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Evaluation of Fracture toughness of fine- and coarse-grain graphite. J. Sumita 1 , T. Shibata 1 , Y . Tachibana 1 , M. Kuroda 2 1 : Nuclear Hydrogen and Application Research Center , JAEA 2 : Kumamoto University. 14 th International Nuclear Graphite Specialist Meeting - PowerPoint PPT Presentation
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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 , JAEA2 : Kumamoto University
14th International Nuclear Graphite Specialist Meeting 15-18th September, 2013, Seattle , USA
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Contents
1. Introduction
2. Experiments
3. Results
4. Summary
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Methodology to confirm the integrity of graphite component in HTTR -Acceptance inspection-
UT : Ultra sonic test, ET : Eddy current test
Structure
Non-destructive inspection
Material After machining
Control rod guide block UT ET
Hot plenum block - ET
Support post/seat UT ET
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.
• Components without harmful defects do not fracture through the in-service period on the basis of fracture mechanics.
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Surveillance testMandatory or non-mandatory (depending on necessity)
Visual inspection by TV camera etc.Mandatory (for HTTR)
Causes shall be investigated
Propriety of continuous use shall be judged by application of fracture mechanics etc.
If a defect ( > design assumption) is found in graphite components by in-service inspection,
In order to confirm the integrity of the graphite components in in-service inspection,
It is necessary to understand the fracture mechanism of graphite.
Methodology to confirm the integrity of graphite component in HTTR -In-service inspection-
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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
Objectives
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.
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Contents
1. Introduction
2. Experiments
3. Results
4. Summary
Material
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Typical properties of G347 and FE250
Fine-grained isotropic graphite
Grade Bulk density(g/cm3)
Bending strength
(MPa)
Tensile strength
(MPa)
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
G347 (Manufactured by TOKAI CARBON)
Coarse-grained extruded graphite
FE250 (Manufactured by TOKAI CARBON)
http://www.tokaicarbon.co.jp/*RT – 1000oC
Specimen preparation
Introduce of notch: razor bladeNotch angle: approximately 15o
Depth and width of notch: profile projectorCleaning: ethanol and acetone in the ultrasound bathRoot radius and notch angle: laser microscope
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Single edge notched beam specimen200 15
20
6.9
8.0
+0.
00-0
.05
0.510.38
15º or 30º
A
Detail of A
B
0.25
45º
Detail of B
Specimen geometry
Straight-through notch
Fracture toughness test
Outer support span: 160 mm Crosshead speed: 0.1 mm/min Sampling rate to record load and displacement: 200 Hz
Three-point-bending test setup
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Loading direction
Rollers free to rotate outwards
Load cell
Laser displacement sensor
A/D converter
Amplifier
Computer
Three-point-bending test
Calculation of fracture toughness
Fracture toughness KC is given by linear fracture mechanics
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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)
Calculation of value of fracture toughness
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6max
12
310
Wa
Wa
BW
SPgKC
23861.120947.59381.1 WaWag
543 1270.57747.152142.19 WaWaWa
The value of the fracture toughness was calculated using the maximum force on the load-displacement curve obtained by the three-point-bending test.
Contents
1. Introduction
2. Experiments
3. Results
4. Summary
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Load-displacement curves
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Displacement (mm)
Loa
d (N
)
Slope Maximum load Pmax
Slope Maximum load Pmax
FE250A G347
<
A typical load-displacement curve
A : across grain
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Grade Pmax(N) Kc(MPa/m0.5)(Obtained in this study)
G347 148.3 1.15FE250A 125.0 0.96FE250W 132.4 1.02
Value of fracture toughness
*1 Ekinaga, at. El., INGSM 9
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)
Nearly equal
A : across grain, W : with grain
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Grade Pmax(N) Kc(MPa/m0.5)(ASTM)
Kc(MPa/m0.5)(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 -
Fracture toughness of fine-grained graphite
*1 Yamada, et. al., HTR2012-4-020*2 Kurumada, et. al., Transaction of the Japanese society of the mechanical
engineeras, A,63(608), 838-844, (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.
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Application to HTGR components
Fracture stress
𝜎 𝑐=𝐾 𝐼𝐶
𝛼√𝜋 ∙𝑎σc : Fracture stress (MPa)KIC : Fracture toughness (MPa/m1/2)a : Harmful defect size(m)α : Shape factor
The fracture stress depends on the harmful defect size.
Small harmful defect High fracture stress
Fine-grained graphite
The employment of fine-grained graphite for core components of the HTGR has some advantages.
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Fracture mechanism
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.
Grain Binder (pitch)PoreCrack
Grain size LargeSmall
In order to confirm this mechanism, the cracks propagation are observed.
Method
Notch
Crack
Observed area
Stereomicroscope : OLYMPUS SZX7
Material :G347, FE250W, FE250A
Observation of crack propagation
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Crack observation by stereomicroscope ( G347 )
1000um×
×
Crack
The crack seems to propagate straight.
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Crack observation by stereomicroscope ( FE250W )
×
×
Crack
1000um
Inside grain?
The crack seems to propagate in pores and binders.Some cracks seem to propagate inside grain.
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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 worksPolarization microscope , EBSD (electron back scattering diffraction).
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Contents
1. Introduction
2. Experiments
3. Results
4. Summary
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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.
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Thank you for your attention!