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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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23

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!