抄録
We studied experimentally the effects of grain size on brittle properties of two kinds of granite with quite different mean grain size. These granite samples in much the same mineral composition (mode) were prepared, and these were classified into two groups in terms of their mean grain size; one with grain sizes between 3.0 and 5.0mm and the other with grain sizes between 0.5 and 1.0mm. In order to investigate the behavior of microcracks in the sample, we measured the axial stress, axial and circumferential strains, elastic-wave velocities along the direction perpendicular to the loading axis and the number of AE events (acoustic emission activity) during loading. The results were analyzed in conjunction with the growth of stress-induced cracks during loading. We analyzed the relation between the degree of dilatancy and the cumulative number of AE events. We also analyzed the velocity change during loading in terms of the increase in penny-shaped cracks during loading with a simple assumption about the orientation of these cracks. These analyses indicate that the coarsegrained granite produces larger volumetric cracks than the fine-grained granite and that, for both the granites, the crack volume decreases when the normalized stress (stress relative to fracture stress) reaches about 0.95. Crack density of the cracks with their fiat planes parallel to the final fracture plane increases acceleratively just before the ultimate fracture. The crack density of coarse-grained granite is higher than that of fine-grained granite. The aspect ratio of stress-induced cracks was estimated to be about 1/400 and 1/150 in coarse- and fine-grained granites, respectively. These results suggest that the stress state in rocks depends on scale of inhomogeneity in them.
