Chapter4-Innovation

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Chapter4

Innovation

Structuraldesignshouldbe,andoftenis,acreativeprocessforwhichempiricismhas

limitedvalue.Innovationisthe‘‘life-blood’’ofbetterdesigns,butcomeswithconsid-

erablechallengetostructuralsafety,because‘‘thevalidatingserviceexperience’’is

notavailable.

Onewaytocompensateforthelackofserviceexperienceistointroduceexplicit

safetyconstraintsintothestructuraldesignprocess.Todesignsothataspecified

levelofsafetyisreachedandthenmaintainedinservicebyacomplementingrisk

managementprocess.Theriskmanagementcanbeestablishedasacontrolprocess

thatusesflexibleinspectionprogramsthat,basedonfeedbackfromservice,canbe

changedtomethodandfrequencytokeepprobabilityofanunsafeflightwithin

requiredlevels.

4.1.SERVICEEXPERIENCE

Pertinentserviceexperienceprovidesavalidationofdesignmethods,designcriteria,

detaildesigns,structuralintegrityrules,definitionsofloads,environmentalrequire-

ments,manufacturingprocessesandcriticalitydecisions.Consequently,thelack

ofserviceexperiencemustbeovercome.Aprudentintroduction,intothedesign

process,ofexplicitsafetyconstraints,‘‘high-fidelity-analysis’’methodsanddesign

datatestingthatiscalibratedtotheseconstraintscould‘‘goalongway’’toward

compensatingforlackofserviceexperience.

Emergingserviceexperienceand‘‘new’’dataareimportantfactorsinmaintaining

safetylevels,butalsointhelearningprocessthatproducesanimprovedfuture.

Arationalprocessmonitors,collects,analyzesandproducesfeedbackintorisk

managementanddesignprocesses.

Anotherimportantaspectofserviceexperienceisthevalidationof‘‘Fail-Safe

design’’principles.FARandJARbothrequirefail-safety,ifB-value‘‘allowables’’

aretobeusedinthedesign.Demonstrationofcompliancewithfail-saferequire-

mentsisaverycomplexanddifficultundertaking.Successisverysensitiveto

materialselection,processes,materialmixturesanddetaildesigns.Thesheernature

offail-safetyprecludesa‘‘totalcompliancedemonstration’’bytest.So,high-fidelity-

analysisalternativesfordamagedstructuremustbemadeavailable.Breakthroughs

producedbyNASALangleyStructuralMechanicsDepartment(formerlyleadby

DrJ.H.Starnes,Jr)inthe‘‘high-fidelity-analysis’’field,provideanaturalavenue.

59Scenario-basedinductivedesignmethodsanddetaildesignapproachesfor

damagecontainmentmustalsobepartofthefutureofsafeinnovation.Lackof

serviceexperienceisanintegralpartofinnovation,andthemethodstobeusedin

designmustbeabletoaccountforsafetyexplicitly.

4.2.CRITICALITY

Modernaluminumstructureshavedevelopedtothepointwhere‘‘quality’’and

validateddetaildesignshavebecometheanswertofatigue,whilesizingof‘‘acreage

structures’’isbasedonultimatestrength.Composites,howeverrequireamore

complexapproach.AtypicalcaseforcompositesisshowninFigure4.1.Figure4.1

shows,

1:5ÁLLR ULCð4:1Þ

whereULCreferstoultimateloadcapability.Eq.(4.1)thenisanillustrationofa

damagetolerancecriticalPSE,becausesizingtoULCwouldproducelessthickness

thanwhatdamagetolerancerequires.Asthisisatypicalsituationforcomposites,an

optimizedselectionofthedamagesizesMADandMUDwouldrequireaselectionof

requiredprobabilitylevelsofthecurveinFigure4.1.ULCisobviouslydefinedby:

Prðs ULCÞ¼0:10ð4:2Þ

(wheresrepresentsstrength),asB-valuesarerequired.Theprobabilityrequirements

forLLRmustbederivedfromtherequirementofamaximumlevelofthe

probabilityofanunsafeflight.

RS

ULC

1.5 · LLRLLR

MUD GDDEDDMADDamage size

Region:3456

Figure4.1.Damagesizevsresidualstrength,RS.60CompositeStructures,Design,SafetyandInnovationTheguidedevolutionofthe2000-seriesaluminumin‘‘traditional,tension–critical

structure’’hasresultedinasituationwhere,

1:5ÁLLR%ULC

basedonthetime-honoredrequirementforresidualstrengthofa‘‘panel’’withone

failed‘‘centralstringer’’andacrackedskin-widthequaltothestringerspacing.For

composites,inbothtensionandcompressioncriticalstructure,thereisalotof

variety.Anengineeringprocesssupportingabalancedselectionofdamagesize

intervalsisamustfortheefficientformulationofdesigncriteria.

Theoverallsafetyrequirementsforthevehiclesimposevaluesforpin,

Pr½RS LLRjD5󰀄 p

where

D5isEDD5Ds5MAD

andcouldbeconsideredthefocusintervalforresidualstrengthintegrity.Then,

D4isGDD5Ds5EDD

istheintervalwheretherequirementsofprobabilitiesforgrowthratesarethemost

important.Adetaileddiscussionofeffectsandordersofmagnitudecanbefoundin

Chapter2.

Adamagetolerancecriteriondevelopmentresultinginhighconfidencelevels

wouldpayattentiontotheslopeof,

RS¼RSðdsÞ

inthepertinentregion.Forregionswhere,

RS0ðdsÞ¼0

thepotentialforaverydamagetolerantdesignisattractive,howevertheresidual

strengthmaybetoosmall,ordetectabilitymayfallshortofdesirable.So,againthe

needforabalancedselectionofintervalshastobeemphasized.

Anothermajorconcernistheshapeofthefunctioninregions3and4.Region3

couldbebasedonthedefinitionthatemergesfrom:

ULCbeingbasedonFBð1þMS)andLLRonFlim(limitallowable)Innovation61