not) And most metallic alloyshave a ductility of 20% or better Certain high-strength alloys (spring steel, for in-stance) and components made by powder methods, have less – as little as 2% Buteven this is enough to ensure that an unnotched component yields before it fractures,and that fracture, when it occurs, is of a tough, ductile, type But – partly because oftheir ductility – metals are prey to cyclic fatigue and, of all the classes of materials,they are the least resistant to corrosion and oxidation.

Historically, design with ceramics has been empirical The great gothic cathedrals,still the most impressive of all ceramic designs, have an aura of stable permanence Butmany collapsed during construction; the designs we know evolved from these failures.Most ceramic design is like that Only recently, and because of more demanding struc-tural applications, have design methods evolved

In designing with ductile materials, a safety-factor approach is used Metals can be

used under static loads within a small margin of their ultimate strength with ence that they will not fail prematurely Ceramics cannot As we saw earlier, brittlematerials always have a wide scatter in strength, and the strength itself depends onthe time of loading and the volume of material under stress The use of a single,constant, safety factor is no longer adequate, and the statistical approach of Chapter 18must be used instead

confid-We have seen that the “strength” of a ceramic means, almost always, the fracture orcrushing strength Then (unlike metals) the compressive strength is 10 to 20 times largerthan the tensile strength And because ceramics have no ductility, they have a lowtolerance for stress concentrations (such as holes and flaws) or for high contact stresses(at clamping or loading points, for instance) If the pin of a pin-jointed frame, made ofmetal, fits poorly, then the metal deforms locally, and the pin beds down, redistribut-ing the load But if the pin and frame are made of a brittle material, the local contactstresses nucleate cracks which then propagate, causing sudden collapse Obviously,the process of design with ceramics differs in detail from that of design with metals.That for polymers is different again When polymers first became available to theengineer, it was common to find them misused A “cheap plastic” product was onewhich, more than likely, would break the first time you picked it up Almost alwaysthis happened because the designer used a polymer to replace a metal component,without redesign to allow for the totally different properties of the polymer Briefly,there are three:

(a) Polymers have much lower moduli than metals – roughly 100 times lower Soelastic deflections may be large

(b) The deflection of polymers depends on the time of loading: they creep at roomtemperature A polymer component under load may, with time, acquire a perman-ent set

(c) The strengths of polymers change rapidly with temperature near room ature A polymer which is tough and flexible at 20°C may be brittle at the temper-ature of a household refrigerator, 4°C

temper-With all these problems, why use polymers at all? Well, complicated parts ing several functions can be moulded in a single operation Polymer components can

perform-Design with materials 291

be designed to snap together, making assembly fast and cheap And by accuratelysizing the mould, and using pre-coloured polymer, no finishing operations are neces-sary So great economies of manufacture are possible: polymer parts really can becheap But are they inferior? Not necessarily Polymer densities are low (all are near

1 Mg m−3); they are corrosion-resistant; they have abnormally low coefficients of tion; and the low modulus and high strength allows very large elastic deformations.Because of these special properties, polymer parts may be distinctly superior

fric-Composites overcome many of the remaining deficiencies They are stiff, strong andtough Their problem lies in their cost: composite components are usually expensive,and they are difficult and expensive to form and join So, despite their attractiveproperties, the designer will use them only when the added performance offsets theadded expense

New materials are appearing all the time New polymers with greater stiffness andtoughness appear every year; composites are becoming cheaper as the volume of theirproduction increases Ceramics with enough toughness to be used in conventionaldesign are becoming available, and even in the metals field, which is a slowly devel-oping one, better quality control, and better understanding of alloying, leads tomaterials with reliably better properties All of these offer new opportunities to thedesigner who can frequently redesign an established product, making use of the prop-erties of new materials, to reduce its cost or its size and improve its performance andappearance

Design methodology

Books on design often strike the reader as vague and qualitative; there is an tion that the ability to design is like the ability to write music: a gift given to few And

implica-it is true that there is an element of creative thinking (as opposed to logical reasoning

or analysis) in good design But a design methodology can be formulated, and whenfollowed, it will lead to a practical solution to the design problem

Figure 27.1 summarises the methodology for designing a component which mustcarry load At the start there are two parallel streams: materials selection and com-ponent design A tentative material is chosen and data for it are assembled from datasheets like the ones given in this book or from data books (referred to at the end of thischapter) At the same time, a tentative component design is drawn up, able to fill thefunction (which must be carefully defined at the start); and an approximate stressanalysis is carried out to assess the stresses, moments, and stress concentrations towhich it will be subjected

The two streams merge in an assessment of the material performance in the ive design If the material can bear the loads, moments, concentrated stresses (etc.)without deflecting too much, collapsing or failing in some other way, then the designcan proceed If the material cannot perform adequately, the first iteration takes place:either a new material is chosen, or the component design is changed (or both) toovercome the failing

tentat-The next step is a detailed specification of the design and of the material This mayrequire a detailed stress analysis, analysis of the dynamics of the system, its response

Fig 27.1.Design methodology.

Design with materials 293

to temperature and environment, and a detailed consideration of the appearance andfeel (the aesthetics of the product) And it will require better material data: at this point

it may be necessary to get detailed material properties from possible suppliers, or toconduct tests yourself

The design is viable only if it can be produced economically The choice of tion and fabrication method is largely determined by the choice of material But theproduction route will also be influenced by the size of the production run, and howthe component will be finished and joined to other components; each class of materialhas its own special problems here; they were discussed in Chapters 14, 19, 24 and 25.The choice of material and production route will, ultimately, determine the price of theproduct, so a second major iteration may be required if the costing shows the price to

produc-be too high Then a new choice of material or component design, allowing an ative production path, may have to be considered

altern-At this stage a prototype product is produced, and its performance in the market isassessed If this is satisfactory, full-scale production is established But the designer’srole does not end at this point Continuous analysis of the performance of a compon-ent usually reveals weaknesses or ways in which it could be improved or made morecheaply And there is always scope for further innovation: for a radically new design,

or for a radical change in the material which the component is made from Successfuldesigns evolve continuously, and only in this way does the product retain a competit-ive position in the market place

Further reading

(a) Design

G Pahl and W Beitz, Engineering Design, The Design Council, 1984.

V Papanek, Design for the Real World, Random House, 1971.

(b) Metals

ASM Metals Handbook, 8th edition, American Society for Metals, 1973.

Smithells’ Metals Reference Book, 7th edition, Butterworth-Heinemann, 1992.

DuPont Design Handbooks, DuPont de Nemours and Co., Polymer Products Department,

Wilmington, Delaware 19898, USA, 1981.

ICI Technical Services Notes, ICI Plastics Division, Engineering Plastics Group, Welwyn Garden

City, Herts., England, 1981.

(e) Materials selection

J A Charles and F A A Crane, Selection and Use of Engineering Materials, 2nd edition,

Butterworth-Heinemann, 1989.

M F Ashby, Materials Selection in Mechanical Design, Pergamon, 1992.

M F Ashby and D Cebon, Case Studies in Materials Selection, Granta Design, 1996.

By eliminating t from the equations, show that the minimum mass of the hull is

given by the expressions

E b

Answers: E0.5/ρ for external-pressure buckling; σf/ρ for yield or brittle compressivefailure

27.2 For each material listed in the following table, calculate the minimum mass andwall thickness of the pressure hull of Problem 27.1 for both failure mechanisms atthe design pressure

Design with materials 295

Material E (GPa) sf (MPa) Density, r (kg m −3 )

Hence determine the limiting failure mechanism for each material [Hint: this is

the failure mechanism which gives the larger of the two values of t.]

What is the optimum material for the pressure hull? What are the mass, wallthickness and limiting failure mechanism of the optimum pressure hull?

Answers:

Material m b (tonne) t b (mm) m f (tonne) t f (mm) Limiting failure mechanism

The optimum material is alumina, with a mass of 2.02 tonne, a wall thickness of

41 mm and a limiting failure mechanism of external-pressure buckling

27.3 Briefly describe the processing route which you would specify for making thepressure hull of Problem 27.2 from each of the materials listed in the table Com-ment on any particular problems which might be encountered [You may assumethat the detailed design will call for a number of apertures in the wall of thepressure hull.]

Chapter 28

Case studies in design

1 DESIGNING WITH METALS: CONVEYOR DRUMS FOR AN IRON ORE TERINAL

Introduction

The conveyor belt is one of the most efficient devices available for moving goods overshort distances Billions of tons of minerals, foodstuffs and consumer goods arehandled in this way every year Figure 28.1 shows the essentials of a typical conveyorsystem The following data are typical of the largest conveyors, which are used forhandling coal, iron ore and other heavy minerals

Distance between centres of tail drum and drive drum: 200 m

Fig 28.1. Schematic of a typical conveyor system Because the belt tends to sag between the support rollers

it must be kept under a constant tension T This is done by hanging a large weight on the tension drum The drive is supplied by coupling a large electric motor to the shaft of the drive drum via a suitable gearbox and overload clutch.

Case studies in design 297

It is important that conveyor systems of this size are designed to operate continuouslyfor long periods with minimum “down-time” for routine maintenance: the unsched-uled breakdown of a single unit in an integrated plant could lead to a total loss ofproduction Large conveyors include a number of critical components which aredesigned and built essentially as “one-offs” for a particular installation: it is doublyimportant to check these at the design stage because a failure here could lead to adamagingly long down-time while a harassed technical manager phones the length

of the country looking for fabrication shops with manoeuvrable capacity, and steelmerchants with the right sections in stock

Tail drum design

The tail drum (Fig 28.1) is a good example of a critical component Figure 28.2 showsthe general arrangement of the drum in its working environment and Fig 28.3 shows

a detailed design proposal We begin our design check by looking at the stresses in theshaft The maximum stress comes at the surface of the shaft next to the shaft-plateweld (Fig 28.4) We can calculate the maximum stress from the standard formula

This stress is only a quarter of the yield stress of a typical structural steel, and the

shaft therefore has an ample factor of safety against failure by plastic overload.

Fig 28.2. Close-up of the tail drum The belt tension applies a uniformly distributed sideways loading

to the drum.

Fig 28.3. Cross-section through the tail drum All dimensions are in mm We have assumed a belt tension

of 5 tonnes, giving a total loading of 10 tonnes.

Fig 28.4. Shaft-plate detail.

The second failure mode to consider is fatigue The drum will revolve about once

every second, and each part of the shaft surface will go alternately into tension andcompression The maximum fatigue stress range (of 2 × 56 = 112 MPa) is, however,only a quarter of the fatigue limit for structural steel (Fig 28.5); and the shaft shouldtherefore last indefinitely But what about the welds? There are in fact a number ofreasons for expecting them to have fatigue properties that are poorer than those of theparent steel (see Table 28.1)

Figure 28.6 shows the fatigue properties of structural steel welds The fatigue limitstress range of 120 MPa for the best class of weld is a good deal less than the limitingrange of 440 MPa for the parent steel (Fig 28.5) And the worst class of weld has alimiting range of only 32 MPa!

Case studies in design 299

Fig 28.5. Fatigue data for a typical structural steel in dry air Note that, if the fatigue stress range is less than 440 MPa (the fatigue limit) the component should last indefinitely The data relate to a fatigue stress cycle with a zero mean stress, which is what we have in the case of our tail drum.

The shaft-plate weld can be identified as a class E/F weld with a limiting stressrange of 69 to 55 MPa This is a good deal less than the stress range of 112 MPaexperienced by the shaft We thus have the curious situation where a weld which ismerely an attachment to the shaft has weakened it so much that it will only last forabout 2 × 106 cycles – or 1 month of operation The obvious way of solving this problem

is to remove the attachment weld from the surface of the shaft Figure 28.7 shows oneway of doing this

Gives stress concentration In the case of butt welds this can be removed by grinding back the weld until flush with the parent plates Grinding marks must be parallel to loading direction otherwise they can initiate fatigue cracks.

Helps initiate fatigue cracks Improve finish by grinding Weld liable to fatigue even when applied stress cycle is wholly compressive Reduce residual stresses by stress relieving, hammering or shot peening.

Help initiate fatigue cracks Critical welds must be tested non-destructively and defects must be gouged out Sharp changes in mechanical properties give local stress concentrations.

Table 28.1. Weld characteristics giving adverse fatigue properties

Change in section at weld bead.

Poor surface finish of weld bead.

Contain tensile residual stresses which are usually

as large as the yield stress.

Often contain defects (hydrogen cracks, slag

inclusions, stop–start marks).

Large differences in microstructure between parent

metal, heat-affected zone and weld bead.

Fig 28.6. Fatigue data for welded joints in clean air The class given to a weld depends critically on the weld detail and the loading direction Classes B and C must be free from cracks and must be ground flush with the surface to remove stress concentrations These conditions are rarely met in practice, and most welds used in general construction have comparatively poor fatigue properties.

Fig 28.7. Modification to remove attachment weld from surface of shaft The collar can be pressed on to the shaft and secured with a feather key, but we must remember that the keyway will weaken the shaft.