HIGH-TEMPERATURE YOUNG MODULUS OF A FINE-GRAINED NUCLEAR GRAPHITE OXIDIZED OR PRESTRESSED TO VARIOUS LEVELS

Effects of oxidation and compressive prestress on high temperature Young's modulus were examined on a fine-grained isotropic graphite, IG-110. Oxidation was performed in air at 500°C up to a burn-off level of 10%. Prestress level was either 60% or 80% of the mean compressive strength. Both oxidation and prestressing caused decreases both in the room temperature and high temperature moduli, which were explained by a crack opening and growth model. An empirical equation by which the high temperature modulus of oxidized specimens can be estimated from the room temperature modulus of the unoxidized specimen was obtained as E (T, d) E(RT, do) = f{hook}(T)( d do)n(T). Here, E(T, d) and E(RT, do) are the high temperature modulus of the oxidized specimen with apparent density d, and the room temperature modulus of the unoxidized specimen with apparent density do, f{hook}(T) is a polynomial in the third order of T, and n(T), a linear function of T. Annealing treatment or heat-up and cool-down process caused a partial recovery of the modulus of the oxidized specimen. This recovery was explained with the aid of a model: The internal stress generated as a result of the anisotropy of the thermal expansion of crystallites becomes smaller for the oxidized material because of the preferential oxidation of the binder region. © 1990.