Mechanism Analysis of Tooth Sink into Rock Based on Unified Strength Theory

In order to figure out the rock breaking mechanism in complex formation conditions,the teeth cutting into rock in bottom was studied. Considering the condition of the bottom hole pressure,based on unified strength theory,the mechanical process of teeth cutting into rock was analyzed,bottomhole rock sinking equation with unified strength parameters was established and sinking coefficient without the impact of axial load,invasion depth and bit angle have been worked out. The equation considers the impact of intermediate stress on sinking coefficient. The results of the equation and Mohr-Coulomb strength theory are identical when the effect coefficient of intermediate principal shear stress equals 0. The impact of angle of internal friction,fluid column pressure,pore pressure and bit tooth angle on sinking coefficient were analyzed based on sinking equation. The results show that the sinking coefficient increases with the increase of effect coefficient of intermediate principal shear stress,increases exponentially with the increase of angle of internal friction and bit tooth angle,increases linearly with the increase of fluid column pressure,and decreases linearly with the increase of pore pressure. The analysis of rock breaking mechanism of teeth can provide further theory guidance for cone bit onsite application.