Large-scale Weibull analysis of H-451 nuclear-grade graphite rupture strength

Abstract

A Weibull analysis was performed of the strength distribution and size effects for 2000 specimens of H-451 nuclear-grade graphite. The data, generated elsewhere, measured the tensile and four-point-flexure room-temperature rupture strength of specimens cut from a single extruded graphite log. Strength variation versus specimen location, size, and orientation relative to the parent body were compared. In our study, data were progressively and extensively pooled into larger data sets to discriminate overall trends from local variations and investigate the strength distribution. Issues regarding size effect, Weibull parameter consistency, and nonlinear stress–strain response were investigated using the Ceramics Analysis and Reliability Evaluation of Structures Life Prediction Program (CARES/Life) and WeibPar codes. Overall, the Weibull distribution described the behavior of the pooled data very well. The Weibull modulus was shown to be clearly consistent between different tensile specimen sizes and orientations. However, the issue regarding the smaller-than-expected size effect remained. This exercise illustrated that a conservative approach using a two-parameter Weibull distribution is best for designing graphite components with low probability of failure for the in-core structures in the proposed Generation IV high-temperature gas-cooled nuclear reactors. This exercise also demonstrated the continuing need to better understand the mechanisms driving stochastic strength response.