Fracture instability in nuclear graphite

Abstract

This dissertation considers the fracture instability of nuclear graphite, specifically of isotropic Gilsocarbon, grade IM1-24, which acts as a structural component and neutron moderator within reactors. The presence of cracks within this graphite informs its behaviour and necessitates a study of fracture properties and instability. Amongst the factors studied, a major finding was that the size effect was the most prevalent. Two aspects of instability were also examined: the crack driving force or energy release rate and the fracture resistance or the incremental work of fracture. The conditions between the extremes of load control and displacement control affecting the energy release rate were studied, based on the compliance of the surrounding components or additional elastic material, generally known as elastic follow-up. The effects of elastic follow-up and specimen geometry on fracture instability was investigated in an idealised model. Two sets of experiments were presented to quantify the effect and to validate the idealised benchmark study. No measurable differences were exhibited at the equivalent degrees of elastic follow-up achieved in the experimental work. Additionally, the effects of load multiaxiality on the fracture of graphite were investigated. Despite the influence of load multiaxiality on fracture stress of graphite, there was little effect in post-peak fracture behaviour indicating the lack of influence on fracture stability. Moreover, to evaluate fracture resistance, this work investigated the crack growth resistance curves, KR and R. To produce these curves, a considerable number of experiments of cyclic load and unload, with crack propagation, is presented. Different sized compact tension specimens were tested, to investigate the size effect typically exhibited in quasi-brittle materials which describes the fracture behaviour of IM1-24. The rising KR and R-curve behaviour observed in all sizes, especially in the more distinct initial fracture stages of KR, can be attributed to the formation of a bridging zone in the wake of the propagating crack. A mismatch between the scaling of the fracture process zone and the specimens was also exhibited, evident from the considerable differences in apparent toughness KQ as well as the linear elastic contributions to the work of fracture. The results indicated that the fracture stability of IM1-24 graphite is only marginally affected by elastic follow-up, whilst size effect is a more prominent contributor.

Date of Award	25 Sept 2018
Original language	English
Awarding Institution	University of Bristol
Supervisor	Mahmoud Mostafavi (Supervisor) & Martyn J Pavier (Supervisor)