Time- and frequency-domain evolution characteristics of high-temperature coal specimens subjected to liquid nitrogen freeze-thaw cycles

Liquid nitrogen (LN 2 ) freeze-thaw cycling is a promising waterless stimulation technology for enhancing coalbed methane recovery by degrading the mechanical properties of coal and inducing fracture networks. In this study, coal specimens heated to 100 °C and 300 °C were subjected to 0-24 LN 2 freeze-thaw cycles. Macroscopic damage evolution was investigated through time- and frequency-domain analysis of acoustic emission (AE) signals, while microscopic fracture evolution was examined using CT scanning. Results show that AE counts remain low during compaction and elastic deformation but increase exponentially during the yield and failure, accompanied by wide b -value fluctuations. Coal specimens subjected to fewer cycles (≤12) exhibit predominantly tensile failure, with the shear proportion increasing slightly from 8.5% to 19.7% due to microcrack friction. After multiple cycles (≥18), intergranular bonding weakens, causing earlier energy release and stabilization of AE counts and b -values, while the shear proportion declines to 4.9%. Frequency-domain analysis reveals that, with increasing cycle number, the proportion of low-frequency (LF) signals decreases, whereas that of high-frequency (HF) signals increase. After 24 cycles, the proportion of HF signals exceeds 35%, while that of LF signals drops to 15.3%. The “multi-frequency simultaneous response” and high-amplitude signals (>60 mV) are observed primarily during yield and failure stages, serving as potential failure precursors. CT images reveal progressive fracture propagation and interconnection after 6 and 24 cycles. Fractal dimensions increase consistently across three threshold segmentation methods. These findings elucidate the damage evolution mechanisms in high-temperature coal under LN 2 freeze-thaw treatment.