Considerations on the fatigue strength of an LPC-Vessel

,DELFT UNIVERSITY OF TECHNOLOGY Faculty of Mechänical Engineering and Marine Technology

Ship Structures taboratory

SSL 350

CONSIDERATIONS ON TH:E FATIGUE

STRENGTH OF AN LPCVÈSSEL

by ir. H..G. Scholte o Report Nô. SSL 350 November 1994

Contents list

Summary.

Estimation of the number of stress cycles. Fatigue strength cf butt welded plate mater.iàl. 11:1. Fatigue strength of welded structural detaIls.

IV.. Estimation of the fattgue stress range at the top of the structure.

Y. Fatigue strength of the deck, structure of the LPC-vessel. Improvement of the fatigue strength.

Conclusion.

Literature..

Tables and figures.

Summary

An assessment. h'a's been made on the fat igue strength of an LPC-vessel according t'o the disposed preliminary design information. The fatigue

life of the LPC-vessel is estimated, inrelatlion to the well-known fatigue

strength of typical structural details in maritime structures The

assessment of the original design showed a rather low fatigue profile Recommendations are given for improvement of the design, resulting in

CONSIDERATIONS ON THE FATIGUE STRENGTH OF AN .LPC-VESSE[.

by ir. H.G. Scholte,

'November 1994

I'. 'Estimation of the number of stress cycles

The following' assumptions have been made:

The LPC-vessel will be in operation for 80% of the time, fifty-fifty

sailing on inland waterways respectively on open se'a.

The mean peri'od of stress Variations due to wave bending is about

6 seconds.

3..

The relation between stress variations and the number of cycles may

be presented by a straight line on a linear'-lo.g basi:s (f'igure 1).

4.

The assessment of the fatigue strength of t'he deck structure at the

location of the transverse bulkhead at the midship may 'be based on

comparison with the fatigue strength of identical structural details.

Verification may be made by1 usi:ng 'S-N curves and the Mimer cumulative

rule, in accordance with' the. Rules of Bureau Ventas.

nc

-nl

in which: n is the number of stress classes of mean values o,, as a standard equa.! to 20.

The assumptions i and'2 result in .4 *365 * 24 * 3600/6= àbt. 2 * 106

stress variatimons per year'.

The relationship between the magnitude o.f the double amplitude.

f

these

stress variations and the number of stress cycles respectively the number

of exceedings is shown in figure 1.

The number of exceedings as Well as the number of stress variations in

each load class is presented 'in table I.

Fatigue strength of butt welded plate material

Considerations with regard' to the fati:gue strength of the deck structure

will be based primarily on S-N' curves for Structural' Details according

to an inest'igati'on.ofMunse.a.o. [1]. The considered details are presented

in figure 2

At first the S-N curves of non-welded plates of base materia1

(mild steel and hi.gh strength low alloy steel.) under axial loading 'a:re

presenied (1M and' 1H.). Also curves are presented for the same plates

containing full' penetra.tion butt Welds (10M and 10H).. Additionally the

S-.N curve is given for plates wi;th butt welds Which have been ground flush

(10(G)). A significant reduction in fatigue strength due to the butt welds

can be noticed.

Fatigue strength of welded structural' details

A further reduction in fatigue strength will be. found fr longitudinals

structural detail may be considered as a normal standard for longitudinal members in the upper deck strUcture of a ship.

A maximum stress variation of 200 MPa results in a fatigue life of abt,. 4750 years (see table lia). However, it should be realized that these

S-N curves present the regression line or mean values of the test results.

So, 50% of ail specimens Will last a longer life, while also 50% will

fail at a smaller number of cycles. Therefore, the lower tolerance limit

-the line of 99% survival with a 95% confidence level -will result in a significantly lower fatigue strength. The exact value will be dependent on the scatter of the results. The lower tolerance limit as presented by Munse [1] for detail 25 resulted ma reduction of the fatigue endurance with a factor 50, decreasing a mean fatigue life in a lower limit design life of abt. 100 years.

However, considering the fatigue strength of the deck of the LCP-vessel it is preferable to base a first estimation of the fatigue life on the

resUlts of other structural details (15 and 32A) and even better on detail 40 in which axial stresses are combined with shear and transverse stresses.

And now a maximum stress variation of 200 MPa results in a mean fati:gue life of 20 years. Whereas for these tests on detail 40 no lower tolerance limit is reported, it is decided to use a tolerance limit as given for tests on details 15 and 32A with a reduction factor of abt. 4. With this rather low reduction factor the design fatigue life will become not more than 5 years.

Assuming a maximum stress amplitude of o= ± 150 MPa, thus a stress range AOmax = 300 N/mm2, then the design fatigue life will be reduced to less than 5 years for detail 25 and to about one year for structural detail 40. (See table Jib).

IV. Estimation of the fatigue stress range at the top of the structure The expected loads and stress ranges in the LPC-vessel are, based on references [2, 3 and 4].

Especially the connection of the top plate of the transverse boxgirder

to the top of the longitudinal coaming of the hatch corner i.n the midships

will be most susceptible to fatigue damage. The reason is that this spot will meet the highest nominal stresses and stress ranges due to the

relatively largest distance to the neutral axis in the midship section. Besides, the geometry will cause a relatively large stress concentration, while also a plate thickness up to 60 mm is not favourable.

From [2] the following stress ranges due to vertical wave bending can

be derived:

- probability of exceeding 1.0 E -8.: Aa = 213.5 N/mm2; - prObability of exceeding 1.0 E -5: Ao = 133.7 N/mm2.

This is in accordance with a linear-log relationship between stress range and number of cycles as was assumed and presented in figure 1, which will result in:

- probability of exceeding 5 * 10 E -6: Ao = 170 N/mm2.

From [3] a maximum significant amplitude of the vertical bending moment of 99450 kNm has been determined for the hi.p saili:ng in fully loaded condition with 10 knots in head seas (ji = 180 ) in Beaufort 12 Same values

may be concluded that the influence of _{loading condition and ship speed} is almost neglectable. The resulting maximum significant _{amplitude of} the wave bending stress range becömes Ao = 123 N/mm2.

The maximum stress _{range for 50 years or yearly is estimated:} - probability of exceedi;ng 10 E -8: _{10 = 2 * 123 = 246 N/mm2;} - probability of exceeding 0.5 E -6: Aa

_{= J7}

_{* 123 = 210 N/mm'.} For further considerations the stress range due to wave bending will be

taken as the mean; value of the _{calculated stress range according to} data

from [2] and [3], resulting _in:

- probability of exceeding .5 * 10 E -6: _{one year max., Ao = 190 N/mm2.} The preliminary analysis of torsion and transverse strength _{aspects [4]} shows that the warping stresses _{in the. top of the coaming}

are small and probably not in phase with the vertical wave bending, so that they may

be neglected. However, regarding 'the transverse strength, a stress variation of Ao = 28.4 N/mm2 has been found, _{which stress variation will be}

in phase

with the wave bending stress. _{So the one year maximum stress}

range to be considered become,s at least: _{Ao = 218.4 N/mm2. For compensation of}

the influence of other stress raising factors as variations of stili _water bending moment due to temperature and loading, torsion, horizontal _bending

and especially slamming,

a correction factor of 1.1 is taken.. This resulits

in a rather realistic one year maximum stress range of Aa _{= 240 N/mm2.} V. Fatigue strength of the deck _{structure of the t.PC-vessel}

In tables II a and b the fatigue lives

of structural details 25 and 40 have been calculated for _{a maximum yearly stress range of 200 respectively} 300 N/mm2.

With a 'stress range of 240 N/mm2 _{the design life will become 23}

years for detail 25 and about 2.5 _{years for detail 40. (Table lila).}

However', as mentioned before cornparing' with the fatigue strength of structural detail 25 will result _{in too optimistic conclusions, while}

a comparison with. detail 40 may be too _{conservative. Regarding the geometry} of the structural details and' the _{specific loading condition, structural}

detail 30 and especially details 15 and 32A are preferred. Table Ilib presents a design life of 12 _{years for' structural detail' 30 and 6.5 years} for detail 32A.

For the purpose of fati:gue design, welded joints are di'vided into several

classes, each with a corresponding design _{S-N curve, [5]. The design S-N}

curves are presented in figure 3A. The curves are based on the

mean-minus-tWo-s;tandard-deviatjon curves for relevant experimental data. (In this

figure is also given the 99%.survi'val', _{95% confidence limit for structural}

detail 32A). The design life of the deck structure of the LPC-vessei has 'to be related to class G (fig. 3B). _{Comparison of the design curve for} class Gwith the 99% survival, 95% confidence limit for structural detail 32A Shows only a marginal difference.

The l'ast aspect tobe considered! is _{the influence of the plate thickness.} The S-N' curves presented in fig. _{2 and 3 are related to plate thicknesses}

up to 22 mm. However, 'the fatigue strength Is decreasing with increasing thickness. Therefore the equation of the S-N curves has to be corrected according the formula [5]:

Log(N) = Log(Iç)

_{- mLog}

_{C (/l/ì}

in which: t _{= 22 mm and t = actual thickness.} This results in

a correction factor f = ((22/t)"4r for the calculated fatigue ]i;fe. So a plate thickness of 40 _{mm needs a correction factor} f = 0.64 for the class

G respectively f = 0.53 for detail 32A,, While _a plate thickness of 60 mm needs a correction factor f = 0.47 respectively 0.35. (For detail 25 _{this wil1 become f = 0.35. and f = 0.17).}

This means that the _{fatigue life for the considered top structures}

on

an average will be half the values

calculated before. Summarizing it can be concluded

that according to the fatigue strength of structural detail 32A _{and class G for}

a one year maximum stress range Ao = 240 N/mm2 and _{a plate thickness of abt. 50}

mm. the 99% survival, 95%

confidence limit fatigue life

of the top structure amidships will be about

3.5 years.

vi. Improvement of the _{fatique strength}

The fatigue strength of a structure and its structural details _{is depending}

on nomi:nal stress and strain _{ranges as well as on local stress}

raising factors due to local geometries and surface conditions. _{So, to improve} the fatigue strength and with that the fatigue life, _{it. is important to} reduce the nominal stress

ranges as Well as to avoid or at least to reduce

the stress concentration _{factors as much as possible.}

With respect to the stress _{ranges, it is quite clear that these}

are determined primarily by the _{wave bending stresses and thus by the horizontal}

wave bending moment and the section modulus. Of these Is the horizontal wave 'bending moment almost entirely

depending on wave length and wave

height. The moment i's only _{for a smaller part dëpending on the principal}

dimensions of the ship and not at all on the particulars of the _midship section.

So, in order to reduce the

wave bending stresses, it will be necessary to increase the section modulus.

This mi:ght be realized by an increase of the hei:ght of the longitudinal

coaming and/or by enlargement of the depth of the LPC-vessel. Otherwise an increase of the _{section might be} realized by an enlargement of _{the width of the gangway and the}

upper part of the double hull section _{in the midships.}

With respect to the negative influence of stress _{concentrations, it is} recommended to improve the local geometry at the hatch corners _{as well} as to apply weld finish, methods, _{as follows:,}

extension of top plate of transverse box girder over the vertical sides along the longitudinal

coarning as far as possible with a radius

as large as possible, tapering off 'to' _zero;

- gri;nding of the weldments _{of ali}

transverse members

to the

longitudinal coaniing, in particular _{the welds of the transverse} box

girder;

- grinding of the welds of the _{support ing brackets}

over a reasonable length amidships should be considered;

- _{additionally to the grinding of}

treatment should be considered.

With the forementioned actions l:t is assumed that the L'PC-vessel also in the top structure amidships will obtain sufficient fatigue resistance Nevertheless, it is recommended that at regular inspections special?

attention wilibe paid to the eventual initiation and propagation of fatigue

cracks at the critical areas. VII. Conclusion

The fatigue strength of the considered .LPC-vessel is low due to rather large wave bending stresses and high stress concentrations at the hatch corners amidships.

The nominal stresses can be decreased by enlargement of the section modulus

The negative influence of the stress concéntrati.on can be reduced by improvement of geometry and wel:d finishing methods.

With forementioned actions the fatigue strength can be increased to an

acceptable level. At regular inspecti?ons special, attention should be paid

Literature

W.I. Munseet ai., Fatigue Characterization of Fabricated Ship Details

for Design, Ship Structure Committee Report SSC-3i8, 1983.

Document: MARS Cl 12.92:. P1 94. Section: PROJ.179 TEK.1302C. 'Containervessel 18/10/94

[3,} J.M.J. Journée, Motions and Loads on a. LP.C-Vessel, Deift University

of Technology, Ship Hydromechanics Laboratory, Report No 1009-O,

NOvember 1994.

[.4] J.H. Vilnk, Conclus tons and Recommendations of a Preliminary Analysis

for Torsion and Transverse Strength Aspects of a 1110m Low Profile Deep Sea Coaster, Delft University of Technology, November 1994.

[5] Extract from Offshore installations: Guidance on design and

construction (Fatigue), U K Department of Energy, April 1984 ISBN 0 11 411457 9.

Delft, November 1994.,

iabie I, Stress ranges versus number of cycles.

Stress distri.buti.on: linear-log scale,.

Stress range

âo:

linear.

Number of cycles: log 10. - Number of cycles per annum:

0.8

*

0.5

*

365

*

24

*

3600/6 2

* 106.

Exceedings Load classes

100. P

-100o

. n max ÀQmax 1 95 2

95-100

2 2 : 90 4 90- 95 2 3 . 85 9

85- 90

5. 4 80 18 80- 85 9 5 75 38 75- 80 20. 6 70 78 70- 75 40 7 65 160 65- 70 .82 8 60 . 331 60- 65 171 9 55 685 55- 60 354 10 50 1 414 50 55 729 il 45 2: 921 45- 50. 1 507 12 40 6 034 40- 45 3 113 13 35 12 4.64 35- 4.0 6 430 14 30 25 747 30- 35 13 283 15 25 53 183 25- 30 27 436 16. . 20 109 856 20- 25 56 673 17 . 15 226 921-

15-

20

Ui 065

18 10 468 735 10- 15 241: 814 19 5 968 230. 5- 10 49.9 495 20 O

2 * 106

0-

5 . 1.031 770 2000 000

Table lia. Fatigue strength of welded structural details.; maximum stress range Ao,.= 200 N/mm2.

- Stress distribution: linear-log scale.

Stress range Ao linear, Ao, = 200 N/mm2.

Number of cycles: 0.8 * 0.5 * 365 * 24 * 3600/6 2 * 106 per annum.

- Fatigue strength: 99% survival, 95% confidence limit. Detail 25: S-N curVe: N, = 5.40 * 1021/S11090.

Mean fatigue life 4750 years;. lower limit design life 95 years.

Detail 40: S-N curve: N1 = 4.63 * 1012/S3.533.

Mean fatigue life 20 y ars; lower limit design life 5 years. Load classes :. Struct.Det. 25 S:truct.Det.. 40

100'o

n, Aa N/mm2 N n1/N1 * E 6 N, * E 6 95.:100 2 193

337E.3

6

39E3

51, 90- 95 2 183 492 E 3 4 4.7 E 3 43 85- 90 5 Ï73 732 E 3 7 . 5.7 E 3 . 88 80-r 85 9 163 112 E 4 8 71 E 3 127 75- 80 20 153 175 E 4 11 89 E 3 . 225 70- 75 40 143 283 E 4 14 112 E 3 357 65- 70. 82 1.33 . 472 E 4 17 145 E 3 566 60- 65 171 123 822 E 4 21 191 E 3 895 55- 60 . 354 113 150 E 5 24 258 E 3 1372 50- 55 729 103 .289 E 5 25 358 E 3 2036 45 50 .1 507 93 597 E 5 25 513 E 3 2938 4.0- 45 3 113 83 134 E 6 23 768 E 3 4053 35- 40 6 430 73 322 E 6 19 12.1 E 4 5314 30- 3,5 13 283 63 944 E 6 . 14 203 E 4 6543 25- 3.0 27 4.36 53 320 E 7 9 375 E 4 7316 20- 25. 56 673 43 142 E 8 4 784 E 4 230 .15- 20 117 065 33 920 E 8 1 . 20 E 6 5853 10- 15 241 814 . 23 120 Ei..0 -. 72 E 6 3360 5- 10 499 4.95 13 700 Eli - 537 E 6 930 0- 5 H 103.1 '770 3 200 El.6 - 95 'E 9 H oli 2000 000 207 49308

Table lib. Fatigue strength of welded structural details.; maximum stress range Ao = 300 NJmm2.

- Stress distribution: linear-log scale. Stress range Ao: linear, Ao,, = 300 N/mm2.

Number of cycles: 0.8 * 0.5 * 365 * 24 * 3600/6 2 * 106 per annum.

- Fatigue strength.: 99% survival, 95% con:fi:dence' limit.

Detail 25: S-N curve: N1 = 5.40 *

Mean fati.gue 1ife 236 years; lower limit design life 4.7 years. Detail 40: S-N, curve: N, = 4.63 *

Mean fatigue life 4 7 years, lower limit design life 1 2 years

Load classes Struct.Det. 25 Struct.Det. 40

100*o

n1 AO, N/mm2 N, n,/N, * E 6 N1 * E 6 àOmaK 95-100 2 290 19 E 3 1,05 92 E 2 21,7 90- 95 2 275

27E 3

74 112 E 2 179 85- 90 5' 260 '41 E 3 122 136. ,E 2 368 80- 85 9 245 62 E 3 '145 168 E' .2 I 536 75- 80 20 230 97 E 3 206' 210 E 2 952 70- 75 40 215 157 E 3 255 266 E 2 1504 65- 70 82 200 262 E 3 31:3 344 E 2 2384 60- 65 171 185 455 E 3 376 453 E 2 ' 3775 55- 60 354 170 829 E 3 427 610 E 2 5803 50- 55 ' 729 155 160 E 4 455 846 E 2 8611 45- 50 i 50,7 14.0 328 E 4 459 121 E 3 12455 40- 45 3 113 125 733 E 4 425 181 E 3 1.71.99 '35- 40 ' 6 430 110 182 E 5 284 E 3 22641. 30- 35 13 283 95' 51:3 E 5 259 477 E 3 27847 25- 30 27 436 80 174 E 6 158 875 E 3 31355 20- 25' 56 673 65 757 E 6 75 182 E 4 31139 15- 20' 117 065 50 486 E 7 24 460 E 4 25449 10- 15. 241 814 35 609 E 8 4 162 E 5 14927 5- 10 ' 499 495 20 322 E.10 2 11.7 E 6 4.269 0- 5 11031 770 5 600 E14 ' - 157 E 8' 66 2000 000 . ' 4237 211682

Table lila.. Fatigue strength of welded structural details;

ifiaximum stress range Ao, = ?'40 N/mm2.

-. Stress distribution: linearLlog scale. Stress range a: linear, Ao, = 240 N/mm2.

Number of cycles: 0.8 * 0.5 * 365 * 24 * 3600/6 2 * 106 per annum.

- Fatigue strength: 99% survival, 95% confidence limit. Detail 25: S-N curve:: N, = 5.40 *

Mean fati:gue life 1150 years; lower limit design life. 23 years. Detail .40: S-N curve: N, = 4.63 * 10'2/S35".

Mean fatigue life 10 years; lower lImit design life 2.5 years. Load classes Struct.DeIt. 25 Struct..Det. 40

100*Ao, _{ni 40,.} N/mm2 N, n,/N, *' 'E 6 N, .* E 6

Aa,

95-100 2 232 .91 E 3 22 20 E 3 100 90- 95 2 220 133 E 3 1.5 25 E 3 80 85- 9.0 . 5 208 198 E 3 25 30' E 3 167 80- 85 9. 196 302 E 3 30 37 E 3 243 75- 80 20 184 473 E 3 . 4:2 46 E .3 . 435 70- 75 '' 40 172 763 E 3 52 59 E 3 678 65- 70 82 16Ó 127 E 4 65 76 E 3 1079 60- 65 171' ₁₄₈ 221 E 4 77 . 100 E 3 1710 55- 60 354 136 403 E 4 88 134 E 3 2642 50- '55 729 124 776 E 4 94 186 E 3 3919 45- 50 1 507 112 160 E 5 ' 266 E 3 5665 40'- 4'S 3 113 100 357 E 5 87 398 E 3 7822 35- 40 6 430 88 883 E 5 73 625 E 3 , 10288 30- 35 13 283 76 . 248 E 6 54 105 E 4 12650 25 30 27 436 64 84'4 E 6 33 192 E 4 14290 20- 2'5 56 673 52 368 E 7 15. 401 E 4 14133 15- 20 117 065 40 236 E 8 5 10.1 E 5 115.91 10- 15 241 814 28 297' E 9 1 357 E 5 ' 6774 5- 10 499 495 16 .157 £10 - 258 'E 6 1936 0- 5 ' ' 1031 770 4' 291 [14 -' 346 E 8 30 2000 000 872 96232

maximum stress range Ao = 240 N/mm2.

- Stress distributiÔn: linear-log scale. Stress range o.: linear, Aa 240 N/mm2.

Number of cycles: 0.8 * 0.5 * 365 * 24 * 3600/6 2 * 106 per annum.

- Fatigue strength: 99% sUrvival, 95% confidence limit. Detail 30: S-N curve: N1 = 3.3OE * oI2,s13i59

Mean fatigue life 35 years; lower limit design life 12 years. Detail 32A: S-N curve: N1 = 2.22 * 10'4/S,4200.

Mean fatigue life 29 years; lower limit design life 6.5 years. Load classes Struct.Det.. 30 Strûct,.Det. 32A

100*a1 n1 Ao, 'N/mm2 N1 n1/N1 * E 6 n1/N1 * E 6 Ao 95-100 2 232 .111' E 3 H 18 26 E 3 77 9.0- 95 2 220 131 E 3 15 32 E 3 63 85- 90 5 208 157 E 3 32 41 E 3 122 80- 85 9 196 189 E 3 48 52 E 3 173 75- 80 20 184 _{231 E 3 87 68 E 3 294} 70- 75 410,, 172 286 E 3 140 91 E 3 44.0 65- 0 82 160 360 E 3 228 123 E 3 667 60- 65 171 148 460 E 3 . 372. 1.70 E 3 1.006 55- 60, 354 ' 136 601 E 3 '589 243 E 3 1457 50- 55 729 . 124 804 E 3 90.7 358 E 3 2036 4i5 50 1 507 112 111 .E 4 1358 549 E 3 2745 40- 45 . 3' 113 100 159 E 4 1958 884 E 3 3521 35- 40 . 6 430 88 238 E 4 2702 151 E 4 4258 30- 35 13 283 76 378 E 4 3514 280 E 4 4744 25- 30 27 436 64 650 E 4 4221 576 E 4' 4763 20- 25 56 673 52 125 E 5 4534 138. E 5 4107 15- 20 117 065 40 287 E 5 4079 415 E 5 2821 10- 15 241 814 28 889 'E 5 2732 185 E 6' 1307 5- 10 499 495 16 518 E 6 964 195 E 7 256 0- 5 103.1 770 ' 4 413 E 8 25 657 E 9 2 2000 000 ' 28523 34859

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Part II Section 4

Fig. 3A. _{Basic design S-N curves for non-nodal joints [5].}

VI T-O O O C

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_QW.

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a,

V) w, Li VI ciD u T-

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w C,

.u-a) With attachment length (parallel to the

the direction of the applied stress) l50mm and with edge distance l0mm.

(b) With attachment length (parallel to

the direction of the applied stress) >150mm and with edge distance

Ç10mm.

3.3 Parent metal adjacent to. or weld metal

In. full penetration butt welded joints

made from bothsides between plates of unequal width, with the weld ends ground to a radius not less than 1.25 times the thickness t.

TYPE 4 WELDED AITACHMENTSON THESURFACE OR EDGE OF A STRESSED MEMBER

Notes on potentIal modes of failure

When the weld s parallel to the direction of the applied stress fatigue cracks normally initiate at the weld ends but when it is transverse to the direction of stressing they usually initiate at the weld toc; for attachments involvingasingle, as opposed to a double, weld cracks may also Initiate at the weld root. The cracks then propagate into the stressed member. When the welds arc on or-adjacent to the edge, of the stressed member the stress concentration us increased and the fatigue strength is reduced this is the reason for specifying an 'edge distance' in some of these joints (see also noteon edge distance in joint Type 2).

4. 1 Parent metal (of the stressed member)

adjacent to tocs or ends of bevel-butt or fillet welded attachments, regardless

of thà orientation of the weld to the direction of appliedstress. and whether or not the welds arc continuous round

the attachment.

4.2 Parent metal (of the stressed member)

at the loes or the ends of butt or fillet

welded attachments on or within 10mm of the-edges-or corners of a stressed member and regardless òf the shape of the attachment.

4.3 Parent metal (of the stressed member)

at the toe of a butt weld connecting the

siressed member to another member

slotted through it.

Vith the length of the stoned-through F

member, parallel to the direction of the

applied stress, 150mm and with edgedistance ) 10mm.

With the length of the slotted-through F2

member, parallel to the direction of the

applied, stress, > 150mm and with

edge distance l0mrn.

With edge distance <10mm. G

F2 Step changes in width can often be avoided by the use of shaped transition plates, arranged so as to enable butt welds to be made between plates of equal width.

Note that for this detail the stress concentration has been taken into account in the joint classification.

Butt welded joints should be made with

an additional reinforcing fillet so as to provide asimilar toc profile to that

which would exist in a fillet welded joint.

F The decrease in fatigue strength with increasing attachment length is because more load is transferred into the longer gusset giving an increase in stress conceniraiioñ.

F2

G Note that the cla5sification applies to allsizes ofattachrnent.1t wouldiherefore inclùde, for example, the junctionof two flanges al right angles. Insuchsituations. a low fatigue classification can often be avoided by the use of a transition plate (see also joint Type 3.3).

Note that this classification does not

apply to fillet welded joints (see joint

Type 5. lb). However it does apply to loading in either direction (L or T in-the

sketch).

Fig. 3B. Classification of weld attachments [5]. Type number, description and

notes on mode of failure Class Explanatory comments Examples, including failure modes

Edge

distance

L

TYPE 5 LOAD-CARRYING FILLETAND T BUUWELDS

Note on potentIal modes oC failUre

-Failure in cruciform or T joints with full penetration welds will normally initiate at the weld toc but in joints made with load carrying fillet or panial penetration butt welds cracking may Initiate either at the weld toe and propagate Into the plate or at the weld root and propagate through the weld In welds parallel to the direction of the applied stress however weld failure is uncommon cracks normally

initiate at the weld end and propagate into ihe plate perpendicular io the direction of applied stress. The stress concentraiion is increased, and the fatigue strength is therefore reduced, if the weld end is located on or adjacent to the edge of a stressed member rather

than, on its surface. -

-L

DELFT UNIVERSITY OF TECHNOLOGY Faculty of Mechanical _Engineering and Marine Technology

Ship Structures Laboratory

Additional notes on report No. SSL 350

'CONSIDERATIONS ON THE FATIGUE

STRENGTH OF AN

_LPCVESSEL»

by

ir. H.G. Scholte

SSL 350a

Report No. SSL 350 A January 1995

ADDITIONAL NOTES ON REPORT NO. SSI.. 350

"CONSIDERATIONS ON THE FATIGUE STRENGTH _{OF AN LPC-VESSE[."}

by ir. H.G. Scholte, _{January 1995}

A. A first estimation of the fatigue _strength

In report SSL 350 an assessment _{has been made on the fatigue strength} of an LPC-vessel. Summarizing _{it was concluded that the 99% survival,}

95% confidence 1 imit fatigue life of the top structure at the hatch corners amidships will be about 3.5 years. This conclusion was the result of the following starting points and considerations:

1.

The LPC-vessel will be in operation for 80% of the time, fifty-fifty sailing on inland waterways respectively on open sea.

2. _{The mean period of stress variations due to wave bending is about}

6 seconds. This results in 2 * 106 stress cycles a year.

3. _{The relation between stress variations and the number of cycles} or

cumulative frequency diagram can be presented by a straight line on linear-log basis.

4. _{Verification of the fatigue strength}

may be made by using S-N curves

and the Miner cumulative rule.

5. _{The top of the longitudinal coaming at the location of the hatch}

corner amidships is considered to be the weakest point with regards to the fatigue strength, due to largest nominal bending stress and poor local geometry.

6. _{Verification of the fatigue strength was based on S-N curves for}

similar structural details [1, detail 32A] and on basic design S-N curves [5, class G].

7. _{The fatigue life was related to a one year maximum nominal stress}

range Ao = 240 N/mm2.

This stress range was determined as follows: maximum stresses according the rules [2]:

wave bending including still water _{Ao = 250 N/mm2}

maximum still water bending _{80 N/mm2}

resulting wave bendi:ng stress _{Ao = 170 N/mm2} wave bending estimation according [3] _{Ao = 210 N/mm2} wave bending (mean value of a and b) _{Ao = 190 N/mm2} allowance due to transverse load [4] _{Ao = 28 N/mm2} correction for slamming, temperature etc. _10% resulting stress range _{Ao = 1.1 * (190 + 28) = 240 N/mm2}

8. _{In order to avoid too optimistic expectations and starting from the}

available information in November 1994, a conservative approach was preferred for all assumptions to be made.

The report SSL 350 also contains _{recommendations with respect to the nominal}

stresses, the local geometry and weld fi:nishing in order to enlarge the fatigue resistance to an acceptable level.

B. Reconsideration on the fatigue strength

Whereas in consequence of the comments and discussions the preliminary design has been modified and _{improved a reconsideration on the fatigue} strength was thought to be recommendable.

For the assessment of the fatigue strength the following starting points and considerations have been used:

1-4.. Conformable to points 1-4 in section A.

5. _{The top of the longitudinal}

coaming at the location of the hatch

corner amidships is still considered to be most susceptible to fatigue damage due to the large nominal bending stress and the stress raising

local geometry of the hatch corner.

6. _{For the preliminary design the verification of the fatigue strength} has been related to the

mean-minus-two-standard deviation basic design

S-N curve of class G [5]. In the meantime the hatch

corner structure has been improved by a

rounding of the top plate of the transverse girder at the hatch corner

and an extension in longitudinal _{direction along the coaming [6].}

So, the determination of the fatigue life is now related to the basic design S-N curve class F.

However, as discussed and agreed upon in a meeting on January 17th 1995, for the application of class F it will be essential also that the rounding of the hatch corner with a radius R = 500 mm will be enlarged in _{an elliptical form by a further} extension of the top plate along the longitudinal coaming to a total length of about 1000 mm.

Besides it is required that all weldments in, and áttachments to the longitudinal coaming are made with full penetration and with any undercutting at the corners of the members dressed out by local grinding.

C) _{Furtheron, it should be avoided that the longitudinal guide}

strip for the hatch covers is dropped abruptly at the hatch corners. These strips should be tapered off from full height of 40 mm to zero or the strips should be continued between the hatches.

7. _{With respect to the loading condition the one year maximum nominal}

stress range is taken to be Ao = 210 N/mm2. This stress range is determined as follows:

maximum stresses according the rules _[7]:

wave bending including still water _{Ao = 220 N/mm2}

maximum still water bending _{44 N/mm2}

resulting wave bending stress _{Ao = 176 N/mm2} allowance due to transverse load [8] _{Ao = 14 N/mm2} correction for slamming, temperature,

still water loading etc. _10%

resulting stress range _{Ac = 1.1 * (176 + 14) = 210 N/mm2}

8. _{With respect to the thickness of}

the longitudinal coaming, exceeding 22 mm, a correction factor of f _{= ((22/t)hh'4}m has to be applied on} the calculated fatigue life. _{For a thickness t = 40 mm and m = 3,} this factor becomes f = 0.64.

In table A the basic design fatigue _{life is determined according to the} mean-minus-two-standard_deviation S-N curve. This results in an expected

Final conclusions

With the forementioned starting points and considerations with respect to loading condition, structural _{details and welding, the fatigue}

life

is determined according design class F, resulting in a basic design life of 15 years and an expected mean life of 40 years.

This means that serious fatigue _{damage and even complete failure}

of the

structure must be expected after _{a life time between 15 and 40 years.} This means also that the _{initiation of fatigue cracks can be expected} in an earlier stage, _{say within a period of 5 years}

or even sooner, depending on load history, overloads _etc.

So it is recommended to _{carry out regular and thorough inspections}

of

the critical locations, in order that eventually necessary repairs can be carried out in an early stage..

If for one or another reason the required improvements or weld quality

could not be met, then the fatigue life of the structure has to be reviewed according to design class G, which results in a decrease of the fatigue life to abt. 33% of the fatigue _{life according to class F.}

[.1 terature

W.H. Munse et al., Fatigue Characterization of Fabricated Ship Details

for Design, Ship Structure Committee Report SSC-318, 1983.

Document: MARS Cl 12.92: P1 94. Section: _{PROJ.179 TEK.1302C.} Containervessel 18/10/94.

J.M.J. Journée, Motions and Loads on a LPC-Vessel, Delft University of Technology, Ship Hydromechanics Laboratory, Report No. 1009-0, November 1994.

J.H. Vink, Conclusions and Recommendations of a Preliminary Analysis for Torsion and Transverse Strength Aspects of a 110 m Low Profile Deep Sea Coaster, Deift University of Technology, November 1994. Extract from Offshore installations: Guidance on design and construction (Fatigue), U.K. Department of Energy, April 1984. ISBN 0 11 411457 9.

Midship section; drawing No. 179/95-1302 E dated 11-1-95. Document: MARS C2 12.92: W 148 Containervessel

Tek. 94-1302 E Midship section dated 9-1-95.

Information from Boon, Vink.

(J.H. Vink, Preliminary analysis of torsion and transverse strength aspects of an LPC-vessel, Delft University _{of Technology, Ship} Structures Laboratory, Report No. SSL 351-0, December 1994).

Table A. _{Fatigue strength according to design class F} and a maximum stress range Ao = 200 N/mm2.

- Stress distribution: linear-log scale.

Stress range Ao: linear, Ao, = 200 N/mm2.

Number of cycles: 0.8 * 0.5 * 365 * 24 * 3600/6 2 * 106

per annum. - Fatigue strength: design class F.

Mean-minus-two-Standard_deviation S-N curve:

N * s13 _{= 0.63 * 1012; design}

life: 1/37665 E-6 = 26.5 years; with t = 40 mm this becomes 0.64 * 26.5 = 17 years.

Mean fatigue life S-N curve:

N, * s13 = 1.726 * 1012; _{mean life: 0.64 * (1.726/0.63) * 26.5}

= 46 years.

- Fatigue strength for other stress ranges to be determined by multiplying with a factor (S1/S2)m:

AOnax = 210 N/mm2 results in

Load classes _{Class F}

100*Ao1 n, _Ao1 N/mm2 N, n1/N1 * E 6 AOma, 95-100 2 193 _{876 E 2 23} 90- 95 2 183 103 E 3 19 85- 90 5 173 122 E 3 41 80- 85 9 163 145 E 3 62 75- 80 _{20 153 176 E 3} 114 70- 75 _{40 143 215 E 3 186} 65- 70 82 133 268 E 3 306 60- 65 171 123 339 E 3 504 55- 60 354 113 _{437 E 3 810} 50- 55 729 103 577 E 3 1 263 45- 50 1 507 93 783 E 3 1 925 40- 45 _{3 113 83 110 E 4} 2 830 35- 40 6 430 73 162 E 4 3 969 30- 35 13 283 63 252 E 4 5 271 25-j 30 27 436 53 423 E 4 6 486 20- 25 56 673 43 792 E 4 7 156 15- 20 117 065 33 225 E 5 5 200 10- 15 241 814 23 150 E 6 1 500 5- 10 499 495 13 > E 9 -0- 5

_1031770

₃ -2000 000 _{37 665}

design life: (2OQ/210) * 17 _{= 15 vears;} mean life : (200/210) * 46 = 40 years.