In-Situ Observation of Crack Nucleation in Nuclear Graphite by Digital Image Correlation

To study the fracture behaviour of nuclear graphite, a full-field digital image correlation technique has been applied to large specimens of isotropic Gilsocarbon graphite. Optical images of the tensile surface in four-point bend tests were recorded throughout the loading history, with a 100 × 100 mm viewing area. Although crack nucleation was not observable in these raw images, the high sensitivity of digital image correlation to the small displacements allows cracks to be detected. Strain maps are derived from the displacements, and surface cracks with lengths from 1 mm can be seen due to the high effective strain that results from crack opening. Post-processing of the strain maps can track the development of every such defect. These unique observations show the distribution of cracks and their sub-critical development and interactions prior to unstable fracture. This information may be used to validate models for the effects of sample size and stress gradient on component fracture strength.