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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
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19th-20th September, 2013
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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19th-20th September, 2013
Symposium on Graphite Testing for Nuclear Applications
Background
• Quasi-brittle behaviour
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PGA graphite
Gilsocarbon graphite
19th-20th September, 2013
Symposium on Graphite Testing for Nuclear Applications
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
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150 mm
50 mm
19th-20th September, 2013
Symposium on Graphite Testing for Nuclear Applications
Four-point bending (PGA graphite)
• 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
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600 N
900 N
1200 N
Peak load = 1370 N
0.07 mm
19th-20th September, 2013
Symposium on Graphite Testing for Nuclear Applications
Four-point bending (PGA graphite)
• 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
19th-20th September, 2013
Symposium on Graphite Testing for Nuclear Applications
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
19th-20th September, 2013
Symposium on Graphite Testing for Nuclear Applications
Brazilian disc (Gilsocarbon and PGA graphite)
• 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
19th-20th September, 2013
Symposium on Graphite Testing for Nuclear Applications
Brazilian disc (Gilsocarbon and PGA graphite)
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• Load-displacement curve (12 mm dia. x 6 mm disc)
19th-20th September, 2013
Symposium on Graphite Testing for Nuclear Applications
Brazilian disc (Gilsocarbon and PGA graphite)
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• Load-displacement curve (12 mm dia. x 4 mm disc)
GILSO
19th-20th September, 2013
Symposium on Graphite Testing for Nuclear Applications
Brazilian disc (Gilsocarbon and PGA graphite)
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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
19th-20th September, 2013
Symposium on Graphite Testing for Nuclear Applications
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
19th-20th September, 2013
Symposium on Graphite Testing for Nuclear Applications
Micro-scale cantilever beams (Gilsocarbon)
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5 μm
19th-20th September, 2013
Symposium on Graphite Testing for Nuclear Applications
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
19th-20th September, 2013
Symposium on Graphite Testing for Nuclear Applications
Loading approach
• Multiple loading cycles
• Vary loading directions
• Imaging at a load on hold
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4 μm
19th-20th September, 2013
Symposium on Graphite Testing for Nuclear Applications
Loading curves
• Linear → softening → Linear
• Direction dependent properties
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0.0 0.5 1.0 1.5 2.0 2.50
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20
30
40
Load
(m
N)
Displacement (m)
Load 1 Load 2
Seperatedunloading
Softening region
Load 3
Linear region
Overlapped loading
19th-20th September, 2013
Symposium on Graphite Testing for Nuclear Applications
Mechanical properties
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Elastic modulus Tensile strength Flexural strength
Gilsocarbon graphite > 20 GPa > 9.8 GPa > 10 GPa
19th-20th September, 2013
Symposium on Graphite Testing for Nuclear Applications
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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19th-20th September, 2013
Symposium on Graphite Testing for Nuclear Applications
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Nano-indentation
Elastic modulus
Gilsocarbon graphite 7.1 - 9.3 GPa
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Symposium on Graphite Testing for Nuclear Applications
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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 - -
19th-20th September, 2013
Symposium on Graphite Testing for Nuclear Applications
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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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Symposium on Graphite Testing for Nuclear Applications
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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.
19th-20th September, 2013
Symposium on Graphite Testing for Nuclear Applications
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19th-20th September, 2013
Symposium on Graphite Testing for Nuclear Applications
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