Evaluation and Comparison of Fracture Behavior of Selected

Nuclear Graphite by Small Size SENB Specimens

Se-Hwan Chi. Ph. D.

15th International Nuclear Graphite Specialist Meeting (INGSM-15), September 15-18, 2014, Hangzhou, China

NHDD Project (Nuclear Graphite Study), KAERI

Content

1.Background2.Materials, Specimen, Test Jig3.Fracture Toughness Testing and KIC, G-Δa Deter-

mination4. Results 4.1 Specimen size effects on KIC and G-Δa of selected nuclear graphite grades 4.2 Comparison of Fracture Behavior of selected nuclear graphite grades. 5. Conclusion

.

1.Background

o

Graphite core components in HTGR experience dimensional, mechanical, physical property changes owing to neutron irradiation and oxidation during operation resulting in an increase in fracture probability of the components.

For safety, monitoring and reflecting the mechanical property change of the core components on reactor operating condition are important as seen in the PWR RPV surveillance program.

Surveillance Program of Core MaterialsPWR

Limited capsule space for surveillance spec-imens requires small volume specimen and small specimen test techniques

Small specimen test techniques

Various kinds of small specimens and small specimen test techniques are well developed in PWR technology

In HTGR, while limited space for surveillance specimens also needs graphite small specimen and small specimen test techniques as in PWR, related study and developments are progressing and further studies are required *

*ASTM Symposium on Graphite Testing for Nuclear Applications: the Significance of Test Speci-men Volume and Geometry and the Statistical Significance of Test Specimen Population (Sept. 19-20, 2013 Hilton Seatle, Seatle, Washington, USA)

HTTR (Japan)

Surveillance Program of Core Materials

No guide on strength measurements yet !

HTGR

Background / Purpose of Study

In this study, the fracture toughness (KIC) and G-Δa behavior of selected nuclear graphite grades were determined by small size fracture toughness specimens in size of 50.0 x 10.0 x 4.0 (4T) mm and 52 x 12 x 6.5 mm (6.5T)* based on a new ASTM fracture toughness testing procedure, ASTM D 7779-11**.

Obtained results ((KIC) and G-Δa) were discussed in view from specimen size and microstructure of the grades. * The 4T and 6.5T specimen correspond to 1/15 (6.5T) and 1/30 (4T) of the ASTM D 7779-11 recommendation

**ASTM D 7779-11 The Standard Test Method for the Determination of Fracture Toughness of Graphite at Ambient Temperature

2. Materials, Specimen and Testing Fixture

IG-110 IG-430 PCEA NBG-17 NBG-18 NBG-25

Grain Size(㎛ )

10 10 360 Max. 800Max: 1,600

20

Forming Method Iso-static Iso-static ExtrusionVibration molding

Vibration molding

Iso-static

Number of

Speci-men*

4T 13 10a c a c a c a c

10 8 10 10 4 7 10 10

6.5T 8 8 8 8 7 8 8 8 8

2. Materials, Specimen and Testing Fixture

Notch depth: 1.6 mm, Angle: 30° (EDM)

Loading Rate: 0.1 mm/min

3. Fracture Toughness Testing and KIC, G-Δa Determination

- g (a/W) for S/W = 8

0.80 0.88 0.96

0

10

20

30

40

50

Load (

N)

Displacement (mm)

IG-430

0.80 0.88 0.96

0

10

20

30

40

50

Load (N

)

Displacement (mm)

NBG-18

Load-Displacement curve

(IG-430)

(NBG-18)

G-Δa Curve

Cn = Dn/Pn

Cn: Compliance for the point n (m/N)Dn: displacement for the point n (m)Pn : loading force for the point n (N).

Initial Crack length, ao = notch depth an = an-1+ [(W-an-1)/2*((Cn–Cn-1)/Cn-1)]

G(an) = P2/2B*δC/δa [J/m2], δC= Cn– Cn-1, δa= an- an-1

G-Δa, where Δa = an – a0

4. Results/Discussion

4.1 KICMPa(m)1/2

(1) M. Eto, et al, Int. Sym. On Carbon (1990), 8 type of specimens.(2) S. Fazluddin and B. Rand (2002, Univ. of Leeds), TB: 100 x 10 x

12, CT: 50 x 48 x 10 (3) Haiyan Li, CARBON (2013) 46, TB: 45 x 10 x 5 mm.(4) T. D Burchell, Proc. HTR2012 (2012), TB: 50 x 6 x 6 mm

IG-110 IG-430 NBG- 2 5-a

NBG- 25-c

NBG- 17-a

NBG- 17-c

NBG- 18-a

NBG- 18-c PCEA-a PCEA-c

KIC

[MPa(m)1/2

]

4T0.76 ± 0.02

0.91 ± 0.03

0.95 ± 0.02

0.93 ± 0.02

0.85 ±0.07

0.96 ±0.05

1.11 ±0.04

1.02 ±0.04

0.90 ±0.03

0.88 ±0.05

6.5T

0.78 ± 0.02

1.02± 0.01 0.99 ±0.01 1.01

± 0.051.06

± 0.031.10

± 0.071.04 ±0.13

1.15 ±0.02

1.07 ±0.04

Ref

0.83 -1.20(1)

1.00 -1.13(2)

1.27 (3)

±0.090.971± 0.038 (4)

0.941±0.060(4)

4. Results/Discussion

Observation1. KIC tends to increases with specimen size 2. Observed anisotropy in KIC except IG-110 and 430.3. Observed smaller value for IG-110 and larger value for NBG-18 and PCEA

4.1 KIC

4. Results/Discussion 4.2 G-Δa (4T)

4. Results/Discussion 4.2 G-Δa (6.5T)

4. Results/Discussion4.2 G-Δa (Grade)

6.5T

4.3 Analysis of G-Δa Curve

Based on the corresponding G and Δa of the intersection point A and B, the GIC, Δa, stable crack growth length to initial ligament (SCL)(%), an increase in ΔG dur-ing stable crack growth against GIC , i.e.,(GB-GA) 100/ GIC, and stable strain energy release rate (ΔGB-A/ ΔaB-A) were determined and compared.

GIC

AB

4. Results/Discussion

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0

0

1000

2000

3000

4000

5000

Str

ain E

ner

gy

Rel

ease

Rat

e, G

(J

/m2 )

Crack Extension, Da

PCEA-a test1 test2 test3 test4 test5 test6 test7 test8

GICA

B

750 J/m2

2525 J/m2

0.25 mm

2.06 mm

ΔG = (2525-750)/750 J.m2

= 237 %

SCL=100*(2.06-0.25)/3.9 = 46 %

Determination of SCL and Δ G

4. Results/Discussion4.4 G-Δa Analysis Results

IG-110 IG-430 NBG- 2 5-a

NBG- 25-c

NBG- 17-a

NBG- 17-c

NBG- 18-a

NBG- 18-c

PCEA- a

PCEA - c

GIC

[J/m2

],

Δa

(mm)

4T 67 100 100 117 175 175 233 167 163 225

Δa 0.18 0.10 0.13 0.13 0.21 0.22 0.20 0.16 0.16 0.24

6.5T 200 300 300 690 690 800 560 750 670

Δa 0.25 0.13 0.19 0.28 0.25 0.44 0.25 0.25 0.20

SCL

(%)

4T 48.0 58.0 62.5 52.0 50.00 52.10 62.0 62.5 46.7 39.6

6.5T 32.0 45.0 48.0 47.0 48.0 47.0 40.0 46.0 45.0

ΔG

(%)

4T 90 (133)

150 (120)

175 (175)

99 (85)

175 (100)

292 (167)

400 (172)

375

(225)

404 (248)

225 (100)

6.5T 225 (112)

575 (192)

600 (200)

1435 (208)

1310 (190)

1400 (175)

878

(157)

1775 (237)

1455 (217)

(ΔGB-

A /ΔaB-A) J/m3

4T 73.0 88.4 116.7 94.0 168.3 211.6 112.0 250.0 360.7 ?

6.5T 180.0 326.7 317.5 775.7 696.8 773.5 566.5 980.7 ?

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.00

500

1000

1500

2000

2500

3000

3500

4000

4500

Str

ain

Energ

y R

ele

ase

Rate

, G

(J

/m

2)

Crack Extension, Da (mm)

IG-110 IG-430 NBG-17a NBG-17c NBG-18a NBG-18c NBG-25 PCEA a PCEA c

4.5 Comparison of G-Δa curves

G- Δa from the extruded (or vibration molded) medium particle size grades is higher than the iso-molded fine particle size grades.

4. Observation

1. More or less smaller value due to small dimension 2. Observed anisotropy in KIC except IG-110 and 430.3. Observed the smallest value for IG-110, the largest value for NBG-18. 4. GIC : NBG-17, 18, PCEA > IG-110, -430, NBG-25

5. Differences between the fine grain, isostatic molding and medium grain, vibration molded or extruded in G-Δa “global behavior” is far larger and apparent than the differences in “local parameter” KIC .

6. No large differences in SCL.7. No correlation between the KIC and G-Δa.

The results show that, though the KIC value from 6.5T was largely a little higher than the KIC

from 4T for 2-28%, the 6.5 T KIC values appear a little smaller (15-30 % for IG-110, NBG-18) or

larger (12.0 – 15.6 % for PCEA) than the reported values from larger size specimens.Overall, the KIC and GIC from the extruded (or vibration molded) medium particle size grades

were a little higher than the iso-molded fine particle size grades.

Differences in GIC were larger than the differences in KIC between grades. While the changes

in KIC were small or negligible, a large increase in the GIC was observed with increasing

specimen size from 4T to 6.5T. Detailed analysis of G-Δa curves, showed that, on average, SCL was 53% and 44%, and ΔG was 152.5% and 187.5% for 4T and 6.5T specimens, respectively.

Present results show that both the local (KIC) and gross (G) fracture characteristics of the

Extruded (or vibration molded) medium particle size grades may largely be better than the iso-molded fine particle size grades, and the fracture parameter G seems to be appropriate in describing the fracture characteristics of nuclear graphite after crack initiation.

5. Summary

23

Surface Filometry (Model: Dektak 150)

IG-110 NBG-18

G-Δa curves will be compared with surface filometry measurements

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.00

500

1000

1500

2000

2500

3000

3500

4000

4500

Str

ain

Energ

y R

ele

ase

Rate

, G

(J

/m

2)

Crack Extension, Da (mm)

IG-110 IG-430 NBG-17a NBG-17c NBG-18a NBG-18c NBG-25 PCEA a PCEA c

NBG-18

IG-430