Study on the rock-breaking behavior and fracture characteristics of PDC cutters in penetration-shearing hybrid rock-breaking

The rock-breaking efficiency and durability of PDC cutters are critical to improving the rate of penetration and drilling efficiency. Previous studies on PDC cutter rock breaking have mainly been conducted at a constant depth of cut, and the penetration-shearing hybrid rock-breaking behavior and associated shear fracture characteristics during cutter penetration into the formation remain insufficiently investigated. In this work, a penetration-shearing hybrid rock-breaking experiment was developed on a vertical turret lathe to investigate the effects of lithology, depth of cut, back rake angle, and cutter geometry on the rock-breaking performance of PDC cutters. Pearson correlation analysis was further introduced to quantitatively characterize the correlations between the influencing factors and rock-breaking efficiency, and shearing fracture tests were also conducted at back rake angles of 30° and 35°. During penetration-shearing hybrid rock-breaking, lithology was the primary factor controlling the rock-breaking efficiency of PDC cutters, with both the cutting forces and mechanical specific energy during granite cutting being significantly higher than those during sandstone cutting. Depth of cut was another key variable affecting rock-breaking efficiency. As the depth of cut increased, the mechanical specific energy decreased rapidly and gradually approached a stable value, whereas aggressiveness increased monotonically and remained independent of lithology. Shaped cutters required less energy for rock breaking, exhibited higher rock-breaking efficiency, and showed stronger resistance to shear fracture. However, the correlations of cutter geometry and back rake angle with rock-breaking efficiency were weaker than those of lithology and depth of cut. At back rake angles of 30° or higher, the rock-breaking mode gradually shifted from shearing to crushing. Meanwhile, cuttings became more difficult to remove, the cutting forces increased, and periodic dynamic impacts intensified, which readily induced shear-fracture failure of the PDC cutter. And rational selection of the back rake angle and proper control of the depth of cut were effective measures for preventing premature failure of PDC cutters. These findings provided theoretical guidance for the design optimization and field application of PDC cutters.