tips on fatigue - navweps 00-25-559
TRANSCRIPT
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
1/123
NAVWEPS00-25-559
58 & *
TIPS ONFATIGUE
r atBU*nO&STATEWEHT A D!S s Sitsu11eleaseApproved rorD"VitpdD'FtributonUnlimitedPUBLISHEDBY DIRECTIONO F
THEHIEFO FTHE BUREAUO FNAVALWEA P ON S
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
2/123
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
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
3/123
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
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
4/123
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
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
5/123
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
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
6/123
TIPSO NFATIGUE
CONTENTS
PREFACEChapter1heFatigueProblemChapter2asicPrinciplesChapter3ointsndJoiningChapter4evelopinganIntuitionforFatigueChapter5astExperienceChapter6akingtheMostofaBadSituationChapter7heckListAPPENDICES
A.atigueTestDataB.tressConcentrationsC.uggestedReadingVll
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
7/123
COOLER|..tHll(< (H I I I I V iii r:iin tuniori 'Mijjliilj .,.!. Mitny ait.tiinith r.Mili-r liiinornm, H .- tiinl ri|ii>n in I'iiuc-ll. ItlOtt HNAL ****
ESTABLISHEDMI SA N I EGO,CAL IFORNIA.MONDAYMORN ING .MARCH 5,19 DAILY0CENTSAirlinerCrashesWith10AboardjBritishCharter ,
|lightHitsNea r AtlanticnAfrica LONDON'.Minvh5Mo nda y )*#*-AcharteredBril-\rtedlylnijromMm|BKa*'ll " " " ". hlquericutJrlrauU X -vmbourgon charterto Lon- donirm.rnim-Afrieartir Coach.Urt .NK A BAUt r OH TAnAirMlntwrypoluwwuiii.Midhecruhonewilbo- Hovedowwcr ttwDotiala airportotarfiemheAt- lande ooaat.He aidthtplan* OUTtad101 p M S t K I t f t l M . .,ApokonaanorODMtotdfanim*,ownerfW plan.klhnunbaruanaiaijirj u" ' a b o u t 1 0 0 . ! * . ' ,Thrt* atownjrdottai'.wfttroperta* to^hOofhThe crathc a m throedayitwrao fhowor*.i*MU on dUaaMr In W itory.i*rtcrath earNowYork'*a% wUdirporthicho o * : Jlives. ', " Notalloll I fcWB- tan* ol Oi Camarooo crash jwre hnnadja latxavailable a i r * .DETAILSSOt 'OUT $--4.JHo t mawntern*DB.UKattomaityotn,Uera.OffidalaaaldaU M E f t r t t , werebolusmado :**-. . :*Milsbomcaaiwan alow.' -'"H'mM CaledonianAirway, f lyr officesIn Prtatwlck .S
,wiumm***.*****Ita*.orhoeaael.Douda*CTCo to^itt*j|-: "- - -olaleettar* * " " *
otcoo^rlfory ocUh^J
BIGGER HEADLINEIN THE HIKING
j t e t B o u n c e s n A i r ; 1 0 H u r t WKtO-SUrths:iUonday> m-~k Pan;American orldwrnsa^.iarjot aldtnerarty-
mg-vtiemtata a |tfhairaftertaantfni\Hono?ijMiryoaieiwirwitt|tBtat
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
8/123
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
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
9/123
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
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
10/123
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)
L-3
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
11/123
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.
1-4
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
12/123
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 .
2-1
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
13/123
TIPSONFATIGUE
+ 90 . willlastor5r6bendsoffrom o90degrees.
15 CYCLES0o90-DEGREE BENDING
Kbentonlyfrom5o90degrees,tmaylastfrom60to70cycles.
70CYCLES45-to90-DEGREEBENDING
2-2
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
14/123
TIPSONFATIGUE
83,000POUNDS
Similarly,heifeofastructuremaydependmoreonhowtisoadedthanonhetotalnumberoftimestisoaded,ronthemaximumamountoftheloadsthemselves.
Thissbestshownbyloa xgannch-squarebarofaluminumalkyhatbreaksat83,000poundswhenloadedonce.
ONE-INCH-SQUAREBARBREAKSAT83,000POUNDS
2-3
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
15/123
TIPSONFATIGUE
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
2-4
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
16/123
TIPSO NFATIGUE
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
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
17/123
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.
2-6
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
18/123
TIPSONFATIGUE
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
2-7
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
19/123
TIPSONFATIGUE
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.
2-8
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
20/123
TIPSONFATIGUE
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
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
21/123
TIPSONFATIGUE
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
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
22/123
TIPSONFATIGUE
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
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
23/123
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.
2-12
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
24/123
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
3-1
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
25/123
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
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
26/123
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
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
27/123
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
3-4
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
28/123
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.
3-5
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
29/123
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-
3-6
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
30/123
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.
3-7
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
31/123
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
3-8
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
32/123
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
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
33/123
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
7.0U
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
34/123
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
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
35/123
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
3-12
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
36/123
TIPSONFATIGUE
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.
3-13
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
37/123
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.
3-14
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
38/123
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
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
39/123
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.
3-16
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
40/123
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
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
41/123
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
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
42/123
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.
4-3
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
43/123
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.
4-4
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
44/123
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
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
45/123
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.
5-2
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
46/123
TIPSONFATIGUE
Shopboo-boosnvolvepoorcraftmanship,ailuretorecognizedesigngoofs,ndfailuretoalertengineeringwhenadesignorshopmistakeisdis-covered. Especiallycriticalaredingsresultingfromcarelesshandlingpractices.Whileshortrivetorboltedgedistancescanusuallybeblamedontheshop,re-quentlyengineeringdrawingsdonotmakeallowanceforsufficientspacetoinstallthenumberofrivetsorboltsrequired.Nomatterwho wasatfault,goodcraftsmanwouldneverleaveinsu-
fficientedgedistanceormismatchedmachinedsurfaces,veniftheblueprintcalledforthem. Evenapoorcraftsmanwouldblushifhewereshownthefailurethatresultedbecausehehadinstalledaboltofthewronglength.
5-3
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
47/123
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.
5-4
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
48/123
TIPSONFATIGUE
730-755-644 5-5
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
49/123
TIPSONFATIGUE
5.2.2 DINGS- Dingscanbententionaloraccidental. Uppereftshowsheatiguefailureresultingfromnaccidentalblowbyabluntobject. Lowereftshowsfailureresultingfromdentificationmarks,ndupperrightshowshowmarkseftbyarivetbuckingbarcauseawingsparocrackintheradius.
Failureatlowerrightissimilartoupperright,xcepttheoolwasanimpactscrewdriverusedforremovingthewingfuelcellcoverattachmentscrews.Precautionsshouldbeakennhandlingpartsoavoiddentingandnicking.
Shoulddingssuchasshownbediscovered,theyshouldbecalledoheattentionoftheforeman andfatiguespecialist. Thesecanfrequentlybecorrected.
5-6
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
50/123
TIPSONFATIGUE
O o.
5-7
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
51/123
TIPSONFATIGUE
5.2.3 FILLETRADIUS-Thinkbigwhentcomesofilletradii. Radiusoftheugntheuppereftphotowouldseemample,utitonlyhadafifthofitsex-pectedlife. Asimilarugntheupperrightphotohasaradiusthatwasncreasedbyremovingsomeofthematerial. Thislugwiththeargerradiushadafatiguelifeeighttimesgreater.
Thelowerphotoshowsatiguefail-urehroughafilletthatwouldhaveseemedtobeOK ,utthefactthatthepartfailedinservicendicatesalargerradiusshouldhavebeenused,speciallysincetherewasnoproblemofclearancenhisarea.
A goodpolicyistouseasargearadiusasspaceandpracticalitypermit,especiallyatchangesofsectionthicknesses,
5-8
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
52/123
TIPSONFATIGUE
OriginalLife N Cycles RedesignedLife NCycles
5-9
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
53/123
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.
5-10
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
54/123
TIPSONFATIGUE
5-11
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
55/123
TIPSONFATIGUE
5.2.5 SQUARE HOLES-Theresusuallyapurposenmakingsquareholes. Some-timesholesaremadesquareforopur-poseatall.
Theholentheupperphotographwasmadeoaccommodatearectangularduct. Solutiontotheproblemwasomakeanellipticalholeencompassingtheoriginalrectangularhole. Fatigueifewasivetimeshatwiththerectangularhole.Theholeshownntheowerphoto-graphwasmadefornopurposeotherthantosaveesshantwoouncesofweight. Itcouldhavebeenround,betteryet,orgottenaltogether. Thepartfaileddespiteheseeminglygenerouscornerradii.
"RectangularDuct"
*ssga
5-12
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
56/123
TIPSONFATIGUE
^S 5-L3
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
57/123
TIPSONFATIGUE
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.
5-14
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
58/123
TIPSONFATIGUE
l-*&
5-15
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
59/123
TIPSONFATIGUE
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
todoaman'sjob!5-16
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
60/123
TIPSONFATIGUE
,#SW*
m H |jb t?i
THIN FLANGECRACK
5-17
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
61/123
TIPSONFATIGUE
5.3.2 TROUBLESOME SPOTFACE -Spotfacingsacommoncauseoffatiguefailure. Whilenotquiteasbadasheexam-pleshownonthepreviouspagenthatnovulnerableoutstandingegswereeft thesharpcornercombinedwithafeatherededgesasurecauseoftrouble.
5-1!
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
62/123
TIPSONFATIGUE
*..:.:*
' . '."''"'. :.' : ; 'y.V'''^':\/ 'V0V ;.V''''V'' ;', "^'V^:;. . % itvi; - '...,-
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
63/123
TIPSONFATIGUE
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.
5-20
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
64/123
TIPSONFATIGUE
%- > - ' - ~\$et
^
730-755-B45 5-21
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
65/123
TIPSONFATIGUE
5.4.2 SPLICEThissplicecontainsthesameproblemasthedaggerfittingon thepreviouspage. Beingmorecompli-cated,heproblemisnotsoeasilydiag-nosed. Thus,heusuallyproposedsolutionisoincreasethethicknessofthematerialbeingspliced. Whilethismaysolvetheatigueproblem,heextraweightpenaltymaybeextreme.
Solutionforhisproblemshesameasorhestringerattachmentforging,xcepthattheoadsonallfirstlinerivetsmustberelieved. Acommonpracticestocutfinger-likeextensionstothesplicedoubleraswillbeshownonthenextpage. This,owever,snotsoeffectiveasaperingthedoubleror,betteryet,uttingthefingersandalsotaperingthedoubler.
5-22
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
66/123
TIPSONFATIGUE
ste-.':''' '- J mm U-^i
5-23
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
67/123
TIPSONFATIGUE
5.4.3 THECONTINUOUSMEMBER-Itwouldbefinetohaveallcontinuousmemberswithnonterveningsplices.Thiswouldendmanyofourfatigueprob-lems. Sincewedohavesplices,hedeaofmakingonlyapartialspliceatagivenlocationeadstotrouble.
Thisexamplewouldappearatfirsttohavebeencausedbyusingspliceplateshatweretoothick*,snthepre-viousexample. Theprimaryfailure,however,wasnotattheendrivetasthephotographwouldindicate. Thisfailurehappenedafterthecontinuousstringerbrokedirectlyoverhesplicearea,dumpingtsshareoftheoadontheskin.
Whathappenedwashattherewasoomuchgiventherivetedjoint,othattheskinwasunableocarrytsfairshareoftheoad,ausingthestringertofail. Theskinfailurewassecondary.
*Theyreallywereoothickattheendrivets. Thismightultimatelyhavecausedthefailureasshownhadnottheinitialfailurebeeninthestringer.
5-24
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
68/123
TIPSONFATIGUE
m m m m m m 5-25
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
69/123
TIPSONFATIGUE
5.4. HEAVYFORGING--Thesearetypicalsituationswhereeavyiecesarettachedoelativelyhinunderstructure, resultingnheailurefthestructureWhereverituationsikehese
arise, theolutionsoeeduthestresshroughuccessive layersfthinnermaterialrorovideradualtaperoheirstointfattachment.Ifhissotossible, perhapstheeavyieceaneplitnormalotheirectionftress sohenderstructureanave "BreathingRoom,"
Toowerhe stressluctuationesultingfrom theoletself, taperoltstpointsfttachmentwillelp.
5-26
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
70/123
TIPSONFATIGUE
I * k,fe
CR *** "
0tfc
.-.--; siipiPipspisiiiPi w& 5-27
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
71/123
TIPSONFATIGUE 5.4.5 NOBACKUPSTRUCTURE-Oneoftheundamentalsofsciencesthatforeveryaction,herehastobeareaction.Here, huskycontrolbracketwasattachedtoaflimsysparwebstructurewithresultsasshown. Solutionstoprovidesupportstructurebackupstructure)havingstrengthequaltothatofthefitting. Thisdiffersfromtheexamplesshownn5.4.4,whichwereknowntohaveadequatesupportstructure.
Thissatypicalproblemwhereonepersondesignsacomponentwhilean-otherdesignsthesupportingstructure. In thisparticularcase,tsdoubtfulhatmuchthoughtwasgiventosupportingstruc-ture. Rather,hebracketwasattachedtoastructurethatwasdesignedforanotherpurpose.
5-28
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
72/123
TIPSONFATIGUE
5-29
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
73/123
TIPSONFATIGUE
5.5UPERIMPOSEDSTRESSCONCENTRATION
Ashoughnotcheswerenotbadenough,theycanbemadeworsebysuperimposingoneuponanother. Thus,wehaveexam-plesoffeatherededgeshatalsoterminateatheavysectionsandfilletradiithatdon'tmatchtherestofthemachinedsurface.
5.5.1 HOLEPARTLYTHROUGH-Theopposite photoshowshowatiguecanstartataholethatwasnotdrilledallthewaythrough. Thesolutionssimple:finishdrillinghehole.
5-30
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
74/123
TIPSONFATIGUE
-K v .- - ' c wJPI'./A.-^^^Sw" . *>^yH1 1,'."w -
* ~ 'v .'viv''cK ^ ' " -I *_**'^^^1'.* a-*?'-'*m > . . ' -*
I.."* 'i
5-31
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
75/123
TIPSONFATIGUE
5.5.2 INTERSECTING HOLES-Heresacasewhereaholewasappedntohehighlystressedregionofanotherholentheensionflangeofawingbeam. Thisssimilarothecaseofagreasefittingholeforabearing.Ifyoumusthaveahole,hedeaisomovettotheeastdamagingposition.Aittleocalreinforcingbeefp)salso
helpful.
POOR
beefup
ALSO ACCEPTABLE
5-32
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
76/123
TIPSONFATIGUE
f:,?:J'f?Pii
FORWARD
I1
5-33
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
77/123
TIPSONFATIGUE
5.5.3 RADIUSATCHANGEOFSECTIONTherealwayshasobearadiusofsomesortatanychangeofsection. Likewise,herehasobearadiusatachangeofdirection. However,youdon'thavetomakeoneradiusrighton topoftheother. Theradiusforchangensectionshouldhavebeenmadeatanotherlocation. Ifthiswerempossible,othradiishouldhavebeenenlargedtopermitamoregentletransition.
5-34
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
78/123
TIPSONFATIGUE
J**'-;f.
*V$>
5-35
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
79/123
TIPNFATIGUE
5. . ROUGHSURFACEFINISH-A finishsuchasthatillustratedherealsoconstitutesasuperimpositionofstress.Whereastressraiseralreadyexists,sinthiscase,tisoolishoetaroughsurfaceikethisgetby,speciallywithtoolmarksnormalothedirectionofloading.
5-36
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
80/123
TIPSONFATIGUE
7 3 0 -7 5 50-64--G 5-37
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
81/123
TIPSONFATIGUE
5. . ROSS GRAIN --hepartsshownfailedorworeasons, roughsurfaces, andhematerial's grainstructuresnormaloheirectionofLoading. Whiletmighthavebeenpossibleoavertfailurebymachiningasmoothsurface, thewrongdirectionofgrainmakesuchaolutionhighLyspeculative.
5-38
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
82/123
TIPSONFATIGUE
5-39
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
83/123
TIPSONFATIGUE
5.6UXILIARYATTACHMENTSThepartshownwasfromaandinggearassembly. Failureoccurredthrougharivetholeusedforattachingaschafingshield. Similarthingshappentoframeshavingattachmentscrewsosupporthy- draulicines,upholstery,orwhathaveyou? Itwouldhavebeenbettertotiehepartonwithropealsoryadhesivebond-ing)hantotakechancesonfatiguingas
shown.
5-40
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
84/123
TIPSONFATIGUE
5-41
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
85/123
TIPSONFATIGUE
5.7 HARDLATINGHeresatypicalexampleofwhereapartwaschromeplatedtomaketmorewearresistant. Itwasn'tmoreatigueresistant.Thecracksntheplatingactasstressraisershateventuallyfailtheparttheplatingssupposedtoprotect.
Shotpeeningprioroplatingsacommoninhibitoroffatiguecrackingnchromeplatedparts. Itsunwiseochromeplatepaintsfordimensionalbuilduporwearresistancewithoutthehelpofthespecialist.
5-42
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
86/123
TIPSONFATIGUE
5-43
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
87/123
TIPSONFATIGUE
5. SHOPBLUNDERSWhilemostoftheexamplespreviouslyshowncanbeblameddirectlyondesign,anumbercouldequallywellhavebeencausedbyshopblunders. Thus,wehavetheholethatwasnottappedallthewaythroughandtheroughsurfacehatwasnotsmoothed. Theollowingexamplescanbeblamedalmostentirelyonshoppractices.5.8.1 NOEDGEDISTANCE-Theengineeringdrawingmaynothavespeci-fiedtheexactlocationsofholesfornutplates;however,tandardshoppracticesshouldbesuchthatthiswouldneverhappen. Asshown,herewasnsufficientroomfornutplatestobespacedinsuchamannerthatholeswouldfallbetweennutplates. Also,otethattheedgesurfacefinishwasnothingtobragabout.
5-44
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
88/123
TIPSONFATIGUE
5-45
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
89/123
TIPSONFATIGUE 5.8.2 MISMATCH-- Evenxcellentmachinistsftenmachine urvedsurfacehatdoesn'tmeettstraightcounterpart, leavingwhatamountsoauperimposedtressaiser. Whileitsotoadwherehewourfacesareonvex, theoncavenessuallyresultnailuresndicatednheillustration.
Manyrawingoommanualsspecifyiemaximumllowablemis-match. Itoappensnotnhesecases)hatatigueailuresaveresultedwherehemismatchwaswithinolerance. Carehouldeexercisednermittingmismatchesinriticalareas- evenwithinspecifiedolerances. Whiletwouldeirtuallympossibleodefinehe.amountfmismatchhatcaneermittednveryase, arulefhumbsosextremecarewithoncaveurfaces.
5-46
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
90/123
TIPSONFATIGUE
l%^'M4a*i'
5-47
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
91/123
TIPSONFATIGUE
5.8.3 EXCESSIVECLAMPING-Theboltonhispartwastightenedwithouthavingtheproperspacerbushing. Fatiguefailurefinallysetn,asmightbeexpected. MakesureyouhaveherightengthbushingandtherightengthboltandTHINKTWICEBEFORETIGHTENINGsee4.4).
5-4*
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
92/123
TIPSONFATIGUE
5-49
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
93/123
TIPSONFATIGUE
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.
5-50
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
94/123
TIPSONFATIGUE
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.
6-1
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
95/123
TIPSONFATIGUE 6.2 DECISIONS
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.
DECISIONS -- ALWAYS DECISIONS6-2
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
96/123
TIPSONFATIGUE
W etartoutwith a stresslikehis zerostress
whichcouldpossiblybehifted totilLSo zerooragaintmightbehangedtoactLikethis...
orsomethingmightbeoneomaketbehaveikehis zeroF 21.6-3730- 7550-647
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
97/123
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).
6-4
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
98/123
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
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
99/123
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
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
100/123
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
6-7
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
101/123
TIPSONFATIGUE
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
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
102/123
TIPSONFATIGUE
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
W
n
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
103/123
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.
6-10
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
104/123
TIPSONFATIGUE
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.
7-1
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
105/123
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
7-2
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
106/123
TIPSONFATIGUE
SHARP NOTCHES (5.2. )7-3
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
107/123
TIPSONFATIGUE
GOUGES (5 .5.4)7-4
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
108/123
TIPSONFATIGUE
NICKS (5.2.2)7-5
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
109/123
TIPSONFATIGUE
ALLOW STRETCH BETWEEN FIRSTTWOOWS OFFASTENERS2. .3)7-6
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
110/123
TIPSONFATIGUE
SHARP EDGES (5.3)
7-7
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
111/123
TIPSONFATIGUE
RAPID CHANGE IN SECTION (5.5.3)
7-8
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
112/123
TIPSONFATIGUE
CHEWED UP SURFACES (5.8. )730-755-648 7-9
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
113/123
TIPSONFATIGUE
FAT GOBS OFEXCESS MATERIAL6.3.2)7-10
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
114/123
TIPSONFATIGUE
LONG SPANS OFUNSUPPORTEDSTRUCTURE7-11
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
115/123
TIPSONFATIGUE
A. - -... .yi-
DANGLING MEMBERS
7-12
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
116/123
TIPSONFATIGUE
HIDDENMISTAKES7-13
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
117/123
TIPSONFATIGUE
HARD PLATING (5 )7-14
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
118/123
TIPSONFATIGUE
TROUBLESOME SPOTFACE (5.3.2)
7-15
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
119/123
TIPSONFATIGUE
EXTRA HUMPS (6.3.2)
7-16
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
120/123
TIPSO NFATIGUE APPENDIX A-FATIGUE LIFE OF TYPICAL STRUCTURES
10
S
NH
00
ooo-
wX P5IDWingO Wing& VerticolTailD HorizontalTail
i-i 70H
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
121/123
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
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
122/123
TIPSONFATIGUE
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
-
7/28/2019 Tips on Fatigue - NAVWEPS 00-25-559
123/123
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