Dynamic Strain Rate Effect and Macro–Micro-Fracture Mechanism of Raw Coal Under True Triaxial Conditions

The stress wave propagation and energy evolution of coal and rock masses under complex stress states hold significant implications for the efficient extraction of deep resources and the prevention and management of dynamic disasters. To investigate the propagation characteristics of stress waves and the energy dissipation in raw coal under true triaxial conditions, this study employed the self-constructed true triaxial split Hopkinson pressure bar test system in conjunction with a scanning electron microscope. Dynamic and static combined impact tests were conducted on raw coal samples. The findings indicate that σ₂ and σ₃ under true triaxial prestress strengthen the sample's resistance, facilitating stress wave propagation but hampering energy conversion. Both σ₂ and σ₃ enhance transmission stress and strain, increasing from 11.0 MPa and 0.53 × 10⁻⁴ in sample tr#1 to 16.3 MPa and 0.78 × 10⁻⁴ in sample tr#5. Reflected energy constitutes the largest proportion of incident energy, followed by dissipation energy, with transmission energy being the smallest. Moreover, two inflection points in the change rate of energy ratio were observed in sample tr#2 (initial increase stage of intermediate principal stress) and sample tr#4 (initial increase stage of minimum principal stress). The spectrum of the stress wave exhibited an initial increase followed by a decrease, and the peak value of the reflected wave spectrum was an order of magnitude greater than that of the transmission wave. The frequency at which the transmission wave spectrum reached the peak point and the stationary phase was lower. The macroscopic failure degree of the sample exhibited a gradual weakening trend under the influence of σ₂ and σ₃. The micro-crack fracture pattern shifted from river-like cracks to steplike cracks, eventually forming herringbone macroscopic fractures, indicating that the coal body failure under stress waves was attributed to brittle fracture.