SSC-178
ASurveyofSomeRecentBritishWorkontheBehaviourofWarshipStructures
byL Clarkson
SHIPSTRUCTURECOMMITTEE
——-—....—— . —— .-
SHIP STRUCTURE COMMITTEE
MEMBER AGENCIES,
6“,s.” 0,5“,,s,D.P,.o, NAVY
M, I.,TARY SrA TRANSPORTATION SERVICE, 0.,7. 0, NAVY
UN IT,. STATES COAST G“,.., TR,ASURY Dir,,
MARITIME ADM, NIsmAT,.aN, D,,,, OF C.. ”,...
..,.,..?4 8“.,.” 0, s.,,,,..
November 1966
ADDRESS CORRESPONDENCE TO.
S...,,..”S.,Ps,.”.,..,co..,,,,,U.S.COASI GUARD Hmc..avAn7Kns
WA, V,.G’10., Q. c, 2022Q
Dear Sir:
“A Survey of Some Recent British Work on theBehaviour of Warship Structures” by J. W. Clark son(ssc- 178) reflects the excel lent remarks and data theabove author presented to the Ship Structure Subcom-mittee at its Annual New York Meeting held May 4, 1966.
The Ship Structure Committee appreciates the
close cooperat ion and information exchange that hasdeveloped through the many years of its existence with
overseas research establ ishments.
Rear Admiral, U. S. Coast GuardChairman, Ship Structure
Comnittee
SSC-178
SpecialReport
A SURVEYOFSOMERECENTBRITISHWORKONTHEBEHAVIOUR
OFWARSHIPSTRUCTURES
by
J.Clarkson,B.SC.A.M.R.I.N.A.SeniorPrincipalScientificOfficerattheNavalConstructionResearch
Establishment,Dunfermline,Scotland
Washington,D.C.U. S. CoastGuardNovember1966
CONTENTS
I!z$!s
Introduction. . . . . . . . . . . . . . . . . . . . .
MeasurementofServiceStresses. . . . . . . . . . .
ElasticAnalysis.. .. . . .. .. .. . . . . . . .
PlasticCollapse. . . . . . . .. . . . . . . . . .
StrengthofBottomStructures. . . . .. . . . . . .
ExperimentalTechniques. . . . .. .. . . . .. . .
ProtectionStructuresforMarineNuclearReactors. .
Conclusions. . . . . . . .. .. . . . . . . . . .
Acknowledgement. . .. . . . . .. . . . . . . . ..References. .. . .. . . . . . . . .. . . . , .
1
1
4
8
1012
1516
16
16
ThaSHIPSTRUCTUEEstructure ofshipsfabtieat-ion.
SllIPSTRUCTURECOIIVI1’1’EE
COLMYITYEE{s constituted to prosecutea ~esearchprogramto improvethe hullby an extensionofknow2edgepe~tain-ingto design, mater<a2sandmethodsof
RearAdmiralJohnB.Or@n,USCGChief,OfficeofEngineeringU.S.CoastGuardHeadquarters
CaptainW.M.Nicholson,USNAssistantChiefofBureauofDesign
ShipbuildingandFleetMaintenanceNavalShipEngineeringCenter
Mr.D.B.Bannerman,Jr.VicePresident- TechnicalAmericanBureauofShipping
Chairman
CaptainP.E.Shetenhelm,USNMaintenanceandRepairOfficerMilitarySeaTransportationService
HHPSTRUCTURESUBCOMMITTEE
TheSflIPSTRUCTURESUBCOMMITTEEacts for the Ship Structure Committeeon technicalmattersb.yprovid{ngtechnieaLcoo~dinationfor the determinationof goa2sandobjectives of the progmm,andby wakating andintap~et{ng tk resu2ts in termsof ship structural des{gn,constructionandoperation.
NAVALSHIPENGINEERINGCENTER OFFICEOFNAVALRESEARCHCaptainS.R.Hell@r,USN- Chairman Mr.J.M.Crowley- MemberMr.JohnVasta- ContractAdministrator Dr.G.R.Irwin- AlternateMr.GeorgeSorkin.MemberMr.T.J.Griffin- AlternateMr.IvoFioriti-Alternate
MARITIMEADMINISTRATIONMr.R.W.Black-MemberMr.AnatoleMaillar- MemberMr.R.Falls- AlternateMr.W.G.Frederick- Alternate
AMERICANBUREAUOFSHIPPINGMr.G.F.Casey- MemberMr.F.J.Crum- Member
DAVIDTAYLORMODELBASINMr.A.B.Stavovy- Alternate
NATIONALACADEMYOFSCIENCES-NATIONALRESEARCHCOUNCIL
Dr.Wm.G.Rauch-“Al~ernate
MILITARYSEATRANSPORTATIONSERVICELCDRDonaldB.Bosley,USN- MemberMr.R.R.Askren- Member
U.S.COASTGUARDCDRClaudeR.Thompson,USCG-MemberLCDRR.Nielsen,Jr.,USCG- MemberMr.J.B.Robertson,Jr.-MemberLCDRJ.F.Lobkovich,USCG- AlternateLCDRJamesL.Howard,USCG- Alternate
LIAISONREPRESENTATIVES
BRITISHNAVYSTAFFMr.A.C.Law
Mr.A.R.Lytle- Director,Shipl-lull ConstructionCDRT.R.Rumens,RCNCResearchCommittee
Mr.R.W.Rumke,ExecutiveSecretary,SHRC WELDINGRESEARCHCOUNCIL
AMERICANIRONANDSTEELINSTITUTE Mr.K.K.Koopman,DirectorMr.CharlesLarson,Asst.Director
Mr.J.R.LeCron
INTRODUCTION
Forsurfaceshipstructures,entirelyra-tionalelasticorplasticdesignprocedureshavenotyetbeenachieved,although,sinceWorldWar11,researchactivityhasbeencon-siderablyincreased.Theunderlyingreasonsforthepresentratherempiricalmethodsofdesignaretobefoundintheconsiderablecomplexityofthestructuresandthe~resentlackofknowledgeonloadingsatsea.Itisinterestingtoobservethatsurfaceshipstruc-turesareusuallyfarmorecomplexthanthesubmarineortheaircraft,andthiscomplexityresultsina requirementforappreciablein-genuitytoreducetheoreticalanalysestoman-ageableproportions.Ontheotherhand,thecomparat~velackofknowledgeonsurf-aceshipstructuresisnotparticularlycritical,sincethedesignofstructureshasevolvedquiteslowly,beingbasedmainlyonpreviousexperi-ence,andthesafetyoftheshipandpersonnelisseldomcalledintoquestion.Newdevelop-mentssuchastheintroductionofmorebrittleorfatiguesensitivematerials,includingthehigh-strengthsteels,aluminiumalloysorglass-reinforcedplastics,mayalterthissit-uationandleadtoa morepressingrequirementforrationaltreatment.
ThepresentauthorhasbeenworkingattheNavalConstructionResearchEstablishment(N.C.R.E.)Dunfermline,Scotland,duringthepast15years,mainlyinthefieldofsurfaceshipstructures.Thisperiodhasseen.con-siderableadvancesanddevelopmentbothintheunderstandingofthemechanicsofshipstruc-turesandintheapplicationofdigitalcomput-erstoshipproblems.A numberoflong-termprojectshavebeeninitiatedtomeasurethestrainsexperiencedatsea.Althoughtheworkfamiliartotheauthorhasbeenmainlycon-cernedwithwarshipstructures,muchofitispotentiallyapplicabletosurfaceshipstruc-turesgenerally.Thepresentreviewisanat-tempttohighlightsomeofthemoresignificantadvances.
MEASUREMENTOFSERVICESTRESSES
Thereisconsiderableagreementthatthepresentlackofknowledgeoftheloadingonsurfaceshipsconstitutesthelargestunknownaffectingthestructuraldesignofsurfaceships,andN.C.R.E.isnowdevotingquiteasignificantefforttowardsthemeasurementofloadsatsea,usingstatisticalstraingauges.Itiswidelyrecognizedthatloadingsmaybemostconvenientlymeasuredusingstra~ngaugesplacedontheship’sdeckorbottomstructureatpositionsunaffectedbystressconcentra-tions.TheworkatN.C.R.E.hasbeenreviewedina recentpaperbySmith.1
Itisconsideredthatthemeasurementofservicestressesshouldbedirectedtowardstwoobjectives.Thefirstrequirementistoobtaininformationontheextremevaluesofstressex-periencedatsea,whilethesecondistoobtainhistogramsofstressreversals.Itisbynomeansclearthatthesameinstrumentwouldbeequallysuitableforboththesepurposes.
Morerealisticinformationonextremesofstressisrequiredforplacingthelongitudinalstrengthcalculationona soundbasis,andYuille2firstproposedusinga verysimplemaximum-readingstraingaugewhichhassubse-quentlybeenadoptedforextensivemeasurementsbyN.C.R.E.(Fig.1). Theinstrumentispure-lymechanicalinactionandrequiresnopowersupply.Wheninoperationthegaugeisboltedinpositionbybearingagainstthesurfaceoftheteststructureontwopairsofhardenedconicalstudswhichgiveaneffectivebaselengthof10in. Anychangeintheseparationofthesebearingpointsismagnifiedbya sim-pleleversystemandismarkedbya recordingpenona stationaryreelofrecordingpaper.Themagnificationfactorisabout100,sothata line1 in.longontherecordingpapercor-respondstoa strainvariationofabout0,001ora stresschangeinsteelofabout13ton/in.2.Thereelofpaperismovedforwardman-uallyatfixedtimeintervalsleavinga seriesoflinesonthepapereachofwhichcorres-pondstothemaximumstrainvariationduringoneinterval.Thegaugeisdesignedsothatbysimplemodificationitsbaselengthcanbeincreasedtoanyrequiredspan.Theshort10-in.baselengthhashoweverprovedsufficient-lyaccurateandhasinanycaseprovedneces-sarybecauseoftherestrictedspaceavailableforfittingofthegaugesinwarships.
Twenty-fivemaximumreadinggaugeshavenowbeenoperatedforsometime,andrecordshavebeenobtainedfrommorethanfortyBritishwarshipsofvariousclassesincludingfrigates,destroyers,cruisersandanaircraftcarrier.Animprovedproductionversionofthegaugehasrecentlybeendeveloped,andafurtherfiftyoftheseinstrumentsarenowcomingintouse.Itisintended,ultimately,thateveryoperativeshipintheNavyshouldbefittedwithoneofthesegauges.Thegaugesarenormallypositionedonthewebofa longitudinalstiffenerunderthestrengthdeck,wellawayfrompositionsofstressconcentration.
Intheanalysisofresults,hoggingandsaggingstressesareseparated,Thisisac-complishedbymarkingstillwaterdat’umsim-mediatelybeforeandaftereachvoyage.Lowamplitude5trainvariationscorrespondingtoperiodsofcalmweatherarealsoused,tofixintermediatedatums,andtheseareconnected
.-L-REEL OF RECORDING
e . . . . . .
i /“rHkLUHIJINbCROSSED SPRING PEN.
PIVOT. \J
CROSSEDPIVOT.
l-.
SPRING
PA,ROF CONICAL/-’BEARING PoINTS.
TESTSTRUCTURE’(STIFFENERWEB)
T\PAIR OF CONICALBEARING POINTS,
t= EFFECTIVEBASELENGTH=IO”
FIG.1. N.C.R.E.MAXIMUMREADINGSTRAINGAUGESHOWINGLEVERSYSTEM.
RETURN PERIOD (DAYS)
1.001
20
10
Id
‘g,4:----12tJII‘1 1,
10LnVIwtin+ I‘/20SAG(* .—. .6 STRESS=
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d.b 2 3 4 !! 13
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10—
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.001.01 .10.23.30 .50 .70 .00 .90 .95 .97.VB ,99 .995.997.990 .999.9995 .9998.9999FREQUENCYF(x)
-2 –1 o I 2 3 4 3 b 7 0 9 9.5
REDuCEDVARIATE(y)
FIG.2. TYPICALEXTREMEVALUEPROBABILITYPAPER.
bya datumline.Mostofthemaximumstrainreadingssofarhavecorrespondedto24-hourperiodsatsea.Withtheobjectofrepresent-ingaccumulateddataina conciseform,allow-inginterpolationbetweentheresultsforshipsofdifferenttypesandpossiblyallowingex-trapolationbeyondtheobservedrangesofstrain,thedailymaximahavebeenanalysedandplottedusinganextreme-valuestatisticaltheorydescribedbyGumbel.3Inthisanalysis,thecumulativefrequencyoftheobservedex-
tremevaluesisrepresentedbya seriesofpointsplottedona specialprobabilitypaper.Anextreme-valueprobabilityfunction,whichappearsasa straightlineontheprobabilitypaper,isfittedtotheobserveddatabyaleastsquaresmethod,togetherwithcontrolcurveswhichdefinelimitswithinwhichaspecifiedproportionofthedatashouldlie.A typicalprobabilitypaper,showingthere-sultsfroma groupoffourshipsofthesameclass,isshowninFig.2. Itmaybeseen
-3-
TRUE DATUM(zERO WAVE-
~STRAIN INDUCEDSTRAIN) m h
TIMEjI II 1 111 1I I 11 I1
I 1 [11
( 1 !
ARBITRARYFIXEb DATUM—.—. -.Ililllllllll
-—– ~-~- --
/ INDICATES
X INDICATES
FIG.3. SAMPLESTRAINRECORD.
m- Xxxx Xxxx xxx
1ISECONDINSIGNIFICANTSTRAINREVERSAL.NON-ZEROFREQUENCYDIGIT.
thatthemeasuredstressesbearlittlerelationtothosepredictedbythestandardL/20staticwavecalculation,andalsothatthesaggingstressesareappreciablyhigherthanthehog-gingstresses,possiblyduetotheeffectsofslamming.Ithasbeenfoundthatextrapola-tiontolongerperiodsusingtheGumbelplotproducesresultswhicharenotgenerallycon-firmedbysubsequentmeasurements.Oftentheextrapolationpredictsextremeswhicharetoohigh,andforthisreasonithasbeendecidedtomeasureextremesforverylongperiodsoftime,possiblyfortenyearsormoreoneachship.Untilmoredataareobtained,itwillnotbepossibletoestablishanystatisticalpatternoftheextremes.
Themainreasonforinvestigatingcyclicleadingasdistinctfrommaximumleadsistoprovideameansofensuringthatnewshipswillnotrununacceptablerisksoffatiguefailure.Inex~stlngships,theproblemoffatiguehasnotprovedcritical,andminorfatiguecrackshavebeentroublesomeratherthancatastrophic.Infuture,thepossibleuseoffatigue-sensitivematerialssuchashigh-yieldsteel,aluminiumandglass-reinforcedplastics,mayleadtomoreseverefatigueproblems.Inmild-steelships,anyincreaseintheacceptablegeneralstresslevelmustbeaccompaniedbyimprovedfatiguestrengthofstructuralcom-ponents.Itisalsopossiblethatmajorstruc-turaldiscontinuitiesofanunfamiliartypemayoccur,inevitablycausingstressconcen-trationsandrequiringanestimateoflocalfatiguestrength.
Itisconsideredthattheexistingtheo-riesofcumulativedamagesuchasMiner’sLawarenotfullysatisfactory,andfutureimprove-mentsmaybeexpected.Equally,itiscon-sidereddesirabletobeabletoseparatevibra-torystressesinordertoindentlfytheeffectsofslammingandbowflareimmersion.A sampleofa continuousstraincurveisshowninFig.3,anditisclearthata simplecountinggaugewouldberatherunsuitable,theresultinghis-togrambeingdependentontheparticularmethodofcounting.AtN.C.R.E.,a strain-recorderde-sign‘isbeingsponsored,whichwillsupplytheequivalentofa continuousstrainrecordinaformsuitableforinterpretationandanalysisbya digitalcomputer.Theinstrumentwillrecordtheamplitudesofmaximumandminimumpointsonthestraincurverelatlvetoanar-bitraryfixeddatumwhichwillsubsequentlybecorrectedtothestillwaterdatum.Specialprovisionwillbemadetoidentifyvibratorystrainswitha periodoflessthantwoseconds.Minorstrainvariationslessthana specifiedamount6willbesuppressed,and6maybead-justedintherange0.00002to0.0002(0.27to2.7tons/in.2forsteel).Thelengthofthedigitizedrecordwilldependquitecriticallyonthevalueof6.
Twopossiblesystemshavebeenconsidered,thefirstbasedona ship-borneslowmagnetic-taperecorderinconjunctionwitha shore-basedanaloguetodigitalconverteranddata-process-ingunit.Thesecondconsistsofa self-containedship-bornedigitalrecorderincorpor-atinganalogue-digitalconversionanda data
-4-
processingunitemployingmicro-logiccir-cuits.Ineachcasetheanaloguestrainsignalwouldbeobtainedusingelectricalresistancestraingauges.Atthepresenttime,thesecondtypeofsystemisfavoured,since~treducestheeffortindataprocess-ing,ismorecompact,andisthoughttobepotentiallymorereliable.A designhasbeenproducedbyPlesseyLimited,anditislikelythattwoprototypeunitswillbeorderedinthenearfuture.
ELASTICANALYSIS
MostoftherecentworkInthisfieldhasbeenconcernedwithtransversestrength,though,withthesecondgenerationofdigitalcomputersnowbecomingavailable,completestructuralanalysisincludinginteractionbe-tweenlongitudinalandtransverseload”ingsmayshortlybecomefeasible.Althoughtrans-versestrengthanalyslshasbeentraditionallybasedontreatmentofa singleringframe,thesubjectismorecorrectlyconcernedwiththeanalysisofflatorcurvedplatedgrillages.Untiltheadventofthedigitalcomputer,grillageanalysiswasrestrictedtoverysim-plecases,andmucheffortwasdevotedtofindingshortcutstotheveryheavyarithme-ticinvolved.Nowadays,straightforwardgrillageanalysisisquitecommonplace.Atextrev~ewlngelasticgrillageanalysisap-pliedtoshipstructureshasrecentlybeenpublishedbythepresentauthor.q
Ideally,eachplatepanelofa grillageshouldbeanalysedbythewell-knownlinearplatetheoryequations,
V4$= o (1)V4W= q/D (2)
where$ istheAirystressfunction,w isthebendingdeflectionq isthelateralpressureandD = Eh~/12(1-u2).Thebeamsmaybean-alysed”bytheusualEuler-Bernoullitheory,andtheboundaryconditionsforequations(1)and(2)areobtainedbyspecifyingequilibriumandcompatibilityateachbeamposition.Thisapproachwasadoptedbythepresentauthortoanalysea simplysupportedpanelstiffenedinonedirectiononly,undera singleconcentrat-edloads TheanalysishasbeengeneralisedmorerecentlybySmithGtoapplytoa verywidevarietyofinterconnectedsystemsofrectangularplatesandparallelbeamsinonedirection,simplysupportedattheends.Structureswhichcanbeanalysedincludeun-symmetricalorskewstiffeners,swedgedorcorrugatedplating,boxbeamsortheVee bot-tomregionofa frigatewitha riseoffloor,asshowninFig.4. A rathersimilarana-lysisfororthogonallyIntersectingbeamgril-lageswasattemptedabout10yearsagoby
Kendrick7whoassumedthattheorthogonalcom-ponentsofthein-planedisplacementcouldbeconsideredseparately,theonlyinteractionarisingfromtheverticalshearforcesatthestiffenerintersections.Kendrick’sworkin-dicatedthatveryaccurateresultscouldbeobtainedbytreatingthestiffenersaloneusingbeamtheoryandincludingtheplatingasanef-fectiveflangetothelongitudinalandtrans-versebars.Theeffectivebreadthofplatingwastakeninthisinstanceasthefullstiff-enerspacing.Inpractice,theplatepanelsofwarshipgrillagesareseldomperfectlyflat,andtheremaybepermanentsetuptoaboutaplatethicknessdeepcausedeitherbyweldingdistortionsorearlyloadingsonthestructure.Thishastheeffectofrenderingthesimplelinearplateequationsinvalid,and,inviewofthecomplexityoftheproblem,thepracticeatN.C.R.E.istoestablishmethodsofchoos~ngstructuralidealisationsbycomparisonwithexperiment.
Nowadays,transversestrengthandgrillageanalysisisbasedonfiniteelementidealisa-tionswiththeproblemsbeingsolvednumerical-lyona digitalcomputer.Untilrecently,acomparativelysmallfirstgenerationcomputer,theFerrantiPegasus,hasbeenusedatN.C.R.E.Initiallytherewasa 4096wordhigh-speedstorewhichhasnowbeenincreasedto7168words.Generalstructuralanalysisprogramshavebeendevelopedforthiscomputer,includ-inga planeframeprogram8suitableforthetraditionaltransversestrengthcalculation.Thisprogramwilltreatanyrigid-jointedmulti-connectedframeuptoabout20intersections.A facilityexistsfortreatingmoreintersec-tionsbypartitioning,andsheardeflectionscanbeincluded.Twoprogramshavebeenwrittenforflatgrillages.g-~oThefirstisverygen-eralandwilltreatanynetworkofstraightelements,notnecessarilyorthogonal,uptoabout20intersections,includingbothshearandtorsion.Thesecondislimitedtoortho-gonalgrillageswitha rectangularboundaryignoringtorsion,butitcantackleupto72intersections.AtthepresenttimeprogramsarebeingwrittenfortheEnglishElectricKDF9 computerwhichwilltreatgrillageswithuptoabout200intersections.Theplaneframeandgrillageprogramsconsidertheplatingtoberepresentedasaneffectiveflangetothebeamelements.A programhasalsobeenwrittentoanalysethetransversestrengthofa com-pletemidshipcompartmentofa ship’shull.11-12Thisprogramincludesshearpanelelements,inadditionto~hebeamelements,toallowfortransferofin-planeforcesbetweenadjacentframes.Duetothelargenumberofunknownsandthelimitedsizeofcomputer,theanalysisisrestrictedtocylindricalcompartmentsiden-t~calthoughnotnecessarilyevenlyspacedframes,andlongitudinaluniformalongtheir
-5-
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A-’’-”L
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FIG.4, TYPICALBEAM-PLATINGsTRUCTURESFORPLATETHEORYANALYSIS.
lengththoughnotequalinsize.
Throughoutthegrillageandtransversestrengthinvestigations,experimentalworkhasbeencarriedoutoftenatfullscaleontypi-calsteelstructures,toassistinformulatingstructuralidealisations,Inparticular,thechoiceofaneffectivebreadthofplatingandoneffectiverigidityofshearpanels.Theprocedurehasusuallybeentocarryoutcal-culationsfora wholerangeofassumedvaluesfortheseparameters,andchoosethevaluesgivingthebestagreement.Generally,forflatgrillages,theassumptionthateffectivebreadthisequaltohalfthebeamspacinggivesresultswhicharesufficientlyaccuratefordesignpurposes,andsometypicalresultsfora grillageunderuniformpressureareshowninFig.5. Thisgraphisparticularlyinterestingasitillustratesthepronouncedeffectoflocalbendingbetweentheintersec-tionsina gr~llagehavingwidelydifferentlongitudinalandtransversemembers.Thechoiceofidealisationfortransversestrengthcurvedgrillagecalculationshasbeensupportedbyexperimentsbothonsmall-scalemodelsinxylonite(aplasticmaterial)andona nearfull-scalesteelmodelofa frigatesection.~3Th~smodelisillustratedinFig.6,andtheresultsformsometrialcalculationsforaloadatthekeel,usingvariouseffectivebreadths(PLandPT)andshearrigidities(G),areshownirIFig.7. Itwasfoundthatcalcu-lation2,shownasthefull-linecurves,gavethebestoverallagreementwithexperiment.
GRILLAGEN..3-STRESSESON TABLESOFOUTERLONGITUDINALBEAMS.
Q FXPERIMmmL Pomm. roP GRILLAGELOAPED 0. PLhTlH6 s,m.= bOTTOM GmlLLAGE LOADED 0?46TIFFEMER SIDE.
_ TWORCTICAL =..”= FOliCFFCCTIVC BR!2?.DTH. HAIX BEAM GPAC!WG6.
)4,.SPAN rDGE
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i----HALT L.NG7H 0. GRILLAG.=100IN.—
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m (’)
L_,,FIG.5. STRESSMEASUREMENTSONGRILLAGEUNDERUNIFORMPRESSURE.
ThestructuralidealisationcloselyfollowedtheshapeoftheframeandincludedalltheIongitudinalsintheregionoftheappliedloadatthekeel,butamoreapproximateide-alisationwasadequateelsewhere.
Theverys~gnificantadvanceusinggril-Iageanalysis,overtheoldsingle-ringframecalculations,canbeseeninFig.8 fora con-centratedkeelloadandFig.9 fora hydrosta-ticwaterpressureloadingextendingto28ftabovethekeel.Theseresultsarefora com-parativelyshortcompartmentmeasuring13%ftlongby24-3/4ftwide.Thecurvedgrillagetransversestrengthanalysisagreesverywellwiththemeasurementsinbothcases,whereastheringframecalculationisout.byfactorsfrom5 to25,andbearslittleornorelationtotheactualbehaviour.Fora longercom-partmentora structureonlytransverselyframed,theagreementusingthesinglering
-6-~w-TT-:-” ““-! ~“”
,.,,;,r::yT~framecalculationwould,ofcourse,becloser.
t ‘,1!!:!!: ~-”.,
FIG.6. FRIGATEMODELINLARGETESTINGFRAME.
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L ,5
Nowthatconsiderablylargerandfastercomputersofthesecondgenerationai-ebecomingavailable,generalstructuralanalysisprogramsarebeingdevelopedforthree-dimensionalstruc-turescomposedofstiffenedplatepanelsorplatedgrillages.OnesuchprogramisbeingwrittenbytheEnglishElectricCompany,Ltd.atKidsgrove,England.Usingthistypeofpro-gram,itshouldbepossibletoexaminethein-teractionbetweenlongitudinalandtransverseloading,andalsoanyeffectduetotaperoftheshipsection.Equallywell, generalstruc-turalanalysisprogramsareapplicabletoprob-lemsmoreconcernedwithlongitudinalstrengthsuchasthebreakofforecastleindestroyersandfrigates,theeffectofsuperstructures,andtheanalysisinwayoflargehatchesandopeningssuchasthesidel~ftopeningsinair-craftcarriers.Atthepresenttime,theEnglishElectric(Kidsgrove)programisbeingevaluatedatN.C.R.E.forthetypeofproblemswhichariseinshipstructures.Asyet,itisnotclearthatthegeneralanalysisprogramre-ferredtoisIdeallysuitedtoallproblems,anditisquitepossiblethatprogramsforpar-ticulartypesofproblemwillstillbere-quired.Inviewoftheconsiderablerecentde-velopmentsinthecomputeranalyslsofshipstructures,particularlyinregardtotransversestrength,thecommitteeoftheInternationalShipStructuresCongressdealingwithstiffened
I
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(,,0,,,0NOWJALLYTO OUTLINEOF HULL)
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FIG.7.THEORETICALANDEXPERIMENTALCOM-PARISONSFORLOADINGONKEELOFFRIGATEMODEL.
f COMPRE5510M
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FIG.8. COMPARISONBETWEENCURVEDGRILLAGEANALYSISANDRINGCALCULATIONFORKEELLOADINGONFRIGATEMODEL.
~“~–Ll~G.6
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\FRAMERINGCALC. ——- — \\CURVEDGRILLAGECALC. : \
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II
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FIG.9. COMPARISONBETWEENCURVEDGRILLAGEANALYSISANDRINGCALCULATIONFORHYDROSTATICLOADINGONFRIGATEMODEL.
-8-
platepanelsinthree-dimensionalstructuresissponsoringanenquiryregardingcomputerpro-gramsinvariouscountries.
Toconcludethissectiononelasticanaly-sis,Iwouldliketorefertotheanalysisofashipina floatingdockbyVaughan.15Inthisanalysis,theshipisrepresentedasa beamconsistingofa numberoffiniteelements,eachofwhichisreactedbya dockblockregardedasanelasticspringrestingonthedockfloor,itselfa grillage.Thecompletesystemisre-actedbybuoyancyforces,andinitiallackoffitbetweenthedockblocksandfloorisin-cluded.Thisisa veryusefulexampleofaparticularproblemwhichcouldnotbetackledusingoneofthestandardcomputerprograms.Nevertheless,realisticanalysiscouldnothavebeenaccomplishedat.allwithouta computer.Ina previousanalysls,AmerikianlGassumedthatthefloorofthedockwasrigidandthattheshiphaduniformrigidityandwassupportedbyuniformdockblocks,inordertobeabletocalculatethedockblockloads.Induecourse,itisintendedtocarryoutparametricnumeri-calstudiesusingtheN.C.R.E.computerprogramtoexaminemorewidelytheeffectoflackoffitandnon-uniformityofthedockblockswhichareimponderableaffectingthepracticalproblem.
PLASTICCOLLAPSE
Themostimportantpotentialapplicationof plasticcollapsetheoryto,ships’structuresisinlongitudinalstrength,wherethemomenttobreakthebackoftheshipisequaltothefullyplasticmomentoftheship’scross-section.This“strength”approachtolongitu-dinalbendinghasbeenproposedina recentpaperbyCaldwell.~7
Inalmostallothershipstructuralprob-lems,themethodsofbendingcollapseanalysisarenotsoobviouslyapplicable.Thisisbe-causea stiffenedpanelsupportedalongallitsedgescannotbelaterallydeformedwithoutde-flectionswhichhavecurvatureintwodirec-tions,andthisinturnleadstoa significantmembranestretchingaction,oncethedeflec-tionsbecome“large”.Fora singleplatepanelbetweenstiffeners,“large”deflectionsImplyamagnitudefromaboutoneplatethick-ness,sayhalfaninchinthecaseofa war-ship.Membranestretchingarisesevenifthesupportingstructureattheedgescannotpro-videanymembranerestraint,andtheactioninsidethegrillageisthenoneofmembranetensionaroundthecentreandtangentialcom-pressionattheedges.Theplatingofshipgrillagesaddsappreciablytothemembranestrengthofthesestructures.Longafterthestructurehasexhausteditsbendingstrength,thestructurewilldeformasamembrane,and
thefinalfailurewouldusuallybeexpectedatsomeweakpointinthedetailsoftheconnec-tions.Itwouldbedifficulttopredictwhe-thergrossplasticdeformationwouldoccurbe-foreanylocalfailures.
Althoughthesimplecollapsetheoryisnotconsideredtobeparticularlyrelevanttoshipstructures,thepresentlackofknowledgeonstrengthandcollapsebehav-iourmakesitalmostmandatorytoloadallexperimentalormodelstructurestocollapse,andthispolicyhasbeeninvariablyfollowedatN.C.R.E.
Threeflatgrillagesweretestedtocol-lapseunderconcentratedloads,thegrillagesbeingsupportedonlyatthecornersforexperi-mentalconvenience.18Twoofthegrillagesfailedina simplemechanisminvolvingonlybendingaction,generallyconfirmingthesimpleplastictheory,butthedeformationsoftheothergrillageinvolvedmembraneaction.Theload-deflectioncurveforthislattergrillageisshowninFig.10,anditcanbeseenthatthereisnoindicationofanycollapsearoundthetheoreticalbendingtheoryvalueof47.5tons.Finalfailureoccurredduetofractureofa weldatanintersectionjoint.Single-directionstiffenedpanelshavealsobeentestedtocollapseunderconcentratedloadinglgandfairlyconsiderablestrengthduetomem-braneactionwasindicated,whichwouldbeex-pectedintheabsenceofanyintersectionjoints.Someflatgrillageshavealsobeenloadedtocollapseunderuniformpressure.LOInonecase,collapsewasbyplasticlateralinstabilityoftheheavytransversebeams,asshowninFig.11,ata loadhigherthanthetheoreticalbendingcollapsepressure.
Morerecently,thelarge-scalesteelfri-gatemodelreferredtoearlierhasbeenloadedtocollapseundera uniformpressureappliedtothebottomstructure.~3 Thestressduetotheappliedloadfirstreachedyieldlocallyinthelongitudinalatabout20lb./in.2,buttherewasnosignoffailureuntil28lb./in.Zwasreached.Atthisstage,a numberoflocalbucklesandcracksoccurredinthemainlongi-tudinalandframes.Thestructurewasloadedseveraltimesfromzeroto28lb./in.2,andthefailuresbecamesuccessivelymoremarked.Eventually,itwasonlypossibletomaintaina pressureof22lb./in.2.A generalviewofthedamage,seenfromtheinsideofthemodel,inFig.12,showsmarkedshearbucklingof“theframewebsadjacenttothekeelinwayofscal-lopholestotakethe2-3/4in.deepsmalllongitudinalstiffeners.Thesewebsweresome-whatslenderbeing0.22in.thickand9%in.deep.Afterthefinalloading,thewebswereverybadlybuckledandfractured,asshowninFig.13. Inassessmentofthecollapsetest,itwasconcludedthatthesewebshadfailedby
-9-
$00 Io 20 30 40 50 60 70 HO 90DEFLECTION,INCHESFIG.10.LOAD-DEFLECTIONCURVEFORCOLLAPSEOFGRILLAGEUNDERCONCENTRATEDLOAD.
a combinationof”plasticinstabilityandfrac-turecausedbyhigh-stressconcentrationsatthescallopsandintermittentweldsbetweenthewebandflange.Otherpositionsofhighstresswereonthemainlongitudinalflangesadjacenttotheendbulkheads.Theseflangesbuckledplasticallyinwayofgapsinthein-termittentweldruns,asshowninFig.14.Similarbucklesoccurredinthetransverseframeflanaesat the bilae wherethere wasareduction. . ;nthesection:Inaddition,there
werea fairnumberofminorcracksandbucklesthroughoutthestructure.
Althoughthecollapseofthefrigatemodeltooktheformofa seriesoflocalfailures,allofwhichoccurredatapproximatelythesamepressure,itisofi[lteresttocomparethe28lb/in.2maximumpressureattained,withthetheoreticalmechanismcollapsepressure.Thetheoreticalcollapsepressure,inthisinstance,wasmodifiedtoinclude,veryapproximately,theeffectofin-planedisplacementsormem-braneaction.Thesecalculationsindicatedthatthetruecollapsepressureintheabsenceofanylocalinstabilityorfractures,orfailureofthesupportingsidestructureorbulkheads,wouldbesomethinglike45lb/in.~,andthatthemodeofcollapsewoulddiffersomewhatfromthatobserved.Thetheoreticalcollapsepressurefortheobservedmodewouldbe70lb/in.2withallmemberstakingthefullplasticmoment,andwithoutallowingforstrengthduetoa planeaction.Thisresultemphasisesthattheratherlowcollapsepres-sureofthemodelwasentirelycausedbyin-adequatedetaildesign.
Theimportanceofrationaldesignofde-tails,particularlyweldedconnections,hasbeenrecognizedatN.C.R.E.forsometimepast,andFaulkner2~haspresenteda reviewofworkonthestaticstrengthofsimpleweldedkneejoints,longitudinaltobulkheadconnections,
FIG.J1. PORTIONOFGRILLAGELOADEDTOCOLLAPSEUNDERUNIFORMPRESSURE.
-1o-
FIG.12. GENERALVIEWOFDAMAGETOFRIGATEMODELAFTERCOLLAPSETEST.
andgrillageintersections.Theobjectofthisworkwastoexaminethestrengthofa numberofsimplejointdesignsandplacethemintoanorderofmerit.Asa resultofthiswork,jointsofvariousdegreesofsophisticationarenowrecommendedforuseinshipdesign,accord-ingtothemagnitudeofloadinganticipated.Itisappreciatedthat“thefatiguestrengthofconnectionsisalsoanimportantissueaffect-ingsurfaceshipsandfatiguetestsona num-berofgrillageintersectionjointsareplannedforthenearfuture.
STRENGTHOFBOTTOMSTRUCTURES
A knowledgeoftheultimatestrengthofthestiffenedplatepanelsorgrillagesform-ingship’sbottomstructuresisrequiredtodeterminethefullv~lasticmomenttobreakaship’sback.Thisl~nconjunctionwithaknowledgeofthemaximumbendingmomenta-swillpermitthetruemarginofsafetyorfactoragainstcollapsetobedetermined.isalsonecessarytounderstandthebehavoftheindividualplatepanelsundersimu’taneouslyappliedlateralandendloadbe-thedesignofthemid-shipcross-sect~on(beoptimised.
sea,oadItour
orean
Withregardtotheplatingofwarships,fewquantitativeresultsapplicableinde-signhaveyetbeenobtained,despitea greatdealofworkatN.C.R.E.andelsewhere.Theprincipalachievementssofarhavebeenforthecaseoflateralpressurealone.A concept
FIG.13.DAMAGETOFRAMEWEBAFTERCOLLAPSETEST.
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FIG.14. BUCKLESINKEELANDLONGITUDINALADJACENTTOBULKHEAD.
FIG,15. TESTARRANGEMENTFORGRILLAGEUNDERCOMBINEDLATERALANDAXIALLOADING.
-12-
FIG.16. FIRSTGRILLAGEAFTERCOLLAPSEUNDERAXIALLOAD.
hasbeenadvancedofdesigningtoa smallbutacceptablepermanentset,assumingthat.theedgesareclampedagainstrotationbutfreetoslideinwards.22Ithasrecentlybeenes-tablishedthattherearesomecaseswhereacompressivestressfieldintheplatingduetobendingofthestiffeningmembersmaysig-nificantlyaffectthepermanentset.20
ExperimentalworkIsnowinhandintheLargeTestingFrameatN.C.R.E.tostudythebehaviouroffull-scaleweldedgrillagesundersimultaneouslyappliedlateralpressureandendload.Fivepairsofmild-steelgrillagesarebeingtestedinitially,eachwithoveralldimensionsof20ftby10ft,coveringa rangeofwidthtothicknessratiosforplatepanelsbetweenstiffenersfrom40to100,anda rangeofspantoradiusofgyrationratiosforlongi-tudinalgirders(betweenframes)from20to60.Thefirst’grillageofeachpairisbeingtestedtocollapseunderendloadonly,whilethesecondwillbecollapsedundera combinationoflateralandendload.In-planeloadingisappliedthroughsix150-.tonhydraulicjacksactingthroughloadcellsandballcaps;later-alloadisappliedbya waterfilledrubberbag,asshowninFig.15. Approximatecondi-tionsofsimplesupportareprovidedbylight
flexureplatesattheendsofeachgrillageandbytie-barsatthesides.ThecompletetestassemblyisshowninFig.16,whichalsoillus-tratesthecollapseofthefirstgrillagebyelasto-plasticinstabilitybetweentwoadjacentframes.Inparallelwiththeexperimentalwork,numericalcomputerprogramshavebeendevelopedfortheelastlcanalysisofgrillagesundercom-binedlateraland~n-planeloading,andtocal-culatethebucklingloadsandmodes.ProgramsforPegasusarecomplete,andprogramsforthelargerKDF9 computerareinpreparation.
EXPERIMENTALTECHNIQUES
Evenwiththelargeandhigh-speeddigitalcomputersnowavailable,thecomplexityofshipstructuresissuchthatconsiderablesimpliflca-t.ionandapproximationareusuallyrequiredbe-foretheoreticalanalysisisfeasible.AtN.C.R.E.,allthestructuralinvestigationshaveincludedexperimentalworktoassistinformu-latingidealisationsandtocheckontheaccu-racyoftheresults.Ideally,large-scalemodelsusingfull-scalematerialsandfabrica-tiontechniquesaredesirablesothatthede-tailsatjointsandtheweldingdistortionsaretypicaloffull-scalepractice.Thesedistor-tionshaveanimportantbearingontheeffec-
_13-
F.IG.17. SMALL-SCALEGRILLAGEMODELAFTERCOLLAPSE.
ofthe~latinuasstiffenerflanaestivenessandundershear.Becafiseoflimitations&thesmallestsizeofstraingaugeavailable,large-scalemodelsarealsonecessarytocarryoutdetailedstrainsurveys.Informationoncollapsestrengthcanonlybeobtainedfromrealistic,preferablylarge-scalemodels.
N.C.R.E.isfortunateinhavinga largetestingframemeasuring69ftlongby33ftwideby39ftfthigh,capableofwithstandingloadsupto500tons(or2,000tonsaxially).!!3Thefrigatemodelmentionedearlierwastestedinthislargetestingframe.
Large-scaletestshavetheunfortunatedisadvantageofprovingextremelycostlyanddifficulttocarryoutduetotheverylargeloadsandtestrigsrequired.A largenumberofsmallmodelscanbeconstructedandtestedforthesameoutlayasonelargemodel,andsmallmodelsareusuallyadoptedbyN.C.R.E.wheneveritisdesiredtostudytheeffectsoffairlywidechangesofparameters.Ontheotherhand,small-scaleweldedsteelmodelshaveoftensufferedfromlargeandcompletelyunacceptableweldingdistortionswhichrenderstrainmeasurementsintheelasticrangeofverylimitedvalue.Small-scalesteelmodelsmayalsobefabricatedusingsilversoldertechniquesorbyintermittentorspotwelding,buttheseconstructionsareseldomsufficient-ly strong for tests in the plastic range, Inrecentyears,thediptransfergasshieldedelectrodemethodofweldinghasbeenadoptedforcomparativelysmall-scalesteelmodels.Thisprocessproducesverymuchsmallerweld-ingdistortionsthanthemanualprocesses,
owingtothesmallerheatinput.Partlyasatestonthediptransferprocedures,twosmallgrillageshaverecentlybeenconstructedatN.C.R.E.usingmaterialrangingfrom0.15to0.27in.thick,whichmightberegardedas1/3or1/4offullscale.zqThemodelsincorpor-atedfilletwelds,buttweldsintheplating,andcorrectlyscaleddetailsattheintersec-tions.Onlyverysmallweldingdistrotionswereobtainedwhichwerequitetypicalofgoodfull-scalepractice.Themodelswereloadedelasticallyandthenintotheplasticrange.Themaximumloadsappliedwereabovethetheoreticalplasticcollapseloadsbuttherewerenocracksorsignsoffailure.A viewofthefirstmodelaftertestingisshowninFig.17. Thepresenceofbuttweldsintheplatingdidnothaveanynoticeableeffectoneitherthebehaviourofa panelbetweenstiff-enersundera localizedloadingorontheover-allbehaviouruptothemaximumloadapplied.Asa resultofthesedevelopments,it~san-ticipatedthatfuturestructuraltestingmaytendtobecarriedoutmoreonsmall-scalesteelmodels.
Forpurelyelasticmeasurements,small-scalemodelsina plasticmaterialsuchasXy-lonite(cellulosenitrate),celastoid(celluloseacetate),perspex(polymethylmethacrylate),orvybak(rigidpolyvinylchloride)havemanyad-vantages.Despitetheirnomenclatureasplasticmaterials,themechanicalbehaviourofthesematerialsiselastic,obeyingHooke’sLawuptoverylargestrains,say0.6to0.8%,pro-videdthemeasurementsaretakenata constanttimeintervalafterapplicationoftheloadtoallowforcreep.Oftheseplasticmaterials,
-14-
FIG.18. GRILLAGEMODELINVYBAK.
celastoidcanberuledoutsinceitisverysensitivetohumiditychanges,whileperspexisonlyavailablefora fairlylimitednumberofthicknesses.Xyloniteispossiblythemostcommonlyusedplasticmaterialformodelsus-ingstraingaugetechniques.Incommonwithmostplastics,xylonitehastheadvantagesofa veryuniformthicknesswithsmoothsurfacesreadyforcementingstraingauges,softnessmakingiteasytocutandwork,andeaseofformingbymouldingatabout120”C,somewhataboveitssofteningpoint.ThelowYoung’smodulus,roughly1/100ofthatofsteel,leadstoappreciablysmallertestloadsandsimpli-fiestheproblemofprov~dingstiffsupportingstructures.Unfortunately,Xylonitehaspoordimensionalstability,andthematerialisfrequentlyfoundtowarp,distortandcontractoverperiodsofmonths.Itselasticconstantsatanyonetimeareaffectedbytemperatureandhumiditychanges,andtheyalsovarywiththelifeofthesheet.Also,thebondsatconnectionswhicharemadebywettingwiththesolventacetoneora mixtureofacetoneandamylacetatearenotfullyreliableandmay“give”orevenfailatquitelowstresses.AnotherdisadvantageofXyloniteisitsin-flammablenature.
AtN.C.R.E.,a rigidpolyvinylchloridematerialmanufacturedbyBakelite,Ltd.underthetradename“Vybak~’,hasbeenadoptedasasuperiorreplacementtoXylonite.Vybakhascompletedimensionalstability;itisnotin-flammable;it’sYoung’smodulusismoreuni-formthanXylonite;anditisnotaffectedbyatmospherichumidity.Therigidityofthematerialisaffectedbytemperature,rathermorethanXylonite,sothattemperaturecon-trolofthelaboratoryisrequired.VybakcanbebondedveryeasilyusingthecementsTensolNo.6 andTensolNo.7,manufacturedbythePlasticsDivisionofImperialChemicalIndus-tries,Ltd.forusewithperspex.Boththesecementsarecoldsetting,TensolNo.6 issuitableforshearconnectionsbutshrinksoncuring,whilecementNo.7 canbeusedtofillgapsandwilldevelopalmostthefulltensilestrengthofthevybak.Vybakhasbeensuccess-fullyusedatN.C.R.E.fora numberofgrillageandtransversestrengthexperiments,(suchastheaircraftcarrierflightdeckstructureshowninFig.18)andmodelstoinvestigatestressconcentrationsathatchopenings.
A reviewofmodelproceduresfornavalstructureswaspublishedbythepresentauthor
-15-
in1962,25butthispaperwaswrittenbeforethediptransferweldingwasintroducedforsmall-scalesteelmodels.Anothermodeltechnique,currentlyunderdevelopment,Istousevybakmodelsforelasticinstabilityex-periments.Duetotheverylargeelasticrangeofstrainforvybak,bucklinganalyslscanbecheckedforrealisticfull-scalegeo-metriesofdesignswhichwouldfailbyyieldwhenconstructedinsteel.Inthisway,thetruemarginofsafetyagainstbucklingmaybedetermined.Vybakisalsoverysuitableforinstabilityexperimentsbecauseitcanbebondedtogetherwithonlyminordistortions,evenwhenverysmallthicknessesareused.Thismakesitpossibletoapproachthebuck-lingloadmore closelywithoutlargeeffectscausedbymagnificationofinitialimperfec-tions.
PROTECTIONSTRUCTURESFORMARINENUCLEARREACTORS
N.C.R.E.hasmadeanimportantcontribu-tiontothedesignofcollisionbarrierstoprotectthereactorofa proposednuclearpoweredship.TheworkhasbeensponsoredbytheMinistryofTransportTechnicalCommitteeontheStructuralProtectionofMarineNuclearReactorsandthereportofthiscommitteeisnowbeingdrafted.Fourpossiblemethodsofprovidingstructuralprotectionhavebeenconsidered;namely,a decksystem,grillagesystem,membranetensionbarrierandmembranecompressionbarrier.Ofthese,thedecksys-temwasfavouredsince,bystiffeningtopre-ventinstability,thestrikingshipwouldberesistedbyforcesneartoyieldstressthroughoutthedeckmaterial,andthespacerequirementswouldnotbetoosevere.TheprincipleofthedeckresistingstructureisillustratedinFig.19,thestrikingwedgebe-ingresistedbya directcripplingstressinthedeck,normaltothewedge,anda frictionalforcetangentialtothewedge.Thedeckre-sistanceattheleadingedgeofthebow,andthebendingrigldltyandfrictionfromtheoutershellplatingwereconsideredtobesecondaryterms.Withtheseassumptions,someverysimpleexpressionswerederivedforthepenetratingforceandworkdone,andtheseweresubsequentlyconfirmedbystatictestsona seriesofmodels.Inordertoachieveashighaspossiblea valueforo,thecripplingstressnormaltothewedge,thebreadthtothicknessratioforeachplatepanelbetweenstiffenerswaslimitedto50,andtheeffec-tivespantoradiusofgyrationratioforthestiffenerswaslimitedto40.
Thepossibilityofcarryingoutdynamictestswasconsidered,butwasruledoutduetodifficultiesincorrectlyscalingboththevelocityofloadapplication,andinertiaef-
\/ Hedge
l-#\Crippling Stressi \.Frictional Stress= uainDeck= o
(Takena. 90%of 0.3%ProofStress)
PenetratingForce= 2oA(sin~ + UCOE~) foreachdeck
= 2kut(tml; + U)X
where A = effectiTemea of contactoneachBideofwedge
k = (effectiveareaofCOntact)i(areaofplating)= A/APt - thicknessof plating
Workdone .I
Fdx . kmt(tan; + “)X2
ResultB~or54in.penetrationonModel2:
k“+ u IF tons workEme tonsin. I1.0 0.2 105 284o1.0 0.3 128 34401,85 0.2 ‘- 5250
Experimental\ 126.5I 3200-l
FIG.19. PRINCIPLEOFDECKPROTECTIONSTRUCTUREFORNUCLEARREACTOR.
fects.Itwasthoughtthatdynamiceffectswouldbesmallina practicalprobleminvol-vinggrossplasticdeformationofmanysquarefeetofmaterial.Thetestswerethereforecarriedoutonsmall-scalemodelsloadedstaticallyina 500-tontestingmachine.Statictestsalsoprovideda continuousrecordoftherelationbetweenloadanddepthofpenetrationwhichwouldrequirea largenumberofmodelsifdeterminedbydynamictests.Thepossibil-ityofa finaldynamictestatfullscalewasconsidered,buthasnotbeenfollowedupduetotheprohibitivecostsinvolved.
Themodelswerebasedona full-scalebarrierdepthof20ftanddeep-framespacingof10ft. Twodesignswereconsidered,onehavinga deckplatingthicknessof1/2in.andthesecond1 in. Thefirstmodelwasa 1/8scalemodelofl/2-in,thickdeckstructure,constructedinmildsteelusinga silversol-dertechnique.Itwasloadedbya rigidwedgewitha 30°includedangleand3/4in.radiusnose.Thistestwasnotfullysuccessful,duetoinsufficientendconstraintinthetest
-16-
ring,andfailureofsomeofthesolderedjoints.Thesecondmodelwas1/4scaleofweldedconstruction,andthismodelfailedprogressivelybylocalcripplingofthestif-feneddeckstructureinwayofthepenetratingwedge.Theresultsarecomparedwithpredic-.tionsfromthesimpletheory,fora 54-in.penetrationinFig.19.
Thethirdmodelwassimilartothesecondbutrepresenteda l-in.thickdeckatfullscale,andthemodelscalewas1/8.Thismodelwasfabricatedbyweldingusingthediptransferweldingprocedure.Themodelwasloadedbya rigidwedgewitha 20°rakeandanincludedangleof30°.Inthismodeltherewasmorepronouncedcurlingofthedeckasitwaspushedbackbythesidesofthewedge,andtheenergyabsorbedwasabout70%ofthevalueestimatedfora verticalwedge,basedonthetestofModel2. Thefourthmodelwasidenti-caltothethird,andwasloadedbya bowrepre-sentativeoftheactualstructureIna fastcarcargoship.Inthiscase,thebowwassevere-lycrumpledwithlittlepenetrationofthebar-rierdecks.ThisIndicatesthatthetypesofdeckstructurebarrierinvestigatedwouldbemosteffectiveinfull-scalecollisions,andthattheenergyabsorbedwouldbesignificant-lygreaterthanthatderivedfromcripplingofthecollisionprotectionstructurealone.Theexperimentswiththerakedrigidbowhavein-dicatedthat,atfullscale,theenergywhichcouldbeabsorbedbytheprotectionstructurewouldbe32,000tonsft(or360,000tons- knot2)foreachdeck,anditiscon-sideredthatthiswouldbeincreasedbyatleast1/3foranysimulatedactualbowstruc-ture.Bearinginmindthatpartoftheenergyofanycollisionwillremainaskineticenergy,itisfeltthata satisfactoryprotectionstructurecanbebasedintheN.C.R.E.designusingsaysixdecks,andthatthiswillbeadequateagainstalmostallpossiblecolli-sionsatsea.
CONCLUSIONS
Itmaybeseenthatsignificantprogresshasbeenmadeintheunderstandingofshipstructures,particularlybydeveloping.methodsofelasticanalysisusingelectroniccomputers.Furthermore,researchisinprogresstoobtaininformationonloadingsatsea,withregardtobothextremesandrepeatedcyclicloading.Itis consideredthatthemostimportantoutstand-ingproblemaffectingsurfaceshipstructuresistoformulaterationaldesigncriteria.Withmoreinformationontheloadingbecomingavailable,thechoiceofmaximumallowablede-signstressmustbere-examined,forboththemeanfieldstress,andlocallyatstresscon-centrations.Atthepresenttimethereap-pearstobesomeinconsistencyindesigners’
attitudestostressconcentrations.Whilemajorstructuraldiscontinuitiessuchasthebreakofforecastleormajorhatchopeningsreceivecarefulattention,itisquitecommonforsmallerdiscontinuitiessuchasscallopholes,jointsandconnectionseithertobede-signedbyeyeorhardlytobeconsideredatall.Ourshipdesignersmustalsoassesswhetherminimumweightdesign,possiblyac-companiedbyincreasulmanufacturingcosts,isofinterest.Minimumweightandcostingstudiesarenowquitefeasibleusingelectron-iccomputers,thoughdifficultiesmaybeex-periencedinobtainingdataonthebreakdownofmanufacturingcosts.
ACKNOWLEDGEMENT
ThispaperispublishedbypermissionoftheMinistryofDefence(NavyDepartment).
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23.“TestingFrameforShips’Structures.”
Hi5-!7:: ’76’‘0’456”‘“’y24’
C..,...,i... r-’l,.””:c:-n.:--ticLLIL,Ly L,,az,F, LLILe LA”,L
DOCUMENTCONTROLDATA- R&D(Securityclaaai{icationof title,bodyofabatractand,r!dexingartmfat?ofirnuatbcenteredwhentheoverallreporris classified)
1 ORIGINATINGACTIVITY(Corporateauthor) 2a. REPORT SECURITY CLASSIFICATION
NoneShip Structure Committee 2b GROUP
3.REPORTTITLE
A SURVEYOFSOMERECENTBRITISHWORKONTHEBEHAVIOUROFWARSHIPSTRUCTURES
4.DESCRIPTIvE NOTES(Typeofreportand)nclusivodates)
Special Reportj AUTHORI’S)(Laxtriruna,firstn.am~,inlficd)
Clarkson, J.
;.REPORTDATE 7a. TOTALNo. OF PAGES 7b. NO.OFREFSNovember1966 25Ja.CONTRACTORGRANT NO. 9a. ORIGINATOR-S REPORT NUMBER(S)
Noneb. PROJECT No. SSC-178c, gb. OTHEW REPORT NO(S) (Anyothernumbersfhafmayboaaaignod
fhisreport)
d.10.AVAILABILITY/LIMITATIONNOTICES
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Il. SUPPLEMENTARYNOTES 12.SPONSORINGMILITARYACTIVITY
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13,ABSTRACT
Entirely rational elastic or plastic design procedures for surfaceship structures have not beenachieved. During the past 15 years, considerableadvancesand developmentboth in the understandingof the mechanicsof shipstructures and in the application of digital computersto ship problemshavebeenmade. This is a report orI research in progress to obtain information onloadings at sea, with regard to both extreme and repeated cyclic loading.
. - . ..., . . - ArUKM1JAN64 14/3
SecurityClassification
.. -,. .. ..securl~y~lasslncatlon4- LINKA LINK~ LINKC
KEYWORDS ROLE WT ROLE WT ROLE ‘#T
Ship Hull Structures
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SecurityClassification
NATIONALACADEMYOFSCIENCES-NATIONALRESEARCHCOUNCILDIVISIONOFENGINEERING
TheShipHullResearchCommitteeundertakesresearchserviceactivitiesinthegeneralfieldsofmaterials,design,andfabrication,as.relatingtoimprovedshiphullstructure,whensuchactivitiesareacceptedbytheAcademyaspartofitsfunctions.TheConnnitteerecommendsresearchobjectivesandprojects;providesliaisonandtechnicalguidancetosuchstudies;re-viewsprojectreports;andstimulatesproductiveavenuesofresearch.
S!+IPHULLRESEARCHCOMMITTEEChairman:
ViceChaimnan:
Mr.MauriceL.SellersNewportNewsShipbuildingandDryDockCompany
Dr.H.NormanAbramsonDirector,Dept.ofMechanicalSciences
SouthwestResearchInstituteMr.HaroldG.AckerShipbuildingDivisionBethlehemSteelCorporation
Mr.W.H.BuckleyChief,AirframeCriteriaStructuralSystemsDepartmentBellAerosystemsCompanyMr.A.E.CoxAssistantNavalArchitectNewportNewsShipbuildingandDryDockCompany
Dr.N.H.JasperTechnicalDirectorU.S.NavyMineDefenseLaboratory
Mr.F.J.JoyceManager,MarineDesignNationalBulkCarriers,Inc.
ArthurR.LytleDirector
Mr.T.M.BuermannGibbs&Cox,Inc.
ViceChairman:
Dr.J.M.FranklandRetiredNationalBureauofStandards
Members
Mr.WilliamR.JensenStructuralMethodsEngineerGrummanAircraftEngineeringCorporation
Mr.J.A.KiesHead,BallisticsBranchMechanicsDivisionNavalResearchLaboratoryDr.WilliamR.OsgoodProfessorofMechanicsCatholicUniversityofAmerica
Dr.G.M.SinclairResearchProfessorofTheoreticalandAppliedMechanics
UniversityofIllinoisMr.MervilleWillisNavalArchitectNewYorkShipbuildingCorporation
ProfessorRaymondA.YagleDept.ofNavalArchitectureandMarineEngineering
UniversityofMichigan
R.W.RumkeExecutiveSecretary
SHIP STRUCTURE COMMITTEE PUBLICATIONS
Theseat%umentsare distributedby the Clearinghouse,Sptingfie2d,Vu. 22151. Thesedoctunentshavebeen announcedin the TechnicalAbstractBu22etin(TAB)of the DefenseDocumentationCenter(DDC),CameronStation,ALexandria,Vu. 22314,underthe indicatedADnwnbers. Thereis no chargefor doetenentsfor registeredusersofthe DDC services. Otherusersmustpay the prescribedrateset bythe Clearinghouse.
Indexof Ship StructureCommitteePublications(1946 - Apri 1 1965 )
SSC- 165, LocalYZeZdingand Extensionof a CraokUnderPlaneStressby
G. T. Hahn and A. R, Rosenfield. December 1964. AD 610039.
SSC-166, Reversed-BendTestsof ABS-CSteelwithAs-Rolledand MachinedSurfacesby K. Satoh and C. Mylonas. April 1965. AD 460575.
SSC-167, Restorationof Ductilityof Hot or ColdStrainedABS-BSteelbyTpeatmentat 700 to 1150F by C. Mylonas and R. J. Beaul ieu.
April 1965. AO 461705.
SSC-168, RollingHisto~ in Re.Zationto the Toughnessof ShipPlate byB. M. Kapadia and W. A. Backofen. May 1965. AO 465025.
SSC-169, InterpretativeReporton Weid-MetalToughnessby K. Masubuchi ,R. E. Monroe and D. C. Martin. July 1965. AO 466805.
SSC-170, Studiesof SomeBrittleFractureConceptsby R. N.Hall, S. W. Terry, W. J. Nordell and G. R. Erhard.AD 476684.
SSC-171, Micro-and MacrocraokFormationby B. L. Averbach.AD 473496.
SSC-172, CrackExtensionand PropagationUnderPlaneStressRosenfield, P. K. Dai and G. T. Hahn. March 1966.
Wright, W. J.September 1965.
October 1965.
by A. R.AD 480619.
SSC-173, Exhaustionof DuctilityunderNotchConstraintFolZ.owingUniformPrestraZningby C. Mylonas, S. Kobayashi and A. Armenakas.August 1966. AD 637143.
SSC-174, Investigationof ResidualStressesin SteelWeZdmentsby K.Masubuchi and D. C. Martin. September 1966.
SSC-175, MechanicalPropertiesof a High-Manganese,Low-CarbonSteelforWeZdedHeavy-SectionS7zipPlate by R. O. Stout and C. R. Roper, Jr.August 1966. AD 637211.
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