Experimental study on dynamic mechanical responses of coal specimens under the combined dynamic-static loading

Dynamic mechanical responses of the coal and rock mass under the combined dynamic-static loading are critical issues for exploring the disaster-causing mechanism of rock burst in coal mining. In this paper, the acoustic emission (AE) technique and high-speed camera imaging were employed, and the burst proneness of the coal specimen, the variation of AE parameters, the specimen failure mode, and the particle distribution of the fractured specimens were investigated by the combined dynamic-static loading experiments. The results indicate that the burst proneness of the coal specimen increases non-linearly as the dynamic loading rate increases. Both the energy input rate, the ringdown counts, and the hits undergo the slow increase, fast increase, and abrupt increase stages. The failure mode of the specimen is changed from shear failure to axial splitting and the final bursting failure. The relationships between the fractal dimension of the specimen fragmentation mass and the dynamic loading rate can be described by a quadratic function. When the specimen reaches the maximum damage degree, an extreme value of the dynamic loading rate (2.8 x 10(-3) s(-1)) is obtained in experiments. The disaster-causing mechanism of rock burst induced by the combined dynamic-static load is discussed from the aspects of the burst proneness of coal specimens, dynamic variation of AE parameters, the change of failure modes of specimens, and the propagation and coalescence of microcracks.