Determining the Shear Fracture Toughness, K IIc , for two grades of graphite

1. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite Tim Burchell Oak Ridge National Laboratory ASTM Symposium on Graphite Testing for Nuclear Applications: The Significance of Test Specimen Volume and Geometry and the Statistical Significance of Test Specimen Population

2. Acknowledgments This work is sponsored by the U.S. Department of Energy, Office of Nuclear Energy Science and Technology under contract DE-AC05-00OR22725 with Oak Ridge National Laboratory, managed by UT-Battelle, LLC.

3. Overview of Presentation Objectives of study Introduction Specimen geometries Experimental Results & Discussion (DEN Compression Specimens) Conclusions

4. Objectives of Study To model graphite biaxial failure data we need a failure criteria The Shetty mixed mode fracture mechanics criteria when combined with a Microstructural fracture model can describe the biaxial data, but require knowledge of KIIc Thus we need to define KIIc for the graphite grades of interest Preferably measure KIIc by two techniques (verification) Determine if KIIc is subject to influence from texture and specimen volume

5. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite Introduction

6. Shetty Mixed Mode Fracture Mechanics Shetty mixed mode fracture criteria and Burchell fracture model for graphite are combined to predict bi-axial failure envelope and failure probabilities D. K. Shetty, Trans ASME 109 (1987) 282-289 KI is the mode I stress-intensity factor, KII is the mode II stress-intensity factor, and KIc is the mode I critical stress-intensity factor (or mode I fracture toughness) and C is an empirical constant (the Shetty shear-sensitivity coefficient)

7. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite Mode I or crack opening mode, KIc: Mode II, plane shear mode, KIIc: Mode III, Anti-plane shear, KIIIc

8. Biaxial Test Facility

9. Specimen and Grip Alignment

10. Defining the Biaxial Stress Quadrants σ1 (Axial) EXTERNAL PRESSURE PLUS TENSILE LOADING -σ2 (Hoop) σ2 (Hoop) EXTERNAL PRESSURE PLUS COMPRESSIVE LOADING INTERNAL PRESSURE PLUS COMPRESSIVE LOADING -σ1 (Axial)

11. First and Fourth Stress Quadrant Biaxial Strength Data for NBG-18 EXPERIMENTAL RESULTS PREDICTION & MODEL FIT

12. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite What experimental methods and specimen geometries exist for the determination of critical shear stress intensity factor, KIIc, or shear fracture toughness?

13. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite • Mode II Testing Configurations • Shear stress along a crack • Iosipescu specimen • Push-off specimen • Punch-through specimen • Four notch cylinder • Mixed – mode device according to Richard • Mixed – mode device according to Arcan • Mixed-mode disc loading (cracked chevron notched Brazilian disc) • Off center notched beam Otto Graff Journal, Vol. 16, 2005

14. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite Specimen Geometries

15. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite Two promising specimen geometries were selected Cracked Chevron Notched Brazilian Disc (CCNBD) Specimen Similar to centrally slotted disc which has been used in the past for graphite, hence some literature data – central notch is cut from both sides with slitting saw to form chevron, thus difficult to vary specimen volume & geometry. Mixed mode thus can measure KIc or KIIc Double Edge Notched Compression (DENC) Specimen Relatively simple slotted rectangular geometry, thus easy to vary specimen volume & geometry. Has been used to test concrete, but NO graphite literature.

16. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite CCNBD Specimen The notch is cut with a circular slitting saw from both sides of the specimen, thus the solid ligament has a “chevron” shape on either end of the central slot. Same geometry gives mixed fracture modes or pure KIc or KIIc depending on the test angle φ.

17. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite 96 specimens being machined 2 graphite grades KIc and KIIC 4 notch variants, disc diameter 75 or 100 mm 6 replicates specimens Testing to commence in Dec/Jan timeframe

18. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite

19. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite The specimen volume varies over two orders of magnitude!

20. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite

21. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite Experimental

22. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite • MTS servo-hydraulic 4-post 110 kip load frame • 100 kip load cell • Crosshead speed 0.001 in/sec or 25 μm/sec • Hemispherical compression platens • Upper platen “floats’ to self level • Specimen compressed between two square steel plates • Lab-view control software

23. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite

24. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite MATERAILS: Nuclear grade NBG-18 graphite. Manufactured by SGL Carbon, vibrationally molded, filler particle size 1.6 mm (max) Nuclear grade PCEA graphite. Manufactured by GrafTech International (GTI), extruded, filler particle size 0.8 mm (max). AREVA NGNP reference grade SPECIMENS Four DEN Compression specimen geometries, 2w=20, 50, 100 & 130 mm 48 specimens, 26 tested.

25. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite

26. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite Results & Discussion

27. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite Type 1 specimens, 2w = 20 mm, compressive failure

28. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite Type 2 specimens, 2w = 50 mm, compressive failure

29. Compression behavior of concrete, samples, showing load discontinuity or critical shear load, i,e., load at which the shear failure occurs BEHAVIOR OF CONCRETE

30. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite

31. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite Type 3 specimen, 2w = 100 mm, shear failure

32. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite

33. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite Type 3 specimen, 2w = 100 mm, shear failure

34. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite

35. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite Type 3 specimen, 2w = 100 mm, shear failure

36. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite

37. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite Type 3 specimen, 2w = 100 mm, shear failure

38. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite

39. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite Type 3 specimen, 2w = 100 mm, shear failure

40. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite Type 3 specimen, 2w = 100 mm, partial shear failure

41. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite Type 4 specimen, 2w = 130 mm, compressive failure

42. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite

43. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite Type 4 specimens, 2w = 130 mm, shear failure

44. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite

45. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite From the concrete literature KIIc ≈ 2KIc (Xu & Reinhardt) For nuclear graphite KIc ≈ 0.8 to 2.5 MPa√m Hence KIIc ≈ 1.6 to 5.0 MPa√m

46. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite

47. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite

48. Conclusions

49. Determining the Shear Fracture Toughness, KIIc, for two grades of graphite