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1、MaterialSelection:AshbyDiagrams1Modulusvs.DensityChart21SelectionStrategyAllMaterialsScreening:applypropertylimitsRanking:applymaterialindicesSubsetofMaterialsSupportinginformationPrimeCandidatesLocalconditionsFinalMaterialChoice3DerivingpropertylimitsandmaterialindicesDesignrequirementsFunctions–
2、e.g.supportload,containpressure,transmitheatObjective–e.g.cheap,light,safeConstraints–e.g.fixeddimensions,operatingtemperatureMaterialindex–acombinationofmaterialpropertieswhichcharacterisestheperformanceofamaterialinagivenapplication42Ageneralstatementforthedesignofastructuralelementfunctionalge
3、ometricmaterialp=f,,requirments,Fparameters,Gproperties,Mp–describessomeaspectoftheperformanceofthecomponent:itsmass,orvolume,orcostf–afunctionofOptimumdesignistheselectionofthematerialandgeometrywhichmaximumorminimisep,accordingtodesirability5Example:Thematerialindexforalight,strong
4、tieFunctionTierodObjectiveMinimisethemassConstraints(a)lengthlspecified(b)supporttensileloadFwithoutfailingMinimisemassm=Alr(1)m–massSectionareaAmustFl–lengthbesufficienttocarryF£sf(2)AF–tensileloadCombine(1)and(2)r-densityEliminateAm³(F)(l)s(3)A–cross-sectionalfarea–failurestrengthfsMaterial
5、index,M=r63DATA:STRONG&LIGHTTENSIONMEMBERS104CeramicsCermetsStrength,sf(MPa)103PMCsSteels
6、
7、grainMetal102alloysdo10woPolymersgrainIncreasingP1=1eforstrongsloptension0.1members0.111030AdaptedfromFig.22.2,Callister7e.(Fig.22.2adaptedfromM.F.Ashby,Materials3Density,r(Mg/m)SelectioninMechanicalDesig
8、n,Butterworth-7HeinemannLtd.,1992.)Yourproblem:Beamsinbending•Bridgedeck•Buildingfloor•AirplanefloorAssumethree-pointbendingforsquarebeaminallthreeapplicationsL84Beamsinbending1:Lightstiffbeam•Functionbeam•Objectiveminimisemass•Constraints(a)lengthlspecified(b)supportbendingloadFwithoutdeflecti
9、ngtoomuch,i.e.stiffness,Sspecifiedm-massFA–cross-sectionalareaS–stiffness–densityt–deflectiondbF–forceE–YoungsmodulusF/2F/2C–constant1I–secondmomentofarea4-bI=129Beamsinbending1:LightstiffbeamMinimisemass2Equat
