Tips on Fatigue - NAVWEPS 00-25-559

7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559

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NAVWEPS00-25-559

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TIPSONFATIGUE

by ClarenceR.mithStructuresDesignSpecialistFatigueLaboratory-GeneralDynamics/Convair

PreparedfortheBureauofNavalW eaponsDEPARTMENTO FTHE NAVY 1963

ForsalebytheSuperintendentofDocumentsU.S.GovernmentPrintingOfficeWashington25 ,D.C. -- Price0ents

ReproducedFrom BestAvailableCopy 2 0 0 1 1 1 3 02 7


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TIPSO NFA T IGU E

PREFACE

/Soonerorlater,metalstructuresunderrepeatedloadwearout. Theprob-lemistobecertainthatitislater,ratherthansooner./Forthisreason,theNavyhasalwaysencouragedresearchanddevelop-menttofindwaysofmakingstructureslastlonger.Ithasbeenfound,however,hatitisnotenoughforscientistsandresearchen-gineerstoknowthesecretsoffatigue. If designers,hopmen,andinspectorsdonotrecognizethesignsoffatigue,thenthepurposeofresearchanddevelopmenthasnotbeenrealized.Togetthismessageacross,plainsimplelanguageandforthrightpictures,

areused,unhamperedbysuperfluoustech-nicaljargon,theory,nddetaileddatadis-plays.Thisapproachison eway toensurethatthefindingsofresearchbecometheusableknowledgeofthemanintheshopandthemanon thedrawingboard. Thepremiseisthatresearchan ddevelopmentardwortheverycenttheycostifandonlyifwemakefulluseofthenew ideas

theyproduce.Lackofcommunicationbetweenthosewhoknowandthosewhoneedtoknowisoftentheprimecauseofstructuralfailure.

Iftheknowledgegainedthroughfatigueresearchoverthelast100yearswereap-plied,manyfatigueproblemswouldneveroccur(orrecur).Thousandsofdocumentsonclassicandappliedresearchofmetalandstruc-

turalfatigueliterallyburyfactsbytheirweightandprofundity. Thesedocumentsshouldbelefttotheexperts.O ntheotherhand,themanatthedrawingboardisapracticalman. Heneedspracticalanswerstosuchfunda-mentalquestionsas: Ishecontinuallymakingerrorsthatwillresultinfatigueproblems? Thefactthatnew airplanesarestillfailinginfatigueindicatesthisto

betrue. Examinationofsuchfailuresin-dicatesthatmanydesignersarenotevenawarethatsharpnotchesarefatigue-prone. Correctionsforthison efaultcouldsavemillionsofdollarsperyearandpos-siblyafewlives.HencejJJiisbookproposesto benoth-ingmorenorlessthanasimpleguideon howto :1 .Recognizepotentialfatigueprob-lems.2.Rectifyexistingproblems.3.voidgettingintosituationsthatmaycauseproblems.

?-U'1 in


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TIPSONFATIGUE fpr:Principlesmentionedhereinhavetodowith1 )relationshipsofonestructuralmembertoanother;and2)pathsofload

carriedwithinthendividualmembers.Thissnottomplyhatthemoreabstruseprinciplesofsolidstatephysicswouldnotalsobehelpful,butthesearefarbeyondthescopeofthiswork.]Beforefatiguewasconsidered,airplanesweredesignedtowithstandagivenstaticoad. ThismaybeintermsofthenumberofG'stheairplanemaybeexpectedtoencounterduringamaneuver,orinsomecasesthelargestgustthatmaybeencounteredoncenalifetime).

Inanyevent,herewassomedesignnumber. Knowingthestrengthofthema-terial,twaspossibletofigureouthowmuchmaterialwasrequiredtocarrytheload. A perfectdesignwasonewhereinthestructurewouldcarry100percentofthedesignloadandfailat101percent. Notonlythat,nocomponentwouldberelativelystrongerthanthenext. Justlikethe"won-derfulone-hossshaythatwasbuiltnsuchaogicalwaythattranforahundredyearstoaday." Inhispoem,"TheDea-con'sMasterpiece,"OliverWendellHolmes(fatherofSupremeCourtJustice)chroni-cleshatnopartcouldfailfirstbecauseeachwasconstructedoftheverybestma-terialfortheunctiontobeserved. Un-doubtedly,designalsohadsomethingtodowitht.

.n _w) Whileairplanesare/stillIdesignedtocarryacertainstaticoad,atigueposestheadditionalproblemofestimatinghowlongtheairplanewilllast. Ideally,tshouldlastasongashedesignerntendeditshould.)

Indesigningforstaticstrength,hedesignerwasgivenasetofrulesgoverning

thestressevelstowhichhismaterialscouldbeworked. Thisgavesomeuniform-ityndesign. Thenominalstressevelsmayhavedifferedwithlocationorpurpose.Forexample,hecompressionallowablewoulddependonstringerandbulkheadspacing,whiletensionallowablesmightde-pendonthetypeoffastener.

Infatigue,designingtoauniformnom-inalstresswouldnotensureauniformfa-tiguestrength. A uniformdesignforfatiguewouldinvolveheproductofthenominalstresstimesthestressconcentration. Notknowingthestressconcentrations,hiswouldbeanimpossibility. Acknowledg-mentsduetoallthosewhoseencourage-mentandassistancehavemadethisbookpossible. Whiletheirssthegloryforanymeritnthework,blameforanyfaulthereinbelongstotheauthoralone.

Assistancecamefrommanysources,allrememberedanddeeplyappreciated,thoughspaceimitsmentiontoM .Rosenfeld,NavalAirEngineeringCenter,R.L.CreelandC.P.Baum,NavyBureauofWeaponsforreviewoftheentirebook.Theauthorwishestoespeciallyac-knowledgehecartoonsofW .GoldsmithandT.Adams;heeditorialassistanceof R.J.Prichard;heorganizationalhelpofRalphDeSolaintheearlystagesofthework;andtheechnicalassistanceofG.G .Green.Forthedataandphotographsthatgivethisnformalworkaconcretesetofex-

amples,especiallynChapter5,heauthorwantstothankthefollowingaircraftman-ufacturers,operatorsandorganizations:--> AeronauticalResearchLaboratories,Melbourne,AustraliaIV


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TIPSONFATIGUE AeronavesdeMexico,S.A.AerospaceDevelopmentCenter,Wright-PattersonAirForceBaseAircraftPlatingCo.AmericanAirlinesOverhaulBaseAmericanAirmotiveCo.BeechAircraftCorp.BellHelicopterCo.BoeingAirplaneCo.BraniffAirlinesBristolAircraft,Winnepeg,OntarioBritishEmbassy,Washington,D.C.Canadair,Ltd.CessnaAircraftCo.AeronauticsandMissileDivisionChance-VoughtCorporationADivisionofLing-Temco-Vought,Inc.ChapmanLaboratories,nc.ContinentalAirlinesDeltaAirlinesDouglasAircraftCo.,nc.EasternAirlinesFairchildEngine&AirplaneCorp.GeneralDynamicsCorp.GeneralElectricCo.GrummanAircraftEngineeringCo.HillerAircraftCorp.KamanAircraftCorp.LockheedAircraftCo.MartinCo.MetalImprovementCo.MexicanadeAviacionMcDonnellAircraftCorp.NationalAeronauticalEstablishment,Ottawa,OntarioNationalAirlinesNationalLuchtvaartlaboratorium,Amsterdam,TheNetherlandsNavalAirEngineeringCenterASL )NorthAmericanAviation,nc.

NortheastAirlinesNorthropAircraft,nc.PanAmericanAirlinesRepublicAviationCorp.StandardPressedSteelCo.TranscanadaAirlinesTrans-WorldAirlinesUnitedAirlinesWesternAirlines/Thisbookrepresentsaninitialattempttomakeallevelsofpersonnelawareofthefatigueproblemthatexistsnaircraft

structures./Itisanticipatedthatrevision$4.willberequiredinthefuture;consequently-userscommentsaresolicitedsothatameaningfulrevisionmaybeaccomplished.Similarly,hephotographsrepresentthebestillustrationspresentlyavailabletodepicttheproblemsdiscussed. Thesephotographswerenotspecificallytakenforthispurpose;hencetheyarenotallasclearandunclutteredaswouldbedesiredPhotographsthatllustratemoreclearlytheproblemsdiscussedhereinoranyotherfatigueproblemsoccurringinaircraftstructuresaredesired. Userscommentsandnewphotographssuitableforillustra-tionshouldbeforwardedto:

DirectorS-3)AeronauticalStructuresLaboratoryNavalAirEngineeringCenterPhiladelphia,Pa.,19112SanDiego,California30Oct1963 C.R.SMITH


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TIPSO NFATIGUE

CONTENTS

PREFACEChapter1heFatigueProblemChapter2asicPrinciplesChapter3ointsndJoiningChapter4evelopinganIntuitionforFatigueChapter5astExperienceChapter6akingtheMostofaBadSituationChapter7heckListAPPENDICES

A.atigueTestDataB.tressConcentrationsC.uggestedReadingVll


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TIPSONFATIGUE

THEFAT IGUEPROBLEM

1.1 A CENTURY OFSTUDYND FAT IGUE STILLFAILS STRUCTURES Thefatigueproblemrelatingtometalsandstructureshasbeeninvestigatedexperi-mentallyformorethanacentury. In 849 ,JonesandGaltoninvestigatedcastironbarsin bending. Theyfoundthatfailureoccur-redin lessthan100,000cycles ifloadedtomorethanone-thirdofultimatebendingstrength. Similarworkon wroughtironbuilt-upgirdersbyFairborn(1860-1861)showedsimilarresults. Wohler'sworkforthePrussianStateRailwaysgoesbacktothe1850'swhenhemadeanextensiveseriesof testsof variousgradesofironandsteelsubjectedtorepeateddirecttensile andcompressiveoads,orepeatedbendingloads,andtorepeatedtorsionalloads. Yetwecontinuetoreadaboutandhearaboutrailroadwrecks,automobilesmashups,airlinercrashes,andothercatastrophesdirectlyattributabletofatiguenmetallicstructures.

anotherreportin muchsmallertype: "JetBouncesnAir: 10Hurt." Howmanysuchbouncescanastructuresustainbeforetfailsandbecomeshefactbehindan evenbiggerheadline?

W hyanevenbiggerheadline? Thefirstplanewasacharteredplanenanunscheduledflight. Itwentdownnearth ecoastofWestAfrica,andwhateverhappensin remoteplacesneverseemsasrealorcriticalaswhathappenscloserohome.Thesecondplanecarriedmorepassengersandwason aregularlyscheduledflight.Itsrouteinvolvedtheivesandemotionsofhundredsofthousandsofpeoplelocally.Whenfatiguefailureovercomesthesecondplane,astmayin thecourseoftime,tissafeopredictthattheheadlinewillbebigger,hecasualtyfiguresmorestartling,andtheimpactontheravelingpublicevengreater.1.3 TODAYFAT IGUESA BIGGERPROBLEMTHAN EV ER

1.2 FAT IGUECAN BE BIG,BA DNEWS "AirlinerCrasheswith10Aboard:"readtheblackheadlineson 5March1962. O nthesamefrontpage,andatthefootofthecolumndescribingthelossoftheairliner,hercrew,andallherpassengers,was

Airplanesnthepastwerenotsubjectedtoloadsexperiencedbypresentdayhighspeedaircraft. Also,theywerebuiltof materialswhosetensilestrengthsweresoowhatin ordertosatisfystaticstrengthrequire-ments,stressesforserviceloadingwouldautomaticallyfallwithinrangesthatwouldprovideanadequatefatigueife.1-1


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TIPSONFATIGUE Thefatigueproblemhasrisenatanalarmingratewithpresentdayairplaneshavinghighspeedandperformance. Tomakemattersworse,hematerialsusedinpresentdayairplanesarestronger,yethavenobetterandinsomecases,poorer)fatiguepropertieshanthoseusedformerly,Theesulthaseenanaccumulationofservicefailures,omewithfatalitiesinvolved. Suchsituationshavenvolvedthedesignernunfamiliarareas. Besides,moreaccuratemethodsofstressanalysisenableustodesignstructureswithgreaterefficiencyandprecision.

1.4 THEPANICFACTORDesignersareusuallyshockedbyservicefailures;henceheirsubsequentdesignsandmodificationsfailedpartsftenincludeahighpanicfactor. Thisanicfactorislikelytobefaroutofproportionothedesignmprovementneeded.hepanicfactorsbornofsuddenfright. Some-timestiscompoundedwithignorance,andcertainlyitsusescontrarytoalltheprinciplesofgooddesign.

Unanticipatedfatiguefailurescausedesignersobecomeappalledattheamountofinformationthatseemsneces-sarytoestimateheserviceifeofanypartorstructure. Thefactthat theirknowledgeoftheordinarymechanicalpropertiesofmaterials ultimateandyieldstrengths,longation,modulusofelasticity hasfailedthem,eadshemtofeelfullyjustifiedinusingthehighpanicfactor. Thenaturaltendencyso"beef-up"hestructurehatfailed,venthoughhischangemaynotbehesolutionandtheweightpenaltyextreme, hisomecases, removalofmaterialmight

solveheproblem,whereasa"beef-up"maycreateanewproblemjustoutsideofthe"beefed-up"area.1.5HEVICIOUSCIRCLEIn aircraftdesign,verypoundofstructureaddedrequiresadditionalweightinheformofaddedpowerandfuelnecessarytocarrytheaddedstructure. Thisquicklybecomesaviciouscirclebecausemoresupportstructureshenneededtosustainheaddedengineandfuelrequiredtocarrythe"beefed-up"redesignedstructure.1.6REAKTHEBIG ONENTOLITTLEONESAsongasfatiguestreatedasoneenor-mousproblem,tneverseemsogetsolved. However,whenfatiguesconsid-eredasanumberofsmallproblems,hesolutionofeachproblembecomesappar-ent. Aninitialapproachtoanyproblemstolistthefactorsnvolved,uchas:

1.hataretheoads?2.hatarehestresses?3.hatarethe stressconcentrations ?4.ow muchisthematerialgoodfor?5.ouldweuseabettermaterial?6.shatshapenecessary?7.hataboutcarelessshoppractices?8.hatisthematterwithnspection?9.hydidn'ttheengineersaysofthatiswhathewanted?10. What"birdbrain"calledoutthisheattreat?

Thislistouldgonndn.hepointis,hatinlookingverventhisshortlist,hedesigner,henspector,andtheshopmancaneachfindateastoneandprobablymoretemshathecanpersonallydosomethingabout.1-2


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TIPSONFATIGUE

1.7 IFEV ERYONE DOESHIS OW N JOBWELL...Accordingly, ifeveryoneookpainsocorrectfaultsnareaswherehehasinfluence, lesseningof fatiguefailureswouldsurelyresult.

Takehecaseoffeatherededges. Justbecausedesignersdidnottakethetroubletocallforcornerradii,ndinspectorsdidnotrejectpartshavingsharporners,repairswerenecessaryonseveralrecent-lybuiltairplanes. Whi letherepairsnthemselvesmaynothavebeencostly,heinterestat6percenton anidleairplanecosting$5,000,000willamounttoover$800 .00perday. A ddtothattherental

valueoffacilitiesforrepairandwagesofanidlecrew,andthedailycostisappalling.

A fleetof300militaryairplaneswasrecentlymodifiedtobringthemuptode-siredfatiguelife. Itcost3200manhoursperairplaneothetuneofmorethan$11,000,000forthejob. Downtimeamountedtoabout3monthsperairplane.1 . 8ALLFOR WANT OF FATIGUE

RESISTANCE, THE BATTLECOULDBE LOST

Inthecaseofamilitaryairplane,hecostcanbefailuretocompleteamission,whichinacriticalsituationwouldbempossibletomeasurentermsofdollarsandcents.

WM& Figure1.1."BeachMarks"dentifyProgressiveFatigueFailureSeeSection2.8)

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TIPSONFATIGUE 1.9 IFYOUCA NGETTHEMWHENatigue designsuperfluous andTHEY'RELITTLE, FEWostly. HereswhereheatiguePROBLEMSWILLGETBIGxperts should be consulted. Inother cases, theptional fatigueThemportanceofconsideringfatigueesign is so simple that little orindesigncannotbeoveremphasized.o costs involved. These areInmanycases, itmayseemhatainlyhe items discussedntheffortnecessaryforanadequatehis book.

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TIPSONFATIGUE

BASICPRINCIPLES

2.1 LET'SKEEPITSIMPLEThischapterwillpresentonlythoseprinciplesthatpracticingengineers,nspectors,andshoppersonnelcanapply. Indoingso,someofthemorebasicfundamentalsofcrystallinestructure,uchasslipplanes,dislocations,ndotherswillbeomitted.Thesearebeyondthescopeofthiswork.Whiletheprinciplesdiscussedareadequateforthepurposeintended,hereader isdirectedtoAppendixC,SuggestedFurtherReading,"formoredetailedtechnicalinformation.

Knowingthebehaviorofmetalsunderloadisvitaltotheunderstandingoffatigue.Everyday,ommonoccurrencesshowhowmetalsactwhenloaded. A thoughtfulcon-siderationoftheseexamplessprobablytheeasiestwaytosummarizesomeofthemorebasicprinciplesofmetalfatigue.

Haveyouevernoticedthatyoucanbreakawirequickerbyincreasingthebendangle?

2.2T'SNOTNLYWHATOUDO,IT'SHOWOUDOITANDHOWMANY TIMES2.2.1UNIDIRECTIONALVERSUSREVERSEBENDINGRepeatedbend-ingsafamiliarexampleoffatigue.

5CYCLES90-DEGREEREVERSEBENDING

Agalvanized14-gagewirehatbreaksnour oriveyclesf 90-degreeeverseending .

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+ 90 . willlastor5r6bendsoffrom o90degrees.

15 CYCLES0o90-DEGREE BENDING

Kbentonlyfrom5o90degrees,tmaylastfrom60to70cycles.

70CYCLES45-to90-DEGREEBENDING

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83,000POUNDS

Similarly,heifeofastructuremaydependmoreonhowtisoadedthanonhetotalnumberoftimestisoaded,ronthemaximumamountoftheloadsthemselves.

Thissbestshownbyloa xgannch-squarebarofaluminumalkyhatbreaksat83,000poundswhenloadedonce.

ONE-INCH-SQUAREBARBREAKSAT83,000POUNDS

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60,000OUNDStension)

AAA--)-e ycle60,000POUNDSAPPLIED25,000TIMESWILLBREAKTHEBAR

60,000POUNDS (tension)~H*1cycle

60,000POUNDS (compression)FatigueFailureIdentifiedby"BeachMark"AppearanceSeeSection2.8

However,floadsoffrom to60,000poundswereapplied,twouldlastabout25,000cycles.

Loadingsfrom60,000poundsensiono60,000poundscompression(commonlycalled"plusominus60,000pounds")wouldfailhebarinabout4,000cycles.

REVERSING THE 6 0 , 000-POUND LOADSBREAKS THE BAR I N 4 , 0 0 0 CYCLES

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C/3 Q I D o0.o

1,000 L0.00000,000CYCLES TOFAILURE 1,000,000Figure2.1 LoadVersusFatigueLifeforOne-Inch-SquareBarof7075-T6AluminumAlloy-Similarrelationshipsbetweenre-peatedtensionandreversedloadsareplottednFigure2.1. Loadisshownon

theverticalaxisordinate)andhenumberofcyclesonthehorizontalaxisabscissa).Sohattheifetimescalecouldbecondensedtofitononepage,heabscissahasbeencompressedinwhatiscommonlyknownasaogarithmicscale. Notehateachmajordivisionrepresents0timeshevalueofthepreviousdivision. Schematicdiagramsofloadcyclesareshowntofacilitateread-ing.2.3 CONVERTINGLOADTOSTRESSThench-squarebarwasusedinthepre-cedingexampleforworeasons. First,squarenchisastandardunitofmeasure.Second,whenfatigueeffectsareunderstoodinermsofasquare-inchcross-section,t

iseasytocompareheoad-carryingabilityof-ourknownexamplewiththeload-carrying abilitiesofstructureshavingotherdimen-sions. Inotherwords,oadcarryingabilityishenexpressedinermsofpoundspersquarenchofcross-sectionalarea.

Commonlyexpressedintermsofloaddividedbycross-sectionalarea,heshort-handoralgebraicdescriptionforstresss

whereS =S = stressnpoundspersquareinchP =oadinpoundsA =ross-sectionalareansquarenches

730-7550-642 2-5


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TIPSONFATIGUE 2.4 FATIGUESHORTHANDSincefatigueifesnotonlydependentontheamountofstress,utalsoonhowhestresssapplied,asystemhasbeende-visedidentifyingtheypeofloading,hus

R = 'mm JmaxThisssimplytheratiooftheminimumstressdividedbythemaximumtress.Usingthisnotation,hecurveforrepeatedtensionloadinginFigure2.1wouldbeidentifiedasR=0,ecauseheminimumloadwaszeroandzerodividedbyanythingisstillzero.2.5 TENSIONANDCOMPRESSIONLOADINGAccordingtoconvention,ensionstressesarealwaysdentifiedasplus(+) and

compressionstressesareminus-). In Figure2.1hecurveforreversedloadingwouldhaveastressratioofJmm = -'max

sinceS oppositesignsmnisequalomax,exceptforhe

2.6

COMPRESSION

S-NCURVESFORSMOOTHSPECIMENSTopresentfatiguedatainbriefform,curvesofstressversusthenumberofcy- clestofailure,calledS-Ncurves)areused. SincethedimensionofthebarsnFigure2.1wasone-inchsquare,pplyingS=P/A,hecurvesshownarealsoS-NCurves. Frequently,wholefamilyofcurvessgiveninordertoshowlivesforotherratiosofstressR). Figure2.2 showsafamilyofcurves. AppendixC,"SuggestedFurtherReading,"containsreferencestoS-Ndataforothermaterials.CurvesfortypicalairplanestructuresaregivennAppendixA.

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80,0001-. 70,000a0,000C O

f0,000Ig0,000w0,000R

20,00010,000 1,000 10,000 100,000 1,000,000

CYCLESTOFAILUREFigure2.2 A WholeFamilyofCurves,ShowingLivesforVariousRatiosof

Stress,R.S P/A

2.7 STRESSCONCENTRATIONSInthecaseofairplanestructures,hefatiguebehaviorwillbesubstantiallydiffer-entfromhatobservednthesquare-inch bar,becausefreeflowofstresssnter-ruptedbyobstaclessuchasholes,notches,bumps,andchangesofsection. Pilingupofstressatobstaclessuchashesescommonlycalledaconcentrationofstress,andtheobstacleshemselvesareknownasstressraisers.2.7.1NFILLEDHOLE- Inthecasefhene-inchar, asmallholethroughtsenterwouldauseheactualtressthedgefheoletoebouthreeimeshatawayromtheole. S P/AFigure2.3. BarwithCentrallyDrilledHole

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Figure.4 PhotoelasticModels,howingStressatEdgeofHoleandatV NotchThetressdistributionsaroundtwodifferentconcentrationsarehowninFigure.4. TheeftpicturehowsthetressdistributionatheedgeofanunfilledholeandheightpictureshowswhathappensaroundaV-notch.Here,anexperimentaltressanalysistechnique,alledphotoelasticity,susedovisuallydemonstrateocationsofhighlystressedareas. Thisech-

niquemployspolarizedightandclearplasticmodelsnwhichstressedareasbecomeopaque. Thissanes-peciallyvaluableoolfordemonstrat-ingrelativemeritsofdesign. Amountofstresssdirectlyrelatedohenumberofopaqueinesandconcentra-tionsproportionaloheinepacing.

Asnjudgingheteepnessoftheer-rainbythecontourinesofaopo-graphicalmap,photoelasticpatternstellheteepnessofstress.2.7.2 WATCHUTFOR OPENHOLES- Moststructureshaveholes. Openholesaresuallyworsehanrivet-filledholes. Stressatanopenholeshreeimeshatawayfromhehole. Forhiseason,openholeshouldbeavoidednregionsofhighstress. Where locationholesareanabsolutenecessity, plughemwithrivets ifpossible. NeverplugholeswithweLd, ashisreateshighresiduallocked p)ensiletresses.

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VinylPlastici Polaroids&=ZZS S

Figure2.5. PhotoelasticModel,howingDistributionofLoadinFastenersofaClevisJoint

2.7.3 RIVETSANDBOLTS-Rivetedorboltedjointsalwaysconstituteproblems.Onereasonishattheoadintroducedbytherivetorboltincreaseshestressatpointsofconcentration.Thesecondreasonshatitisvirtuallyimpossibleodistributetheloadevenlybetweenrivetsorbolts,irre-spectiveofworkmanship. Thissbecause

thesecondandsuccessiverowsofrivetscannotcarrytheirshareofloadwithoutsomestretchinthesplicingmaterialbetweenhefirsttworows. Infact,hestretchinthesplicingmaterialshouldbegreaterhanthatofthematerialbeingsplicedatthispoint. NoteinFigure2.5 thatthefastenernearesttheoadhashehigheststress. Asolutiontothisproblem

wouldbeomakehisfastenerincapableof carryingsomuchoftheload. Unfortunate-ly,educingthesizeofthefastenerisnotalwaysasolution. Infactitsannvitationtoroublewiththefastenershemselves.Perhapsawiserchoicewouldbeoremovesomeofthesplicematerialsothatitwouldnotbeableooverloadthefirstfastenerorfasteners. Thehinnersplicematerialstretches,husallowingsomeoftheoadtobecarriedbythesecondowoffasteners.ThissllustratedinFigures2.6and2.7,whereedgeviewsofphotoelasticmodelsareshown. ThemodelinFigure2.6sverysimilartotheclevisjointshownnFigure2.5. Beingcutfromonepieceofmaterial,hemodelinFigure2.6clearlyshowshatagoodloaddistributioncannotbeattainedbyprovidingabetterfit.

Figure2.6. PhotoelasticModelofClevisJoint,howingEdgeViewofLoadDistributionBetweenFasteners2-9


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Figure2.7. ImprovedLoadDistributionbyScarfing2.7.4CARFINGFigure2.7showsandealjointwhereintheoadatthefirstfastenersrelievedbyscarfingthema-terial. Practically,hisshardtodo.Othermethodsofrelieving theloadatthefirstfastenerwillbediscussedunderjointsandjoininginChapter3. Inthemeantime,thiswouldbeoneofthecaseswheretheseniorengineerorfatiguespecialistshouldbeconsulted.2.7.5INGLEVERSUSDOUBLESHEARThephotoelasticmodelsofjointssofarhavebeenoftheclevisordoublesheartype. Unfortunately,erody-namicsmoothnessrequiresdesigningmanyairplanejointsnsingleshear,heworstexamplebeingthesimpleapjoint. Themajorfaultwithalapjointsthatthesheetsbeingjoinedtendtoalignwitheachother,ausingseverebendingstressesatthefirstfastener. ThissshownbythephotoelasticmodelnFigure2.8. Figure2.9. sanimprovedlapachievedthrough

scarfing. Lapjointswillbediscussedfur-therinChapter3.2.7.6 NOTCHES-Liketheopenhole,thenotchconstitutesoneofthemostharassingproblemsnfatigue. Figurative-lyspeaking,notchscomprisedofanykindofadiscontinuity,heholebeingoneofthemanytypes. Thus,tscommontospeakofthe"notcheffect"whenthenotchbeingconsideredsnrealityabump.

Accordingly,heermnotch"canbeappliedoholes,grooves,notches,bumps,tc. Perhapshemosttrouble-someofallnotchesshefilletradius.Machinistsseemoenjoymachiningneatcorners,nsteadofprovidinggenerousradiiattheunctionsoftwosurfaces. TheimportanceofprovidinggenerousradiisshowninFigure2.10,whereaphotoelasticmodelshowshowstressatafilletcanberelievedbyprovidingamoregenerousradius.

Figure2.8. SimpleLapJointNoteBending2-10


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Figure2.9. ScarfedLapJointA properadiussespeciallycriticalnmachinedpartswherenly alightchangeoulddeterminewhetherhepartwereatisfactoryornot. Mostmportant,owever,isomakeertainhatanadditionalnotchsotcreatedyhemachinedradiiailingomeethelatsurfacesmoothlyashownnhephotoelasticmodelnFigure2.11. Further

examplesftheffectsfradiion fatiguearegiveninChapter5.2.7.7 FRETT INGTheerosionoftwosurfacesrubbingagainsteachOtherisknownas"fretting." Thenotcheffectof thepittedsurfacestendstoexaggeratetheeffectofothernotchessothatthecombina-tionisasuperimposedstressconcentra-tion. Frettingiseasilyrecognizedby

Figure.0 PhotoelasticModeLsShowingEffectofFilletRadiionStress

Figure2.1 PhotoelasticViewofMachinedRadiiNotMeetingStresssndicatedbyNumberofDarkLines2-11


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TIPSONFATIGUE powderthatsiftsoutfrombetweenthesurfaces.

Sincefrettingscausedbyrub-bing,anythingthatwillreducetheamountofrubbingwillreducefretting. Equalizingtheamountofstretchbetweenmaterialbe-ingsplicedandsplicematerial,asllus-tratednthephotoelasticmodelsofscarfedjoints,willhelp. AdhesivebondingseeSection3.9)alsohelps. Lubricantscanbehelpfulnspecialcases. Consultthespe-cialistonthis.2.8 IDENTIFYINGFATIGUEFAILUREThoseofuswhohavehadthedubiouspleasureoflookingatthebrokenendofadriveshaftorarearautomobileaxle,canneverforgetwhatitlookedlike. W emayalsorememberhemechanicsayingthattheshaftwasoldandcrystallized,andthatwaswhytfailed.

Thisexplanation,owever,snotnecessarilyaccurate. Allmetalscrystalline. However, becausefatiguecrackspropagatethroughthecrystalsnsteadofaroundthemasnthecaseoftheone-timeoading,rstaticfailureinaductilematerial)outlinesofthecrystalscomentoclearview.

Fatiguecrackspropagateatvariousrates,ependingonthematerialandloading. Thestressconcentrationattheendofacrack,eingextremelyhigh,causesthematerialtofatiguelocallysothatthecrackcontinuesuntilenoughfreshmaterialnotyetfatigued)sengagedtoresistloadingforanotherinterval. Thisgivesrisetothe"beach"markappearanceoffatiguefailedpartsasshowninFigure1.1. Sometimescalled"tide"marks,theyareusefulforlocatingoriginsoffatiguefailure. Thebeachmarkseftbyhighloadingareusuallyspacedfartherapartthanthosecausedbylowoading.Ineithercase,heoriginisusuallyatapointofstressconcentrationornucleus,andthebeachmarkspropagatencircularpatternswiththenucleusashecenter.Usually,hemarksneartheoriginareobliteratedbyrubbingoffracturedsur-facesagainsteachother. ThusnFigure1.1,heoriginswereprobablyatlowercornersofthehole;however,eachmarksdonotappearuntilsomedistanceaway. Afterfatiguecrackinghadpro-gressedtothelastbeachmarkattheright,herewasnsufficientremainingareatocarrytheload. Staticfailurefinallyresultedasndicatedbytheroughsurfaceattheright.

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TIPSONFATIGUE

3 JO INTSANDJO INING

3.1OO MUCHSTRESS IN THE WRONGPLACES Therewasnofatigueprobleminaircraftduringtheerawhentheyweremadeof wood. Itwasonlywiththeadventofall-metalairplanes,ndinparticularof high-strengthmetalairplanes,hatfatiguebecameaproblem. Why? Toomuchstressinthewrongplaces!

Thereasonwehaveoo muchstressinhewrongplacesshatwehavehrownawaythesimpleapproachusedingluingwood,andinsteadweresorttotheobviousboilerplateconstruction. Now,boilerplateconstructionisfinewhenusedon boilers,utwedon't haveobuildair-planesikehat.W oodenairplaneswerebuiltwithcabinetmakertechniques,ndthecabinet-makertriedtojoinhisstructureinsuchamannerthatthejoint wasnotapparentto theeye-nortohestress. Whenthesameechniquewasusedon woodenair-planes,hestressflowedfromonepiece

toanotherasftheywereone. Maybeitwasuck,buttheresultwasacontinuityofstressflow.3.2HYHAVE JO INTS?Joining,obeginwith,s techniqueusedonlywhenthestructurecannotbebuiltin

on epiece. Ideally,heloadisevenlydistributedthroughoutthestructuretoaffordacontinuityofstressflow. Accord-ingly,hemorenearlythejoiningresemblesasinglepiecenmisrespect,hebetterthejoint. Thecabinetmaker 's long-scarfjointverynearlysatisfiedthiscondition.3.3 TYPESO FJO INTS Themostobviouswayto jointwosheetsofmaterialtogetherisoaptheedgesof onepieceovertheotherandfastenthem

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TIPSONFATIGUE withsomedevice. Historiansellushatman'sfirstattemptofthissortwaspro-bablyfasteningtwopiecesofanimalskintogetherwithafishbone. Theresultwouldbeknowntodayasaapjoint.

4^LAP JOINT

DOUBLESHEARBUTTOINTAnotherypeofbuttjointholdsthetwopiecesofmaterialwithtwospliceplates. Thisscalledadoubleshearbuttjoint. Beingsymmetrical,thasafatigue

strengthsuperiortohatofeithertheapjointorsingleshearbuttjoint. Thedoubleshearbuttjointispreferablewherevercostandaerodynamicspermit.A neaterwayisobuttthewosheetsedge-to-edgeandfasteneachtoathirdsheet(spliceplate)underneath.Thisgivesasmoothsurfaceononesidethatisnotonlypleasing,utalsosaero-dynamicallysuperiorifthesheethappenstobeheoutsideskinofanairplane. Thebuttjointwithasinglespliceplateordou-blersknownasasingleshearbuttjoint.

* = f * = * SINGLESHEAR BUTTJOINT

LUG

A fourthtypeofjointishesimplelug. Itusuallyconsistsofaclevisandasinglefittingthatispinnedbetweentheclevisbyasingleboltorotherfastener.Thistypesgenerallyusedformovingpartswherebushingsorbearingsareusedforesseningfriction.3-2


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TIPSONFATIGUE Manyvariationsofth eabovejointscouldbementioned;however,heirproblemsandsolutionsaresimilar.

Severalexceptions,suchashooksandpianohinges,needaspecialist'sattention.

3.4HAT 'STHE MATTERWITH JO INTS?

CLAMPS?-N otsobadifbendingisawayfromrivets

3.4.1 LAPJ O INTS -Whilethelapjointisthesimplestofalljoints,tsmainproblemismat,whenthetw osheetsof materialarejoined,heytendtoalignthemselveswitheachother. Thiscausesthesheettobebentatthefirstfastener,whichisalreadysufferingfromtoomuchloadseeFigure2.8). Thisoffsetinalign-mentiscommonlycalledeccentricity.

Thelogicalsolutionwouldbetoletthesheetbend,asongastdidn'tbendrightwheretheoadwasgreatest.Ideally,oucouldclampthesheetto maketbendatsomeotherpoint;hiswouldseparatethebendingstressfromtheload-carryingshearstress. A tricksuchasthissfrequentlycalled"confus-in gthestress"or"stressconfuser." S ee Chapter6orotherstressconfusers.

LAPJOINT-heetbendsightwherethurtsmost THFSSONFUSER

STRESSCONFUSER3-3


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TIPSONFATIGUE WhileusingaC-clampwouldbehighlympracticalnanairplane,hesame

resultscanbeobtainedbydrivingextrarivetshroughtheedgesofdoublers.Infatiguetests,anordinaryapjointwithtwocountersunkrivetsasnA

sameproblemashelapjoint. Oneofitsadvantagessthatthedoublercanbemadethickerthanthematerialbeingspliced.Thisreducestheeffectsofbending,utitcreatesanadditionalproblem: herivetnearestthedoubler'sedgenowcarriesmostoftheloadjustasnthecaseoftheclevisjointshowninFigure2.5.ZBZB2ZBB02.

A=55,000Cycles>>->>^>.>.>.-*VVVN--V^

1WMML s sB=248,000CyclesSameA sA "ExceptExtra

RivetsAreDrivenThroughEdgesO fSplicePlates.

Tovisualizethis,stakeheendofawiderubberbandtoaboardwiththreethumbtacksandpull. NoteheamountofdeformationrequiredattheNo. fastenerbeforeheNo. startsocarryheoad.Thiswouldindicatethatsomeprovisionmustbemadeforthespliceplatetostretchfailedafter55,000cyclesofrepeatedload-ing. Bydrivingextrarivetsthroughdou-bleredgesasnB,heatigueifewasraisedto248,000cycles. Thesecondjointlastedlongerbecauseheedge-drivenrivetcouldtakenoloadotherhanthatcausedbysheetbending,huspassingtheshearoadontohenextrivet,whichwashusre-lievedofthebendingload.

ThumbTacks Rubber

Wood3.4.2 SINGLESHEARBUTTJOINTS-Thesingleshearbuttjointisreallytwolapjointsacingeachother,oithasthe

EXPERIMENT --howingoaddivisionbetweenfasteners

i: n HEAVYSPLICEPLATE-Reducesbending, butmakesivetcarryoomuchload

iftheNo. andNo. fastenersareocarryheirfairshareoftheload.Somedegreeofdeformationcanbeachievedbyhinningthedoublermaterialbetweenthefirsttworowsoffastenerssothesecondrowcancarry

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TIPSONFATIGUE

Very thin atfastenerNo. 1 Use exrafastenersif necessary tocarry static load

Bending stressis relievedat firstfastenerand also someof the load

TAPEREDSPLICEPLATE

someoftheoad. Sincedoublermaterialmuststretchin ordertodothis,hethicknessatthefirstfastenershouldbelessthanhalfthatofthematerialbeingspliced.Thevalueofone-halfisarbi-trary;hepointbeingthat,withthisthickness,heN o.fastenerwon'tover-loadthesplicedmaterial. Highloads

wouldcausedoublermaterialattheN o.fastenertoyieldinbearing--whichisgood,ftheremainingfastenerscancarrythedesignload. Thus,orstaticstrengthitmightbeagoodpolicynottorelynthefirstrowof rivetsbut,nsteadtoprovideextrafastenersforthejob.

Theoretically,hedoublershouldtapertoalmostnothingsothattheN o.fastenercarriesninfinitesimalpartof theloadsomethingon theorderofthecabinetmaker'sscarfedwoodjoint. Gen-erally,heextramachiningisimpractical,sometimes,however,heweightsavingdoesmakeitworthwhile. Almostasgoodasthethick,aperedspliceplateisthethinauxiliarydoublernexttothematerialbeingspliced. Theauxiliarydoublershouldbelongenoughtoengageanextrarowofrivetsoutsidethemainsplicearea.Hereagain,hereisacompromisebetweenthepracticalandtheoreticaloptimumthicknessofauxiliarydoublers.

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TIPSONFATIGUE

AUXILIARYDOUBLER

TWOAUXILIARYDOUBLERS

Forsplicesnaluminumalloy,analuminumalloyauxiliarydoubleraboutonethirdashickashesplicedmaterialisaboutright. Wherewoauxiliarydoublersareused,heonenearestthebuttshouldbeaboutonefifthashickasthematerialspliced.

doublesheartypealsohasheproblemofloaddistributionbetweenfasteners. (SeephotoelasticmodelsnFigures2.5and2.7). Scarfing,rprovidingauxiliarydoublersasforsingleshearjoints,willimprovefatigueife.

Auxiliarythindoublerswhenproperlyusedwillincreaseheifetimemorehantenimes.3.4.3 DOUBLESHEARBUTTJOINTSDoubleshearbuttjointsaresuperiortothoseofthesinglesheartype. Thissbecausethesymmetryofthedoublesheartypeeliminatesthebendingeffectsfoundinthesingleshear However,he

3.4.4 LUGSThelugisasimpleformofthedoubleshearjoint. Sincethejointhasbutonefastener,heproblemofloaddistributionbetweenfastenersdoesnotarise. Whetherornotthefastenerusu-allyabolt)itstightlyhasmadeasub-stantialdifferenceintestresults. Aoosebolttendstobendmoreandwillsome-timesfailinthemiddleofthetongue,rmalefitting. Italsointroducesanex-

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TIPSONFATIGUE

WIDE THINUG-asyonboltbutbadon fatiguetremelyhighbendingstressonthecornersof theclevis,orfemalefitting. Thicklugswithcloselyfittedboltsorbushingswillhavetwicethefatiguelifeoflugswithsloppyfits. A goodnterferenceitwillmprovehelifemanyimes. SeeChapter ormorenhis.

Becausefhebendingeffect,itsagooddeanototintnboltsize. Indicationsrehattwouldbehelpfulohaveheboltevenwicesstrongasheug. Thiseepsromoverloadingheornersfheug.

NARROWTHICKLUG--etterthanwide, thinlug, butbendsbolttoomuch;

Also,om won't haveoworryaboutthebolt.Shapeof thelug'scrosssectionisveryimportant. A widethinlug,whilerelievingboltbending,causesthestressattheedgeof theholetobemanytimestheaveragestressawayfromthehole. (Seeconcentrationfactorsin AppendixB).O nheotherhand, anarrow,thicklugrequiresaongbolt,whichbendsandoverloadshelugedges.

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TIPSONFATIGUE

Testshavehownthatthelug withcrosssections withratiosof A /B between 1 and3 wouldbebestwheretheboltshear-ingstrengthisequaltothestrengthofthelug.

3m U U

1,000,000

100,000

10,000 i

CalculatedifBoltDidn'tBend

Testforlughavingstaticstrengthequaltothatofbolt

StrongerboltspermitusinglargerA /B forbetterfatigueperformance

A /B

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TIPSONFATIGUE 3.5 FA ST ENER PACING Muchhasbeenwrittenaboutspacingfastenersnajoint,mostofitfromastaticstrengthpointofview. Accordingly,handbooksrecommendfastenersizesandspacingthatleaveasmuchoftheareaaspossibleinthesplicedmaterial. Forstaticstrength,henetcrosssectionalareaisusuallynolessthan75percentof theareaawayfromthesplice. (Netcrosssectionisthearearemainingafterremovalofmaterialforholes.) Thiswouldgiveafastenerspacingcommonlycalledpitch)ofaboutfourtimesthefas-tenerdiameter. Tofurtherenhancethestaticstrength,hefirstfastenernearesttheloadissometimesreducedinsize.

ThisRowofFastenershas5.7-DiameterSpacingo

Frequently,astenersarestag-gered. Jointsofthistypehaveneverproventobeanybetterthanthetandempattern. Staggering,however,sde-sirableforfuelsealingorotheruseswherejointsshouldnotleak.

TheseRowsHave4-DiameterSpacing

TANDEMPATTERN GoodStaticStrengthJointSTAGGERED PATTERN OK forleakprevention

730-7550-643 3-9


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TIPSONFATIGUE

Design practicesusedoroptimumtatic strengths seldomapplyofatigue. Again, inastenerspacing, theestpracticeortaticstrengthfall sarhortfgoodfatigueesign. Asas been shown,the stressatthedgefheirstfastenerholeshemostcriticalforfatigue. Accordingly, the bestfastenerpatternwoulde thatwhichwouldessenhetress here.

Inermsfheveragetressawayromheirstowffasteners,theatternanbeikenedo eriesoflugs. Thiswouldndicatehattheptimumpacingorfastenersnormaloheirectionfoadingwouldebout2.5iameters. In termsftatictrength, thiswouldamounto ointwhosetrengthwas60percentfhetructuraltrength

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TIPSO NFATIGUE Edgedistanceshouldalwaysbeconsideredalongwithfastenerspacing.Exceptwherethefastenersusedthroughtheedgeofadoublerforclampingpur-posesseeSection3.4.1),aminimumspacingoftwo-diametersfromthecenterofthefastenertotheedgeofthedoublerisrecommended.Thenumberofrowsofrivetssusuallygovernedbystaticstrengthrequirementsandavailablespace. Forfatigueresistance,tsdesirableousetwoormorerows. Neverone! Thiss

particularlyruewheresplicesaremadeoverribflanges. Often,onerowsusedontheexcusehattheflangestoonarrowtousewo. Youcanalwaysfindroomforanotherrowawayfromtheflange,oryoucanstaggerarow.

TnTWO ORMORE ROWS OFFASTENERS ARE AMUST

3NEVERUSE A SINGLEROW OF FASTENERS STAGGERED ROW3-11


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TIPSONFATIGUE 3.6 COUNTERSUNKFASTENERSCountersunkfastenersareusedforaerodynamicorhydrodynamicsmooth-ness. Fewothercaseswillustifytheuseofacountersunkfastener noteventhedoorsnthegalley. Wherematerialthicknesspermits,holesforfastenerheadsaremachined. Thesearecalled"machine countersunkfasteners." Wherehecourtersinkisprovidedforbydimplingarecessintheskin,heprocessscalled"dimplecountersinking."3.6.1 MAC HINEC OUNTERSINKING-Useofmachinecountersinkingisnotrecommended wherethematerialthicknesssesshan1.5timesthedepthofthehead. Theraggededgecausedbythecountersinktoolcreatesasuperimposedstressconcentrationthatnvariablyresultsnfatiguecracks.(See"FeatheredEdges,"Chapter )

BETTER

SharpEdgesPOOR

BEST

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DIMPLE COUNTERSINK OVERHEAVYSUBSTRUCTURE DIMPLECOUNTERSINKOVERLIGHTSUBSTRUCTURE

3.6.2 DIMPLECOUNTERSUNKFASTENERS--areobeusedwherever skin thickness doesnotpermitmachinecountersinking.Whenassembled overheavy sub-structure, partofthesubstructureismachine countersunktoacceptheprotrusion caused bydimplingheskin.

Whenassembledoverightsubstructure, theubstructuresdimpledalso.

Whilemostdimpledjointshaveasuperiorfatigueifeohosewithmachineountersink, extremecaresneededinformingdimplestopreventcrackingaroundheperi-pheryofthedimple. Thissusuallycausedyoosharpadimpleradiusor.bynot supplyingheproperamountofheatonforming.

Itisagoodpolicyouseamoregenerousendradiushanheminimumspecifiedinhandbooks.

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TIPSONFATIGUE

3.7 WELDINGBookshavebeenwrittenonweldingandweldingprocesses. N oattemptwillbemadehereogontothemanyramifica-tionsofwelding: theadviceofaspecialistshouldbesoughtwhendesigningweldedjoints. However,afewpertinentfactsre-gardingeffectsofweldingonfatiguearenorder.

Tobeginwith,tisnotnthecardsforaweldtodevelopfatiguestrengthequaltothatoftheparentmaterial. Manyvendorsofspecialequipmentandmaterialswillmakesuchclaims;however,heirclaimsarebasedonlaboratorytestspeci-mens,atherthanonfull-scalestructures.Weldsnspecimensdon'thaveasmanyde-fectsasthosenthefield. Defectssuchasvoids,laginclusions,nddirtaregoodplacesforfatiguetostart.

Sincefatiguefailuresalwaysstartatstressconcentrations,weldingshouldbedonenamannerthataffordstheeastcon-centration. Figure3.1showsacompari-sonofvariousweldconfigurationsforstaticstrength,atiguestrength,ndcost.Forpurposesofcomparison, buttweldhavingcompletepenetrationandaflushsurfacesconsideredas100percentnotnecessarily100percentofthestrengthforparentmaterial). Photoelasticmodelsn-dicatestateofstressatthewelds.

ComparisonsofweldsnstainlesssteelarepresentedinFigure3.2. Itwillbenotedthatthebuttweldhasverynearlythesamefatiguestrengthasthatofthebasemetal. Aspreviouslystated,autionshouldbeexercisednusingdatafromsmallspecimentestsforestimatingstrengthoffabricatedstructures. How-ever,hedataservetollustratepossibili-tiesofgoodwelding.

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TIPSONFATIGUE

Figure.1 "WeldedJointsComparedforStaticStrength, FatigueStrengthandCost.

g 100w a > 80

tokCO hH sST 40 Bo

60

H 0

BASE.METALUTTWELDFILLET W LLD

SPOTWELD^ ._

10000000,00000,000CYCLESTOFAILUREFigure3.2 Fatigue StrengthComparisonsfWeldsnStainlessSteel3-15


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TIPSONFATIGUE

HEAVYANDSFORATTACHMENTS\

EROUSRADIUS

SUBSTRUCTURE

3.8CULPTUREDTRUCTUREInkeepingwithhedealhataperfectstructurewouLdbebuiltfonepiece,sculpturedstructuresmachinedfrom onepiecefmaterial)haveomentouse.

effectofconcentratingthestress. Atthetimeofthepresentwriting,ogoodap-proximationsareavailablefortherightthicknessoflandmaterialneeded. Inallcases,eedforextremelygenerousfilletradiiandgoodworkmanshipsparamount.3.9 ADHESIVEBONDING

Theseculpturedpartsareputtogethernerector-like fashion, extrathickmaterialbeingprovidedatattach-mentpointsonegateheeffectofholes.Fortaticonsiderations, a25percentincreasenthicknessmightbeatisfact-ory, butforfatigueonsiderations,theamountfbuildupland)ependsnthetressaiser. Forheimplehole, abuildupofthreeimeshenom-inalskinthicknessmght beatisfactory;thenagain, itmightnot. Theeasonsthathehickandbuildspamisalign-mentatheenterfhematerialandtheesultingeccentricityaccentuateshe

Finally,gettingbacktoourcabinet-maker'sechnique,adhesivebondingformetalshasbecomequitecommon. Whenusedwithcabinetmakertechnique (scarfedverythinattheedges),bondedjointshavesuperiorfatiguestrength. Onereasonitisnotmorewidelyusedishatitisdifficulttonspectfordefects. Also,mostad-hesivesdeterioratewithtimeandenviron-mentalexposure. Asaresult,acombina-tionofbondingandmechanicalfasteningisoftenusednprimarystructures,ortoflikewearingbeltandsuspenders. Insuchcases,however,hebondingsusedforfuelsealing,vibrationdamping,orotherspecialpurposes.

Mostadhesivesecomebrittlewhencoldandscorchwhenhot.Consulthepecialistonthis.

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TIPSONFATIGUE

DEVELOP ING AN INTUITION FO RFAT IGUE

4 . 1 SEAT OF THE PANTS TECHNIQUEW ehavefrequentlyheardtheterm"flyingbytheseatofthepants." Thishasmoremeritthansomeoftheinstrumentspecial-istswouldcaretoadmit. CertainlythenumberofG'sowhichtheairplaneisexposedwouldlargelybeafunctionofthecomfortordistressexperiencedbythepilot. Yetthefeelfortheamountoftrimrequiredforspecialsituationssargely

aresultofpracticeandintuition. Simi-larly,on ecandevelopanintuitionforfatigueresistance.4.2 NATURE ' SWAYO FDES IGNINGBoilerplateconstructionisman-conceived.You neverseetreefrondsconnecteddirectlytothehugetrunkofatree;nordoyouseebranchesattachedtothetrunk withhugegobsofextrawood. Yetman,nsplicing

NATURE DOESN'T DOHIS NOR THIS4-1


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TIPSONFATIGUE twopiecesfmaterial, oftenusesadoublerheamethicknessrhickerthanhematerialbeingpliced. Con-sequently,mostofheoadisumpedonhefirstrowoffastenersnthesplice.

Intuitively, wehouldknowthatheecondrowoffastenersan-notbeginocarryoadwithoutome"give"athefirstrow.Naturally, wedon'twanthegive"ooccurnhestructurethatwearetryingoprotect.Intuitionhouldtellushatheoublermaterialbetweenhefirstworowsffastenershouldbehinneddownoitwillstretchwithoutoverloadinghefirstrowoffasteners. SeeChapterformoredetail.

Similaroheheavyplicedoubler, thehugegobofmaterialthatismachinedawayoalmostnothingnspotshouldrubourintuitionresponses

thewrongway. Sometimeshemallfingerlikeextensionsaremadeforattachmentpurposes. Thereisnothingwrongwiththis. What'swrongishatthefingeriikeextensionendsabruptlyinagobofmaterialwheretressan-nothelpbutconcentrate.

4.3 SHARPCORNERSSharpcorners,eithernternalorexternal,haveproducedmorefatiguefailuresnmetallicstructureshananyotheronecommonfault. Theresnothingaesthetic

aboutasharpcorner. Infact,nywork-manwhotakesprideinhiswork.takesspecialpainsnsmoothingoffsharpedgesandcorners. Examplesofwhathappenswhensharpcornersareeftun-finishedaregiveninthenextchapter.4-2


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TIPSONFATIGUE

4.4 DON'TFORCE ITA biggerhammersrarelyheanswer. Intuitionhouldellusthatwhensomethinghaso eforced, everythingsnotwell.A nxamplefhissryingofitaheavyugnto levishatisoomall. Thisaultsusuallyobvious. Justasad, butnotoobvious,sheasewhereheclevisaseenprungybolttighteningbecausehepacerbush-ingwasoohort.

4.5 ROUGHFINISH Similaroheharpornerssheroughinish. Likehe cheapautomobilepaintobpecial, theroughfinishreflectssloppyworkman-ship. Intuitionhouldellushatanexcessivelyoughmachineobsnotright. Whereurntuitionmayneedfurtherrainingsnellingusow muchoughnesssolerable. Whi lesawtoothinishesrenevertolerableanywhereseeChapter),weshouldsensehatongtraightmachinedsurfaceswouldnothaveo eosmoothasilletsurfaces. A goodrulefhumbsousexceptionalcaretchangesnectionsometimesevenohepoint fhandinishing.

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TIPSONFATIGUE 4.6 POWEROFKNOWINGIntuitionas been defined as the giftof knowing without recourse to infer-ence or reasoning, but it takes alotofobservationandogicoethatgift.

Previoushaptershavepre-sentedsomeofhepureeasonofwhatfatigues andhowoavoidt. Thenextchapterwillpresentsomeexamplesoffatigue.Whiletshopedhathesuggestionspresented herein will

prepareheeader to better copewithwhatever situation arises, thereader will have to acquire his ownexperience.As the father who tries toprepare his sonor the pitfalls inlife --none seem tobeike thenesdescribed.Experiencewith similarsituations, however, shouldelp.The important thing to rememberis not the individual problem, butthe logic usedn its solution. Later,

you'll get to the point that you auto-matically knowwhat's right-- that'sintuition.

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TIPSONFATIGUE

5 PAST EXPERIENCE

5 . 1 STRUCTURAL BLUNDERS ANDHOW TO AVOID THEM

A stheParsonsaid,Everybodyfeelswhat'sright,utdon'talwaysdoright."Thefeelingforwhat'srightcanbestimu-latedbyobjectlessonsof what'swrong.

That'swhyheParsonpreachedfireandbrimstoneonSunday-- andthat'swhyhishaptersdevotedothosewrongshatareknownysuchtermsasblunders, butches, botches,

boo-boos, genfs, mistakes, andwhathave ou Whenweseetheconsequencesof wrong,wearedriventodowhat'sright.Takeblunders,orexample. Thereare

designblunders,hopblunders,ndthathorriblegroupofatrocitiesthatarecon-ceivedasdesignblundersandperpetuatedasshopblunders. Thedesignerwhosays"Idon'tneedtospecifydetails,heshopmancantakecareofthat"wouldbehorri-fiedtoheartheshopmansay"Ifthefooldesigneditthatway,'llbuilditthatway.5-1


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TIPSONFATIGUE

Thephotographsoffatiguefailurescollectedin thischapterhavebeenfurn-ishedbyvariousmanufacturersandair-planeoperatorswhohopethatseeingwillmakebelieversofdesigners,nspectors,andshopmen.

Examplesofdesignbutchesstartof fwithnotchesinvariousforms,includingdesignswithnoradii,hortfilletradii,ndsharpbendradii. Specialcasesofnotchesincludesquareholesandfeatherededges. Featherededgesmaymoreogicallybeblamedontheshop,sincemoststandardshopproceduresin-cluderoundingoffallcorners. Never-theless,wheresharpedgesareantici-pated,pecialpainsshouldbetakentoseethatthedrawingclearlyspellsoutroundingof fallcorners. Thisisespe-ciallycriticalwherecarelesshandlingmightresultinnickingsharpexposededges. Poorloaddistributionisanotherdesignbotch,ndcloselyrelatedtothisisthecaseofsuperimposedstresscon-centrationinitsvariousforms.

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TIPSONFATIGUE

Shopboo-boosnvolvepoorcraftmanship,ailuretorecognizedesigngoofs,ndfailuretoalertengineeringwhenadesignorshopmistakeisdis-covered. Especiallycriticalaredingsresultingfromcarelesshandlingpractices.Whileshortrivetorboltedgedistancescanusuallybeblamedontheshop,re-quentlyengineeringdrawingsdonotmakeallowanceforsufficientspacetoinstallthenumberofrivetsorboltsrequired.Nomatterwho wasatfault,goodcraftsmanwouldneverleaveinsu-

fficientedgedistanceormismatchedmachinedsurfaces,veniftheblueprintcalledforthem. Evenapoorcraftsmanwouldblushifhewereshownthefailurethatresultedbecausehehadinstalledaboltofthewronglength.

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TIPSONFATIGUE

5.2OTCHES 5.2.1 SHARP NOTCH-Theveryexistenceofthesharpnotchisaresultof stupidity,arelessness,ndunwisepenny-pinching.Themanufacturermayaimatsavingapenny,uttheuserwillalwayspayheavily,ndoftenthemanufacturer

willpaylaterintheformofwarranties.Inanyevent,machinetimesavedispeanutswhencomparedwithoverallstruc-turalintegritycosts and,nmanycases,usingmachinecutterswithproperedgeradiiwillavoidthesharpnotchesatnoextracost.Thousandsuponthousandsof fatiguefailuressimilartothosehownon theoppositepagecouldhavebeenavoided

bygooddesignan dcraftsmanship.everytoolsalesmanwiththeneed,machinetoolmakerswouldsoondesigncutterssothatsuchcrimeswouldbeimpossibletocommit. Thesharpradius,swellasthemeansformakingit,houldbeoutlawedthewaytheyoutlawedgu nslingers. Toolsformakingsharpradiishouldbekeptunderlockandkeyforuseon nonstructuralparts,uchasashtrays.

Similarfailurescanalsooccurin wellfilletedpartswherethefillethasoolmarksorscratches.

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5.2.2 DINGS- Dingscanbententionaloraccidental. Uppereftshowsheatiguefailureresultingfromnaccidentalblowbyabluntobject. Lowereftshowsfailureresultingfromdentificationmarks,ndupperrightshowshowmarkseftbyarivetbuckingbarcauseawingsparocrackintheradius.

Failureatlowerrightissimilartoupperright,xcepttheoolwasanimpactscrewdriverusedforremovingthewingfuelcellcoverattachmentscrews.Precautionsshouldbeakennhandlingpartsoavoiddentingandnicking.

Shoulddingssuchasshownbediscovered,theyshouldbecalledoheattentionoftheforeman andfatiguespecialist. Thesecanfrequentlybecorrected.

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O o.

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5.2.3 FILLETRADIUS-Thinkbigwhentcomesofilletradii. Radiusoftheugntheuppereftphotowouldseemample,utitonlyhadafifthofitsex-pectedlife. Asimilarugntheupperrightphotohasaradiusthatwasncreasedbyremovingsomeofthematerial. Thislugwiththeargerradiushadafatiguelifeeighttimesgreater.

Thelowerphotoshowsatiguefail-urehroughafilletthatwouldhaveseemedtobeOK ,utthefactthatthepartfailedinservicendicatesalargerradiusshouldhavebeenused,speciallysincetherewasnoproblemofclearancenhisarea.

A goodpolicyistouseasargearadiusasspaceandpracticalitypermit,especiallyatchangesofsectionthicknesses,

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OriginalLife N Cycles RedesignedLife NCycles

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TIPSONFATIGUE ^5.2.4 SHARPBENDSORNUISANCEFAILURES--Theclipnthesketchcostsabout 0cents,ut,uriedinhema^eshownnhephotograph,took32man-hoursoreplace Whilefailuresofclipssuchashismayneverconstitutemorethananuisance,hereplacementcostsappalling.

Theproblemhereshatsomeonethoughtthesharpbendintheclipradiuslookedcleanerandwaswithintheallowablebendradiusforhematerial. Unfortu-nately,pecificationsforbendradiiwerebasedontheabilitytoformwithoutcrack-ingatthetimeofbending. Littlehought,ifany,wasgiventofatigueperformance.

Useabendradiushatisasgen-erousaspossiblewithoutdestroyingfunctionalperformance. Insomecases,usingastainlesssteelclipthatisonegagethinnerthantheoriginalaluminumclipwouldbeinorder. Consultthespecialistonhis.

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5.2.5 SQUARE HOLES-Theresusuallyapurposenmakingsquareholes. Some-timesholesaremadesquareforopur-poseatall.

Theholentheupperphotographwasmadeoaccommodatearectangularduct. Solutiontotheproblemwasomakeanellipticalholeencompassingtheoriginalrectangularhole. Fatigueifewasivetimeshatwiththerectangularhole.Theholeshownntheowerphoto-graphwasmadefornopurposeotherthantosaveesshantwoouncesofweight. Itcouldhavebeenround,betteryet,orgottenaltogether. Thepartfaileddespiteheseeminglygenerouscornerradii.

"RectangularDuct"

*ssga

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5 . 3 FEATHERED EDGE AND SHARPCORNERS

N oonenhisrightmindplansohavesharpcornersorfeatherededges. Theyusuallyoccurbecausehedesignerdidn'tpayattentiontowhatwouldhappenwhenhistwo-orthree-drawingviewswerente-grated. Hewouldbesurprisedtoindrawedgesofthekindthatcausedservicefail-urentheseexamples.Besidesbeingastressraiser

initself,hefeatherededgeorsharpcornerseasilynicked. Thus,structureemployingsimilarpartsmayfailmostfrequentlyatthenickson featherededges,andsomeonemightwronglynferthatfeatherededgeswereallright--twasonlythenickshatwerewrong.However,ftherewerenofeatherededges,herewouldprobably

beewernicks. A nickontopofafeatherededgeconstitutesasuperimposedstressconcentration. Whileeverycon-centrationdoesn'tcausefailure,whyhaveconcentrationshatcouldbesoeasilyavoided. Thetimecouldbetterbespentreducingtheeffectofconcen-trationsthatcan'tbeavoided.

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Don'tsendaboy-

5.3.1 THINFLANGES-Thisparthasfaultsshownintheprecedingexamplebecausethespotfacesresultedinfeatherededges. Thesefeatherededgesmightultimatelyhavecausedafatiguefailure,butanadditionalaultthatof usingthinflangesocarryoadnbend-ingcausedthefailureevensooner.Thisdoesnotmeanthattsalwaysundesirableohaveareasalong

distancefromthecenterofbending. Theoppositestrue. TakethecaseoftheI-beam. Thelangesremotefromthecenterofbending;but,heresenoughbeeftomorethancompensatefortsdistancefromhecenterofbending.

Intheexampleshown,hehinflangeaddedittleothestrengthoftheforging. Thiscauseshestressattheoutermostflangefiberstobehigherhanitwouldhavebeenntheplatesurfacewithouttheflange. Whereashinflangesattachedtoheavymainstructurescansometimesbecondonedonthebasisofincreasedrigidity,hissdoneattheex-penseofncreasedstressandshouldnotbeattemptedwithouttheapprovalofhespecialist.

Thelangeonan-beamsanexampleofsupplyingmaterialwhereneeded- Justasmportantsheremov-alofmaterialwherenotneeded. Thus,removingsomeofthematerialprovidesagenerousfillet,asnSection5.2.3)andmprovesfatigueife. AnotherexampleoftheneedformaterialremovalwillbeshownnSection5.4

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THIN FLANGECRACK

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5.3.2 TROUBLESOME SPOTFACE -Spotfacingsacommoncauseoffatiguefailure. Whilenotquiteasbadasheexam-pleshownonthepreviouspagenthatnovulnerableoutstandingegswereeft thesharpcornercombinedwithafeatherededgesasurecauseoftrouble.

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i.4 BADOADDISTRIBUTION5.4.1 STRINGERFORGING-Thesestringersfailedbecauseherewasoomuchstressattheendfasteners. Sincethestressherescausedbyanumberof items,ncludingaxialoading,bending,andfretting,areductionnanyorallwouldimproveife.

Anattemptwasmadeoreducethestressherebyusingsmallerrivets.Asyouseeitdidn'thelp. Whileusingsmallerrivetsattheendsofsplicesscommonpracticeforstaticstrength,heiruseforfatiguesituationshasneverprovenhelpful.

Onesolutionishinningtheforg-ingseeSection3.4.2)orelievehebend-ingeffectsofthesingleshearattachment.Thisalsopreventsoverloadingthefirstfastenersandatthesameimereducesfrettingbyequalizingthestretchnstringerandforging.

Anothersolutionsoreplaceheendrivetswithnterferencefitfasteners(seeSection6.3). Thissespeciallyhelp-fulwhereaneasyfixiswantedtobringexistingstructuresuporequiredlife.Replacingendrivetswithsomewhatlargerdiametertaperedboltswillensureremov-ingfatiguedamagedmaterialprovidednocracksremainvisible)besidesprovidingabetter-than-newfatiguestrength.

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5.4.2 SPLICEThissplicecontainsthesameproblemasthedaggerfittingon thepreviouspage. Beingmorecompli-cated,heproblemisnotsoeasilydiag-nosed. Thus,heusuallyproposedsolutionisoincreasethethicknessofthematerialbeingspliced. Whilethismaysolvetheatigueproblem,heextraweightpenaltymaybeextreme.

Solutionforhisproblemshesameasorhestringerattachmentforging,xcepthattheoadsonallfirstlinerivetsmustberelieved. Acommonpracticestocutfinger-likeextensionstothesplicedoubleraswillbeshownonthenextpage. This,owever,snotsoeffectiveasaperingthedoubleror,betteryet,uttingthefingersandalsotaperingthedoubler.

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ste-.':''' '- J mm U-^i

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5.4.3 THECONTINUOUSMEMBER-Itwouldbefinetohaveallcontinuousmemberswithnonterveningsplices.Thiswouldendmanyofourfatigueprob-lems. Sincewedohavesplices,hedeaofmakingonlyapartialspliceatagivenlocationeadstotrouble.

Thisexamplewouldappearatfirsttohavebeencausedbyusingspliceplateshatweretoothick*,snthepre-viousexample. Theprimaryfailure,however,wasnotattheendrivetasthephotographwouldindicate. Thisfailurehappenedafterthecontinuousstringerbrokedirectlyoverhesplicearea,dumpingtsshareoftheoadontheskin.

Whathappenedwashattherewasoomuchgiventherivetedjoint,othattheskinwasunableocarrytsfairshareoftheoad,ausingthestringertofail. Theskinfailurewassecondary.

*Theyreallywereoothickattheendrivets. Thismightultimatelyhavecausedthefailureasshownhadnottheinitialfailurebeeninthestringer.

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5.4. HEAVYFORGING--Thesearetypicalsituationswhereeavyiecesarettachedoelativelyhinunderstructure, resultingnheailurefthestructureWhereverituationsikehese

arise, theolutionsoeeduthestresshroughuccessive layersfthinnermaterialrorovideradualtaperoheirstointfattachment.Ifhissotossible, perhapstheeavyieceaneplitnormalotheirectionftress sohenderstructureanave "BreathingRoom,"

Toowerhe stressluctuationesultingfrom theoletself, taperoltstpointsfttachmentwillelp.

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TIPSONFATIGUE 5.4.5 NOBACKUPSTRUCTURE-Oneoftheundamentalsofsciencesthatforeveryaction,herehastobeareaction.Here, huskycontrolbracketwasattachedtoaflimsysparwebstructurewithresultsasshown. Solutionstoprovidesupportstructurebackupstructure)havingstrengthequaltothatofthefitting. Thisdiffersfromtheexamplesshownn5.4.4,whichwereknowntohaveadequatesupportstructure.

Thissatypicalproblemwhereonepersondesignsacomponentwhilean-otherdesignsthesupportingstructure. In thisparticularcase,tsdoubtfulhatmuchthoughtwasgiventosupportingstruc-ture. Rather,hebracketwasattachedtoastructurethatwasdesignedforanotherpurpose.

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5.5UPERIMPOSEDSTRESSCONCENTRATION

Ashoughnotcheswerenotbadenough,theycanbemadeworsebysuperimposingoneuponanother. Thus,wehaveexam-plesoffeatherededgeshatalsoterminateatheavysectionsandfilletradiithatdon'tmatchtherestofthemachinedsurface.

5.5.1 HOLEPARTLYTHROUGH-Theopposite photoshowshowatiguecanstartataholethatwasnotdrilledallthewaythrough. Thesolutionssimple:finishdrillinghehole.

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-K v .- - ' c wJPI'./A.-^^^Sw" . *>^yH1 1,'."w -

* ~ 'v .'viv''cK ^ ' " -I *_**'^^^1'.* a-*?'-'*m > . . ' -*

I.."* 'i

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5.5.2 INTERSECTING HOLES-Heresacasewhereaholewasappedntohehighlystressedregionofanotherholentheensionflangeofawingbeam. Thisssimilarothecaseofagreasefittingholeforabearing.Ifyoumusthaveahole,hedeaisomovettotheeastdamagingposition.Aittleocalreinforcingbeefp)salso

helpful.

POOR

beefup

ALSO ACCEPTABLE

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f:,?:J'f?Pii

FORWARD

I1

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5.5.3 RADIUSATCHANGEOFSECTIONTherealwayshasobearadiusofsomesortatanychangeofsection. Likewise,herehasobearadiusatachangeofdirection. However,youdon'thavetomakeoneradiusrighton topoftheother. Theradiusforchangensectionshouldhavebeenmadeatanotherlocation. Ifthiswerempossible,othradiishouldhavebeenenlargedtopermitamoregentletransition.

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5. . ROUGHSURFACEFINISH-A finishsuchasthatillustratedherealsoconstitutesasuperimpositionofstress.Whereastressraiseralreadyexists,sinthiscase,tisoolishoetaroughsurfaceikethisgetby,speciallywithtoolmarksnormalothedirectionofloading.

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5. . ROSS GRAIN --hepartsshownfailedorworeasons, roughsurfaces, andhematerial's grainstructuresnormaloheirectionofLoading. Whiletmighthavebeenpossibleoavertfailurebymachiningasmoothsurface, thewrongdirectionofgrainmakesuchaolutionhighLyspeculative.

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5.6UXILIARYATTACHMENTSThepartshownwasfromaandinggearassembly. Failureoccurredthrougharivetholeusedforattachingaschafingshield. Similarthingshappentoframeshavingattachmentscrewsosupporthy- draulicines,upholstery,orwhathaveyou? Itwouldhavebeenbettertotiehepartonwithropealsoryadhesivebond-ing)hantotakechancesonfatiguingas

shown.

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5.7 HARDLATINGHeresatypicalexampleofwhereapartwaschromeplatedtomaketmorewearresistant. Itwasn'tmoreatigueresistant.Thecracksntheplatingactasstressraisershateventuallyfailtheparttheplatingssupposedtoprotect.

Shotpeeningprioroplatingsacommoninhibitoroffatiguecrackingnchromeplatedparts. Itsunwiseochromeplatepaintsfordimensionalbuilduporwearresistancewithoutthehelpofthespecialist.

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5. SHOPBLUNDERSWhilemostoftheexamplespreviouslyshowncanbeblameddirectlyondesign,anumbercouldequallywellhavebeencausedbyshopblunders. Thus,wehavetheholethatwasnottappedallthewaythroughandtheroughsurfacehatwasnotsmoothed. Theollowingexamplescanbeblamedalmostentirelyonshoppractices.5.8.1 NOEDGEDISTANCE-Theengineeringdrawingmaynothavespeci-fiedtheexactlocationsofholesfornutplates;however,tandardshoppracticesshouldbesuchthatthiswouldneverhappen. Asshown,herewasnsufficientroomfornutplatestobespacedinsuchamannerthatholeswouldfallbetweennutplates. Also,otethattheedgesurfacefinishwasnothingtobragabout.

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TIPSONFATIGUE 5.8.2 MISMATCH-- Evenxcellentmachinistsftenmachine urvedsurfacehatdoesn'tmeettstraightcounterpart, leavingwhatamountsoauperimposedtressaiser. Whileitsotoadwherehewourfacesareonvex, theoncavenessuallyresultnailuresndicatednheillustration.

Manyrawingoommanualsspecifyiemaximumllowablemis-match. Itoappensnotnhesecases)hatatigueailuresaveresultedwherehemismatchwaswithinolerance. Carehouldeexercisednermittingmismatchesinriticalareas- evenwithinspecifiedolerances. Whiletwouldeirtuallympossibleodefinehe.amountfmismatchhatcaneermittednveryase, arulefhumbsosextremecarewithoncaveurfaces.

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5.8.3 EXCESSIVECLAMPING-Theboltonhispartwastightenedwithouthavingtheproperspacerbushing. Fatiguefailurefinallysetn,asmightbeexpected. MakesureyouhaveherightengthbushingandtherightengthboltandTHINKTWICEBEFORETIGHTENINGsee4.4).

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i.9

ORTGTNALFITTINGSA .RONGGRAINDIRECTIONB.POTFACESC.NADEQUATEFILLETRADIUS D.HARPCORNERSE.EATHEREDEDGES F.OUGHSURFACEG.TS=240-250ksl

MURPHY'S LAW Murphy'sLawstateshatftwerepossi-bleobotchupajob,omeonewillsurelyfindawayodot. DON'TUNDERESTI-MATEGROUPEFFORT Asshownntheuppereftphotograph,hecombinedeffortsofengineeringandshopverynearlysuc-ceededndoingeverythingwrong.

Botchesnthisonepartncludesharpedges,badspotfaces,mallfilletradii,oughsurface,crossgrain,andothers. Inaddition,hereweresignsofhydrogenembrittlementdueocyanide-bathcadmiumplating. Hydrogenembrit-tlementsatermusedforowductilitycausedbyabsorptionoftoomuchhydrogenduringprocessing. Thesenicebigwords

REDESIGNEDFITTINGH.NLARGEDFILLETRADIUS I.OUND EDCORNERSJ.MOOTHSURFACE IN ISH K .O SPOTFACESL.TS=210-220ksi

soundauthoritativewhenusedtoexplainfailuresforwhichnorealreasonotherthanpoordesignorworkmanship,whichwehateoadmit)sapparent.ecarburi-zationanothermouthfulhatmeansossofcarbondueopoorprocessing)wasalsoapparenttoaminorextent.Properprocessingwasnsuffi-cienttobringthepartuptorequiredife,sotwasnecessarytoperformamajoroverhaul. Thisncludedprovidingabettersurfaceinish,removingsharpedges,andprovidingmoregenerousfilletradii. Thereworkedpartsshownntheupperrightphotograph. Testsonsimilarpartsre-vealedalifeofapproximatelyfourtimesthatsustainedbyoriginalpartswithnon-

creasenweight.

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MAKING THE MOST OF A BADITUATION

6. BADSITUATIONS6. . THECASE OF THE

FLAT TIRE- Haveouever had fLat tire while drivingalongthecountrysideanddiscoveredthatyoursparewasalsoflat?...opumporpatchingmaterial... oyoufinallydecidedtodrivetflat. Thenherewastheokerwhopassedyouandyelled,"Don'tyouknowyougotaflattire?" Andyoufeltlikestickingabigsignonyourrearbumpersaying,Iknowit'sflat owhat?"

Ifyouhavehadsuchanexperi-ence,henyoucanmaginewhatit'sikeohaveaservicefailurenanareawheretherejustisn'tenoughroomorreplace-mentwithahuskierpart. Maybeyouarealreadyusingmaterialasstrongasyoudare.Whatnow? Shopisstillturningoutpartslikehosethatbroke)bythebarrel-

full,ndyou'refacedwiththeneedforaquickdecision. Youhavehreepossibledecisions: (1 )youcandonothingandhopethattherestofthepartswon'tbesobad;(2)youcantrytofixtupandhopehattheixsO K ;or,3)youcanfixafewsamplesandestthepartsoseefthefixsanygood.

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OardecisiononastructuralfixshouLdbebasedonhefactswe'veearnedsofar. The purpose ofhishapterisoarrangehesefactsnsuchamannerhatourdecisionsanbeeasier. Whatarehesefacts?

fatigueife. InChapter weoundwhathappenswhenwevioLateprinciplesofgooddesignandfabrication. As in vioLationofprinciplesfgoodhealth,theorrectivemeasuresmaybelow,painfuL, andbadasting.6.3 SIZINGP THE SITUATION

SomeofhebasicprincipleswerepresentednChapter Alsoshownwere methods for reducingstress atfilletsbyprovidingamoregenerousadius.

Theubjectofjointswasintroducedn Chapter3. O fpartic-ular importancewasheacthat asmall changenbasic design couldresuLt in a vast improvement in

Asfarasatiguesoncerned,remember that structure willneverfail exceptata stress concen-tration. Accordingly, Let'sworryaboutstressatheconcentrationandnever mindaboutwhatappens else-where, at leastotor the timebeing. Thisimplifiesurproblem.ThenexthingsovisuaLizewhatcanbedoneohisparticularstressomakehe mostofhe situation.

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W etartoutwith a stresslikehis zerostress

whichcouldpossiblybehifted totilLSo zerooragaintmightbehangedtoactLikethis...

orsomethingmightbeoneomaketbehaveikehis zeroF 21.6-3730- 7550-647


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TIPSONFATIGUE Whileheremaybeothers thesecaseswillsufficeostart. Supposeapartwereoadedsohatstressattheconcen-trationfluctuatesrom to60,000psi

(R 0). Suchapartcouldbeexpectedto 60,000J__J 90,000Cycles, StressRange 0,000psi, R 190,000cyclescurveforR -1)andmaxi-mumstress=30,000psi,igure2.2.

30,000Cycles, StressRange 60,000psi, R

lastforabout30,000cycles,ccordingtotheS-NcurveforR 0shownnFigure2.2.Wherehestressrangesdefinedashedifferencebetweenhemaximumandminimumtress,otalstressrangewouldbe60,000psi.

Thentroductionofacompressivestressattheconcentrationsoundsikeanicerickifyoucandot. Thatis,hecompressivestressshouldbeapermanentaffair ockedupsoitcan'tgetawayandshouldbenjustexactlytherightspot.Suchstressesarecommonlyknownasresidualstresses. Residualstressescaneitherbentensionorcompression.

Similarly,fthestresscouldbemadeobehaveashoughitwerecycling

6.3. MOVETHEWHOLESTRESSD OWN

Ifitwerepossibleodosomethingtothestructureocallysothatthestressattheconcentrationwouldbe30,000psincom-pression-30,000psi)whenthepartwasunloaded,heoriginalloadingshouldcausetheocalizedstressofluctuatebetween-30,000psiand +30,000psi 30,000psi).Thiswouldcorrespondtothesamestressrangeasbefore,uttheifenowwouldbe

200,000Cycles,StressRange 60,000psi, 2fromminus40,000psitoplus20,000psi,alifetimeof200,000cycleswouldresult(curveforR -2andmaximumstress20,000psi,igure2.2).

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TIPSONFATIGUE Whenaparthavingastressraisersoadedintensionsoasocausehematerialatheconcentra-tiontoyieldlocally,hepermanentlydeformedmaterialmustgontocom-pressionwhentheoadsemovedandthepartspringsback. Suchtemsashooksendthemselvesothisypeofcorrection. Generally,heamountofoverloadscriticalandshouldbespecifiedbythepecialist.A practicalmethodcalled"shotpeening"susedtontroduceresidualcompressivetressfora

longerife. Inthefirstnstance,acompressiveayerathenotch,amountingo0,000psi,willdohetrick. Intheecond,alayerof40,000psiwouldbeequired. Bothareeasilyachieved,tbeingcommonpracticeo

introduceesidualcompressivestressesashighaswohirdsofthematerial'scompressiveyield strength.Othermethodsofintroducingprotec-tivecompressivetressayersncludecontrolledmechanicalpeening,vaporblasting,urfaceolling,andaprocesscalled"coining." Surfaceollingsespeciallysuitableforcylindricalob-jectsuchasbolts. Figure.1 showshowthreadrollingwasusedtom-proveheatigueifeofbolts.Fatigueifeofapartcanbe

improvedbyprovidingabetterfinish.Thissparticularlytruewhentheoriginalpartfailedbecausefmachinemismatch. (A mismatchoccurswherethewomachineurfacesdonotmeet-seeection5.8.Z).200 ,0 00

180 , 000(Boltshavinghreadsrolledprioroheattreatmenthaveaboutheamefatiguestrengthasboltswithmachinedhreads)

10 , 000 100 , 000 1 , 000 , 000 10 , 000 , 000

Figure. ThreadRolling6-5


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TIPSONFATIGUE FigureZ.llshowshowstressescanbeverysevereasaesultofthis.Simplymoothingoutheadiuswouldbeaolutionouchamismatch.Otherasesmayoteoimple. Wemayind a part made with a sharpnotchohat bearingoulditlose

Let'sakeheasefaug. AssumehathemachiningdirectionwerenormalohedirectionofheLoad AswehaveaLreadyseen, thiswouldconstituteauperimposedstressoncentration. However, ifhemachininghadbeenparaLLeLo the

POOR POORDirectionofMachining

BETTER BETTER tohehoulderofahaft. Eveno,tis sometimespossibleoimprovehedistributionofstressbyremovingsomematerialoeliminateheotchandstillhave closeit,ashownnthelowerketch.

directionofhe Loading, aubstan-tialIncreasenfatigueifewouLdresuLt. ThisotoaLyappLlesoroughsurfacesutwouLdbequaLlyappLLcabLe tonysurfacefinishing.6-6


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TIPSONFATIGUE 6.3.2 CUTOFFTHEOPOFTHECURVE--orrdinaryroughsurfaces, sometimesmoothingwilldoherick;uthemannernwhichthemoothingsonewillhaveaottodowithhowsatisfactoryhepartwillbe. Forxample, theparthatfailedinSection5.4 mighthavebeensatisfactoryfhedirectionofthemachininghadbeenwithheradius.

Themportanceofremovingmaterialomproveatigueifecannotbeovermphasized. Thisnotonlyresultsnaighterstructure, butonethatsroublefree.Ourexamples sofar havebeenforremovingmaterialnfillets.Equallymportant, perhapsmoreo,isremovingundesirableexternal humps,

PhotoelasticmodelsnFigure6.2howhowabeamcanbereworkedtoowerhetressatheconcentration.Exceptforremovalofmaterialshowninhecrosshatchedarea, thelowermodelsdenticaloheupper. Beingidenticallyloadedoneagainstheother)theevisedmodelhadhetressloweredtowohirdsofheoriginalvalue. Thiswouldincreasefatiguelifemorehanenimes. Removing

NOTE : Stresssdirectlypropor-tionaltonumberffringesFigure6.2 RemovingMaterialLowersStress

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0S T R E S SR E F . )

Figure.3 AnalogyofStressCycleasnfluencedbyInterferenceFit

asmallamountofmaterialmakesthedif-ferencebetweensatisfactoryandunsatis-factoryperformance.6.3.3 CUTOFFTHEBOTTOMOFTHECURVEAnothermethodofshiftingthestresscyclesbymeansofaninter-ferencefitfastenerorapressed-inbush-ing. Here'showtworks. TurnbacktoFigure2.2. Let'smaginethatourpartfailedbecauseheactualstresswascy-clingatR 0withamaximumstressof50,000psi. IfwecouldmakehestressbehaveasthoughloadingwereatR +0.5,weseehattheifewouldthenbeabout10,000,000cycles. Inotherwords,fwecouldblocktheholeandkeephestressfromreturningtozero,heoadcouldbereducedtozero,yetthestresswouldremainat25,000psintheexamplewe

haveselected. Practically,hisseasytodo. Figure6.3illustratestheprincipleinvolvedntheuseofannterferencefit.Wherethedeflectionofthespringslikenedtoastresscycle,tcanbeseenthatanntroductionofahosesegmenttopreventthereturntozerowouldlowertheexcursionwithoutaddingtohemaximumdeflection.Figure6.4showsheeffectofvariousamountsofpininterferenceonthefatigueifeofsmalllugspecimens. While

fatigueifedoesnotseemtobeoweredappreciablybyexcessiveamountsofinter-ference,ookoutforpossiblestresscor-rosion. Withtaperedbolts,nowcommer-ciallyavailable,heamountofnterferencecanbecloselycontrolled. Amountsofn-terferenceshouldbehoserecommendedbyheaperedboltmanufacturer. 6-8


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1,000,000

HCO /o , 3/8-InchDiametero Jj; TaperPinH B|[\-4100,000 e|;)M

" l*

o jCO

a o in.mn1

0 .0 0 0 0.002. 004 ..006TAPER INNTERFERENCE NCH 0 .0 0 8Figure.4 EffectofnterferenceFitonFatigueLifeFigure.5showsagraphforlargeugswithinterference-fitbush-ings. Inorderoachieveheinterfer-encehown,hebushingmustbeongerthantheughickness,withexcesslengthchamfered0degrees. Excesslengthsgroundoffafterpressing.K^ L vj

.75-

1 0

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TIPSONFATIGUE ExamplesnChaptershowhowfatigueifecouldbem-proved yaddingedge-drivenrivetstorelieveheeffectsfbendingnarivetedjoint. Theproblemareananyrivetedointwillusuallybeatherowoffastenersnearheoad.Figure.6howsowhiscanbeachievedyprovidingalargerholenhedoublerathefirstoadedfastener)whileusinganinterference-fitboltnhematerialbeingprotected.Thisactsverymuchike theedge-drivenrivetdescribednChapter:

tight

looseFigure.6 InterferenceBoltInstallation

axialandbendingoadsareeparated,carryingbendingoadsonlyathefirstrowandpassinghearoadsotheecondrow. Bothdevicesareexamplesfwhatarecommonlycalledstressonfusers, butthenterference-fitbolthasheadvantagehatitcanstillcarryshearoadnanemergency,whichheedge-drivenrivetcannotdo.

FatigueestsfjointssimilarohatshownnFigure6.7showaatigueifencreaseoffrom117,000cycleso,371 ,000cyclesoverimilarointswith1/4-inchdiameterivets. Usinganinter-ferencefitfastenerwithoutheover-sizeholenthedoublerresultednafatigueifeofZ3,000cycles.Figuresgivenareaveragesorivetestseach.

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7 CHECKIS TFO RAT IGUERES I STANCE

7 .1GREMLIN 'S MENAGER IE OFCOMMON OVERS IGHTS Inthelightofwhatwehavelearnedaboutfatigue,et'sreviewsomeofthemostcommonoversightsthatcausefatiguefailure.

Whenthingsgowronginanair-plane,tiscommonpracticetoblameitonagremlinon eofthosemythicalfoot-high,ll-humoredimpsthathavebeendisruptingtheworkssincemanfirsttooktotheair. Inthecaseof fatigue,however,twouldseemhatevenagremlinwould needsomehelp. It'sasthoughthegremlinhasamenagerieofcommonoversights,each"animal"withitsownstructuralchar-acteristicsthatonlyagremlincouldlove.

Sincenoonelovesthesecritters,itiscommonpracticetosaytheybelongtosomeoneelse thatthereforetheyaresomeoneelse'sresponsibilityandthatiftheyareignored,maybetheywillgo away. Thecrittersneverdogoaway.In fact,fletalone,heytendto multiply.

Therearewaysobreedthismenagerieofoversights,and therearewaystoreduceit. Whenthedesignerfeelsheisbeingesteredbyaseemingly

stupidquestionfromashopmanwhohasfoundwhattheshopthoughtwasanover-sight,tisrecommendedthattheloftyengineerlistenwithinterest,espect,andappreciation.

Maybethistimetheshopmanhasn'tfoundanoversight,butit'sacinchhewon'tkeeplookingforon eifhiseffortsaren'tproperlyappreciated-ifhecomesntentimesandonlythetenthisanoversight,tmaybetheonethatsavesthedesigner'sreputationand10 0lives.

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TIPSONFATIGUE Onbehalfoftheshopman,hereundoubtedlywillbecaseswhereapoordesignhasbeenallowedtogetbybecause

ofcostorotherexcuses. In areasofverylowstress,hismightnotbeserious,utalwaysrememberthatNEWAIRPLANESSTILLFAILNFATIGUEbecauseover-sightshavebeenallowedtogetby.Onlyafewofthecritterswill

beshown,utenoughtorelaythedea.Thehingtorememberishatwheneveronesfound,hetimeomakehechangeisnow. Youmaysometimeschangeapartthatmighthaveasteditsrequiredlifetime,utit'sbettertobesafethansorry.

Thegremlin'smenagerieisn-tendedtojogyourmemorybyassociatingrelatedideas. Numbersnparenthesisattherightrefertoappropriatesectionsinthepreviousext.Whiletheremaybemanyotheritemshatneedchecking,hefewmentionedhereshouldsufficeforstimu-latingthealertnessrequiredtocatchmistakesbeforetheybecomeserious.A goodplanwouldbeomakeagameoflistingallpossibledesignandshoperrors. Trytocategorizehem. Youwillfindmanyimesthefewistedherein. Also,bythisimeyouwillknowwhatneedsobedonetofixit. Goodluck'.

SHARP NOTCHESGOUGESNICKSALLOW STRETCHBETWEENIRST TWOROWS OF FASTENERSSHARP EDGESRAPID CHANGENSECTIONCHEWED UPSURFACESFAT GOBS OF EXCESSMATERIALLONG SPANS OF UN-SUPPORTEDTRUCTURE_DANGLINGMEMBERS

HIDDENMISTAKESHARD PLATINGTROUBLESOMESPOTFACEEXTRA HUMPS

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SHARP NOTCHES (5.2. )7-3


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GOUGES (5 .5.4)7-4


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NICKS (5.2.2)7-5


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ALLOW STRETCH BETWEEN FIRSTTWOOWS OFFASTENERS2. .3)7-6


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SHARP EDGES (5.3)

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RAPID CHANGE IN SECTION (5.5.3)

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CHEWED UP SURFACES (5.8. )730-755-648 7-9


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FAT GOBS OFEXCESS MATERIAL6.3.2)7-10


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LONG SPANS OFUNSUPPORTEDSTRUCTURE7-11


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A. - -... .yi-

DANGLING MEMBERS

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HIDDENMISTAKES7-13


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HARD PLATING (5 )7-14


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TROUBLESOME SPOTFACE (5.3.2)

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EXTRA HUMPS (6.3.2)

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TIPSO NFATIGUE APPENDIX A-FATIGUE LIFE OF TYPICAL STRUCTURES

10

S

NH

00

ooo-

wX P5IDWingO Wing& VerticolTailD HorizontalTail

i-i 70H


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TIPSONFATIGUE APPENDIX --STRESS CONCENTRATIONS

W 4 1

r? 3

OHO < !hii H

w O oCO CO W tfHc o

L

< J 4W < i < ^ Se n c o c oW CO

0.40


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APPENDIXC-SUGGESTEDREADINGFollowing isalistofreferencesOnfatigueforthosewhowishtopursuehesubjectfurther. Whileherearemanyothervaluablereferences,thoseistedrepresentacrosssectionofthehinkingduringtheastdecade.

STP9,ReferencesonFatigueSTP91,ManualonFatigueTesting

A.S.T.M. SpecialPublications,PublishedbyTheAmericanSocietyforTestingMaterials,916RaceStreet,hiladelphia

STP203,atigueon AircraftStructures,1957STP237,ymposiumonBasicMechanism

ofFatigue,959STP274,ymposiumonFatigueofAircraftStructures,960STP284,ymposiumonAcousticalFatigue,961 STP338,SymposiumonFatigueTestsofAircraftStructures: Low-cycle,Full-Scale,andHelicopters

AmericanSocietyforMechanicalEngineers. ConferenceInternational)onFatigueofMetals-Proceedings, 29West39thSt.,ewYork18,956AmericanSocietyforMetals. MetalsHandbook,Vol1,thEdition,MetalsPark,Novelty,Ohio,961 Forrest,P.G. FatigueofMetals,PergamonPress,ewYork,962Freudenthal,A.M.Editor). FatiguenAircraftStructureProceedingsoftheInternationalConferenceHeldatColumbiaUniversity), January30,1,ndFebruary1,956


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TIPSONFATIGUE Grover,Gordon,&Jackson. TheFatigueofMetals&Structures,U.S.GovernmentPrintingOffice,WashingtonD.C.,1954Harris,W .J.etalicFatigueWithParticularReferenceoSignificanceofCertainStandardAircraftFabricationandFinishingProcess,PergamonPress,NewYork,1961Heywood,R.B. DesigningbyPhotoelasticity,AcademicPressInc.,Publishers,

NewYork,952 Heywood,R.B. DesigningAgainstFatigueofMetals,ReinholdPublishingCo.,

NewYork,1962Peterson,R.E. StressConcentrationDesignFactors,JohnWiley&Sons,nc.,

NewYork,1953Plantema,F.J.&Schijve,Editors). Full-ScaleFatigueTestingofAircraftStructures,PergamonPress,NewYork,1961SocietyforExperimentalS

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