The effects of gravity on rock fractures caused by PDC bits

To understand the crack propagation and chip formation mechanism caused by a PDC bit for various gravity values, a numerical study using particle flow code in 2 dimensions (PFC 2D) is performed. The numerical results indicate that the cracks, caused by stress concentrations, are responsible for chip formation. In addition, the fracture area and cutting efficiency first increase and then decrease with the increase in gravity. The dynamic analysis of stress evolution in the cutting process indicates that the input energy per step curve consists of many fluctuations, accompanying with crack propagation and stress concentration-dissipation cycles. Moreover, the theoretical analysis using the maximum circumferential stress theory successfully explains the reason why the increase in gravity first increases and then decreases the fracture area and cutting efficiency.