高強度黒鉛の破壊じん性と破壊抵抗の評価

抄録

Effects of specimen size and initial notch on fracture toughness and fracture resistance characteristics were examined for three high strength graphite materials, IG-11, IG-110 and IG-110U. A characteristic R-curve behavior and specimen size requirements for fracture mechanics parameters were discussed. An effect of elevated temperature environment on the fracture toughness was also examined at elevated temperatures from room temperature upto 820°C in a vacuum environment. The temperature effect was discussed in comparison with the result at room temperature in an air environment. The results obtained are summarized as follows:
(1) Graphite materials show a characteristic R-curve behavior in which the fracture resistance increases in the initial stage of crack extension near the initial notch tip, takes a constant value in the intermediate stage, and then decreases in the final stage near the specimen back face. The effect of initial notch is caused by an underestimation of the stress intensity factor, K, for a corner crack. It is shown that the true fracture resistance is constant and equals to the fracture toughness.
(2) The stress intensity factor, K, is valid as the fracture mechanics parameter for the fracture toughness testing. A well-known specimen size requirement for the metallic materials can be extended to the graphite materials by replacing the yield stress by the tensile strength.
(3) The fracture toughness increases slightly with increasing temperature in the vacuum environment and it has a larger value than that obtained at room temperature in the air environment. So, the fracture toughness and the fracture resistance obtained at room temperature in air can give conservative estimations for the evaluation of the structural integrity of the high temperature gas cooled reactor.