Characteristics of instability and ejection modes of coal exhibiting strong impact inclination under uniaxial compression

Mechanical behaviors of deformation, rupture, and ejection failure of coal with impact inclination under uniaxial compression were investigated. Uniaxial compression tests were conducted on coal with impact inclination at different loading rates. The classification characteristics of post-peak stress drop in coal samples have been identified. Based on the 3D reconstruction of the primary fracture of coal samples through CT scanning, the distinct mechanisms of instability failure of coal samples with various stress drop types are analyzed. Based on the non-contact full-field strain measurement results, the pre-peak and post-peak fracture evolution and the fracture-ejection rule of coal samples are analyzed. The stress change and transformation characteristics of the coal sample ejection region are statistically calculated. The ejection mode of the coal body is disclosed, and the classification characteristics of the fracture-ejection and the stress drop type of the coal sample ejection mode are obtained. Additionally, the correlation between the existing impact tendency index of coal sample and the impact kinetic energy of post-peak ejection is analyzed, and W_sdr is established to characterize the intensity of post-peak ejection of coal sample. The results show that: Post-peak stress drop of coal samples can be divided into three types: continuous drop, stepped drop and instantaneous drop, and the post-peak stress drop type of coal samples is closely related to the development of primary fractures. Surface crack propagation before the impact inclined coal ejection has the nonlinear characteristics of slow and accelerated crack propagation, and the incubation time of class Ⅰ, class Ⅱ and class Ⅲ impact ejection decreases successively. Coal with impact inclination can be classified into local ejection, regional through-out ejection and global instability ejection. Coal samples of Class Ⅰ, Ⅱ and Ⅲ are dominated by local ejection, regional through-out ejection and global instability ejection respectively. The coal sample has two types of ejection modes: parent power source and ejection block power source. The same stress drop type coal sample can be caused by parent power source or ejection block power source. Proposed index W_sdr has a linear positive correlation with the ejection kinetic energy.