Small-scale Approaches to Evaluate the Mechanical Properties of Quasi-brittle Reactor Core Graphite 1 Dong Liu, 1 Peter Heard, 2 Soheil Nakhodchi, 1,3

Small-scale Approaches to Evaluate the Mechanical Properties of Quasi-brittle Reactor

Core Graphite

1Dong Liu, 1Peter Heard, 2Soheil Nakhodchi, 1,3Peter EJ Flewitt

1Interface Analysis Centre, School of Physics, University of Bristol, Bristol, BS8 1TL, UK2Department of Engineering, University of Bristol, Bristol, BS8 1TR, UK

3HH Wills Physics Laboratory, University of Bristol, Bristol, BS8 1TL, UK

19th-20th September, 2013

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Symposium on Graphite Testing for Nuclear Applications: the Significance of Test Specimen Volume and Geometry and the Statistical Significance of Test Specimen Population

Content

• Background

• Measurement methods• Four-point bending (centimetre)• Brazilian disc (millimetre)• Micro-scale cantilever (micrometre)• Nano-indentation (micrometre)

• Concluding comments

2


Symposium on Graphite Testing for Nuclear Applications

Background

• Important to be able to measure the mechanical properties of both virgin and irradiated graphites.

• Test samples obtained by trepanning from bricks or from monitoring schemes. These provide centimetre scale test specimens.

• Ideally like to reduce specimen size. The use of small scale test specimens offers benefits when handling irradiated reactor core graphite.

• How small can specimens be to achieve representative data.

• Scaling over length is always a challenge.

• Aim to explore the potential benefits, difficulties and value of small scale mechanical tests for this particular application.

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Background

• Quasi-brittle behaviour

4

PGA graphite

Gilsocarbon graphite



Four-point bending (PGA graphite)

• Virgin and irradiated PGA graphite

• Cross-section: 25 mm x 25 mm• 20% porosity – virgin PGA graphite• 48% porosity – irradiated PGA graphite

5

150 mm

50 mm




• Load-displacement curve

• Reloading follows the linear route of the previous cycle

• Residual permanent displacement• Peak load and post peak

softening is visible• Linear variable differential

transformer (LVDT) for vertical displacement measurement

6

600 N

900 N

1200 N

Peak load = 1370 N

0.07 mm




• Mechanical properties (Along the extrusion direction for PGA)

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Virgin PGA graphite Irradiated PGA graphite

Elastic modulus (GPa) 7.5 ± 0.4 (Ref: 5-15 GPa) < 7.5

Flexural strength (MPa) 13.8 ± 0.6 (Ref: 10-20 MPa) 6.5



Brazilian disc (Gilsocarbon and PGA graphite)

• Experimental setup

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12 mm

Gilsocarbon graphite

• PGA graphite disc • 12 mm dia. x 6 mm

• Gilsocarbon graphite disc• 12 mm dia. x 6.0 mm• 12 mm dia. x 4.3 mm• 12 mm dia. x 4.0 mm• 12 mm dia. x 3.8 mm




• Experimental setup

• South Bay Technology Inc. Model 650 low speed diamond wheel saw

• Deben compression / tensile stage (MicroTest 2000 model, Gatan Ltd., Abingdon, Oxon, UK)

• Compression speeds of between 0.033 and 0.4 mm/min

• Load cell with maximum of 2 KN• Curved anvils with distributed load

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Curved anvilCurved anvil




10

• Load-displacement curve (12 mm dia. x 6 mm disc)




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• Load-displacement curve (12 mm dia. x 4 mm disc)

GILSO




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• Mechanical properties (VE=vertical to extrusion direction)

Gilsocarbon graphite PGA graphite (VE)

Elastic modulus (GPa) 2.2 ± 0.4 (Ref: 10 GPa) 1.4 ± 0.3 (Ref: 4.8 GPa)

Tensile strength (MPa) 15.6 ± 1.3 (Ref: 20 MPa) 8.5 ± 1 (Ref: 9 MPa)

400 μm

Fractured surfaceof Gilsocarbon graphite



Micro-scale cantilever beams (Gilsocarbon)

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• A new approach is developed to prepare beams

• Dualbeam workstation

(FEI Helios NanoLab 600i Workstation)

• Force measurement system (FMS)

(Kleindiek Nanotechnik)

• Select specific microstructure features

Step I

Step II



Micro-scale cantilever beams (Gilsocarbon)

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5 μm



Merits of new method

• No tapering

• The root is visible

• Fracture surface exposed

• View specimen throughout the test

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5 μm1 μm



Loading approach

• Multiple loading cycles

• Vary loading directions

• Imaging at a load on hold

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4 μm



Loading curves

• Linear → softening → Linear

• Direction dependent properties

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0.0 0.5 1.0 1.5 2.0 2.50

10

20

30

40

Load

(m

N)

Displacement (m)

Load 1 Load 2

Seperatedunloading

Softening region

Load 3

Linear region

Overlapped loading



Mechanical properties

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Elastic modulus Tensile strength Flexural strength

Gilsocarbon graphite > 20 GPa > 9.8 GPa > 10 GPa



Nano-indentation

Experimental and specimens

• Cylinder Gilsocarbon graphite specimen (22 mm dia. x 30 mm thickness)

• Agilent Nano Indenter G200(Culham Centre for Fusion Energy, Materials Facility, UK)

• A square array of 5 x 5 of distributed indents are performed on the flat surface

• Berkovich diamond tip

• Load control with the maximum load of 10 mN and peak hold time of 10 s

• Select specific microstructure features

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20

Nano-indentation

Elastic modulus

Gilsocarbon graphite 7.1 - 9.3 GPa



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Summary of data

Materials Elastic modulus (GPa)

Tensile strength Flexural strength

Four-point bending

PGA (virgin) 7.5 ± 0.4 - 13.8 ± 0.6

PGA (irradiated) < 7.5 - -

Brazilian disc

PGA (virgin) 1.4 ± 0.3 8.5 ± 1 -

Gilsocarbon 2.2 ± 0.4 15.6 ± 1.3 -

Micro-scale cantilever beam

Gilsocarbon > 20 GPa > 9.8 GPa > 10 GPa

Nano-indentation

Gilsocarbon 7.1 - 9.3 - -



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Concluding comments

• Small-scale testing is a powerful approach to explore mechanical properties of quasi-brittle materials

• Properties measured depend upon length scale of tests

• Centimetre scale tests provide representative global measurements

• Millimetre scale tests could provide representative global values if sufficient tests conducted to sample the material – requires statistical analysis of these data as porosity increases this becomes more difficult

• Micro-scale and below provide measurements for specific micro-scale features

• Important to understand what information is required for a particular application



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Acknowledgement

We acknowledge the financial support from EPSRC funded project – QUBE

(QUasi-Brittle fracture: a 3D Experimentally-validated approach). Grant number:

EP/J019801/1. The Materials Research Laboratory at the Culham Centre for

Fusion Energy was used for the nano-indentation on Gilsocarbon graphite.



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Documents

Small-scale Approaches to Evaluate the Mechanical Properties of Quasi-brittle Reactor Core Graphite 1 Dong Liu, 1 Peter Heard, 2 Soheil Nakhodchi, 1,3