Fracture behavior of nuclear graphite NBG-18

This paper aimed to study the fracture behavior of nuclear graphite NBG-18 through three-point bend tests with single-edge-notched beams. The size effect on its fracture toughness and associated statistical characteristics was studied by using three different specimen sizes: (I) 200 mm(Span) × 50 mm(Width) × 25 mm(Thickness), (II) 100 mm(S) × 20 mm(W) × 10 mm(T) and (III) 40 mm(S) × 10 mm(W) × 5 mm(T). Digital image correlation and acoustic emission techniques were employed to monitor the damage evolution during loading. The mean and standard deviation of critical stress intensity factor KIC   (MPam) measured from each group were: (I) 1.69 (0.04), (II) 1.36 (0.05), and (III) 1.27 (0.09). The obtained curve of KIC against crack length showed three stages: an initial increase, a plateau and a final decrease, similar to a typical crack growth resistance curve. An analytical model based on the fracture process zone ahead of the crack tip was used to examine the size effect and to derive the length of fracture process zone cf and size-independent constant KIf. The predicted cf and KIf were 9.74 mm and 1.76 MPam, respectively. The test results were correlated with the micro-structure and unique fracture characteristics of NBG-18 were concluded.