Rock breaking characteristics and mechanism of conical polycrystalline diamond compact cutter

Conical polycrystalline diamond compact (PDC) cutter is a new type of PDC cutter with high impact and wear resistance, which has a very good drilling effect in hard, strong abrasion, and soft-hard interbedded formations. In order to reveal the hard-rock breaking mechanism of conical PDC cutters, the laboratory test and numerical simulation on the granite broken by the conical PDC cutter were carried out. The influence law of cutting depth and front rake angle on cutting force and rock breaking specific energy of the conical PDC cutter was analyzed. In the laboratory test, a high-speed camera and transparent K9 glass were used to observe the formation process of cuttings and the initiation and propagation of microcracks under the action of conical PDC cutters. The stress response and damage evolution characteristics during the rock breaking process were analyzed by numerical simulation. The surface morphology and fracture microscopic characteristics of cutting grooves and large-size cuttings were analyzed, and the mechanical model of conical PDC cutters to break granite was established. The results show that the granite breaking process can be divided into two stages: crushing and chipping. The effect of the front rake angle on the rock breaking process is relatively small, while the influence of cutting depth is significant. The cracks around the conical PDC cutter are mainly composed of compaction nucleation, longitudinal crack and transverse crack. The maximum propagation depth of the longitudinal and transverse cracks is 6.69 and 4.53 times the cutting depth, respectively. The compressive stress around the cutter tip is concentrated, the shear-compression failure occurs, an arc-shaped strip-like tensile stress zone is formed at the periphery of the compressive stress zone, and the tensile micro-cracks are induced at the boundary of the cutter tip and compressive stress zone. When the micro-crack propagates to the front of conical PDC cutters to form an arc-shaped tensile main crack, the block debris cracking occurs, which improves the rock breaking efficiency. Meanwhile, the tensile micro-cracks propagate to the rock inside and deteriorate rock strength, a bottom damaged area is formed, which improves the rock breakage efficiency of the subsequent cutting.