Behaviour of mild steel under very low-frequent loading in seawater

ECHNISCHE HOGESCHOOL DELFT

AFDELING DER SCHEEPSßOUW- EN SCHEEPVAARTKUNDE

LABORATORIUM VOOR SCHEEPSCONSTRUCTIES SHIP STRUCTURES LABORATORY

SSL 255

BEHAVIOUR OF MILD STEEL UNDER

VERY LOW-FREQUENT LOADING IN

SEAWATER

by

prof.ir. J.J.W. Nibbering

Report presented at the EFC Meeting on LobJ Frequency Cyclic Loading Effects in Environment Sensitive Fracture

(Milans

9-11

March 1982)

March 1982

BEHAVIOUR OF MILD STEEL UNDER VERY LOW-FREQUENT LOADING IN SEAWATER

By prof.ir. J.J.W. Nibbering

Summary

A widely held opinion about high stress, low cycle fatigue is that the

deterioration promoted by seawater is small. The idea is that crack growth

is faster than the penetration rate of the corrosive medium.

This is probably true when the cyclic frequency is moderate, for instance

about 0.1 Hz.

En ships large changes of the still water bending moment may occur when

the loading condition goes from ballast to fully loaded and back. Wave

bending is superposed on the still water stresses. Consequently the

absolute maxima and minima of the combined bending moments may occur

only about once a week. Other very low-frequent changes of stresses are

connected to temperature changes (day night) and for offshore structures

-changes of wind and wave directions.

Experiments have been carried out in the Delf t Ship Structures Laboratory

with Fe 410 and Fe 510 at frequencies between 0.05 Hz and 0.0003 Hz.

For the latter the reduction in lifetime as compared to loading in air

at the same frequency was in the order of magnitude of 10.

The crack growth curves for air and seawater remained practically parallel

in a log da/dn - log AK plot. In other words the power m remained more or

less the same but the c-value increased greatly.

For simple programmed loading in seawater (one peak among 200 low-stress

cycles) the difference with results obtained in air was for Fe 410 about

1:20 in terms of da/dn; for Fe 510 it was 1:10.

In air the peaks were very beneficient. In seawater there was no advantage

BEhAVIOUR OF FÍILD STEEL UNDER VERY LOW-FREQUENT. LOADING IN SEAWATER by prof.i.r. J.J.W. Nibbering)

i. Introduction.

Corrosion fatigue behaviour of stee. offshore structures has become a main

area of research in many institutes dealing with the danger of cracking

and fracture of welded structures. All has started when the Nbrth Sea

became a hunting field for oil-companies some 15 years ago. One may ask

oneself why the problem had not been recognize.d as a serious otte much earlier. For, offshore structures existed already for a long time both in

the form of ,ships as in the form of oil-exploring and -exploiting

struc-tures, either floating, movable or fixed The ie.son was that formerly

the offshore structures generally voirked in regions of good weather. Fatigue was no problem. Only static strength. was important in connection

to earthquakes and typhoons. Moreover they were situated in shallow waters

which allowed inspetion and repair (as in ships).

in the:.North Sea the conditions are quite different. Seaway causes fatigue n a similar way asin ships But a difference with ships is that. the

life-time of fixed offshore structures may be long and. that inspection and repair are difficult, ('safe lifet design).

Ships are clearly 'fail-safe' structures. (Failures may develop, b.ut may

not render the ship unsafe between inspection periods).

The development .afte.r World War H of ships. becoming bigger, faster and more sophisticated, together with reductions in harbour times and tight

schedules, has brought he problem of fatigue crac:king to the foreground.

t has taken some time before it was understood that, although cracking

occurs mainly inside a shi:p, the real problem is yet corrosion fatigue. in regions where condense

water appears. Very conspicuous are tanks which are alterna'tively filled

with oil and ,ballastwater.

In connection to offshore structures the European Community of Steel and

Coal and Ministries of Energy Education., Economic Affairs etc. in many

countries have sponsored corrosion fatigue investigations up to amounts

of money in the order of magnitude of a hundred million dollars.. Much of

the reséarch has already been published in public reports., conference papers etc. The author may be forgiven in mentioning only a recent one

as /1/, It covers five years of work in the Netherlands on thick plate

specimeis - either or not welded - , large tubular connections, influence

of heat treatment., cathodic protection, weld finish, type of loading and

test temperature. 1w general .the reduction in fatigue life., as compared to lifetime. in dry air, amounted to a factor. two to three. Cathodic

pro-tection restored the fatigue strength.to a great extent., especially for

low stress ranges. (it will be seen further that the last restriction may

nót longer be necessary in the light of the results given in this paper).

Because offshore structures have to live long the number of load cycles

they meet is at least 108 cycles...In cases of resonances leading to vibra-tions of structural parts the number may increase with another factor 10.

it is not surprising that a lot of people worry about eventual corrosion

fatigue caused by. large numbers of very small amplitudes of cyclic stresses. In air such low stresses do not give any troublie., because they are below the 'fàtigue limit'. But for corrosive conditions it ishighly probable

that the fatigue strength diminishes when the number o.f cycles increases Unfortunately the relation between strength and number of cycles is not

well' known. This is simply due to the fact that nobody has run testi in

seawater for decades at the proper loads and frequencies. Indeed the

my serve to illustrate some aspects /'2i. Testing at >1 Hz generally

leads to fatigue lives which are practically the same for air and sea

environment.

For 0.2 Hz -the results for seawater become worse, although still falling

within the 99% scatter band for air. The difference between results of

1 Hz, 4 Hz and 0.2 Hz corresponds to a. factor two in crack growth rate.

in the high K-.region all results converge. This suggests that at high

loads the environment has no adverse influence But this generallyheld.

-conclusion may be completely wrong. The only -conclusion which may be drawn

is, that.fo.r frequencies higher than 0.2 Hz seawater has little effect on.

the fatigue life for high cyclic loads The important po]nt now is that

hi h loads occur at lower frequencies than small loads.. This will he

ex-pl'ained in §2.

As a consequence of the considerations given there, a test programme has,

been set up which comprises high-stress/ultra--low-cycle fatigue loading

and experiments in-which high and low loads have been combined,. The results are given in §3.

§2.. Low freuent high loads at sea

it has become a more or less accepted practice to- apply test frequencies of about 0.2 Hz for corrosion-fatigue investigations, for stecl offshore

structures. Yet,.. looking at sea spectra it is obvious that the peaks occur lower freqúencies, In moderate conditions 0.1 Hz'is a characteristic

value. In extreme 'conditions it. is about 0.07 Hz. In case ôf long lasting storms, the peak frequency maybe as low as 0.05 'Hz. The length of the

cor-responding wave component is more than 600 rn. A ship will not be much

disturbed by such a wave. It travels over it asif' it was a hill. But fixed

offshore structures. and particularly anchored floating structures may 'be loaded heavily when such waves, pass. But of course in reality the various

real high waves is much lower than that of the peak of the wave spectrurn. But perhaps equaIly important are the changes which occur as a consequence

of thérmal effects andchanges of loading condition (f. i,, ballast - loaded),

(see fig. 2 from /3/).' From 7.3.T969 to 21.3.1969 the maximum change of stress was +8 5/-12 kpsi 20 5 kpsi 145 N/mm2 (One might also say that

it changed from -10 to +6to -12 in that period).

The còrrespondi'ng frequency is in the order of magnitude of 1: (.1 4X24X3600,Y Hz

1:O.

Another source of. high-low-frequent-loads in ships 'is slamming. The whipping stresses due to it add to those of the wave bending. In fig. 3 a total

double amplitude of stress of 270 N/mm2 is found. According to Aertssen /19/

sevére slamming occurred only two or thre& times per hour.. This sfrequency is less than 0.00.1 Hz. When a ships' life conforms to some 108 cycles and bad weather is present some 'I;O% of that time, extreme. stress reversals occur about 0.001 X' 0.1 X 108 _{10" cycles. Chan'gs of temperatute and loading'}

conditions will enlarge these stress values hut' reduce the relevant number

of cycles'. On the whole it is assumed that there are 10" cycles of 000'1' Hz; of '250 N/mm2 (20 'N/mm2 lower than i'n fig. 3) ot perhaps 1000 cycles ..of

30.0 'N/mm2 of' very low frequency (fig. 2) in ships.

'For offshore' structures 'the situation"is similar. Storms in the N6rth S'ea

may come from northern directions as we'll as from southern directions.

Tide streams change daily. Slow vibrations of the structure as a' whole and

quick vibrations of components due to resonancè, wave impacts and vortex

shedding again enlarge the primary stresses' incidentally.

§3. Low f reqiency experirneùts

Constant amplitude tests r

In 197.8. the experiments' started' on an p.ld 'Losenhausen machine (table Ii:

-5-n casual i-5-nterruptio-5-ns of the loadi-5-ng may occur, have bee-5-n a reason for

carrying out all tests as a twin-set, one specimen in air and one in

sea-water. This has remained so when later another testing machine, capable

of programme loading, was used. With that type of loading the need of a

reference result in air is even more needed. Two materials were tested:

Fe 410 and Fe 510 (normalised). Particulars are given in table I. The test

set-up is shown in fig. 4.

The test procedure consisted of fatigue loading at high frequency (4 Hz)

up to the moment a 1-2 mm long crack had developed. From then on the factual

experiment started at low frequency.

In figs. 5 and 6 are summarized all constant amplitude test results for

Fe 510. The influence of loading frequency is mnifest. Indeed the results

for 0.01 Hz are much better than for 0.0017 lIz. The two curves for 0.0017 Hz

(WBIO, WB21) fall closely together, which may show that the different

test-ing machines have done their job well. (Fig. 6 curves for da/dn = c(AK)m). The maximum difference in the factor c between air and seawater amounts to

10, but the slopes of the various curves do not differ substantially.

This tendency is confirmed in fig. 8, which together with fig. 7 summarizes

the results for Fe 410. The difference between the curves for 0.0003 Hz and

0.05 lIz seems to be even greater than that between 0.05 Hz and air.

Comparing the air curves for both steels, Fe 510 is not better than Fe 410.

Also in corrosion fatigue there is little difference.

For Fe 410 a curious result was obtained with VD1. It was first loaded at

0.0003 Hz up to 1400 cycles. Next the frequency was i.ncreased with a factor

3. This caused a great reduction in crack growth rate. It took some 5 nun

crack extension before the former crack speed was more or 1.ess re-established.

b. (Simple) programmed tests

high-load/constant-

-6-amplitude fatigue in seawater in §3a, the question remainswhethe high

loads have a 'simi]ar damaging effect when mixed with lower loads.. it is evident that it has no sense to'start with andom-type loading,

because even for air this field has not yet sufficiently been, covered.

The most simple and yt purpose-setving programme corisists of repeating.

sequences of I high load and à number of low loads (see table II and fig 9).

Of cóurse the number of low löads is very important.Fo'r the first test

200 cycles have been chOsen. Thenext experimentswill be doìe with 2000

low-load cycles pro high-load cycle. After that the loading of fig. 13 will

be tested. '

The resûlts of a Fe 410 and 'a Fe 50 specimen are shown in fig. 9. The

dif-ference between both steels is appreciable,.

An essential comparison to be made is' that between the results for constant-'

amplitude and for programmed 'loading. This can be done 'in two ways: either the low-load or the high-load cycle's' are taken into account.

Most logical is, tO use the low ioàd' cycles for the a - n and da/dn' - AK

plots. FOr whenthe high, loads (peaks) we"re 'only 'taken into account d'a/dn

Suffers from two sides: dà is'not only dueto the high loads hut alsoto

the. low loacís (is taken too large'), while dn is very small because only the

number of the peak-cycles is taken into account. The result is a twofold'

too high quotient da/du. Yet there 'is reason for plotting the results also in terms of t'he peak loading. At this moment t is not knOwn whether the

crack growth occurs mainly during, the peak load-cycles or during the lage

number of, smàil l-oad cycles. In future elctr,on-m'icrdscoic investigations must bring some light in this problem. But even when crack prooagation

mainly occurs during, the small load cycles, the cause f it might he the

damaging 'effect (stress corrosion effect) of the high loads. These high

loads leave behind' compressive' residual stresses a't the crack tip. Ii air

7

was reduced by a factor of about 10.

In seawater hydrogen may be attracted by the plastic deformation at the

crack tip and the high residual compressive stresses in the plastic zone.

This may completely counterbalance the favourable effect of the compressive

stresses in air. Indeed this was found in our experiments.

In this view the peaks are the driving cause of the crack propagation and

the mall load cycles act more as the vehicle. It is of interest to see whether the high load cycles alone could he made responsible for the whoe

crack development by comparingwith the constant-load tests.

Figures 10 and 11 allow that comparison. Figure 10 is for Fe 410, fig. 11

for Fe 510.

The lower curve for VD17 in fig. 10 is the one in which the peaks have been

neglected. It lays very well in line with the curve VEI for

constant-amplitude loading in sea. It seems indeed as if the peaks have not had any

positive or negative influence!

On the other hand, this is not the right comparison to be made. Specimen VDI6

is the one which has obtained exactly the same load-programme as \T1)17 - but in air. When the results of these two are brought together, a large

diffr-ence becomes manifest. (Lower curve VDI7 with right curve VDI6; the latter

is an extension of the line through the results in the low AK domain).

The difference between both lines amounts to a factor even higher than 10.

Indeed the large beneficial effect of overloads on fatigue performance in

air completely disappears in seawater.

Van Lecuwen and de Back /6/ have found that for rod-steel, electro-quality,

normalised (0.21 C, 0.2 Si, 1.48 Mn), one single high-stress cycle had an

unfavourable effect in corrosion fatigue (at 2 Hz). However, repeating of

the high-stress was favourable with respect to crack propagation.

But these high stresses were only applied once in 10 OflO and 20 000 cycles.

loads appeared once in 200 cycles. Another difference is the frequency of

the low load cycles: 2 Hz while our tests had 0.2 Hz.

The foregoing observations for Fe 41:0 do not fully apply to Fe 510 (fig.. 11).

It is evident that Fe 510 behaves bettet under programmed loading than

Fe 410. From fig. H it might be concluded that the peaks bave even a

slightly favouràble effect (compare iówer curve F2with WB.11 and 1''F14

fòr air ànd with WF16 for seawater). . .

§4; Discussion and conclusions . . - ..

The da/dn-curves given in the various figures are not always beautiful,

especially for the part corresponding to small crack lengths.. in order to

get a fair impression about the dependency of crack growth rate on test

frequency, the part of the a-N-curves between 5 and F0 mm crack length was

sel'ected 'För each specimen was calculated the relation 5 mm diided by

s (N - N ). Thèse Aa/N-values ere corrected for magnitude. of load

1.0mm

5mm

for which. the p wer m was taken 2.' In fig.. I2 these Aa/AN-values are

plotted in function of frequency:. The,fü'l.l curve is seini-logaritlìmic,

the dotted one is double-logarithmic, the thin broken line is, a

log suare root one. The log - log presentation comes closest to a

straight' line. 'In the región of low frequencies app'iie more or less

log da/dN. -0.47 logfreq4- 3.8.

Roughly spoken, a frequency reduction from

102

to iO gives an increase

in da/dn of 2. For constant-amplitudes loading little difference appears

between Fe 410. ana Fe 510. But in the same figure it can be seen that for

progrthnmed loading the difference is signiícant. In this figure the

pro-grammed load test results have' been plotted on the basis of the number of

paks. It is curiois to see th'at both in fig. 10 and fig. 12 these peaks'

alone can be 'made fully responsible fòr th,e crack growth. behaviour for

loading in seawater.

In §3 possible reasons for the behaviour of both materials under progranmied

loading have ben given.. Apart from, the need of an electron-microscopic

investigation of the cracked surfaces, the causes of cracking under pro-'

granuned loading should also be clarified by additional tests. When it is

true, that the beneficial effect of tensile peak loads - in giving

com-pressive residual stresses at the crack tip - is counterbalanced by a

stress-corrosion effect of the peaks, it might be expected' that a doubly negátive.

result appears 'whenrse peaks would be applied., for 'this, result

in tensile residual stresses, However, compressive peaks' are far less effective than tensile peaks, because under compress:ion the crack closes. A method to meet this difficulty consists in applying first a tensile peak

leading to some resid'ual crack opening, and next a compressive peak. Ii this way the plastic zone will indeed' be, compressed more or less, leaving residual tensile stresses after unloading (figé 13a)a

Other tests will be carried out with wider spaced, peaks '(1:2000). Furthermore the loading of fig. 1.3b will be applied with respectively

10 and 100 sinal.l load cycles per large load cycle at different frequencies.

Conc'].usions

i. At very low frequencies the corrosion fatigue strength of. unalloyed steel at high. loads is very b'ad.

Below 102 Hz the crack growth rate seems to increase a factor 2 for

every 10-fold reduction in frequency (log da/dn = -0.47 logfreq. - 3.8)

This applied both to Fe 410 and Fe 510 Nb (normalis'ed) for constant.

amplitude loading. '

For'programined loading the performánce. of Fe. 510 was 'clearly better .than''

10

-In air, the crack growth rate under programmed loading was an order of

magnitude lower than for constant amplitude loading.

In seawater the crack growth rate was not favourably affected by the

presence of peaks for Fe 410 and slightly for Fe 510.

The crack propagation behaviour under programmed loading can

be attributed completely Vto the low frequency peaks in the programme, o',,

to the higher frequency (0.2 Hz) low loads. (This should be made

more clear by tests for one peak in 2000 cycles).

In practice there is some fear for the very large numbers of very

low-load cycles in the low-load spectrum of sea-structures from a corrosion

fatigue point of view. The results of the present paper suggest that

there are reasons to worry also about the less frequently occurring

References

/1/ J. de Back, C.H.C. Vaessen et al., Fatigue and corrosion fatigue

behaviour of offshore steel structures.

Final report ECSC Convention 7210-KB/6/602 (J.7. I f176).

Deift/Apeldoorn, April 1981.

/2/ J,J.W. Nibbering, La fatigue des constructions soudées (3e partie). Journal de la Soudure/Zeitschrift fir Schweisstechnik, 70, No. 3 (1980),

pp. 41-52.

/3/ _{R.S. Little and E.V. Lew'is, A statistical study of wave-induced}

bending moments on large tankers and bulk-carriers.

Transactions Society of Naval Architects and Marine Engineers (1971).

/4/ C. Aertssen, An estimate of whipping vibraÍon stress based on slamming

data of 4 ships.

-International Shipbuilding Progress, 26, Feb. 1979.

/5/ J.J.W. Nibbering, Design against fatigue -and fracture for marine.

structures.

Proceedings of International Symposium on Advances in Marine TechnoLogy,

Trondheim, 13-15 June 1979. Trondheim (1979), _{pp. 721-738.}

/6/ J.L. van Lecuwen and J. de Back, Effect of load history on the fatigue

behaviour of a low alloy steel in a saltwater environment.

Report 6-74-13; subject: C.V.-1. Stevin Laboratory, Delft University

ofTechnology. Deift (1974).

17/ D. Dillingh, Enige aspecten van bet scheurgroeigedrag in plaatstaal

Fe 52 onder variabele vermoeiingsbelasting. Dccl I: tekst.

Report 6-78-2; subject: S.6-1. Stevin Laboratory, Delft University

Table I. MATERIAL

Fe 510 Nb, normalised: = 401 N/mm2;

0B 556

N/mm2; 6

33.5%.

0.18 C; 0.13Mn; 0.43 Si; 0.12 P; 0.11 S; 0.15 Al; 0.26 Nb.

ASTM ferrite grain size lo.

o Charpy energy -20 C: 62 J; -40°C: 41 J. Fe 410: a = 313 N/mm2; a = 492 N/mm2; 6 = 34%. y B 0.16 C; 0.021 P; 0.017 S.

Tab(e.fl - Summary of' corrosion fatigue tests. (repeated Loading)

-(mm)

c;M(m (Ñmm mat spec-imen environm ent ' minjma Nimm2

'.

Cycles 15mm crac.kL. ' freq Hz. m ' C .

loading condf ions

' 10mm

:

.,. ' , WB'-21, WB-9 ' ! seawater ' air ' 21.51237 21:5/237 335Ó - ' ' OOO17 OOO17 . 1,46

-:7.21O_8 '

-A .

'

Gn.'

' '

---o

H ' ' ' ' 2,5mm. Fe 510 V/B 10 seawater 21 5/215 3 950 00017 1 77 8 2x iO - _H . ' 100:sec... _. ' WBL1I . air ,' 215/215 ₂₇₀₀₀ O99i7 2,38 8,2x

1O2

Wß-20 seawater 21.51215 8 900 OO1 1,89 ' 14x 1O cmin

W15:

Uif (see

below)

,

-

--- 0

WF-16 seawa(er--(see below)'

' ' 1

80'C.

60mm. '0,0003Hz. -' ' .' 20mm. ' 0,000/t Hz.

1760400003

264 25x10_11

-VO-1 seawater 71237 87OOO,O0O84 ''4,17... 2;1x1Ó17 . ' . VB 9 air 7/237 JO,0003and

-

-0,00084 . ø. Fe.410

---0

' 1,5 ' 2,5 min. .

1/4sec4Hz

VE 1 seawater 4/184 13900 005 3,23 46x10_14)

2Osec005lz

VE-i air 4/186 27000 0,05 3,08 8x10-141

w

--VE-7 air 4/184 32350 0,1 318 38X10_14 I Fe.1O .WF-15 WF-1 6'

afr

seawater 4/184' 4/164-.30000 72600

4.-'Od 2.27 0.97 219x10_h1J 6Ox1O4J ' ' '

--

-

O VO-17. seawater 4/66/186 7.30 000 <

10,2-[.J.'3,8

' . 1.02x1o:15, .. ' ' .. ' . Fe 610 t0001 A. Pt 3,7 16x10-15 U200cyctes l000sec.

r

₂ :, 12 . H VO 16 air 4/66/186 s000000 0 2,3 1,19x10 (0,001 Hz)

- max

°°'

/V./I 2,29 2,9.xiO. i -' Fe 510 WF 2 seawater 4/66/184

0,2_ ¡jj!3i8

16 3,2x10 .' Umax. -

-__ornln

1160 O0Ot0001._4,. _A. 297104x10_12

5sec.0,2Hz.

WF-3 . air 6/66/1134

-

-

-

-i_sec.4Hz.

' Fe.410 VB-13 air _{4166 6050000} , 4,-, 4,82 3,6x10:19 ' ' ' :

----max..

V13-16 , air 4166 '.6,- . . '' '' ''

- - Gnin

9 e 7 6 9 8 7 6

5-5 ?1 T TI 1100 T0 o o o .1 O r A DÂ (U/mm2 100 o WC-24 150 £ WD-3 150 VID-0

Thoso Lines envotop 50V. of tho reuLt5 obtzined in oir (18 spocunons).

4 lIz. 300 WI3-01 250 C] vC- 25 - 1 Hz. 350 0 VID 05 350 A wo- os 200 280 D WO-12 A WO- 15 ?-0,2H0. S WD-07 T WOlt

-

3/ S Ak (Nmrro 12) t !

loi

, i

Ill

53799,

2 3

156799

o

/

t,

C, Steen (lipi) + u' +

Depart Perth Amboy

ant ant Hope _{pman Gulf}

baflaut 3/11/69 Shifted ballast 3/12/69 -Shifted

r .-3/7/69

-

T.

___J

-::::;jf-;l:

Cross Equator 3/17/6 _3121/69

-Chnnged bal _{-Discharged ballast -Replaced bal} -Finish ballasting

3/22/69

Cape of Good 3/25/69 4/2/69

Enter Calf of Enter Persian Arrive Ras Tanura

. Pass 4/5/69 -4/6/69 -4/6/69

-"7f_

-a il I C

HOGGING

SAGGING

o i I

O 10 20 30 40 0 60 90 sec.

Fig. 3 Stresses caused by wave bending and sLamming.

(Aertssen [4]

).

k

_t

t

-'

.'

..

p,.-

-r'-,:

Fig.4, Test set-up

Upper specimen in seawater Lower specimen in air.

InitiaL fatigue Loading at

4.Corrosion fatigue test

Fe.5l0normaLised _{; Gmin./Crnx} 21,5/215 N/mm2 for WB1O;W3"1l;WB20 4Hz. in air. 1mm initiaL crack. WB10+11: 31960 cycles WB20: 39190 cycles

30-20

nin./max.21.5/23.7 N/mm2 for WB9;W3-21.

--00J

0,25 C,,

ÏV

- -

-Fig.5

Constanf-2mp1itue results for Fe.510

5000

load cycles

0,0017 Hz. for W39; WB-10 WB11; W5-21 N/mm2 a'

-

21,5 -O 20 sec. 215 N/mm2 0,01 Hz. for WB-20 I J t I t I 10000

15000

E

E

20

C

G) -'J ra U I I L 21,5 O

-c

ci

-c

'-J

-I-o

C 2mm initial crack (32370 cycLes)

1nitia

fatigue toa6ing at;

Fatigue test; Fe.510; G'nj/ max

4/184 N/mm2

4Hz. in air;

G7ijníGrnax A

30.

20 4/147 N/mm2

-E

-

E

- _"k' ai ai WF-15 and WF-16 1mm initial -crack 31 000 cycles I-., ro n E E

Fig.

-

ResUlts

L:

Li

o

Li 20 10

10.4=0

0 25

10000

toad cycles

r cònstant amplitude tests; Fe.510

Q 1I4sec. 4 Hz.(WF- 15) l0sec.!0,1 Hz.(WF-16)

20000

4N/mm o

.1

- -

t-30000

₄₀₀₀₀

-u, CD (mm) _p.4 w -s a. -1 . u, -'..n a. .o

!nitial fatigue (oad?r,g at

.-L----Corrosion

fatigue test; Fe.410 4Hz. in air.(VE-i;VE-2;VE-7;

r

WF15)

₃₀

"min.! "max.-4/167 N/mm2 VE -7 1mm initial crack (22000 cycles)

VE-i and VE-2

1mm initial crack (24900 cycles) WF-1S 1mm initial crack (31000 cycles) VO-1 No initial crack

in./Ginax.=4/184 N/mm2 (VE-I ;VE2;VE7)

20

Gmin/nax7/237N/mm2

(VO-I) -l--___

WC

D _IQ. q: U

20

lOj

10-4-0

0,25

10000

4.-load cycles

20000

Loading conditions for

V39;V31

W310; WB11 ; WB-20 W3-9; W321

Fig.7

Resul.ts for constant amplitude tests

Fe.410 (also included air Fe.510).

.

30000

0,05 Hz.

/

/

'

/

/

.1

/

/

"

,/

s,

ip

/

,2

2

/

i nprinr, rrr,r1ifinr,e Çnr

VE1;VE2 and VE7; VO-16 and VO-17

,'

/

- --.--...

. ..0 .,.o . I

a1 i a. WF-2 and WF-15

-e.,-

-'

----

.p

--Stress C Nf m m2 184(VEI;VE2;VE7 237(VD-1) wF-15 7(VD-1) - 4(VE1;VE2;WF15) o

40000

1mm initial crack

II

0.25

lo

4Hz.

.-.--max.66 N/mm2

ltJUJ\JL_._.Gmin.4N/mm2

- -. - 0

toad cyctes

.

(n

w->

I I T I I I I I

1xlO

2xlO

3xlO

4jØ.6

5x1O 6x1O

7xlO

8xlO

-I, L-%a,

L-- .

w Fej0...

I I air 3 3 ¿1F (AI s,'

_y

w z- u, o.. _-J (mm) 'JI

w

a

U, . -4 '0 e. -...Seawaer Ve_y

InitiaL fatigue Loading at 4Hz. in air. Gmin./max. 4/116 N/mm2 1mm initial crack VD17: 54000 cycles WF-2 ¡ 121900 cycLes

Corrosion fatigue test Fe.410 and Fe.5lQnormatised

4-ci

a

t-, '-J

E

E

-t I t I 0,25

0,5x10.6

J-0,2510. t-.

load cycLes

Fig.9

Results for programmed loading Fe.410 and Fe.510.

Load (kN

'

Stress (Nimm2).

:

A

184

«lillA

Ti

o I -'-198 cycles

-I 0.2 Hz.

L!

lHzj

at 5

-..

5x10.

CC5

ar

Fe''°

J___ ____L

0,5 0,7510.

Number of.. peak cycLes

66

-4

o vb-16 i t t i t. I I I I t t t

l

-l o.6

1.5x10.6

2x10.6

30

20

10

s Q -s -s 4-p,, u, 4-r.,,

u

4-u, s -a 4

Q -n 1 m £3 Q Aa/LN (mm/cycle) C

(córrected for differences in toad)

'Ji *

a

a

Q

a

N W 4-¿J 0'

J

) 'o IL) I I I I I t I

Aa/LN (mm/cycLe) -- - -)

(corrected for differences ¡n Load)

Ji

X

a

a

Q 9 1L3 IL) 0..

1

03 ..o sJ Isa/AN (mm/cycle)

(---(corrected for differences in Load)

(D 4-

a

Q t I I , i t

ti

w

0' J CO

IL)

w

a

P

a

Fig.13a

Fig

Fig.l3 Coming Loading programs. 200 or 2000 cycLes

'Ç

Summary of resuLts of [ow frequency-experiments

(corrosion science voL 23 N2 .6

J.J.W. Nibbering).

a OnLy high stress cycles counted Number of 1adcycLes xlO.6 for 10mm crackLength 20 mm totaL notchtength.

jair

seawater Loading j Load-cycles io6 Loading Load-cycles )cW6 Loading Load-cycles

xlü6

Loading Load cycles io.6

Type _Gax(N/mm2) Type G(N/rnm2) Type _Cax(N'mm2) Type G(NImm2)

Fe.410

58

66

14

---66

2 ₀₆

r n r

UJ

---184

0.022

¡-.--0J\.í\/d.l:__.+6

184

0.07

-.

-.

-. 184

T4

0.011

- --184

66 0,003 Fe .510 ±6.5

--184

0JVV\dLI6

-

-184

0,02 /.

. .J._66

---184

--

---184

L..4

0,01

----184

oo5

o