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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.

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Presentation on theme: "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."— Presentation transcript:

1 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 Peter EJ Flewitt 1 Interface Analysis Centre, School of Physics, University of Bristol, Bristol, BS8 1TL, UK 2 Department of Engineering, University of Bristol, Bristol, BS8 1TR, UK 3 HH Wills Physics Laboratory, University of Bristol, Bristol, BS8 1TL, UK 19 th -20 th September, 2013 1 Symposium on Graphite Testing for Nuclear Applications: the Significance of Test Specimen Volume and Geometry and the Statistical Significance of Test Specimen Population

2 Content Background Measurement methods Four-point bending (centimetre) Brazilian disc (millimetre) Micro-scale cantilever (micrometre) Nano-indentation (micrometre) Concluding comments 2 19 th -20 th September, 2013 Symposium on Graphite Testing for Nuclear Applications

3 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. 3 19 th -20 th September, 2013 Symposium on Graphite Testing for Nuclear Applications

4 Background Quasi-brittle behaviour 4 PGA graphite Gilsocarbon graphite 19 th -20 th September, 2013 Symposium on Graphite Testing for Nuclear Applications

5 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 19 th -20 th September, 2013 Symposium on Graphite Testing for Nuclear Applications

6 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 6 600 N 900 N 1200 N Peak load = 1370 N 0.07 mm 19 th -20 th September, 2013 Symposium on Graphite Testing for Nuclear Applications

7 Four-point bending (PGA graphite) Mechanical properties (Along the extrusion direction for PGA) 7 Virgin PGA graphiteIrradiated 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 19 th -20 th September, 2013 Symposium on Graphite Testing for Nuclear Applications

8 Brazilian disc (Gilsocarbon and PGA graphite) Experimental setup 8 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 19 th -20 th September, 2013 Symposium on Graphite Testing for Nuclear Applications

9 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 9 Curved anvil 19 th -20 th September, 2013 Symposium on Graphite Testing for Nuclear Applications

10 Brazilian disc (Gilsocarbon and PGA graphite) 10 Load-displacement curve (12 mm dia. x 6 mm disc) 19 th -20 th September, 2013 Symposium on Graphite Testing for Nuclear Applications

11 Brazilian disc (Gilsocarbon and PGA graphite) 11 Load-displacement curve (12 mm dia. x 4 mm disc) GILSO 19 th -20 th September, 2013 Symposium on Graphite Testing for Nuclear Applications

12 Brazilian disc (Gilsocarbon and PGA graphite) 12 Mechanical properties (VE=vertical to extrusion direction) Gilsocarbon graphitePGA 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 surface of Gilsocarbon graphite 19 th -20 th September, 2013 Symposium on Graphite Testing for Nuclear Applications

13 Micro-scale cantilever beams (Gilsocarbon) 13 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 19 th -20 th September, 2013 Symposium on Graphite Testing for Nuclear Applications

14 Micro-scale cantilever beams (Gilsocarbon) 14 5 μm 19 th -20 th September, 2013 Symposium on Graphite Testing for Nuclear Applications

15 Merits of new method No tapering The root is visible Fracture surface exposed View specimen throughout the test 15 5 μm 1 μm 19 th -20 th September, 2013 Symposium on Graphite Testing for Nuclear Applications

16 Loading approach Multiple loading cycles Vary loading directions Imaging at a load on hold 16 4 μm 19 th -20 th September, 2013 Symposium on Graphite Testing for Nuclear Applications

17 Loading curves Linear → softening → Linear Direction dependent properties 17 19 th -20 th September, 2013 Symposium on Graphite Testing for Nuclear Applications

18 Mechanical properties 18 Elastic modulusTensile strengthFlexural strength Gilsocarbon graphite> 20 GPa> 9.8 GPa> 10 GPa 19 th -20 th September, 2013 Symposium on Graphite Testing for Nuclear Applications

19 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 19 19 th -20 th September, 2013 Symposium on Graphite Testing for Nuclear Applications

20 20 Nano-indentation Elastic modulus Gilsocarbon graphite7.1 - 9.3 GPa 19 th -20 th September, 2013 Symposium on Graphite Testing for Nuclear Applications

21 21 Summary of data MaterialsElastic modulus (GPa) Tensile strengthFlexural strength Four-point bending PGA (virgin)7.5 ± 0.4-13.8 ± 0.6 PGA (irradiated)< 7.5-- Brazilian disc PGA (virgin)1.4 ± 0.38.5 ± 1- Gilsocarbon2.2 ± 0.415.6 ± 1.3- Micro-scale cantilever beam Gilsocarbon> 20 GPa> 9.8 GPa> 10 GPa Nano- indentation Gilsocarbon7.1 - 9.3-- 19 th -20 th September, 2013 Symposium on Graphite Testing for Nuclear Applications

22 22 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 19 th -20 th September, 2013 Symposium on Graphite Testing for Nuclear Applications

23 23 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. 19 th -20 th September, 2013 Symposium on Graphite Testing for Nuclear Applications

24 24 19 th -20 th September, 2013 Symposium on Graphite Testing for Nuclear Applications Guess which is the author

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