"Double Peak" of Dynamic Strengths and Acoustic Emission Responses of Coal Masses Under Dynamic Loading

In recent years, the depth, intensity and complexity of coal mining in China has continued to increase. Meanwhile rockburst disasters, especially fault-slip bursts, have become even more severe. Research on precursor characteristics and early warning methods need to be in-depth, as rock burst disasters severely restrict the safety production of coal mines. Stress waves (dynamic loading) caused by the rupture of the roof of overlying rock layers, or the slipping activity of faults act on coal and rock masses, can easily cause the sudden instability of coal and rock masses and induce rockburst disasters in a high in situ stress environment. Therefore, this article is about a study of fracture characteristics of coal mass and its acoustic emission (AE) signal response under dynamic loading. The results show that the dynamic strength presented a "double peak" pattern. It rapidly dripped after two crests appear, and the second crest was higher than the first crest. With increase in bullet velocity (and applied axial static load conditions), the dynamic strength-strain curve shifted upward gradually. The difference between the two peaks was relatively stable. The coal sample with cracks and pores was damaged significantly at the first wave crest, the damage area was concentrated mainly around the ring of the sample. The proportions of the meta-instability stage in total process time increased with increase in impact velocity applied on the samples, which ranged from 3.17 to 5.94%. The axial static load did not show a significant role affecting proportion of the meta-instability phase in whole process time. The AE peak count and energy increased linearly with increase in impact velocity, while they decreased with increase in axial static load. When employing a same impact velocity, the difference between the two peaks increased with increase in axial static load. Because the first wave crest appeared when the coal body ruptured, the accurate rupture time of the coal body can be monitored by the AE count and energy. The paper sheds some light on exploring the fractural evolution process of coal and rock masses under dynamic loading, as well as being a reference basis for the evolutional mechanism of rockburst under a far-field fluctuational load condition. The research results are helpful to further improve the prediction accuracy and control of rock burst in coal mines.