TECHNISCHE HOGESCHOOL DELFT
AFDELING DER SCHEEPSBOUW- EN SCHEEPVAARTKUNDEDELFT UNIVERSITY OF TECHNOLOGY
Department of Shipbuilding and Shipping Ship Structures Laboratory
FITNESS FOR PURPOSE ANALYSIS OF M.V. ARCTIC
by
Prof.Ir. J.J.W. Nibbering
Fatigue and Charpy tests at low temperature
on deck plate material
November 1981
-
I-FITNESS FOR PURPOSE ANALYSIS OF M.V. ARCTIC
Prof.Ir. J.J.W. Nibbering
Fatigue and Charpy tests at low temperature on deck plate material
Introduction
From the deck plate three longitudinal specimens have been cut for fatigue
testing at low temperature (Fig.
1). The specimens have been tested in a
loo tons Ainsler pulsator (Fig. 2).
The specimens were surrounded by a cooling box (small white one on the left
hand side of the photograph), which was connected by a flexible tube to the
cooling machine (large white one). (The philosophy behind fatigue loading
at low temperature is given in the appendix).
Fatigue testing at low temperature
Specimen No.
I was tested in repeated loading; R = a
.¡a
= 0.1;
a
= 220 N/mm2;
min
maxmax 2
a.
=20N/nim.
min
The temperature should have been -10°C but was somewhat lower in reality
due to some inconveniences with the cooling apparatus. During the stage of
crack propagation the temperature was about -12 C. The number of cycles to
fracture was 331,000; the loading frequency was 4 Hertz.
The fatigue crack started at the upper edge of the specimen at the fusion
line of the back weld. It went straight through the butt weld (Fig. 3).
The fatigue crack obtained an average length of 4.8 cm, being 32% of the
surface (Fig. 4). The average stress at the moment a brittle fracture
start-ed was 320 N/uuu2, which is far above the yield point of 270 N/mm2.
)The development of a brittle fracture at this high stress after a few
hundredthousand cycles of high stress fatigue loading is not alarming at all.
(The criterion in this type of testing is that a brittle fracture should not
occur at average (net) stresses lower than yield point).
Specimen No. 2 started with a cyclic load conforming to nominal stresses of
240 N/mm' and 20 N/mm2. This is 10% higher than in specimen
Iin order to
accelerate crack initiation. After 280,000 cycles the crack depth was about
10 mm, (see Fig. 5: dark part) in the thickness direction.
Then the load was reduced to 200/20 N/mm2. The fiffal fracture occurred at
-IO C when the section had been reduced to only 30% of the original value.
This result is very satisfactory. The fatigue crack started at the fusion
line at the top of the weld and consequently propagated completely in the
plate material (Fig. 6), both in the thickness and transverse direction.
Therefore this test reflects the fracture properties of the base metal.
Specimen No. 3 was loaded at 200/20 N/mm2 from the start because of the
presence of serious defects at the plate surface (end of longitudinal?,
Fig. 7). Despite these 'favourable' starting points it took 470,500 cycles
before the specimen fractured. There were a number of initiation points as
can be seen in Figs. 8 and 9, mostly at the fusion line of the back weld.
The final fracture was a shear fracture and developed at -10 C when about
50% of the surface had cracked due to fatigue. This result is very good.
-2--Reviewing all fatigue experiments it can be said that the results are satisfactory both from the viewpoint of fatigue nd brittle fracture.
Some small defects in the welds, and rather serious surface irregularities at the fusion line will certainly not cause trouble in practice, because the fatigue resistance is quite adequate. Indeed the stress values applied during fatigue loading were very high. They will not occur more often in
the ships' life than a few tenths of times. This is not even enough for crack initiation, because in the tests the number of cycles to crack initiation was in the order of magnitude of some 100,000.
3. Charpy-V-testing
12 Charpy-bars have been fabricated. 6 of these were made from the base metal, 3 were taken out of the weld and 3 from the Heat Affected Zone (HAZ). The position of the bars is indicated in Fig. 10. The results are given in
Fig. 11.
The results for weld metal and HAZ are very good. Only one is somewhat below the general trend, due to a gas inclusion. But it is still good as compared to the plate values. It is evident that the plate material is of A-quality, which should give some concern about the safety of the ship in arctic
con-ditions. (A value of 15 ftlb at 0°C is generally considered only sufficient
for 'normal' conditions). But a positive point is that the weldability of
the steel seems to be good.
Keeping in mind the good results of the fatigue experiments, the conclusion may be that the ship is fit for the job.
But of course this is only an indication because the amount of material
tested was really little.
Deift, 4 November 1981,
Chairman of the Ship Structures
Laboratory of the Delft University
of Technology, The Netherlands.
Figure 1.
Note: the words 'front side' and 'back side' refer to the position in the testing machine. In fact the front side is the side of the back weld and the back side the side of the wide end of the
V-butt-weld.
Figure 4.
t'
I"
L
I
Figure 5.
Figure 6.
SC.8102
Figure 7.
S C. 8 102
L
Figure 8.
t pr
Figure 9.
SC. 8102
Charpy-V notch bars.
i
2Position relative to weld and H.A.Z.
3 4 5 6
nrs.i1l213,14,15 and 16 from base metal.
Test temperature
1°C)-x Charpy-values of original plate material.
a Charpy-values of heat affected zone.
O Charpy-values of weld material.
Test bar nr. Notch position Test temperature Energy (Joules) % Fibrous. 11 plate material -15 °C 7.84 0 12 - -
-9 oc
11.75 2 13 0 Ct 30.38 10 14 - +5 ° 14.7 15 15 +15 oc 19.6 30 16 +19 oc 49.0 40 1 weLd material o +19 C 99.96 --- 100-3 CZ ,, O Ct_A 0f
89.181.1 1 90 4->' a, C LU 75 so 25 o -G) 100 75 50 25 V, o o/
i
/
/
/
/
/
/
/
/
X,
/
,
X -X--
-X O-(o)-.Gas inclusion.
X _200 -iO° 0° +10e + 20°APPENDIX al
-Testing of heterogeneous metals
One disadvantage of the C.O.D.-test is that only a few cubic millimeters
of the material of the whole specimen are tested.
The position of the notch-tip is quite haphazard in multirun welds and H.A. Zones. Therefore a reliable estimate of the fracture toughness can
only be obtained by using quite a number of specimens.
This problem can partly be met by using hardness-measurements and
micro-scopic analysis for finding the worst places in the weld. But especially for thick plates this cannot be done satisfactorily, because the welds can only be examined at the outer surfaces of the specimens. A possible way to increase the efficiency of the C.O.D.-test is starting with a
shallow notch, say 10 nun, and loading the specimen up to a C.O.D. of f.i. 0.2 mm. After that the fatigue crack should grow an other 5 imii and the
next C.Q.D.-test (up to 0.2 mm) can be carried out. When again no fracture occurs the crack should be deepened another 5 mm and so on.
Although not ideal, the method improves the reliability of the C.O.D.-test
subs t an t i ally.
A logical consequence of the foregoing is fatigue loading at low temper-ature. This procedure has been introduced in 1969 by Nibbering /1/, /2/,
/3/. Since then the Deift Ship Structures Laboratory has used the test often for fitness for purpose analysis for industry. The main advantage of the method is that during fatigue loading a crack travels through the
various zones of welded joints. When for instance 20,001) cycles lead to
a crack of 20 mm length, the material of the specimen has been analyzed at every 0.001 mm along the crack line. One might say that the result is
equivalent to that of 20,000 static tests. Additional advantages:
I. From maximum (critical) crack length and applied load a K1 -value can
be calculated. c
C.0.D.-measurements are not necessary.
Due to 2 an irregular crack front in case of residual stresses is not
objectionable.
When the fatigue crack has grown to maximum length (about one third of specimen height) without brittle fracturing, the test may be ended
by a static
C.O.D.-test.---Test programme
a. It will be obvious that for simulating service conditions, high fatigue load testing is required. This has the additional advantages that
test a2 test
-applied in order to accelerate the crack initiation. When notch plus crack have got a length of 20 mm the nominal stress should be reduced
to one half of yield point.
c. The length of the specimens should be at least four times the height
for three point bending. Literature
/1/ I.I.W.-doc. X-593-70. Low cycle fatigue tests at low temperature witt' E,G.-welded plates. S.S.L.-report No. 143a.
J.J.W. Nibbering & A.W. Lalleman.
/2/ Low cycle fatigue problems in shipbuilding. Craci.- propagation in
coarse grained zones. Paper 16, Fatigue of Welded Structures Conference, Brighton 1970.
J.J.W. Nibbering & A.W. Lalleman.
/3/ Some observations on C.0.D.-testing. I.I.W.-doc. X-901-78. J.J.W. Nibbering.
