The objectives of this research are to reduce energy consumption and dust generation during rock cutting process. The paper deals with the numerical analysis of conical bit penetration into rock. During this penetration process, the rock is subjected to two different kinds of damaging stress in succession: the first is dynamic shock wave stress induced by bit impact associated with high speed loading; the second is static stress induced by contact pressure associated with slow speed loading. Both impact and contact effects contribute to rock fragmentation. In this paper two types of numerical methods, namely finite difference and finite element methods, were applied to investigate the rock fragmentation mechanism under dynamic bit impact and Contact effects respectively.
Understanding the fragmentation mechanics of rock is important for many applications in rock engineering. Many mechanical excavators have been designed to cut the rock in mining engineering and underground construction. The essential mechanism involved in the mechanical fracture of rocks by indentation is the development of a "crushed zone" as a prelude to the formation of shear cracks. It has been shown that it is this action which eventually causes the chipping and ultimate fracture of the rock. The existence of these crushed zones imposes constraints on efficiency and increases dust generation (Khair, 2001). This paper will primarily discuss the numerical study on the fragmentation mechanics of the cutting process. Some measurements will be prompted to reduce the crushed zone in order to improve the cutting efficiency.
When bits impact rock, the effects of forces are applied for very short periods of time. The effects must be considered in terms of the propagation of stress waves. FLAC 4.0 was utilized to analyze the stress wave effects. This model investigated the propagation of a spherical wave in the rock due to an impulsive pressure induced by the impact of the bit at the top center of the rock. Three aspects were considered in preparing this finite difference model. These were:
dynamic loading and boundary conditions;
mechanical damping; and
wave transmission through the model.
In an unbounded media, two types of waves can exist: compression and shear waves. In this model, an axisymmetric boundary was used and the axisymmetric nature eliminates the shear waves, Therefore, only the solution for the compression wave needs to be sought. The quiet boundary condition was imposed on the outer boundary of the rock cylinder to absorb the reflected waves. For a dynamic analysis, Ole damping in the numerical simulation should reproduce in magnitude and form the energy losses in the natural system when subjected to a dynamic loading. Rayleigh damping was used in this model. Since Rayleigh damping is frequency dependent, the natural frequency of this model was evaluated first. This curve demonstrates the cyclic loading due to impact pressure wave. In the first loading cycle this zone is damaged and a short time later the zone is unloaded, the shear strain decreases.
