Fractal characteristics and energy transfer mechanism of coal-rock combined body fragments under different loading rates
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Abstract:
In order to study the distribution, fractal characteristics and instability failure mechanism of coal-rock assemblages under different loading rates, uniaxial compression tests of fine sandstone-coal (FC), coarse sandstone-coal (GC) and fine sandstone-coal-coarse sandstone (FCG) were carried out under 0.001 mm/s, 0.005 mm/s, 0.01 mm/s, 0.05 mm/s and 0.1 mm/s loading rates. The results show that:1) at the 0.001 mm/s rate, the particle size of the broken coal is small, which is a complete and full failure, and the failure type belongs to plastic failure. Under 0.1 mm/s loading rate, the size of the failure fragment is the largest and the shape is irregular, which is an incomplete and inadequate failure, and the failure type belongs to brittle failure. The influence of loading rate on the failure of specimens is mainly shown in the degree of fracture development, the particle size of failure blocks, the number of failure blocks, the rate of energy release, the form of failure and the mechanism of instability. 2) The sample fragments have obvious classification characteristics. With the increase of loading rate, the number of fragments from 4.75 mm to less than 10 mm and 10 mm to less than 20 mm decreases gradually, and the fragmentation degree of specimens decreases, and the length to thickness ratio of three kinds of specimens increase at first and then decrease with the decrease of fragment size. For the fragments with the same particle size, the length to thickness ratio increases with the increase of loading rate. Increasing the loading rate will promote the number of thin-shaped fragments. 3) At the five loading rates, the particle size-quantity fractal dimensions of FC, GC and FCG composites are from 1.53 to 0.55, 1.27 to 0.26, and 1.45 to 0.46, respectively. The granularity-quantity fractal dimension decreases with the increase of loading rate, and the higher the loading rate is, the smaller the fractal dimension is. The particle size-mass fractal dimensions of FC, GC and FCG composites are from 2.35 to 1.48, 2.36 to 1.34, and 2.34 to 1.58, respectively, and the particle size-mass fractal dimensions decrease with the increase of loading rate. 4) According to the failure form of coal-rock assembly, the energy transfer mechanism of the failure process is analyzed. With the continuous loading of the assembly, the coal component is the first to be destroyed, and the released energy is directly transferred to the rock component. If it reaches the energy storage limit of the rock component, it will lead to the failure of the rock component. The energy transfer mechanism in the failure process of coal-rock assemblage reveals the lag phenomenon of rock component failure.
