., PERMISSIBLE STRESSES AND THEIR LIMITATIONS

1 SSC-206.,PERMISSIBLESTRESSESANDTHEIRLIMI TATIONST hisdocumenthasbeenapprovedforpublicrelea seandsale; : , , , ,withthepermissionoftheauthor, , ,111 RADM, , ,TheNetherland~Washington, .. ABSTRACTV ariousaspectsofca~abilityanddemandofship ssuchasextremeloads,cyclicloads,plasticd esign,crackdesign, ,theyshouldbeboundedbyprobability-concep ts, , failsafe , ( PERMISSIBLE ) ,permissiblestressesareofrestrictedvalue inplasticdesign, , ,infuturethedangeroflargecrackswillbecom eaJ@@ slength(L/20);latertheexorientofLwasredu ced,firstto0,5(l,l~L)andlaterto0, ,wherecisaconstantandinwhichx,accord-ing toNordenstrom~shouldbeequalto- :whatisrealistic?Shouldthestandardwavesr epresentthemostsevereconditionsashipcane vermeetinherlife?

2 Certainlynot, ; !Manyinvestigationshaveprovedthatthestan dardwavemethodsnowadaysinuserepre-senton theaverageratherwellthemaximumwavebendin gmomentsocurringinships(fig. 1).Fromthis, 22r141210m . eIIvIIIII516,9761014 /mI[14 Calculatedstressrange(h=l,l~)+ ( )14r201210a6420 ~- Gi~ T221-120-m+.,,,7,,05.,, \1(.1(A) , >+ ~ ,ONE fEAR-1\ \! ~ !IOF105,.4,0310?NI,01 IIIIIIIIIYo,.,-1010:910:810:710:6~Q(X>Xj ) :410-310+10:1probabilityofExceedingXjtoT roughStressValuesinSeveralStateandHoos~s rStCZte.(I_ewisz) ~subjectwasmainlylong-termwhichareshownt hehighestexpectedbendingdistributionsofb endingmoments,whilethemomentsin20yearsNo rthA c ,opeakstressis16 Kpsi, ~ ~curvea reservationISSC-committee2bIIitisshown , ,butalsooncomparisonsmadebyAertssen, papermadebyHoffmanin-Ferencesaresolarqet hatevenwhenallowance.)]

3 -3-iSmadeforthefactthatLewis calculationsaremorerefinedthanthelatter- mentionedones(headingandangularspreadofe nergyhavebeentakenintoaccount) , ,--thatisataboutQ=10-6-,thetheoreticalup per95%confidencelimitmightdeviatesome20t o40% :20that20to40% see , ,especiallywhen capability inthesamewayas demand ~psduringtheirwholelifetime, , ,Swedish,American,Norwegian,Belgian,Japa nese,French,Ru:,sianandDutchinstitutegat herssufficientmaterialbeforeareliableest imateabouttheextremeloadsfortheshipsthey investigatedcanbemade? ,itwouldcer-tainlycoverafewhundred--anda ftersta-tisticaltreatment--afewthousands hlpyear~Thenitispossibletostatethatthela rgestwavebendingmomentsevermeasuredinall theseships, ,asfarastheauthorknowsthehighestvertical wavebendingstresses(peak-to-trouh)everme asuredareabout17 (Bennet;Johnson,LarkinG).

4 Itisinterestingtonotethatinbothasesmenti onedthestresseswerestill2kg/mm2 lowerthancalculatedforastandard-wavemt1, lK( ).Neverthelesstheyarerealextremesbecause formostshipsthemeasuredmaximawere~ntheor derof10 , ,trades, ~ ; ( [a+b]> , ,bothequalto12 , >m~~~~r~~~?r~~ ( ).Thesumofthelargestindividualwavebendin gtensilestressesandthelargestcompressive onesduringastorm-periodcanbesome10%large r( ).Thisleadsto22 ,5 , ,5kN/cm2isaddedwhichresultsinanabsolutee x-tremetensilepeakof20 ,butwhatmattersisthatthecombinationofthe 12,5and7,5 underestim-ation andthatconditioniscertainlyful-filledwhe n20 , ~ (+)andmax(0)andthecomparisonvalues1,1,25 and1, Althoughthistreatmentoftheresultsistheor eticallynotcorrect, [I1+ Max.)]

5 (+} :*+kt oo ok =JEEax (og ~ -+l*,000 0002P4-Q 2L,ZeroSpeed+,DriftSpeedA0 Heading, ( ; )(Indications Max (O),(+)andComparisonNumbers1, ). ,vibrationsandcorrosionalsoplayedarole, ,shouldbedefinedintermsofchance, ,unfairnessofplating,quali-tyofdesignand welding, (inasense!)theinfluenceofthesevariablesc anhardlybejudgedonaccountofpracticalexpe rience,becausefewstructurescollapseandwh entheydo,therelevantloadingcondi-tionsar emostlyunknown.[180 l-leading, , ,DriftSpeedA0 Heading, ~+5 O. OMeanofvaluesMeanofvaluesforh/A< > , { Forabeamwithrectangularcrosssectionteloa dnecessaryforthede-velopmentofafullplast ichingeis1, ,butahollow-6-1%elaskic+plasticdeformati on(local)0,12%elashcdeformationatG=GYIII DECK realstressandstraindistribution idealplastlcstressdistributionat1%locals trainindeck\Differencebetweenrealidealpl astlcstraindistributionand idea~straindi5trib-1rat1%Local,t~al~j d~~kutionduetoshear-lag\\qOTTOM +strfbutioninth+?))]

6 Side-PlatlngWhen10 StrainHas!) ,likebulk-carriersandcontainerships,some 30%.However, , , Plasticdesign thatdeformat~onslargerthan1% (E=W,0,125%formildSteel).5 Butitshouldbereaizedthatthereisabigdiffe rencebetweenthecaseo-F1%straindevel-oped overalengthof0;5minaship sdeck* ;onlyasmallpartofthesideplat; *Inthefollowlngonly deck ismentionedalthoughoften deckorbottom ,.h,t=dl, ,m(a)Dlstr,butlonoflo , ,d, (Caldwell)Thisformsabigcontrasttothecase whenalargepartofthelengthofadeck,saysome 10m,isplasticallydeformedby1% , ,istooshort, , weakchain intheformofalocallylessefficientstructur aldesigndetail,acorrodedarea,asectioncon tainingopeningsorwithunfair, ,viz.,onwhattestsortheorythementioned1%i sbased,anextension, ~nvolvedlikebeamspacing,platethicknesset c.

7 , ---r?$l plastic hingehasdeveloped!). (PD=1) , , ,conformwelltothecurves(AZbUera,F , ,beingafullydevelopedhinge, , ,wasonly0,2%.Locally,-whichmeansclosetot hebulk-head,Itwasmore,say1%,butithasalre adybeenexplainedbeforethatplasticdeforma tionsinsuchanarrowstriparenotabletobring thewholetransversesectionoftheshipina..~ ~onbetweenthecollapseloadandtheidealplas ticloadwillbelowerthanac-cordingtoCaldwe ll.(Mult+)/(Mpl,)J0,77 Insteadof0, , ,yielding(bending)startedalreadyat250ton s,bWhentheloadisincreasedaverticalplas-~ ,plasticbuck-ling( ). , ,3%abouthalfofthemaximumloadwasleft( ).ItmustbeconcludedthatCald-well shypothesisthatthebehavlourofshipstructu resundercompressionissimilartoidealplast icbehaviourisnotalwaysjusti-fied, ,asymmetricalsect~ons, ,thecollapseloadwouldhavebeenmuchlarger; some95%andnot67%.

8 ,13%I0,1%0,2%T/~ Strain// ; (20kgf/mm2) (26kgf/mm2) ,with(Mult.)/(MPlast,)aslowas0,77and(Mul t.)/(Muy):0,9(Muy=Wxuy)thecol-lapsestres sisstill0,9x26,00=23,40kN/cm2(kgf/mm2).~ ,sothechancethatsuchashipi~theonethatwi1 1meettheextremeof20 F). +10+20+30+Lo + (1!*tbTWOSPECIMENSHOUTFATIGUE,CRACKso AIncb[:)~-,..Jq~~ ,i1:TESTTEMPERATURE[CENTIGRADE) !d:;!.vNDTv,dVEEN 1OO%CRKTALLINEd$udbSTRAIGHTCURVEDSYMBOLS SYMBOLSJ.~~AAVERAGESTRAINATZZINREGION Kz-zL-1-jdmmI~,D1cl? +u----e+-- z z m---m- --- +-----$,,,,z-z,,,$,-c-RoEERTSONARREST(IS UTHERMTE51) ,,4,z-z-,,D1+Dz~racturestarts, thecapacityofthestructurefortakingupdefo rmationenergyisimmediate-lyexhausted;the chanceofcompletecollapsethroughfracturei slarge.]]

9 ~Thehighfrequencyofslammingstresses,whic hwasbenef icientincaseofcompressioncanbedisadvanta geousfortension.~ sopinionthatthedangerofbrit-tlefracturei nshipsmustbeeliminatedbyus-ingcrack-arre stingsteelshasbeenexplainedIastyearatthe Construction-day. l Itseemstoconflictwiththefactthat90% ~ (andhighertensilesteels) ,providedthattheywillnotbeweldedwithweld ingmethodsgivingexcessiveheat-input(one- runElectro-GasandElectro-Slagsystems}For inthatcase, ,cracks,initiat-edintheheat-affectedcoar segrainedzoneadjacenttotheweld,donotdevi atefromthiszoneasusualandstopintheunaffe ctedplate, ,Infactitdoessoalreadynowadaystogetherwi thbrittlefracture, ,dedicatingtheirtimetowavebendingmoments , ] ,whatistheuseofstatisticalmaterial,calcu lationmethods,modeltestsandwhatelse, ,Thereforethepertin-entinvestigationsint heDelftShipStruc-turesLaboratoryhavebeen devotedtothequestionifthepresenceofsmall fatiguecraclsenlargestheriskofbrittlefra cture.)

10 ,thestresslevelwillnolonger(partly) (Ofcoursedamageduetoleakagealsoplaysarol ebutthisisexcludedfromthepresentdiscus-s ion.)Itisafavorablecircumstancethatfatig ue-crackspropagateveryslowlyinships ,wherepermissiblecracklengthsaremuchsmal ler,andconsequentlylesseasilydetectable, thisphilosophyof fail-safe ,JapanandGreatBrit~~ , , , ,b,c,d, (strainenrgyre-5leaserate)andisequaltoG= ~i~;a= , (shearfracturing)isa-bout5,00 cracklengthplushalfsizeofplasticzone= 13 -ix! , ~2a+ , !,< ,because(horizontal)connectionsbetweensp ringsareleftaway,(- YCRACK/~ , (kgf/rnm2)~u~22,50 ,8u!ForsomehigherstrenqthsteelsGrcanyeve nbesmallerthanform; ~ticalstressUc~.! enouY :5k icm;;:::f; , crack-design , , +Asecondproblemistheelasto-plasticbehavi ouroftheship shullincaseofsmallpermissibleplas-ticdef ormations(limitedplasticdesign).)