Two nuclear grade graphites, IM1-24 and PGA-AGL, have been thermally oxidised In an atmosphere of 5% CO/CO2 at 900°C. Under these conditions the oxidation rate is controlled by the rate of chemical reaction. This produces an even weight loss principally from the Internal surfaces. The effect of oxidation upon four mechanical properties has been studied; these are bend strength, ob, critical stress Intensity factor, Kc, the elastic modulus, E and effective surface energy, y. it was found that these properties were reduced by 50% as a result of approximately 7% weight loss. Thermal oxidation is highly selective, attacking prismatic edges rather than basal planes. This results in the preferential development of filamentary shaped pores in domain structure in the binder phase of the graphites. It has been shown that the development of these high aspect ratio pores is extremely detrimental to the elastic modulus and effective surface energy (more detrimental than if there had been equal development of all classes of pore). The reduction in E and y account for the observed reduction in Kc. The critical defect size was found to be larger than any Individual microstructural feature and thus required sub-critical development of the flaw. Upon oxidation the critical defect size did not significantly alter, nor did the observed fracture mechanism. The strength reduced as a consequence of reduced tolerance to flaws as Kc reduced with oxidation. An analytical fracture model was used to relate the oxidation induced changes in microstructure with mechanical property changes. Stress induced acoustic emission was monitored during flexural strength testing to aid evaluation of sub-critical failure events. It was found that acoustic emission. In terms of' onset stress, total counts to failure and amplitude distribution, was characteristic of the graphite microstructure, more specifically the fine scale optical texture. The onset stress is believed to be representative of the cleavage strength of contiguous areas in the grains of the graphites which have common basal plane orientation. These areas are believed to dominate sub-critical flaw development.
|Date of Award||1984|