Mechanical damage mechanism of frozen coal subjected to liquid nitrogen freezing

•Mechanical changes of frozen coals under different liquid nitrogen treatments.•The freezing-thaw damage effect of LN2 and the principle of freezing enhancement.•The mechanics principle of liquid nitrogen changing the pore structure of coal. Anhydrous liquid nitrogen cracking low-permeability coal seams can improve the efficiency of coalbed methane extraction. Under normal pressure, liquid nitrogen (-196℃) contacts the coal body, and the temperature stress and frost heave force generated will change the mechanical properties of the coal body, cause internal structural damage, and increase the seepage channel of coalbed methane. In order to study the influence of different liquid nitrogen freezing variables on the mechanical properties of coal, Brazilian splitting, uniaxial compression, and ultrasonic velocity measurement tests of saturated frozen coal were carried out. The results show that a single freezing of liquid nitrogen has a certain enhancement effect on the mechanical properties of coal: the tensile strength increases by 64.0%, the uniaxial compressive strength increases by 54.6%, but as the freezing time increases, the coal strength increases first and then decreases The strength of coal under the action of freeze–thaw cycles shows an exponential decline in varying degrees: tensile strength has dropped by 81.3%, uniaxial compressive strength has dropped by 68.9%, and the degree of decline is positively correlated with the number of freeze–thaw cycles. The change in mechanical strength changes the elastic stage. The freezing enhancement factor I and the freeze–thaw damage factor D are defined by the change of elastic modulus. I presents a quadratic function relationship that first increases and then decreases with freezing time, and D continues to decrease as the number of freeze–thaw cycles increases. The longitudinal wave velocity first increases and then decreases with the freezing times, and decreases with the increase of the number of freeze–thaw cycles. The porosity is negatively correlated with the wave velocity. The wave speed decreases, the porosity increases, and the internal pores of the coal are connected to form a fracture network, which accelerates the damage of the coal and reduces the mechanical properties of the coal. Through analysis, it is concluded that when liquid nitrogen acts on coal, the change in mechanical properties is the result of the combined action of multiple forces.