Effect of stress holding on fatigue strength of nuclear graphite

Since graphite components in the HTTR (=High Temperature Engineering Test Reactor) are subjected to cyclic stress during reactor start-up, operation or shut-down, the design of the reactor requires the knowledge of cumulative fatigue damage of graphite materials. However, the number of reports on the cumulative fatigue damage of graphites are very limited. In this paper, the results of fatigue test in the mixed fatigue mode of cyclic and static stresses were reported on HTTR components graphite, IG -110, regarding the effects of stress hold time and static fatigue-cyclic fatigue interaction on cumulative fatigue damage of the graphite. The following results were derived: (1) Static fatigue failure of IG-110 graphite was observed at the stress level of more than 0.95 times as large as the mean tensile strength. (2) Fatigue life under cyclic stress was longer than that of mixed fatigue mode of cyclic and static stresses at the stress level of more than 0.85 times as large as the mean tensile strength. (3) Fatigue life of mixed fatigue mode decreased with increasing stress hold time. (4) Damage factor of static stress in the mixed fatigue mode was very small, and cumulative fatigue damage in this was smaller than 1. (author)