Effect of Cumulative Damage on Fatigue Strength of VHTR's Graphite

Cumulative fatigue tests of fine-grained isotropic graphite IG-110 were performed on the conditions of high-low stress and low-high stress and the cumulative fatigue damage were evaluated. The following conclusions were derived: (1) Fatigue damage tested on the conditions of high-low stress was larger than that of lowhigh stress. (2) The evaluations of fatigue damage according to Miner's and Hashin's laws overestimated the real fatigue damage. On the other hand, the evaluations according to following equations were excellent to the real damage: (n1/N1)(lnS1/lnS3)3+n2/N2=1 (2 step test) (n1/N1)(lnS1/lnS3)3+(n2/N2)(lnS2/lnS3)(n2/N2)(lnS2/lnS3)2+n3/N3=1 (3 step test) where Si was applied stress in each step i (i=1, 2, 3), ni and Ni were applied cycle and fatigue life in each step i.