DEM numerical investigation on the effect of back rake angle of PDC bit in drilling hard rock strata

Abstract

Polycrystalline diamond compact (PDC) drill bits used in the Deep Borehole Placement (DBP) approach for nuclear waste disposal are subjected to highly competent, low-porosity, and low-permeability rock mass that cause severe wear and damage. To augment the penetration rate and minimize the wear-induced cost, a novel interaction rock cutting model with breakable granite and unbreakable cutter was established using two-dimensional discrete element method (2D-DEM). The particle element was used to simulate the granite sample, and the micro parameters of the contact elements between them were calibrated based on the uniaxial compression and Brazilian tests. While the rigid block element was used to reproduce the single PDC cutter including its elastic properties and geometric structure (i.e., back rake angle). The cutting process was simulated by applying normal force and tangential velocity on the single cutter to explore the effect of the back rake angle (5, 10, 15, 20 and 25°) on the stress condition of the cutter and penetration rate. The simulation results show that with the increase of the back rake angle, the maximum shear stress magnitude and stress concentration area decrease simultaneously, in according with the decrease of material removal rate, which do good help to decrease the wear and unexpected damage of bit cutter. In addition, the cutting process can be divided into the effective cutting stage when the elastic energy continuously stores and the fragmentation initiation stage when the stored energy suddenly releases and numerous cracks generate. In long term, the established DEM model can be used to optimize the structure of the PDC bit.