Mechanical behavior and acoustic emission characteristics of initially damaged coal under triaxial cyclic loading and unloading

During deep coal mining, an instability failure of coal usually occurs under the combined effect of initial damage and triaxial cyclic loading and unloading (TCLU). Therefore, this study investigated the impact of initial damage on mechanical behavior and acoustic emission (AE) characteristics of coal under TCLU. Initial damage variables (IDVs) of coal specimens were quantified using preloading, followed by TCLU experiments to assess the deformation, energy distribution, and fracture development. The results revealed that the increase in IDVs significantly reduced the structural integrity of coal specimens, increased the cumulative irreversible strain, and enhanced the dissipated energy owing to microfracture expansion. Moreover, AE monitoring showed earlier activation of fractures and a higher occurrence of large-scale rupture events of coal specimens with high IDVs, which correlated with decreasing AE b values (reflecting the different scales of fracture within specimens) and increasing S values (reflecting the AE activity within specimens). Additionally, computed tomography analysis revealed intensified fracture networks and increasing three-dimensional fractal dimensions of coal specimens with higher IDVs. Finally, the coupling effect of TCLU and initial damage on the weakening mechanism of coal was investigated. Initial damage significantly reduced the structural integrity of coal by increasing the number of weak planes within coal specimens, contributing to the earlier activation and rapid expansion of fractures at low stress levels under TCLU and eventually accelerating the weakening process of coal. This study provides a scientific basis and theoretical support for the prevention and control of dynamic disasters in deep coal mining.