Interim-report on low cycle fatigue investigations with ship structural components made of higher strength steel, Committee Fatigue et Construction Type, Liege, 1973

LABORATORIUM VOOR

SCH EEPSCONSTRUCTI ES

TECHNISCHE HOGESCHOOL

-

DELFT

/

RAPPORT Nr.

SSL 178

BETREFFENDE:

Interim-report on low cycle fatigue investigations with ship structurai components made of higher strength steel.

Ship Structures Laboratory Deift University of Technology Mekeiweg 2

DELFT - The Netherlands.

INTERIM-REPORT ON LOW CYCLE FATIGUE INVESTIGATIONS WITH SHIP STRUCTURAL COMPONENTS MADE OF HIGHER STRENGTH STEEL

Compiler: ir. H.G. Scholte

Interim-report of Dutch part in the - EGKS program 6210-55/2/162

-of the Commit-tee -"Fa-tigue et Construction Type".

Prepared for the meeting at Liege inOctober 1973.

V

Report no.

Introduction

This second interim-report is restricted to the progress since May 1973

of the low-cycle fatigue investigation at Deift on ship structural

com-ponents made of. higher strength steel,. corresponding to the ECKS-program

6210-55/2/162. ..

As already mentioned in the first interim-report /1/ the investigation has not been limited to the original program /2/of low-cycle fatigue but,also the influence of low temperature upon fatigue and the possibility

of brittle crack initiation have been included.

This of course meant a. complication for the test procedures and caused

time delay. . .

.

Regarding the total progress of the Dùtch part of the investigation., the

testing ofthe last specimen is expected tObe finished at the end of

October 1973. . . .

With regard to the. results as reported in. the first interim-report, a

further analysis of the crack surfaces was made. Together with the data of specimens 6, 7 and 8 these crack analyses led to the necessity of preparing a new diagram as presented in this report. .

With respect to the investigations carried out in Belgium., France, Germany

and Italy it was not possible to compare already these results in a more

detailed way with. the results of the full-scale test carried out in Deift.,

or to build them in by further analysis Ïnfd thè results .of thfufl-scaire

specimens for a further theoretical compilation..

2. Test procedures.

The test procedures for specimens 1 to 5 are already given in report /1/. For specimens 6, 7 and 8 the following data can be mentioned.

Specimen 6 was tested under .a nominal fatigue stress of +1760/-880 kg/cm2

at a temperature--of -37°C. The testing was stopped when very large cracks

3. Test results.

T.htest results arie given in the form of diagrams with W6hler curves. in figure 1 and 2 the points of 0, lOO and 500 mm2 crack area are presented for bottomplate and brackets. Looking for these points with their location as determined after a thorough analysis of crack surface and development

of crack area it is clear that the differences in result between the

spec-imen 'tested at low temperature and' the specspec-imen tested at room temperature

have to be considered as significant. So two lines can be drawn for the different phases of the development of the crack area; one line for room

temperature and one line for' a temperature of, about -37°C.

In figures 3 up to 8 the lines of 0,, 100 and 500 mm2 are compared for the

earlier results of St. '+2 at room temperature and the new-found lines for

St. 52 at room temperature and at -37°C.

Comparing these lines of St. 52 for room temperature and temperatures of -37°C, we see that they are in good agreement with the results of earlier had developed. During the fatigue test, impact loading was generated

after regular intervals of about 1000 load cycles; The impact loading was carried out with a dropweight of L5 kg at heights of

1,5

meter in

such a way that the specimen was hit at the top of the bulkhead plate, just when in fatigue loading the amplitude reached a value of about 95%

of maximum tensile load. In this way, any critical point in view of brittle

crack initiation due to sensitiveness to impact at lowtemperatures should

be discovered.

The specimens 7 and 8 were tested at room temperature under a nominal fatigue stress of 1-810/-LW5 kg/cm2 respectively +1260/-630 kg/cm2.

Specimen 7 was tested until the point that very large. fatigue cracks had

developed, while fatiguing of specimen 8 was finished when cracks had de-veloped With estimated values of crack surfaces between 100 and 500 mm2. After the fatigue test specimen 8 was loaded under axial tensile load of

4

investigations as mentioned by Munse /3/. Extrapolating the data mentioned

by Munse the decrease of the temperature from i-20°C up to -37°C may effect

an increase of the fatigue limit of 20-30%.

Comparing St. Li.2- and St. 52 under fatigue at the same temperature (room

temperature) it. can be stated that the results confirm the expectatioñ that

steels with higher yield point and ultimate strength would not behave much

better under fatigue loading. In this way the results are -in entire

agree-ment with, the conclusions of Committee 11 of the 5th International Ship

Structures Congress /14/. These conclusions are based largely upon the

in-vestigat'ion of Fisher and others /5/, who have conducted nearly 400 bending

tests of welded beams with'3 grades of steel resulting in- the- -conclusion

that "structural steels- with yield- points between 250. and 700 N/mm2 did not

exhibit any significant difference. in fatigue strength."

At first sight it may appear that there is some difference between the.

-- results of St. - 52 and St,. 142 for 0 mm2 crack area.

It should be kept in mind that especi-ally.crack initiation is not.easily

definable and the determination of-the initiation point is somewhat more arbitrary. Even with extrapolation of the curves of crack growth the initi-ation almost never can be determined with the same accuracy as can be done in the case of the propagation through the points of l00and 500 mm2. Besides, differences in :cr< initiation between specimens of different steel will be caused by difference in welding details and imperfections of the weidment.- In the same way there will exist more scatter in crack miti-

--ation for -the--s-ame--'kin&-of'- spec-imen--However- -t-he-

-l-ines-of-zero-crack-sur-f ace are in -l-ines-of-zero-crack-sur-fact o-l-ines-of-zero-crack-sur-f no practice value out o-l-ines-of-zero-crack-sur-f a viewpoint o-l-ines-of-zero-crack-sur-f strength.

But although the initiation: in St. 52 seems.to happen somewhat earlier,

the behaviour during crack propagation seems to be somewhat better when

looking for the lines of 100 and 500 mm2. However the differences in.

behavi-- our are rather of practical interest. -

-With- regard to the investigation in Belgium, France, Germany and Italy w.e

must state, that a good comparation of the -results could not be made- up to

- now, partly due to the fact that not all the tests are finished and not all

the results are available, partly due to a' lack of time and to the fact that

many results of the investigations of the other countries have 'to be- adapted

to another form of presentation before- a comparison and total analysis will

be possi-ble. However, as far as a comparison could be made already the

References.

suits are in very good agreement with each other. As well the results of

'France as reported in /6/ as the results of Germany reported in /7/ give

the same conclusion; there is no significant difference between St. 52

and St. 142 in behaviour under fatigue lading.

/1/ First interim-report of Dutch part in the - EGKS program 6210-55/2/162

of the Committee "Fatigue et Construction Type".

H.G. SchoLlte:

"Low cycle fatigue investigations with ship structural components

made of higher.strength steel".

Report no, 175, S.S.L. De.lft,Aprii 1973'.

/2/ J.J.W. Nibbering:

"A program for low-cycle fatigue investigations with ship structural

components made of higher strength steel".

Report no.' 1142a, S.S.L. Deift, March 1970.

j/"Fatigue of welded steel structures".

Prepared by W.H. Munse.

Edited by LaMotte Grover.

Welding Research Council, New York, 19614.

/14/ 5th International Ship Structures Cóngress, September 1973 - Hamburg.

Report of committee 11: "Fat:igue and brittle fracture".

/5/ J.W. Fisher, K.H. Fränk, M.A. Hirt and B.M. McNamee:

"Effect of weidments on the fatigue strength of steel beams".

NCHRP Report no. 102, Highway Researcth Board, 1970.

/6/ "Compte-rendu d'essais de fatigue en traction ondule et

haute resistance".

Par Lopard - 10-5--1973..

Ministre dttat chargé de la défense nationale, digation minist-nelle pour l'armement, direction technique des constructions navales.

/7/ "Vergleichende Untersuchungen an gekerbten Proben aus Schiffbaustahl

GUtegrad A und hherfestem Schiffbaustahi DH 36",

von H. Paetzold.

6000 5000 4000 3000 2000 1890 1790 1000 1500 1200

I

o

uz

o

z

w w io.2

703-08--703-05 703 02 703-07--N

--\ o

o

ç Number of cyc'es

Fig.'

2.

Fatigzelinès for brackets of H.S. steel.

Ordinates are total. ranges of cyclic strain

Fatue test ended

Extrapolated vaLues 73 -017 3000 - Specimen N 2800 703-03---2640

703062500 -2350

70304

-2190

70301

-8000 7000 9000

Crack area is ZERO

--- Crack area is

100mm2

EJ

Crack area is 500mm2

-D Test

temperature

+20°C

Crack area is ZERO

area is 100mm2

--'--Crack area is 500mm2

Tesi:

terperature

-37°C

T tn

2

i

C X C L I-. u,

w

w

z

o

II

z

o

z

w

w o.2 Number of cycles

Fig. 2.

Fatigue lines for bottom of S.S. steel.

'\

,49 % o.

D

t z' I I I l-_J io I I ¡ I I 10.

Ordinates are total ranges of cyclic strain

O

Crack area is ZERÓ

--- Crack area

is 100 mm

Crack area is ZERO

area is 100mm2

Q

Crack area is 500mm2

Fatigue test ended

ExtrapoLated values Crack area is 500mm2 Test temperature

20°C

Test temperature

-37°C

73-018 30004 Specimen 7000

1Q00-

_{2800 --703-03---}

-2640

--703-06--10000 6000 2500 9000

2350

--703-04---2190 --

703-01 8000 -- 5000 2000

1890

--703-08-7000 ₁₇₉₀

-703-

OS-4000 6000

1500

--703-02-5000 3000 1200 --

703-07---4000 1000 2000 3000

St.42

Test temp:20°C

Number of cycles 44

_St52

4

tjest temperature:-37

4

at

St.52

'N.

Test temp:20°C'

e I I I I I I io.4

Fig.

3

Fati'ue 4inesfor bracket.: crack area is

o

Conroarisbn of H.S

steel with mild steel.

I I I

-i

I I I

73-022

3000 -2500 2000

-i

o E ,.w 1500 C L. in 1000

o

bZ

o

Z

w w io.2

3000 2500 2000 'rs o E 1500 C X. C I-1%) 1000

o

s

10.2 St. 42

Test temp+20°C

I I

liii

1Q3 Number of cycLes

Fig. 4. Fátigue lines for bracket:. crack area is 100 mm2..

Comparison of H.S. steel wit1, mild steel.

s

I i i St.52 Test temperature:_37Ct St.52 Test temp.: +20°C i

Iii

73-023

3000 2500 E

-. 2000

o, -x u., o E

,

1500 O) C X C L 4-, Lfl 1000

io.I

Number öf cycles St.42

Tst temp:+20°C

_St.52

% Test temperature:-37°C i J

11111

11JIIJJ

I.

I

11111

io. l0 io5

Fig. 5.

Fatiue

lines

for bracket: crack area is 500

Comparison of B.S. steel with mild steel.

St.52

Jest temp.:20

3000 -2500 2000 1500 1000

o

o

z

w w o. j G 4

'j

'

St52

\Test temperature :-37

St.42

Test temp :20°C

'

4 St.52

Test temp:20°C

¡ I 1:1 J I i I I i i I i i I i L i i

io. io.4 io.5

Number of cycles

Fig. 6.

Fatigue lines for bottom: crack area is

o

Comparison of H.S

steel with mild steel.

3000 2500 E 2000 UI -J -D o E j/I 1500 C X C 4-. 1000

z

_o

bz

z

o

z

w

u

St.42 Test temperature :+20 °C St.52 Test temp:20°C 44 44 4 4 4 4, St.52

Test temperature: 37 oc.

44 4 8 4 4 4 s 44444 $5 s S

t

I

III!!

I I I

IIll

I

11111!

io. io. Number of cycl.es

Fig.

7. Fatigue lines for bottom: crack area is 100 mm2.

Comparison of S.S. steel with mild steel.

2500 2000 o E .y 1500 C X

-c

L 1000

i

_o

s

- I I I. I i --- _- -i- I L I I io.2 _{r o.3 -}

--St42

Test temp -: 20°C D St.52

%jest temperature:-37°C

Number of cyctes

-Fig.

8.

Fatige lines for bottom: crack area is 500

Comparison of S.S. steel with mild steel.

St. 52

Test temp :20°C

I I t -i I

10.