The effect of compressive pre-stress on the thermal expansion behaviour of anisotropic

nuclear grade graphite

M. Haverty, W. Bodel, B.J. MarsdenNuclear Graphite Research Group

The University of Manchestermaureen.haverty@postgrad.manchester.ac.uk

Contents

1. General Methodology used2. Preliminary Study3. Main Study4. Comparison between two studies5. High Resolution images

Motivation

• Previous studies have shown a change in CTE with stress– Gilsocarbon: Applied uniaxial compressive and

tensile stress 1

– Steels: Applied uniaxial tensile stress and pre-stress beyond elastic limit2

• Are these changes observed in PGA?• What causes these changes?

1. Preston, S.D. & Marsden, B.J., 2006. Changes in the coefficient of thermal expansion in stressed Gilsocarbon graphite. Carbon, 44(7), pp.1250–1257

2. Rosenfield, A.R. & Averbach, B.L., 1956. Effect of Stress on the Expansion Coefficient. Journal of Applied Physics, 27(2), pp.154–156.

3. .

CTE MEASUREMENT

Standards and Methodologies Used

• CN821-1 “Advanced technical ceramics-Monolithic ceramics-Thermo-physical properties-Part 1: Determination of thermal expansion” – Provides test method including, sample size;

reference standards and heating rates• Accuracy if followed:

– 5K/min 0.5 x 10-6 K-1

– 2K/min 0.1 x 10-6 K-1

CTE Measurement equipment

• Netzsch Proteus DIL– Pushrod dilatometer– Nitrogen atmosphere

• Al2O3 Reference standard– Glow runs– Filler piece for short samples (Al2O3)

Thermal Expansion Measurements

Al2O3 Sample Holder

Al2O3 Pushrod

Al2O3 Filler Piece

Graphite sample

PRELIMINARY TEST

Method

• Samples were cut in the AG and WG directions • Uniaxial compressive stress applied– Compressive strength: 27 MPa

• Max temp: 250 °CSample No. Stress (%)AG1/WG1 0AG2/WG2 20AG3/WG3 40AG4/WG4 60AG5/WG5 80AG6/WG6 90

Applying Stress

• Universal Load Testing Machine

• Compressive stress• Sample subdivided into

two sister samples• Excess used to face off

to correct tolerance

Φ =12 mm

H =18 mm

Φ =12 mm

H =6 mm

Comparison of unstressed samples with the literature

• Comparing unstressed sample values with Sutton and Howard 3

• Two blocks of PGA, measured in WG and AG directions

• Average CTE for 50°C increments, e.g. 100-150°C

• CTE plotted at midpoint of increment e.g. 125 °C

3. Sutton, A.L. & Howard, V.C., 1962. The role of porosity in the accommodation of thermal expansion in graphite. Journal of Nuclear Materials, 7(1), pp.58–71.

CTE Preliminary Results

• Average CTE plotted• Reference temperature of 50 °C used, e.g.

50-250 °C• CTE plotted at midpoint of temperature

increment

60 80 100 120 140 160 180 2002

2.5

3

3.5

4

4.5

5

AG Average CTE vs Temperature

AG3 (40%)AG2 (20%)AG1 (0%)

Temperature (°C)

CTE

(X 1

0-6

K-1)

60 80 100 120 140 160 180 2002

2.5

3

3.5

4

4.5

5

AG Average CTE vs Temperature

AG5 (80%)AG4 (60%)AG3 (40%)AG2 (20%)AG1 (0%)

Temperature (°C)

CTE

(X 1

0-6

K-1)

60 80 100 120 140 160 180 2002

2.5

3

3.5

4

4.5

5

AG Average CTE vs Temperature

AG6 (90%)AG5 (80%)AG4 (60%)AG3 (40%)AG2 (20%)AG1 (0%)

Temperature (°C)

CTE

(X 1

0-6

K-1)

60 80 100 120 140 160 180 2000

0.5

1

1.5

2

2.5

WG Average CTE vs Temperature

WG1 (0%)

Temperature (°C)

CTE

(X 1

0-6

K-1)

60 80 100 120 140 160 180 2000

0.5

1

1.5

2

2.5

WG Average CTE vs Temperature

WG2 (20%)WG1 (0%)

Temperature (°C)

CTE

(X 1

0-6

K-1)

MAIN STUDY

SamplesΦ =6 mm

H =6 mm

• CTE measured on 10 samples to ascertain sample variability

• Two samples in each direction selected randomly for pre-stressing

Method

• CTE measured• Pre-stress applied (fraction of compressive

strength)• Properties re-measured• Max temp: 450 °C• AG: 0%; 20%; 40%; 90%• WG: 0%; 20%; 40%

Comparison of unstressed samples with the literature

• Comparing unstressed sample values with Sutton and Howard 3

• Two blocks of PGA, measured in WG and AG directions

• Average CTE for 50°C increments, e.g. 100-150°C

• CTE plotted at midpoint of increment e.g. 125 °C

3. Sutton, A.L. & Howard, V.C., 1962. The role of porosity in the accommodation of thermal expansion in graphite. Journal of Nuclear Materials, 7(1), pp.58–71.

Sample VariabilityTemp Range (°C) AG CTE

(x 10-6 K-1)St. Dev (x 10-6 K-

1)

WG CTE (x 10-6 K-1)

St. Dev (x 10-6 K-

1)50-100 0.9 ± 0.1 0.150-150 3.4 ± 0.1 0.1 1.1± 0.1 0.250-200 3.5 ± 0.1 0.1 1.2 ± 0.1 0.150-250 3.6 ± 0.1 0.1 1.3 ± 0.1 0.150-300 3.7 ± 0.1 0.1 1.5 ± 0.1 0.150-350 3.8 ± 0.1 0.1 1.6 ± 0.1 0.150-400 3.9 ± 0.1 0.1 1.7 ± 0.1 0.150-450 4.0 ± 0.1 0.1 1.9 ± 0.1 0.1

0 10 20 30 40 50 60 70 80 90 1000

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Residual Strain vs. Pre-Stress

AG2AG13WG7WG13

Compressive Pre-stress (%)

Resid

ual S

trai

n (%

)

COMPARISON BETWEEN PRELIMINARY AND MAIN STUDY

Observations

• Both preliminary and main studies show good agreement in CTE measurements

• Pre-stress causes increase in CTE– Approximately 20% increase in CTE at 90% Pre-stress in AG

direction– Approximately 10% increase in CTE at 40% Pre-stress in AG

direction• Minimum pre-stress required to cause change has yet

to be determined• Residual strains not directly proportional to pre-stress

What causes change in CTE?

• Crystal re-orientation may be occurring• Other authors have attributed the increase in

CTE in steels to lattice distortion 2 • Elastic limit must be reached in steels before

permanent changes to CTE occur– Residual strains in graphite– Is there a minimum residual strain for permanent

changes in CTE to occur?

Acknowledgements

• This project is funded by EPSRC through the FunGraphite consortium

• The author would like to thank the following for their advice and help with the project: Dr. Marc Schmidt; Gary Harrison; the staff at FEI; David Mortimer; Judith Shackleton

• The SEM images were taken at the Dalton Cumbrian Facility and the author would like to thank DCF for access to the equipment