Dynamic fracture characteristics of selected rocks

Fragmentation by blasting is distinct from other method of rock breakage due mainly to the short time scale involved in the application of stresses. In the present work, the dynamic rock properties, applicable to non-static fragmentation process have been measured and compared with measured values of other fracture related properties. Further, these are examined to establish correlation with respect to their physical, mineralogical and micro-structural characteristics. The rock types selected for the present work ranged from nearly homogenous isotropic rock to an-isotropic rocks. The nearly isotropic rock were represented by three different types of granites. The an-isotropic rocks consisted of gneissic granite, gneiss, marbles, limestone, and quartz.The dynamic compressive strength, measured under a strain rate of 10 3/sec, has been found to be about 2.5--4.6 times the compressive strength measured under static conditions (strain rate of 10-6 /sec) for similar dimensions of rock samples in a wide variety of rock types. It has also been found that this ratio is higher for low strength rocks, and lower for high strength rocks.The particle size distribution resulting from high velocity impact breakage is much smaller than in the static case. The degree of fineness (50% passing) generated under dynamic breakage is well correlated with the dynamic compressive strength; however, there appears to be no correlation between static compressive strength and the corresponding fragment size distribution.Except for static compressive strength, the dynamic strength was found to have no significant correlation with the measured values of fracture toughness, tensile strength, or comminution work index.The fracture toughness value is found to be controlled by the largest crack or grain size and to some extent, by porosity. The effect of crack density appeared to be non-linear; the fracture toughness initially decreases with increase in crack density, but further increase in the latter results an increase in toughness. This suggests the behaviour of rocks in dynamic compressive breakage is different than that due to static single crack growth.The work index (WI), which represents a fracture process at an intermediate strain rate correlates better with the Brazilian tensile strength than the static and high strain rate compressive strengths. (Abstract shortened by UMI.