Experimental Study on Rock Crack Characteristics of PDC Cutter in the Process of Rock Breaking

Polycrystalline diamond compact (PDC) cutters are an important rock-breaking unit of PDC bits, which include indentation and rotary cutting in rock breaking process. However, existing studies ignore the rock damage during indentation. The objective of this paper is to investigate the indentation ability of PDC cutters into the rock and explore the mechanisms of rock damage. This study aims to provide a theoretical basis for the parameter selection of PDC bits. The rock-breaking process of PDC cutters with different rake angles pressed into blue sandstone and granite was studied by laboratory test methods. Macroscopic and mesoscopic cracks in the rock were analyzed using non-destructive microscopic detection technology. The study found that the crushing of sandstone was caused by the falling off between fine sand particles and the binder, and the crushing of granite was manifested as the brittle fracture of crystals. After the rock was loaded, a single micro-crack would first appear in the weak part of the rock, and the micro-crack would continuously expand to form a main crack. The main crack would continue to expand to form a macroscopic and visible crack. The part near the main crack would become a weak area containing many micro-cracks that have not yet formed. The cutter tip in the contact area was the stress concentration area, and the main crack was developed along this area. The process of rock damage varies with the rake angle. The ability of PDC cutter with a rake angle of 20° to press into blue sandstone is the strongest, and the ability of PDC cutter with a rake angle of 25° to press into granite is the strongest. PDC cutter with a rake angle of 5° and an indentation depth of 4 mm had the worst ability to press into the rock. The research results are of great significance for revealing the mesoscopic and macroscopic damage mechanisms of rock, establishing the evaluation method of PDC bits for rock breaking, and selecting and optimizing the design and working parameters of PDC bits.