Dynamic Mechanical Behavior and Constitutive Model of CO₂‑Exposed Coal under Three-Dimensional Stress

During CO₂ sequestration in coal seams, coal stability is affected by CO₂ adsorption, three-dimensional in situ stress, and mining-induced disturbances. In this study, three-dimensional impact tests were conducted on coal under different CO₂ pressures and confining pressures using a self-developed high-temperature and high-pressure gas-bearing coal-rock dynamic-static combined loading system. X-ray computed tomography (CT) was employed to characterize fracture evolution before and after CO₂ exposure. The results demonstrate that CO₂ exposure promotes the propagation of primary fractures and the formation of new fractures while reducing the dynamic peak strength and dynamic elastic modulus of coal. In contrast, confining pressure suppresses fracture propagation and enhances the dynamic load-bearing capacity of coal. On this basis, a damage constitutive model for CO₂-exposed coal under three-dimensional stress was developed by coupling CO₂ adsorption-induced damage with mechanical loading-induced damage. The model effectively describes prepeak deformation, damage evolution, and peak strength and provides a partial representation of the postpeak stress attenuation trend.