Studies of the crack initiation and notch sensitivity behaviour of gilsocarbon

Thesis

To better understand the initiation of axial cracks in the graphite moderator bricks of advanced gas-cooled nuclear reactors, the development of a test suitable for measuring notch sensitivity from small specimens would be valuable. The test geometry is constrained, currently, to be under 20 mm in two dimensions. There is limited data on the notch sensitivity of Gilsocarbon, particularly after exposure to a nuclear environment. For the potential small specimen test geometries the microstructural scale is expected to significantly affect the fracture behaviour, which could make understanding of the component-scale behaviour difficult. The material science challenges in defining a suitable test will be outlined in this thesis, with studies of how crack initiation of the Gilsocarbon-grade graphite depends on the notch sensitivity.

Geometries based on a notched 6 x 6 x 19 mm beam will be prototyped, with over 100 beams of seven different notch geometries tested. Multi-scale, multi-camera in situ imaging will be combined with image correlation and finite element techniques to calculate the specimens’ states at fracture, and potential fracture criteria evaluated (notch strengthening factors and strain energy density, SED). Comparison with historical results will demonstrate an increased insensitivity to notch radius (from 0.05 mm to 2.00 mm), and notch depth (up to 4.50 mm). The need for non-linear simulations will be highlighted and a potential material model tested, with limited success. SED calculation from digital image correlation measurements will be evaluated as a suitable monitoring technique, agreeing with the simulations but suffering from lack of reliable coverage for predictive capability. In situ volumetric imaging will be used to understand the interactions of the microstructural phases. A non-linear, non-recoverable stress-strain response will be observed, with anomalous deformation identified and attributed to damage at the filler particle peripheries. The origin of the insensitivity of Gilsocarbon to notch radii less than 2.0 mm will be explained by the observation of a notch radius-independent fracture process zone that extends over several millimetres. This understanding will support the development of a notch sensitivity test, and the outstanding challenges and potential future work will also be identified.

Authors: Jordan, MSL

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Radiolytic oxidation; Advanced gas-cooled reactor; Strain energy density; Nuclear graphite; Fracture; Diamond Light Source; Notch sensitivity; Finite element modelling; X-ray tomography; Digital volume correlation; Gilsocarbon; Digital image correlation; Neutron irradiation
• Subjects: Digital image correlation; Materials science; Notch sensitivity; Fracture properties; Nuclear power; Graphite properties