Evaluating the dynamic fracture toughness of graphite via accurate determination of the dynamic crack propagation length

Nuclear graphite is widely used as a moderator, reflector, and core structure material in very/high temperature gas-cooled reactors. However, because graphite may experience impacts with medium-low velocities during its transport and installation or under extreme conditions such as severe earthquakes, it is necessary to evaluate the dynamic fracture toughness of graphite to assess the safety of the reactors. The ASTM standard recommends a formula to calculate the fracture toughness; however, this formula includes the crack length parameter, which cannot be measured accurately in a dynamic process. To enable the accurate measurement of the dynamic crack propagation length of graphite and subsequently obtain the fracture toughness, a virtual extensometer method based on digital image correlation was established in this study. Three-point bending experiments were conducted using pre-cracked specimens to obtain the dynamic fracture toughness of graphite IG11 under medium-low impact velocities. The results indicated that the dynamic fracture toughness of graphite IG11 increased with the increase in the impact velocities, and its value was substantially higher than the static fracture toughness in certain cases. The proposed approach involves a simple operation, yields a high accuracy, and can enable automatic calculation. Moreover, this approach can be used to evaluate the dynamic fracture performance of other brittle or quasi-brittle materials.