Effect of coarse-grain contents on strength and fracture toughness of fine-grained graphite

To investigate the effect of the coarse-grain content on strength and fracture toughness of fine-grained graphite, bending and fracture toughness tests were conducted for specimens with different contents of coarse-grains. In the study the standard specimen was made of fine-grained isotropic graphite (IG-11) with a mean grain size of 20 μm, and two kinds of different grain size specimens were prepared by 20 and 40% mixing of coarse-grains with a mean grain size of 125 μm. The bending test revealed a strength increase for the 40% specimen with a small deviation compared with that for the standard specimen. As for the fracture toughness, two kinds of fracture toughnesses were investigated on the basis of the crack initiation load and the maximum applied load. The initiation load based fracture toughness for the 20% and 40% specimens was higher than that for the standard one; however, the difference for the 20% and 40% specimens was not observed clearly. These results suggest that the fracture toughness tends to saturate at a relatively low coarse-grain content, which is below 20% in the present study. On the other hand, the maximum load based fracture toughness increased with increasing coarse-grain content; thus the difference with different coarse-grain contents was observed. Moreover, the present authors applied a probabilistic strength model to the bending test results, using the pore size distributions obtained by image analysis of microstructures observed by optical microscopy. The model had been proposed by Burchell under uniaxial stress conditions, in consideration of pore size distributions. The prediction by the present model indicated a good correlation with the experimental results. (author)