Flexural strength and defect behaviour of polygranular graphite under different states of stress

The effect of stress state on the fracture behaviour of Gilsocarbon, an isotropic nuclear grade polygranular graphite, has been studied by employing four-point bend and ring-on-ring loading configurations to achieve uniaxial and equi-biaxial flexural stress states, respectively. Optical images of the specimens' tensile surface were analysed by digital image correlation 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 J-integral 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 combined with digital volume correlation: 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 in agreement with the effect of stress state on the measured strain energy release rates of sub-critical crack propagation. Both are attributed to the plastic constraint effect, which varies with the stress state in materials that can undergo inelastic deformation.