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Se-Hwan Chi. Ph. D.

15 th International Nuclear Graphite Specialist Meeting (INGSM-15), September 15-18, 2014, Hangzhou, China. Evaluation and Comparison of Fracture Behavior of Selected Nuclear Graphite by Small Size SENB Specimens. Se-Hwan Chi. Ph. D. NHDD Project (Nuclear Graphite Study), KAERI. Content.

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Se-Hwan Chi. Ph. D.

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  1. 15th International Nuclear Graphite Specialist Meeting (INGSM-15), September 15-18, 2014, Hangzhou, China Evaluation and Comparison of Fracture Behavior of Selected Nuclear Graphite by Small Size SENB Specimens • Se-Hwan Chi. Ph. D. NHDD Project (Nuclear Graphite Study), KAERI

  2. Content Background Materials, Specimen, Test Jig Fracture Toughness Testing and KIC, G-Δa Determination 4. 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 .

  3. Background 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. o

  4. Surveillance Program of Core Materials PWR 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. Limited capsule space for surveillance specimens requires small volume specimen and small specimen test techniques

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

  6. Surveillance Program of Core Materials • HTGR HTTR (Japan) No guide on strength measurements yet ! In HTGR, while limited space for surveillance specimensalsoneeds 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 Specimen Volume and Geometry and the Statistical Significance of Test Specimen Population (Sept. 19-20, 2013 Hilton Seatle, Seatle, Washington, USA)

  7. 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

  8. 2. Materials, Specimen and Testing Fixture

  9. 2. Materials, Specimen and Testing Fixture Notchdepth: 1.6 mm, Angle: 30° (EDM) Loading Rate: 0.1 mm/min

  10. 3. Fracture Toughness Testing and KIC, G-Δa Determination - g (a/W) for S/W = 8 (IG-430) (NBG-18) Load-Displacement curve

  11. 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 = notchdepth 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

  12. 4. Results/Discussion 4.1 KIC MPa(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

  13. 4. Results/Discussion 4.1 KIC Observation KIC tends to increases with specimen size Observed anisotropy in KIC except IG-110 and 430. Observed smaller value for IG-110 and larger value for NBG-18 and PCEA

  14. 4. Results/Discussion • 4.2 G-Δa (4T)

  15. 4. Results/Discussion • 4.2 G-Δa (6.5T)

  16. 4. Results/Discussion • 4.2 G-Δa (Grade) 6.5T

  17. 4. Results/Discussion 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 during 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. • B A GIC

  18. Determination of SCL and Δ G • B SCL=100*(2.06-0.25)/3.9 = 46 % 2525 J/m2 ΔG = (2525-750)/750 J.m2 = 237 % A GIC 2.06 mm 750 J/m2 0.25 mm

  19. 4. Results/Discussion 4.4 G-Δa Analysis Results

  20. 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.

  21. 4. Observation More or less smaller value due to small dimension Observed anisotropy in KIC except IG-110 and 430. 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.

  22. 5. Summary 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.

  23. Surface Filometry(Model: Dektak 150) IG-110 NBG-18 G-Δa curves will be compared with surface filometry measurements

  24. IG-430 NBG-18

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