Study on mechanical properties and energy evolution of coal under liquid nitrogen freezing

To investigate coal's mechanical properties and energy evolution laws under different liquid nitrogen (LN2) freezing conditions, uniaxial compression experiments were carried out on frozen coal samples, with water saturation and LN2 freezing time as variables. Based on the experimental results, the mechanical properties, acoustic emission characteristics, and energy release laws of coal were analyzed, and the mechanism of mechanical strength enhancement of coal frozen by LN2 was also discussed. The results prove the positive role of LN2 freezing in enhancing coal's mechanical strength, that is, under the action of LN2 freezing, both the uniaxial compressive strength and elastic modulus of coal increase with the increase in water saturation and LN2 freezing time. Besides, the longer the LN2 freezing time, the longer the time the loaded coal maintains the elastic stage. At the compaction stage, acoustic emission events with large ampli-tude frequently occur due to the sliding friction between ice particles and the coal matrix. Under the action of LN2 freezing, the water in coal pores turns into the ice with an expanded volume and then fill the pores, which weakens the stress concentration effect at the crack tip and further prevents the crack extension of the loaded coal. With the increase in water saturation and LN2 freezing time, two peak points gradually appear on the curve of elastic energy variation of frozen coal with strain, indicating that LN2 freezing enables water-containing coal to withstand defor-mation even after the initial load damage. The formation of pore ice has three positive effects on coal's mechanical strength enhancement, i.e., pore ice bears a certain mechanical strength, plays a cementing role, and reduces the stress concentration at the crack tip.