An Uncertainty Evaluation on the ΔKth and ΔKIC of Fine-grained Isotropic Graphite

This paper intensively explores the critical issues related to the quantitative and accurate evaluations of FCG behavior in the early stage, macro fatigue fracture toughness, and the critical crack size for damage tolerance in nuclear graphite. To address these issues, scale-span FCG tests were carried out using two typical specimens, CT and SEM in-situ specimens. These results indicate that the FCG threshold and the effective FCG length have a significant correlation with the modified maximum loop stress theory for a mixed I/II mode. In particular, the effective FCG length (a_eq) and the applied stress threshold of polycrystalline graphite are important parameters for fatigue damage tolerance design in engineering application. The influencing factors of ΔK_th,eq and a_eq were discussed in detail using the mixed I/II mode, respectively. In addition, the scattered values of ΔK_IC for this graphite can be quantitatively estimated using the Weibull distribution equation. The predicated parameters and experimental results demonstrate a strong correlation.