1chap1 mater

Design For Manufacturing and Assembly Effect of Materials and Manufacturing processes on Design

1.EFFECT OF MATERIAL AND MANUFACTURING PROCESSES IN DESIGN 1. Introduction: Design is the process of translating a new idea or a market need into the detailed information from which a product can be manufactured. Each of its stages requires decisions about the materials from which the product is to be made made and and the the proc proces ess s for for maki making ng it. it. The The numb number er of mate materi rial als s avail availabl able e to the engine engineer er is vast: vast: betwe between en 4000 40000 0 and and 8000 80000. 0. At the beginning the design is fluid and the options are wide; all materials must be considered. As the design becomes more focused and takes shape, the selection criteria sharpen and the shortlist of materials, which can satisfy them, narrows. Then more accurate data are required and a different way of analyzing the choice must be used. In final stages of design, precise data are needed and the search finally comes to only one. The procedure must recognize the initial initial choice choice,, the narrow narrow this this to a small small subset subset,, and provide provide the precision and detail on which final design calculations can be based. The choice of material cannot be made independently of the choice of proc proces ess s by whic which h the the mate materi rial al to be form formed ed,, joine joined, d, fini finish shed ed,, and and otherwise treated. Cost enters, both in the choice of material is processed. Good design alone alone will not sell sell a product. Industrial design is one that, if neglected, can also loss the manufacturer his market. So, Engineering materials are evolving faster, so there are wide options, which pave pave way for new innovati innovations. ons. It is important important in the early part part of design to examine the full materials, which fulfill the requirements, and subsequently deciding upon the manufacturing processes. For this, the know knowle ledg dge e of the the Effe Effect ct of mate materi rial al prop proper erti ties es and and manu manufa fact ctur urin ing g processes is required.

1.1. Major Phases of Design: Introduction: Engineering design work is usually performed on three different levels: 1. Development of existing products or designs, i.e., redesign, by introducing minor modifications in size, shape or materials to improve performance. 2. Adaptation of of an existing product or design to operate in new environment or to perform a different function.

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3. Creation of of to totally ne new de design th that ha has no no pr precedent. This work is more demanding in experience and creativity of the designer.

1.1.1. Major Phases of Design: Engineering design is usually an iterative process, which involves a series of decision-making steps where each decision establishes the framework for the next one. There is no single, universally recognized sequence of steps that leads to a workable design as these depends on nature of the problem being solved as well as the size and structure of the organization. However, a design usually passes through most of the phases, which are shown in the Fig 1. 1. Identi Identific ficat ation ion of the proble problem m and and evalua evaluatin ting g the need need in order order to define the objective of the design represent the first phase of the design in most cases. 2. Functi Functiona onall requi requirem rement ents s and opera operatio tional nal limita limitatio tions ns are are direct directly ly rela relate ted d to the the requ requir ired ed char charac acte teri rist stic ics s of the the prod produc uctt and and are are specified as a result of the active phase I. 3. System System definition, definition, concept concept formulatio formulation, n, and preliminary preliminary layout layout are usually completed, in this order, before evaluating the operating loads loads and deter determin mining ing the form form of the diffe differen rentt compo componen nents ts or structural members. 4. Cons Consul ulti ting ng desi design gn code codes s and and colle collect ctin ing g info inform rmat atio ion n on mate materi rial al properties will allow the designer to perform preliminary material selection, preliminary design calculations, and rough estimation of manufacturing requirements. requirements. 5. The The eval evalua uati tion on pha phase invo involv lves es a comp compar aris ison on of the the expe expect cted ed perfo performa rmance nce of the design design with with the perfor performa mance nce requir requireme ements nts estab establish lished ed in phase phase 2.Eva 2.Evalua luatio tion n of the differ differen entt solut solution ion and and sele select ctio ion n of the the opti optimu mum m alte altern rnat ativ ive e can can be perf perfor orme med d usin using g decision-m decision-makin aking g techniques techniques,, modeling modeling technique techniques, s, experime experimental ntal work and /or prototypes. 6. In some some cases, cases, it is not possible possible to arriv arrive e at a design design that fulfills fulfills all all the requirements and compiles with all the limitations established in phase2. This means that these requirements and compiles with all the limitations established in phase 2. 7. Having Having arrived arrived at final design, design, the project project then then enters the the detailed detailed design stage where it is converted in to a detailed and finished fi nished form for suitable for use in manufacturing. The preliminary design layout, any available detail drawings, models and prototypes, and access to the developer of the preliminary design usually form the basis of the detailed design. 8. The The next next step step in the the deta detail iled ed desi design gn phas phase e is deta detail ilin ing, g, which hich involv involves es the crea creatio tion n of detail detail drawin drawings gs for for every every part part .All .All the infor informa matio tion n that that is necess necessar ary y to unambi unambiguo guousl usly y define define the part part should be recorded in detailed drawing. The material of the part should should also also be selec selected ted and specif specified ied by refer referenc ence e to standa standard rd codes.

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Major phases of design Constraints Safety, LOP Fig 1 1. Identification of the problem

Unavailabl e informatio n

Yes

Files R&D Patents

No 2. Functional requirements

3. Concept formulation and preliminary layout.

4. Preliminary material and process selection.

Material properties, Design Codes

No Information sufficient to reach feasible solution?

Modeling and simulation Prototype Expt.Work.

Yes 5. Evaluate solution with functional requirements. requirements.

Sales Marketing Prospective customers

No Revise Functional requirements.

Acceptable Design?

Yes Detail Design

Detailing Materials and processes specified.

Specifications for standard items.

Yes Design Changes necessary

No 4. Bill of Materials

3

Manufacturing Customer

Marketing Purchase and Accounting.


9. An importan importantt part of the the detailed detailed design design phase is the prepar preparatio ation n of the bill of materials, sometimes sometimes called parts list .The bill of materials is a hierarchical listing of everything that goes into the final product including fasteners and purchased parts. Close interaction between design, manufacturing, and materials engineers is important at this stage. 10.The relationship between the designer and the product does not usuall usually y end at the manuf manufac actur turing ing or even even delive delivery ry stages stages.. The The manufacturing engineer may ask the detailed designer for a change in some parts to make fabrication easier or cheaper. Finally when the product gets in to use, the reaction of the consumer and the perf perfo orma rmance nce of the the prod produc uctt in serv servic ice e are are of conc concer ern n to the the desi design gner er as the the feed feedba back ck repr repres esen ents ts an impo import rtan antt sour source ce of information information for the future design modifications. modifications.

1.2. Effect of Material Properties on Design: Introduction: Materials are the food of design. A successful product is one that performs well, is good value for money and gives pleasure to the user. A successful design should take in to account the function, material properties and manufacturing processes, as shown in the following fig., in the context of sele select ctio ion n of mate materi rial al,, ther there e are are many many clas classe ses s of mate materi rial als s meta metals ls,, polymers, and ceramics but in the end, what we seek is a profile of properties. Function And Consumer Requirement

Component Design

Manufacturing Process

Material Properties

Fig 2 Factors that should be considered in component design. 4


This figure shows that there are other secondary relationships between material properties and manufacturing processes, and between function and material properties. The The relat relation ionshi ship p betwe between en design design and and mater material ial proper propertie ties s is compl complex ex becaus because e the behav behavior ior of the mater material ial in the finish finished ed produc productt is quite quite diff differ eren entt from from that that of stoc stock k mate materi rial al used used in maki making ng it. it. This This poin pointt is illustrated in the following Fig.3

Properties of Stock materials.

Behavior of material in the Component

Component Geometry and External forces

Effect of fabrication method

Fig 3, Factors that should be considered in anticipating the behavior of material in the component. This This figure figure shows shows the direct direct influe influence nce of the stock stock mater material ial proper propertie ties s production method, and component geometry and external forces on the beha behavi vior or of mate materi rial als s in the the fini finish shed ed comp compon onen ent. t. It also also show shows s the the secondary relationships exist between geometry and production method, and between stock materials and component geometry.

1.2.1 Effect of Component Geometry: In most cases, engineering components and machine elements have to incorpora incorporate te design design features features,, which introduce introduce changes changes in cross-se cross-sectio ction. n. These These changes changes cause cause localized localized stress stress concentr concentratio ations, ns, which are higher higher than those, based upon the nominal cross-section of the part.

1.2.2 Stress Concentration Factor: A geometrical or theoretical stress concentration factor K t,t, is usually used to relat relate e the maxim maximum um stres stress, s, Smax , at the discon discontin tinuit uity y to nomina nominall stress, Sav , according to the relationship: 5


K t = Smax / Sav In making a design, K t is usually determined from the geometry of the par part. Under nder stat tatic loa loadin ding Kt give ives an uppe upperr lim limit to the the stre tress concen concentr trati ation on value value and and applie applies s only only to brittl brittle e and notch notch –sensi –sensitiv tive e materials. With more ductile materials, local yielding in the very small area area of maxi maximu mum m stre stress ss caus causes es a cons conside idera rable ble reli relief ef in the the stre stress ss concen concentr trati ation. on. So, for for ductile ductile mater material ials s under under static static loading loading,, it is not usually necessary to consider the stress concentration factor. Guidelines for design:

Stress Stress conce concentr ntrat ation ion can can be a sourc source e of failu failure re in many many cases cases,, especi especial ally ly when when desig designin ning g with with the high-s high-str treng ength th mater material ials s and under under fatigue loading. In such cases, the following guidelines should be observed if the stress concentrations are to be kept minimum. 1. Abru Abrupt pt chan change ges s in cros crosss-se sect ctio ion n shou should ld be avoi avoide ded. d. If they they are are necessary, necessary, generous fillet radii or stress-relieving grooves should be provided. 2. Slots Slots and grooves grooves should should be provid provided ed with the generous generous run-ou run-outt radii in all corners. 3. Stress Stress-re -relie lievin ving g groov grooves es or under undercut cuts s should should be provid provided ed at the ends of threads and spines. 4. Sharp Sharp internal internal corners corners and extern external al edges should should be avoide avoided. d. 5. Oil holes holes and simila similarr featur features es should should be chamfe chamfered red and the bore should be smooth. 6. Weakening Weakening feature features s like the bolt and oil holes, holes, identifica identification tion marks, marks, and the part part number numbers s should should not be locat located ed in highly highly stres stressed sed areas. 7. Weaken Weakening ing featur features es should should be stagger staggered ed to avoid avoid the addition addition of their stress concentration factors.

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Fig 4 Stress concentration factor on Design.

1.2.3 Designing for Static Strength: Designs bases on static strength usually aims at avoiding yielding of the component in the case of soft, ductile materials and at avoiding fracture in the case of strong, low-toughness materials. materials. Designing for Simple Axial Loading:

Compon Component ents s and struct structure ures s made made from from ductil ductile e mater material ials s are are usuall usually y designed so that no yield will take place under the expected static loading conditions. When a component is subjected to uniaxial stress, yielding will take place when the local stress reaches the yield strength of the material. The critical cross-sectional area, A, Of such a component can be estimated as : A= K tnL/YS Where Kt = Stress concentration factor, L = applied Load, N = factor of safety, YS= yield strength of the material Designing for Torsional Loading:

The critical cross-sectional area of a circular shaft subjected to torsional torsional loading can be determined from the relationship: 7


2Ip/d = Kt nT/ ‫ح‬ where d = shaft diameter at the critical cross-section, cross-section, ‫ =ح‬Maximum shear strength of the material T = transmitted Torque, Ip = polar moment of inertia of the cross-section = π d 4/ 32 for a solid circular shaft = π(d4o – d4i)/ 32 for a hollow shaft of inner i nner dia d i and outer dia d o Design for Bending:

When a relatively long beam is subjected to ending, the bending moment, the maximum allowable stress, and the dimensions of the cross-section are related by the equation: Z = (nM)/ YS YS where M = bending moment. Z = section modulus = I/c, I = moment of inertia of the cross-section cross-section with respect to the neutral axis normal to the direction of the load. c = distance from the center of gravity of the cross-section to the outermost outermost fiber.

1.2.4 Designing for Stiffness: In addition to being strong enough to resist the expected service loads, there there may may also also be the added added requir requireme ement nt of stiffn stiffness ess to ensure ensure that that deflections do not exceed certain limits. When an initially straight beam is loaded, it becomes curved as a result of its deflection. As the deflection at a given point increases, the radius of curvature at this point decreases. The radius of curvature, r , at any point on the curve is given by the relationship: r = EI /M

The equation shows us that the stiffness of a beam under bending is proportional to the elastic constant of the material, E, and the moment of inertia of the cross-section, I. Therefore, selecting materials with higher elastic constant and efficient disposition of material in the cross-section are essential in designing beams for stiffness. Torsional Rigidity of Shafts:

The torsional rigidity of a component is usually measured by the angle of twist, ø, per unit length, where Ø = T/ G Ip Where G = modulus of elasticity in shear = E/2(1+v) Where v = Poisson’s ratio. The usual practice is to limit the angular deflection in shafts to about 1 degree in a length of 20 times the diameter.

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1.2.5 Designing With High-Strength, High- Strength, Low Toughness Materials: High-strength is being increasingly used in designing critical components to save weight or to meet difficult service conditions. These materials tend to be less tolerant of defects than the traditional lower-strength, tougher materials. While a crack-like defect can safely exist in a part of lowerstrength ductile material, it can cause a catastrophic failure if the same part is made of a high-strength, low toughness material. material. Guidelines for design:

In desi design gnin ing g with with the the high high-s -str tren engt gth, h, low low toug toughn hnes ess s mate materi rial als, s, the the inter interact action ion betwee between n frac fractur ture e tough toughnes ness s of the mater material ial,, the allow allowabl able e crack size, and the design stress should be considered. In the case of highstrength, low-toughness material, as the design stress increases (or as the size of the flaw increases) the stress concentration at the edge of the crack, the stress intensity K I increases until it reaches K IC IC and fracture occurs. 1/2 K I = K IC IC = YFs(πa)

where Fs = fracture stress (controlled (controlled by the applied load and shape of the part) a = quality control parameter (controlled (controlled by the manufacturing method) Y = dimensionless shape factor. (Estimated experimentally, analytically or numerically)

1.2.6 Designing against Fatigue: In majority of cases the reported fatigue strengths or endurance limits of materials are based on tests of carefully prepared small samples under labor laborat ator ory y condi conditio tions. ns. Such Such value values s canno cannott be direct directly ly used used for for design design purpo purposes ses becaus because e the behav behavior ior of the compo componen nentt or struct structure ure under under fatigue loading does depend not only on the fatigue or endurance limit of the material used in making it, but also on several other factors including:  Size and shape of the component or structure  Type of loading and state of stress. concentration  Stress concentration  Surface finish  Operating temperature temperature  Service environment  Method of Fabrication. The The infl influe uenc nce e of the the above bove fact factor ors s on the the fati fatigu gue e beha behavi vior or of the the component can be accounted for by modifying the endurance limit of the material using a number of factors. Each of these factors is less than unity and each one is intended i ntended to account for a single effect.

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Se = ka kb kc kd ke kf kg kh S′ e Where, Se = endurance limit of the material in the component. S′ e = endurance limit of the material as determined by laboratory fatigue test. ka = surface finish factor. Surface finish factor varies between unity and 0.2 depending upon surface finish and strength of the material. kb = size factor. Size factor is 1.0 for component diameter less than 10mm; 0.9 for component diameter in the range of 10 to 50 mm. kc = reliability factor. Reliability factor is 0.900 for 90% reliability 0.814 For 99% reliability 0.752 For 99.9% reliability kd = operating temperature factor. Operating temperature temperature value is 1.0 in the range of -45 ° to 450°C Its value is 1- 5800(T-450) for T between 450 ° - 550°C Its value is 1- 3200(T- 840) for T between 840 °- 1020°C ke = loading factor. Loading factor is equal to 1 for applications involving bending. It is equal to 0.9 for axial loading. It is equal to 0.58 for torsional torsional loading. kf =stress concentration factor. kg = service environment factor. Service Environment factor varies from 0.72 to 0.19 kh = manufacturing process factor. Manufacturing factor is generally taken as 0.3-0.5. The above equation can be used to predict the behavior of the component or a structure under fatigue conditions provided that the values of the different modifying factors are known. Cumulative Fatigue Damage:

Engineering components and structures are often subjected to different fatigue stresses in service. Estimation of the fatigue life under variable loading conditions is normally based on the concept of cumulative fatigue damage, which assumes that successive stress cycles cause a progressive deterioration deterioration in the component. The Palmgren -Miner rule, also called Miner's rule proposes that if a cyclic stressing occurs at a series of stress levels S 1, S2, S3…..Si each of which would correspond to a failure life of N 1, N2, N3,….N i if applied singly, then the fraction of total life used a each stress level is the actual number of cycles applied at this level n 1, n2, n3, .n i divided by the corresponding life. The part is expected to fail when the cumulative damage satisfies the relationship: n1 N1

n2 +

N 2

n3 +

N3

+ ......... +

10

ni Ni Ni

=

C


The constant C can be determined experimentally and is usually found to be in the range of 0.7-2.2. The Palmgren - Miner rule does not take in to account the sequence of loading nor the effect of mean stress and it should be taken as rough guide to design.

1.2.7 Designing Conditions:

under

High-Temperature

Servic Service e temper temperatu ature re has a consid considera erable ble influe influence nce on the stren strength gth of mate materi rial als s and and cons conseq eque uent ntly ly,, on the the work workin ing g stre stress ss used used in desi design. gn. Depending on the temperature range, the design can be based on: 1. Shor Shortt-ti time me prop proper erti ties es of the the mate materi rial al,, i.e. i.e.,, ulti ultima mate te tens tensile ile strength, yield strength for moderate temperatures. temperatures. 2. Both Both the the shor shortt time time and and cree creep p prop proper erti ties es for for inte interm rmed edia iate te temperature range. 3. Creep properties of the materials for high temperatures. In addit dditio ion n to cree creep, p, the the othe otherr fact facto ors, rs, whic which h must must be take taken n in to consideration when designing for elevated temperatures, temperatures, include: 1. Metall Metallurg urgica icall and micro micro struc structur tural al change changes, s, which occur occur in the mate materi ria al owing ing to long long-t -tim ime e expo expos sure ure to elev eleva ated ted temperature. 2. Influ Influen ence ce of meth method od of fabr fabric icat atio ion, n, espe especi cial ally ly weldi welding ng,, on creep behavior. 3. Oxidat Oxidation ion and hot corros corrosion ion,, which which may take, take, place place during during service and shutdown periods. Design guidelines:

For design design purpos purposes, es, creep creep proper propertie ties s are are usuall usually y prese presente nted d on plots plots,, which yield reasonable straight lines. Common methods of presentation include log-log plots of stress vs. steady state creep rates and stress vs. time to produce different amounts of total strain as shown in the Fig.5. A change in the microstructure of the material is usually accompanied by a change in creep properties, and consequently a change in the slope of the line.

) le a cs

g lo (

Increasing temperature

ss er t

S

11


Creep rate (%/1000h) (log scale) Fig5 Fig5,, Vari Variat atio ion n of stre stress ss with ith stea steady dy-s -sta tate te cree creep p rate rate at vari variou ous s temperatures.

Rupture Strength ) el a cs

g ol ( ss er t

S

Increasing total strain

Time (h)

(Log scale)

Fig.6, Variation of stress with time to produced different amounts of total strain at a given temperature. Larsen- Miller Parameter:

In many cases, creep data are incomplete and have to supplemented or extended extended by interpola interpolation tion or, more more hazardou hazardously, sly, extrapo extrapolatio lation. n. This is particula particularly rly true of long-time long-time creep and stress-ru stress-rupture pture data where where the 100,000 hour (11.4 years) creep resistance of newly developed materials is required. Reliable extrapolation of creep and stress-rupture curves to longer times can be made only when no structural changes occur in the region of extrapolation. Such changes can affect the creep resistance, which would result in considerable errors in the extrapolated extrapolated values. The basic idea of these parameters is that they permit the prediction of long-time creep behavior from the results of shorter time tests at higher temperatures at the same stress. A widely used parameter for correlating the stress rupture data is the Larson-Miller parameter (LMP), where LMP is described as, LMP = T(C + log t r)

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Where T= the test temperature in kelvin ( °C+273) or degrees Rankine ( °F+ 460) tr= time to rupture in hours (the log is to the base 10) C= the Larson- Miller constant which generally falls between 17 and 23, but is often taken to be be 20. 20.

100

) a

p M( ss

10

er t

S

20

30

40

T(C+logt)

Fig.7 Larsen-Miller Plots.

Life under Variable Loading

The stress-rupture life of a part or a structure, which is subjected to a variable loading, can be roughly estimated if the expected life at each stress level is known. Under such conditions, the l ife fraction rule assumes that rupture occurs when: t1 tr 1

t2 +

tr 2

t3 +

tr 3

+ .... + = 1 .

Where t1, t2, t3, are the times spent by the part under stress levels 1, 2, 3… respectively. tr1, tr2, tr3…. are the rupture lives of the part under stress levels 1, 2, 3... respectively. Life under Combined Fatigue And Creep Loading:

Similar reasoning can also be applied to predict the life of a part or a stru struct ctur ure e when when subj subjec ecte ted d to comb combin ined ed cree creep p and and fati fatigu gue e load loadin ing. g. Cumu Cumula lati tive ve fati fatigu gue e dama damage ge laws laws,e ,e.g .g.. Palm Palmgr gren en-M -Min iner er Law, Law, can can be 13


combined with the life fraction rule, given in the equation, to give a rough estimate of expected life under combined creep-fatigue loading. Thus: t1 tr 1

t2 +

tr 2

t3 +

tr 3

+... +

n1

N1

n2 +

N 2

n3 +

N3

+.....

=1

Where n1, n2, n3... are the number of cycles at stress levels 1, 2, 3… respectively. N1, N2, N3… are the fatigue lives at stress levels 1, 2, 3… respectively.

1.3 Effect of Manufacturing Process on Design Introduction It is now now wide widely ly reco recogn gniz ized ed that that desi design gn,, mate materi rial als s sele select ctio ion, n, and and manu manufa fact ctur urin ing g are are inti intima mate tely ly rela relate ted d activ ctivit itie ies, s, which hich cann cannot ot be performed in isolation of each other. Creative designs may never develop into marketable products unless they can be manufactured economically at the required level of performance. In many cases, design modifications are made to achieve production economy or to suit existing production facilities and environment. Modifications of design may also be made in order order to impro improve ve quality quality and perfo performa rmance nce,, in which which case case the cost of production may increase.

1.3.1 Design Considerations for Cast Components Casting covers a wide range of processes which can be used to shape almo almost st any any meta metalli llic c and and some some plas plasti tics cs in a vari variet ety y of shap shapes es,, size sizes, s, accu accura racy cy,, and and surf surfac ace e finis finish. h. In some some case cases, s, cast casting ing repr repres esen ents ts the the obvious and only way of manufacturing, as in the case of components made of the different types of cast iron or cast alloys. In many other appli pplica cattions ions,, how however ver a decis ecisio ion n has has to be made whethe etherr it as advantageous advantageous to cast a product or to use another method of manufacture. manufacture. In such cases, the following factors should be considered: 1. Casti Casting ng is partic particula ularly rly suited suited for parts parts which which conta contain in inter internal nal cavitie cavities s that are inaccessible, too complex, or too large to be easily produced by machining. 2. It is adva advant ntag ageo eous us to cast cast comp comple lex x part parts s when when requ requir ired ed in larg large e numbers, especially if they are to be made of aluminum or zinc alloys. 3. Casti Casting ng techniq techniques ues can can be used used to produce produce a part, which which is one of a kind in a variety of materials, especially especially when it is not feasible to make it by machining. 4. Precious Precious metals metals are are usually usually shaped shaped by casting, casting, since there there is little little or no loss of materials. 5. Part Parts s produc produced ed by casti casting ng have have isotro isotropic pic propert properties ies,, which which could could be important requirements in some applications. 14


6. Casting ting is not not comp compe etiti titive ve when hen the par parts can be pro produc duced by punching from sheet or by deep drawing. 7. Extrusio Extrusion n can be preferable preferable to casting casting in some some cases, cases, especially especially in the case of lower- melting nonferrous alloys. 8. Cast Castin ings gs are are not not usua usually lly a viab viable le solu soluti tion on when when the the mate materi rial al is not not easily melted, as in the case of metals with very high melting points such as tungsten. Guidelines for design:

A general rule of solidification is that the shape of the casting should allow the solidification front to move uniformly from one end toward the feeding end, i.e. directional solidification. This can most easily be achieved when the casting has virtually uniform thickness in all sections. In most cases this is not possible. possible. However, However, when section section thickness must change, change, such change should be gradual, in order to give rise to stress concentration and possible hot tears in the casting. Figure 8.gives some guidelines to avoid these defects. Another problem, which arises in solidification, is caused by sharp corners; these also give rise to stress concentration and should be replaced by larger radii. When two sections cross or join, the solidification process is inter interrup rupted ted and a hot spot spot resul results. ts. Hot spots spots retar retard d solidi solidific ficat ation ion and usually cause porosity and shrinkage cavities.

Effect of material properties The type and composition of the material play an important part in determining the shape, minimum section thickness, and strength of the casting. Materials, which have large solidification shrinkage and contain low – melting phases are susceptible susceptible to hot tears. tears. Another Another material material variable is cast ability, which can be related to the minimum section thickness, which can be achieved. It should be noted that the shape and size of the the casting as well as the casting casting process process and foundry foundry practice practice could affect the minimum section thickness. Correct Designs

Incorrect designs

Solidifications of intersecting sections results in hot spots and shrinkage activities

15 Stagger section

Use a core or internal chill

Use External Chills

Use a riser


Fig 8

1.3.2 Design Considerations for Molded Plastic Components Compression, transfer, and injection molding processes are the commonly used methods of molding plastic pl astic components. These processes processes involve the introd introduct uction ion of fluid fluid or a semi semi fluid fluid mater material ial into a mould mould cavity cavity and permitting it to solidify into the desired shape. Guidelines for design

Experience shows that the mechanical, electrical, and chemical properties of molded components are influenced by the flow of the molten plastic as it fills the mold cavity. Streamlined flow will avoid gas pockets in heavy – sectioned areas. An impor importan tantt commo common n featur feature e in moldin molding g proces processes ses is draft, draft, which which is required for easy ejection of molded parts from the mold cavity. A taper of 1 to 4 degree is usually used for polymers, but tapers of less than 1 degree can be used for deep articles. Another common feature is the uniform thickness. Non-uniformity of thickness in a molded piece tends to produce non-uniform cooling and unbalanced shrinkage leading to internal stresses and warpage. If thickness variations are necessary, generous fillets should be used to allow a gradual change in thickness. The effect of junctions and corners can also be reduced by using a radius instead, as shown in Fig 9.The nomina nominall wall wall thickn thickness ess must must obviou obviously sly such such that that the part part is suffi sufficie cient nt strong to carry the expected service loads. However, it is better to adjust the shape of the part to cope with the applied load than to increase the wall thickness. This is because thick sections retard the molding cycle and require more materials. The presence of holes disturbs the flow of the material during molding and a weld line occurs in the side of the hole away from the direction of flow. This results in a potentially weak point and some from of strengthening, such as bosses may be necessary as in Fig 10.Through 10.Through holes are preferred to blind holes from a manufacturing standpoint. This is because core prints can often be supported in both halves of the mold in the case of through holes, but can only be supported from one end in the case of blind holes. Accuracy of molded parts.

Dimensional tolerances in molded plastic parts are affected by the type and const constitut itution ion of the mater material ial,, shrink shrinkage age of the mater material ial,, heat heat and 16


pressure variables in the molding process, and the toolmaker’s tolerances on the mold manufacture. manufacture. Shrinkage has two components: Mold shrinkage, which occurs upon solidification; and After shrinkage, this occurs in some materials after 24 hours. For example, a thermosetting plastic like melamine has mold shrinkage of about 0.7 to 0.9 %, and an after shrinkage of 0.6 to 0.8%. Thus a total shrinkage of about 1.3 to 1.7 % should be considered. On the other hand, a thermoplastic like polyethylene may shrink as much as 5% and nylon as much as 4%. In addition, the value of tolerance depends on the size of the part. Larger dimensions are normally accompanied by larger tolerances. For example, dimensions less than 25mm (1 in) can be held within ± 50 µ m. Larger dimensions are usually given tolerances of ± 10 to 20 µ m/cm. The value of tolerances also depends on the direction in relation to the parting plane.

Better Design

Poor Design

(a)

Fig 9 some design features of plastic parts. (a) Using radii instead of sharp corners.

(b)

Fig 10 some design features of plastic parts (b) Use of bosses to strengthen areas round holes and slots.

1.3.3 Design Considerations for Forged Components

17


Forging processes represent an important means of producing relatively complex parts for high-performance applications. In many cases forging represents a serious competitor to casting especially for solid parts that have no internal cavities. Forged parts have wrought structures, which are usually stronger, more ductile, contain less segregation, and are likely to have less internal defects than cast parts. This is because the extensive hot working, which is usually involved in forging, closes existing porosity, refines the grains, and homogenizes the structure. On the other hand, cast parts are more isotropic than forged parts, which usually have directional properties. This directionality is due to the fibre structure, which results from grain flow and elongation of second phases in the direction of deformation. Forged components components are generally stronger and more ductile in the direction of fibres than across the fibres. Guidelines for Design

Rapid changes in thickness should be avoided because these could result in laps and cracks in the forged metal as it flows in the die cavity. To prevent these defects, generous radii must be provided at the locations of large changes in thickness. Another similarity with casting is that vertical surfaces of a forging must be tapered to permit removal from the die cavity. A draft of 5 to 10 degrees is usually provided. It is better to locate the parting line near the middle of the part in order to avoid deep impression in either of the two halves of the die and allows easier filling of the die cavity cavity.. A design design would would be more more econo economic micall ally y produc produced ed by forgin forging g if dimen dimensi sion ons s acro across ss the the part partin ing g line line are are given given appr approp opri riat ate e mism mismat atch ch allow allowanc ance, e, and parall parallel el dimens dimension ion are are given given a reaso reasonab nable le die closur closure e allowance. Specifying close tolerances to these dimensions could require extensive machining which would be expensive.

Machined

Forged

Fig 11 Schematic comparison of the grain flow in forged and machined components.

18


1.3.4 Design Considerations For Powder Metallurgy Parts Powde Powderr metal metallur lurgy gy (P/M) (P/M) techniq techniques ues can can be used used to produc produce e a large large number of small parts to the final shape in few steps, with little or no machining, and at high rates. Many metallic alloys, ceramic materials, materials, and particulate reinforced composites can be processed by P/M techniques. Generally, parts produced by the traditional P/M techniques contain 4 to 10 vol % porosity. The amount of porosity depends on part shape, type and and size size of powd powder er,, lubr lubric icat atio ion n used used,, pres pressi sing ng pres pressu sure re,, sint sinter ering ing temperature and time, and finishing treatments. treatments. The The dist distri ribu buti tion on and and volu volume me frac fracti tion on of poro porosi sity ty grea greatl tly y affe affect ct the the mechanical, chemical, and physical properties of parts prepared by P/M techniques. An added advantage of P/M is versatility. Materials that can be combined in no other way can be produced by P/M. Aluminum - graphite bearings, copper - graphite electrical brushes, cobalt - tungsten carbide cutting tools (cermets), and porous bearings and filters are such. Guidelines for design

The Powder Powder Metallurg Metallurgy y Parts Parts Associa Association tion and Metal Metal Powder Powder Industries Industries Federation have made certain rules. They are: 1. The shape of the part must permit ejection from the die, Fig 12 2. Parts with straight walls are preferred. No draft is required for ejection from lubricated dies. 3. Parts with undercuts or holes at right angles to the direction di rection of pressing cannot be made, Fig 13. 4. Straight serrations can be made easily, but diamond knurls cannot, Fig 14. 5. Since Since pressu pressure re is not trans transmit mitted ted unifor uniformly mly throug through h a deep deep bed bed of powder, the length/diameter ratio of a mechanical pressed part should not exceed about 2.5: 1.

Fig 12 Reverse taper should be avoided, use parallel sides and machine the Required taper after sintering.

19


Fig 13 undercuts and holes at right angles to pressing direction should be avoided; if necessary such features are introduced by machining after sintering.

Fig 14 Diamond knurls should be replaced by straight serrations. serrations.

1.3.5 Design of Sheet - Metal Parts Parts made from sheet metal cover a wide variety of shapes, sizes, and mater material ials. s. Many Many exampl examples es are are found found in the the autom automot otive ive,, aircr aircraf aft, t, and cons consum umer er indus industr trie ies. s. Gene Genera rall lly, y, shee sheett-me meta tall part parts s are are prod produc uced ed by shearing, bending, and/or drawing. The grain size of the sheet material is important and should be closely controlled. Steel of 0.035 - 0.040 mm (0.00 (0.001 1 - 0.001 0.0016) 6) grain grain size size is genera generally lly accep accepta table ble for for deepdeep- drawin drawing g appli applica cati tion ons. s. When When form formab abil ilit ity y is the the main main requ requir irem emen entt in a shee sheett mate materi rial al,, draw drawin ing g - qual qualit ity y low low carb carbon on stee steels ls repr repres esen ents ts the the most most economic alternative. Guidelines for design

The most important factor, which should be considered when designing parts that are to be made by bending, is bend ability. This is related to the ductility of the material and is expressed in terms of the smallest bend radius that does not crack the material. Bend ability of a sheet is usually expressed as 2T, 3T, 4T, etc. A 2T material has greater bend ability than a 3T material. Another factor which should be considered when designing for bending is spring back, which is caused by the elastic recovery of the material when the bending forces are removed. One way of compensating for spring back is to over bend the sheet. Another method is bottoming which eliminates the elastic recovery by subjecting the bend area to high-localized stresses.

20


1.3.6 Designs Involving Joining Processes The major function of a joint is to transmit stress from one part to another and in such case the strength of the joint should be sufficient to carry the expected service loads. In some applications, tightness of the joint is also nece necess ssar ary y to prev preven entt leak leaka age. ge. Beca Becaus use e join joints ts repr repres ese ent area areas s of disco discontin ntinuit uities ies in the assem assembly bly,, they they should should be locat located ed in low-st low-stres ress s regions especially in dynamically loads l oads structures. Welding

Weld Weldin ing g has has repl replac aced ed rive riveti ting ng in many many appl applic icat atio ions ns incl includ uding ing stee steell structures, boilers, tanks, and motorcar chassis. This is because riveting is less versatile and always requires lap joint. Also, the holes and rivets subtr subtract act from from streng strength, th, and and a rivete riveted d joint joint can can only only be about about 85%a 85%as s strong, whereas a welded joint can be as strong as the parent metal. Welded Welded joints joints are easier easier to inspect inspect and can be made gas and liquid-tight liquid-tight with withou outt the the caul caulki king ng whic which h has has to be done done in rive rivete ted d joint joints. s. On the the negative side, however, structures produced by welding are monolithic and behave as one piece. This could adversely affect affect the fracture behavior of the structure. For example, a crack in one piece of a multipiece riveted structure may not be serious, as it will seldom progress beyond the piece without detection. However, in the case of a welded structure, a crack that starts in a single plate or weld may progress for a large distance and cause complete failure. Anothe Anotherr factor factor,, which which should should be consi consider dered ed when when design designing ing a welde welded d structure, is the effect of size on the energy-absorption ability to steel. A Charpy impact specimen could show a much lower brittle-ductile transition temperature than a large welded structure made of the same material.

Guidelines for design of weldments weldments

1.Weld 1.Welded ed struct structure ures s and joints joints should should be design designed ed to have have suffi sufficie cient nt flexibility. Structures that are too rigid do not allow shrinkage of the weld metal, have restricted ability to redistribute stress, and are subjected to distortions and failure. 2. Accessibility of the joint for welding, welding position and component match-up are important elements of the design. 3. Thin sections are easier to weld than thick ones. 4. Welded section should be about the same thickness to avoid excessive heat distortion. 5. It is better to locate welded joints symmetrically around the axis of an assembly in order to reduce distortion. 6. Whenever possible the meet of several welds should be avoided. 21


7. Use weld fixtures and clamps to avoid distortion. Adhesive Bonding

Adhesi Adhesive ves s repre represen sentt an attra attracti ctive ve method method of joinin joining g and their their use is increasing in many applications. Some of main advantages in using Adhesives are as follows: 1. Thin sheets and parts of dissimilar thickness can be easily bonded. 2. Adhesive bonding is the most logical l ogical method of joining polymerMatrix composites. 3. Adhesives are electrical insulators and can prevent galvanic Action in joints between dissimilar metals. 4. Flexible adhesives spread bonding stresses over wide areas and Accommodate Accommodate differential thermal operation. 5. Flexible adhesives can absorb shocks and vibrations, which Increases Increases fatigue life. 6. The preparation of bonded joins requires no fastener holds, which Gives better structural integrity and allows thinner gage materials to be used. The main limitations of adhesives are as follows: 1. Bonded joints are weaker under cleavage and peel loading than under tension or shear. 2. Most adhesives cannot be used at service temperatures above 300 degree C(600 degree F). 3. Solvents can attack adhesive-bonded joints. 4. Some adhesives are attacked by ultraviolet light, water, and ozone. 5. The designer should also be aware of the adhesive's impact resistance and creep, or cold flow, strength. Design of adhesive joints

The strength of the adhesive joint depends on the geometry, the direction of loading in relation to the adhesive material, surface preparation, and application and curing technique. As the bonded area limits the strength of an adhesive joint, lab and double-strap joints are generally prepared to butt joints. If the geometry constrains do not allow for such joints, a scarf or double -scarf joint should be made. When When a lab lab joint joint is used used to bond bond thin thin sect sectio ions ns,, tens tensil ile e shea shearr caus causes es deflection, and this results in stress concentration at the end of the lab. Tapering the ends of the joints, gives more uniform loading throughout the joint. Since adhesive joints are weaker under peeling forces, joint design should avoid this type of loading. l oading.

1.3.7 Designs Involving Heat Treatment: Heat trea Heat treatm tmen entt repr repres esen ents ts an impo import rtan antt step step in the the sequ sequen ence ce of processes that are usually performed in the manufacture of metallic parts. Almost all ferrous and many nonferrous alloys can be heat treated to 22


achieve certain desired properties. Heat treatment can be used to make the material hard and brittle, as in the case of annealing. Gene Genera rall lly, y, hard harden enin ing g of stee steels ls invo involv lves es heat heatin ing g to the the aust austen enit itic ic temperature range, usually 750 to 900 °C (1400 to 1650 ° F), and then que quenchi nching ng to form the the har hard mart marten ens sitic itic pha phase. The non nonunif unifo orm temperatu temperature re distributio distribution n that occurs during during quenching quenching and the volume volume change that accompanies the martenstic transformation can combine to cause distortions, internal stresses, and even cracks in the heat treated part part.. Inte Intern rnal al stre stress sses es can can warp warp or dime dimens nsio iona nall chan change ges s when when the the quenched part is subsequently machined or can combine with externally appli applied ed stre stress sses es to caus cause e fail failur ure. e. Corr Corros osio ion n prob proble lems ms can can also also be aggravated owing to the presence of internal stresses. These difficulties can be reduced or eliminated by selecting steels with hardenability as they they requ requir ire e a less less cool cooling ing rate rate to achie achieve ve a given given hard hardne ness ss valu value. e. Manganese, chromium, molybdenum are commonly added to steels to increase their hardenability.

1.3.8 Designs Involving Machining Processes Guidelines for design The following discussion illustrates some component shapes and features which can cause difficulties in machining, take an undue length of time to machine, call for precision and skill that may not be available, or which may may even even be impo imposs ssibl ible e to mach machine ine by stan standa dard rd mach machin ine e tool tools s and and cutting tools. 1. The workpi workpiece ece must must have have a refere reference nce surface surface,, which which is suitable suitable for holding it on the machine tool or in a fixture. This could be a flat base or a cylindrical surface. 2. Whe Whenev never pos possibl sible e, the the desi desig gn shoul hould d allo llow all the machin hining ing operations to be completed without resetting or reclamping. 3. Whenever Whenever possible, possible, the radii radii between between the different different machined machined surfaces surfaces should be equal to the nose radius of the cutting tool. 4. If the part part is to be machined machined by tradition traditional al cutting cutting methods, methods, deflectio deflection n under cutting forces should be taken into account. For the same cutting force, the deflection is higher for thinner parts and for lower elastic moduli. Under these conditions, some means of support is necessary to ensure the accuracy of the machined part. 5. Feat Featur ures es at an angl angle e to the the main main mach machin inin ing g dire direct ctio ion n shou should ld be avoided as they may require special attachments or tooling. Fig 15 6. To reduce reduce the cost cost of machining, machining, machine machined d areas areas should be minimum minimum as shown Fig 16 7. Cutting Cutting tools often often require require run-out run-out space, space, as they cannot cannot be retracted retracted immed immediat iately ely.. This This is partic particula ularly rly impor importa tant nt in the case case of grindi grinding ng where the edges of the grinding wheel wear out faster than the center. Fig 17 gives some examples to illustrate this point.

23


Fig 15 (a) Poor design design as drill enters and exists at at an angle to the surface surface. (b) Better design, but drilling the holes need a special attachment. (c) Best design. Poor design

(a )

Better Design

(b)

(c )

Added materials to reduce machine area

Relief’s to reduce machined areas

Fig 16 Some design details which can be introduced to reduce machining.

Fig 17 Some design details which can be introduced to give run-out for grinding wheels.

1.4.The Materials Selection Process: 24


One of the most important requisites for the development of a satisfactory prod produc uctt at a comp compet etit itiv ive e cost cost is maki making ng soun sound d econ econom omic ic choi choice ces s of engineering designs, materials, and manufacturing processes. The large number of materials and the many manufacturing process available to the engineer, coupled with the complex relationships between the different selection parameters, often make the selection process a difficult task. A rigorous and through approach to materials selection is, however, often not followed in industry and much selection is based on past experience. It is often said, “When in doubt make it stout out of the stuff you know about.” While it is unwise to totally ignore past experience, the frequent introduction of new materials and manufacturing process, in addition to the increa increasin sing g pressu pressure re to produc produce e more more econo economic mic and compe competit titive ive products, make it necessary for the engineer to be always on the lookout for possible improvement. The reasons for reviewing the types of material and processes used in making an existing product are: 1. Taking Taking advanta advantage ge of new new materia materials ls or process processes. es. 2. Impr Improv ovin ing g serv servic ice e perf perfor orma manc nce, e, incl includ uding ing long longer er life life and and higher reliability. 3. Meeting Meeting new legal legal requir requirement ements. s. 4. Accounting Accounting for changed changed operating operating condition conditions. s. 5. Reducing Reducing cost cost and making making the product product more more competit competitive. ive. Selecting the optimum combination of material and process can be perf perfor orme med d at one one cert certai ain n stag stage e in the the histo history ry of a proj projec ect; t; it shou should ld gradua gradually lly evolve evolve during during the differ different ent stages stages of produc productt develo developme pment. nt. These are: 1. 2. 3. 4.

Analysis Analysis of of the perfo performa rmance nce require requiremen ments. ts. Developm Development ent of alterna alternative tive solutio solutions ns to the problem. problem. Evaluatio Evaluation n of the differ different ent soluti solutions. ons. Decision on the optimum solution

1.4.1 Analysis of the Material Performance Requirements: Functional Requirements: Requirements:

Functional requirements are directly related to the required characteristics of the part or the product. For example, if the part carries a uniaxial tensile load, the yield strength of the material can be directly related to the load-carrying capacity of the product. For the evaluation process of the characteristics of material properties like thermal shock resistance, wear resistance, reliability etc., and simulation service tests are employed. Processability Requirements: Requirements:

The processability of the material is a measure of its ability to be worked and and shap shaped ed in to a fini finish shed ed part part.. With With the the refe refere renc nce e to a spec specif ific ic manuf manufac actur turing ing method method,, proces processa sabili bility ty can can be define defined d as a castab castabilit ility, y, 25


weldability, machinability etc.,Ductility and hardenability can be relevant to processability if the material is to be deformed or hardened by heat treatment respectively. The closeness of the stock form to the required product form can be taken as a measure of processability in some cases. The material properties are closely related to functional requirements. requirements. Cost:

Cost is usually the controlling factor in evaluating materials, because in many applications there is a cost limit for a material intended to meet the application requirements. When the cost limit is exceeded, the design may have to be changed to allow the use of a less l ess expensive material. The cost of the processing often exceeds the cost of the stock material. Reliability Requirements: Requirements:

The reliability of the material can be defined as the probability that it will perf perfor orm m the the inte intend nded ed func functi tion on for for the the expe expect cted ed life life with withou outt failu failure re.. Material reliability is difficult to measure, because it is not only dependent upon the material’s inherent properties, but also greatly affected by its production and processing history. Tho Thoug ugh h ther there e are are diff diffic icul ulti ties es in eval evalua uatin ting g relia reliabil bilit ity, y, it is ofte often n an impor importa tant nt select selection ion facto factorr that that must must be taken taken in to acco account unt.. Failur Failure e analysis techniques are usually used to predict the different ways in which a product can fail, and can be considered as a systematic approach to reliability evaluation. Resistance to Service Conditions: Conditions:

The The envi enviro ronm nmen entt in whic which h the the prod produc uctt or part part will will oper operat ate e play plays s an important important role in determinin determining g the material material perform performance ance requirem requirements ents.. Corr Corros osiv ive e envi enviro ronm nmen ents ts,, as well well as high high or low low temp temper erat atur ures es,, can can adversely affect the performance of most materials in service. Whenever there is more than one material involved in an application, compatibility becomes a selection consideration. For example, In thermal environment, the coefficient of thermal expansion of all the materials involved may have to be similar in order to avoid thermal stresses. In applications where relative movements exist between different parts, wear resistance of the materials involved should be considered.

1.4.2 Cost per Unit Property Method: In simplest cases of optimizing the selection of materials, one property stands out as the most critical service requirement. In such simple cases the cost per unit property can use as a criterion for selecting the optimum material. Consider the case of a bar of given length (L) to support a tensile force (F). The cross-sectional area (A) of the bar is given by: A=F/S

()

Where S is the working stress of the material, which is related to its yield strength by an appropriate factor of safety. The cost of the bar is given by: 26


C′ = Cρ AL = (Cρ FL)/S Where C = cost of the material per unit mass, and ρ = Density of the material. In compa comparin ring g differ different ent candid candidate ate mater material ials, s, only only the quanti quantity ty (Cρ )/S, which is the cost of unit strength, needs to be compared, as F and L are cons consta tant nt for for all all mate materi rial al.. The The mate materi rial al with with the the lowe lowest st cost cost per per unit unit strength is the optimum material. When one material is considered as a substitute for an existing material, the two materials a and b can be compared on the basis of relative cost per unit strength (RC′ ): RC′ = (C′ )a (C′ )b which is equal to Caρ aSb Cbρ bSa RC′ less than unity indicates that the material a is preferable to material b. Equations similar to () an () can be used to compare the materials on cost basis.

1.4.3 Weighted Properties Method: The The weight weighted ed proper propertie ties s method method can can be used used in optimi optimizin zing g mater material ials s selection when several properties should be taken into consideration. In this method each material requirement, or property, is assigned a certain weig weight ht,, depe depend ndin ing g on its its impo import rtan ance ce.. A weig weight hted ed prop proper erty ty valu value e is obta obtain ined ed by mult multip iply lyin ing g the the nume numeri rica call valu value e of the the prop proper erty ty by the the weighting weighting factor factor (α ). The The indivi individua duall weight weighted ed proper property ty value values s of each each material are then summed to give a comparative materials performance inde index x (γ ). The The mate materi rial al with with the the high highes estt perf perfor orma manc nce e inde index x (γ ) is considered as the optimum for the application. When evaluating a list of candidate materials, one property is considered at a time. The best value in this list is rated as 100 and the others are scaled proportionally. proportionally. B= scaled property = Numerical value of property x 100 Maximum value in the list For proper propertie ties s like like cost, cost, corr corros osion ion or wear wear loss, loss, weight weight gain gain in oxidation, etc., a lower value is more desirable. In such cases, the lowest value is rated as 100 and B is calculated as: B= scaled property = Minimum value in the l ist x 100 Numerical value of property For mater material ial proper propertie ties s that that can be repres represent ented ed by numer numerica icall value values, s, applying applying the above procedur procedure e is simple. simple. However, However, with properti properties es like corrosio corrosion n and wear wear resistanc resistance, e, machinab machinability ility and weldability weldability,, etc., etc., are rarely given and materials are usually rated as very good, good, fair, poor etc. In such cases, the rating can be converted to numerical values using an arbitrary scale. For example, a corrosion resistance rating of excellent, 27


very good, good, fair and poor can be given numerical values of 5,4,3,2 and 1 respectively. Then, n

Material performance index,γ = ∑ Biα i i =1 =1 Where i is summed over all the n relevant properties. In the cases where numerous material properties are specified, the digital logic logic appro approach ach is used used as a system systemati atic c tool tool to determ determine ine α . In this procedur procedure e evaluatio evaluations ns are arranged arranged such that only two propertie properties s are considered at a time. Every possible combination of the properties or performance performance goals is compared and no shades of choice are required, only a yes yes or no deci decisi sion on for for each each eval evalua uati tion on.. To dete determ rmin ine e the the rela relati tive ve impo import rta ance nce of each each prop proper erti ties es or goal goal a tabl table e is cons constr truc ucte ted, d, the the properties or goals are listed in the left hand column, and comparisons are made in the columns to the right, as shown in the table. Table 5.1 Determination of the relative importance of performance goals using the digital logic method Goals

1 2 3 4 5

Number of possible decisions [N=n(n-1)/2] 1 2 3 4 5 6 7 8 9 1 0 1 1 0 1 0 1 0 1 0 0 1 0 1 1 0 0 0 0 1 1 Total number of positive decisions

Positive decisions

3 2 1 2 2 =10

Relative Emphasis Coefficient (α )

α 1=0.3 α 2=0.2 α 3=0.1 α 4=0.2 α 5=0.2 ∑ α =1.0

In comparing two properties or performance goals, the more important is given numerical one (1) and the less important is given zero(0).The total number of possible decisions N=n(n-1)/2 , where n is the number of the properties or goals under consideration. A relative emphasis coefficient or weighting factor,α for each goal is obtained by dividing the number of positiv positive e divisio divisions ns for for each each goal goal (m) into into the total total number number of possib possible le decisions (N). In this case ∑ α =1. Howe Howeve ver, r, if ther there e are are larg large e numb number ers s of prop proper erti ties es to cons conside iderr the the importance of cost may be emphasized by considering it separately as a modifier to the material performance index ( γ ). In the cases where the material is used for space filling, cost can be introduced on a per unit volume basis. A figure of merit (M) for the material can then be defined as: M=γ /(Cρ ) Where C= total cost of the material per unit weight (stock, processing, finishing, etc.) 28

Design For Manufacturing and Assembly Effect of Materials and Manufacturing processes on Design ρ = Density of the material

The The weig weight hted ed pro propert pertie ies s meth metho od can can be used used when when a mate materi rial al is considered as a substitute for an existing one. This is done by computing the relative figure of merit (RM), which is defined as, RM = Mn/Mc Wher Where e Mn and and Mc are are the the figu figure res s of meri meritt of the the new new and and exis existi ting ng materials respectively. If the RM is greater than unity, the new material is more suitable than the existing material. The steps involved in the weighted properties method can be written in the form of a simple computer program to select materials from the data bank. An interactive program can also include the digital logic method to help in determining weighting factors.

1.4.4 Limits On Properties Method: In the limits on properties method, the performance performance requirements are divided into three categories: 1. Lower Lower limit limit proper propertie ties s 2. Upper Upper limit limit proper propertie ties s 3. Targe Targett value value proper propertie ties s The limits on properties method are usually suitable for optimizing material and process selection when the number of possible alternatives is relatively large. This is because the limits, which are specified for the different properties, can be used for eliminating unsuitable materials from data bank. The remaining materials are those whose properties are above the lower limits, below the upper, and within the limits of target values of the respective specified requirements. After the screening stage, the limits on properties method can be used to optimize the selection from among the remaining materials. As in the the case of the the weigh eighte ted d pro proper perties ies metho ethod, d, eac each of the the requirements or properties is assigned a weighted factor, α , which can be determined using the digital logic method, as discussed earlier. A merit para parame mete ter, r, m,is m,is then then calc calcul ulat ated ed for for each each mate materi rial al acco accord rdin ing g to the the relationship:

 

m =  ∑ α

i

X X       α 1  + ∑ −  +  ∑ α   X  Y   Y Y

j

i

j

i

j

1

k

k

k

where where l,u, l,u, and t stand stand for lower lower limit, limit, upper upper limit, limit, and target target value value properties respectively. nl,nu,and nt are the numbers of the lower limit, upper limit, and target value properties respectively. α i, α j, α k are the weighting factors of the lower limit, upper limit, and target value properties respectively. Xi,X j and Xk are the candidate material lower limit, upper limit, and target value properties respectively.

29


Yi,Y j,and Yk are the specified lower limit, upper limit, and target value properties respectively. According to the equation the lower the value of the merit parameter m, the better the material. As in the weighted properties method, the cost can be considered in two ways: 1. Cost Cost is trea treate ted d as an uppe upperr lim limit pro propert perty y and give given n the the appropriate weight. 2.Cost is included as a modifier to the merit parameter as follows: m´ = (CX/CY) m

Where CY and CX are the specified cost upper limit and candidate material cost, In this case the material with the lowest cost-modified merit parameter, parameter, m ´, is the optimum.

1.5. Case Study for Material Selection: 1.5.1 Materials for springs: Springs come in many shapes as shown in the Fig 18, and have many purposes: one thinks of axial springs, leaf springs, helical springs, spiral springs, torsion bars. Regardless of their shape or use, the best material for a spring of minimum volume is that with the greatest value of 2 2 σ / E , and for minimum weight it is that with the greatest valueσ / E ρ . f f

30


Fig 18 springs store energy. The best material for any spring, regardless of its shape or the way in which it is loaded, is that with the highest value of 2 2 σ / E Or if weight is important,σ / E . f f The primary function of the spring is that of storing elastic energy and when required releasing it again.The elastic energy stored per unit volume of material stressed uniformly to a stress σ is 2

1

W v

=

σ

E

2

Where E is young’s modulus. It is Wv that to be maximize. The spring will be damaged if the stress σ exceeds the yield stress or the failure stress σ f ; the constraint is σ <=σ f . So the maximum energy density is W v

1 σ =

2

f

E

2

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

W v

2

1

=

σ

f

E

3

And for leaf springs,

W v

1

σ

=

2

f

E

4

But this has no influence on the choice of the material. The best material for springs is that the biggest value of σ

M 1

2

=

f

E

If weight rather than the volume, matters, we must divide this by the density ρ (giving energy stored per unit weight) and seek materials with the high value of 2

M 2

=

σ

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f

ρ E


Fig 19 Materials for for small springs. High strength strength (‘spring’) steel is good. good. Glass, CFRP and GFRP all under right circumstances, circumstances, make good springs. Elastomers are excellent. Ceramics are eliminated by their l ow tensile strength.

1.5.2 The Selection The choice of materials for springs of minimum volume is shown in the Fig 19 family lines of slope ½ link materials with equal values of σ

M 1

=

2

f

E

Those with the highest values of M 1 lie towards the bottom right. The heavy line is one of the families; it is positioned so that a subset of materials is left exposed. The best choices are a high0strength steel (spring steel) lying near the top end of the line, and at the other end,

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rubber. But certain other materials are suggested too: GFRP (truck springs), titanium alloys, glass and Nylon.

1.6.Problem : 1. Suggest a suitable operation sequence for the stub carrier shown in Fig.20 Fig.20 and redra redraw w the compo componen nentt incor incorpo pora ratin ting g featur features es to facilit facilitate ate manufacture. The carrier is to be produced from a steel casting and the symbol indicates a ground surface for the 30 mm diameter f8 limits.

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2. The proposed machining procedure for the plate Fig 21 (1) Bore and face, reverse, face other side - turret. (2) Drill and ream four 25 mm H8holes - drill, drill jig. Suggest a design modification which will permit of an alternative procedure to achieve a substantial reduction in machining time. State the procedure for producing the modified design.

3. A Cast iron bearing bracket is shown in i n Fig 22. Indicate the preferred parting line and any necessary sand cores. Offer a design modification that will reduce or eliminate the need for sand cores.

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1chap1 mater

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