The dynamical response characteristics of elastic–plastic coal under dynamic load

• The generation of tensile stress waves in elastic–plastic coal under instantaneous unloading.
• The developing processes of the coal and gas outbursts began inside of the elastic–plastic coal.
• The generation process of spallation phenomena was revealed in the coal and gas outbursts.

At present, it has been confirmed that tensile stress can be generated in coal bodies through instantaneous unloading, which leads to severe damage in certain areas of the coal. However, the process for the generation of this tensile stress is still unclear. In this study, by taking the elastic–plastic coal in the front of the tunneling face and the rock cross-cut coal uncovering region as examples, and by applying the rules of reflection and transmission to the stress waves, the concrete propagation process of stress waves in coal under dynamic loading was examined. At the same time, the formation and acting mechanisms of the tensile stress waves under the interaction of the loading and unloading stress waves were studied. The results indicated that compression-shear, tension, and tension-shear failures in the elastic–plastic coal were evident, under the role of the instantaneous loading and unloading. Due to the fact that the wave impedance of the coal tended to distribute unevenly in the front of the tunneling face, the loading and unloading stress waves were thereby transmitted and reflected in the propagation process of the coal body, and formed unloading stress waves. The tensile stress generated in the coal under the unloading waves, which were prone to severe damage to coal body, led to dramatic increases of the gas desorption rate, as well as a significant rising of the gas pressure. These results led to the occurrences of the coal and gas outburst accidents. Therefore, the development process of the coal and gas outbursts was determined to have begun on the inside of the coal, then to gradually develop towards the tunneling face. Multi-beam unloading waves were formed by the instantaneous unloading of the blast-hole wall, as well as the pressure released around the coal. Radial tensile stress waves in the coal were prominent, under the role of the unloading waves, which led to tension failure in the coal, which in turn generated circumferential cracks. This caused the spallation phenomena of the coal around the outburst-hole wall during the process of the coal and gas outbursts. The conclusions of this study have important theoretical guiding significance for the prevention of coal and rock dynamic disasters.