First results of wide plate testing under fatigue loading at low temperature

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Report

S5i298

_FIRST

_{RESULTS OF WDDE}

PG 86-29

PLATE TESTING

UNDER

FATI:GUE LOADING AT

LOW

TEMPERATURE

Deift, 1986

TUD 'ft

Department of Marine Engineering

-

ei

Ship Structure laboratory

Technical University DeIf t

ir. H.G. Schoite

ir. E.vanRletbergeh

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CONTENTS INTRODUCTION SPECIMEN 2

2.1 Materi1

2 2.2 Geometry and instrumentation

2 TEST PAOCEDURE

3 DISCUSSION OF THE RESULTS

4

4.1Meagurement data

₄

4.2 Failure assesment evaluation

4

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Nomenclature a - c:racklength mm ac - critical cracklength mm ap - prefatigue cracIc.length mm An - net-section area B thickness mm

CTOD - crack tip opening displacement mm

Fa - reserve factor on cracklength

FK - reserv.e factor of fracture toughness

FL - load factor

K - stress intensit;y factor

MPam

Kc - critical stress intensity factor MPa'lm

Kr - maasure of proximity to LEFM failure

Lr - measure of proximity to plastic yielding,

Pf - fractùre ìoad MN

Pum

- limit load MN

Pnet - load at yielding in net-section MN

Pt:ol - tolarable load _MÑ

Temp - test temperature degree.s C

W - haïf width of wide plate mm

Ey

-

yield straIn

a - overall stress MPa

of - flòw stress (ay+outs)/2 MPa

o.net - net-section stress MPa

aûts - ultimate tensile strength MPa

ay - yield strength MPa

- d'imensíònless ClOD

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1. INTRODUCTION

Testing. wide plates under fatigue load òondition.s at low

temperature is a continuation of _{the small scale} _{fatigue bending}

tests at low temperature (fabalt-test) in the NIL, fracture

programme 1 1) . The wide plate experiments are carried out to

verify if _{the results .òf the small scale. tests are applicable, to}

components of more complexity. For this _{purpose two} _{wide plates}

are to be tested in the _{Sh:ip Structure Laboratory (SSL) of the}

De.l'ft University of Technology. This report prese'nts the first

resulte of the first wide plate that was tested.

The experiment on the first Wide. . plate has not been completed

yet. In consequence. of. some considerable time delay due to

building-in and rapairing problems, the experiments are stopped

for the time ben.g on behalf of other tests of higher priority.

The experiments will 'be continued as soon as possible.

After present1ng the test data a:nd a short discussion of the

results, the first fracture result.will als.o be evaluated Using

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2 'SPECIMEN

2.. 1 Material

The wide plate was made of a Fe 51ONb steel which is the same

steel that was usad in the small scale. fabalt-tests.

The welding procedure of. the three welds in the wide plate is

also similar to that of the small scalé tests. For further

details see [1] and [2].

2. 2 Geometry and ins'trurnen:tation

The specimen was composed of

four

welded plates as shown in

fig.1. This means that three welds can be tested under the same

load conditio'ns. Each weld was provided with a mechanical notöh

with a geometry shown in fig.2. Likethe smal.l scale specimens

the notch was also situated in the center of the weld and can be

consideradas a flat through the thickness defect.

The instrumentation of the plate consisted of strain gauges on

three plate sections and three weld sGctions, and COD spring

gauges ovez the notch. The position

of

the strain gau;ges and COD

spriñg gauges is shown in fig.3.. Strain gauges nr. 1-6 were

placed to record the s.tPess distribution of the plate. The other

strain gau:ges, in line with the notch, weré placed to give an

indication

of

the amount of crackgrowthduring fatigua at low

temperature. The temperature was controléd by 7 thermocouples.

The COD spring gauges were placed to indicate the amount of

deformation at the cracktip (fig.4).

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3. TEST PROCEDURE

The wide plate was tested in _{the 6 MN fatigue machine of the SGL.}

The e:xperiment _{started with precracicing the defects}

in the three

welds. The amount of _fatigue _precracking _and _other

test

conditions, are listed in table 1.

After precracking, the specimen was insulated and cooled to the

test temperatura by a nitrogen cooling device at the canter of

the plate and a sup:p.orting alcohol _{òooling device for both the.}

side welds. The temperature for the center weld _{was selected at}

-80

oc. This temperature was chosen to verify fracturing during

fatigue w.hich occured in the, small scale fabalt-tests. After

cooling the center weld, the side welds appeared to have a

temperature o-P -60 °C. This temperatura was adapted as test

temperature for the side welds.

The plate. was tasted at a _{frequency of} _{2 Hz} _{and with} _{a strass}

ratio-of R=0.1. The test was _{performed by} _{applying a.constant}

maximum fatigue. load of 4.403 MN. _{Adjusting the. fatigue load to}

maintain a constant stress sïtuation at the cracictip was. not

applied because of the _absence _of _a _reliable _cracklength

measuring method. This way o-P testing _{results in a critical}

region where fracture is most likely _{to occur.} _{Hodever, compared}

to small scale specimen _{this region. is considerably larger,} _and

is estimated as 30-40 _{mm for the present wide plate.}

The plates were welded in the _{test-rigs in} _{as welded condition.}

To avoid introducing secundary stresses the plates were not

stretched; Unfortunately the deflection from the load line

appeared to be larger as expected, and therefore high banding

stresses ware introduced during _{fatigue testing.. Because strain}

gauges were placed at both sides of the plate, the bending _effect

could be quantified and will be taken into account in the

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4. DISCUSSION OF THE RESULTS 4.1 Measurement data

In fig..5 the strain gauge measurements ore plotted as a function

oP the number of cycles for the variuo.s sections. As can be

concluded from fig.S high bending stresses of the some magnitude

as the axial tension stress have occured in the center of the

plate. The bending effect at the side welds is considerably less.

The sudden change in strain gauge measurement data le caused by

fracture in one 0f the connecting welds. Because of repairing

this weld and placing of additional brackets the stress

distribution appeared to. have changed. The stress distribution

over the wide plate can be il.lustrate.d by fig.6 at three

different moments during this experiment.

Fig.? shows the COD values as a function of the number of cycles.

The cracktip value was not calculated because no reliable

calculatiOn method was available for this geometry. Hcwev'er, the

COD-values indicate fracture at low toùghness values which

correspond to the resultsof the small scale tests.

The geometry of the fracture surface, shown in fig.8, illustrates

the great influence of the bending effect. The fracture surface

was examined and a fracture initiation point was located as

indicated in fig.6. The position df this point w.illbe used for

calculating the critical c.racklength.

4.2 Failure assasment evaluation

The moment 0f fracture of the center weld and the current

situation et the side welds hove been evaluated us;ing two failüra assessment methods, the COD design cùrve (3]. with the revision of

Dawe:s [4), and the' R6 Rev.3 method (5]. For this purpose the

critical c:r'acklength was calculated 'from the Initiation point on

the fracture surfoàe. The applied stress was evaluated taking.

into account the bending effect derived from the strain gauge

measurement data. '

Table 2 and fig.9 presents the relevant data and calculation

results for the COO design curve method. For

thecritical

ClOD-value use was made of the average ClOD-value at that temperature

(indicated as av.) and of the minimum ClOD-value (min.). As can

be seen the moment of fracture on the certer of the weld would

'hove been predicted safely using this methòd. in contrast With

the results of the stati,c wide plates in [6], fracture would not

have been predicted safely when yielding in the net-section would

have been used as a criterium. Comparing critical and actual

cracklength of both side welds, fracture at one of thOse welds

was 'very unlikely to occur and would have meant a very bad

result.

The relevant test doto and calculation results foi- the CEOB R6

Rev.3 method are gathered in table 3 and illustrated in fig.1O.

The, results 0f this calculation show similar results as the COD

design curve. Fracture a.t the center weld is predicted safely and

fracture occuring at the side welds Was very onlikely. The

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results are _{however less conservative compared to those from the} COD design curve.

LITERATURE

Schalte, H.G., Rietbergen, _E1 _van,"Fatigue

bending-tests at

low temperature _{on welded specimens of 30mm and}

70mm

thickness", Ship Structure _{Laboratory Report No. 302,}

Deift 1986.

Koning,, C.,".NIL-breuktaajhejdsonderzoek/Nederlöflds

program-ma. Uitnameplan en type pPoefstuIcken", TNO-rep;ort.

85M/35/08862/KoÑ[p,EV BRE 84-11, 2 juli 1954 (in _Dutch).

British Standards Institution, "Guidance _{on some method.s for}

the acceptance levels for defects _{in fUsion welded joints",}

PD 6493, B.S.I. 1980.

Dawes, M.G.,"The ClOD design _{curve approach: limitQtlons,}

finite size and application."; _{The Welding I:nstitute,}

July 1985.

Mime, I.

_{et al ,"Assessment o-F the i.ntégrity of structure}

containing defects", Central _{Electricit.y Generating Board,}

R/H/R6-Revision 3, May 1986.

[6) Rangen, H.J.M. van,."Prediction _{of tolarabÏe loads, for 17}

wide plate tests in the _{NIL-fract:u.e.program, üsing the}

design curve _{(ClOD) approach.", TNOreport} _{BSM/O11959/RON}

/SCN, BRE 85-25, 26 september _1985.

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Tb1e I.

Te5t partioulor.

Weld test prefatlgue

crack1enth

ai-ca netto load at

temp. cracklength at fracture area fracture

ap a 28W An

Pf

Oc mm mm mm2 mm2 MN

Li -60 17.3 34.8 20014 18884 4.403

L2 -80 19.8 92.3 20160 14660 4.403

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Table 2. _{CTOD design curve1} Weld Temp 28w Li L2 L3 weld Li L2 L3 oc mm2 mm2 average ClOD

O/ay

a MPo

with last revision of' Daweà :

n( 1-a/W)

Pf aret a Pnet PÇ/Pn CTOO ClOD

ay*An _av. _min.

MN MPa MPa MN mm mm p2.3 35 minimum CTOD o/ay Weld a Temp MPA °C matérial prôperties ay o-F' outs Ey

MPA MPa MPa %

266 -60 592 639 686 0.276.6 83 -80 620 666 711 0.2884 --60 22196 19690 -80 20150 14560 -60 21328 1892Ó 4.403 224 1.98 . 11 68 0.38 0.246 0.160 4.403 302 219 9.03 0.49 0.070 0.043 4.403 232 206 11.20 0.39 0.246 0.160 0.359 0.609 361 0.233 0.483 0.029 0.170 105 0.018 0.134 0.359 0.609

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Table 3. Failure assesment method CEGB R6 - Revision 3.

Weld Temp. Pf B W a Kc ay Pnet Lr a K Kr

MN mm mm mm MPa[m MPa MN MPa MPaifm

Li -60 4.403 36.8 310 35 178.1 592 11.68 0.377 223 74.4 0.418

L2 -80 4.403 32.5 310 92.3 101.1 620 9.03 0.488 285.2 161.8 1.60

L3 -60 4.403 34.4 310 35 178 592 11.20 0.394 233 77.8 0.447

Limit condition Reserve factors

Weld Lr K Plim a FK FL Fa

MN mm

Li 0.488 0.961 5.70: 97.5 0.43 0.77 0.36

L2 0.305 0.987 2.75 38.6 1.62 1.60 2.39

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connecting weLd

notched X-wetds fo be tested.

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4 Strain gages (.0.0. clip-gage Theèmö couple 3/4 P4C 3/4 7a 9 11/12 1410 lOa -/T218 T54 2

qig.3., Instrumertat.iOflQf tche wide plate.

I Irontside/bachside F /1147 (thermo couplel T3/-,A-/C.1 11/12 '7:C2/(3 (c:oo.-bridgesl

T

C.0.-bridge. (.1 C. 3 /C.2 s

f'ig.4. Detail 0f COD spring gauge Instrumentation Inst rume n ta t ion

D'l 3/6 ,/ Fitt4i C2/ 410 t1Oa -/12 +5/6 A-A 8-B

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p_ IO 2U000 L.1 L.1 30000 i..GOi.IiciOw_7 :1 j200

1.001

P.2 30000

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n 6442 P3 L.1 P.1 1.2 P.2 1.3 n =15851 P.3 Li P.1 12 P2 13 e Froottide of plate Backtide of pille P3 1.1 Pl 1.2 P.2 13 n= 22894

.fig.6. Stress distribution over the wide plate at three

different stages during, the experiment. Numbers refer to eE/1000 RN values.

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¡ nat fracture n 92 o:i Section 1.2. + (.2 .c.1 + + + 100 20000 n + + + s .30000 0,1- 0,00- 0.06- 0,04-0,02 o Section L.1 +_s

++

.

mm .c.Q0 + C.2 (.1 s n 30h00 mm ato 0.10- 0.08- 006- 0.04- 0.02-Section 1.3

*

'e 'e

fig.7 _{COD measurements}

welded se9tl.ons. t + C2 o (.1

t

o 000 20000 30.000

fatigue notch sawcut notch

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+HInhiuim C.T.OEO. Hean C.T.O.ft

Unsafe Area

C.T.O.D. design curve

0.0101 I I

1.0 10,0

fig.9.

COD design cUrv

diagram.

Safe Area

0.2 0,6 0.6 0,8 1,0 1,2

Lt

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