Experimental study of the fragmentation characteristics of brittle rocks by the effect of a freefall round hammer

The dynamic fragmentation of brittle rocks (slate, granite, marble and limestone) under low-velocity impact was investigated using freefall round hammer experiments. A three-parameter generalized extreme value distribution was used to characterize the fragment sizes, and the finest particles were neglected. A transformation of fragment size was adopted to overcome the problem that the interval of adjacent fragments size increases as the fragment size grows. The results reveal that the frequency distributions and num-based cumulative frequency distributions based on number have a certain change law with variations in impact energy and hammer size; the frequency distribution curve became narrower, and the cumulative frequency distribution curve moved toward the left as the impact energy increased, and the hammer size decreased. Marble was most easily broken followed by granite, limestone and slate, which was proved by scanning electron microscope images of the fracture surfaces of the four rocks. Because fragmentation is a stochastic process, the equivalent diameters of the fragments always had a more obvious regularity than the major and minor axes in the histograms. Therefore, the mean value of the equivalent diameter was used as the experimental average fragment size. Comparing the experimental results with the theoretical results of average fragment sizes that were predicted by four normalized models, it was determined that Levy and Molinari's model and Zhou et al.'s model better forecasted the fragment sizes.