Inelastic and fracture behaviour of nuclear graphite

Understanding nuclear graphite's inelastic and fracture behaviour is essential for current and future reactor technologies using graphite-based engineering components. This study compares the behaviour of three nuclear graphite grades, fine-grained IG-110, coarse-grained NBG-18 and medium-grained PCEA, subjected to the uniaxial compression (UC) and the splitting tensile (ST) tests. It was found that the IG-110 graphite has a more favourable combination of ultimate strength and ductility when compared to the NBG-18 and PCEA grades containing large pores acting as strain concentrators. The formation of shear cracks was the primary failure mode under compression, while the formation of a main tension crack in the middle of the specimen was the primary failure mode during the ST test. The inelastic and fracture response was modelled using finite element simulations employing the concrete damaged plasticity (CDP) material model with the dilation angle parameter value selected by two different optimisation processes; a decoupled optimisation was run on the UC and ST models separately, and a coupled optimisation was performed on the UC and ST models running simultaneously. The best predictions were obtained when the value from the coupled optimisation was used. The results showed that the CDP model accurately describes the inelastic behaviour and peak force of all graphite grades and could also capture the failure modes observed experimentally in both UC and ST tests. In particular, the numerical model could capture the crack initiation and propagation path observed in the ST test reasonably well for the IG-110 and PCEA graphite grades.