Investigation into the Effect of Physical Size on Crack Initiation in Brittle Materials Subject to Mixed Mode Fracture (With Focus on Graphite Nuclear Reactor Cores)
This work explores experimentally and numerically the topic of size effect in simple and complex loading scenarios with a view to determining a suitable fracture prediction criterion for advanced gas-cooled reactor (AGR) graphite bricks. In this context size effect is taken to mean the non-constant nature of material properties when specimens are tested at different sizes. The literature shows that typically strength correlates negatively with increasing size. Conversely, the literature also shows that fracture toughness typically correlates positively with increasing size. The work presented here concentrates on concrete as an analogy of graphite. Specimens are tested at a range of sizes for splitting strength, flexural strength, compressive strength and fracture toughness. Scaling behaviour of mixed mode (I/II) fracture is also examined experimentally and through FEA. In total, data from over 750 specimens is presented. The project’s experimental work is in agreement with the literature; showing a negative correlation between size and splitting strength while showing a positive correlation between size and fracture toughness. The testing and theoretical work is used to demonstrate the applicability of the FEA approach to the stress states found in late life AGR reactor graphite bricks; with the criterion shown to be size insensitive for Mode-II dominant stress states through the experimental testing and FEA modelling of three sizes of disc specimens with 90° re-entrant corners.
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