Research on Composite Rock-Breaking Mechanism of PDC Cutters with Impact and Cutting Based on Three-Dimensional Discrete Element Method

Current research on the matching relationship between the axial-torsional impact velocity amplitude and frequency of PDC cutters (including their worn states) under composite impact is still insufficient, leading to a lack of theoretical basis for impact parameter optimization and limited improvement in rock-breaking efficiency. To this end, based on the discrete element method, a heterogeneous granite model was constructed; its dynamic fragmentation behavior under the composite action of impact and cutting by PDC cutters with different axial-torsional impact velocity amplitude ratios and axial-torsional impact frequency ratios was numerically simulated, and the rock fragmentation characteristics were analyzed from multiple aspects. The results indicate that rock fragmentation effectiveness is better under the composite impact and cutting action with a larger axial-torsional impact velocity amplitude ratio and a smaller axial-torsional impact frequency ratio, and it is recommended to adopt 4∶1, 2∶1 or 3∶1, 1∶1. Under these conditions, PDC cutters with a certain wear amount can promote rock fragmentation to a certain extent, and this effect disappears when the wear of PDC cutters becomes excessive. When the axial impact velocity amplitude is large, the axial impact frequency can be appropriately increased to obtain a larger rock debris volume; conversely, when the amplitude is small, the frequency should not be increased. Under the composite action of impact and cutting, rock failure is predominantly tensile, and over 70% of tensile cracks tend to propagate in the vertical direction, among which cracks with a dip angle exceeding 80° account for 20%. The research results have a certain reference value for improving rock-breaking efficiency in deep and ultra-deep formations.