Chp 2 - Design Engineering and Selection of Materi

ME215–EngineeringMaterialsIDr.OğuzhanYILMAZAssistantProfessorMechanicalEngineeringUniversityofGaziantepChapter2–DesignEngineeringandSelectionofMaterials1IntroductionDesignofnewproductsanddevelopmentoftheexistingonesistheessentialpurposeofengineering.Forthedesigningofamachineelementoracomponent,anengineerhastoconsidermanyrequirements.Theselectionofthematerialfromwhichapartistobeproducedhasalwaysbeenapredominantfactorintheoverallperformanceofadesignbecauseofitsinfluenceonotherfactors.Hence,itisnotusuallypossibletomakethefinaldecisiononthegeometryanddimensionsofapartuntilthematerialisselected.2FundamentalAspectsoftheDesignProcedureMakingtherightdistinctionbetweendemandsthataretrulyappropriatetomaterialpropertiesandcertaindesignfeatures.Forexample,strengthofapartdependsuponthestrengthofmaterialandgeometricalparameters.Thisdoesnotmeanthathighstrengthmaterialsareneededtoachievetherequiredstrengthofthepart.Instead,designercanpreferaweakermaterialbutlargerdimensionsaslongastherearenospaceorweightrestrictions.Whensuchrestrictionsaretight,thenstrengthofmaterialitselfbecomesconsiderablyimportant.Donotunderstimateme!3FundamentalAspectsoftheDesignProcedureInaddition,materialselectionhassuchagreatinfluence;-theselectionofatotallydifferentmaterialwouldresultinanewdesignapproach.-Inevitably,chosingthemostsuitablematerialforapartispossiblebygoodstorehouseknowledgeconcerningthematerialproperties.-Themostdecisivefactorformakingaproperselectionistheexperience,whichnobookcouldprovide.4FundamentalAspectsoftheDesignProcedureAsimpleflowdiagramofdesignthinkingformaterialselection:NEEDFUNCTIONALREQUIREMENTS–OrderofImportance–LevelofSatisfaction–FailureCriteriaDESIGNLIMITATIONS–ProductionRequirements–EconomicRequirements–MaintenanceRequirementsPROBLEMDEFINITION–MaterialSelectionMATERIALALTERNATIVES–StorehouseKnowledge–ExperienceFINALCHOICE5FundamentalAspectsoftheDesignProcedureEverydesigneffortisaimedatsatisfyinganexistentorpotentialneed.Fromtheanalysisoftheneed,adesignerdeterminesessentialanddesirablefeaturesofthedesignthatareexpressedintheformof“functionalrequirements”.Asitisimpossibleforadesigntosatisfyalloftherequirementstothesamedegree,theyarethenarrangedintheorderofimportanceinordertoidentifytheareasofcomprimise.6FundamentalAspectsoftheDesignProcedureFurthermore,adesignhastobeincompliancewithcertaininevitable“designlimitations”thataregroupedas:Manufacturing(production),Money(economic)andMaintenancerequirements(i.e.“3Mrule”).Dependinguponthenatureofdesign,itissometimesthefunctionalrequirementsandsometimesthedesignlimitationsthatdictatethepropertiestobedesiredinthemetarialforthedesignworkathand.7FundamentalAspectsoftheDesignProcedureFunctionalrequirementsconcernthemechanicalpropertiesofmaterial(suchasstrength,stiffness,resilience,toughness,dimensionalstability,hardness,etc.)andthephysicalproperties(e.g.coefficientoflinearexpansion,thermalandelectricalconductivity,andsoon).Productionrequirementsarelogicallythefirsttobeconsidered.Hence,thedesignermustconsiderfunctionalmeritsofthematerialaswellasitsabilitytobemachined,shaped,formed,cast,welded,andsoon.8FundamentalAspectsoftheDesignProcedureEconomicrequirementsarebasedonthefinalproductcostcomposedofrawmaterialcostandproductioncostswithoverheads.Thecostofanyproductshouldbeashighasthecustomerscanpayforit.Finally,maintenancerequirements(i.e.whetherreplacementorrepairisrequired)dependuponsizeofthepart,extentofpossibledamage,facilitiesofthecustomersandtheacceptablelevelofcosts.9ProductionRequirementsAdesignisrealizedonlyafteritisproduced.Hence,thedesignermustbeawareofthefactthatproductioniscarriedoutaccordingtodrawingsandspecifications,wheretheproductiongroupmaygiveusefulhints.Materialselectiondependsuponthefunctionaldemands,howmanypartswillbeproduced,whichmaterialscanbeused,andwhatpropertiesarerelatedforthatdesign.Theproductionrequirementscanbegatheredinthefollowinggroups:1.Machinability2.Formability3.Castability4.SuitabilityforCompacting5.Weldability6.HeatTreatability7.AdaptabilitytoSpecialProcesses8.AdaptabilitytoFormsofProtection10ProductionRequirements–MachinabilityMachiningisshapingapartbyremovingtheunwantedmaterialintheformofchipstoachievethedesiredshape.Turning,milling,shaping,drillingandboringarethefamiliarexamplesofchipremovalprocesses.Inaddition;grinding,honingandlappingremovethematerialwithabrasives.Speedofchipremoval,toollifeandqualityofmachinedsurfacesareusedjointlytodescribethe“machinability”.Quantitativelyspeaking,ahighlymachinablematerialistheonethatallowsthemaximumamountofchipremovalwiththeminimumtoolwear,yieldingahighsurfacequality.Theabovefactorsvarynotonlyfromonematerialtoanother,butalsofromonemachiningprocesstoanother.11ProductionRequirements–FormabilityFormingprocesses(e.g.rolling,forging,coldheading,stamping,pressing,anddrawing)providespecialadvantageofenablingthedesiredshapetobeobtainedwithease,withoutmachiningthesurfacesthatarenotmating.Thisisagreatadvantageoverchipremovalprocesses.Anotheradvantageofsuchprocessesisthat,unlikecastingprocess