Objective: Graphite components in a Very High Temperature Reactor (VHTR) may fracture under the actions of external and internal (irradiation-induced) stresses. Measurement of the fracture properties of graphite is therefore essential in the design and structural integrity assessment of VHTR cores. This study aimed to evaluate the fracture toughness and its associated statistical characteristics of nuclear graphite NBG-18, for which there had been very little data. The effect of specimen size on its fracture toughness was also studied.

Materials and methods: Three-point-bending tests were conducted with a MTS machine (858 Mini Bionix II, MTS, US) on single-edge-notched beams (SENB) of NBG-18 to measure its fracture toughness. Three different specimen sizes were considered: (I) 200mm (Support Span S) × 50mm (Width W) × 25mm (Thickness T), (II) 100mm (S) × 20mm (W) × 10mm (T), and (III) 40mm (S) × 10mm (W) × 5mm (T). A notch was machined into each specimen to give a crack length-to-width (a/w) ratio of 0.4 using a 0.3mm-thick diamond blade. The acoustic emission (AE) technique was applied to monitor the damage evolution process during loading.

Results: The mean and standard deviation (in brackets) of the critical stress intensity factor KIC (MPa·m1/2) for each group were: (I) 1.69 (0.04), (II) 1.36 (0.05), and (III) 1.33 (0.06). Specimen size was found to significantly influence the fracture toughness results: the smaller the specimen, the lower the mean fracture toughness and the larger the variation.

Conclusions: The fracture toughness of NBG-18 appears to reduce with the specimen size. The lower fracture toughness of the smaller specimens could be attributed to a finite process zone ahead of the crack tip.

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