Fracture processes in graphite and the effects of oxidation

Measurements were made of 3-point bend strength, σsub(b), critical stress intensity factor, Ksub(lc), dynamic elastic modulus, E, and specific fracture surface energy, γ, for two graphites before and after oxidation up to 7% weight loss in CO2/5% CO gas at 1173 K. The fracture path cleaves grains parallel to basal planes and traverses pores in the binder phase. Calculated critical flaw sizes were larger than any single microstructural feature, implying that they develop by sub-critical cracking. The decrements in σsub(b), E and Ksub(lc) with increases in fractional porosity P following oxidation are described by exponential equations; the decrements in γ are better described by linear equations; each of the properties is reduced by 40-50% for up to 7% weight loss. There is no fractographic evidence for changes in fracture mechanism upon oxidation and the calculated flaw size does not change significantly; the decrements in Ksub(lc) are entirely accounted for by decreases in E and γ and there is evidence for nonideal brittle behaviour in both graphites. The most significant change in the microstructure after oxidation is the development of narrow, slit-shaped shrinkage cracks in the binder phase which are mainly oriented parallel to basal planes. The elongation of these cracks is attributed to the reaction anisotropy of graphite. The severe decrement in E after oxidation is attributed to the development of these slit-shaped pores of high aspect ratio. (author)