SPH Simulation of Coal Cutting by a Single Pick under Compressive Pre-stress

The rock fragmentation mechanism during the cutting process of a conical pick is influenced by the complex interaction of tool parameters, material properties, and stress environments. Addressing the current insufficient understanding of compressive pre-stress, this study developed a novel Smoothed Particle Hydrodynamics (SPH) model. This approach constructs a hybrid failure model based on the Drucker-Prager model and the Grady-Kipp model, capable of naturally simulating large deformations and crack propagation under multi-axial stress states. By establishing an SPH numerical model for coal-rock, the entire process of pick cutting under compressive pre-stress pressures ranging from 1 to 6 MPa was simulated. The results indicate that compressive pre-stress significantly influences the force characteristics during rock fragmentation by the pick. As the compressive pre-stress increases, the peak value of the average acting force initially increases and then decreases. When the compressive pre-stress exceeds 4 MPa, the peak average force decreases, even showing an 8% reduction compared to the 1 MPa case. Simultaneously, compressive pre-stress affects crack development. With increasing compressive pre-stress, crack patterns evolve from being shallow and dense to forming internal networks. The proportion of the crack development zone relative to the entire coal-rock mass first decreases and then increases, with 2 MPa identified as a critical compressive pre-stress. At this pressure, the crack development zone decreases by 3% compared to the 1 MPa case, after which cracks develop rapidly, and the affected zone increases fivefold. This demonstrates that regulating compressive pre-stress can alter the internal rock fragmentation mechanism. From the perspective of the stress field, increased compressive pre-stress leads to a sixfold increase in the overall average stress within the coal-rock and a threefold decrease in the stress concentration coefficient. This confirms the fundamental mechanism by which compressive pre-stress alters the internal stress state, thereby changing the internal rock fragmentation process. Furthermore, compressive pre-stress promotes the directional propagation of fractures towards the free surface of the pick's operation, effectively confining the fractured zone near the excavation face, which is beneficial for surrounding rock stability. This study provides a theoretical basis for optimizing cutting techniques under different stress environments and validates the effectiveness and advantages of the SPH method in simulating geotechnical engineering problems involving large deformations and multiple failure modes. It also establishes a new computational paradigm by linking continuum damage mechanics with discrete fragmentation processes in confined coal-rock cutting systems.