Crack growth resistance in nuclear graphites

Crack growth resistance curves for the non-linear fracture parameters K sub R , J sub R and R were measured for unirradiated PGA and IM1-24 graphites that are used as moderators in Brithis Magnox and AGR nuclear reactors, respectively. All the curves show an initial rising part, followed by a plateau region where the measured parameter is independent of crack length. J sub R and R decreased at large crack lengths. The initial rising curves were attributed to development of crack bridges in the wake of the crack front, while, in the plateau region, th ecrack bridging zone and the frontal process zone, ahead of the crack tip, reached steady state values. The decreases at large crack lengths were attributed to interaction of the frontal zone with the specimen end face. Microscopical evidence for graphite fragments acting as crack bridges showed that they were much smaller than filler particles, indicating that the graphite fragments are broken down during crack propagation. There was also evidence for fraction points in the crack wake zone and shear cracking of some larger fragments.