Mechanical and Acoustic Emission Characteristics of Coal at Temperature Impact

Coal and rock mass constitute a type of porous medium. This study investigated the influence of temperature impact on the mechanical properties and acoustic emission (AE) characteristics of coal. A mechanism analysis was performed from the perspective of microstructure. The results show that the temperature impact causes the development of pores and cracks in the coal, which reduces the strength of coal. The elastic modulus of coal generally decreases with increasing temperature gradient. AE parameters increase with the increase in the load and reach the maximum value at the peak stress. AE parameters and cumulative parameters decrease with increasing temperature gradient. Not only does temperature impact change the fracture structure of the coal surface, but also the internal fracture structure of the coal is significantly affected. After temperature impact, the cracks expand and new cracks are initiated, and the fracture volume of the coal increases. Temperature impact causes the volume and specific surface area of small pores and meso-pores in coal to increase, and promotes the opening of the necking pores within the coal. The impact causes macro-pores to penetrate through to form cracks, which increases the transport of coal gas and significantly improves the permeability of coal. The thermal stress generated by coal under temperature impact is greater than its tensile strength, which promotes the cracking of coal, along with the initiation, widening, extension, and expansion of crack networks, which significantly change the fracture structure of coal. The research results lay a certain theoretical and experimental foundation for further study of mechanical properties of coal affected by liquid nitrogen.