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