Fracture Characterisation of Reactor Core Graphite under Biaxial Loading

Fracture Characterisation of Reactor Core Graphite under Biaxial Loading

Dong Liu1,2, Mahmoud Mostafavi3 , Peter EJ Flewitt1,2, James Marrow3, David Smith4

1Interface Analysis Centre, School of Physics, University of Bristol, Bristol, BS8 1TL, UK2 HH Wills Physics Laboratory, School of Physics, University of Bristol, Bristol, BS8 1TL, UK

3Department of Materials and Oxford Martin School, University of Oxford, Oxford OX1 3BD, UK4Department of Mechanical Engineering, University of Bristol, Bristol, BS8 1TR, UK

Contact: [email protected]

15th - 18th September, 2013

114th International Nuclear Graphite Specialists MeetingSeattle, USA

Content

• Background

• Materials

• Finite Element Analysis

• Experimental

• Results

• Concluding comments

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Background

• Quasi-brittle material

• Heterogeneous

• Polygranular

• Aggregate

• Porous

3



Background

• Gilsocarbon graphite

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12 mm

GilsocarbonLeft: Optical image; Right: Electron image;



Background

• Gilsocarbon graphite• The reactor core graphite is subject to (i) a range of

loading conditions that develop over the service life and (ii) change of microstructure due to radiolytic oxidation.

• It was established that the strain energy release rates of stable propagating shallow cracks (0.5 to 1 mm depth but up to tens of mm in surface length) are very different under uniaxial and equi-biaxial loadings.

• The average initiation toughness is Jc = 176 ± 44 J/m2 under uniaxial and Jc = 779 ± 97 J/m2 under equi-biaxial stress.

5

M. Mostafavi, S. A. McDonald, H. Çetinel, P. M. Mummery and T. J. Marrow: Carbon Vol. 59 (2013), p. 325.



Background

• Gilsocarbon graphite

• We wish to explore a range of biaxial loading conditions

• Test based on a cruciform geometry specimen with four-point loading. Vary loading positions on the arms to change biaxiality.

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Material I

• Cruciform virgin Gilsocarbon graphite • Pre-slot specimen with θ = 0 to 45°

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Slot (~ 10 mm x 5 mm x 0.6 mm)(a) (b)

50 mm

50 mm

25 mm

θ



Material I

• Five-point loading • Clip gauges to measure crack mouth opening• Acoustic emission• Camera imaging

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Loading jig

Roller supporter

Camera with LED lights Acoustic emission

Strain gauges

Clip gauges



Material II

• Cruciform virgin Gilsocarbon graphite

• Plain specimen

• Strain gauges

• Acoustic emission

• Camera imaging

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Strain gauges



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Finite Element Analysis



• FEA model showing the stress distribution and that the cracks initiate at the slot root (E=9.5GPa).

• Stress intensity factor, K Distribution of K when a = 5 mm K change with a (5 to 10 mm)

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Finite Element Analysis



Calibration using Perspex specimen

• Change of the roller positions to change the loading biaxiality. These selected positions are from 1 to 3 (min, mid and max).

• Pre-calibrated using Perspex specimens with four strain gauges.

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33

3

3

32

3

2

50 100 150 2000

500

1000 Channel I Channel II Channel III Channel IV

Str

ain

Time (s)50 100 150 200

0

600

1200

Strai

n

Time (s)

Channel I Channel II Channel III Channel IV



• Specimen with pre-slots but different biaxiality

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σo

σv

σo

σv

0.0 0.1 0.2 0.30

-1

-2

-3

-4

-5

Load

(K

N)

Crack mouth opening displacement (mm)

3-0 CMOD

3-3 CMOD

3-2 CMODσo:σv = 1:2

σo:σv = 1:0σo:σv = 1:1

Results



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Strain ratioσo:σv

Peak load(KN)

Max crack opening before nonlinear (mm)

Crack mouth opening compliance

1:2 4.87 0.10 -

1:1 4.01 0.07 2.5 x 10-5 mm/N

1:0 1.66 0.03 1.7 x 10-5 mm/N

Results





• Fracture surface varies with loading condition.

• The crack propagate at the crack tip then deflects due to the biaxiality.

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Results



• Plain specimen

• Load-displacement curve under 3-3 equi-biaxial loading

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Results

-

-

-



• Load-displacement curve under 3-3 equi-biaxial loading - Acoustic emission graph

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Initial calibration

• Plain specimen

Results



• Load-displacement curve under 3-2 loading and acoustic emission graph

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0.0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 -0.80

-1

-2

-3

-4

-5

-6

-7

-8

Load

(K

N)

Displacement (mm)

cycle 1

Higher than this load, micro-cracking is detected by AE

cycle 2 cycle 3

cycle 4 cycle 5 cycle 6

• Plain specimen

Results



Concluding comments

• The current experimental arrangement demonstrates that different degrees of biaxiality may be applied to a notched and plain cruciform specimen, and the load-displacement behaviour monitored.

• Acoustic emission provides evidence that micro-scale cracking occurs within the specimen prior to the peak load.

• Under multiple loading cycles, the curve follows the gradient of the previous cycle.

• The crack geometry will be verified by examination of interrupted tests in the non-linear range of the load-displacement curve. In addition the specimens will be wedged open and observed by X-ray tomography and high spatial resolution serial sectioning. The effect of notch orientation relative to the cruciform specimen axes will also be studied to verify the stress states that are developed in this specimen.

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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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Rhossili bay, Gower

Fracture Characterisation of Reactor Core Graphite under Biaxial Loading

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