Influence of Liquid Nitrogen Cooling State on Mechanical Properties and Fracture Characteristics of Coal

To determine the role of LN2-cooling in the fracturing process of the coal, the LN2-cooling process of the coal samples is divided into three states: initial state, frozen state, and freeze–thaw state. Changes in the mechanical properties and fracture behaviors of the coal samples under three states are systematically evaluated by a series of laboratory experiments. The thermal cracking behavior of the coal during LN2 freeze–thaw is revealed through a crack phase-field model. The results indicate that the compressive strength, elastic modulus, and fracture toughness of the frozen coal significantly increase, while they decrease for the freeze–thaw coal. The tensile strength of the coal under the freeze and freeze–thaw states has an obvious reduction, where a greater decrease for the freeze–thaw coal is induced. The fracture propagation process and induced fracture morphology of the coal under both the freeze and freeze–thaw states become complex, in which a greater change for the freeze–thaw coal is presented. The micro-fracture in the coal during LN2-cooling mainly comes from the temperature gradient and mismatch of thermal stress between adjacent mineral particles. Both fracture growth rate and fracture area in the LN2 thaw process are larger than that in the LN2 freeze process. The variations in the fracturing behaviors of the coal with different LN2 treatment states in the mechanical experiments are well explained by the numerical simulation results.