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7/30/2019 Fracture behavior of straight pipe and elbow with local wall thinning
1/13
Nuclear Engineering and Design 211 (2002) 91103
Fracture behavior of straight pipe and elbow with local walthinning
Seok-Hwan Ahn a,*, Ki-Woo Nam a, Yeon-Sik Yoo b, Kotoji Ando b,Su-Hwan Ji b, Masayuki Ishiwata c, Kunio Hasegawa c
a School of Engineering, College of Engineering, Pukyong National Uni6ersity, 100, Yongdang-dong, Nam-Gu,
Busan 608-739, South Koreab Faculty of Engineering, Yokohama National Uni6ersity, 79-1, Tokiwadai, Hodogayagu, Yokohama 240, Japan
c Power and Industrial Systems Nuclear System Di6ision, Hitachi, Ltd., Hitachi 317-8511, Japan
Received 24 April 2001; received in revised form 10 September 2001; accepted 12 September 2001
Abstract
Fracture behavior of pipes with local wall thinning is very important for the integrity of nuclear power plant. The
we studied the fracture behavior of straight pipe and elbow with local wall thinning. For the straight pipe, failur
mode, limit load and allowable wall thinning limit based on plastic deformation ability have been studie
systematically. Twenty two straight pipe specimens were tested. The failure mode was divided into four type
cracking, local buckling, ovalization and plastic collapse (ovalization+buckling). Maximum load was successful
evaluated using plastic section modulus and modified flow stress, in dependent to failure mode. For the elbow, plasti
collapse and low cycle fatigue fracture by reversed loading have been tested using ten specimens. Observed failurmodes were ovalization and local buckling under monotonic loading, and were local buckling and cracking unde
cyclic loading, especially local buckling promoted crack initiation. Test results were compared with ASME desig
curve and allowable limit of local wall thinning will be discussed. 2002 Elsevier Science B.V. All rights reserve
www.elsevier.com/locate/nucengd
1. Introduction
High energy carbon steel pipes and elbow etc.
are used extensively in piping systems of power
plants. For the service periods, high temperature
and high pressure water and steam flow at highvelocity through these piping systems. Sometimes,
these pipes and elbows are subjected to a wall
thickness thinning at the inside wall by erosion
corrosion (E/C). Therefore, it is important t
evaluate the strength of the pipe and elbow wit
local wall thinning to maintain the integrity of th
piping systems. Up to now, some tests of carbo
steel pipes with locally thinned area have beeperformed to evaluate plastic collapse behavio
and strength of pipes by researchers or researc
institutes (Japan Atomic Energy Research Inst
tute, 1993; Roy et al., 1997; Ahn et al., 199
Miyazaki et al., 1999). However, the acceptab
values of local wall thinning are not well known
* Corresponding author. Tel.: +82-51-620-1617; fax: +82-
51-620-1405.
E-mail address: [email protected] (S.-H. Ahn).
0029-5493/02/$ - see front matter 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 9 - 5 4 9 3 ( 0 1 ) 0 0 4 4 7 - 2
mailto:[email protected]:[email protected]7/30/2019 Fracture behavior of straight pipe and elbow with local wall thinning
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S.-H. Ahn et al. /Nuclear Engineering and Design 211 (2002) 9110392
Table 1
Chemical compositions of tested materials (wt.%)
Si Mn P SMaterials C
0.25 0.43STS370 0.0080.15 0.007
STPT410 0.18 0.22 0.43 0.013 0.004
ning. The effect of the location of wall thinnin
on fracture behavior was not considered, becaus
the results between inside wall thinning and out
side wall thinning were obtained identically fo
fracture behavior (Miyazaki et al., 1999). There
fore, the local wall thinning was machined on th
outside of both pipes. There are many varieties o
types of possible wall thinning in pipes. Thosmay be irregular according to defect size etc. O
them, especially, we simulated various types o
local wall thinning that can be occurred at pip
surfaces due to coolant flow. Locally wall thinne
shapes were machined to be different in size alon
the circumferential and axial direction of straigh
pipes. Straight pipe bending tests were conducted
one in which the locally thinned area was locate
at the tensile direction side and the other in whic
it was located at the compressive direction sid
against loading direction. And the locally thinne
area was located at the neutral axis side agains
loading direction to promote ovalization type fai
ure. Four types of straight pipe specimens an
one type of elbow specimens with local wall thin
ning were made. Straight pipe specimens and e
bow specimen are shown in Fig. 1ae and Fig. 2
respectively. The thinned sizes are shown in Ta
bles 35, respectively.
A monotonic bending load was applied t
straight pipe specimens by four-point loading a
ambient temperature without internal pressur
The major and miner spans of the four-poin
loading were 900 and 245 mm, respectively. Test
were carried out under displacement control o
cross head speed 0.1 mm s1.
On the other hand, elbow tests were performe
on two types of controlled mode, displacemen
controlled monotonic load and displacement con
trolled cyclic load. The controlled displacement o
cyclic load test was determined by displacemen
corresponding to about 90% of maximum load
ASME has considered the need to provide appro-priate guidance to steel pipes subjected to erosion/
corrosion damage (Deardorff and Bush, 1990).
Acceptance rules for local wall thinning have been
established for high energy carbon steel pipes
based on design construction codes (Mathonet et
al., 1995).
This study was performed to evaluate the frac-
ture behavior of carbon steel straight pipes and
elbows with local wall thinning under monotonic
and cyclic load. Based on the failure mode and
fracture strength for carbon steel straight pipesand elbows, the allowable level for local wall
thinning with erosion/corrosion is proposed.
2. Material and experimental procedure
The materials used in the experiments are car-
bon steel straight pipes and elbows called carbon
steel pipes for high pressure service, STS370 and
for high temperature service, STPT410 in JIS
(Japanese Industrial Standards). Both are com-
monly used in piping systems of nuclear power
plants in Japan. STS370 and STPT410 are similar
to ASME A333 Gr.6. The chemical compositions
and mechanical properties of STS370 and
STPT410 are shown in Tables 1 and 2, respec-
tively. Full-scale experiments were performed on
3.5 in. diameter Schedule 80 STS370 carbon steel
straight pipes and 4 in. diameter Schedule 40
STPT410 carbon steel elbows with local wall thin-
Table 2Mechanical properties of tested materials
Materials Tensile strength |u (MPa) Yield strength |y (MPa) Elongation (%)
406STS370 227 25.3
450STPT410 301 39.0
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Table 3
Specimen geometries and test results of straight pipes
Plastic collpaseThinned orAxially thinnedThinned orSpecimen Maximum Plastic collp
moment raticracked depth, moment bylength, l (mm)number cracked angle, moment by
Mmaxd (mm) experiment, calculation, MPO2q ()
(experiment)(kN m)Mmax (kN m)
MPO(calculation)
1.589 360 25.92 16.3125.02.0LWT-1
3.0 13.34 1.720 25.0 360LWT-2 22.95
LWT-3 1.7594.0 25.0 360 18.45 10.49
1.218 5.1436025.0 6.26LWT-4 6.0
16.312.0 1.432 102.0 360 23.35LWT-5
19.02 13.34 1.430 LWT-6 3.0 102.0 360
10.494.0 1.477 102.0 360 15.49LWT-7
1.233 5.14LWT-8 6.0 6.34360102.020.874.0 1.380 25.0 46.1 28.81LWT-9
26.64 19.41 1.372LWT-10 25.06.0 56.7
1.292 20.874.0 102.0LWT-11 26.9646.1
19.416.0 1.307102.0 56.7 25.36LWT-12
27.04 16.61 1.630 LWT-13 6.0 102.0 56.7
20.545.0 1.26037.5 48.9 25.84LWT-14
1.290 19.85LWT-15 6.0 25.5253.941.0
19.227.0 1.360 44.0 57.9 26.08LWT-16
LWT-17 1.2705.0 37.5 48.9 26.16 20.54
1.230 19.8553.941.0 24.39LWT-18 6.0
19.227.0 1.160 44.0 57.9 22.31LWT-19
25.27 21.95 1.151 LWT-20 5.81 33.1
23.75 20.87 1.138 5.54LWT-21 46.11.099 19.41 56.7LWT-22 6.79 21.34
O, Ovalization; B, Buckling; O+B, Ovalization+Buckling; C, Cracking; Rs, Outer half diameter (=51.0 mm); t, wall t
7/30/2019 Fracture behavior of straight pipe and elbow with local wall thinning
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S.-H. Ahn et al. /Nuclear Engineering and Design 211 (2002) 9110394
Fig. 1. Straight pipe specimen with local wall thinning and
pre-crack:(a) Type of specimen LWT-1LWT-8; (b) Type of
specimen LWT-9LWT-12; (c) Type of specimen LWT-13;
(d) Type of specimen LWT-14LWT-19; (e) Type of speci-
men LWT-20LWT-22.
3. Test results and consideration
3.1. Fracture beha6ior for locally wall thinned
pipes
3.1.1. Straight pipe
The test results are summarized in Table 3. I
this study, the failure mode was divided into fou
types; cracking, local buckling, ovalization an
plastic collapse (ovalization+buckling). Fou
types of failure modes are shown in Fig. 3, respec
tively. Typical momentdisplacement curves fo
straight pipe specimens are shown in Fig. 3. Fig
3a shows momentload point displacemen
curves for the case of local buckling only. In th
case, the load was increased a little after bucklin
and the specimen showed an enough ductility. I
this case, it is characterized that the type of loca
buckling indicates form such as wrinkle of el
phant leg. In the case of ovalization only (Fig
Fig. 2. Elbow specimen with local wall thinning.
Cyclic load test was finished, when the maximum
load (that is a load of 1st cycle after the cyclic
load is applied to the elbow) reached its 75% or
cyclic number to the fracture of 300 cycles. In the
same way, tests were carried out under ambient
temperature without internal pressure.
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S.-H. Ahn et al. /Nuclear Engineering and Design 211 (2002) 91103 9
Table 4
Specimen geometries and test results of elbows under monotonic test
Thinned depth Failure modeThinned angleSpecimen Pmax SMDisplacement Moment
() (kN) at Pmax (mm)(d/t)number (kN m) (kN mm2)
87.14 25.14 35.28BP-1 14420.0 O
0.0 BP-2 O
BP-3 3600.3 61.28 23.71 33.28 1363 O
360 48.66 23.180.5 32.54BP-5 1334 OBP-7 3600.8 18.13 6.42 9.01 366 B
O, Ovalization; B, Buckling; t, wall thickness (=6.0 mm).
Fig. 3. Moment (M)-load point displacement (l) curves for the local wall thinning of straight pipes: (a) Case of buckling; (b) Cas
of ovalization; (c) Case of ovalization+buckling; (d) Case of cracking.
3b), the applied moment has a tendency to de-
crease slowly after the maximum moment except
for the case of local wall thinning with shallowly
thinned wall which is located at the tension side
toward loading direction. In the case of ovaliza-
tion+buckling (Fig. 3c), buckling occurred an
then the maximum moment was determined b
ovalization. However, all specimens showe
enough ductility. Especially, it can be seen whe
the local wall thinning is located at the compres
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Table 5
Specimen geometries and test results of elbows under cyclic test
d/tSpeci-men PE (kN) Mis/Z (MN) ME/Z (MN) SP (MN) SEP (MN) NiP1 (kN) Actual
number displacement
(mm)
9581 +1383BP-4 918760.3 +83.5 916.4 9113.2 +428.3
1064329.464.2+2240.5 9294 +724 9950 285+43.6 98.3 957.3BP-6
227 73344.1
9115 +205 9371 30+63.5 93.24 922.4BP-8 0.8 +12.37
60.5 19511.76
BP-9 +1190.8 +269 +384 15+16.17 +3.35 +23.2 +83.2
261 269 26983.27.3716.17 50.9
O, Ovalization; B+C, Buckling+Fatigue crack; t, Wall thickness (=6.0 mm).
7/30/2019 Fracture behavior of straight pipe and elbow with local wall thinning
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S.-H. Ahn et al. /Nuclear Engineering and Design 211 (2002) 91103 9
Fig. 5. Loaddisplacement curve of elastic analysis for no
local wall thinned elbow and monotonic loaddisplaceme
curves of local wall thinned elbow.
Fig. 4. Fracture surface obtained from straight pipe specimens:
(a) Case of buckling (LWT-4); (b) Case of buckling (LWT-7);
(c) Case of cracking (LWT-19).
sion side toward loading direction. In the case o
cracking (Fig. 3d), three specimens (LWT-20, 2
22) with crack were failed by cracking and compared to locally wall thinned specimens occurre
crack initiation. Three specimens with crac
showed that load increases slowly after crac
penetration, and plastic deformation occurs. Tw
locally wall thinned specimens failed by cracking
and the load decreased rapidly after crack pene
tration. Typical failure modes obtained from
straight pipe specimens were shown in Fig. 4a
3.1.2. Elbow
The results of monotonic load tests are showin Table 4. Pmax is the maximum load and lmaxthe displacement at Pmax. Loaddisplacemen
curves of elastic analysis for the non-local wa
thinned elbow and monotonic loaddisplacemen
curves of the local wall thinned elbow are show
in Fig. 5. Two types of failure modes were ob
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S.-H. Ahn et al. /Nuclear Engineering and Design 211 (2002) 9110398
served, that is, ovalization and buckling. It seem
that overstrain was concentrated at local wa
thinning when the locally thinned limits hav
some value. The BP-2 under the tensile loa
shows a different tendency compared to the BP-
under the compressive load. In the case of BP-2
the load is increasing with the displacement. Th
shows that the strength of thinned part retain
one until fracture of some extent if the tensile loa
work on local wall thinning as the external force
In case of all-circumferential thinned elbows, ac
cordingly, it is considered that the compressiv
force has an effect on the thinned part rather tha
tensile force is so. The local buckling occurred a
elbow specimens indicated the elephant leg type
too.
The results of displacement controlled cycl
tests are shown in Table 5. In this Table, P1 is th
maximum load at 1st cycle, Ni is the crack initia
tion cycle and Nf25 is the cycle at 75% of P1. Fig6 shows three types of failure modes obtaine
from displacement controlled cyclic test of elbow
Fig. 6a,b,c are hysteresis curves of loaddisplace
ment from BP-4 (d/t=0.3), BP-6 (d/t=0.5) an
BP-9 (d/t=0.8), respectively. Hysteric curve o
BP-4 did not decrease the load suddenly. From
this result, an elbow with d/t=0.3 had a hig
strength against the fracture and showed enoug
ductility. However, an elbow with d/t=0
showed that the load decreases remarkable afte
buckling. An elbow with d/t=0.5 showed thathe fatigue crack occurred at 285th cycle in spit
of buckling, after that, the decrease of load wa
not so observed. Therefore, in this case, th
strength of local wall thinning has a high valu
and the specimen shows enough ductility. Fig. 7
and b show surface obtained from test result o
elbows with d/t=0.8.
3.2. Fracture strength for locally wall thinned
pipes
3.2.1. Straight pipe
Maximum moments (Mmax) for each thinne
configuration were obtained from experiment
Plastic collapse moments (Mpo) were calculated b
the net-section stress criterion (Kanninen et al
1982) using the following equation:
Fig. 6. Hysteresis curves of loaddisplacement for the elbow
with local wall thinning: (a) Case of BP-4; (b) Case of BP-6; (c)
Case of BP-9.
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S.-H. Ahn et al. /Nuclear Engineering and Design 211 (2002) 91103 9
Fig. 7. Fracture surface obtained from test result of elbows
with d/t=0.8: (a) Case of monotonic test (buckling: BP-7); (b)
Case of cyclic test (fatigue crack after buckling: BP-9).
Mpo=2R2t|f[2 sin i+(y2i) cos i] [Ar|f]
(1
where
A=R s2qR s
2 cos q sin q
i=y
2 A
4Rt
r=R cos i
+2
3Rs sin3 qqsin q cos q
n:!
R=Rst
2
"where, i is one half of the neutral angle of th
pipe at bending moment, A is the locally thinne
area, R is the mean radius of the pipe, Rs is th
outer radius of the pipe, q is a half of the thinne
angle, r is the moment arm and |f is the flo
stress given by [yield strength+tensile strength]/2
Maximum moments for thinned pipes are evalu
ated very conservatively by the net-section stres
criteria as shown in Table 2.
Fig. 8 shows the relation between experimenta
maximum moments (Mmax) and calculated plasti
collapse moments (Mpo). In figure, the correlatio
of Mmax=1.25Mpo was shown by dash line. Th
figure shows that the calculation is still conserva
tive even when Mpo is calculated by usin
modified flow stress 1.25|f. In this figure, uppe
arrows mean that the maximum moment obtaine
from the experiment is higher than the actua
maximum moment showed in Fig. 8 if the exper
ment is continued. MY is the general yieldin
moment of the pipe and it was calculated using |
denoted yield strength of STS370 of the materia
used in this experiment.
For the locally thinned pipe, the general yield
ing condition was evaluated as a function of flaw
depth, flaw angle and flow stress. The result
shown in Fig. 9.
Plastic rotation capacity is a very importan
factor for the integrity of indeterminate pipin
system. Therefore, the relation between plast
rotation angle (Pmax) and plastic collapse moment by the modified flow stress 1.25|f (Mtp) ar
shown in Fig. 10. In this case, the plastic rotatio
angle was calculated by Eq. (2).
Pmax=4lPmax
LOLI(2
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S.-H. Ahn et al. /Nuclear Engineering and Design 211 (2002) 91103100
Fig. 8. Relation between experimental maximum moments (Mmax) and calculated collapse Moments (Mpo).
Where, Pmax is the plastic rotation angle, lPmax is
the plastic displacement at the maximum load
point obtained from the experiments, LO is the
outer span length and LI is inner span length.
From this figure, Pmax of 0.150.2 rad may be
required to form the plastic collapse mechanism
of piping system that is regarded as indeterminate
one (Liu and Ando, 2000). When Mtp is larger
than MY, Pmax is larger than 0.2 rad, except forpipes with crack. Then, it can be concluded that is
a necessary acceptance condition for local wall
thinning if Mpo should be larger than MY.
3.2.2. Elbow
The moment by elastic FEM analysis was cal-
culated for the stress evaluation of monotonic
load test and displacement controlled cyclic test.
The elbow was represented elastic beam element
in order to perform FEM analysis. The relation
between load and displacement was obtained byapplying moment of inertia to a pipe. Fig. 11
shows schematic of FEM analysis model. In FEM
analysis, each parameter is; outer diameter (D)
114.3 mm, wall thickness (t) 6.0 mm, radius of the
elbow (Re) 228.6 mm, Youngs modulus (E) is 206
kN mm2 and Poissons ratio (w) is 0.3. From
FEM analysis, the moment at assessment point
was calculated with elastic load (reactions) obtained from non-local wall thinning. The momenof point 3 was 28.1 kN m and the displacemen
corresponding to one was 20 mm. And the forcof point 1 (or point 6) was 138.1 kN. From thi
result, the relation between moment and controlled displacement is shown in Fig. 12. Th
Fig. 9. General yielding region to show enough ductility.
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S.-H. Ahn et al. /Nuclear Engineering and Design 211 (2002) 91103 10
Fig. 10. Relation between plastic rotation angle (Pmax) and calculated plastic collapse moment by the modi fied flow stress 1.25
(Mtp).
moment (M) of monotonic load test and the
moment (ME) of displacement controlled cyclic
test were calculated from Fig. 12. Stress intensityvalue for the evaluation of monotonic load test
was obtained from Eq. (3).
SM=B2M
Z(3)
where, SM is assessment stress, B2 is 2.16, M is the
moment and Z is the modulus of section with
t=6 mm. The fracture under monotonic test oc-
curred at the maximum load point. The results are
shown in Table 4. The allowable limit was evalu-
ated with SM and Sm under each operating condi-tion. Sm indicates the design stress intensity value
in ASME Code SEC.III (ASME, 1992) 1.5Sm and
3.0Sm showed in order to compare with SM. Sm is
0.137 kN mm2 in carbon steel pipes for high
pressure service STS410. Comparison between SMand Sm is shown in Fig. 13. Using Sm, SM can be Fig. 11. Schematic of FEM analysis model.
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S.-H. Ahn et al. /Nuclear Engineering and Design 211 (2002) 91103102
Fig. 12. Moment vs. controlled displacement by elastic FEM
analysis.
evaluation of displacement controlled cyclic te
was calculated by Eq. (4).
SEP=K2C2ME
Z(4
where, stress index of a bended pipe K2 is 1.0, C
is 3.23 as stress index calculated from 1.95/h2
(h=tRe
/r2; t=wall thickness, Re
=radius of th
elbow, r=pipe mean radius), ME is the momen
calculated from PEle(PE=elastic load derive
from elastic analysis, le=269.5 mm) and Z is th
modulus of section with t=6 mm. Also, the stres
(SP) obtained from the displacement controlle
cyclic test can be written by substituting Mis (=
P1le) for ME and P1 (load obtained from th
experiment) for PE in Eq. (4). The results ar
shown in Table 5. Cyclic failure stresses of fict
tious stress amplitude (Sa) for elbows with loca
wall thinning are shown in Fig. 14. The momen
of each specimen is calculated from the elastload. Failure stresses derived from moments ar
shown in Table 5. The failure stress of BP-
specimen is not clearly plotted, because the stres
amplitude is skew (R"1).
4. Conclusions
This study was performed to evaluate the frac
ture behavior of carbon steel straight pipes an
elbows with local wall thinning under monotoniand cyclic load. The results obtained from th
present study can be summarized as follows:
1. The four types of failure modes were observe
in straight pipes, that is, ovalization, buckling
ovalization+buckling and cracking. Also, th
failure modes of elbows were ovalization an
buckling under the monotonic load test an
were ovalization and buckling+fatigue crac
under the cyclic load test.
2. For locally wall thinned specimens, maximum
moments (Mmax) were estimated by using thmodified flow stress(|tf=1.25|f) and the ne
section stress criterion (when Mmax]MYThese conditions were evaluated as a functio
of flaw depth, flaw angle and flow stress.
3. The allowable limit of elbows was derive
from the comparison between assessed stres
Fig. 13. Comparison between SM and Sm.
allowed when 1.5Sm50SM53.0Sm. That is, the
ovalization after buckling or the ovalization as the
failure mode can be allowable. However, a locally
thinned elbow with d/t=0.8 cannot allow be-
cause of the fatigue crack occurred after buckling.
When the locally thinned area occurs in the
piping system, the modulus of section decreases
and the applied stress increases. In this study, the
moment is obtained from experimental resultsbased on ASME Code SEC.III (ASME, 1992). In
this case, the decrease of modulus of section by
local wall thinning is not considered. This means
conservative evaluation. The current piping sys-
tem design uses the elastic FEM analysis and
stress index. Therefore, the elastic stress for the
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S.-H. Ahn et al. /Nuclear Engineering and Design 211 (2002) 91103 10
Fig. 14. Comparison between elbow strength and ASME design fatigue curve.
(SM) and design stress intensity value (Sm).
The failure stress can be allowed 1.5Sm=
SM=3.0Sm in the case of ovalization or oval-
ization after buckling as the failure mode.
However, the case of d/t=0.8 cannot be al-
lowed because the fatigue crack occurred after
buckling.
4. From the comparison between elbow strength
and ASME design fatigue curve, the strength
of elbow for the case of d/t=0.5 is
conservative.
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