Validation of ring-on-ring flexural test for nuclear ceramics using miniaturized specimens

For the purpose of evaluating fracture strength of the inner pyrocarbon (IPyC) layer of TRISO fuel particles, a miniature disc equibiaxial flexural test technique was developed. It was applied to nuclear graphite specimens in a variety of combination of sample thickness and loading ring diameter to investigate the influences on the stress uniformity within the loading ring. Although similar and relatively high Weibull modulus were observed for all conditions (m ≇ 20), the significant stress concentration associated with the large deflection was observed at the loading location for thinner specimens (t;<0.2mm). However, the true local fracture stress was successfully estimated from the measured fracture load using finite element analysis. The method for estimating true local fracture stress appear reasonable for evaluating the fracture strength of dense poly-crystalline graphite, and can be used for determination of the statistical parameters for fracture stress of graphite and pyrocarbon.