Predicting and visualizing crack propagation in nuclear graphite

Nuclear graphite exhibits a complex fracture behavior due to the presence of defects ranging from nanoscale basal cracks to sub-millimeter scale voids as well as the heterogeneity of constituents. This study aims to develop insights on whether finite element (FE) modeling can effectively reproduce fracture behavior observed in the experiments coupled with micro X-ray computed tomography (CT). Two-phase (graphite-defect) models and grayscale-based models with elastic properties based on constituents were employed, respectively, to compute the stress and strain distribution in an NBG-18 single-edge notched beam under three-point bending. Compared with the two-phase models, the grayscale-based models revealed more variations in the stress and strain distribution because they captured the sub-voxel heterogeneity in the material properties. Particularly, the strain distribution pattern exhibited good agreement with the crack propagation paths shown in the micro-CT images. The results proved the feasibility of predicting and visualizing the crack propagation process using grayscale-based FE models. However, the prediction of crack initiation location using FE modeling remained difficult. The residual stress on the notch surface was measured to be from 1.1 to 2.5 GPa using Raman spectroscopy. It indicated that the large values of highly localized residual stress could be a factor that influenced crack initiation.