Crack nuclei in nuclear graphite
Studies of hard-to-detect crack nuclei by tomography and DVC. The crack is visualized by its opening strain (top right) when loaded; its depth can then be measured.
The effect of stress state on the fracture behaviour of Gilsocarbon, an isotropic nuclear grade polygranular graphite, has been studied under uniaxial and equi-biaxial flexural stress states respectively. Optical images of the specimens’ tensile surface were analysed by digital image correlation (DIC) to measure the full-field displacements: these were used to identify the fracture initiation sites, analyse crack geometry (surface length and opening displacements) and also to calculate the resistance to fracture (i.e. the strain energy release rate associated with surface crack propagation). Surface cracks that did not propagate to failure were identified and subsequently examined by X-ray computed tomography (XCT) combined with digital volume correlation (DVC): measurements were made of their three-dimensional displacement fields when subjected to an opening tensile stress using a modified (flat) Brazilian Disk test geometry. The crack opening behaviour is explained by an effect of stress state on the development of the crack tip fracture process zone, which is consistent with the measured strain energy release rates of sub-critical crack propagation.
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