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In this study, the fractal characteristics and energy dissipation behavior of gas-bearing coal are explored under both dynamic and static loads. Multifactor coupling mechanical tests are conducted using visualized static and dynamic loading experiment systems. The effects of impact velocity, axial compressive stress ratio, and initial gas pressure on energy dissipation and fragmentation characteristics of gas-bearing coal are analyzed. An internal correlation between the debris fractal and the energy dissipation density is clarified considering the fact that debris distribution has prominent fractal characteristics in the log–logarithmic coordinates. It is discovered that the fractal dimension experiences exponential growth with the strain rate in the impact velocity group, while showing that it rises linearly in the gas pressure group. There exists a critical axial compressive stress ratio that causes the formation of the debris fractal segment in the axial compressive stress ratio group. The debris fractal dimension change with the energy dissipation density is divided into two stages, and it is found that the dissipated energy value and its ratio decrease with the rise of initial gas pressure; debris fractal exhibits a quadratic decreasing relationship to the density of energy dissipation. Furthermore, by considering the dimension of the strain rate, the relationship of the aforementioned three indicators are uniformly quantified and compared. The research results reinforce the fundamentals of gas-bearing coal dynamics and provide a theoretical reference for further research on gas composite dynamic disasters in mines. [ABSTRACT FROM AUTHOR]

Coal seams are often affected by gas effects in addition to static and dynamic superimposed loads in the process of coal mining. Understanding the mechanical properties and energy dissipation of impacted coal under different initial gas pressures is extremely important. Therefore, dynamic compression experimentation of coal samples was conducted using a self-developed observable combined dynamic and static loading test system of gas-bearing coal. Furthermore, based on the crack volume increment, the crack energy dissipation density (CEDD) was defined to measure the energy required for a unit volume of cracks under different gas pressures, and the following conclusions were drawn: the deformation of gas-bearing coal can be divided into the elastic stage, elastic‒plastic stage, plastic stage and failure stage, and the dynamic strength and secant modulus decrease with increasing initial gas pressure. The impacted coal contains annular parallel cracks that run through the cross section and axial splitting/thoroughgoing cracks, and the two forms of cracks become more obvious with increasing initial gas pressure. The reflected energy of gas-bearing coal increases and the transmitted energy, dissipated energy and energy dissipation ratio decrease with increasing gas pressure. In addition, the CEDD decreases with increasing gas pressure because free gas and desorbed gas participate in external expansion work at the moment of impact, which results in a decrease in the CEDD required for gas-bearing coal failure. These conclusions enrich the basic theory of the dynamic coal–rock–gas disaster induction mechanism and can provide theoretical support for the monitoring, early warning and prevention technology of dynamic disasters in composites. Highlights: An observable combined dynamic and static loading test system of gas-bearing coal is successfully developed. Considering the strain–stress-dissipated energy curve of gas-bearing coal, the time-dissipated energy curve can be divided into the elastic stage, elastic‒plastic stage, plastic stage and failure stage. The relationship between the crack energy dissipation density (CEDD) of impacted coal samples and the initial gas pressure is clarified. The reason for the decrease in the CEDD with increasing gas pressure is revealed. [ABSTRACT FROM AUTHOR]

Coal samples having the same diameter (50 mm) and different length-diameter ratios (l/d), i.e., 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0 were tested under dynamic uniaxial impact compression using the Split Hopkinson Pressure Bar (SHPB) experimental system. This study evaluates: (a) The effects of l/d on the energy consumption law and fractal characteristics of coal crushing; (b) The effects of l/d and stress balance on energy dissipation; (c) The effects of l/d and energy consumption density on the fractal characteristics of coal crushing. The findings under different l/d are as follows: (1) The coal samples show similar stress–strain curve shapes in stages including elastic, plastic, and failure stage, which is an "open" shape, the proportion of plastic stage increases, and strain-softening occurs; (2) The dynamic compression dissipation energy and energy consumption ratio of coal shows the same trend, showing two stages with the increase of length-diameter ratio, which increases linearly in the first stage and overall decreases step-by-step; (3) The average particle size increases while fractal dimension of fragmentation decreases linearly, which endorses the decreasing trend of fragmentation degree; (4) It is determined that there is a power relationship between fractal dimension and energy dissipation density; (5) A new index Crushing Density Energy Efficiency (CDEE) is proposed, which can be used to characterize the rock-breaking efficiency of crushing energy consumption under different conditions. This index is inversely proportional to l/d. The research results can provide a basis for the design of top coal caving mining, and the determination of blasting parameters. [ABSTRACT FROM AUTHOR]

Gas-containing coal seams are highly susceptible to coal and gas outburst and composite dynamic disaster due to dynamic loads generated by roof breaks and blasting. This is an energy-driven state destabilization phenomenon, and is intricately correlated to the development of the cracks (i.e. damage evolution) within the coal. To accurately understand the energy evolution and damage principle of gas-containing coal under impact loading condition, we conducted impact compression experiments on the coal body under varying gas pressures. We analyzed the compression energy dissipation characteristics of the coal body under different gas pressures. Additionally, we utilized the CT scanning system to obtain the fracture evolution laws of gas-containing coal under the impact loading condition, and we explored the damage law of the gas to the coal body. In addition, the energy required for crack energy dissipation density (CEDD) is introduced to quantitatively characterize the relationship between energy dissipation and crack propagation. [Display omitted] • A visual gas-bearing coal dynamic-static combined loading system is self-developed. • We analyzed the failure characteristics of impacted coal under varying gas pressures. • The fine damage law of gas-bearing coal under impact loading conditions was analyzed. • The relationship between dissipation energy and fracture expansion is described. [ABSTRACT FROM AUTHOR]