| 초록 |
The fracture toughness (KIc) and strain energy release rate (GI) for five kinds of nuclear graphite were investigated. Fracture tests were performed using a three point bending specimen with a straight-through notch at the crosshead speed of 0.01 mm/min. KIc was determined at the maximum load and strain energy release rate-crack extension (GI-Δa) curve was obtained by a compliance method. As the apparent density and the coke particle size of the graphite increased, the KIc value increased which may be releated withan inherent crack density and size. As the coke particle size increased the GI value was higher for a given crack extension. This trend reflects that more energy was dissipated in the coarser-grained graphite during crack propagation by irreversible processes. After thermal oxidation at 600oC, KIc decreases of approximately 25 and 50% occurred for burn-off levels of 5 and 10 %, respectively. The decrease in KIc of the oxidized graphite can be attributed soley to a decrease in the strength, not increases in intrinsic flaw size. GI decreased with increasing the burn-off for a given crack extension. The dependence of GI on burn-off correlated with the apparent density and consistents with a simple mechanical failure model. That is, as the density decreases, the amount of material occupying the area of the fracture surface also decreases, requring less energy to propagate the crack through a unit area whose density of atomic bonds is lowered correspondingly. |
