Fatigue criteria for HTGR core support graphite structure are presented. The criteria takes into consideration the brittle nature of the material, and emphasizes the probabilistic approach in the treatment of strength data. The stress analysis is still deterministic. The conventional cumulative damage approach is adopted here. A specified minimum S-N curve is defined as the curve with 99% probability of survival at a 95% confidence level to accommodate random variability of the material

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... , A constant life diagram is constructed to reconcile the effect of mean stress. The linear damage rule is assumed to account for the effect of random cycles. An additional factor of safety of three on cycles is recommended. The uniaxial S-N curve is modified in the medlum-to-high 3 cycle range (>2 x 10 cycles) for multiaxial fatigue effects. The strength of the weak axis is recommended for use in all the orientations to conservatively avoid the uncertainties of the orientations of cyclic stresses with respect to the material axes. The effects are also discussed of other factors which influence fatigue, such as temperature, irradiation, oxidation, stress concentration, etc. Finally, recoimnendations for future work in this field are listed. iii