CYCLIC FATIGUE PROPERTIES OF ISOTROPIC GRAPHITE AT ELEVATED-TEMPERATURES

Fatigue properties of two kinds of isotropic graphite were investigated at elevated temperatures. After the temperature region applicable to the crack propagation rate equation was estimated, V = A(K(I)/K(IC))n, three-point bend bar test specimens were subjected to cyclic loading at room temperature, 473 K or 673 K. The results under different stress ratios were normalized by the effective lifetime. Reduction of lifetime and weak bonding of particles due to oxidation were observed with the increase of temperature. Reducing the stress ratio caused a drastic decrease in lifetime, in which graphite particles acted as the debris instead of grain-bridging at the fracture surface. These fatigue behaviors were discussed quantitatively with fatigue parameters n and DELTAA; the latter was the ratio of A value under cyclic fatigue to that under static fatigue.