1. Evaluation of Fracture toughness of fine- and coarse-grain graphite
J. Sumita1, T. Shibata1, Y. Tachibana1, M. Kuroda2
1 : Nuclear Hydrogen and Application Research Center , JAEA
2 : Kumamoto University
14th International Nuclear Graphite Specialist Meeting
15-18th September, 2013, Seattle , USA 1 1

2. Contents
Introduction
Experiments
Results
Summary

3. Non-destructive inspection
Methodology to confirm the integrity of graphite component in HTTR -Acceptance inspection-
In order to remove the components with harmful defects, the non-destructive inspection UT and ET are carried out for the components in acceptance inspection.
Structure
Non-destructive inspection
Material
After machining
Control rod guide block UT ET
Hot plenum block -
Support post/seat
UT : Ultra sonic test, ET : Eddy current test
Components without harmful defects do not fracture through the in-service period on the basis of fracture mechanics.

4. Methodology to confirm the integrity of graphite component in HTTR -In-service inspection-
In order to confirm the integrity of the graphite components in in-service inspection,
Surveillance test
Mandatory or non-mandatory (depending on necessity)
Visual inspection by TV camera etc.
Mandatory (for HTTR)
If a defect ( > design assumption) is found in graphite components by in-service inspection,
Causes shall be investigated
Propriety of continuous use shall be judged by application of fracture mechanics etc.
It is necessary to understand the fracture mechanism of graphite.

5. Objectives
Previous studies of evaluation on fracture features of graphite
Effect of notch sharpness and size of specimen
Effect of oxidation, etc
Experimental methodology to determine fracture　toughness of graphite (ASTM D7779)
Some qualitative theories for fracture of graphite
To characterize fracture features of fine- and coarse-grained graphites
The three-point-bending test is carried out for two kinds of specimens, a fine- and coarse-grained graphite.
The fracture mechanism of both fine- and coarse-grained graphites is investigated.

6. Contents
Introduction
Experiments
Results
Summary

7. Typical properties of G347 and FE250
Material
Fine-grained isotropic graphite
G347 (Manufactured by TOKAI CARBON)
Coarse-grained extruded graphite
FE250 (Manufactured by TOKAI CARBON)
Typical properties of G347 and FE250
Grade
Bulk density
(g/cm3)
Bending strength
(MPa)
Tensile strength
Thermal expansion*
(10-6/K)
Thermal conductivity
(W/m・K)
G347
1.85
49.0
31.4
5.5
116
FE250
1.75
24.5 -
3.3
162
*RT – 1000oC

8. Specimen preparation Single edge notched beam specimen
Straight-through notch
Specimen geometry
Introduce of notch: razor blade
Notch angle: approximately 15o
Depth and width of notch: profile projector
Cleaning: ethanol and acetone in the ultrasound bath
Root radius and notch angle: laser microscope

9. Fracture toughness test
Three-point-bending test
Three-point-bending test setup
Outer support span: 160 mm
Crosshead speed: 0.1 mm/min
Sampling rate to record load and displacement: 200 Hz

10. Calculation of fracture toughness
Calculation of value of fracture toughness
Fracture toughness KC is given by linear fracture mechanics
KC : Fracture toughness (MPa/m1/2)
Pmax : Maximum force (N)
S : Support span (m)
B : Specimen breadth (m)
W : Specimen width (m)
a : Notch depth (m)
The value of the fracture toughness was calculated using the maximum force on the load-displacement curve obtained by the three-point-bending test.

11. Contents
Introduction
Experiments
Results
Summary

12. Load-displacement curves
A typical load-displacement curve
Displacement (mm)
Load (N)
A : across grain
<
FE250A
G347
Slope
Maximum load Pmax
Slope
Maximum load Pmax

13. Value of fracture toughness (Obtained in this study)
Grade
Pmax(N)
Kc(MPa/m0.5)
(Obtained in this study)
G347
148.3
1.15
FE250A
125.0
0.96
FE250W
132.4
1.02
A : across grain, W : with grain
Previous study for G347*1
Fracture toughness obtained by CT specimen (1.06 MPa/ m0.5)
Fracture toughness obtained by SENB specimen (1.09 MPa/ m0.5)
*1 Ekinaga, at. El., INGSM 9
Nearly equal

14. Fracture toughness of fine-grained graphite
Grade
Pmax(N)
Kc(MPa/m0.5)
(ASTM)
(Previous study)
IG110
115.3
0.89*1
0.85 *2
IG430
136.0
1.04*1
0.91*2
ETU10
121.0
0.93*3 －
*1 Yamada, et. al., HTR
*2 Kurumada, et. al., Transaction of the Japanese society of the mechanical engineeras, A,63(608), , (1997)
*3 Matsushima, et. al., M&M 2012 (in Japanese)
The fracture　toughness of fine-grained graphite obtained in accordance with ASTM D7779 is almost the same as that reported in the previous studies.
The value of fracture toughness of the fine-grain graphite is not different from that of the coarse-grain graphite.

15. Application to HTGR components
Fracture stress
σc : Fracture stress (MPa)
KIC : Fracture toughness (MPa/m1/2)
a : Harmful defect size(m)
α : Shape factor
𝜎 𝑐 = 𝐾 𝐼𝐶 𝛼 𝜋∙𝑎
The fracture stress depends on the harmful defect size.
Fine-grained graphite
Small harmful defect
High fracture stress
The employment of fine-grained graphite for core components of the HTGR has some advantages.

16. Fracture mechanism
Grain
Binder (pitch)
Pore
Crack
Grain size
Large
Small
The fracture of graphtie is influenced by pre-existing defects or weak region.
The crack would basically propagate in pore.
If higher stress applied, the crack would propagate in binder.
The crack in both fine-grained and coarse-grained graphite would propagate in pores and binders along with grain boundary.
If the direction of crack growth corresponds to the grain with proper orientation, the crack would propagate inside grain.
In order to confirm this mechanism, the cracks propagation are observed.

17. Observation of crack propagation
Method
Stereomicroscope ：OLYMPUS SZX7
Material :G347, FE250W, FE250A
Observed area
Notch
Crack

18. Crack observation by stereomicroscope （G347）
1000um ×
Crack
The crack seems to propagate straight.

19. Crack observation by stereomicroscope （FE250W）×
Crack
Inside grain? ×
The crack seems to propagate in pores and binders.
Some cracks seem to propagate inside grain.
1000um

20. Crack observation by stereomicroscope （FE250A）×
1000um
Crack
Inside grain?
The crack seems to propagate in pores and binders.
Some cracks seem to propagate inside grain.
Future works
Polarization microscope , EBSD (electron back scattering diffraction).

21. Contents
Introduction
Experiments
Results
Summary

22. Summary
The fracture　toughness of nuclear grade graphite obtained in accordance with ASTM D7779 is almost the same as that reported in the previous studies.
The value of fracture toughness of the fine-grained graphite is not different from that of the coarse-grained graphite.
The crack in coarse-grained graphite seems to propagate along with grain boundary and some cracks seem to propagate inside grain.
It is planned that the crack propagation in graphite is observed using the polarization microscope and EBSD to confirm the direction of crack growth.

23. Thank you for your attention!