Biaxial fatigue strength of a fine-grained isotropic graphite for HTTR

Biaxial fatigue tests on an isotropic graphite IG-11 used in the HTTR were done in the stress regimes of tension-tension and compression-tension at room temperature to evaluate its fatigue strength at the biaxial state. Biaxial fatigue tests at 1,273 K in a vacuum were also done in the latter stress regime. Applied stress levels in the longitudinal direction ranged from 75 to 90% of the mean static strength, and in the transverse direction, from 53 to 74% of the mean strength. It was indicated from the tests that (1) the difference in the fatigue strength between the uniaxial and biaxial was observed when the applied stress levels were smaller than about 90% of the mean strength, (2) the biaxial fatigue strength was found to be larger than that for the uniaxial if the biaxial applied stresses were normalized to the mean biaxial static strength, (3) the biaxial fatigue strength at 1,273 K was equal to or larger than that at room temperature, (4) the evaluation of the integrity of graphite components in the HTTR, from the aspect of the biaxial fatigue, was believed to be conservative on the basis of the uniaxial fatigue strength and the biaxial static strength. (author)