Fatigue and Brittle Fracture, Report of Committee 11, International Ship Structures Congress, ISSC73, Hamburg

ji 1í

LABORATORIUM VOOR

SCHEEPSCONSTRUCTI ES

TECHNISCHE HOGESCHOOL

- DELFT

BETREFFENDE:

International Ship Structures Congress 1973 - Hamburg.

Report of Committee 11: "Fatigue and Brittle Fracture".

SSL 172

5th INTERNATIONAL SHIP STRUCTURES CONGRESS September 1973 - Hamburg

Report of Committee ii:

"Fatigue and Brittle Fracture".

CommIttee Members: Prof. ir. J .J.W. Nibbering (chairman) Mr. M.P.M. Ballet Prof.dr. K. lida Prof.dr. T. Kanazawa Dr. A.I. Maximadji Prof. W.H. Munse Prof.dr. H. Petershagen Prof. E. Steneroth Mr. W.H. Winn Mr. H. Wintermark.

Issued by the Ship Structures Laboratory, Mekeiweg 2, Deift 8,

CONTENTS:

Page

Section I.. INTRODUCTION. . . 3

Section II. FATIGUE. 5

a. Cenéral.

b,. Service performance.

Fatigue strength of welds with defects. Fatigue design rules and defects.

Cumulative damage. H.S.-steels.

Experiments with structural models.

influence, of mean stress and mean strain.. impröving fatigue strength.

Crack propagation. Welded connections. i. Influence of corrosion.

m. Prediction of low cycle fatigue strength.

Section III, BRITTLE FRACTURE. . 52

Generai.

Significance of defects. Fracture mechanics.

d.. Acceptance testing; correlations among

results of different tests.

influence of residual stresses, unfairness of plating and structurai discontinuities. Brittle crack propagation and arrest..

Section IV. CONCLUSIONS AND RECOMMENDATIONS.' . 95

A. remarkable fact in relation to the work of this committee is the large increase in the interest of ship designers for fracture problems, particularly fatigue. No doubt this is a consequence of the growing uncertainty a]out the crack-safety of ships in view of the steady in-crease of service stresses /1/.

Also the fact that cracks hvè. developed at vital places in container ships soon after entering service has caused much conçern /1/, /2/.. Both aspects will be treated in section lIa.

it is most fortunate that some shipowners have freely published their design methods and service experience. It shows the way to ther ship-designers and offers the possibility of checking whether the applied calculation methods are sufficiently reliable or not. It has beçome obvious that intelli:gent and careful approaches can indeed lead to satisfactory predictions, but it also is obvious that both on the side of loads as on the side of realistic fatigue experiments much remains to be desired.

This has already been emphasized in the 1970 report, but without the urge from practice it did not disturb too many people.

Another problem area in which the interest has jumped forward is the significance of defects and the. connected application of fracture mechanics., /3/, /4/, /5/, /6/.

important conferences have been held of which the proceedings contain awealthof information of interest to shipbuilders.

First of all should be mentioned the second Conference on Fatigue of Structures held in 1970 in Brighton /7/. Next the Conference on the Practical Application of Fracture Mechanics to Pressure Vessel

Technol-ogy /8/4 London 1971.

The I.I.W.-colloquium (public session) on "Acceptable Level of Defects in Welding of Bridges and Pressure Vessels" is most recent /9/,

(Toronto 1972). Bearing in mind that /7/ as well as /8/ and /9/ were mainly concerned with bridges and pressure vessels,, one wonders whether no conferences. and books. appear for ships., being far morefl cyclically

-4-loaded. This is undoubtedly due to the existance of Classification Societies, who make it superfluous for many shipbuilders to take great interest in fracture (and welding technology) problems. This is not true for the whole world. The situation, in Japan is different, which is not only reflected in a great number of papers from centers of naval architecture, but also ¿bvious from personal contacts with Japanese shipyard- and, research-people.

In Japan the shipbuilding education embraces to a far greater extent than in Europe welding technology and fracture problems. There is little doubt that the success of Japanese shipbuilding is partly due to this "engineering" attitude in teaching. In this connection it is

of interest to quote one. of the main points discussed during a British seminar held on 'Sept. 8, 1971 at the University' of Strathclyde. It was devoted to "Recent Observations on Japanese Shipbuilding and. Shipping". It was concluded that: "Considerable financial resouröes are being de-voted to applied research with particular emphasis on welding and mat-erials", /10/.

Papers with a highly instructive value are always of great general interest. The 1971 Adams lecture by Fellini /11/ is a good example of such a paper.

Somewhat on the border of the scope of this committee are the proced-ures of the 1971 I.I.W. public session "Welding in Shipbuildingt1. But it deserves' a place in this introductiOn because everybody has to realize that fatigue and brittle fracture problems cannot be separated

from welding technology. ' .

Other books of value are "Brittle fracture in steel structures" /24/ and "Critical review of fracture and fa.tigue analysis" 1.25/.

The field to be covered by this committee is relatively large. It has been discussed whether it would be useful to extend it also to steels for low-témperature applications like Ni-steels. But it has been abandoned because it is thought that what is covered by. a particular 1SSC-committee should preferably meet the problems emerged in other committees. Moreover, owing to the importance of light metals for low temperature applications it woUld be better to discuss other materials

Section II. FATIGUE.

a. General.

In the introduction of the 1970 report has been stated what this com-mittee expects from those who work on loads. This same theme has been treated further in the reply to a discussion by Mr. Soejadi. Afterwards he brought these points under the attention öf committee 3. There is little use in repeating everything here, despite its actuality.

It will only be emphasized that fatigué specialists and load speòial-ists have to do a combined job. The load experts need to know which parameters in the tótal load spectrum are important and which may be neglected. Onlythen they will be able to collect and present the data in an efficient and economical way. Moreover they may select a number of simplified but characteristic load configurations with indicated for the same purpose in committee 12: "Materials other than steel"; perhaps it could be renamed "Special structural materials".

The application of higher strength steels is widespread nowadays, and many of the present practical problems aré connected to these. Yet the steels are still of moderate strength, unalloyed (in the usual

sense of the word) and not quenched and tempered (Q and T). Normalizing is quite popular now in order to obtain very fine grain in steels con-taming slight amounts of Niobium, Vanadium and such.

In shipbuilding steels of higher strength than 550 N/mm2 are only used for warships and special strUctures like off-shore rigs, Therefore the Q and T-steels are partly left out of the report. But the attention is called for the proceedings of the I.i.W.-colloquium 1972, specially de-voted to the welding of these steels /12/.

o)

probability of o'ccurrn'ce for use in fatigue testing. But it is a han-dicap that fatigue experts need to have these load configurations be-fore they can quantify by experiments the influence of the many para-meters involved (like shifts of the mean., rest periods, incidental ten-sile or compressive loads, incidental vibratory load fluctuations

superposed on main loading system, etc.).

The whole process is iterative. How the loads may be simplified forex-. periments can only follow from experiments with these loads...

Sometimes it is argued that selected load programs are less realist.ic than random loading. This is .only .true for those cases where the load-ing is completély random and where very large numbers of specimens can be used in order to eliminate the chance that haphazardly rather severe or mild load sequences are.applied. If we only think of the great num-ber of different structural welded details in ships it is obvious that the latter is a most uneóónomical solution,. But more important is that thé loading of a ship is not completely random. Many ships alternative-ly navigate n loaded and bal±asted conditions, in calm (summer) and stormy (winter)' periods etc. The. distributions of peak to trough values

for each period will be. different and the still water values of the bending moments will change in a rather systematic way.

There is a fair amount of experimental results with respect to the nf1uence of the mentione.d load parameters. Most of it is. however of limited value for shipbuilding because it has mainly been obtained with small, mildly notched or unnotched specimens in polished condì-tion..This means that the information at best applies to the initia-tion of cracks, and not to what is needed. more: propagainitia-tion. This

brings us to another theme, also thoroughly çovered in the 1970 report: welded structures will alwáys contain defects. Some of these will be - so severe that the number of cycles neèded to initiate a crack there,

is small as. compared to the number of' cycles needed to extend them, for instance to plate thickness. 'On account of this, fatigue experiments may be devoted to the. study .of the phenomenon oE crack propagation only

and the results should be evaluated using the formula of Paris and

Most of the new literature on this subject is in support of this view (See section IIj).

Additional adyantages are, that the problem: when does a crack start to grows is eliminated and the main source ofscatter is excluded. This does not mean that it has no sense to try to delay the initiation moment by grinding, polishing, T. I .G.-welding, peening etc. (This sub-ject will be treated in section Iii). No, it should be realized that the more macro and micro stress concentrations are reduced, the higher the stresses may be before initiation starts. On the other hand it should also be. realized that in. such cases (in the extreme comparable to polished bars) eventual cracks do propagate much faster than cracks. starting from severe stress raisers, because the material ahead of the initiationpoint will be more deteriorated the longer the time has been before crack initiation. (In constructions with high stress concentra-tions, a quick started crack travels through practically undamaged mat-erial.). Improving the resistance to initiation is of interest for those

structures which actually are very difficult to construct satisfacto-rily, like hatch-corners of containerships. in this connection it is in a way fortunate that the most crack-proof defects are those which can most easily be "repaired" or better improved viz, undercuts. The fact that these form the weakest link of most welds is partly caused by. the circumstance that most structural. parts are not purely axially loaded, but also hended.

In many cases this bending is simply due. to unfairness of plating and distortion due to welding. (An out-of-plate displacement of 1/6 plate thickness suffices to double the stress at the surfacó of an .xiafly. loaded plate). Of course only those defects lying near the surface, like undercut., will "feel" this effect.

lida /13/ is much in favour.of a combined crack initiation- and pro-pagation-approach in the fatigue design of ship structure.. He argues that the fatigue design based on the propagation criterion is more complicated than that based on the initiation criterion by reason of the difficulty in knowing exactly in advance at the time of design,

the start point of a vital crack followed by preliminary cracks and the redistribution of the stress field in comparatively wide areas, which results from the change of load-carrying-capacity caused by the propagation of a straight or curved crack in unknown directions. His observation shows that in laboratory specimens the initiation life of a crack of 0..2 to 0.4 mm in length. takes about '40 to 60% of the total failure life in low cycle fatigue. range /93/, while the ratio of crack.

initiation'life to complete failure life becomes 0.7 to 0.9 in high cycle fatigue. This all is valid for small and unnotched .or mildly notched specimens. in a small specimen a slight extension of the 'crack

reduces the cross section sensiblyand'. raises the load stresses. in.

larger specimens the situation is far more favourable and particularly in ships the load can be shifted to still intact parts.

Another point is that 50% of life may sound important, but in terms of logarithm of numbers to fracture., in which we should think, it hardly.

in 50.000 4 7' . does:

100.000 - 0,94.

lida mentions transverse load-carrying fillet welds as an example where. fatigue cracks may start simultaneously from many points, when the f

il-P,

let welded member (e.g.. bulkhead to deck) is subjected. to bending. Then crack length along the weld may be larger than critical before the full thickness is penetrated. The same situation may appear with transverse butt wlds having long lacks of penetration..

The crucial point is., that in bothcases the. residual stresses will not always succeed in f6rcing the cracks, to leave the welded region; the cracks are "guided" along the defect, because the. stresses in the sec-tion concerned are much larger than at a small distance away.

Indeed "general" rules always have exceptions, and the unfortunate thing is that undercuts at the edge of fillet welds not 'only are 'rather common, but also more dangerous (sharper) than other types of defects. On the other hand an optimistic sound can be heard because as said be-fore, undercuts can easily be improved with great success by grinding (expensive), T.I.G.-weiding (less expensive) and particularly .peening (cheap), /14/. (See section IIi).

In the intròduòtion (section I) of this report the structural strength analysis of the O.'C.L. containerships has been referred to,

/1./.

The great value of this work cannot easily be overemphasized. Various items of the report are:

Load calculations. Longitudinal vertical and horizontal wavé bending stresses and torsonal stresses were calculated by 0er-manischer Lloyd on the basis of a North Atlantic Sea spectrum fOr various speeds. The maximum values for a probability of 108 were significantly larger than those calculated with a standard wave with height 1,l/L.

Horizontal wave bending moments were equal to 60-75% of the ver-tical ones.

Full scale experiments. (Stresses caused by .torsion. Service data about longitudinal wave bending and slamming). The comparison be-tween the latter, (which correlated satisfactorily with the pre'

dictions under a)) and similar ones obtained from recordings on, a large number of dry cargo ships and tankers, by Johnson and Larkin /15/ is interesting. Even when allowance is made for the, current wide spread use of. higher strength steels,, nearly 15% greater stresses occurred inthe Encounter Bay class.

Fatigue strength estimates: use of B.S. 153 fatigue strengths limits for bridges.

Fatigue experiments with hat ch corner details.

Observation of service, behaviour of the ships' structure, (cracking).

Effect of repairs and improvements of design.

It is easy to critisize some weak spots in the analysis., but this would be unfair in view of the overall quality of the work and the

ex-tensive publishing of every step made. Everybody who likes to make. ad-justments in those. .parts where he is an expert may do so. We rather like to-point, to perhaps the most important outcome of the work viz. that predicted and actual service behaviour' (cracking) 'we'e in

satis-- lo.satis--

lo.-factory agreement. A similar conclusion was arrived at in a paper by Alte' /79/.

In another interesting paper by Hur /2/ about stresses in large. tank-ers, the importance of fatigue is again emphasized; it is worth quoting the very justified observation that: "it is comparatively easy to post-analyse structural details in which cracks are observed and recognize their faulty design.

It is much more diffIcult to predict the expected life of a structure which ppears to be well designed."

This brings us to the chapter: significance of defects

A general disciission on this topic including brittle fracture, is given in section III. The next section will mainly report experiméntal re-suits and their.influence on fatigue designrules.

c. Fatigue strength of welds with defects.

A work of great practical value has been conducted by Soete and Sys /6/. Fatigue tests havé been carried out on mud steel cylindrical pressure vessels with systematic weld defects: slag inclusions, porosities. and

lack of penetration. All defects were located in longitudinal welds. Dimensions of vessels were: length 3 m, diaxneter.0,6 m, thickness 18 mm. 566 Slag inclusions have' been surveyed; they varied between i and 150 ¡p2 Further 6000 porosities up to 2 mm were present.

On the, basis of a fatigue life of 100.000 cycles, slag inclusions of 30-50 mm2 showed a failure probability of 2% at a nominal stress of, 21.10 N/mm2. Porosities did not impair the fatigue strength. Lack of pe-netration became already important at lengths larger than 10 mm'.

it should be observed that this is not in conformity with. fracture mechanics., where depth of the lack of penetration instead of length is, the dominant parameter. The cause may be that in general the ionger,a lack of penetration,, the greater its depth is.

In' /22/ Koch and Schrader compared 21 different standards on judgment of x-ray films. Literature on relation between weld defect and fatigue strength is mentioned and 42 references are given.

1.00 0.90 0.80 0.70 0.60

L'

050 0.45 Q40 0.35

4 0.30

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w

I-0.20

w

>

-i .1

w

'X 0.10 lO

_io

_io

FAILURE

LIFE, Nf

FIG. i

PROPOSED FATIGUE DESIGN CURVES FOR

SOUND AND DEFECTIVE BUTT JOINTS

(PARAMETER IS THE WELD DEFECT

SEVERITV DEFINED AS AREA PERCENTAGE)

la

(WITHOUT REINFORCEMENT)

PARAMETER: OÇj0Io)

L

O

i

L

i---SOUND

WITH

REINFORCEMENT

--WITH UNDERCUT

I

h11 -' i I ₁₁₁₁₁₁ I I

In /23/ general observations for testing of welds with defects (pores. and slag inclusions) are reviewed by Eiro: .

s

The effécis of porosity and lack of penetration in m.s. weidinents were also studied by Ekstr6m and Munse /26/. It was found that only large

clusters of porosity had an effect upon the fatigue resistance, al-though not so strong as lack of penetration.

Cùriously enough, Hirt and Fisher /27/ found that the most common in-itial flaw condition in welded beams leading to fatigue failure was porosity! They computed the life of the structures by using fracture

mechanics for an equivalent crack of 1 mm and obtained satisfaclory agreemen with the test results. .

Ishii and Ilda /941 re-analyzed the range of scatter of data reported in the previous, paper, and proposed conservative fatigue design curves for defective butt welds containing porosity, slag inclusions, incöm-plete penetratión and cracks. An example is shown in f ig. 1, in which the ordinate stands for the ratio of the maximum stress in repeated axial tension fatigue to the ultimate strength of the base metal of A533B steel. The parameter means the defect severity defined as the area ratio of total weld defects to a. fracture surfäce. Lines V-W, and X-W show the upper and lower lines öf the range of quality "W", which was proposed by Harrison /5/ as the quality level of porosity of 3% by volume percentage. One point which should be emphasized is that the

fue strength of 'a transverse sound..butt joint with reinforceiñent TABLE I (bending.tests, 18 kp/mm2).

11W-class 'Number of specimens N Defect size (%)

1 13' 215.000 23

2 . 17 290.000 18

3 22 ' 765.000 7,5

4 . 20 850.000 6,6

s

is lower than that of a reinforcement-less, butt joint containing weld defects of critical value of defect severity. The critical value at the failure life of 2 X 05 cycles is 5% for porosity and slag inclusions and 2% for incomplete penetration.

The important problems in discussing the fatigue strength of defective welds may be how does the defect severity defined by observation of radiográphs correlate with the estimation of fatigue strength as well as 'how to find the most reasonable size of a given area for defining defect severity, on the radiograph.

Further information on defects and fatigue is given in. the section on.

H.S.-steels'..

Fatigue design rules have everything to do with significance of de-fects; therefore in the next section both are discussed in combination.

Fatigue design rüles and defects.

One. of the important items of the Brighton Fatigue of Welded Structures Conference was the critical discussion on fatigue design rules /7/. In the first paper by Gurney /80/ a comparison was made between' the rules in use in both Germanies, Italy, Japan,.Sweden, U.K.' and U.S.A. There were great differences between them. Although the joints were rated reasonably similar, the design stresses varied widely. It should be recálled that the fatigue data are valid for bridges loaded to high numbers of cycles. Fig. 18 and 19 of the 1970 ISSC-report give an idea of the types of curves and wide. scatter. Further details are given in

the. section onH.S.-steels..

Paper 3 by Munse /81/ discusses the changing American design specifica-tions. Many of his ideas correspond to those of this ISSC-committee, which may be comforting to both. Serious concern about. the lack of ac-curaté information about loads is expressed.

Paper t by McLester and Norman /82/ reflects British railway experience of BS 153. They consider BS 153 to be. generally very much on the safe side because:

- 1t

-b) The practical experience with structures designed according to BS 153 is favorable.

These opinions do not correspond to those of Ogle in paper 5 /83/. Figure 2 is a probability plot from which is evident that the chances of a test specimen having a shorter life than BS 153's is in the order of i in 10 for the lower classes of detail.

Striking is further that a small error in estimating the quality class of a structural detail may result in a large error, in life. A struc-tural detail classed F for 100 years life at a million cycles a year

has a 1% chance of failing within 2 years when its class would in real-ity be G.

Fig. 3 (Ogle) shows a comparison of the effect of planar defects (lack of penetration in transverse welds) in 1" thick plate on fatigue

strength at 2 X 106 cycles compared with details in classes D, E, F

and G of BS 153. The proposed safe design curves are from a paper by Harrison, Burdekin and Young. They represent a probability of 1:5000 at about 2 x 106 cycles. Plots of 1:100 and 1:8 risk are also shown of which the latter gives a direct comparison with BS 153 class D, E, F and G. It appears that surface defects are two to three times as severe

as central ones. The point where each curve intersects the D, E, F or G level represents the maximum size of defect that can be tolerated at the indicated risk. The results for two probability levels are shown in table 2, together with an assessment of the likelyhood of the defect being found by NDT-methods. 'For class D and E (ordinary plain butt weld) the minimum permissible defect size is mostly too low for its discovery to be guaranteed! Fillet welding is only accepted as class F when flaws > 1,5 mm are not tolerated at a 1:8 probability level. This of course is very high; 1:100 would be more realistic. Then flaws of only 0,75 mm in depth must be detectable and repaired!

Finally an extensive study by Serensen et al. should be mentioned here /76/ which covers a number of subjects, combined with the problem of fatigue design rules.

The foregoing had mainly the purpose to show how much permissible stresses should depend on quality of welds. But it should be reminded that detail design is an at least equally important factor.

200

E

z

C

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Class D

Class E

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ClassE

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Cont. long, FW(manual)

o Trans. BW (manual)

L Non L-C FW

Long. L-C FW

Gurney

Proc,Inst->

Civ. Eng._

zt

April'63

OOÓ1

0'l

i

10

50

Probability.of occurrence.

90

9g

99.9

o

99.9999.

FIG.2

PROBABILITY OF OCCURRENCE OF FATIGUE STRENGTHS LOWER THAN STRESS SHOWN.

C

o

4-. C

.2

'n

io C

_w I I - I I I I I I

0,001 a,

-J

ro

u

(n c) -.

a

o

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TI

1::

-s-Defect height 2a, ¡n (log scale)

0,01 0,1 I I I I i I

II

I i

II

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Central defect

Edge defect

D

FIG.3

EFFECTS OF PLANAR DEFECTS ON FATIGUE STRENGTH.

's-i I 1

11111

I I

1.111111

I I

Ii itIiï

I J

0,05

0,1 0,5 1

5

10

Defect height 2a, mm(log scale)

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. too small to guarantee detection by ultrasonics

® not generally applicable to these details

+ detectable in butt welds if ground flush both sides G dye penetrants must be used for detection

4 not normally to be expected in single run fillets

Class of detail dictating fatigue strength of member

containing, defect

Critical defect size 2a above which class of member is reduced, mm (in.)

Probability i in 8 Probability. i in 100

Edge Central Edge ' Ceñtral

0.3 (1/6-) * 0.7 (1/32) IC 0.07 (.003) * 0.2 (1/128) E X 0.8 (1/32)

+ 2

(1/16+) * 0.2 (1/128) IC 0.6(1/32-)

F 2 (1/16) (1/8-i-,) 0 0.6 (1/32-) ® 1.5 (1/16)

18

-In shipbuilding Classes of Quality are not in operation. Something of the kind has ,béen proposed in the ISSC-report 3d of l96 and has met

serious opposition.

Maybe the, time is more ripe now to take the point once again in con-sideration.

e. Cumulative d'amage.

Since' 1970 a great number of papers has' appeared in the field of pro-grammed and random loading and the influence of load variables. The importance of this branch of the fatigue science has already been em-phasized in section lIa. in the 1970 report and the printed

discus-sions, the basic problems have sufficiently. been put forward. The fol-lowing therefore will mainly point to some useful contributions (in, connection to ships) in the field.

Petershagen ./Lt8/ has reviewed the related literature of both

Germanies ànd selected some 21' papers. Some of the papers will be men-tioned here, in order to familiarize t1e reader with the numerous pro-blems tobe attacked..

Lehmann /29/, /30/, /31/ emphasizes the advantages of strength criteria on the basis of service fatigue life in terms of probability of failure. The scatter of .loa&' and strength is taken into account by a "statistic-al" factor 8:

max. strength (50% prob. of survival). statistical average of service load. SG, SK standard deviations.

Increase in 8 means decrease of probability of failure.

The relevant parameters and their influenôe on fatigue under 'ser-.

em-phasized. The difficulties of derivation of load blocks for programmed fatigue tests from measured random load sequence is treated in

/33/.

It is shown that the, shape of the load population depends on the method òf classification of the measured load amplitudes.

One of the pioneers in the field of applying service loading in fatigue, Gassner, is not optimistic aboüt the effect of simplifying service

loads for fatigue testing. But once again it should be emphasized that ships' loads are essentially different from for instance car loads.. (See section lIa).

The influence of block size and load distribution within these blocks for programmed fatigue tests is discussed in ¡3V and /35/.

In

/36/ a summary of calculation methods for fatigue under service load is given.

In /37/

a new calculation method is developed, taking into considera-tion that each block has a damageing effect, which modifies the SN-curve to be used fór calculation. The result is in accordance with test results in contrast to Miner's rule which overestimated life 8-fold:

Nt = (N1 + NMd - N1) C.

NMred modified cycle number according to Miner.

N1 cycle number of SN-curve for max. load of service population..

C

= factor to be obtained from individual tests or to

b&

systematically developed for relevant influences.

Paper ¡8V by Whitman and Webber discusses in general "The problem of cumulative damage in welded structures". Apart from being a good philosophical introduction it also contains a literature review aboutj cumulative fatigue damage "rules". The paper is in support of Sherratt's view /38/ that the application of the linear damage rule E . 1 is

justified when:

) All stress levels would. individually give N-valUes between ÏO

and 106. cycles. . . .

20

-The loading spectrum is. well mixed.

No infrequent load applications causingvery high stresses occur.

5.) Large numbers of low stress cycles causing crack propagation are not present.

To these restrictions we may add that:

Large differences in average stress (e.g. still water stress) should not occur;

Low and high frequency cyclic loads do'not occur together,

(e.g.. wave bending springing). (See /51/ .by Bugiov and Kolikov

From all these considerations it emerges that the

linear damage

rule will probably be very inaccurate for naval

architecture but that

for the. time being it is virtually impossible to indicate

whether, and

which of, the existing damàge "rules" would constitute a substantial improvement. One additional and important argument that little data exist for crack Qroaation in welded steel structures.

Another p per by Haibach /85/ contains results of experiments with welded joints of St. 37 and st. 52. The applied cumulative frequency distribution was

varied

systematically.

Mention

should be made of the relatively recent work conducted at tle coie Polytechnique de Montréal by Dubuc and cooperators.

/39/.

In aeroplane building much

attention is devoted to cumulative damage.

Although oad histories and materials differ appreciably from those for

ships, a number of roblems is closely related. Therefore the second Piantema Memorial Lecture by Schijve /1.1.0/ is also of.interest for ship constructors. A very extensive review, is /141/.

The Amerióan literature has been reviewed by Munse /142/. A páper by bowling /43/ is mentioned that considers the important aspects of load séquence and .mèan stress. The. "rain flow" counting metIod and the.

"range

pair" method gave

results in accordance with, the experimentally

obtained oneso All the' other counting methods resulted

'in large

differ-ences between predicted

nd natural lives.

Another paper by' Topper et al. /144/ shows that summations ]ased upon completely reversed

strain

versus life' must be. reduced if the mean

1,4

'1,2

1,0 0,8

Smooth

_-o--.---o-_

exfoliation of

specimens

With through drilling

Fig.4

Test results and summing of damages.

scatter area

]

Variant

Seguence

of blocks

I

Steel

.

the average for. 3 specimens

fracthre

crack

Cm4C

D9r2

CX1T-4

V

O

V

22

-stress is tensile and generally increased if it is compressive. The se-quence of straining is relatively insignificant when the strain levels are close to one another.

Bussa et al. /145/ studied the random problem 'with a mathematical model of the form of life being equal to a function of stress concentration, maximum dynamic stress factor, probability of maximum load occurrence and low load elimination factor.

Tang and Yao /146/ explored the use of a fatigue damage factor in struc-tural design .in order to arrive at design diagrams.

Maximadji /147/ reviewed the Russian literature. Malygin and Jan-kovskiy /149/ concluded that a linear damage rule is only sufficiently accurate when the number of blocks is 20-140 (repeated loading). Hypo-theses of non-linear summation are put forward but have the disadvan-tage that it is necessary to carry out a great number of tests for the estimation of the. coefficients of the formulae.. (The word disadvantage. is not completely justified because the authors cannot be blamed for

"the nature of things"). Peculiar and important differen ces were found between the behaviour of mild and higher strength steels, especially with regard to overloading. For the higher strength steeisthe values

of summarized cyclic fatigue life were lower for specimens with stress concentrations than for smooth ones. For mild steel the opposite. was found.. (The stress concentrator was not a sharp notch but, a round hole).

Maximadji and Shekhovtsev

/50/

studied 'the hypothesis of linear summing of damages under low cycle fatigue. In fig. 14 the results are shown for four variants of load programs:

n,2. n.

C

E'-.

C

cr

N.

e.

N.

cri ci

n. ' real number of cycles in a given i-block.

'N ., N . number of cycles before the appearance of a visible crack

cri ci . '

p G 4 2

.10

-2 Tb -4 -G -e

FULL LOAD CONO n

w

I VOYAGE

FIG.5

LOAD PROGRAM

MEAN STRESS

BALLAST CONDn

ON

ILU CYU.4

FIG. 6.

PROGRAMME

LOADING.

50

I

-

]IE 1000

CycLes:

_Umin

O

kg/2

'

°max. =

15

k,1

50 CycLes:

_{°'min. =}

_'

max.

18.5

kg/

lIt.

1000

Cyctus:

-

_,

_a,0X

_{= 75}

I

Sequence of

Loading. 1 2 _{3 4 5 6 1}

IIIIt

]t

I

itlItI

It]lt

I

it

lIIlEr

I

t1Jt]It

0

_-

_21.

-1000 U) ₁₀₀₀ U)

Fig. t shows that for all steels the damage sum for the appearance of

a visible crack is more stable than that for full fracture. lida /96/ reviewed the Japanese literatures on fatigue..

Minaini, Itagaki and Ogawa concluded from 10w cycle rotating bend tests under random loading that the fatigue lives under independent random load sequences were shorter and showed smaller coefficients of varia-tion than those under correlated random, load sequences. Another con-clusion reached tO is that the. effect of randomness of applied load is greater than that of variations in material properties in case of

larger R.M.S. value. ' .

Nakamura et al. /99/ who carried out fatigue tests by double, triple and quadruple superposed' stress waves as well as random waves with superposed.waves, confirmed the reliability of an estimation formula of fatigue life.

The problem of cumulative damage for shipbuilding can only satïs-factorily be solved by combined,efforts in a number'of countries in order to be.. able to cope with all parameters involved.

Although not complete. agreement on this point exists, only for reasons of economy and feasibility the attention should be concentrated Qn crack propagation.. . '

A number of typical weidments and structural details should, be select-ed and a restrictselect-ed: number of typical load, programs should be agreselect-ed upon. As examples are shown fig. 5 of /53/ and the presumed "severe" loading system applied in a combined investigation of 5 countries of the European Community /52/, (fig. 6.).

The various load parts particularly, simulate an unfavorable combination, of wave and still water loads,, but such 'that the stresses applied are predominantly tensile because only for those cases fatigue is a problem in shipbuilding. (The fatigue strength of ship parts' is, certainly

satisfactory when the loading is more or less "compressive"). In gener-al regener-alistic loading should comprise combined' axigener-al loading andb,ending of' different frequenàies. Biaxial conditions should'also be studied and finally corrosive influences should not be excuded. Maybe this task' could be devoted to a special committee: "Wave loads. and,cumulative'.,

The use of steels with high yield point in shipbuilding has been dis-cussed many times and there is no room here for repeating the, arguments

again..

Reference' is made to the Conference on the Manufacture and Application of H.Y. Steels in 1970 /61/ and particularly to paper 606 by Soete, called: "Limitations in the use of H.Y. steels".

For the rest only a few pertinent conclusions from recent experiments of a highly realistic nature will be mentioned:

1) Fisher et al. /514/ who have cònducted nearly 1400 bending tests

of welded beams with 3 grades of steel (ASTM A36; A14141; A514) concluded that "structural steels with yield points 'between 250 and 700 N/mm2 did not exhibit any significant difference in fatigue strengths".

Radziminski et al.1/59/ reported results of tests of butt-weld-inents in high strength Q- and T-steels, (o: 550-900 N/mm2). The welds were full penetration transverse ones, fabricated by gas-metal-arc and shielded-metal-arc processes. The range of life was iO_iO6 cycles. It was evident that weld flaws had a signif-icant effect iipon the fatigue-resistance when subjected to high levels of repeated stress.'(Obs.: This is the domain for which is generally thought that the use of H.S.-steels is advantageous). The question of inspection is very important for these steels. 3) Radziminski and Lawrence /60/ found a large effect of internal

weld discontinuities. Fig. 7 shows the wide range in fatigue lives obtained in these tests.

o

'4) Ødegard and Hembre /62/ have published.results of experiments

with notched and welded test pieces of 3 hih-tensi1e (o '400

N/nun2) and 2 'ordinary ship steels (a 300 N/mm2). Basic and 26

-damage", or a working group of members of these committees.

loo

70

'i:

020

a, C ro

10

Sound Weld

Severe Poro

Lack of penet

sit

rat

y

on

io5

106

_iü

Number of cycles to failure, in thousands

28

-rutile welding was applied. The tests were repeated tension. Advantage for the higher strength steels was mainly found in the low-cycle region. It amounted on the average, to some 40 N/mm2 at 3.10 cycles. For the specimens containing a hole + notch it was only 20 N/mm2 in the low-cycle range, (3.l0' cycles).

Boilarii /63/ found that quenching and tempering, - which increas-ed the yield point from about 430 tollO N/mm2, - did not improve the fatigue strength of a C-Mn steel.

Munse /42/ states that "the fatigue crack propagation character-istics of steels seldom vary even for a wide range of materials and other parameters".

Gurney in /80/ found that the fatigue design rules of most coun-tries result in little benefit for users of H.S.-steels.

Exceptions were both Gèrmanies and Sweden. A reasonable and real-istic item of these rules was that the advantage to be obtained by using H.S..-steels was greater the better the weld.

Paper /86/ by Harrison et al. atiributed the relatively low fatigue strength of H.Y.-steels to "small sharp defects at the weld.periphery, derived from the welding slag (slag intrusions)". These defects exist in the majority of conventional weld

pro-cesses.

T.]'.G. andS.A. weids'showed the best appearance but T.I.G. welds behaved far.better. Christopher Crabbe and Cargill in /87/.

io) Watanabe et al. /100/ reported results of reversed bending fatigue tests of machined plate specimens of transverse welded joint of a HT8O steel (a, 765 N/mm2). The 'endurance limit of the base metal was slightly higher than that of the weld metal. A crack generally initiated at the softened zone in thè cases that the applied

stresses, were comparatively high and at the fusion line in case of'lower applied stresses. They proposed the next formula for the fatigue strength on.the basis of Vickers hardness H and grain size D (mm) as follows:

0.017 H + 5.3 + 1.1 H + 34 (N/mm2).

11) Effects of angular distortion and offset in thickness direction on the pulsating fatigue strength of 22 nun thick butt welded joints of five high strength steels (a 3L9, 882, 716, 716 and 963N/mm) were investigated by Kurlyama et al. /101/. The re-suits showed a linear relationship in a log-log plot, between the amount of angular distortion and the failure life (fig. 8).

g. Experiments with structurai models.

An extensive investigation has been carried out by Mataba and Kawasaki /53/ with bilge structural models of bulk-carriers. Programmed fatigue loading was applied. The models were on reduced scale which seemed to have little effect on the results. A summary of the test data is given in fig. 9.

Careful studies had been made about the aspect of crack propagation. (See fig. lo).

Another important investigation by Saete and Sys has already been dis-cussed in section lIc,, /6/.

Fisher et al. /5Ll/ have conducted nearly L0O tests on flexural members of three grades of teel.

They concluded that:

Stress range is the dominant variable for the welded details tested.

Structural steels with 250-700 N/cm2 yield point did not exhibit any significant difference in fatiue strength.

e) Partial length cover plates have a very significant effect upon the fatigue.resistance of a flexural member.

Fig. 11 is a summary of the results.

It is not so long ago that the increase in height of deep webs as a consequence of the increase of the dimensions of ships led to

troub-19

L:

o

0E

o

o

E

_o

t I

N

A

_oNc

Offset (mm)

o

2.4 to 3.0

O

2f. to 3.0

S. I J t I

Mark Specimen Smáx(N/mm2)

SM5OB

Joint

186

O

A

HT8OA

Joint

284

2x10'

5x104

2x105

5x105

Number of stress cycLes

(N)

Fig. 8

PULSATING FATIGUE STRENGTH INFLUENCED BY ANGULAR DISTORTION AND OFFSET

IN THICKNESS DIRECTION

o -T YPE o . T YPE 4.0 o o o LI4ICAVPU \

*

NLLL

C 'T' c'l.:..c £ 7.0

s'

s s s V

\'

-\s D- - - - DIL OP pp

\

'. CORDIER Typp

'

_s:::... P - ---MDpK 0E PULP

'

---3.0 CAERlE.,, TYPE I s - - .. "EDIL OF .. -IHIP TYPE _{I '.} ... t--- AT CROCI((Dl.IG

2.0 at P.0 LAC!Ç --.ICALI pN PAo4

I1IE - ACArI nOICoa(js 13*3 ACTV4L JHIP

i.0 _i

o

J

4OTlA!.?

N

40 60 80 lOO IZO 140 160 180 200 220 vo y A cîE NO

FIG.9

SUMMARY OF TEST RESULTS

e

r

I

10.0 q.0 8.0

00 lo ¿0 so 40 50 2Q lo

FIG.1O ASPECT OF CRACK PROPAGATION .(RIGHT ANGLED TYPE)

CR/ICI( Of WELD 10E (-SIEPE)

0 CRACK OF CENTER OF WELD(-5 TYPE) O (RuCK OF CENTER 01 00

qe CRACM 0F BU TOE

70-60 _{Rotted beams} 50 40 -

-t..

4A Lb 0 O A . AA A A 30 OLI I) A A A%q A A

- i:.

20 - o

-

WeLded beams End weLded cover pLates

Mean regression Line

95 % Confidence Limit

for. 95% survivaL.

o

iii

I

II ¡iii-

I

Ii iii.I

0,07 0,1 0,5

1,

5 10

CYCLES TO FAILURE (106)

FiGli SUMMARY OF FATIGUE TESTS OF WELDED BEAMS.9

-

1'

314

-les, as a consequence of the fact that the thickness of- the webs had

not been increased correspondingly. In this connection experiments by

Patterson et al. .155/ and Parsanejad and Ostapenko /56/ with about

lo ni x 1,25 m girders are of interest.

The results particularly demonstrated the importance of initial

out-of-flatness of the webs. Fatigue mainly developed due to lateral

flex-ing of the web plates.

Toprac and Natarajan /57/ conducted fatigue tests with hybrid plate

girders (M.S. web + Q.T. flange), 36 Specimens of 12 X 0,9 in were used.

The effect of openings in webs of rolled sections was studied by Frost.

and Leffler /58/. The sections were 130 cm deep.

Investigations by Akita, Matsui and Uchino /102/ in which fatigue

de-sign diagrams were obtained for tube-to-gusset joints with the stress

ratio as a variable,, experiments by Maeda, Uchino and Sakurai /103/,

/1014/, and tests by Saiga et al. /105/ are of interest in relation to

the fatigue strength of offshore structures.

Teramoto and Matoba /106/ carried out in-plane bending and axial

load-Ing low cycle displacement-controlled fatigue tests of pipe connections

of SM5O steel (a

363 N/mm2). They consisted of 6 inni thick branch

y

. o

pipe welded with an intersecting angle of 90

or 60° to 6 nun thick. main

pipes of 1400 mm inner diameter. The diameter ratio of branch to main

pipes was 0.6 or 1.0. The strain amplitüde at the crack initiation

point was estimated, from the measurement of the strain distribution,

and.plotted against crack initiation life.. All results showed little

scatter, and the median line almost agreed with a prediction curve

pro-posed by lida on the basis of strain-controlled fatigue tests of

hour-glass shaped specimens.

Joint studies under sponsorship of thé Shipbuilding Research

Associa-tion.of Japan should be mentioned here which cover many subjects /107/.

In /108/ reported a statistical. survey of cracking in welded joints of

ship hull in. service,, experimental study of the effects of root gap as

well as penetration of fillet welded joint and offset in thickness

di-rection on fatigue strength of 8 to .12 min thick mild and 600

N/mm2 high

strength steels and the fatigue sft'ength of oblique cruciform joint and

cruciform joint with boxing at scalop. Effects of flank angle and radius

at the toe of the reinforcement and the effects of welding position on the high cycle fatigue strength were also investigated. The report deals also with the linear relation between the fatigue strength re,-duction factor on crack initiation life basis and the theoretical

tress concentration factor ranging from 1.08 to 10.8.

in/109/ bending fatigue test results on scale models with a wide vari-ety of configuration of slots, back brackets, òollar plates and stiff-eners in transverse member are described.

The report /110/ is of interest in connection with the propagation be-. haviour of a fatigue crack from a through notch machined along a toe of the fillet weld of two stiffeners which are welded on each face of a plate with the inclination angle of 'i5° to the loading direction. Effect of angular distortion on the fatigue strength was investigated

on transverse butt welded joints of mild, 500 N/mm2 and 600 N/mm2 high strength steels /111/. Results reported in /112/ are related to fatigue strength of a mild steel under corroding environment of wet-and-dry sait water.

In /113/ tests simulating the repetition of impact load due to breaking waves are reported. The specimens, notched plates with a welded stiff-ener and notched panel models, were subjected to repeated dropping onto a water surface or repeated impact loading by release of spring load until the initiation of a crack. Also scale models of hull plate, 1800 mm in length and 2850 min in width, with longitudinal and transverse stiffeners were loaded to the repetition of one side surface pressure.

h. Influence of mean stress and mean strain.

In /70/ Abrahainsen has presented a simple formula for the permissible maximum stress in the midship section with respect to fatigue:

a a (1 + y . a + y

.

a.

.. (1)

endurance limit (apparently for stress ratio R -1, i.e. mean stress a O)

m a actual mean strèss

m

y parameter for Tùean stress influence. Further:

where:

a permissible stress

M wave bending moment (characteristic value)

still water bending moment (characteristic value) W midship section modulus

6 = stress concentration factor.

From (1) and (2) the required section modulus follows:

M +M

W SW 6

a m e e

In /71/ Petershagen and Nibbering have shown that this formula is not in agreement with formula 6 of the paper:

M +M

W SW 6 M a sw e

1+yw-.

-w For as

.M +M

W SW

w=

w 6. ... (2) 36 -a M M .6 a

m sw_ sw

W rn -W M .6 - M .6 e w w

W.

w

W > i follows.

This may be dúe to the fact that Abrahainsen has supposed that:

M a

sw._ in

M

a'

w e

thereby overlooking that a is determined by design plus actual load

(M), while a

is determined by the design only.

For y is taken 2/3. The stress concentration 6 is taken 2. Of. course for both numbers applies that in reality they will not be the same du-ring the stages of crack initiation and crack propagation. Systematic research in this would be welcome.

In /88/ Friis et al. recommend Gunn's method for unnotched specim-ens.

am

m

a =a

a ao _GB

a alternating tension compression strength GB tensile strength.

For notched specimens more complicated formulae are given. In gen-eral the sharper the notch, the smaller the influence of mean stress,

(fig. 12).

Effect of mean stress on the deformation behaviour in low cycle fatigue was investigatedby Yamanouchi, Asada and Imamura /114/, who

concluded that the logarithm of mean stràin increment per cycle is linear to stress amplitude, with mean stress as a parameter.

As for the effects of mean strain and strain ratio on the strain cyc-ling fatigue, lida et al. /115/ obtained linear relations between the relative mean strain and the relative strain range in case of shorter

250

200

E

i50

z

I

(1)50

o

150

100

50

Tonf lin2

Kf = 1,0

FIG.12 FATIGUE DIAGRAM FOR MATERIAL

_{1411 at N=107 cycles.}

10

15

O Parent material Kf =1,06

X Butt weld K=1,7

Filtet weld Kf =3,5

Material 1411

= 460

N/mm2 (29,8tonf/in2)

Gs =253 N/mm2 (16dtonf/in2)

Gao =187 N/mm2 (12,ltonf/(n2)

M =1,7

O 50 100 150

200

250

300

Mean stress, Cm N/mm

15 10 UI C 5

w

w

N co posed by them. i - N /N W1. (e + e ) - W, (e ) 1W c co j tm pa pa f where:

visible crack initiation life in. case of the mean strain of e

tm

visible crack initiation life in case of no-mean strain

W, (e + e ) strain energy required to strain to (e i- e )

a tm pa tm pa

Wf strain energy required to static tensile fractUre

W., (e .) strain energy required to strain to e

. pa pa

e plastic strain amplitude pa

e = mean strain. tm

i. Improving fatigue strength.

in /86/ Harrison et al. reported that"T.I.G. remelting of fillet welds--, undercuts resulted in marked improvements, (at 2 X _{106 cycles from. 110}

to 260 N/mm2).

They also studied deep grinding (0,5 mm) and peening which both proved to be very successful. A cost comparison was made with the. following spectacular result: peening i

TiGmelt

3 .

deep grinding 110.

Fig. 13 shows that peening was stili successful at really high stresses. This is surprising in view of the fact that the. result of the treatment is due to the creation of compressive residual stresses which will be greatly relieved at high stresses. . Christopher et al. in /87/ confirm the beneficient effects of peening in the low cycle range.. -Salkin in /89/ mentions work of Kudryavt.sev and Chudnovski /75/ in which

600

500

400

300

18 16 C

q

_C

o14

-

10

14Q

-3 4. 6

Endurance, cycles

FIG.i3 TFATIGU.E TEST RESULTS FOR MMA FILLET WELDS BEFORE AND

AFTER PEENING THE TOES.

As welded scatter

band.

3456

38

T Failure at

36 P Plate fallu

R Failure at

34 D Failure at

.'

)C10

.2

3 45

-

32

30

28

- 26

24

22

20

ali steels and structural details only between.m 2,8 da of: 3 was generally acceptable. For the relationship

it was fòund that c varied fróm 0,134 to 0,501 X.

1013

(for lack of penetration defects)...

.haimnering of welds was reòornmended. ' .

,Hotta et ai. /73/, Takahashi et al. /74/ and Kanazawa et al. /116/ also reported good results. of TIG and MIG welding. .

j. Crack propagation.

The interest of experimental results in the field of crack propagation has been emphasized in section lIa.

Several important papers connected to it were discussed in section 11g ("experiments with structural elements"). .

As a start for this section it is useful tO quote from /64/ by Crooker and Lange that "there is an abundance of evidence to support the mise that fatigue failure is, in large complex welded structures,

pre-dominantly caused by crack-propagation originating at pre-existing de-fects, which are too small and too numerous to eliminate".

Although lida is not in favour of a pure crack propagation approach for, shipbuilding, one of his papers /78/ illustrates well the great diffic-.uity of distinguishing bet ween initiatioti and propagation.. He strain-cycled electrolytically. polished 10 mm dia. hour-glass shaped. specimens of steel with a tensile. strength 600 N/mm2. Microcracks 0.01 mm length developed, already after 10-24% of the total life.

Lawrence. and Radziminski /65/ found for H. S. steels good agreement be-. tween the basic fracture.mechanics - crack propagation relationship and the .fatigue behaviour for joints with internal defects. But in order to suppress large scatter of the results it is required to eparate number of cycles to initiation and for propagation resp a

In /26/ Ekstr6m and Munse found for welded specimens with defects that the initiation period occupied about one half of the fatigue life; Hirt and Fisher /27/ found fr weided.beams with porosity some 75% for the

life from initial 'défect to a visible crack; the power of .K. varied for and 3,3.. A value.

c

(K)m

in /26/ and m was 5,8

10 E 6 s C G' C

o

o. X w

i.:

N/rn

-o

--o

o

___Q_____Q___

500

1000

1500

2000

.01

i

..,

Structural steels

F i I

50.

100. 150

'200

YieLd strength (ksi)

o

o

00-00

FIG.1'

_{SUMMARY PLOT OF CRACK GROWTH RATE EXONENT VALUES}

VS YIELD STRENGTH FOR A BROAD SAMPLE OF STRUCTURAL

STEELS,(REFEREÑCES 18-33.). of (91) CROOKER AND LANGE

.60

50

w-

'4--I

t40

20

;io

_u

cv L

o

p o QT 35 normal fracture

Ql 35 short transverse fracture A NY 80 normal fracture

A NV 80 short transverse fracture

Marre! normal fracture

O Marre! short transverse fracture & Buffered weld

& Grooved and buttered weld P Peened along weld toes

GB o P o P P A

_A

B8t

A

La

A O A

°

o-B P

A

¿AA

O

₀₆

o

A i i i 0 10

20

.30

40.

50

60

Fuit width crack as percentage of total life.

70

80

90.

FIG.1S.

DATA RELATING FATIGUE CRACK INITIATION AND FULL :wIDTH

-.

CRACK TO TO-TAL -LIFE.

G

LIê

-Jerram in /90/ also concluded from a literature survey and own experim-.ents that for notched steel specimens only the crack propagation part

of the. fatigue life is important. In most cases the. power m of K seemed to be equal to 4.

Fig. l4 of /91/ by Crooker and Lange shows the value of m for a number of. structural st eels of different strengths. It is indeed surprising that an average value of 3 applies for steels of widely differing

strengths. . ..

Fig. 15 of /87/ by Christopher et al. is also very informative about the relative number of cycles spentfor initiation and propagation resp.

Maddox /69/ has not found that the curve v.lòg K is stress dependent. His work further confirmed the validity of the crack-propaga-tion approach for fillet-welded specimens. For a 13 mm thick specimen, the number of cycles for initiating a crack at the toe of a weld and to grow it up to 1 mm is very small as compared to the total life.

Nibbering and Lalleman /21/ investigated crack propaga.tion in H.A. Zones of E.G. and E.. S.. welded., Nb-normall.sed steels of 314 and i46 mm thickness, in accordance with Gurney and others they found "two-branch" curves (fig. 13 of /21/). Great variations in rn were observed depending on the condition of heat-treatment which .served as a simulation for the welding. influences,. For the upper branch m varied from 1,2 to 4 and for

the lower one from. 2 to 8. Striking was the large difference in. crak propagation behavidur cf the heat-simulated material and the H..A..Z. A difference. in cyclic strength of 20-40% was observed for the. region between lO5' and 106 cycles in favour of the H.A..Z1. (welded) specimens.

These results. were attributed to the influence, of the welding stresses. Luith'le' /66/ has studied crack propagation in multi-stage fatigue tests. Elber /67/ investigated .the effect of crack-closing on crack pro-'.pagation.'Saal /68/ 'developed.in this connection an effective strèss

intensity factor, taking into account the residual stress caused bythe

initial load. - .

Shingai, Matsumoto and Imamura 1117/ investigated crack propagation in two stages and multiple blocks pulsating fatigue tests of 24.0 mm wide

of 12 to 20 mm thick, and found m value of 3.0for both steels.

in /118/ Yamaguchi studied the propagation behaviour of a crack started from a notch in a web plate of a box girdar, which wassubjected to bending fatigue. A paper by Nagai. et al. /119/ is of interest which concluded that the incubation period in crack propagation was longer. for a mild steel and very short for two high strength steels.

k. Welded connectiöns.

A lot of experiments have already been discussed in previous sections. To these may be added the following,.

Yamaguchi /77/ has made experiments with fillet welded cross joints. One result was that the heavier thefillet weld., the lower the fatigue strength.

Paper /88/ by Friis et al. gives a lot of results for welds in M.S. and H.T,S.

Fig. 16 shows the great influence of weld e'ectrode type.

Harrison in /92/ reports low cycle fatigue experiments on trans-verse and longitudinal butt welds and non load carrying fillet welds

in 5 H.S. steels.

-Instead of the more or less established relation c .N e Cc

1/3 p p

plastic cyclic strain), he found _cT.N c more valid _(cT total

cyclic strain).

For f iflet welds he came to a similar conclusion as the Swedish in /88/ that when strain range x strain concentralion factor is vertically plotted in a a-N figure., the results correspond to those for plain mat-erials., (strain cycling). . .

For longitudinal fillets in load-controlled tests .the fatigue strength was about equal to the tensile strength

forN < l0.

'450 400;

350

300

E

Z

s X E

b

200

-c

.4J C .4.J Ill w Li.. q 100

i Parent mater at HIS. 36

2 Butt weLd 0K 4830

3 Butt weld 0K 3875

4 Butt weLd 0K 38-75/A

5 Butt weld 0K 3865/5

t t I

iü

L

io6

Number of toad cycles

30

25

20

15

FIG.16

S-N CURVES FOR FINE GRAIN OR MICRO-ALLOY STEEL HTS.36,

PARENT METAL AND BUTT WELDS.

PULSATING TENSILE FATIGUE.

c'J

C C

Paper /87/ by Christopher et al. is especially concerned with H.T. Steels. Fig. 17 summarizes some of the results.

Takahashi et al. /120/ carried out pulsating tension fatigue tests on five series of transverse cruciform K butt welds and obliquely

crossed K butt welds. One of the conclusions they reached is that the endurance limit of the obliquely crossed K butt welds with intersecting angle of 40° is equivalent to about three quarters of that of the cru-ciform K butt welds. An extensive joint research was made ori the fatigue

strength of a plate with transverse fillet welded gussets /121/. Base metals were a mild steel and three high strength steels (o = 392, 530

and 598 N/mm2). lilmenite type as well as low hydrogen type electrodes were used for fillet welding. A buckling preventing jig was developed in order to carry out fatigue tests with the stress ratio ranging from -1 to 0.5. The results showed remarkable effects of electrode type and geometry of fillet, and led to construction of fatigue design diagrams.

1. Influence of corrosion.

In /72/ Kaufmann has investigated the effect of load interruptions on fatigue life in corrosive atmosphere. In contrast to what is generally found, viz, that load interruptions enlarge the fatigue-life, appreci-able reductions were observed (up to 50%).

In /122/ Minami, Ogawa and Kirnura reported the results of rotating bending fatigue tests on a mild and a 600 N/mm2 strength steel in cor-roding environment of salt water. The effect of superposed impulsive loads was found to result in a considerable decrease in fatigue life. Reversed plane bending fatigue tests of submerged arc weld metals of three high strength steels (o 334, 412, 540 N/mm2) were made in 3%

salt water by Nagai, Muraki and Akaishi /123/, who found the lowest fatigue strength in the coarse grained zone of the welds. A study by Takashima et al. reported in /124/ showed considerable influences of sea water. For transverse cruciform K butt welds the endurance limit decreased by the environmental condition of sea water from 127 N/mm2

40

30

20 .4-C

2

s

t'

C.-ru

j40

O t I

Axial butt weld

Kneebend tee butt weld'

'

Axia through thickness

O

N

N

N

Number of cycles, log scale

FIG.17

GENERALISED RESULTS ÊOR 01 35 TYPE TESTS (ZEROTENSION STRESS CYCLES).

I I I

I I I

Four point bend butt weld

FOur, point bend through thickness

'-Three and fou

point bend

through thickness.

I I I

600

to 49 N/mm2 and from 98 N/mm2 to 59 N/mm2 for obliquely crossed K butt welds.

m. Prediction of low c ele fatigue strength.

Available papers were published on the subject of prediction of low cycle fatigue strength. Ando, lida and Soya /125/, /126,1 investigated the correlation of crack initiation life of a notched wide. plate, with

that of a small size specimen. Wide plate specimens, 13 mm thick and 200 mm wide, with a central notch of which the theoretical stress

con-centration factor was 5, 10 or 14, were subjected to repeated axial loading. Hour-glass shaped specimens., 8 mm in diameter and of the same material as used for the wide plate, were loaded so as .to dup1iate into an hour-glass specimen the cyclic history of' longitudinal peak strain at the tip of a notch in wide plate. Good agreement was observed' between both lives of notched wide plate and hour-glass specimens, (fig.. 18).

SiÑilar tests as those by Ando et al. were performed by Fedorov /127/ on 80 sim wide specimens. An investigation by Nagai, Iwata and Nishimura /128/, which deals with the result of Moira measurements 'of cyclic be-haviour of the plastic strain field aroi.md a dull notch in 24 mm. wide

steel specimen, is also of interest.

In /129/Coffin suggested that a strain range versus failure life dï-grain could be represented by a 2-slope relationship with a transition

òccurring at approximately 50 to 100 cycles.

It is of interest to refer to the work 'by Hotta et al. [130/, ¡'131/, which covers reversed strain cycling tests on 18 ferritic materials. They found that strain amplitude versus life curves for some materials showed a 2-slope relationship with a transition occurring at.the strain amplitude level corresponding to the yield strain. One of the conclu-sions is that such a 2-slope phenomenon is closely related to the

ten'-sué

strength above 600 N/mm2 and the metallographic structure of bain-ite and martensbain-ite. Steels of which the tensile strength was less than 600 N/mm2 exhibited a one-slope relationship. They pointed out that the four points method as well as the universal slope method proposed by

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,

,

CRACK INITIATION LIFE OF BAR SPECIMEN

FIG. 18

CORRELATION BETWEEN CRACK

INITIATION LIFE OF NOTCHED PLATE

AND THAT OF HOUR-GLASS SPECIMEN

FATIGUED BY SIMULATED STRAIN

CYCLING.

L

4

Nc

PLATE SPECIMEN

STEEL

Kt

MARK

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Manson remarkably overestimate the fatigue strength.

lida /78/ reconfirmed the dependency of constants in the following re-lation, which was proposed previously, on the static tensile properties:

-k - -k

0.35 c (N ) + (0.000107 a - 0.0021i4)(N e ,

ta f c u e

where:

Cta longitudinal total strain amplitude Cf = static fracture ductility

ultimate tensile strength (kg/mm2)

visible crack (0.2 - 0.5 mm long) initiation life k., ke material constants depending on yield-to-strength

ratio (figured from diagrams).

As for the notch effect in strain cycling fatigue, a paper by Ilda, Urabe and Ando /132/ is of interest which leads to the conclusion that the fatigue strength reduction factor in strain cycling fatigue is ap-proximately equal to the plastic strain concentration factor at a notch.

-52-Section III. BRITTLE FRACTURE.

'General.,

in the field of brittle fracture this committee faces the same problem as in the\field of fàtigue: the literature which has appeared the last 3 years is. so extended that 'afair and justified sélection would re-quire far more space than in the present report is ávailable. This ap-plies particularly to fracture mechanics. In Great .Britain and Japan much attention has been directed to the C.O.,D. approach. in the U.S.A. the path-independent J-integral is thought to be a very promising tool for situations of extensive yielding..

In the 1.1,. W. significance of defects and the search for reliable test-ing methods obtain much attention. The same pplÌes to weldabilityof very high yield.steeis. and, steels for very low temperature applications and also to' the problem of lainellar tearing.. These' latter subjects will not be treated in this report, partly 'because they.are too specialized

and partly for reasons of space. .

. Significance of defects..

A discussion on this subject cannot, stand alone and should be accom-..panied by considerations of overall structural approach. Danger of

fatigue in connection with brittle, fracture., use of fracture mechanic non destructive testing, welding technoIo', codes, use of crack-ar-resting steels etc. It would go 'too far to handle all this systematic-ally and thoroughly; it is. believed that a mere. illustrative discussion 'might 'be as. informative and more easily understandable.

Starting with fracture mechanics,, which by many has 'been 'welcomed as the key for the disclosure of all fracture problems - ., it has

unfor-tunately become clear that crack or defect length is 'often a parameter of secondary importance in the whole problem of 'fracture-safe design,,

(see hic).

Of principal importance are welding technology parameters, of which the effects however cannot be qùantified easily. Hat input is the most im-portant of these. It determines the fracture. toughness and the extent