The effect of pre-stressing and annealing on the young's modulus of some nuclear graphites

Studies have been made on the decrease in the dynamic Young's modulus of some nuclear graphites caused by pre-stressing or cyclic loading in compressive mode and its recovery due to annealing up to 2000 °C. It is found that the decrease in the modulus is well represented by the formulae, $\text{E}\text{E}{}_0\text{= ? Aσ}{}^2\text{+B}$ for ex-truded graphites and $\text{(E ? E}{}_{\mathrm{c}}\text{)}\text{(E}{}_0\text{?E}{}_{\mathrm{c}}\text{) = B}\text{exp}\text{( ? Aσ}{}^2\text{)}$ for moulded ones, where E, E_c and E₀ are the modulus at a pre-stress σ, that at pre-stress equal to fracture stress and that with no pre-stressing, respectively. A and B are constants. Annealing experiments show that the moduli of extruded graphites recover almost completely below 1000 °C, while in the case of moulded graphites complete recovery is not attained even at 2000 °C. The activation energy obtained for the recovery of an extruded graphite below 1000 °C is about 0.6 eV.