Effects of the structural properties of down-the-hole drill bits on the fracture behaviour of rock during rotary-percussive drilling

Down-the-hole (DTH) drill bits play a crucial role in rotary-percussive drilling, a widely used drilling technique for hard brittle rock. The structural properties of DTH drill bits significantly influence the rock fracture behaviour during the drilling. This work investigates these effects using a numerical method that incorporates a recently developed cohesive zone model to simulate rock fracture. A finite element (FE) model of a bit-rock interaction system is developed to simulate the rock cracking process under the impact of a multi-button DTH drill bit. Intrinsic cohesive elements are used for crack initiation and propagation in the rock. The accuracy and effectiveness of the simulation method are validated by comparing the numerical results with experimental observations of rock fracture under the impact of the DTH drill bit. Subsequently, structural properties, including the drill rod length, bore diameter, button geometry, and the number and arrangement of inner and outer buttons are analysed for their effects on impact force, torque, and rock fracture characteristics. The results indicate that, for a given mass, a longer DTH drill bit with a smaller bore hole diameter enhances drilling performance, while a greater number of flatter buttons improve rock fracture efficiency during rotary-percussive drilling.