fracture behavior of straight pipe and elbow with local wall thinning

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]


2/13

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


3/13

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


4/13


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.


5/13

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


6/13

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).


7/13


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


8/13


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.


9/13


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


10/13


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.


11/13


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.


12/13


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


13/13


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.

References

Ahn, S.H., Ando, K., Ishiwata, M., Hasegawa, K., 1998.Plastic collapse behavior of pipes with local wall thinning

subjected to bending load and allowable limit (Japanese).

J. High Pressure Inst. Jpn. 36-4, 225233.

ASME Boiler and Pressure Vessel Code Section III, 1992.

Deardorff, A.F., Bush, S.H., 1990. Development of ASME

Section XI Criteria erosion-corrosion thinning of carbon

steel piping, ASME, PVP, PVP-Vol- 186, NDE-Vol.7,

71-75.

Japan Atomic Energy Research Institute, 1993. Technical r

port on the piping reliability tests at the Japan Atom

Energy Research Institute (Japanese), JAERI-M, 93-07

104-115.Kanninen, M.F., Zahoor, A., Wilkowski, G., Abousayed, I

Marschall, C., Broek, D., Sampath, S., Rhee, H., Ahmad

J., 1982. Instability predictions for circumferential

cracked Type-304 stainless pipes under dynamic loadin

EPRI NP-2347, Vols.1 and 2, EPRI, Palo Alto.

Liu, S.P., Ando, K., 2000. Leak-before-break and plast

collapse behavior of statically indeterminate pipe syste

with circumferential crack. Nucl. Eng. Des. 195, 26127

Miyazaki, K., Kanno, S., Ishiwata, M., Hasegawa, K., Ah

S.H., Ando, K., 1999. Fracture behavior of carbon ste

pipe with local wall thinning subjected to bending loa

Nucl. Eng. Des. 191, 195204.

Mathonet, J.F., Cherasse, J.M., Leblois, C.L., Guyette, M.A1995. Belgian methodology for evaluation of erosion co

rosion phenomena in units 1 2 3 & 4 of the Doel Nuclea

Power Plant (NPP). ASME PVP 303, 393399.

Roy, S., Grigory, S., Smith, M., Kanninen, M.F., Anderson

M., 1997. Numerical simulations of full-scale corrode

pipe tests with combined loading. J. Press Vessel Techno

119, 457466.

fracture behavior of straight pipe and elbow with local wall thinning

Documents