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BendingfatiguetestsonametallicwireropeforaircraftrescuehoistsMarcoGiglio*,AndreaManesDepartmentofMechanics,PolitecnicodiMilano,ViaLaMasa34,20158Milan,ItalyReceived29July2002；accepted24August2002AbstractDuringnormaloperation,theendsofhelicopterrescuehoistropes,towhichahookisattached,canbesubjecttobendingstresscausedbyvibrations.Thishappensintheeventofpartialortotalrecoveryofthehookintoitslodgingwithoutaspring-loadedblockingsystem.Theswingingoftheropeendconsequentlycausesbendingoftheropeneartheterminal,withresultantfatiguestressthatcanleadquicklytobreakageordamageoftherope.Aseriesofalter-natingbendingfatiguetestsusingaconstantload,similartothoseknownasBoS(bendingoversheave)tests,werecarriedout.Thesesimulatetheeectiveworkingconditionsofarescuehoist.Thispaperdescribesmodicationstotestmachinery,thetestmethodandthedataobtained；statisticalanalysisofthisdataenabledustomakearopelifepre-dictionwhichwasthenexperimentallyconrmedbyrun-outsontestropes.#2002ElsevierScienceLtd.Allrightsreserved.Keywords:Wireropes；Sheave:Fatigue；Hoists；Aircraft1.IntroductionWireropes,likeropesingeneralandalsochains,arecommonlyusedfortractionloadtransmission.Despiteitswidespreaduse,wireroperemainsanextremelycomplexandlittle-knownpieceofequipment.Itsconstruction,theconsiderableanddiverseinternalcontactforces,thewearduetotheseforcesandtocontactwithdrums,pulleysorsheaves,allmakeacompletelyanalyticalapproachtoreliabilityextremelydicult.Eventoday,experimentaltestingplaysanimportantroleintheanalysisofropesubjectedtoaparticularload,especiallyasregardsfatigueanalysiswherethebreakingofstrandsresultsfromvaryingtypesoftestandload.Thepurposeofthispaperistoinvestigatethebehaviourofropeusedonahelicopterrescuehoist.Weanalysethecircumstanceinwhichthehookattachedtotheendoftheropehasbeentotallyorpartiallyrecoveredintoitslodgingandisleftfreetoswing.Incorrecthooklodgingorinsucientpre-loadingofthespringblockingsystemresultinbendingoftheendsectionoftherope,whichmaythenhit1350-6307/03/$-seefrontmatter#2002ElsevierScienceLtd.Allrightsreserved.PII:S1350-6307(02)00043-2EngineeringFailureAnalysis10(2003)223235/locate/engfailanal*Correspondingauthor.Tel.:+39-02-399-47354；fax:+39-07-063-8377.E-mailaddress:marco.gigliopolimi.it(M.Giglio).theguidewheelsofthewinch(seeFig.1).Breakageanddamagetoropewithouttheapplicationofanytractionload,andthereforenototherwiseexplainable,havebeenattributedtothisparticularoperatingcondition.Thiswirerope,likeallaeronauticalcomponents,mustbeoptimisedforweight；theropehasasafetyfactorof5onthestaticfailureload(lessthanropesforcivilandindustrialuse)butmustbereplacedafter1500completelower-raisecyclesunderoperatingloads.Itis,therefore,reasonabletoassumethatwearanddamageduetovibrationalswinging,evenwhenthehoistisnotinuse,canleadtoprogressiveandmarkedweakeningoftheresistantsection.Assumingaconstantpreload(equaltotheweightofthehookmulti-pliedbyanappropriatefactor,whichtakesintoaccountinertiaforces),westudiedthebendingfatiguebehaviourofwireropeduringcyclicallyrepeatedswingingontheguidewheels,ataxedalternatingangle,inordertoestablishwhetherthereexistsanangularvariationbelowwhichnodamagetotheropeoccurs.Theinstrumentationusedwasdesignedandconstructedspecicallyforthesetests:itreproducestheswingingoftheropeendandtheconsequentalternatingbendingfatiguestressoftheropeatthesheave.2.BackgroundWefoundnopreviouspublishedstudiesconcerningbendingtestscarriedoutwiththemethodsusedinthisexperiment.Inpublishedresearchonbending,ropehasbeenwoundoandontoapulleyofagivendiameterandthusassumesthebendingradiusforadenedlength.Intheexperimentaltestsperformed,thebendingradiusoftheropeonthesheavediersfrompointtopoint.Moreover,thealternatingbendingoftheropeendinducesavariablesignbendingloadingineverysectionoftherope(ineverycycle),thusdeterminingalternatingfatiguestressofthewiresincontactwiththesheaves.FatiguetestsonpulleysareforeseenundertheMil-W-84140standard1forhelicopterrescuehoistropes.Therope(inourcase,aropewithanominaldiameterof3/16inches),suitablyconditioned,mustFig.1.Terminalbending.224M.Giglio,A.Manes/EngineeringFailureAnalysis10(2003)223235perform150,000cyclesonapulleywithathroatdiameterof1.781inches(45.23mm),withapreloadof22.5pounds(100.08N),andmustsubsequentlyhaveresidualmechanicalcharacteristicstoallowatensilestrengthofover2000pounds(8896.44N).Asimilartest(intermsoftestsystem),aimedatobtainingmoredetailedinformationondeformationsinthewiresmakinguptherope,isthatcarriedoutbyRidgeetal.2,3.Inthisarticle,measurementsaretakenusingstraingaugesplacedonthewires(thickerthan1mm)ofacomplexsectionropeduringwindingoveratestpulley.Thediameterofthepulleyis18timesthatoftheropeandthepreloadis20%ofthetensilestrengthoftherope.Itcanbeseenhow,initially,whentheropeisonlyaxiallyloaded,wiresthatshouldgivesimilarmeasurementsactuallydisplaydierentstrains(probablyduetoimperfectionsintheropeconstruction),andhowthesedier-encestendtodisappearquicklyduringthebendingfatiguetests.Theamplitudeandtheshapeofthestraincyclesrecordeddonotchangesignicantlyduringthefatiguetest.Theauthorsshowexperimen-tallyhowdegradationandimpairedqualityduetowronguse,wearandthemanufacturingprocess,consideringthecongurationandloadusedbyRidgeetal.,haveonlyalimitedinuenceonbendingfatiguelife,whichdependsmainlyontheropesnominalconstructionandontherope/sheavediameterratio.NabijouandHobbs4analyse,inawidevarietyofcases,thefatiguebehaviourofaropewoundonsmalldrum,subjectedtoapreloadproportionaltoitstensilestrength(4060%).Theirstudylooksintotheinuenceofparametersincludingthegeometryofthegrooveinwhichtheroperests,thetypeofworkingofthepulleythroatandthetreatmentcarriedoutonthewiresoftherope,aswellastheeectofthediameteroftheropeandofanylubricatingorcontaminatingagentssuchaswaterorsand.Itisinterestingtonotehowtwoidenticalsheavesmadebytwodierentmanufacturersproducedierentropefatigueendurance.Thisconrmsthatfatiguelifecanalsothereforedependonthemicroscopicchar-acteristicsofthesurfaceswithwhichtheropecomesintocontact.FromNabijouandHobbswork,itcanbeseenthatforlowloadandhighsheavecurvaturecomparedtotheropediameter,thepredominantformofbreakageisthatduetowearcausedbycontactwiththepulley.Inthecaseofhighloadandsmalldiametersheave,ontheotherhand,thersttypeofweartoappearisthatduetoslidingbetweenwires.Theinternalslidingbetweenwires,insuchaconguration,hasalsobeenobservedanalyticallybyNabijouandHobbs5.Thetwogroupsofarticlesoutlinedabovearenotconictingbuthelpprovideamorethoroughunder-standingofthecomplexphenomenonofBoS.ThefatiguelifeandtheconsequentfailureofaBoS-testedropedepend,unequivocally,onfretting(wirewire,wiresheave).Fatiguelifeseems,however,tobeinuencedmainlybythenominalloadconditionsanddiameterratios,andonlyslightlybyparticularunfavourableinitialconditionsorprogressiveinitialyielding.TheseconditionsunderlineasubstantialdierenceoftheBoSfromtheaxialfatigueloadwherenotchesandwearresultinaconsiderablereductioninthefatiguelife.TheimportanceofwearduetofrettingwasrsthighlightedbyStarkeyandCress6.Theirstudywas,infact,thersttoidentifytheexistenceoftwodierentcriticalregionsinarope:therstonerelativetothelinearcontactbetweenadjacentstrandsinthesamelayer,thesecondrelativetolocaland,therefore,moredangerouscontactbetweentwowiresbelongingtotwoadjacentlayers.Instatictests,theeectofwearhasaconsiderableinuenceontensilestrength.DeSilvaandFong7analysethesignicantresidualloadreductioninropessubjecttowearbothinthelaboratoryandduringactualoperation,andcomparetheirresultswithcurrentstandardsonthereplacementofropesubjecttowear,inparticularISO4309.Inasimilarway,Kuruppuetal.8analysethesamereductioninmechanicalcharacteristicsandreachsimilar(butlessdrastic)conclusionstothoseofthestudybyDeSilvaandFong.Inbothstudies,however,itcanbeseenhowametallicarealossofafewpercentagepointsresultsintensilestrengthreductionsofabout10%.Aspreviouslystated,publishedresearchcontainsnostudiesonfatiguebehaviourwhenthewindingangleoftheropeonthesheaveislimitedandwhenthereisaverylowconstantpreload,inotherwords,undertheconditionsappliedinthistestproject.M.Giglio,A.Manes/EngineeringFailureAnalysis10(2003)2232352253.TheropetestedTheropetestedcorrespondstoMil-W-83140specicationsasTypeI,19C27stainlesssteel,preformed,non-rotatingforhelicopterrescuehoists.Cablesofthisconstructionconsistofonecoreofsevenstrandsofsevenwireseachforatotalof49wiresandanouterlayerof12strandsofsevenwireseachforatotalof84wires.Theinnercoreislanglay,leftlay；theouterlayerisregularlay,rightlay.Thetotalnumberofwiresinthecablesis133.Theropediameteris3/16inch(4.76mm).4.Thetestmachine4.1.DescriptionofthemachinebeforemodicationThemachineusedforthetests,theMETROCOMT6AL(testfrequency0.5Hz,maximumangularvar-iation180C14),wasmadetoperformalternatingtorsionfatiguetests.Thetestropesareplacedbetweentwoclamps,oneofwhichturnsarounditsaxis；theotherisxed(seeFig.2).Themachinewasmodiedtobeabletocarryoutthealternatingbendingtests.Themachinesmobileclampingheadwasreplacedbyaropeterminalsupportspeciallydesignedforthetests,andaservicestructure,necessarytocreatethedesiredtestconditions,wasadded.4.2.RopeterminalsupportThesupportallowsbothanoptimallockingoftheropeterminalandthecreationofactualloadcondi-tions.Thesupport(Fig.3)was