Damage nucleation in nuclear graphite

Abstract

This paper reports the use of advanced materials characterisation techniques, X-ray microtomography and surface strain mapping by electronic speckle pattern interferometry (ESPI), to study the mechanisms of damage nucleation in polygranular nuclear graphite. It is found that strain localisation occurs owing to the heterogeneous microstructure, giving rise to microcrack nucleation and coalescence before tensile failure. This produces a substantial damage zone at stress concentrations, such as notches and crack tips, which can be observed directly and in situ. Crack propagation occurs by the coalescence of microcracks in the damage zone. The measured R curve is a consequence of both frictional bridging in the crack wake and energy dissipation in the microcrack process zone.