Analysis of experimental results of underwater ship structural model and full-scale

CHINA SHIP SCIENTIFIC RESEARCH CENTER

ANALYSIS 0F EXPERIIIENTAL RESULTS 0F UNDERWATER SHIP STRUCTURAL

MODEL AND PULL-SCALE

Heu Ping-Han

Hou Wei-Lian

CSSRC Report

June 1985 English version-85001

P. 0

.

BOX 116, WUXI, JIANGSU

Contents _page

Summary 1

I. - The unique characteristics of unders water ship

structural design and tests

It.. Modelling theory and methods ₂

11E. Testing facilities ₅

IV. Stress measurement in full scale tests 8

V.. Comparison of experimental results 13

Future development and suggestions 16

SUMMARY

Undrwater,pressure hull cànsists of framed cylindrical, conicál and

spherical shells with various diameters The purpose of this paper is. to describe several research activities concerning the model and

full scale tests of underwater ship structures cónducted at China Ship Scientific Research Centèr (CSSRC) iú many years:

Modelling (theory and method) of underwater structureS

Testing facilities and measuring system of. underwater structure model

Measuring technique of prototype on initial sea trials and also in yards.

Compàrison between test results and theoretical predictions. Emphasis is placed on seleètiori 'of model scale., the requirement of

model fabrication, the modelling methods and its facilities,

pres-sure tanks, the measuring technique of prototype on initial sea

trials, such as waterproof technique for strain gages during the deep

diving test, the watertight device which bring the meásuring electri-cal leads from the outside to the inside of the hull without

destroy-ing the watertight, integrity of the pressure hull'. The paper

des-cribes also the multipoint automatic strain recorder and, its datä

processing system during the deep diving test in real sea, and the

comparison between test results and theoretical predictions as cor-relations between scale model results, full scale' measurements and

theory, must be considered and reflected in the structure ' design

specifications. Finally, some suggestions and forward looking

as-pects 'of future development are presented1 .

I.

THE .UIQUE CHARACTERISTICS OF UNDERWATER SHIP STRUCTURA DÊSI( AND TESTS ' .

Deep-diving rescúe boats, deep-diving vehicles and. submersibles can

all' belong to the underwater ship category. Although the tnaingtrn_

tures are in common and consist of framed cylin4rical and _conical shells, as well as spherical shells of various diameters, ' it' has

nany differences with the common engineering structure . anr has

the higher-requirements in the design and fabrication procedure. ' Its unique characteristics are: '

It is à thin-shell structure with smaller thicktiess to diameter ratio as' compared to the common engineering structures..

It has a large diameter and is subjected to relatively high

ex-ternal pressure. .

The limitation of the structure weight provides a safety factor for underwater ship. much lower than that for a common structure.

For instance, the safety factôr in land structures are. always' above 4, but in underwater ship it is sometime only 1.5.

All these factors require the underwater pressure hull design to be

based on.a sound analytical foundation verified bymodel and full scale experiments.

-1-Research work for underwater pressure hull had been conducted early in

many countries, and many fruitful results had been achieved in elastic and elasto-plastic stress responses of pressure huÏls i

The designers can usually determine the demensions of the _structure

by using design charts, siuipl _{fromulae and computer programs.}

Espec-ially in recent years wheñ the cmputer and the numerical, methods have

made such rapid progresses,, tikere had appeared not only large _general

computer codes but also special computer codeE for presSure hull struc-ture, such as large computer program

_Roqn

_{4, specially adapted to}

ela.sto-plastic shell buckling calculations 2 Al]. these have made

the pressure hull design and calculations _{more reasouable. One- point} must be mentioned, however, that is the computer codes _{mgy present}

very beautiful results, yet many practical problems still -remain

un--8olved..

Because the construction factórs, such as welding and

as-sembling, will cause residual stresses and initial deflections of the

structure, which will reduce thè buckling capabilify and the fatigue life of the pressure hull significantly. _{Until now it has not} beeúef-fectively solved in theory and in còmputer codes. _{So- on the one side}

much efforts must continually be spent in theoretical _{lúvéstigatiena} and on the other, final validation of new complicated pressure hull design must. presently still rely largely on experiments. Soin Chiha

and in other countries the establishment of new design specification

for underwater pressure hulls stili rely heavily

ona large

áount of

,nodeiandfull scale-tests.

II.

MODELLING TORY AND

TRODS

-As indicated above, the final results for new pressure hull - :designs

-still rely largely on experiments, the fu-il.scale test is too costly,

so model tests become in itself a necessity for evolution of _new.

press-sure hull designs. All models which are tested and fabricated in

-CSSRC laboratory and workshop are subdivided into three categorjs. 1. _{Small-scale, machined models with diameter usually up to}

200-500mm, with she-li thickness aboUt. 1mm are adopted :for basic par-ametric studies and theoretical analysis. _{The model inaterLals}

usually adopted are high tensile stell With yield stress of

40-70kg/n2 or plastic birefringent materical for photo-elastictests.

2.; Intermediate-size fabricated models with

_{diters less than-}

_2m

are used for failure, mode research and failure _pressure

determina-tions. High tension.stell piatewith _{yield stresses of 4OE-7Okg/nnn2}

are usually adopted..

-3. _{Large-size fabricated models with diameters 2-3m.}

It amounts to

full-scale for deep oce _{vehicles and deep, dive rescue}

submer-sibles d _{to 1/3-1/2-scale for other preséurehùll stictures.}

Because such models are more similar to the real structures , so t càn be accepted not only to verify the failure mode, _failure-

pres-sure and to study the response of the structure detáils, such. _as

stress concentratjo _{in Some locations of the structure irnder} in-vestigation, but also to study the effects of _{fabricationvariab1p.,} such as initial deflections, construction _{tolerances, welding}

factors in relation 'to the capability, _{of hüll structure.}

-The designers and the users are more concerned with determination of failure mode, failure pressure (collapse depth in real sea) and its

general stress level in certain compartments which have weaker

struc-ture than others _{Can all these required data be predicted by the} scaled model tests? The answer is in the affirmative, If the

dimen-sions of the model and prototype are in'geometrid similarity, _{and also} duplications with regard to material and tolerances are realized, the

failure pressure and failure mode betwêen the model and the full-scale will be the same. _{By the laws of similitude, it can be proved}

that a i i relationship exists in pressure and strain measurements for

the model and prototype while the deflections vary directly as

afunc-tion of model scale ratio. Por example, in the case of a framed

cy-lindrical shell. 3 :

-2R: Shell diameter

Shell thicimess

Lf: Spacing of frame bay

A: Frame area

Frame möment of inertia e: Initial deflection

E: ElastIc modulus

p: Poisson's ratio Hydrostatic pressUre

Initial Stress in structure

a : Yield stress inmaterial

Stress in structUre

: Deflection in strUcture

Shear force

M: _{Bending moment}

If the scale factor in=Li/L, where the subscript i denotes mo4el, the

following equations are obtained from the láws of similitude:

a Pi Wi Q.i 2

-=- ;

--m; m

P

W.

.q.

a

E e a a0 = 11 ;

=m;

I

-Prom the above equatioùs it is apparent that if the.tested

iodelsátis-fies the condition of duplicatioñ of fabrication tolerances _and

material of the prototype and proper geometric scaling, the i i rela-tionship in stress will be indicated, when the pressure acting on mocl

is the same as that acting on the prototype. _{If the stress in model is} the same as in prototype, then the i i relationship in failure pres-sure will also bé asSured.

3 Mi

m

_M

m2;

Sotnetiines.it is difficult to design the model having the full geometric similarity with the prototype, because the dimension of the tested modd. is very limited. According to our experience the following two pro-blems are often encountered.

î. Thé f lge of the fre for the model is rather small as compared with the shell dimension. So it is not possible to duplicate the

real frame configuration in the tested model, and it _{should be} flexibly modified by the condition L in2

= m.

2. The heavy transverse structure (bù1kheads) vi greatly affect the shell stress distribution. To avoid this effect, it is

neces-sary to modify the shell thickness, the frame area, and the frame

spacing at the end bays.. By this way the stress distribution and

the failure mode and failUre pressuré will not be influenced by the

heavy end stiffening. This has been confirmed by the experimentain

our labólatory and also indicated in reference 4

in fabricáting a model it is

an

iortwit factor is

to reduce the

fabrication tolerances and initial stresses which are very hard to accomplish in small-scale models with thin shells. Large-scale models provide better construction tolerances, but it is too costly and

require large testing facilities, which is also very expensive. So it has been a very interesting research subject to study the model scaling,

which can satisfy all the proposed test requirements. According to the experience in CSSRC laboratory, the stress distribution, the fallure

pressure and the mode.of failuré can not be simulated in t1e tested

model truly tmlessthe plate was at least 5-6nnn and its diameter

wú

about 2m. So it is more common and economical to adopt the fabricated

model with intermediate-size. .

-Fig i 3 shOws the failure modes of intermediate-size model subjected

to the failure pressure.

Fig. I Symmetrical Fig.2 Asymmetrical Éig.3 Overall

III. TESTING FACILITIES

Th testing Of the strUctural models is conducted in hydrostatic

pres-sure tanks, Fig.4.

Fig. 4

There are large amoimts of pressure tanks throughtout the world. For

example, .according.to the statistics in

1975,

the total nunibèrofthe

pressuré tanks in the United States excéeded niore than. 300, 60% of

which belongs to the Navy. Its operating pressure may vary from iQkg/cm2 to several hundreds kg/cm2.

But most of such tks are

in

small-size and are speciálized tanks for equipment tésting. Only few of them are capable of testing structure models of intermediate and large-size 5 The testing of structural models require pressure

tank with large diameter and high pressure capability, which will

cause many techñical difficulties. For example., to cönítrúct a pres-sure tank having a diameter of more than 3m and a prespres-sure of. more than 100kg/cm2 is very expensive and also very difficult. Because the plate thickness of such tanks is more than 50mm generally, and is

more than 100mm in certain places sUch as shell flange. As f or ma-terial selection not only high tensile strength but also high tough-ness are required. There are several difficulties which must be

over-come during its conétruction such as:

The technique of fabricating, assembling and welding, the main shell which is of large diameter, thick plating and is ma4è of

high tension steel.

The technique of casting, forging, welding and machining of other

flanges flange yokes etc; all being fabricated from special high

ten-sion steel.

To accomplish this formidable task,. it is necessary to have _special

fabrication technology, large workshop and, facilities with large.

cap-ability in construction works.

The characteristics of a few of the pressure tanks in China

of 1m or

more in diameter which have been used for structural model tests are

euíTabI1.

'Táble-" 1.

/

Note:. Testing tanks of smaller diameter and lower _{pressure are not}

listed.

Most of the tanks do not have cycling loading system, except

for tanks in

CSSRC -and Ruazhg Institute

of Technology Location Inside' Dimensions Dia z Length (mm) Design Testing Pressure,, Pressure (kg/an2) Media Notes Shanghai JiaoTung Uni-versit lolo z 2100 400 _{Water 6}

Qing Hua

Uni-versity 986 z 3000 45 Water Hùazhong Ins ti tute of tech-nology 1100 z 3000 . -. 135 Water 7 China Ship Scientific Research iwò z '3400 1800 z 6000 ' 3200 z 8000 100 45 75 Water Water Water Institute of Marine In-strumé.nt 800 z 300 Water Ji angN eng 1600 z 3000 45 _Water JiangNing Machinéry

Plnt

3000 z' -

30 _Water

From open informatiön provided by many countries, there are only few pressure tanks,which can be used for structural fatigue tests,

be-cause such tests require pressure tanks of large diameter and high

pressure capability. Fatigue cracks in the underwater structure

al-ways occur in local Structural aréas of high stress distributiona

Duplication of local structural details including assembling and

welding technologies are possible only iii model of sufficiently large-scale. However, materials used in contemporary pressure hulls are ductile and tough, thus providing a margin of forgiveness for

the poor structural detailing and an inherent resistance to uns-table crack growth. However the toughness characteristics of newer high tension steels may be such that brittle behavior may ôccur in

the relatively thicker plates or in thickness equivalent to those in

full scale. In such cases, _{full scale sections vili have :to be}

tested for flaw growth and for fracture determination under repeated

loadings 5 In such tests, small-Scale models vili have no

value.

Full scale section studies may be conducted in two ways: one. is by

the use of full scale mOdel, it requires large diameter' pressure1

tanks, such _{as 30 foot diameter pressure tank in the United States}

which vas used for testing of full-scale models of entire compart-ments of submarines subjected to repeated fatigue loading 8 . The

other is the use of full-thickness but reduced-diameter models,which

can be .tested in a mediUm diameter high pressure tank. Since the stress is a functioii of pressure, diameter and varies inversly as thickness, so it is possible to desi a reduced-diameter full-thickness model, subject to highter pressure th the

proto-type. The purpose isto duplicate the stress values and stress distributions in the prot:otypè, to check the welding

technology,ma-terial property and to estimate the fátigue life of structure ' under investigation.

The fatigue cyclic loading can be devised in two ways:

The tested model and also the pressure tank are subjected

simul-taneously to cyclic

loadings of appreciable maitude.

In

thié case more stringent requirements are imposed òn the tank desi d also on its constructión technology.

The. pressure tk itself 'is not subjected to cyclic _{loading by} maintaining a constant pressure inside the tank

but

varying the pressure inside the model. The difference between the pressure

which is a constant in the

tank anda variable inthemodel

will simulate repeated processes diving of thestructúte from the surfáce down to the test depth and returning back again to

the surface.

Execution of the cycling function can be perfórmed by

_an

autOmatic

cyclic loading syste such asshown in Fig.5 which is conducted at

the laboratory of CSSRC 9 The cycling system in Fig.5 can be

used to generate fatigue loading time-histories of trapezium

con-figuration. The shape of the trapezium and the period of the cycle can be statistically.determined b actually performing

repeateddiv-ing operations from the surface to the test depth and back again. The

period and configuration so determined çan be copied and executed by

electronic circuitry controlling hydraulic valve systems.

Reliábii-ty and stability of this method have been validated by experience of

-7-many model tests subjected to cyclic loading in CSSRC 10

L.

1

y'

] Main pump _{2. Surge tank}

5. Watercon.tajner

Vi,V2 ,

V3 , V+. : Valves

Iva _{STRESS MEASUREMENT IN FULL SCALE TESTS}

There are two kinds of full scale tests, _{one is the internal pressure} test in the berth of shipyard to check the hull watertight _integrity

and the welding technology. _{Theother is the external pressure test}

at sea to check the hull strength and the overall performance of ma-chanical systems. _{In this paper only the external pressure test will} be discussed, because it is _{more complicated in technique than the} internal pressure test. _{The results and the stress data, obtained} from full scale external pressure _{tests can be used tò check. the} strength design in _{general and also in structural details. During} the deep-diving test stress measurements' are necessary also to furnish on-site measurement of data for on-the-spot _{analysis of strength at} various submerged depths, to _{safeguard the structure shall perform} adequatly and safely for _{the next increment of submergence. In}

order to accomplish this, _{it is a mut to provide absolutely. reliable}

and

fast methods of measuring, recording, and evaluating the behavior of the. hull stíucture. _{The measuring sensor may be meèhanical,} elec-trical or optical; _{the recor4ing instrumentation may be any kind}

of

modern automatic data acquisition _{and processing device such}

as mag-netic tape recorders combined with _{Tnicro-computers,.but the}

proper interpretation of data analysis and its final _{evaluation can only be}

made by human intelligence and judged by huthari experience. .

Söme of the problems often encountered _{in full scale trials are}

dis-cussed as below:

4.1. _{The measuring sensor:}

It is common to use strain gages as measuring sensors, but to apply

I

3. Pressure tank 4.Testingmode].

al ,a, a3.:

Pressure indicators Fig.5

V5

it on the prototype is more complicated and more difficult than in the laboratory, because the condition in a fuÏl-scale trial is more rig-orous, and it 'presents some peculiar problems and exacting demands

which must

be properly resolved.

4.1.1. The Scheme of gage location:

In essence, the primary problem is to place the strain gage at the

right location, where the structure is weaker Or the stresses are

greater. These places càn be determined by calcúlations, by the re-suits of model experiments and by considerations of the construction tolerance. In the case of a 'real submersibte it is not easy to deter-mine the accessibility of the desired locations. For instance, in

some areas such as fuel oil tanks, where one often finds intensive

networks of pipelines and valve manifolds. It is often very difficult to grind and polish the surface of the hull plate, and to apply the

gaging operation. So on the one hand, one must compromise betweenwhat is preferable and what is possible, while on the othér hand, one must

not give way'easily and must ask the operator to overcome difficulties

where necessary. It is not uncommon that they are asked to lie un-derneath narrow passages in order to do gaging operations.

The maximum number of gages used is determined not' only by the demand, but also by the state-of-the-art technique. _{For example, in 1937,when}

the U.S. Navy conducted the deep diving test, only 9 measurement points were taken inside the submarine SS-175, later- it vas increased to 34

points for submarine SS-285 in 1943. DUring that' period, all

8train-measuring sensors were mechanical, one strain indicator _{cai. -measure} only one point and it was extremely inconvenient _{to use. The modern} era of instrumented deep submergence tests began with the _{testing of}

SS-KL submarine after 1951 in the U.S. When electrical résistance strain gages vere first used on the inside (49' gage) and outside (39

gage) of the pressure hull 5 _{In 50's the number of gages used in} the submarine is still not large, being limited by the technology of.

that period. _{Because the strain recording vas manual, so that all} data must be painstakingly read, _{recorded and calculated point by} point. _{For a 100 measuring points it is necessary to have 5 sets'}

of

strain recording station. 'Each set required 3 men to'work at thè same

time and more than ten minUtes were used for one diveing depth. So it is apparent, that such conditions _{are not suitable for full-scale}

tri-als. _{Besides, the strain gage itself vas- not so convenient to use}

as

its present day counterpart. _{For example, during that time the strain} gages were required to be dried when .the submarine was in dock, using strong thermal lamps (500 Watt.) for at least' 8 hours, in order to

ob-tain the necessary high electrical insulation between _{the gage d the}

pressure hull. Much time and labour 'had been wasted in these opera-tions in the fifties. But with the present state-of-the-art, _{it is all} different. _{The total number-of strain gages. used in deep diving test} for newly designed submarines in some countries may. be by the hundreds inside and outside of the pressure hull.

-4.1.2. Another problem with the scheme of th'e _{gage location is to} de-termine the gage locations very accurately back-to-back on both -sides of the hull plate. _{Because during the deep diving test the hull} ature-ture'is flot only compressed, _{but is also bent at such location as the} -interface between the cylindrical and the. conical shells, and also in

the end bay of bulkhead. This can be done generally by utilizing the bench marks, such as various kinds of Markings used for locating frame lines in construction, but sometimes this is not successfully employed and is not accurate. This problem has been solved in CSSRC

by using a simple equipment consisting of two small ültrasonic sen-sors and one small amplifier 11 Put one sensor in the required place and move.the other sensor at the opposite side of the hull plate, the maximum indication of theamplifier locates the right po-sitoù for gages onboth sides of the surface.

4.2. Tec1ique of the strain gaging, where exposed directly to ex-ternal high pressuré

There are three problems connected with train gages applied external to the pressure hull:

To maintain the gage waterproof, properly, efficently and for a ling dUration.

To maintain the measuring cables or electrical leads fully wa-terproof. The leads are exposed for long duratioñ directly to

high external pressure outside of the hull.

To bring these leads through the pressure hull safely to

arecord-ing station within the

pressure hll.

The first problem can be solved by attaching a protective layer _over

the gages. This protective layer must have the property of (1) good adhesion to the hull plate, strain gages and electrical leads. ₍₂₎

good waterproof capability under high ambient pressure. (3) capabili-ty of sustaining hydrodynamic force exerted by on-coming flow, the

latter can be significant for high-speed submarines. (4) ease of operation. Over a long period of experirnèntation various kiñdsof the.

protecting materials have been such as special rubber paste, oil with

some plastic polymerizer, the cannon-cleaning tallow, etc. A pro-tective material has been found,. which can maintain the strain gage

in effeciant working condition in sea water for more than three months under high water pressure. _{Of course, before attaching the layer the} hull plate and electrical leads must be eleaned properly.

The second problem is to. maintain the measuring cables in good ele -ctrical insulation. _{This fact is more than often ignored. However,} in practice the. measuring cable is often exposed' to sea water under a flow velocity over long durations of time. So for càmmon electrical

leads, their sheaths may be easily partially corroded, and in casé it

is punctured, water will flow under pressüre through the electrical cable into the pressure hull. In order to cope . with this. pror

bleni, a special kind of marine cable has been manufactured, _which have a double layer of protective Eheátbs, and _good

electricalin-stilation property had been confirmed by eperiinents under. high

pressures for a long period of time.

The third próbleni can be solved by using a simple fitting, designed_by the CSSRC laboratory. 12 a.s in FÎg.6.

"4 Spaser.

-Washer

I

Measuring. cables. S crew Washer. Sealing wasl Rübber áasher. Basé Pressure hull Fig.

There are two types of such fittings. _{Type B with 40mm inside}

diame-ter can bring 44 electrical leads through thé _{pressure hull without} destroying the watertight integrity of the _{pressure hull The}

reli-ability of this type _{fitting bad been proven by pressure tésts in} our laboratory and also confirmed by the _{fú'll scale trials at sea.}

The only nuisance is that holes must be drilled in the pressure hull and fittiñgs must be welded to the hull. But this is mavoidable be-fore remote sensing technique can be developed _{successfully in deep}

divitg tests.

4.3. Recording.

Thespace in the submerged vessel _{and since the number of technicians}

are very limited during a deep-diving test, therefore rapid, _compact and automatic methods of recording strain and post processing are also

required, in order to knowguickly whether the _{structúre will perform} adequatly and safely for the next increment of dépth during _{the test.}

Such data acquisition and processing system had beeñ completed by the CSSRC laboratory in 1977 13 _{All measuring gages are connected to}

the automatic multipoint switch box. (50 or 20 points in one box) Jill

switch boxes are linked by built-in connectors, and _{are connected to}

the automatic digital s'rain indicator, by which the strain data can

be quickly observed and printed. The remaining step is to transirit

thé strain signals to the computer simultaneously in _{order to process} and print out the required parameters as fast as possible. After this interface has been succeeded the computer and the strain _indicator can be operated stab].ely ans simultaneously together. _{The block} dia-gram of this data acquisition system is shown in Fig. 7.

N

_Autom-

-Measuring Gages Bo CI)

NQfrfl-C3) Straino--

I__ Connector

meter Fig. 7 L1 _Power -i Computer

Tape-fwriter

As validated by full-scale trials

in deep sea this system is both com-pact and reliable. _{Presently with the development of}

the minicomputer technique, the above system has _{again been renovated in our laboratory} into one compact equipment _{with dimensions (600}

x 450 x 200mm) in which the strain indicator, the computer, the printer and the electrical

dis-play are all installed compactly _{in one unit. So it is more convenient}

to use _{and also easier to carry it for}

full-scale trials. The reno-vated system of equipment is not only capable _{of recording strains,} but also of recording simultaneously

other physical quantities such as

temperature, pressure. _{The sampling, computation and printing are all} done at a fast speed. _{So it is more suitable for conducting}

large scale experiments with hundreds _{of measuring points during the}

sea

trial.

Some points, however, _{must be emphasized:}

The compartment _{space in a submersible is always very limited, and}

its watertight doors are also of small dimensions, so the dimen-sions of the required _{instrun,entatf on and}

computers must also be small and adaptable to the _space.

The environment condition is severe. _{For example in South China} Sea, the ambiant temperature and hyniidity in summer and _{autuun are} high. _{During the deep diving}

tests the operating accumulators and

engines send out heat continually _{into the enclosed space, so}

in

the tiny instrumentation _{compartment the environment will}

be very

severe. _{So it is apparent that any precise but delicate}

instru-mentation may easily get into _{trouble under such conditions,}

and

this point should be particularly borne in mind in choosing _the

proper measuring equipment. 4.4.Procedure for deep diving test For a newly designed submersible

2-3 dives. Dive i takes the vessel down to approximately one-half the

design test depth for the purpose of obtaining an estimate of. the

sttength of the structure prior do descent to the design test depth.

Dive 2 and 3 then are made to the design test depth. The submersible is always submerged :step by step. It may take approximately(30-40m) in

one step for a depth of approximately 2/3 of design test depth, then (1O-15m).in one step for a depth of approximately 90% of design test

depth. Finally the remaining distance is travelled in steps of

(5-ioni).

Strain data are' recorded and analyzed at each step prior to descent to the next depth. According to the nature of the strain data certain

steps may be eliminated, additional steps may be added. In each step before proceeding to the next increment of depth, watertightness of the presstire bull, and measured data must be carefully checked to assure that the structure performs adequately and safely. The time, during

which the submersible should stay in each depth is determined by the

time necessâry for the checking of hull watertightness and the

meas-urementof stress. Generally, it takes about 5 minutes.

On some occasions, it vas experienced that as the ship was descending down to a certain depth some big banging noise came from the structure.

This is a sywpton of failure of certain structure, so it must be ex-amined and analyzedrcareful1y. In most cases, the sounds may be due to

the light structure, i.e. the stiffeners betwèen the pressure hull and

light hull, bending and buckling. Froni the structure mechanics point of view, the cause is quite calear: when the ship is going down, its

pressure hull will be compressed and radially contracted, so is the 'light hull. The relative radial contraction of the structure along

the length of hull, however, will not the same, because the hull

stif-fness is different in different parts of the structure. For instance,

the radial contraction in the structure near-by a strong buckhead is

lesser than another place. So at that location, the stiffeners

con-necting the light hull with the pressure hull will be subjected to

certain amount of compression. Generally, the stiffeners have the

capability to sustain such compressive forces. But if the initial de-flections caused by the sea trial or the construction are such as

to greatly reduce the resistance to buckling, then the stiffeners

be-tween the pressure hull and the light hull mayaetually reach a mode of buckling failure at certain depth during the tests. On such occa-sions, it is the duty of the experimental.officer.. in charge to _tell

the crew not to be afraid, because the damage of these beams would not affect the strength of the pressure hull.

V.

COMPARISON OF EERflNTAL RESULTS

5.1.

Comparison of stress value

Comparisons of meaningful values of stress measurement for various

sizes of scaled models and protoype are shown in Fig. 8-9.General agree-of elastic stress in frame bay between experiment and theo-retical predication is considered to be good and it is also noted that some discrepancies in tested stress sensitivity exist in various scaled models. For the small scale model with plate thickness less

then 1mm, its stress sensitivity is some what lower than that of the

theoretical predication, the same tendency exists for the

intermediate-scale model but to a lesser degree. As for the full scale tests, the

agreements between experiment and theory are considered to be good.

-Comparing Shell s.tress near the frames, more discrepancies between the test results and theoretical predications are found. It can be ex-plainéd by the welding toe, which actually existed near the frame and which are not considered in the theorctical calculations.

So for the framed cylindirecal shells the following conventional

equa-tion 14 can bé recommended for the strength design:

D+2(+cp

_cLx

_2x2

_(1-)

R 2

The stress value arte checked at mid-bay of the outside. shell sur-face and also at thé end-bay of the inside surface.

As confirmed by our expetirnènts the stress values at maximum working depthcan be estimated with sufficient accuracy by the above equation.

-uiuu

-T

-rènce Stressa,

_{smalj_ o}

Axial StressA

Mdel O

Model

HIGH STRENCH STEEL

EXTERIOR FACE I R10 FACE

..

NU

UIRUUIUI

TT

L -i

hum

Ouj

r. Scéie

1'IEDTUM LOW STEEN(flH STEEL

EXTERIOR FACE

0.20.4

0-80.8

T

L.Small

-:

0Mediuxn Model INTERIOR FACE IO 0.4 0.6 0.6 LO Fig.9 O 0.6 0.8 LO

Fig.8

0.2 DA 0.6 0.8 LO

5.2. _{Comparison of failure pressure}

Fig. IO shows the failure _{pressure for small-scale models}

with

thick'-ness of _{1nm and also for intermediate-scale}

fabricated models with

thickness of S-6mm. The test results are compared with the Von-Mises theory in the 'form of parameters $-. _{curve. The following}

formula due

to Von-Mises, which had been simplified

_{in caiderati of}

the unequal distrubuti.on of _{circumferential stresses, be}

deopted as 'the basis for compariosn

P

E)2

0.6 _0.642

E

R

p-0.37

theoretical failure pressure : buckling coefficient

X : dimensionless _{coefficient, as indicate in Pig.iO}

Pc : the a failure pressure

--- i

then =

a

₌-PER

a

z

'

E .)i

In elastic response. _{The actual buckling coefficient}

will be:

a8h A2

z Exper results- of intermediate-scale o Exper results of sma1i-seale:

j .2 0.2 o 04 0.6 08 1.0 12 1 16 L8 20- 22 2

_{26 2J 10}

A U-0.37 ER 0.6 Fig, 10

The results of intermediate-scale _{fabricated model tests indicated that,}

if A was greater than 0.77, _{an asymmetrical buckling between the - frames} (Fig.2) would probably occur. _{Otherwise, symmetrical bucklings (Fig.1)}

between the frames may occur. _{And, overall instability (Fig. 3) of the}

shell and frames may occur if

15 -o

Pi.1-1.2 P

-P00: cálculate _{overall instability of the shell and frames}

PL: calculated local buckling of the shell between. frames

Fig.1O indicates that the tested failure pressure was less than the

theòretical predication. This is due to:

i. _{Buckling failure was associated with inlastic stress response.}

2. _{Failure pressure was greatly affected by the fabrication varia-!}

ble and residual stresses.

Our small-scale machined models ha undergone annealing treatment.

So its failure pressure will be less affected by _geometrical imper-fections and residual stresses, andwill approach better to Von Mises theory than with the fabricated models.

Becaused the fabrication variable can-not be embodied in the theo-retical calculation very faithfully; _{so some empirical coefficients}

based on large number of experiments _{are used to modify the}

sim-plified Von- Mises equation in actual design applications. In the

new structural design specifications the effect of inelastic _stress

responses and also of geometric imperfections are considered in the-oretical solutions and in computer cOdes 15 16 17 Of

the theòry and its assumptions _{are still based on the experimental} results.

VI. _{FUTURE DEVELOPNENT AND SUGGESTIONS}

The R & D work for pressure hulls had beenstartedquite _early. Although more and more fruitful results both in theoretical _methods and in computer codes for typical structures had _{been achieved.} mty problem areas still remain, especially with respect tòfabrica-tion factors, welding factors and initial stresses, which greatly infÏuence the structures capabilities. Up to the present,

theseef-fects cannot be accounted for properly _{in computer codes. So. it is}

extremely important to interface the structural _{research, the fa}

rication methods and the meterial _{research together in order to} solve this serious problem in practice.

-In order to solve this serious problem in practice, _{great .} ef-fort must continually be spent _{on theoretical research, while} si-multanèously final results for important _{hull designs Should still}

principally reiyon large-scale experiments, _{In China, although a}

largé amount of testing data for _{small-scale models had b..een} col-lected, _{test data for intermediate-size and large-size}

models are

still very limited, so it is necessary to have projects on further R & D of this type of work.

.3. _{One of the important tasks is to study}

fúrther into.the theory

of similarity in modeling

_Although

_{in static stractural tests the}

láw of similitude had been commonly adopted, some problems areas 5tjll remain. _{For instance, pressure hulls always hàve}

large

di-ameters, _{thin plating with frames.and other}

reinforcements. So it

is very hard to have full geometrical and material _{-.:.behavioural} similarities ôn small-scale models. _{Such problem had always been} encountered in strength-testing of models with steel or plastic bj-refringent materials. _{Many difficulties are also confronted with}

quite regularly in modelling of buckling research. So, if a simple and

economical method may be found which does not require the füll

Satis-faction of the law of similarity, it vili be of great value in

modell-ing research. A method using an equivalent material behaviour and an

equivalent structure to modify the model buckling experiment had been

developed at the Harbin Shipbuilding Engineering Institute 18 . A

method using the measurement of vibration parameter instead of the

struc-turai buckling experiment alsohad been conducted in other cotmtries 2

It is apparent taht all these methods should. be further developed

-confirmed by the more tested results.

Many problems exist for the estimation of fatigue life of tmderwater

ship.sturctures. Thè method based on material Specimens and small-scale

plate tests d modified by some kind çf iñfluence coefficients seems to

be poor and rough, since the fatigue life of a structure are largely in-fluenced by welding technology and cannot be represented by a small-scale model properly. It is therefore important to use irge-scale or full-scale Sections for fatigue testing. This implies a need for large pres-sure tanks having high cyclic fatigue loading systems.

Fui-i scale tests are very important for R & D and design york. Al-though some test datahad been ollected, they _{are still quite}

limit-ed1 So more tests should be conducted to accumulate valuable testing data, in spite of the high cost. In order to solve this important man

date. successfully, modern equipment and reliable methódare essential.

Coupled with the prgress in electronics and otherS, more precise

in-strumentation such as laser, remote-sensing technique, may become pos-sible in future deep d-iving tests, and these would finàlly eliminate the

need for wiring through the full and consequent hull penetrations. RoV vér, whatever the _{advance electronics and computer science may be, the}

final interpretation of data analysis and its evaluation can.only be

made by man with human experience and human knowledge of the hull s

truc-ture.

REFERENCES

i. _{"Proceedings of 7th International Ship Structure Congress"1979 Paris}

"Proceedings of 8th International Ship Structure Congress " 1982 Gd an sk

"Comparative Behaviour of. Submarine Pressure Hull Structures of

Dif-ferent Scales under Uniform External Pressure" F.W.Dunhan S.R. Heller

Naval Engineers Journal Vol.75. No.2, May 1963

"An Experimental i:nvestigation of Effect of End- Condition on Streng-th of Stiffened Cylinder Shell" RJ'. Keéfe. Report DTMB-1326 1954

"Experimental Stress Analysis Goes Deep" PM. Palemo, Naval Engi-neers Journal Aug. 1975

6, "400 kg/cm2 High Pressure Tank" Li Long-Yuan et al. Report on

Con-gress Ship Structure Mechanics, Sponsored by.the Chinese Society Of

Naval Architecture, Shanghai May 1981

7. _{"Fatigue Tests and Analysis of Submarine Pressure Hull" Chang Zhuu.Qu}

JoUrnal of RuazhongInstituteof Technology 1980 17

-"Fatigue Testing of Large Scale Models of Submarine Structural Datails" F.W. Dunhan Marine Technology July 1965

"Cycle Loading System of Pressure :ranks

No.110 No.132"

Can Si-Nan et al. Report CS SEC 1982

"Fatigue Tests of Framed Cylindrical Model Subjected to

Ex-ternal Cycling Loäding" Wan Chan-Tao et al. RepOrt CSSRC

1982

il. "Back to Báck Strain Gage Location Indicator" Report CSSRC 1981

"Watértight Device for. Nigh Pressure Null" Liu Yuen-Shan et al. Report CSSRC 1978

"Computer Application in Stress Measurements and Data Process"

Shai. Sho-Li, Tu Zen-Son, Report CSSRC 1978

"Bending and Buckling of Cylindrical Plate and Shell" Y.E.

Xolotkin (in Russ.an)

"Buckling of Framed Cylindrical Shéll with Initial Imperfec-tions" Tong Hua-Xing Report CSSRC 1979

"Inelastic Banding and Buckling of Framed Cylindrical Shell Subjected to Hydrostatic Pressure" Chan Tie-Tun, San Ti-Mm, Wang Chu-Yu Report on 83 Anniversary of Chiai Tung University

"Inelastic Instability of Framed Cylindrical Shell" Wang Chu-Tu Report CSSRC 1980

"Buckling of Spherical Shells and Coluana by the. Ultimate Ana-lysis" Lue

Pci-Lin

Journal of Rarbin Ship Engineering