Mechanical strength change and coal damage analysis of frozen saturated bitumite after cryogenic freezing

The changes in mechanical strength and damage analysis of bitumite under the action of liquid nitrogen (LN₂) freezing are critical issues that need to be addressed in the application of LN₂ in coal seam permeability enhancement. Using an mechanical testing & simulation (MTS) testing machine, strain gauges, and ultrasonic detection instruments, experiments were conducted to obtain stress–strain curves and ultrasonic time-domain images of coal samples after single freezing and cyclic freeze–thaw processes. These data were analyzed to understand the mechanical performance and internal fracture evolution of coal samples subjected to ultra-low temperatures of LN₂. The experimental results indicate that: (1) The uniaxial compressive strength and elastic modulus of coal samples subjected to a single LN₂ freeze are positively correlated with the freezing time from 0 to 50 ​min, and negatively correlated with the freezing time from 50 to 180 ​min. (2) The uniaxial compressive strength and elastic modulus of coal samples subjected to cyclic freezing and thawing with LN₂ decrease as the number of freeze-thaw cycles increases. (3) The Poisson's ratio of coal samples subjected to single freezing and cyclic freezing-thawing is negatively correlated with the absolute freezing time. Additionally, the decrease in Poisson's ratio is greater in coal samples subjected to cyclic freezing-thawing compared to those subjected to single freezing. (4) As the absolute freezing time increases, the ultrasonic waveforms of the coal samples begin to become disordered, which is manifested by a decrease in amplitude and a delay in the arrival time of the first wave. Under ultra-low temperature conditions, LN₂ can deteriorate the mechanical properties of coal samples, with the degree of deterioration being greater under cyclic freezing-thawing than single freezing. This study can provide theoretical guidance for increasing the permeability of coal seams.