FRACTAL CHARACTERISTICS OF CRACK PROPAGATION IN COAL UNDER IMPACT LOADING

摘要

This paper studied the fractal characteristics of crack propagation in coal containing beddings under impact loading condition. Split Hopkinson pressure bar (SHPB) system was applied to determine the prepared notched semi-circular bending specimens. The high-speed camera was used to record the propagation characteristics of cracks. The image processing method and fractal dimension calculation software are combined to further analyze the effects of bedding and loading rate on the fractal characteristics of crack propagation in coal. The experimental results presented that the presence of bedding has a remarkable impact on the crack propagation. The crack velocity of coal samples with 45 degrees bedding angle is of the maximum, however, the crack velocity of samples with 0. bedding angle is of the minimum. Bedding angles also have obvious influence on the fractal dimension of cracking path, the bedding angle of 45 degrees being the largest, and those of 22.5 degrees and 67.5 degrees having the intermediate values, while those of 0 degrees and 90 degrees being the smallest. Several points of instantaneous fractal crack velocity are close to the Rayleigh wave velocity (C-r). The crack velocities of coal specimens with bedding angles of 22.5 degrees, 45 degrees and 67.5 degrees are prone to the high value.