Low cycle fatigue of intersections of bottom longitudinals of tankbarges

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LABORATORIUM VOOR

SCH EEPSCONSTRUCTI ES

TECHNISCHE HOGESCHOOL - DELFT

RAPPORT Nr.

BETREFFENDE:

Low cycle

fatigue of

intersections of

bottom longitudinals of tankbarges. By ir. H.G. Scholte & ir. J.H. Vink.

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r.

Sb i.p Structures Laboratory,

Deift University of Technology, Mekeiweg 2, Deift,

The Netherlands. Report

SSL 168

LOW CYCLE FAkFIGUE OF INTERSECÑONS OF BOTTOM LONGITUDIITALS OF TANKBARGES

by

ir. H.G. Scholte 1)

md

ir. J.H. Vink 2)

Deift Ship Structures Laboratory. Netherlands Ship Research Centre.

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Contents. Summary Introduction Test specimen Test procedures Test results 5.1. Static tests 5.2. Fatigue tests 5.3. Fracture experiments

Conclusions and final observations

References Appendix I Appendix II

Figures nr. i - 50

-

1-LOW CYCLE FATIGUE OF INTERSEC1IONS OF

BOTTO1 LONGITUDINALS OF TANKBARGES by ir. H.G. Scholte and ir. J.H. Vink.

§ 1. Summary.

Lw cycle fatigue tests with full-scale structural sDec±mens have been carried out in the Ship Structures Laboratory at Deift on a 600 ton tension-compression-mchine. The tests were carried out on request of the Netherlands Ship Research Centre and of the n.y. shipyard and

engineeringworks De Biesbosch - Dordrecht.

The specimens represented the interconnection of longitudinal frames and transverse bulkheads constructed in normal shipbuilding steel. One specimen was provided with interrupted brackets another specimen

with through brackets. A third specimen contained a longitudinal

bulk-head. in addition to the longitudinals. All specimens had an overlap in

the bo±tomplate just below the transverse bulkhead. The overlapwelds

of overlapstrip to the bottomplate have been welded pat'tly with a

thick-ñess of6 mm and partly with a thickness of 5 mm.

Axial cyclic loading was.applied (P min./P max.

-i).

Prior to the

fatigue tests some static tests were carried out. After fatigue testing the specimens were pulled to rupture

The way of testing as well as the number of specimens and the desired variation in structural details were determined in discussions between the n.y. shipyard De Biesbosch - Dordrecht, Bureau Ventas, the

Nether-lands Ship Research Centre and the Ship Structures Laboratory. A

con-tinuous help to the authors has been prof.ir. J.J.W. Nibbering (reader T.H.D.), who to a large extent also determined the experimental basi. Attention is given to the distribution of strain, the resistance to

fatigue and the effect of fatigue. ciacks upon the strength of the st'uc-ture. The report is ended with some conclusions and recommendations.

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2

-§ 2. Introduction. .

During more than 10 years many pushharges have been built at n.y. shipyard

and engineeringiiorks De Biesbosch - Dordrecht.

Ali these barges had. conventional intersèctions of bottom longitudinals with flat bottomplates . However, as a result of reorganisation' and

improve-ment of production techniques, the yard likes to change the intersections in such a way that the bottomplates will be connected by means of an over-lapweld, instead of a butt joint. However, insufficient knowledge is avail-able about the influenceof such an overlap on the p1ace of the

inter-sections. Moreover, the barges,, to be built in the future according to

the new production-system, have to be accepted for classification for K1 and K3 cargo. So, it was thought necessary to gain more inforration about the strength of the intersections with overlap weld by means of experiments

on some full scale structural parts.

Two aspects had to be included in the investigation.

At first it was found important to get more detailed information in case of

the distribution of strain over the specimen; especially the points of

dis-continuity, the influence of the overlap upon bending effects in the bottom-plate and the E over the length of the intersection.

virtual

Secondly it was important tobe informed about'the lifetime of the structure

under low cycle fatigue loading. As the barges are used for inland navigation,

only the load amplitudes depending to loading and unloading of the barge will be of interest from a viewpoint of fatigue. With a lifetime of about 50 years, one voyage a week and 2 loadvariations per voyáge, it is clear that only

low-cycle high-stress fatigue is of interest. So it was agreed to restrict

the fatigue experiment to a maximum of 10.. 000 cycles per specimen and to

start the fatigue investigation with a stress amplitude for o . between

2 2 nominal

- kg/mm- and +8 kg/mm

In addition to an investigation concerning the overlap weld in the bottom-plate, also a comparison had to be made between intersections with trough-brackets and intersections with interrupted brdckots and finally the influ-ence of a longitudinal bulkhead had to be looked for. As moreover, for reasons of costs and time the number òf specimens had to be kept as low as possible, the investigation had to be restricted to one specimen of each

type; of course each specimen cöntains two testing parts.

In this way the experiments give not more than an indication about the fatigue resistance of the intersections and the result should be seen mainly as a

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§ 3. Test specimen.

For the experiments three different specimens were available, all of them fabricated under normai working conditions by the n.y. shipyard and engin-eeringworks "De Biesbosch - Dordrecht". . In the first instance a.

test-specimen was preferred composed of bottomplate and transverse bulkhead

with 4 bottomlongitudinals , and in the middle a longitudinal bulkhead .

As this structure is very heavy and probably would cause many difficulties

in testing, it was decided to replace this structure by three other spec-imens..

First two specimens type A and B concerned the intersection of two bottom longitudinale (figure 3), without disturbing influences from other struc-turai longitudinal members. The difference between type A and B is found in the choice of the brackets. Type A has interrupted brackets, in

agree-ment with the most simple way of construction and fabrication and thus most preferable from a viewpointof economicalaspects for the yard. Type B,

as a through-bracket type, is preferred by the classification society. Where after strainmeasuremen-t in specimen A and B, very large strain was

found in the bottomplate at the end of the bottomlongitudinals,it was

de-cided after some discussion to modify specimen B with intent to decrease this large strain. Assuming that a circular cut out at the end of the frames would make the transition of the forces from the longitudinal into the

bot-tompiate less discontinuous, resulting Ip a decrease of the strain, two. frames were modified, as can be seen in figures 2 and 13.

Secondly a specimen type C (fig. 1i) was tested. This specimen was built up with two longitudinals and a longitudinal bulkhead in order to et

informa-tion whether a very rigid longitudinal strudtural part relieves the load

upon the rather flexible intersection of the bottorrilongitudinals, Based upon

the results of strain measurements with specimens A and B, it was decided to build up specimen (with bottom longitudinals) of the through-bracket

type.

When the tests should be restricted to the through-bracket type and the structure would fail in the bottomplate, there would he no information if

the interrupted bracket would have been. strong enough. With a limitation to the interrupted brackets and a failure of the bracket it would he uñknowi

if a through-bracket iou1d be (much) better. So the main reason for

includ-ing two types of brackets was to avoid a possible necessity for a second

experimental testing afterwards. -

-For similar reasons two types of overlap welds were used; one overlap with

a throat of 6,5 mm and one overlap with a throat of 5 mm, as shown in f

1g-ures 3 and 4. Classification rules require a throat thickness of 6,5 mm, being 70% of plate thickness. On the contrary, the yard prefers a throat thickness of 5 mm as in their welding practice has been proved that a weld

of 5 mm can be made in one layer without problems and independent of welding position. However, when the thickness of the weld is larger than 5 mm, it will be necessary to lay down the weld in three layers and this means that

the weld will be much more expensive; may b without necessity. A

restric-tion to only a weld of 6,5 mm throat thickness will not give any informarestric-tion if a weld of 5 mm will behave really worse than a weld of 6,5 mm. And if

there would be no difference in strength behaviour .there would be no need

for the weld of larger thickness. In relation to these considerations it

was decided to join the overlap strip with the bottompi.ates by means of a 5 mm weld at one side of the transverse bulkhêad and by means of a 6,5 mm

weld atthe other side.

For determining the properties of the material the yard delivered a strip

-150 X _{.12. The properties of this strip and also the properties of the}

bot-tomplates are given in Appendix I. For determining the properties of the bottomplates, small testpieces were taken out of the edge of the bottomplate at halfway between transverse bulkhead and the end of the structure.

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- Lt

--§ Li. Test procedures.

All specimens have been tested in the 600-tons tension-compression machine

of the Ship Structures Laboratory of the Technological University at Deift, (figure 2).

The investigation had to be split up into three parts; statical tests,

fatigue tests and loading to failure.

For all three specimens the experiments were started with some static tests to determine the distribution of strain over the structure. Three static tests were carried out. The first test concerned axial tensile- and com-pression load and enclosed two cycles with a maximum amplitude equal to the amplitude of the first series cycles of the fatigue load. For speciren A and B successively strain and E . were measured at loads of 0, -t-30,

virtual

-t-80, +130, -t-80, -t-30, 0, -15, -'-f0, -65, -L0, -15, 0 tonf. For specimen C the

steps were 0, -i-'-0, +130, -i-220, +130, -t-140, 0, --20, -65, -110, -65, -20, 0 tonf. (The locations of the strain gauges are given in figure 5, 6 and 7;

figure 8 shows the location of the points between which a measurement of E . is carried out).

virtual

The second and third static tests concerned bending, successively by means of a vertical load upon the top of the transverse bulkhead and by means of a horizontal load upon the topflange of the bulkhead. For all the specimens as well with the vertical as with the horizontal loading, measurements were

carried out at forces of 0, ±1500, +3000, -t-14500., + 6000, +3000, 0 kgf.

During these bending tests, the specimen could be considered as fixed in at one end, while the other end of the specimen was free to move in axial

direction.

As second part of the experiments the fatigue tests were carried out.

The fatigue loading represented more or less the loading of the bottomlongit-udinal of the barges due to the difference in longitbottomlongit-udinal still water bending moment for the empty and the fully loaded condition. The first part of the fatigue testing énciosed 3000 cycles with a stress amplitude for o .

. 9 -)

nomina-between a . -4 kg/mm and o +8 kg/mm .

-n mi-n n max

Besides in all fatigue tests the tensile component of the fatigue loading was taken two times as large as the compressive component (P mm/P max _-b). The frequency of testing was between Li and 8 cycles per minute depending

on the magnitude of the applied load.

Returning to the agreements of the meeting on 16-11-1971 at Delft, Bureau Ventas proposed afterwards to lower the amplitude of the first 3000 cycles

to a . /a -2/+14 kg/mm2. However, the Ship Structures Laboratory

n mIn n max

preferred the original agreed loading, since it is very difficult to keep the upper- and lower value of the load amplitude constant with enough accur-acy just in that very low region of load capacity of the 600-tons test

machine. Besides, it was not expected that a nominal stress of -2 +14 kg/mm2 would cause any fatigue crack and in that case it would serve no useful pur-pose when 3000 cycles of -1 . ±8 kg/mm2 are being preceded by 3000 cycles

of half value. Moreover, when during the fatiguing the amplitudes háve to be

enlarged, there should be not too large differences between the

load-amplit-udes. So, finally, it was agreed to maintain the original program.

To get the nominal stress of -14/-t-8 kg/mm2 in the first test experiment for

specimen type A the first 3000 cycles were carried out with a load amplitude

of -65/-t-130 tonforce. After 3000 cycles had passed and no crack could be

observed, the load amplitude was increased to -80/+160 tonforces in order to

get a nominal stress of abt. -5/i-lo kg/mm2. The intention was to increase the load amplitude again when, a total number of 6000 cycles should have been

passed and still no crack initiation could be observed. However the first

crack was found after abt. 3750 cycles and after a total number of 14620 cycles the fatigue test was stopped.

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For the specimen of type B the sane loading procedure was applied. Although after 2500 cycles a small crack was found, the amplitude was still increased after 3000 cycles, as otherwise a comparison between type A and B wòuld

become very difficult, if not impossible. Besides, a crack is, initiated and

already over a certain length proìagated before a crack will be observed, so that it would be not impossible that also for specimen type A the crack

initiation took place before. the first 3000 cycles had passed. Based upon

the preliminary results of specimen type A, the fatigue testing of specimen

type B could be continued to a total number of 5800 cycles with a crack

pro-pagation to a much larger extent than was the case with specimen type A. In consequence of a larger area of the transverse section, specimen type C was tested with an amplitude of -1101+220 tonforce for the first 3000 cycles and with an amplitude of

_-1357+275

tonforce for the following cycles in order to have the same nominal stresses in the bottom section.

The fatigue test of specimen type C was ended after a total number of 8500

cycles.

After ending the fatigue part of the test, all specimens were loaded under

axial compression up till the point of local buckling,, hOwever, without accepting a collapse of the specimen. For this purpose the total

lengthen-ing of the specimen was measured by means of a displacement transducer at

the end of the specimenat the point where theaxial load was applied.

As shown in figure 1 thi measured displacement was plotted on a X--Y-recorder

in relation to the applied load; the latter measured by means of pressure

dynamometers. Next to this the specimens were examined visually. In all these tests the compression load was increaed very carefully Until the dis-placement intended to grow even if the load was kept constant. At that moment the load was dropped down immediately. As the last step the specimens were

pulled under axial tensile load to failure.

As already mentioned above, strain measurements were carried out under sta-tic loading to determine the distribution of strain in the specimen,. Four types

of resistance strain gauges are used; all strain gauges of the fabricate

Tokyo Sokki. The rosettes are of the type PC-5, the torsional gauges, of 'the type PR-5 and the single gauges of t'ho type PL-5. So all strain gauges have

a filament length of 5 mm and a resistance of 120 1- 0,3 . Ónly the strain gauges nr. '41, 42, 51 and 52 on specimnen B are of the type PL-2, with a filament length of 2 mm and a resistance of 60 0.

For the. measurement of the E . four clip-gauge devices .weie used, as virtual'

developed in the Ship Structures Laboratory for C.O.D.-rneasurements. The

out-pUt of strain gauges, the clip-gauges and pressure dynamometers was measured by a digital measuring system, fabricate Peekel, composed of some 50-point connecting boxes, a power supply, a scanner and a digital voltmeter. The out-put was registrated by means of a high speed Teletype paper punch. The paper-tape was read out fo direct control, while later on the tape was used as input for an I.B.M. 60/360 computer to determine the Es-values, etc.

During the fatigue test 6 strain gauges of the ±eference section were con-nected to Peekel strain indicators and at intervals registrated with a

6-channel Rikadenki pen-recorder as an extra control in adjusting and cOntrolling

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5. Test results.

A.

Static tests.

The results of the. static tests are given in the figures 9 till 21.

For the axial loading the results are given in EC-values (kg/cm2) for a

tensile load of 100 tonforce. This could he done as all local strain,

measured with the strain gauges, proved to be practically propoitional

to the applied load. This inean that also in the heavily loaded zones,

even in the immediate vicinity of severe stress raisers, little or no

cyclic plastic straining occurred. This can be seen for example on the

computerplots for the strain gauges 13, 14, 15, 25 and 16 of specimen A

in the figures 22 till 26. The only exception in linearity was found fer

some strain gauges at the longitudinal bulkheadof specimen C, which was

due to the unflatness of thebulkhead.

Forthis reason the computerplots for the strain gauges 8V, SA, 9V, 9A,

23V, 23 A, 2t4V and 2A are included in this report in the figures 27 till

34. When comparing the results of specimen A and B with the results of

specimen C it should he noted that all results are given in EE-values

for 100 ton axial tensile load. However, the area of a transverse

sec-tiori cf specimen C is much larger than the area of specimen A and B.

For a comparison based upon equal nominal stresses it will be necessary

to corrigate the values for specimen C with a factor of abt. 1,7. In the

same war, also the axial loads in testing applied upon specimen C were

chosen

1. 7 times as high as was applied upon specimen A and B

, 'in order

to get the same conditions undei' testing. In a first look, this factor

1,7 seems to be somewhat doubtful, as the nominal area of the transverse.

section of specimen C is 315 cm2

,

that is abt .

twice the area of A aìd B

with a nominal area of 166 cm2

.

However , there were two reasons for

taking a testload for specimen C, which was less thanabt. twice the

loading force for specimen A and B.

At first , just next to the transverse bulkhead the area are respectively

380 cm2 for specimen C and 225 cm2 for specimen A and B, thus a ratio of

abt. 1 7. Secondly and of much more importance, is the influence of the

bending aspect in the specimen. In fact, the variation of loads and

stresses in the bottom section of the pushharges are the results of the

differences in bending moment in the transverse section of the barge

during loading and unloading the barge. That

_{means that a tensile force}

and stress in the bottom is accompanied at the

same time by a compressive

force and stress of the same order of magnitude in the

decic. So we have

not a constant value for strain and stress over the height of the

struc-ture. For the transverse section cf the barge we find the neutral axis at

a height of abt. 22'40 mm above the bottomplate. The neutral axis of

spe-cimen A and B lies 143,6 mm above the bottomplate, while the neutral axis

of specimen C lies about 283 mm above the bottomplate. So the

mean value

of stress in specimen C has to be (22LI0_283)/(221o_4I4)

_{0,89 times the}

mean value of specimen A and B, assuming that also the bending effect is

simulated in testing of specirnenC. This results in

an applied load for

specimen C of (315/166) x 0,89

1,685 times the applied load for

speci-men A and B. With the tapering-off of the longitudinal bulkhead towards

the ends of specimen C the distribution of stress and

train over the

nominal transverse section is brought into better agreement with reality,

resulting in a nominal stress of lower value i

specimen C as is the case

for specimen A and B, while stress and strai.n in bottomplate

and

longitu-dinals of specimen C are the senne as for specimen A and B.

From figures 9, 12, 13, and 18 it can be seen, that the ineasured strains

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trans-verse bulkhead and the end of the longitudinal, have much higher values than anywhere else in the structùre. The high strains are the results of axial strain and load bending strain and it can already he concluded but

of the static results. that this point will he thb weakest of the struc-turc; that is to say that in this region the first crack i'nïtitation can be expected.

Comparing smecimen A with specimen B, there seems to be no large differen-ce. In the brckets of type A more bending occurred, hut strain values are not growing really high. However, the strain measured by gauge np. 12

on the bracket of specimen A is about 1 times the strain at the sanie place of specimen -B. _{This is due to local bending in the bracket and from}

this point of view bracket type of specimen B may be preferred. Looking

for the bottoinpIate at the strain gauges 15/25 of specimen A and the

values of strain gauges at the same locations of specimen B, we see that the maximum strain value is about 10% higher for specimen A. So at a

first look snecimen 13 seems to be a little bit better than specimen A, hut this should be handled very carefully.

Concerning the modification of specimen 13, where after te first static measurements two longitudinals were modified hy cutting out a circular hole at the end of the webplate of the longitudinal, it pointed out that the modification was not an improvement of the structure. The mean value, the bending value and the maximum value of strains measured at- tIe end of the longitudinals were enlarged with about 30%.

For determining the E . the displacement between two points as

indi--virtual

--cated in figure 8, was measured in relation to the applied load.

- In accordance with the strain measurements also the displacements proved to be proportional to the applied load, so that the measured elongation

per load of 100 tonforce also are no-ted in the figures 9, 12, 13 and 18. The E . , based upon the mean value of the elongation at points C

virtual _.1

C2, C3 and C1, was found to be 1,52 x l0 kg/mm2 for specimen A.

For specimen B a value of E . was found to be 1,60 x l0 kg/mm2

be-- virtual -

-fore the modification. Due to the modification of the longitudinals, the

E . decreased to a value of 1,50 kg/mm2. virtual

For specimen C also the same elongation measurement has been carried out. However, it is not easy to transfer this measured elongation into a

value for E . , as it is not weil done to define what in this case has virtual

to be understood by E . . However, it can he seen as very pleasant,

- virtual

-that it is proved to be possible to calculate the elongation very

accura-tely out of the measured strain values of the strain gauges. This is

important, since it is therefore possible to calculate the E . or,

- virtual

what is of more importance, the deformation orelonation _{over any}

-length and at any place you like.

The measured elongations are all compared with calculations-, based upon-the distnjbijtiìon of strain in the specimen as measured with the strain _gauges. The measured- - and calculated values were in entire agreement with a

maxi-mum deviation of abt. 3%. Some examples of this calculation as weJ.l as

some controls upon the applied axial load are given in appendix II. As already mentioned in chapter

1,

the - specimens werè also applied to a

vertical load and to a horizontal load upon the - top of the transverse

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-In figure 2 it can he seen how the loads were ap1ied, Just _{as was the case} with the axial lading, the measured strain values _{were all exactly}

pro-portional to the applied, load. The results are pldtted in figures 10, 1'4,

15 and 19 for the vertical loading, while the results for the horizontal loading are given in figures 11, 16. 17 and 20. For both cases the results

are given as E-values (kg/cm2) per 5000 kgf. For comperason of these results for specimen A and B with the results of specimen C again it

should be noted that the structure of specimen C is such heavier s the

structure of specimen A and B in this respect.

B. Fatigue tests

The results of fatigue tests are given in the, figures 35 till '4'4.

As mentioned already in '4 the first 3000 cycles of fatigue testing were carried out with an applied axial cyclic load amplitude of _{-651+130 tf for}

specimen A and B and a load amplitude of -1101+220 _{tf for specimen C,} cor-responding with a nominal stress amplitude in _{the bottomplate of -'4'-f-8} kg/mm2. When the first 3000 cycles in testing specimen A had passed and no crack could be observed, it was decided to increase the applied load with 25% and testing was continUed with _{a load amplitude of -80/-i-160 tf,}

according to' the agreeded test program. In order. -to have the same test

conditions also the load amplitude of specimen B was increased up till -801+160 tf, after 3000 cycles had passed, while the load amplitude of

specimen C was enlarged up till -135/+275 tf.

At a total number of 3750 cycles for specimen A first cracks were observed,

haying some length already. Fatigue was stopped at a total number of '4620

cycles for specimen A, wen no one of the cracks had developed _{over a depth}

equal to the plateth'ic1ness.

in specimen B, first cracks were found at 2500 cycles. Unless this

presen-ce of cracks the load amplitude was changed at 3000 cycles in order _{to be} able to compare the results with those of specimen A.

Fatiguing of specimen B was continued Until cracks _{were developed}

of-con-siderable greater extent than in specimen A, in order to get better

insight into crack growth characteristics. _{At a total number of 5800 cycles} fatiguing of soecimen B had beer, stopped when crack _{A 3, fig. 35 and 38,}

had developed over some length a'depth equal _{to plate thickness.}

In testing specimen C again first cracks were found before 3000 cycles

of --1101+220 tf had passed. However, according _{to the test procedure of} specimen A and B. the load amplitude was changed. into -135/+275 tf for the

next 3000 cycles. At 6000 cycles total it _{was decided to continue}

fatigu-ing as the cracks were s-till rather _{small; may be to small to he sure of}

having fracture at a tension load below 600 _{tf. The lòad amplitude was} now maintained upon -135/+275 tf. Fatiguing was stopped _{at' 8500 cycles} total, lust before collapse during fatigue could _{b.e expected due to crack}

B.'4 (fig. 36 and 39).

Figures 35 and 36 show growth of crack _length ' in relation to number of cycles. The cracks are split up into _{some types and provided with a code} in relation to their location in _{the structure (see fig. 35-39).}

The code encloses a letter A _{or B and a figure 1 up to 6 and is specified}

'as given below:

A : fatigue crack initiated at the toe of the

overlapweld at the top side of the bottomplate, aocI developing

-downwards into the bottomplate

B : fatigue crack initiated in the weld-undercut _{of the} (overhead layed) overlapweld _{at the underside of the} of the overlap stiip and developing upwards, into _the

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-9-i-2 : fatiguecrack is initiated near the end of a longitudirial at that side of the transverse bulkhead, where 6,5 mm overiapweldsare used

3-14 fatiguecrack is initiated near the end of a longitudinal at that hide of the transverse bulkhead, where 5 mm overiapweldsare used

b etc. : fatiguecrack along one of the above mentioned weidments, but outside regious as indicated by thefigures 1, 2, 3 and 14.

If necessary, the code may be preceded by a letter A, B or C indicatirg

the type specimen.

Cracks of type Ai-14 and Bi-14 were discovered at almost the same number of cycles, but they behaved in a quite different manner.

Cracks of type Al-14 gradually propagated in length at both cracktips as

-well as in depth as the number of cycles increased, while cracks of type

Bi-'4 initiated over a larger extent of length, but propagated less in

depth.

Cracks of type A5 etc. are of less importance and became visible in a

later stage of fatigue.

The explanation of these fenomena is quite logical. Due to the interruption

of the longitudinals the largest stress and strain in the bottornplate

will occur between the end of the longitudinals and the transverse bulkhead,

with a stress raiser at the toe of the longitudinal and a tapering off

towards the transverse bulkhead (see appendix II figure AI:[). So, looking along the entire length of the overlapwelds the largest hart of' plate

stress will be found just at the end of the longitudinals in thé thinner bottomplate, see figUre 21. The effect of bending stresses dueto the

overlap will also depend upon the- location in length and breadth with

respect to the end of the longitudinals, as well as to the thickness of the plate. Unfortunately the direction of bending comes through in such a

way that stress and strain in the overlapstrip and bottomplate at the toe of the overlapwelds are increased considerably. In line with the interrupted

longitudinals the bending stress in the thinner bottomplate will be higher than in the bulkhead strip, (see figures 9, 12, 13, 18 and 21) as the length between bulkhead and overlap divided by thickness of strip

is larger than the length between end of longitudinal and overlap divided by thickness of bottomplate. Away from the line of longitudinais the length from bulkhead to overlap does not change while the length from overlap to endpoint can be thought to increase very sharp. This results in a sudden decrease of bending stresses at the toe of the weld at the side of the longitudinal. However the bending stresses at the toe of the weld near the transverse bulkhead will hardly change over the length of the weldment

(which is also found in figure 21).

-Due to the above mentioned effects the stresses will be maximum in the

bottornplate in the line of longitudinals. In that particular region crack

initiation and largest propagation can be expected As is clear these effects will exist for cracks of code A, and indeed the results for- these

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lo

-For the overlap welds at the underside of the bottomplate close to the transverse bulkheads , and with a larger distance to the ends of the

longitudinals , where cracks with COd B are found, the situation is more

complicated.

The hart of plate stress is relatively small because of increased

thickness of overlap-strip with respect to bottomplate; mean levels are related with factor 0.9/1.2.

The hart of plate stress, when looked along the weidment, does not show

a very high peak in way of the longitudinals because of the larger distance of : this overlapweld to the end of longitudinals.

These effects can very clear be seen in the figures of appendix II, where E values for hart ¿f plate are given for the bottomplate along the line of longitudinals, and in figure 21 for distribution along the weidment.

The bending stress alöng this weldment will show some raise when

further away tram the line-of longitudinals while the value of this stress in line with the longitudinal is less than in the hottoniplating.

The resulting stress distribution along the toe of this weidment f

the strip will accordingly show much less a peak in way of the

longitudinal-intersection and the mean value is much less than the peak value at the toe of the overlapweld of the bottornplate just in way of the longitudinal.

So, crack initiation and propagation couldbe expected to be of much

lower value in this region. However, the conditions at the toe of the

weld are much worse as a result of the weld-undercut due to ovehead

welding, which effect is also present along the entire length of the overlap-weld. For this reason, initiation of cracks with code B can also be

expected in an early stage. Initiation will be more extended along thd

weld ìcnt as the difference in conditions in way of longitud inals with respect

to the rest of the weld is little. Propagation in depth will be ess for

this cracks with code B than for cracks with code A. This is due to

the fact that after initiation of cracks the cracktip conditions, are no longer influenced by the weld-undercut as stress raiser.

At the other side of the overlapweld, in the bottomplate on the line of longitudinals the field of the stess raiser due to the interruption of the longitudinal has considerable longitudna1 and transverse dimensions with respect to crack dimensions and stress raiser due to the presence of a crack in this particular region has to be superposed upon the

high stresses that already exist. This gives a situation in which cracks

with code A will propagate faster than cracks wit« code B. 0f course, for cracks with code B the first initiation point along the weldrnent

can be expected in the neighbourhood at the longitudinals. The propagation

into the depth will he fastest in this region too as can be expected but slower than for cracks with code A.

Except for crack B.-i of specimen C the results are in fui.ly agreement with the above given explanation.

For speciment A sorne measurements of length of cracks type Al_11 were made,

but the number was not sufficient to get insight into crack growth

characteristics as fatigue was stopped at an early stage.

For speciment B and C more crack length measurements were carried out and

the results are plotted in figures 35 and 36.

These figures show that crack initiation has taken place before 3000 cycles.

For specimen A there is only one line of crack growth; the other cracks are given with end-points. For specimen B and C the curves of cracks

(14)

After changement of load amplitude when

3000

cyLes. have been applied

crack groth rate starts to increase strongly for most cracks type A partly due to the increased load amplitude. This high crack growth tate however is of limited duration and a period of 1essr drack growth

starts. This is the moment when the crack length reachés beyond the

region of peak stresses due to longitudinal interruption. Sàon a

minimum, crack growth rate is reached and then a continuous increase

of crack growth rate follòws due to a not furthermore neglectible

reduction of cross rectional area.

It seems tht crack growth of speciment B is somewhat faster than for specimen A and C but one has to remember that cracks Al and A3 of specimen B, which show high crack growth., are located near longitudinal-ends of

modificated type where increased stresses exist withrespect to

longitudinal-ends without moditication. Cracks A2 and A-t of specimen B,

however, show crack growth of the same order as specimen A and C.

Comparison of mean crack lengths of different specimen in relation with measured Ec-meanvalues in the region of longitudinal-ends is mad in

table I.

Table I Comparison of mean crack lergths

i) Ec mean values for a load of 100 tonf for specimen A and B and

170

tf for specimen C.

2.) Mean value at upperside of bottomplate..

3) Hart of plate valúe.

Lt) Cracks at longitudinal ends with modification.

Cracks at the endoflongitudinals without transverse brackets. Cracks at the end of longitudinals with transverse brackets.

Length of A3 at Lt600 cycles is guessed at 57 mm with the aid of figure

35.

These values are something extreme because of crack

_{A2 of}

specimen C. From this table it follóws ' that crack growth is in good relation to Ec values at the upperside of bottornplate near the end of the longitudinal.

The modifications of specimen B are disadvantageous with respect to maximum stress level as well as crack propagation. Some of the final cracks type A in specimen B and C extended over all the thickness of the plate for some length. The numbers of cycles at which was observed that the bottom of the crack reached the other side of the plate, are given in figures 35 and

36,

indicated by NTH.

Specimen Crack

no.

mean crack length at

'4600 cycles 5800 cycles Statical I 2) E 1.) ii

3)

A

Al... .4

79 min -

4820

1882 B

Al,A3 6)

₁₂₄ mm 178 mm 5916 1805 A2,A4 75 mm 7) 109 mm 4617 1829 C

Al,A4 4)

₇₅ mm 94 mm

4850

1947

A2,A3 5)

42 mm 8) ', 68 mm.

8)

4660

1817

(15)

12

-Figures 37, 38 and 39 show final crack dimensions for all cracks tyne A,

while particularities of these cracks are shown in the pictures of figires -1-0, '1-1 and 42.

Figure '40a shows crack' Al of specimen A at the side of the 6,5 mm weld,

looking from bulkhead to longitudinal. This is a normal fullr developed

crack at '4620 cycles. Figure '4Db shows cr'arlc M of specimen B at the

side of the 5mm weld, looking from.bulkhead to longitudinal. This is

an other normal fully developed crack at, 5800 cycles. The dark line in the

bottom of the crack to the right is a shearlip due to break out the

piece afterwards for examination.

Figure '4la shows crack AL1 of specimen C at the side of the 5mm weld,

looking from bulkhead to longitudinal. This shows a normal crack which is propagated over the full plate thickness for some length at 8500 cycles. See also cracks in weld on toe of longitudinal which ki,nd of cracks also occured in way of Al and A3 of specimen C in something lesser extent,

but nowhere in speimen A and B due to less cycles, even not in M of

specimen B (figure '40b) which shows much weld-undercut in the longitudinal. Figure '41b shows crack Al of specimen B at the side of the 6,5 mm weld, looking, from bulkhead to longitudinal. This figure shows a lamination in the crack bottom. Number ot cycles N 5800.

Figure '+2 shows some not fuÏly developed cracks of type A:

Figure 42a shows crack A2 of specimen A at the 6,5 mm weidside, looking

from bulkhead to longitudinal at '4620 cycles.

Figure '42b shows rack A3 of specimen A 'at the 5 mm weidside looking from longitudinal to bulkhead at '4620 cycles.

The horizontal cracks in this piece are caused when the other part was

broken out of the plate for examination of the crack surface.

Pieces of figures '-1-2a and 42b are photographed with _{inclination of the}

cracks. BotJ pieces are somewhat damaged because of the great forces for

breaking out them.

The lamination found in specimen B, crack Al, may be seen as a defect

which is rather normal for materials of properties as used. The fatigue

crack is somewhat ce1ayed for growing into the depth by this lamination. The length of this crack is not excessive as crack A3 ot specimen B, which is comparable, has more length. For these short term experiments, a

lamination has not so much influence even when at, present in the regions

of longitudinal ends, in practice, when a corrosive liquid canpenetrate

into the lamination, it becomes more dangerous,

To get some indication about the difference in behaviour of cracks type A3,'4 at the 5tnm weld side with respect to cracks typeAl,2 at the 6,5mm

weld side.see table II for final crack lengths, crack areas and' measured Ec values.

(16)

- 13

-Table

II

Total final lengths and areas on éither side. of

bulkhead arid measured Ec values.

Notes 1), 2) and 3): see table I on page 11.

In most cases cracks A3,Llat the 5 mm weidside have little more length and some more area than cracks Al,2 at the 6,5 mm weidside due to some

more depth at the 5 mm weidside. Looking at strains of specimen B at the 6,5 mm weidside these seem to be something larger but cracks are worse.

at the 5 mm weidside. However, in the

5 mm weld itself, strain is in

excess of strain in the 6,5 mm weld (straingauges 51 and 41).

It is hard to indicate the difference as

siignifidant, but

it looks like

that there is little tendency to somewhat better behavior of

_{6,5 mm welds}

with respect to. cracks of type Al-4.

Though sorne cracks of type B are plotted in figure 36 they are considered in another way as cracks of type Al-4, r

Figure 43 is a picture of an example ot 'cracktype B at. N 4620 for

specimen

A at 6,5 mm weldside. It i's clearly shown in this picture that

this kind of crack is not only concentrated near the longitudinal-bulkhead-intersection, which place is marked by straingauge 28, These _cracks

developed over a much greater extent and are initiated in the bottom

of the weld-undercut due to overhead welding which is found in the

bulkheadstrip. Tese cracks, having little crack opening due to little penetration and being hidden in the bottom of the weld-undercut,, are most difficult to determine in _{an early stage during}

fatigue tests. Length of these cracks during tests _{can hardly be mentioned}

with good accuracy. They appeared on

both sided of the bulkhead in the

bulkheadstrip.

Besides cracks of type Al-4,at the ends 'of longitudinals, hardly any crack of 'type A5etè. appeared because of lag of weld-undercut due to

downhand welding. Only crack AS specimen B (figure 38), and in specimen C near the longitudinal bulkhead some cracks are found.

TabLé UI gives 'some information about 'the distribution of these cracks,

because itgives total final lengths in percentages of weld lengths and final number of cycles for each specimen as found after a thoroughly

inspection of the specimen when experimentswere finished.

Total final length

and area of cracks

Ali-A2 at 6,5 mm

weldside

Mean E

values 1)

Total final length

and area of cracks

A14-t-A3 at 5 min

weld-side

Mean E

values 1)

I 2) II 3)

I 2)

II 3)

Specimen A

NTot 4620

86-F73159 mm

2 358-i-l2348l mm -

-80-t-77l57 mm

2

75228303 mm

. 4820 188» Specimen B NTot 5800 l60f78238 mm 37-1'l50524 mm 5526 1860 l0+195z335 mm ' 49i+'825J3l6 mm

46 154

L' Specimen C NTot 8500 2l9+l55374 mm 2 9l5i-44ll356 mm -250+206456 mm 2 l276±8l42090 mm 4755 1880

(17)

1.) Mostly near crack tips of cracks A1-'4.

Crack A5 figure 38 and vicinity. Near longitudinal bulkhead mainly.

LI.) These values may be extra high as much length

of these cracks is found in the vicinity where final fracture cracks stopped while material over there

shows much elongation and cracks were opened. Outside cracks s drawn in figures 37, 38 and 39. First value is mean value, second value is max. depth in way of B1,3 and the third in way of B2,i..

One can see now that these cracks appear to be somewhat deeper at the

5mm weldside, unless initiation' is hardly the same for both sides.

From figure 36 it clearly can be seen that crack BLI. of specimen C, starting

very slowly, has become unstable at about a number of 8400 cycles total, which is shown in table IV to coincide with the moment that the bottom

of this crack reached the other side of the bulkheadstrip. Table IV Unstable growth of crack B'4 of specimen C

1) This is exclusive a length of 500mm where _{a crack of little depth is}

found afterwards.

Outerside of hull deep weld undercut

Innerside of hull, no weld-undercut

5

Outside regions of longitudinals

Bli-B2

6,5mm weld

B3+B4

5mm weld 6,5mm weld ' 5mm weld

% i depth % 1 depth % i depth % i depth

Specimen A N '4620 98 % 0,38mm 1,40mm 1,00mm 92 % 0,67mm 1,00mm 1,20mm 1) i-10% -i) +10% -Specimen B N 5800 83 % 0,53mm -1,0 mm 73 '% 0,91mm 0,50mm 2,50mm i- 5% -2) i-20% 4) max.l,5 mm Specinien C N 8500 68 % 2,20mm 1,50mm 10,5mm 79 % '4,8 mm -12 mm i-3,5% -3) +22%

-Cycles Length at underside of strip Length at upperside of strip

7000

85mm

'0

'7830 140mm 0 7975 163mm 0 8350 300 mm 0 8470 not measured ' 125 mm 8540 500 mm 1) 320 mm

Table III Length in % of weldlength where cracks are found and depth of cracks B and A5etc, initiating from weld-undercut.

(18)

-. 15

-It has to be remembered however that the moment whn crack B- of _{specimen C} became desastrous, due to unstable crackgrowth atN _{8!400, is well beyond}

the moment of acceptable cracks of type A, as cracks Al, A3 and A'4 of

specimen C had developed over the full plate thickness before N = 7830

(see figure 36).

Final fatigu.e crack dimensions of crack ß14 of specimen C is shown in figure 39. At the same moment crack B2 of specimen C had developed in this

specimen to dimensions as given in figure 39 too. This once more gives an indication that there is only little difference with respect to weld

thickness. It can further be concluded from this that, unless initiation of these cracks is found hardly over all the weld length, the propagation

into the depth is faster in the regions near longitudinal intersections,

which is clear. This is found in something lesser extent in specimen A and B too.

Regarding foregoing results of cracks of type B it is hard to entitle these differences as significant hut it seems that 6,5mm weld behaves somewhat

better due to less stress concentration at the toe.

During fracture of specimen A it became clear that in the welds, connecting the interrupted brackets to the bulkhead, fatigue cracks existed which were not determined during fatigue experiments as they had not yet reached

t.he weld surface.

At a tensile load of 250 tons during fracture test these cracks _{were opened}

over the lower part of the brackets.

Looking at figure 9 it follows that in the interrupted brackets, which are

overlapped to the longitudinals, bending occurs, resulting in higher

stresses in that side of the bracket where the longitudinal is overlapped.

Now looking at the final fractures of the brackets, cracktypes according

to figure 4'-i- are found.. At the side of overlap, with higher stresses, _type

Cl crack is found over all the height of bracket, with _{a mean distribution} of crack kinds over the throatthickness according to specification a; C 1a

At the other side of the brackets a mixture of two fully different crack

types is found: cracktype C 1, with specification b, is mainly located in the upper part of the bracket and cracktype C 2 mainly in the lower part;

see detail of cracks C in bracket i and bracket 2 figure Distribution

of crack types C 2 and C ib over this side in percents _{of bracket height} is given in table V.

Table V

Crack types C i a and 'C 1 b, having somewhat different mean distributions of crack kinds over throat-thickness, _{are of formas given in the}

specification of figure :

- A first brittle initiation part over K% of throat-thickness.t.,

- A second fatigue part over L% of throat-thickness.t.

- At last a fibre or mixed fracture over M% of throat-thickness _t. Mean values of K, L and M for cia and Clb,cracks _{are according.}

Bracket 1 . ' Bracket 2 type C2 .

65mm

20% height '

151mm

47% height type db 80% . 53%

(19)

to table VI

Table VI

It is concluded that a brittle initiation crack has developed from the throat over some 16% of t already in an early stage of fatigue or

before. 'This crack can appear over there due to following combined factors; Ti) Inclusions in the weld material.

Bad material properties due to quick cooling of weld material. Stress concentration due to quick cooling of weld material.

L)

_{Strain concentration when loaded.}

All these factors besides the fourth one are of lesser magnitude when

further away from the corner and.the brittle initiation crack may stop. During fatigue the crack propagates over some LI0% of throat thickness. Finally a rest fracture of fibre or mixed type over some 1f5% of throat thickness took place during fracture test.

It can be expected that most fatigue length will be at present near the lower part of the bracket at the side where the longitudinal overlaps,

but this is not confirmed when. looking at the surfaces of the fractures.

Interesting is that several brittle initiation parts are found in the lower parts of the welds at the other side of the bulkhead too.

Crack type C 2 is as drawn in figure 4: In this case the ciack starts

in the 'H.,A..Z near the throat of the weld, runs over sorne length through the weld-material, crosses through the H.A.Z. and reaches the plate material.

As here the properties are good it is turned back over some 90 degrees

to the plate surface at the toe of the. weld through the 1I.A.Z. again. At first it was thought o,f a fracture due to lammellair tearing but

microscopic inspection pOinted out that it was not, the case.. Both brackets

fractured in the same way and crack parts of type C 2 must be caused by

the same effect which only can be found in the crack sequence of the. welds connecting the brackets to the bulkhead.

Looking, at the effect of the longitudinal bulkhead it can be reviewed that

first and worse fatigue cracks appeared near longitudinal intersections in specimen C. At the end of the fatigue experiments hardly any fatigue crack was found near the longitudinal bulkhead (only some 12 mm at N 8350 at the 5. mm weld-side on each side of the longitudinal bulkhead along the toe

of the inner overlap-weld). From the point of view of fatigue the bulkhead

construction is safer than the longitudinals.

The transverse brackets on one side of the longitudinal bulkhead may have.

effected the results of that longitudinal which they are connected with. Indeed at the side where no brackets are placed crack growth is faster, but it has to be regarded that in specimen A and B a somewhat faster crack growth is found in the same side of the specimen too; see table VII.

01a

_cib

K 16 16

L '+3 . 38

(20)

17

-Table VII Final crack dimensions at either sid of specimen.

Comparing fatigue resistance of longitudinals of specimen C without

transverse brackets (Crack Al,4) with the unmodificated longitudinals of specimen B (cracks A2,4) shows little difference as is shown already in

table I on page 11. So the influence of the longitudinal, bulkhead upon fatigue behavior of an adjacent longitudinal intersection seems to be

negiectible

when no transverse brackets are connected with the

longitudinal.

C. Fracture experiments

Fracture experiments were carried out after fatigue testing of the. specimen.

First the specimen were compressed until a maximum compressive load wìs

reached at which negative eongation takes place without a further increase of compressive load. This could directly be observed in a recording in which P (tf) was plotted against the displacement of the traverse of the 600

tons tension-cômpression machine. As an example to illustrate these kinds

of recordings figure '45 shows the character of one of them.

Having reached the point of unstability the compressive load was released

and followed by tensile loading up till fracture.

Table VIII gives Pmin and P W.T.S. for the three specimen:

Table VIII

Length

and area

of Al

+ AQ

Length and area

of A2

1- A3

relative propertie

1. and A1

_{'r and Ar}

_-1./'r

_Al/Ar

86 -t- 77 163 mm 73 80 153 mm

1,07

Specimen A 358 + 75 '433 mm2 123 228 351 mm2 160-t-.l'40300mm

78+l95273mm

1,10 Specimen B 374 + '491

865 mm2 150'

+ 825 975 mm2 0,94 219 + 250 469 mm 155 + 206 361 mm 1,30 Specimen C 915 +1276 ;2l91 mm2 '4'4l + 814

l255mm

. 1,32

Pmin Puit.tensile _{foregoing total number of cycles}

A - 175 t 395 t '4620 cycles

B - 190 t 340 t 5800 cycles

(21)

18

-Figure '6 gives the final fractures. From this figure it becomes clear that cracks of type B, having not caused limitation of fatigue life with respect to acceptable crack growth in the hottcm, in all casés were involved with final fracture in the bottomplate. Only in specimen A final fracture was not restricted to the bottomplate, but accompanied

and initiated by fracture of the brackets.

Only specimen A showed a total fracture while for specimen B and C the crack in the bottomplate was stopped. In specimen B it stopped because of extra elongation when fatigue crack A,3 opened. In specimen C it may

have been the influence of the longitudinal bulkhead to stop the crack. For specimen B and C the brackets did not fracture.

At 250tf tensile load brackets of specimen A showed opening of cracks

that already existed.

In specimen B some little cracks were found afterwards in the weld at the

end of the longitudinal where the bracket is connected.

It is important that, unless cracks of typ A caused limitation of number of fatigue cycles because of crack depth, no one of these cracks

failed during fracturetests and did not influence ultimate strength. Cracks of type B did so more. In specimen C crack B,14 had become already

unstable during fatigue testing and initiated final fracture.

During the fracture tests the loads were applied stepwise and for each load the strains were measured by the digital measuring system. As a result of computer handling with this information plots of epsilon with respect to load are available for the straingaeges. From these nsight can be got

into fracture sequence and behavior of the structure during the period

of high loads as well as behavior under compressive load. Sonic of the characteristic plots are collected in a separate addendum to this report.

(22)

-- 19

6. Conclusions and final observations.

In all specimen fatigue cracks. are initiated at the toe of the

overlapuelds in the bottom after about 2000 cycles of a

loadamplitude, corresponding with a nominal stress amplitude of -41+8kg/mm2 for the bottomplate in the undj'turbed section. The initiation and propagation of the fatiguecracks at the toe of

the overlapwelds at the upper- or inner side of the bottomplate is the result of the high strainvalues between transverse bulkhead and the end of the longitudinals, partly due to the stress- and strainraising effect of the interruption of the longitudinals and partly due to the large bending effect of the overlap.

The initiation of fatigue cracks at the toe of the overlapwelds at the under- or outerside of the overlapstrip is especially the result of the stress- arid strain-raising effect of the large

weldundercut. in combination with the bending effect of the overlap, while the stress and strain-raising effect of the inter-.

ruption of the longitudinals is of less' importance.

The geometry of the weidments and the applied welding techniques are

very important. In this respect the attention is drawn upon the

weld-undercut, due to overhead welding, which weldundercut can be seen as primarily responsible for the initiation of the cracks mentioned in point 3.

5 _{For all specimen the local strainvalues, determined in staticàl}

measurements before and during fatiguetests, proved to be pràcticall.y

proportional to the applied load. This means that in tse heavily loaded specimen, even in the immediate vicinity of severe

stres-raisers little or no plastic training occurred.

Interrupted brackets show a larger horizontal bending than throug

brackets, resulting in higher local strain and stress. Partly due to this bending effect, but primarily due to the kind of welding

connection between brackets and transverse bulkhead, fatigue cracks

were initiated in this weld. Although fatigue resistance of the

structure was not restricted by these fatigue cracks, the collapse .of the structure under loading to rupture was initiated in the fatigue

cracks in the welds between bracket and bulkhead.

Comparing 6,5 and 5 mm wej.dments of overlapwe.ld with r'espect to their

fatigue resistance, the 6,5 mm weld showed a little hit better behaviour, notwithstanding the larger strainvalues in bottomplate near the 6 mm weldments (see strainmeasurement of specimen 13 figures 12 and 13). However, the difference could not be pointed out to be very significant.

It should be noted that no remarks can be made with respect to the

influence of the bulkhead stiffeners, as the difference in stifines of the intersection on both sides of the transverse bulkhead may be related to the behaviour of the overiapwelds.

(23)

20 -The modification of specimen B proved to have an unfavoura.ble influence

upon the resulting strainvalues and fatigue-resistance.

The total loadvariations applied during the fatigue tests within a

couple of days, will be spread over about 50 years in reality. So

partly due to the short time of testing

but primarily in consequence

of the absence of corrosive elements, the influence of corrosion was

not included in the investigation. Fio:ever, it should be noted that

corrosion fatigue

rack will develop easily as the fatigue resistance

cf unprotected structures in a corrosive environment is very low.

/s could be expected the intersection of the longitudinal bulkhead

behaved much better as the intersections of interrupted longitudinals.

With

_{respect to the influence of the longitudinal bulkhead upon the}

adjacent bottom longitudinals, the results show a tendency that the

longitudinals adjacent to the longitudinal bulkhead and connected

to

the bulkhead with transverse brackets tere in. little advantage compared

with the other longitudinals.

Howeve; the differences are very small and by far not significant.

The measured values of the E virtual over the intersection of the

longitudinals showed a reduct.ion of about 30% with respect to E normal

for the specimen with interrupted brackets and about 25% for the specimen

with through brackets. However, it must be realised, that these

figures are arbitraly as the E virtual depends strongly

upon the distance

over which the value is determined. From calculations based upon the

measurements of strain distribution over the structures, it was found

that E virtual can be determined with good accuracy.

Looking at the results of fatigue testing äs presented in figures 35

and

36 it seems that the fatigue resistance is low. However, the results

are

presented for a loadamplitude during the first 3000 cycles corresponding

with a nominal value of _L/±8 kg/mm2 in the bottorripart of the

_structures

at the underturbed sectional area, while the loadamplitude

was enlarged

up till -51+10 kg/mm2 for the following cycles. Assuming that the real

loadarnplitude as it will be present in practice,

may be of lower value,

the results are translated for other lower values of loadamplitudes. This

translation is based upon earlier results from experimental work carried

out in the shipstruc-tures laboratory, which investigation is published in

report 82S. (7)

In this report crackgrowth results are given fbr initiation of cracks

2

(crack area is zero), crack areas of 100 mm2 and crack

areas of 500 mm

as

a function of nominal. stresses (strain x youngs modulu) and number of

cycles. Some of these results concerned fatiguecracks in bottomplates

between transverse bulkhead and ends of longitudinals

at the place of

intersections. Although these cracks developed at the toe of the

weld of

longitudinals to bottomplate, the. conditions

are the same and a good

comparison is thought to be possible. The results as presented in figures

6 and 8 of report 82 S are used for the translation of fatiguecrack results

of intersections of the pushbarges. The donfiguration of the specimen

as

shown in figure 6 of report 82 S is most according to the specimen

of

the pushbarges and therefore this figure

s used primarily. However, as

the results of the specimen showed in figure 8 of report 82 S

are somewhat

unfavourable with respect to an extra-polat ion into lower stress- and

(24)

21 -For translation of the results it is assumed that the lines for certain

cracklengtb have the

_{sarre character as the lines for the. crack area.}

The first part of the translation enclosed areplacement of the results

of figures 35 and 36 by a new diagram with a line fo mean uppervalues of

fatiguecrack

_{growth. (figure7 )}

In the same figure also

the

growth of fatiguecracks is given for a constant

ioadarnpììtude of -5/-t-10 kg/mm2. The translations were made by

means of two

diagrams based upon the results from figures C and 8 of

_{report 82 S}

(see figures '-18

and

L9 ).

_{From this diagram the numher of cycles could be}

determined for different cracklc-ngth as function of loadap1i.tude.

For predicting the crackgrowth for a ioadarnplitude of O/+'4 it is assumed

that the influence of the compressive parts of the loadamplitude

upon

fatigue damage is half as much as for the tensile part of the amplitude.

In this way a prediction is given for loadamplitudes of -5/-t-10,

_-41+8,

-2/L

and O/' kg/mm2 .

The results are given in figure 50

.

Comparing the different specimen and the testresults

,

it seems evident

to analyse once more this way of construction J)efore it will be used in

normal production. Although the differences between the interrupted- and

the throughbracket

_{structures are not very large, it must be concluded}

that the throughbracket is prferab1e to the interrupted bracket.

Perhaps also the welded connection between :i.nterrupted bracket and

transverse bdlkhead can he improved, using a larger throatthickness of the

weld or using a K-type weld. However, a larger thickness of the weld

ox

a K-type weld will be more expensive from a viewpoint of cost and time.

When using an interrupted bracket, welding must be carried out very

carefully.

With respect to the thickness of the overlapwelds a

6,5

mm. weld seems to

be preferable.

However, the differences

in

_{resistance against fatigue between the 5,5 and}

5 mm weld arc very small. Although the bottomplate fractured

in the

_{5 mm}

welds under loading, to rupture,

it

_{is still the question if the 6,5 mm} weld is much stronger than the 5 mm weld. Concerning the initiation of

fatiguecracks in the overlapwe.lds

cannot

he said that the 5 mm

weld is

worse. Of more importance is the effect of local bending due to the

overlap and the strainraising effect due to we].dundercut. Therefore it is

important to check if these effects can be

reduced

or eliminated. With

respect to the local bending, it can be expected that the large strains

due to the bending effect of the overlap

can be reduced largely

when the

thickness of the overlapstrip is increased

up till about 20 mme

Due to the larger stiffness of the thicker ovenlapstrip

_{the angular}

desplacement at the overlap is reduced, resulting in smaller

_{bending strain}

between overlap and end of longitudinal. Bending strain in the overlap is

decreased even more, due to the larger thickness. Also _{the mean value of}

strain in the overlap is decreased in

_{consequence of the larger thickness.}

This is also very important in connection with the

_{weidundercut dUe}

_o

overhead welding. In order to decrease the strain- and stress-raising

effect of the weldundercut, perhaps the weld

_{can be improved by means cf}

a TIG melt, grinding or peening. From a viewpoint of cost deep grinding

will not

be attractive, but comparing approximate costs peenirig seems to

be much less expensive. (2)

Another improvement of the overlap might be

_{to enlarge the overlap in}

such an extent, that the overlapwelds are located under the longitudinal

(25)

22

-Finally attention has to he paid to corrosion prevention. _{Concerning the}

requirements that the barge must be classified for K1 and _{K3 cargo, it seems} that a corrosive environment can he expected, Looking at the pollution

of the inland waterways we can assume that the possibility of. a Corrosive

environment wilinot be restricted to the innerside _{of the barges. At one} side it may be advisable to look for improvemen-ttechniques of weldments

a. _{peening of TIG welding and at the other side corrosion pretection} _must

be considered. A normal corrosion preventing paint, however,, is. not

sufficient as it can only help to prevent initiating of corrosion fatigue

cracks. Sut the protection against corrosion fatigue is finished when cracks have started already due to other (strainraising) effects. In fact

this corrosion protection is of really importance in the regions of

longitudinal intersections and overlapweids. For this places, where a large chance exist that fatiguecracks are initiated, a corrosion protecting

medium should he used, with so large an elasticity that this surface coating will be kept intact, even if a small fatiguecrack develops uìder the

coating. In this way a kind of rubbercompound _{e.g. at a thiokoll base, may} be of good help. 0f course such a compound must have a good resistance