THE SOCIETY OF NAVAL ARCHITECTS ANO MARINE ENGINEERS74 Trinity Place, New York, N.Y., 10006

PWW t.bepresentedattheshipstructureSynpmiumW.shin@n,D.C.,October6-8,1975

Analysis and Design RequirementsDonald S. Wilson, Member, John J, McMullen Associates, Hyattsville, Maryland

0 Cwvriaht1975byTheSmietYofNwelArchitectsandMmineE.#ineers

ABSTRACT

Major changes in the size, shapeand performance of ships will createa severe challenge to the technicalskills of ship structural designer.Fortunately, the groundwork has al-readY been laid for the technologyrequired to support him. The na~;re andquality of this technical base is dis-cussed and gaps are identified. Theserious need for closing the interfacebetween researcher and designer isnoted. References were selected togive a broad background in progress todate, and an easy initiation into allfacets of the topic .

INTRODUCTION - THE DESIGN PROCESS

The practicing structural navalarchitect is at once blessed and cursedby the thousands of years of experiencegarnered by his precursors -- blessed,because few professions enjoy the bene-fits of so many thousands of full-scale experiments which led to todayr sempirical design methods, and cursed,because in the face of such soundly–based tradition, it is vet’ydifficultto promote innovation. However, inno-vation is required, and must be pro-moted, to meet tomorrow’ s design chal-lenge. Empiricism, which served us wellfor millenia, has been stretched to thebreaking point by our recent exponentialIncreases in the speed and size of $hipsFurther, empiricism based on experiencecannot safely deal with new concepts inhull form, propulsion technology anddYnamic lift. Development of completelyritional structural d~sign method; isessential to insure that structuralefficiency and reliability will keeppace with other performance parametersin the rapidly-developing field ofmarine technology.

Since the 1920’s, major impFow-ments have been made in the structureof ships, including the introductionand refinement of welding and thedevelopment of many excellent high-strength steels and marine aluminumsEven more important, the theoretical

ground-work has been laid for a tOtallYrational process of ship structuraldesign, with the potential for opti-mization and lifetime reliability pre-diction. Between the development of atheoretical concept and its practicalapplication in a design office, however,there is a great gulf which can bespanned only by hard work. The natureand scope of this work, and the newdesign methods which will hopefullyresult, will be the principal subjectsof this paper.

Before discussing the future of theship structural design process, it isnecessary to outline briefly the presentnature of the process, and the environ-ment In which it operates .

The process of overall ship designis an iterative one, proceeding cycli-cally to resolve the conflicts among themany systems which comprise the ship,each of which has its own separateobjective and constraints, but with eachcontributing to the overall goals ofship performance . These conflicts areoften serious, and difficult to resolve,and the compromises which must be madesometimes produce marked changes in theobjectives of individual systems . Thesystem designer may find that he hasdeveloped two or three completely dif-ferent systems for the same ship, withonly the last representing a tOtallYacceptable compromise in itself, andwith other systems The key words inthe ship design process are “time” and“cost”, and the usual demand for speedand productivity leaves little opportu-nity for any system designer to advancethe state of his art.

The structural design process it-self is also an iterative one, sincedirect structural synthesis has beenachieved only for very simple structuralsystems Tbe core of this ite~ativecycle contains these steps: postulationof a geometry; analysis of the responseunder applied loads; comparison ofcalculated response against an estab-lished standard; and return to the geOm-etry, revising it for an improvement inresponse. Other peripheral steps arerequired to support the cycle, including

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the calculation of loads, the carry lng-out of trade-off studies leading tomaterial selection, and optimizationstudies for certain geometric parameters,and the establishment of performancecriteria for materials and structuralelements. Once the general geometry isfixed, fabrication studies will locatebutts and seams, and establish thenature of joints; protection studieswill locate sacrificial anodes anddevelop coating systems; and a periodicmaintenance plan will be developed.

The entire structural design processis normally conducted within the State-of-the-art, or at best with very minorextrapolation. The normal customer hasno interest in supporting tool-sharp-ening, or in advancing the status ofmarine technology. He wants as muchship as his money can buy. Thus, theprincipal advances in the state-of–the-art have come through government orgovernment-funded research programs,such as those of the Ship StructureCommittee, the Navy, the Coast Guardand the Maritime Administration, orthrough the efforts of a few enlightenedowners, operators and builders who cansee the eventual profit accruing froma carefully-directed research task, orthrough the continuing work of theworld!s regulatory agencies, who clearlyhave a vital interest in the structuraladequacy of ships . All these effortsmay be sparked by questions raised, orpreliminary investigations conducted,by technical or trade associations TheSNAME Technical and Research Programmakes si!znificant contributions to thisend. -

It(s appropriate next to look at thedesign techniques now developing, toassess their potentials, and to determinewhat ys left undone There is a vastwealth of research applicable to thestructural design of ships, much of itpublished in the professional journalsof the civil and mechanical engineeringsocieties and the foreign marine soci-eties, and in the foreign and U .S tradeand technical journals, but this paperwill merely identify the salient featuresof developing technology, using thepublications of the Ship Structure Com-mittee and the Society of Naval Architectsand Marine Engineers as the principalsources.

A ma.ior concern throughout thedesign cokmunity is for th~ lack offollow-through among researchers . lhebulk of all research projects culminatesin reports which are not directly useableby the designer, and which require furthertranslation and verification to becomepractical design tools. This translationof research results into design tools isan overhead task, and a time–consumingone, and only the largest and wealthiestof engineering activities can afford theluxury of pursuing it. It is vital that

the translation be accomplished.

LOADS

Our present skills in analysis andmaterials technology far outweigh ouraccuracy in load prediction -– so muchso that the practical structural de-signer has been reluctant to employ someof the more sophisticated tools of anal-ysis available to him, knowing thatinaccuracies inherent in his load criteriawould wipe out any merit the analysisprocess might have to offer. One reasonfor this mismatch is the probabilisticnature of seaway loads, which prevents ,the clean, deterministic statement ofload criteria with ‘which the analyst ismost comfortable The procedures forpredicting wave loading on a statisticalbasis are still under development. Theother reason is the weight of traditionand a century of satisfactory experiencewith empirical methods. The time avail-able to the designer for development ofload criteria is seldom adequate forinvestigation of new prediction techniquesThe inadequacy of empiricism in dealing~ith new hull forms, or even in copingwith recent major increases in ship sizeand speed, has been the driving forcetoward improvement in load predictiontechniques .

For most ships, the controllingload forms are consequences of operatingin waves, and the probabilistic approachto prediction of wave effects , pioneeredby +ierson and St Denis (1) Over tWentYyears ago, clearly represents both amarked improvement in prediction capabilityand a welcome transition from empiricismto a theoretically-supportable procedure .An excellent first exposure to the basicmethod has been developed by Michel (2),and is recommended for anyone interestedin understanding the principles involved.lhe concept is completely rational, andreadily understandable, and it is notsurprising that Gerard and Lewis (3),in recommending a proposed course ofaction for the Ship Structure Committeein 1959, set forth the verification anddevelopment of the statistical approachto wave loading as one of the keystonesto the Committee’ s long-range program.Work in this area has been continuouslyfunded by the Ship Structure Committeesince that time . Prediction of loadresponse to waves of unit height andvarying length, an important step inthis procedure, was initially based onmodel tests . The strip-theory work ofKorvin-Kroukovsky and Jacobs, supportedby several sponsors and summarized in aSNAME monograph (4), provided the basisfor a cOmDletelY analytic replacementfor model” testikg, thereby offering asignificant saving in time and cost.Computerization of the strip approachhas been supported by the Ship StructureCommittee (5, 6, 7), and the SCORES

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program which resulted (8) has since beenextended and adapted for a wide range ofmarine vehicles, including surface effectships and semi-submersible drill rigs

Prediction techniques must be veri-fied experimentally, to give confidencein their outcome, and the techniquesdeveloped to date for unit response towaves leaves somethinE to be desired inthis regard. The SCORES program wasverified initially against model testsin head seas, and correlation was good.Later tests, however, run for ShipStructure Committee in seas fromvarying directions (9) have shown thatstrip theory, as presently applied,runs into trouble in cases where waveencounter frequencies and shiD motionfrequencies a~proach coincidence, asin roll or in following seas (10). TheShip Structure Committee is supportingfurther investigation to tie down thecause of this problem and achievecorrection. In addition, the assumptionof linearity of response with waveheight underlies the entire process, andrecent work (11) indicates that non-linearities. which we recognize toexist, may have important gearing onpredictive accuracy.

.4serious defect in all striptheory approaches, from the designersstandpoint, lies in the format of theirOutput Since they were conceived asreplacements for the static-balancebending moment calculation, and arebased on a conceDt which divides theship lengthwise into discrete elements,they give us gross hull girder shears,moments and torques on the planesdividing these elements Where the shiphull is treated as a simule beam. thisis an admirable presentation. H;wever,for a finite-element hull representation,it requires a very tedious conversionto modify these section forces intosets of forces actirwzat the elementnodes. An entirely ~ifferent approachhas been proposed (12), which providesa set of pressures on the surface nodes,and inertia forces on these and theinternal nodes. to nrovide the sameresult in an oktput’ compatible with afinite-element representation.

AS ships go faster, impulsivebending, or whipping, and impact, orslammin’z. became more imnortant Forexample~’Heller and Kamm;rer (13)identified whipping as responsible foralmost half the maximum total bendingmoment experienced in an aircraftcarrier, operating at high speed inbad weather. Slamming pressures havebeen an important cause of structu~aldamage for years (14), and a SNAMEbibliography, ‘lNotes on Slamming”, ispresently being readied for publishing.Since both of these phenomena aredependent on ship motion, it seemsreasonable to consider whether the strip-theory approach can be expanded to include

them. Kaplan (15) has made a firstattempt to consider whipping, with somesuccess , but the feature is not yetincluded in his SCORES program. AnNSRDC program, IPRESS (16) generatesdesign slamming pressures, using specificgeometries of ship and wave, and pre-determined closing rate, as inputsMating of this program to a good ship-motion program seems to be a possibleway to relate slamming pressures to sea-state, which will solve the response-amplitude operator portion of theproblem (17)

The above procedures Pelate shipgeometry, speed and direction to responsein a given regular sea. This is thenexpanded, by superposition, to give theresponse to a specific random sea. Thelast phase in the total procedure is toconsider the effect of operating theship, for a normal liftime, on itsintended ser”ice route, exposed to ahistorically-suDDorted distribution of. .possible sea states. We are interestedin two outputs from this procedure –- alife-cycle history of loading, for fatigueanalysis; and a lifetime maximum loading,for ;Ompirison against hull girderstrength. For the first, the bulk ofthe area under the distribution curve,figure 1, is of interest . For the latter,

Figure1 Arc..of interestin the di.rri-butio” of load or stress for theser.ice life of a structuralelement.

interest is concentrated on the outerleg of the curve Statistically, thishas no limit, and can only be boundedby accepting some risk of failure out-lines of appropriate calculating pro-cedures have been developed by Lewis etal (18) and ManSour (19) . Ochi hasdeveloped a separate process (20) forpredicting maximum slamming p~essures,incidence of slamming, and deck wetness

Considering the working cost of allof the above procedures , and the intricacyof the irmut data they reauire . they canscarcely ~e considere~ pr~ctical fo;early phases of design, where major

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changes are frequent, and the ancientways still give ‘Iballpark!t figureswhich are adequate for this early work.These recent statistical approaches can

DrOve indispensable. however. for newind novel h;ll form;, or for’any othe~design problem which is outside the rangeof reliable extrapolation from presentpractice. They do require one importantdecision -- the acceptance of a specificrisk of failure The concept of failureas a design goal may be unpalatable tomany old-time structu~al designers whocontrol our destinies, but it is inherentin the statistical design process. Ittsan unspoken element in any design processwhich contains a random function. Eventhe underw~iter 1s rules which governship design today make no promise toavoid failure, and now that we mustdefine, in numbers, the risks we canafford to take, the results of yearsof ship operation under these rules cangive uk S;me clue as to what an accept-able number might be. Some early cutsat this number have been taken (21),but a definitive study is overdue .

Slammlnz and hull-girder bendin=are not the ;nly loads ~eeding con- -slderation -- in fact, for some smaller,faster ships, normal surface forces maybe responsible fo~ more than half thestructural weight. The present approachto these SurfaCe forces involves thearbitrary establishment of lines ofdesign pressure head, based on estimatedeffects of roll and pitch in naves,tempered by experience. Strip-theoTydeals basically with regular waves,not with the ultimate, superposed Com-bination of waves which provides themaximum pressure head. Something newis needed to develop a response pre-diction procedure for surface hydro-static pressure over the entire envelope.

There are many load forms whichthe ship 1s structure must survive .Fortunately, they don-t all reach theirpeak at the same instant. Some can beassigned to a specific part of theoperational cycle, but others occurrandomly. Itrs necessary, particularlyfor finite-element analyses, to establisha load envelope which includes all theload forms active at a given moment.This, too> can be done on a statisticalbasis . usin~ combined Probabilities ofoccurrence, ‘as propose~ by Lewis, etal. (22). At present, It)s done byjudicious but empirical selection ofcombinations of load maxima (23), buta formalized Drocedure Is needed toproduce load ~nvelopes of design maximafor all structure . Abrahamsen (24) hasproposed a deterministic approach to thesuperposition of hull airder loads . andMansour, in work suppo~ted by SNAME andyet to be released, provides a statis-tical summation of secondary verticalhull girder stresses, fen.cmnbinatlo”with similarly expressed primary st~esses.

The relative merits of all these ap-proaches require careful comparison.

ANALYSIS

The function of analysis is simple .It predicts the Teaponse, in terms ofstress or deflection, of a structure orstructural element under a given loadcondition. Based on this prediction,the designer either modifies hlsstructural geometry to produce a moredesirable response, or he defines theDredicted resDOnse as acceptable . andproceeds to the next probl~m. ‘

There is a continuous spectrum ofanalysis complexity and sophisticationavailable. ranxirm from the MC/IaPProach used ~n ?Irst-pass design tothe full-hull finite-element analysis,typified by the American Bureau ofShipping DAISY program (25). All theelements in this spectrum may beinvoked, at some time, in the ship designprocess. The choice of an analysismethod for a specific problem must bebased on a reasonable trade-off amongthe considerations of accu~acv anddetail requlr.ed, the tIme and”skll1savailable, and the cost of the procedure .

In another dimension, the solutionof the differential equations of elementresponse, pioneered by such revered titansas Tlmoshenko, Flugge and Bleich, havelong since progressed to the simultaneoussolutlon of sets of equations for multi-element structures . To deal with struc-tures of awkward shape , which produceunsolvable differential equations, orwhich e“en defy de”elopnrent of anequation, techniques have been developedwhich break these complex structuresinto simpler elements, which can now besolved simultaneously or progressivelyto give a close approximation to thetrue response Both finite-differenceand finite-element methods fit in thislatter category, and these methods formthe bases for most of the powerful com-puterized structural analysis methodsnow available .

Despite the publicity given tofinite-element methods in the technl:alpress, the classical differential-equationprocedure Is far from obsolebe. Forexample, the line-solution procedure,widely used by EuTopean analysts, has 7been summarized i“ hiandbook form byPilk&y (26) under the joint sponsorshipof the Ship StFuctuTe committee, theOffice of Naval Research, and others .Orthotropic-plate solutions, which areessentially differential-equation solu-tions to a much-simplified model of across-stiffened plate, are still beingdeveloped. A recent investigation byMans Our (2?’)into the response of theorthotropic stiffened-plate model beyondthe elastic limit is being extended underSNAME sponsorship (28), to include thepost buckling response of stiffened plate

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under combined loading. This problemof ultimate strength of stiffened plategrillages is under attack from severalother directions, following the ap-proaches used by Ostapenko (29), Chang(30), and Faulkner (31) . The latteraPProach is One of the end-products ofa Year’s effort by a select group ofinvestigators coordinated bv Prof.J. H. Evans of M.I.T. and j~intly sup-ported by the U.S. and Royal Navies andthe Ship Structure Committee. The sum-mation of results of this year of workhas been Dublished bv the ihi~ StructmeCommittee’ (32), and imovides ;n excellentguide to the design of hull girderstructure, using the most up-to-dateworkin~ methods in load Prediction.analys~s and material seiection, i;cludingconsiderations of fatigue and fracture,corrosion, thermal effects and opti-mization. This document should be inevery ship design office .

The biggest change in the analyst’scapabilities has been wrought by thecomputer . Nhile it can do only what theanalyst himself can do, its speed andpatience permit the tackling of thetremendous bookkeeping problems inherentin iterative or simultaneous solutions,on a time scale approaching compatibilitywith the design cycle. The computerreadily solves complex differ-entialequations, using iterative methods ofproven validity, but its most valuableapplication for the ship structuresanal.vsthas been in the field of finite-elemint analysis. Conceived in the com-puter 1s own infancy, developed byArgyris (33), Clough (34), and a hostof followers, this procedure ca” nowprovide bar, beam, shell and solidelements from which a representationcan be built up for any ship structu~alcomponent. These elements have beenused to build up a variety of programsaimed at solution of specific shipproblems. TYPical of recent programsin current use, are those developed forShip Structure Committee by St . Denis(35, 36) and Nielsen and Chang (37, 38,39) for solving the three-dimensionalproblem of stress distribution in trans-versely and longitudinally framed struc-tures. The advent of the latest gene~-ation of computers has made possible thedevelopment of super-programs such asDAISY (Z5), which can analyze an entireship 1s hull structure at one pass.

The application of the finite-element pI.ocess requires that the realstructure be modeled as a complex ofbeams, shell and solid elements havingmechanical properties equivalent tothe real thing. The modeling processis the most critical step in the pro-cedure. It is the last stronghold oftrue engineering judgment in the pl.o-cedure, and deserves the most experiencedtalent that can be made a“ailable .

The newer, better finite-element

programs ask only for the measurablephysical properties of the elements, andsave the analyst from the onerous choreof calculating stiffnesses in alldirections. This is excellent, as longas the analyst can affort to rw a fully-realistic model of the real structure,complete in eve~y detail. Use of acoarser grid, which will save in corn.puter dollars, requires considerableanalytic work to combine the propertiesof all the members within a chosenelement. The mathematical model is theengineers statement to the computer ofthe problem he wants solved. It mustpossess those featu~es of the piwtotypewhich the engineer wants analyzed, a“din the proportions which will produce acomparable response, but it must berestricted in size and complexity to alevel commensurate with the worth of theexpected solution. Despite the criticalnature of the modeling process, it issingularly ignored in the literatweThe only way to learn the art today isthrough painful, expensive experience,although the NASTRAN system does run auser-oriented exchange “hich providesusers with the benefits of the mistakesof their precursors A useable text orcourse on the subject of structur-almathematical modt?lin’zis 10KW overdue.

The problem of ;athemat~cal modelingis worsened by the length of the painfulprocess required to set up the usualanalysis program. In addition to thedecisions involved in selecting the modelvsgridwork, and the characteristics of thespecific elements to be used, a tremendousvolume of wopk is involved in locating thecoordinates of nodal points, and cal-culating the characteristics of theindividual elements . Preprocessors areavailable for some prog~ams, “hich takeover some of this odious chore, and forsome of the simpler programs , interactivesubroutines help the engineer to dealdirectly with the computer. The effec-tiveness of graphical Interaction isnow being investigated (40), followingits aDDarent success in the aircraftindusi;y. The GIFTS system (41),developed by the University of Arizonafor ONR, is specifically designed foruse in ship Structural work, and isbein&! tried out by Coast Guard andothe~s .

The programs themselves are cur-rently the least of our problems Theimportant finite-element pI.ograms pre-sently in use are briefed by Pilkey andothers (42), and the most useful tothe ship design industry are discussedfurther by the contributors to theONR symposium summarized in reference(43). They are being improved, inefficiency and capability, by suchadditions as reduced substructuring,isoparametric elements, varied ‘Vzooming’!techniques for inspecting highly-stressedlocations, and more economical matrix-

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inversion techniques. They have, ingeneral, more power than we can wiselyuse, and discretion is required to avoidoverkill, with resulting cost ineffective-ness. They will continue to be improved,because of the glamo~ swrounding theprogramming game, and these imp~ovementswill not be wasted; but they are not OUPmost urgent need.

So~ution of the usual mediun-to-large sized structural problem usuallyinvolws only about 10% of the totalcost in the computer operation itselfThe other 90% is spent in getting dataready for the computer, and in trans–lating its output into useful conclusionsThe activity in process in input gen-eration has already been discussed. lheoutput problem is of equal magnitude .

The volume of data developed bytoday 7s standard prog~ams is truly anddepressingly huge As W. J. Robertsnotes (Ub). the solution to one shiDgirde~ problem has produced a print; ut~% times the length of the ship. Re-duction of this volume of data to usefulknowledge can be a tedious chore Forstructural data, offline sub–programswhich prepa~e graphical plots of stressdistribution or deflection can be WTYuseful Itts even possible to applydynamic loads progressively, plottingresponses at i-eg”lar time intervals, togenerate a ‘rcartoonvlwhich, with theaid of a movie piwjectorj gives a smn-blance of the real motion under load.This technique is pai-tic”larly usefulfor developing an understanding ofvibration problems. Other time-savingtechniques can be invoked, if some fore-knowledge of the probable outcome isavailable: output can be limited tocritical areas; or it can be reduced byasking for stresses only above a certainlevel. Regardless of the availablemethod used, however, the total timerecluired to encode. calculate. and decode.is “p~esently longe~ than is p~rmlssible ‘in the normal ship design cycle, andthe consequence is that the ship isdesigned by simpler, crude~ methods,and checked, on a not-to-delay basis,by the computer. Techniques are underinvestigation which involve catalogs ofelements, any of which can be pluggedinto a pre-established geometric skeletonby interactive graphic techniques, per-mitting a computerized version of theiterative procedure common to the slide-rule engineer. Within the narrow rangeof geometries and elements used in theaircraft design business, this process isworking The superposed load com-binations, the complex and redundantstructure, and the short design schedulesimposed on the ship design community,make this a much mare difficult problemfor the programmer and the compute~system designer. Intei-action between agraphic console controlled by a mini-computer, and a data bank stored in a

large computer, may makepractical.

Desuite its awesome

this approach

Dower , thecomDuter- is a mindless beast. doing onlywhai it is told to do, but d~ing ii “faithfully and with great precision andspeed. Its user must know what it hasbeen told, in order to assess the val-idity of results. The limits of use-fulness of the algorithms it employs,the ranges of its applied constraints,the quality of data In its catalog andother similar programmer-controlledlimitations, all affect the quality ofits response , particularly in the outerfringes of its applicability, and seem-ingly reasonable but inaccurate answersmay result. Inclusion of lorcical andUnderstandable error-message: in theprogram is,essential. It is alSO theduty of the programmer to include, inhis documentation, the assumptions andconstraints that went into the m’o’aram.to insure that it wonft be applied-out:side its z.ange However, accuracy ofan analysis will always rest with theengineer doing the analysis, and he mustalwavs view conmuter results with sus-pici~n, and ha”; at hand quick methodsto insure that the answers the computergives him are in the right ball-park.This is an area wherein the universitiesowe the technical world a service . Toomany recent graduates believe that thecomputer is the only solution tool nowneeded. Slide rule sales have droppedto zero, and with this trend, interestin the classical methods has alsodwindled. The computer must be fedand interpreted by reasoning and mis–trusting individuals who understandthe algorithm with which the computeri.sworking, who are acquainted withthe engineering principles behind thecomputer method, and who know -- orcan easily calculate -- the rangewherein the answer should be, and whoare ready to investigate if somethinglooks wrong.

Both management and engineer needto be well aware of the practicallimitations of the computer. Forexample, it solves only the problemthat was given it -- the engineersmathematical model -- not the real,nuts-and–bolts problem. It Will doonly what itvs told, and the engineermust take the responsibility to convertthe computer’ s analysis of his modelinto an accurate analysis of the realstructure . Also . it often sDeaks inaverage terms, particularly kor plateelements ThiS is fine for a uniformor slowly-varying stress field, but inthe vicinity of a stress concentration.it doesnvt ~ell the worst Computerapproaches to stress-concentrationsolutions usually involve “zooming”, orfining down the mesh in the criticalspots This is useful. once hhe criticalspot is located -– but”consider the

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plight of the analyst concerned withfatigue He has to get a pictu??e of allthe highly-stressed locations, under eachcritical loading condition, with aquantitative measure of the stress foreach. To do this fully, by computer,may cost more than the structure 1ssafety is worth. Short-cuts to thisprocess must be found.

Structural optimization has beenwith us for years. It is common prac-tice, in trade-off studies, to investi-gate the effect on weight of modifyingframe spacing (45 ) , or of using othermaterials (46, 47) . Our principalinterest In optimization, however, isnot necessarily weight. For most shipdesigns, optimization should probablybe pursued on a basis of life-cyclecost, and this objective has too manyvariables to permit the classic mini-mization procedure to succeed. Evenoverall ship weight optimization has toomany variables to permit a one-passapproach . Fortunately, weight optimi–zation can be effectively pursued, ondifferent segments of the hull girder,by individual actions, without toomuch error. Before it is employed,however, it must be clear that weightoptimization has the potential forimproving the ship.

The possibility of a total prob-abilistic aDDrOach to shiD desire wasmentioned ei;lier. This ;oncep; hasproven successful in estimatingreliability in electronic components,and has been suggested as a possibleaPPPOach tO Predicting the useful lifeof ships, or the risk of possible loss,or any other statistical measure ofintegrity or reliability. Many of the.ioint Probabilities which KO into suchi determination are alread~ beingworked out -- for example, the pre-diction of maximum wave loading, andthe normal distribution of yield strengthTechniques have also been su~s?ested (48)whereb~ these probabilities ~in be cOm-bined to predict a cumulative probabilityof failure (or survival) . This is anappealing prospect However’, Freudenthal(49) warnS that, for large structures,failure statistics canlt be developedexperimentally, for economic reasons,and must be tentatively estimated froma combination of small-element testingand analysis. This is generally theaPProach followed in the aircpaftindustry, except that they often haveproduction runs large enough to supportfatigue-testing of one or two full-scale vehicles to provide experimentaldemonstration of the validity of thereliability prediction. This willseldom be the case for ship design andconstruction. However, we do havethousands of shiDs in service. and itmay be possible Lo tap this lirgereservoir of operational experience to

SUDDIV a statistical base for reliability. . .projections, following the lead ofreference (14).

MATERIAL SELECTION - THE FATIGUE ANDFRACTURE PROBLEMS

Material selection is the Sti7uCtuI’aldesigner 1s prerogative and responsibilityy.For commercial ships, this selection isnormally based on a series of trade-offstudies leading to cost optimization,which must consider lifetime profit-ability FOP military ships, where pro-fit is not a consideration, the goal isa combination of life–cycle cost andperformance optimization.

The basic tools of the process arealready available, in the form ofeconomic models (50. 51) . and simplifiedship design models <52)i‘and these toolshave been used already for the ShipStructure Committee in evaluating thefuture Dotential of aluminum (ti6 ) andclass-reinforced nlastic (47) forw

specific functionsThe conventional static properties

of the available shipbuilding materialsare readily obtainable - often frOM thematerial suppliers, and the designer willseldom have difficulties resolving theproblems associated with strength andstiffness. The properties of toughness,creep and fatigue resistance are not aswell as defined, and their applicationin the design process is far from beingadequately described.

Creep - the long-term deflectionresponse of a material under load - hasnot been recognized as an importantproblem for the shipbuilding profession,since creep in carbon steels is a neg-ligible quantity at normal workingtemperatures. However. as we make wideruse of fiber-rein force~ plastics,aluminums and titaniums, creep willbecome an operating consideration andwe must prepare to design for it.

The mechanisms of fatigue and ofbrittle fracture have been researchedexhaustively, and are fairly well under-stood Both require some form of MateFialdefect for Initiation; fatigue can beginfrom a molecular defect. but fractureFequires a more substantial flaw, some-times as much as sews~al inches in size .In fact, fatigue is often the cause ofthe flaw which ultimately producesfFact“I-e Both can take-Di&Ce ataverage stresses well bel~w yield, sincethe stress concentration at the flawproduces the stress levels requi~ed fordamage The propensity for fracture hasbeen determined (53) to be largely de-pendent on material toughness, a temper-ature-dependent quality which is measur-able by simple testing (54) . Fatiguesensitivity, however, is more difficultto detect, and is usually measured bysubjecting the material to accelerated

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cyclic loadlng, using many specimens atmany stress levels .

While the theoretical explanationfor the phenomena of both fatigue andfracture may be fairly well established,the approach to considering them indesign is not nearly as clea~. TheaPproaCh In the past was simply to avoidall materials which proved to be sensi-tive to either fatigue or fracture .Following the epidemic of b~ittlefractures in ships during and immediatelyafter World War II, material standardswere established for ship steels whichinsured that, even at expected sub-freezing temperatures, any failurewould OCCUP plastically . Also, designand production standards were improvedto avoid sharp corner5, weld undercuts,and other stress concentrations whereeither fatigue cracks or brittlefracture could sta~t . This pragmaticapproach has been quite successful inreducing the number of hull girderfailures However, if it were to eOn-tinue in force. it would Dlace a serious

longer close his mind to the life of theship after launching, but must becomeinvolved in such matters as overhaulcycles, minimum flaw sizes, crack growthrates, and non-destructive test capabil-ities.

The designer has two paths to follow(57). He can produce a “safe-life”structure in which, by judicious selectionof materials and working stresses, hecan insure that . for the suecifiedoperating life ,‘fatigue cricks will notgrow to critical size, or a “fail-safe”structui-e, in which a potential flawand its resulting crack can be containedwithin a limited area. The first willput severe constraints on the materialswhich can be used, and will not permitachieving a particularly efficientstructure . The second sounds risky, butis actually a very common practice . Itcan Yesult in useful weight reduct%on,since higher-strength materials can beemployed.

It is possible to categorizematerials . throuzh their touxhness. bY

strength steels.The greatly increased competition

in the marine field in recent years hasplaced much emphasis on efficiency inall ship systems, and the efficiency ofthe hull structure system can bemeasured by the ratio of strength toweight. The mild steel which has beenin We foI.several decades if,a reliable,forgiving, easily-worked material, butits yield strength is only 33,000 psiThis yield strength is readily Imreasedby either alloying or heat treatment,and the resulting improved strength-to-weight” ratio is atti-active to a designer

restriction on”ship struciu~al efficiency, the size ;f the ?law whic~ c~n pro~ag;tesince it rejects moat of the hlgher- into a brittle fracture . HY 80 steel

can operate in the vicinity of yield witha flaw several inches long: whereas someof the steels used in aircraft landinggear components must spend their lifepolished and oiled, since a scratchbarely visible to the naked eye cancause fracture under load. This relation-ship between toughness and flaw size isone key to the designer?s approach tomaterial selection. He cannot toleratethe possibility of an undetectable flaWw2neratinE a catastrODhic fracture . so~e must l~mit his sel~ction to mat~rialsin which the critical fla” size isreadily detectable by the inspection

or owner seeking to improve his trans–port efficiency. Navy has successfullyused a 45,000 psi alloy (55) to reduceweight in combatant hull structure fo~years, and commercial designers andowners have cautiously been experimentingwith higher-strength steels . A guidefor the use of hi!zh-strenfzthsteels inshipbuilding has ~ecently-been revisedby the Society of Naval Architectsand Marine Engineers (56), and will beI’ePublished shortl!r. Aluminum. also.ha; been used in s~perstructur; and ~nsome high-speed hulls, but seldom at itsfull strength capability. This concernfor unrestricted use of these weight-saving materials is well founded. Ingeneral, as the yield Strength of ametal is raised, either by heat treat-ment, cold working or alloying, itstoughness decreases, its propensitytoward fatigue damage grows, and theplastic range between yield and ruptvreshrinks. The designer who proposes touse a higher-strength metal must con-sider what effects these changes willhave on his design. Since fatigue isa time-dependent phenomenon, he can no

procedure which is expected to beavailable .

In determining critical flaw sizefor the materials under consideration,the designer, with a tight schedule tomeet, has no time for theoretical orexperimental developments . He willnormally use readily available data,such as the Ratio Analysis Diagramsdeveloped by Naval Research Lab (58).These diagrams represent a generalizationof available fracture mechanics knowledceregarding the relationships that tie ‘yield strength and measured toughnessto critical flaw size and probablefailure mode . To enter them, it 1snecessary that toughness test data beavailable for the material in question,and that an assessment be made of probableoperating stress and section thicknessThe conclusions to which the RAD leadsare by no means specific -- they can’tcover the peculiarities of the existingstress field, its modification by thecrack’s progress or the geometry of thecrack tip. On the other hand, the de-signer is concermed with several hundredor several thousand tons of ship structure,

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and he normally can’t investigate thestress state in the vicinity of allflaws-some of which wI1l not developfor months. He has to be satisfied withgeneralities, and must make a quickdecision, without benefit of any de-tailed analysis. If the material he 1sconsidering hasn’t yet been characterizedfor toughness, he must drop it fromfurther considez’ation, unless there tstime for laboratory testin.x in thedesign schedule . “

The other half of this flaw-sizecomparison is the detectability limit,and this is the point where the shipdesigne~ gets into the maintenancebusiness, and also into time-dependentphenomena. Much of the flaw-criticalarea of a ship’s structure is painted,or insulated, or otherwise uninspectableduring noFmal operations, so Inspectionfor flaw or crack development must beplanned for overhaul, when coverings canbe removed. The combination of detect-able crack size and crack growth ratemust be such that the crack can? t growto critical size before the succeedingoverhaul period. Cracks for surfaceships are presently detected by visualinspection alone While this is suitepositive and requires negligible ~nstru-mentation, itts quite restrictive tomaterial selection, since the minimumdetectable size is rather large, andthe flaw must be at the surface.Acoustic and x-ray methods have beenused for flaw detection in submarinesand some high-performance patrol craft,with considerable success, but they aretime-consuming and expensive, and theircost must be factored into the trade-offstudy which considers any materialrequiring such instrumentation. Ob-viously, development of an inexpensive,high-resolution flaw detection systemwill enhance the practicality of usinghigher-strength materials.

The factor of crack-growth involvesthe designer with fatigue, since cyclicloading is the cause of crack growth.For this specific part of the problem,the designer is not interested in thecrack-initiation phase of fatigue, butonly in the time it takes to crow acrack from just under detecta~le sizeto critical size. The traditionalstress-frequency (S-N) diagram doesnttdo a good ,iobof this. since it vs basedon te=ts which include the crack-initiation phase, as well as a definitionof failure based on decrease in strength,rather than fracture or a specific flawsize . The da/dN or crack-growthdiagram, however, is just what isneeded -- if it’s available for thematerial under consideration. Ifnot, there are some general expressions,(59), which apPly to broad categoriesof steel, and are adequate for thispurpose. They require calculation ofthe stress-intensity factor from the

initial I“la”size and Dtress range(yield to yield is the safest assumption) .Integration of the process from initialflaw size to critical flaw size, toobtain number of cycles, is then required.This number of cycles is compared againstan equivalent, estimated number of cycleswhich could occur between overhauls, todetermine whether the possibility ofcatastrophic failure exists .

There is a nraanatic alternative tothis , somewhat m~ri-expensive in con-struction costs, but less demanding ofthe designer. This is the use of crackarresters, of tougher material, strate-gically lor.ated throughout the hull~lrder” to intercept r~rining cracks beforethey become fatal. The concept satesback to the early days of welded ships,and its effectiveness is well-proven.Some suggested geometries are given inreference (59). This is one way ofachieving a “fail-safe” structure, anddoes so without requiring alternate(and heavy), redundant load paths.

Fatigue can have three unpleasantconclusions -- either a through-thicknesscrack which leaks, a reduction in ef–fective area to the point where maximumload can no lonzer be carried. or acrack which gro~S to the critical cracklength in a brittle material For a“safe-life “ structure, the last two mustbe avoided for the ship 15 operating lifeFor a “fail-safe “ structure, crackgrowth to critical size , between inspec-tions, is the major concern, and willusually occur well before loss oreffective area becomes a problem. Thematter of crack growth between inspectionshas been discussed eaplier. This leavesthe problems involving the !!Safe-life”ship as the major concerns .

Fatigue damage is a function oftwo principal variables, cyclic strainhistory, and material properties . Sinceboth of these vary from location tolocation, some way must be found oflimiting the size of the problem to avoidhaving to look at every piece of theship . The ideal structural design is the,,one_hoS. ~hay,,, in which every molecule

gives out at the same instant . In arandom-load, random-strength environment,this ideal is unachievable, and there willbe many areas of the ship’s structurewhich will endure more severe cyclicloading than the rest In some of theseareas,‘a fatigue crack will initiate,grow, and die out, due to lack of a sup-porting stress-cycle field. In otherareas, however, the crack can continueto grow until failure through criticalityor throuzh loss of area occurs . The.-. —.designer must use his stress analysisskill to detect the areas in whichfatigue cracks will probably arise andgrow to cause failure He must thenmake some attemnt to m-edict the DrOcessof crack initiation a~d”development,with time in service .

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An approach to prediction offatigue damage requires three elements:prediction of strain history; character-ization of fatigue response of thematerial involved, based on standard-ization tests; and a cumulative-damagetheory. The usual S-N curve is finefor predicting the expected demise ofa machinery component, under constantcyclic loading, but fails to do thejob under the sort of random loadingto which a ship is exposed. Here,the cumulative effect of distinctly–different levels of loading must beassessed. An early entry in thisfield was the Palmmen-Miner i?ule (60).which makes no dif~erentiation between”varying sequences of loading, and isinexact, at best. Other cumulative-damage theories have been developedor are under development, some of whichhave sound probabilistic basis, someof which separate crack initiation fromcrack growth, but all of which have onebasic fault - to date, they can boastno better accuracy than the Palmgren-Miner procedure . This, then, is today’sprocedw-e, hopefully to be replaced soonby something better.

The strain history must be a jointproduct of stress analysis of the struc-ture, and a load history projectionbased on some form of operationalscenario. If simple fatigue is theonly concern. this can be exDressed inter;s involving only strain intensityand frequency of occurrence Ifstress-corrosion is important to thematerial involved, then time, also,becomes important.

To use any cumulative-damagetheory, it is necessary to haveexperimental data on the fatigueresponse of the material in question,over a complete range of strain ampli-tudes and ratios . For most of thecommon shipbuilding materials, thisis available, in the form of S-N plots,for amplitudes, and Goodman Diagrams,for stress ratio effects. The avail-able data is usually taken from machined-specimen tests in contz-oiled environ–ments . Real structure is often repletewith Ylaw.s, fabrication notches, re-sidual stress, and other stress raiserswhich can?t be assessed for stresslevels using normal analytic techniquesIt would be most useful, therefore, ifthe testin~ vrocess could include someof these f;b;ication-ge ometry character-istics to close the gap between cal-culation and reality Unfortunately,this adds several new dimensions tothe design of test specimens, andgreatly magnifies the volume ofdesirable testing. Some excellentwork has been done in this directionby Nibbering (61) , concentrating onspecific design features which havegiven frequent trouble with fatigueFuture work may well follow his lead.

Since fatigue prediction combinesboth theory and experience, at thisstage, it?s necessary that feedbackbe used to improve predictive accuracy,and thus reduce the need for largefactors of safety in design. Navy istrending in this direction for designof high-performance ships (62 ) Theii-aPProach, however, appears to followthat of the aircraft industry, whichmakes extensive use of full-8calefatigue-test models to guide theirmaintenance and repair procedures andto reinforce theory for later application.Such a procedure is not economicallypractical, In the general marine field,and alternatives are needed.

CONCLUSIONS AND RECOMMENDATIONS

The mushrooming demands being madeof water transportation have placed com-parable requirements on the 6houlderSof the structural designer and thestructural researcher. Over the pastdecade, the researcher has opened UPgreat possibilities in design technology,giving promise that this challenge canbe met. The results have not, however,been completely available for application.Not onlv must the desi~ner learn themechani&I of this new ;echnology, butthe technology itself must be carriedbeyond the mere proof of capability,into the status of a verified, workingtool. useable within the constraintsof a-tight design schedule Of allthe efforts presently needed in thefield of ship structmes, this trans-lation of research results into desixntOOls iS the most ur~ent and mostpotentially profitab~e .

The adoption of probabilistic,rather than deterministic, definitionsof design parameters, and the extensionof Probabilistic aDDrOaches into allfac~ts of Strwtur;i design, may bethe most important single achievementof the past two decades. To be fullyuseful, it requires more than just thedevelopment of technology - it alsoneeds a broad base of large-andfull-scale experimental data, and anindustry which understands the basisand application of the procedure

The computer has overshadowedall other calculating tools, and hasadded tremendous depth to the designer? sanalytic capability . It makes severedemands, however, on the designer’sresources of time and money, and itsoutput is only as reliable as its input.Future engineers must be taught notonly how to use the computer, but howto understand and live with its limita-tions, and a major challenge to theprogramming industry lies in the nee~for improving the techniques of gettingonto and off-the computer.

The improvements in operatingefficiency realized through use of

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materials with higher ratios of strengthto weight are als; purchased at con-siderable pain to the designer andsome added risk to the operator. Theproblems of fatigue and fracture arestill in need of solutions, useablein the design cycle, which will takefullest advantage of the strengthcapabilities of newly-developedmaterials

ACKNOWLEDGEMENTS

Any trained and thinking structuraldesigner must recognize that hisachievements rest on the cumulativeaccomplishments of hundreds of men,working over thousands of years Evenin our own generation, it is impossibleto sort out just a few great minds,since so many intellects have broughtus to this stage in technology. Twomajor repositories of talent haveachieved much, in the past few decades,to get the problem-owners and theproblem-solvers together, and to laycomplete, prioritized plans for thesystematic acquisition of knowledgeThese are the Society of Naval Architectsand Marine Engineers, working throughits Technical and Research groups, andthe interagency Ship Structures Com-mittee, through the Advisory Groupsof its Ship Structure Subcommittee .They, in turn, derive their supportfrom agencies of the government, fromthe American Bureau of Shipping, andfrom the private members and commercialenterprises which support SNAME. ItIS this support, in the long run, whichmakes prbgress possible .

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