Dynamic Characteristics of Coal under Triaxial Constraints Based on the Split-Hopkinson Pressure Bar Test System

Deep coals are mostly in a three-dimensional (3D) unequal stress state. In the hope of revealing dynamic characteristics of deep coals under impact load, dynamic failure experiments were performed on coal samples under dynamic load through the triaxial Split-Hopkinson pressure bar test system. In addition, the dynamic characteristics of coal in the multi-axial pre-stress state were analyzed by sampling the signals of incident, reflected and transmitted waves. Moreover, the relationships of dynamic peak strength, macroscopic fracture morphology of coal with axial pressure and confining pressure were explored. The following conclusions were drawn: The 3D pre-stress state exerts an obvious constraint on the dynamic failure of coal. With increase in strain, the dynamic initial stress of coal increases linearly first, and then grows at a reduced rate until the peak strength. After that, the mechanical curve rebounds notably. With increases in vertical force sigma(2) and horizontal force sigma(3), the dynamic strength of coal increases gradually. Under uniaxial impact, coal is broken into particles or powder, while in 3D pre-stress state, coal presents macroscopic fracturing. The dynamic strength factors of coal vary obviously with the increase of confining pressure. The research results can provide reference for the study on dynamic characteristics of coal under multi-axial constraints and for the prevention and control of dynamic disasters induced by dynamic loads in deep coals.