Fracture behaviour of stainless steel pipes containing circumferential cracks at room temperature and 280°C

Int. J. Pres. Ves. & Piping 43 (1990) 367-377

Fracture Behaviour of Stainless Steel Pipes Containing Circumferential Cracks at Room Temperature and 280°C

C. M a r i c c h i o l o , P. P. Milella & A. Pini ENEA/DISP, Via V. Brancati 48, 00144 Rome, Italy

A BSTRA CT The paper presents the experimental results o f a research programme on fracture behaviour of austenitic stainless steel and TIG welds in pipes containing circumferential through-wall cracks at room temperature and 280 ° C. Pipes were loaded in pure bending using a four-point bend test method. The diameter o f the pipes under investigation was 168 mm and324 mm, with a thickness varying f r o m 10 to 17mm. As opposed to the behaviour o f carbon steel pipes, it is found that the Net Section Collapse ( N S C ) criterion predicts the moment o f instability. Crack mouth opening displacements (COD) and collapse moments calculated using the G E - E P R I engineering approach show a rather high scatter with respect to experimental results.

1 INTRODUCTION The use o f the double ended guillotine break (DEGB) as a design criterion m a y not necessarily lead to a real improvement of the safety of a nuclear power plant. It requires, in fact, the installation of a large number of pipe whip restraints to prevent the possible occurrence of large movements of the pipes assumed to have severed, and the crushing of other pipes and components. It also requires the use of heavy barriers to cut offhigh pressure water flows and avoid jet impingement. Besides the economics of such a design choice, the use o f restraints and barriers results in a considerable loss of accessibility to the piping systems 367 Int. J. Pres. Ves. & Piping 0308-0161/90/$03.50 © 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain

368

C. Maricchiolo, P. P. Milella, A. Pini

during the life of the plant and additional irradiation doses to the workers for their removal, where possible, during scheduled outages. This has led the Directorate for Safety and Protection (DISP) of the Comitato Nazionale per la Ricerca e lo Sviluppo dell'Energia Nucleare e delle Energie Alternative (ENEA) to consider new and more realistic design criteria such as the Leak-Before-Break (LBB) concept. To substantiate the applicability of LBB to P W R nuclear power plants, in 198 ! ENEA/DISP undertook a research programme to improve knowledge regarding the fracture behaviour of circumferentially through-wall and part-through cracked pipes.~ Within the framework of the ENEA research programme two different steels have been studied: A 106 B carbon steel and type 316 stainless steel. Some results on carbon steel pipes were already published. 2 The present paper summarizes the results obtained on stainless steel pipes containing circumferential through-wall cracks, tested at room temperature and 280'C.

2 EXPERIMENTAL PROCEDURE The experiments considered in this study were conducted using the fourpoint bend method schematically shown in Fig. 1. During the test the load was quasi-statically increased under displacement control, until the maximum load was reached. Because of the low compliance of the test rig, unstable crack propagation never occurred.

t

t '(

2Ct

"W

Through-wall crack

Fig. 1.

Schemalic diagram of four-point bend test method.

Fracture behaviour o/'pipes with circumferential cracks

369

The pipes contained through-wall circumferential cracks of various size, introduced by EDM (Electric Discharge Machining) at the notch tip of milled slots, about 3 m m wide; the flaw tip radius was about 0"1 mm. Fatigue pre-cracking was not applied, because the high toughness of the steel causes large blunting of the flaw tip before the propagation of the crack. The crack mouth opening displacement (COD) was measured by means of a clip gage-mounted at the centre of the flaw. In some of the experiments four crack opening gages were placed, evenly spaced, along half of the crack on the outer surface of the pipes to determine the crack shape and measure the leak area. The Direct Current Electric Potential (DC-EP) method was utilized to detect the onset of the crack propagation. Tubes with two different diameters were tested: 168 and 324mm, with a thickness varying from 10 to 17 mm. Table 1 presents the test matrix of all the experiments conducted so far. TABLE 1 Summary of ENEA Fracture Experiments on Stainless Steel Pipes

Test code

Outer diameter (m)

Thickness (m)

Crack angle (~')

Temperature (°C)

0-323 9 0-323 9 0-323 9 0"323 9 0"323 9

0'015 8 0-016 1 0"016 2 0"016 4 0'016 4

60-00 40-00 50-00 90"00 180-00

RT RT RT RT RT

0-1683 0" 168 3 0"168 3 0"168 3 0"168 3 0"168 3 0"168 3 0"168 3

0'0102 0'010 3 0-0105 0"0106 0"010 7 0"010 9 0-014 2 0-014 3

133"00 135"00 180'00 182'00 90-00 90-00 140"00 64'00

RT RT RT RT RT RT 280 280

0'0106 0-010 7 0"010 7 0'0107 0'0108 0'011 0

133"00 90"00 90-00 135'00 180"00 180"00

RT RT RT RT RT RT

Material: SS 316 123160 121 I40 122150 124190 1251180

Material: SS 316L L6BlI135 L6B21135 L6CLI180 L6D11180 L6A2190 L6A 1I90 L6311140 L62II60

Material." SS 316L- W L6CIIS135 L6D 11$90 L6A 1IS90 L6A2IS135 L6FIlS180 L6F2IS180

0-1683 0"168 3 0"168 3 0"1683 0'168 3 0'168 3

370

C. Maricchiolo, P. P. Milella, A. Pini TABLE 2 Summary of Material Properties

Material specification

SS SS SS SS

316L 316 3t6L-W 316L

Test temperature (~C)

Young's modulus (MPa)

Yield strength (MPa)

Ultimate strength (MPa)

(MN/m)

RT RT RT 280

198000 198000 198000 176000

240 358 432 171

560 636 619 440

0.750 0.524 0-249 0.372

Jlc

3 MATERIAL PROPERTIES Three types of stainless steel were analysed; namely, AISI 316, AISI 316 Low Carbon (316 L) and Tungsten Inert Gas (TIG) weld on 316 L. Welds were performed according to ANSI B 31.7 and A S M E Sections III and IX. The material characterization programme involved several tests on compact tension (CT) and tensile specimens cut out of the pipes; the tests were conducted in accordance with the A S T M standards. Table 2 summarizes the average values of the mechanical properties measured, that were used in the fracture mechanics evaluation of the experiments.

4 A N A L Y S I S OF T H E E X P E R I M E N T A L R E S U L T S Two different methods of analysis were used to assess the fracture behaviour of defected pipes and are compared in this study: the Net Section Collapse (NSC) criterion and the G E - E P R I engineering approach.

4.1 Net Section Collapse criterion The Net Section Collapse (NSC) criterion is based on the limit load analysis which assumes that failure occurs when the stress on the pipe cross-section reaches the flow stress of the material. In the case of a pipe with a circumferential through-wall crack under pure bending, the limit m o m e n t is given by 3 NSCL = 2arRZt{2 sin [(rc - c~)/23 - sin c~}

(i)

where af is the flow stress (herein assumed as the average of the yield and ultimate strengths), R and t are the mean radius and the thickness of the pipe, and a is defined in Fig. 1.

Fracture behaviour of pipes with circumferential cracks

371

To compare the experimental results from different materials and geometries, the applied remote stress, arem, was calculated for all the tests as the experimental failure moment divided by the pipe section modulus

O'rem =

Mexp/nR2t

(2)

The NSC criterion estimate of the remote stress at failure may be calculated as

arem.NSC= 20"r{2 sin [(n -- ~)/2] -- sin ~}/n

(3)

The comparison between the experimental results and the NSC predictions are shown in Fig. 2, where the remote stresses are normalized to the flow stress values; the solid line represents the NSC estimation. The NSC criterion seems particularly effective in predicting the fracture behaviour of pipes made of 316 L stainless steel, while it overestimates, by 10-20%, the load carrying capability of pipes of 316 stainless steel, as well as that of the welding material. However, these discrepancies decrease as the crack size increases. This behaviour is completely different from that of carbon steel pipes, whose experimental resistance was always greater than that predicted by the NSC criterion; at least at room temperature where dynamic strain ageing did not o c c u r . 2 To establish the applicability of the NSC criterion, a simple rule based on the evaluation of the dimension of the plastic zone ahead of the crack tip was proposed by Battelle Laboratories: 4 if the dimension of the plastic radius is greater than the distance between the crack tip and the neutral axis, then the fully-plastic condition is reached and the NSC criterion is applicable. Using 1"00 F

0.90~ o.8ol-

0"70

&A

o 0"60 .-.._ 0.50

p 0.40 0.30

"~ 0 2 0 o ."." 0 1 0

s'o 6'0 7'o

0

Jo 16o

Crack half angle (deg)

Fig. 2. Experimental results (open and closed symbols) and NSC moment prediction (solid line) versus crack half angle. - - ,

NSCL; O, 316-L-W; V, 316-L; /k, 316.

372

C. Maricchiolo, P. P. Milella, A. Pini 1.40 -

E 1.30o E J U

1.20

z

1-10

q q,

E E IO0 o E "o 0 " 9 0 6) Q. .~080

7 A

'7

• ee

&"

0.7% o!2 or4 0% 0% 1.'o 1!2 1!4 l'.e 1!8 ~.0 2'.2 2!4 Plastic radius/crack

tip-neutral

axis distance

Fig. 3. Ratio of applied momentto NSC momentversusextensionof plastic zone. O, 316L-W: V, 316-L; A, 316. lrwin's equation of the plastic radius, in pure bending the previous condition becomes EJlc/[rca2(~z -

~)R] > 1

(4)

where E and J~c are the Young's modulus and the toughness of the material, respectively. Figure 3 shows the comparison of the experimental results and the NSC prediction versus the ratio defined in eqn (4). The estimation of the failure load is poor in all the experiments where the above ratio is less than 0-6, i.e. for the tests on the lowest toughness material and small cracks.

4.2 GE-EPRI engineering approach The GE-EPRI engineering approach is a method to predict the fracture behaviour of cracked structures based on the J-integral approach. So far different pipe geometries subjected to different loads were analysed by means of finite element analysis. The solution in terms of functions to be used to calculate fracture mechanics parameters such as J-integral, crack opening displacement (COD) and crack tip opening displacement (CTOD) are available in tabular form. 5'6 The material properties play an important role on the GE-EPRI method predictions. At present, a debate exists whether the engineering or the true stress-strain curve should be used for the calculation of the RambergOsgood coefficients and whether the deformation-J or the modified-J values should be utilized. Since the J-estimation scheme was developed under the assumption of small-scale deformation of the overall structure, the use of the engineering curve along with the deformation-J values seems more

Fracture behaviour of pipes with circumferential cracks

373

700 --

50C

t~ n

g ~c 3oc U)

20C 100 I 01

0

I 02

I 0.3

I 04

I 0'5

I 0.6

Strain

Fig. 4.

Engineering stress-strain curve for 316 L stainless steel, at RT. ~ = 3.042; n = 7"6507; a0 = 358 M P a ; e.0 = ao/E; E= 198000 M P a . - - , Best fit; A , test.

consistent with the theory. Furthermore, from the safety viewpoint, this choice is to be preferred, because it results in a prediction of lower limit loads. As an example, the application of the method is presented for a 168 mm diameter pipe of 316 L material, with a through-wall crack of 90 ° (test L6A 1190). Figure 4 shows the engineering stress-strain curve of the material with the coefficients of the Ramberg-Osgood equation derived from a best fit to the experimental data. The material R-curve is reported in Fig. 5 (open triangles); since the crack growth measured on the CT specimens was always small, the data points were best-fitted by the following equation: (5)

Je = A d a B

The R-curve extrapolated data were always used in tile GE-EPRI analysis. Finally, Fig. 6 shows the results of the J - R analysis using the GE-EPRI engineering approach. 2"50 2'00 ~ z

1.50

n- 100 0.50

0

I

1

I

I

I

0.0010

00020

00030

00040

0.0050

Crack g r o w t h (m)

Fig. 5.

J-Resistance curve for 316 L stainless steel, at RT. JR= A x G r o w t h s. A = 62-90, B=0.6133.

- - ,

Best fit; /N, test.

374

C. Maricchiolo, P. P. Milella, A. Pini 7

"i

6

/ /j/

~_ 5

/

/

/

/

/ /

/

-~ 4 t_

~3

c

~

. . . . . . . . . .

2

i l

1

I

I

I

I

I

I

[

0 0 0 2 0 0 0 0 4 0 0 " 0 0 8 0 0 " 0 0 8 0 0'0100 0"0120 0"0140

C r a c k g r o w t h (m)

Fig. 6. J - R analysis for 168 mm diameter 3 ! 6 L stainless steel pipe with a 90 ~ through-wall crack, a t R T . - - . . . . . . , M = 0 - 5 4 6 8 x 10-1MNm: ...... ,M=0"5340x 10 ~ M N m ; M = 0 " 5 0 8 4 x 10 -~ M N m ; M = 0 - 4 8 2 8 x 10 - 1 M N m ; ....... , M=0'4572 x 10 ' M N m (initiation); - - - , R-curve.

Results o f all the experiments are plotted in Figs 7, 8 and 9. Figure 7 shows the bending moment (BEND) and crack mouth opening displacement (COD) at initiation, as the ratio o f the experimental value to the calculated one, versus the crack angle. It can be seen that there is a large scatter in the results. TIG welds, in particular, seem to behave rather differently from the theoretical expectations, as far as the C O D is concerned, while for the bending moment at initiation the experimental results appear to be close to the calculated values. It is not clear whether the discrepancies are to be ascribed to the uncertainty in the potential drop technique, to the material characterization of TIG welds or to the GE-EPRI method or to a combination of all these factors. 5.0

~>4o ~3

._E3.o ~2.0

o

E

E

× t~ 0

I

I

I

I

I

I

~

I

I

20

40

60

80

100

120

140

160

180

Crack angle (deg)

Fig. 7. Ratio o f experimental C O D and m o m e n t at initiation to the calculated values versus crack angle. 0 , C O D 316 L - W ; . , C O D 316L; A , C O D 316; C), bend. 316 L-W; n , bend. 316 L; /k, bend. 316.

Fracture behaviour qf pipes with circumferential cracks

375

3.0 >2.5 ~2.0 1.5

o

|

6 6 ~o

= ® 10

8

[]

p

E

E ~05 u.I

0

Fig. 8.

t

I

I

20

40

60

I

/

I

80 100 120 C r a c k angle (deg)

I

I

I

140

160

180

Ratio of experimental COD and moment at maximum load to the calculated values versus crack angle. Key as Fig. 7. 3.0

~

2-5

E ~1.5

[]

~.o

8

[]

E

i~0.5

&

x td

0

Fig. 9.

[]

1,,

[] I 20

£ A 40 60

I I 80 100 C r a c k angle

o i 120 (deg)

I 140

I 160

I 180

Ratio of measured stable crack growth to calculated values versus crack angle. O, Growth 316 L-W; r-q, growth 316 L; /k, growth 316.

Figure 8 is the same plot as Fig. 7 referring to COD and bending moment (BEND) at maximum load. Again, the results are spread out although the scatter is much smaller than in Fig. 7. Finally, Fig. 9 shows the subcritical crack growth, in terms of the ratio of experimental measurements to calculated values versus the crack angle.

5 CRACK SHAPE Four crack opening gages were used in all the 316 stainless steel pipes and in the 316 L pipes tested at 280°C, to monitor the flaw opening shape during the loading phase.

376

C. Maricchiolo, P. P. Milella, A. Pini

t 8 ~:

2

L U

O

10 20

30

~ 40

x

5 0 6 0 70 8 0 9 0 100 110 120 f r o m the crack centre (mm)

Distance

Fig. 10. Crack opening on the surface of a 324mm diameter 316 stainless steel pipe for various loading steps at RT. O, M = 0.3855 MNm; +, M = 0 - 3 3 5 4 M N m ; ×, M = 0.2957 MNm; V, M-0"2398 MNm; O, M = 0-1987 MNm; &, M = 0'1620 MNm.

Figure 10 is an example of the results obtained. The plot pertains to the Experiment 124190, namely a 324mm diameter pipe of 316 stainless steel, with a through-wall crack of 90 °. The crack mouth opening displacements (COD) are plotted versus the distance from the crack centre for several values of the applied moment, up to the maximum moment. Subcritical crack propagation commenced at about 80% of the maximum load reaching a value of about 8 mm at each crack tip; the crack tip opening was very large. A different behaviour is shown in Fig. 11, that pertains to a 168mm diameter pipe of 316 L stainless steel with a through-wall crack of 64 ° (Test

E

F

io?-

0

i:

2 u

0

k

?

I

10

210

Distance

d

o

0

I

40

I

50

I

60

I

70

0

f r o m the crack c e n t r e ( m m )

Fig. 11. Crack opening on the surface of a 168 mm diameter 316 L stainless steel pipe for various loading steps at 280°C. O, M = 0.0647 MNm; + , M = 0-0645 MNm; x, M = 0.0628 MNm; O, M = 0-0584 MNm; II, M---0-0534; V, M = 0.0493; O, M = 0.0416.

Fracture behaviour of pipes with circumferential cracks

377

L62II60), tested at 280°C. At variance with all the other tests, for this smaller pipe the subcritical crack growth was practically negligible, while the crack tip blunting appeared to be larger.

6 DISCUSSION A N D C O N C L U S I O N Pipe fracture assessment using the Net Section Collapse (NSC) criterion appears to be effective for ductile materials such as 316 L stainless steel, or for a combination of material toughness and crack size which results in the development of a plastic enclave ahead of the crack tip that extends throughout the ligament. The use of the Battelle screening criterion is recommended to judge whether the NSC criterion can be fully applied. Predictions of crack mouth opening displacement (COD) and bending moment, both at initiation and collapse, using the GE-EPRI method may be different from experimental results; however, the method generally gives conservative predictions of the failure loads. A proper characterization of the mechanical properties of the material is needed. The crack opening area associated with through-wall cracked pipes of stainless steel material loaded under pure bending is fairly approximated by the ellipse-shaped crack model. Large size flaws develop subcritical crack growth before the maximum moment is reached. This stable growth may be completely absent in pipes with small cracks.

REFERENCES 1. Milella, P. P., Outline of nuclear piping research conducted in Italy. NucL Engrg. Des., 98 (1987) 219-29. 2. Maricchiolo, C. & Milella, P. P., Fracture behaviour of carbon steel pipes containing circumferential cracks at room temperature and 300°C. NucL Engrg. Des., 111 (1989) 35~,6. 3. Kanninen, M. F., Broek, D., Marschall, C. W., Rybicki, E. F., Sampath, S. G., Simonen, F. B. & Wilkowski, G. M., Mechanical fracture predictions for sensitized stainless steel piping with circumferential cracks. EPRI NP-192, September 1976. 4. Wilkowski, G. M., et al., Degraded Piping Program Phage II. NUREG/CR-4082, Vol. 2, July 1985. 5. Kumar, V., German, M. D. & Shih, C. F., An engineering approach for elasticplastic fracture analysis. EPRI NP-1931, July 1981. 6. Kumar, V., German, M. D., Wilkening, W. W., Andrews, W. R., deLorenzi, H. G. & Mowbray, D. F., Advances in elastic-plastic fracture analyses. EPRI NP-3607, August 1984.