Abstract: The damage and failure of a rock mass is the inevitable consequence of the initiation, propagation, and coalescence of internal cracks within it. This process can be characterized by changes in acoustic emission and infrared radiation signals, highlighting the importance of studying the acoustic-thermal response characteristics of the damage evolution in fissured rock masses. To investigate the acoustic-thermal response characteristics of crack extension and its precursor law during the cyclic loading and unloading of fissured sandstone, tests were conducted on prefabricated fissured sandstone with varying crack inclination angles. The study analyzed the acoustic-thermal response characteristics of crack propagation, identified precursor laws of various acoustic and thermal signals, and proposed a coefficient of variation index to quantify crack evolution. The main conclusions are as follows: 1) As fissure dip angles increases, the shear-induced damage effect on the specimen becomes more pronounced, with infrared and acoustic emission signals exhibiting distinct stage characteristics during loading and unloading. 2) With increasing cyclic loading and unloading cycles, precursor laws of acoustic and thermal signals became evident, characterized by prefabricated cracks forming around high-energy acoustic emission localization points, accompanied by infrared thermal image temperature anomalies; a sudden change in acoustic emission counts, RA(the ratio of acoustic emission rise time to amplitude), and AF(the ratio of ring counts to duration time) values during unloading, with these parameters maintaining higher values and more concentrated distributions. 3) Infrared temperature and acoustic emission RA, AF anomalies appear earliest, followed by acoustic emission localization. Acoustic emission counts show changes later, with infrared thermal images being the latest indicators. Acoustic emission localization and thermal image anomalies can provide spatial early warnings of crack formation. 4) A novel acoustic-thermal coefficient of variation (CV) index is proposed to quantify crack evolution. This index integrates crack evolution characteristics, optimizes weighting ratios, and reflects fluctuations in acoustic and thermal parameters over time, thereby characterizing crack evolution intensity. It overcomes the limitations of single acoustic or infrared signals, such as hysteresis and spatial constraints. This research provides valuable references for understanding rock disaster mechanisms, rock control strategies, and geological disaster prevention and mitigation.

Key words: rock mechanics, fissured rocks, crack propagation, acoustic emission, thermal infrared, coefficient of variation