Effect of adsorbed gases on mechanical properties of nuclear graphite

Investigations were made of effect of test environments on bend strength and fracture toughness of fine-grained isotropic graphites. Four-point bend tests were carried out in various atmospheres of air, helium and vacuum. When we measured the bend strength of fully outgassed specimens in a vacuum, we obtained values higher than those measured in air by up to 45%. When measurements were made in helium atmosphere containing 1 to 3 ppm of water vapor, the bend strength was higher by 20 to 30% than those measured in air. When the water content in helium increased to about 20 ppm, the strength decreased to the same value with that measured in air. The bend strength of graphite specimens which were fully outgassed at 1200degC decreased with increasing temperature up to 900degC, whereas that of specimens which were not subjected to the outgas treatments showed a positive temperature dependence. Furthermore, the fracture toughness of fully out gassed specimens measured in a vacuum was about 20% higher than that measured in air. These experimental results were interpreted in terms of effect of adsorbed gases on strength of graphite materials. (author)