Relationships between the Microstructure and Mechanical Behaviour of Nuclear Graphite

B.J. Marsden, L. Babout, A. Hodgkins, J. Ali, T.J. Marrow, P.M. Mummery (Sp), A. Fok, University of Manchester (UK) 
A combination of novel (X-ray Microtomography (XMT), Electronic Speckle Pattern Interferometry (ESPI), and image correlation (IC)) and conventional techniques has been used to study the mechanical and fracture behaviour of PGA graphite.

ESPI and IC have been used to study the strain distributions and their development around growing cracks in PGA graphite of two orientations. These have shown strain concentrations at the crack tip and the extent of strain partition between the binder and coke particles. On growth, much of the strain has been relieved. However, there is residual strain and microcracking that promotes crack closure and increases toughness. The R-curve behaviour has been studied.

The mechanical properties (modulus and strength) of the graphite have been determined on small specimens, typical of those removed from reactors during routine shutdown. In addition, a modified three-point sandwich specimen has been developed that enables valid toughness measurements on small specimens. XMT, is a high spatial resolution (5 micron), non-destructive technique that images the interior of a specimen, has been used to characterise the microstructure of the graphites. It was used to determine the point-to-point distribution of density quantitatively within specimens of a range of thermally oxidised materials. These data were used to determine the relationships between the degree of thermal oxidation and Young modulus, fracture strength, and thermal diffusivity.

Crack growth within the bulk of the material has been studied. This has shown crack bridging and both trans- and intergranular failure. Crack growth in compact tension specimens under tension-tension loading was investigated. This allowed the results of R-curve testing to be evaluated. The drop-off in crack growth resistance with increasing crack length can be associated unambiguously with interaction of the crack front with the back face of the specimen. This was clearly demonstrated by tomography.

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