Stress field evolution and crack extension law of coal breaking with flexible cutting tools

Flexible cutting tools reaming is a new method of coal seam unloading pressure and enhancing permeability, in order to resolve the problem of insufficient ability of flexible cutting tools breaking coal at the present period, improve the performance of coal breaking by optimizing the structural parameters of flexible cutting tools under certain kinetic energy. Therefore, 2D numerical model of coal breaking by flexible cutting tools is established to clarify the evolution of coal stress field and crack extension characteristics within the process of coal breaking, investigate the effects of different installation angles, number of cutting teeth and cutting tooth distance on the stress evolution and crack formation of coal body from the mesoscopic scale. Clearly defined the effect of the number of cutting teeth on the energy evolution law of the coal breaking process in kinetic-energy unity condition, and reveal the mechanism of influence between installation angle and cutting tooth distance on the crack extension of the coal body. Results show that the coal body stress wave is affected by the installation angle of cutting tooth as abnormal propagation, the installation angle of 0°～20° is impact breaking coal, the middle cracks and side cracks radiate around by the impact point, and crack quantity is distributed uniformly. At 25°～40° is the impact-abrasive coal breaking, the impact effect is dominated, side cracks are developed, and double peaks of cutting tooth loads occurred, maximum crushing area at 35° installation angle. Under certain kinetic energy, with increase the number of cutting teeth, the energy conversion time is decreased, and the area of coal breaking is increased. The strain energy peak and strain energy concentration of the double cutting teeth become better, improved rapid development and connectivity of fractures. Double cutting tooth impact forms a connecting curved compression stress area inside the coal body. Force chains connect between the two impact points and provide preconditions for the formation of fractures between particles, thus improving the damage effect of coal break. There exists the optimal cutting tooth installation angle to improve the force transmission effect so that the coal body breaking area is maximized, and there exists the ideal cutting tooth spacing to connect the breaking area with each other, fully utilizing the cooperative coal breaking effect of the double cutting teeth.