The ductile-brittle transition of rock cutting: insight from the discrete element method

The rate of penetration (ROP) improvement of the drilling bit is an eternal topic in drilling engineering. For polycrystalline diamond compact (PDC) bits, when the cutting depth exceeds the critical transformation depth (CTD), it will lead to higher rock breaking efficiency and saves more energy. Therefore, it is of great practical engineering significance to study the ductile-brittle transition (DBT) behavior of PDC cutters in rock cutting. Through theoretical analysis, the judging index of DBT rock cutting is determined from four aspects: mechanical specific energy (MSE), energy distribution, cuttings size characteristics and microcrack formation characteristics. Then, a combined distributing method (CDM) is used to reveal the DBT behavior during rock cutting. On this basis, a discrete element method (DEM) numerical model of single-cutter cutting red sandstone is established and laboratory tests are carried out. Finally, combined with the numerical results, CDM is applied and illustrated. The results show that the four DBT determination indicators proposed in this paper can effectively reflect the DBT characteristics during rock breaking. And the brittle energy consumption ratio of PDC bit during rock breaking process is less than 25%, and the ductile energy consumption ratio is greater than 50%. Besides, the fractal dimension reflects the repeated fracture of the cuttings during the rock breaking process, and the volume of the chips meets the statistical fractal characteristics after the single-cutter cutting. The equivalent CTD of the whole drill bit is about 1.12 mm. In the actual drilling process, it is recommended that the cutting depth of the PDC bit should be greater than 1.12 mm, and the rake angle should be in range from 0o to 20o.