An investigation of fracture behaviors of NBG-18 nuclear graphite using in situ mechanical testing coupled with micro-CT

Nuclear graphite contains defects spanning from nanoscale basal cracks to sub-mm scale voids. This study aims to establish an experimental method to investigate pre-existing defects and 3-D crack growth inside nuclear graphite, NBG-18. Three-point bending tests were performed on single-edge notched beam specimens with and without coupling with micro-CT. The fracture toughness was measured to be 1.17 +/- 0.06 MPa root m . Cracks were observed to initiate from the pores and thermal cracks on the edge of the notch and then deflect or twist to grow along the pre-existing defects. Crack bridging, deflection, and twisting were the primary toughening mechanisms. The crack resistance curve exhibited a trend of rising-plateau-rising that can be related to the interaction of crack front and the pre-existing defects. The results highlight the capability of laboratory micro-CT-based experimental method for the visualization of multi-scale defect interactions, which remains to be a challenge in the characterization of nuclear graphite.