A 3D FEM Methodology for Rock Breakage in Rotary-Percussive Drilling

In order to reveal the rock breaking mechanism of the rotary-percussive drilling, considering the coupling effect of the static pressure, impact force and rotary cutting and using the finite element methods (FEM), a three-dimensional (3D) insert-rock mutual action model is established. We use a Mohr-Coulomb type material model to describe when and how rock fails, and a triangular wave to replace the stress wave. Using the finite element analysis software (ANSYS), the 3D contact analysis of the insert and rock under percussion and rotary-percussion is carried out respectively. Considering the inclined plane angle (α) of the bit end face as 0° (vertical impaction), 20°, 25°, 30°, 35°, 40° and 45° (inclined impaction), the stresses of the insert with percussive drilling and rotary-percussive drilling are calculated respectively. The results show that: the torsional shear stress causes volume breaking, tensile stress accelerates the crack extension and volume rock breaking, while the compressive stress contributes as a secondary factor for the rock breaking, 30°, 35° and 40° are the preferred angle for the flat bit with single inclined plane, and the combination angle of 20°-40° is preferred for the flat bit with double inclined planes. The results of the 3D contact analysis are further calibrated with a series of field applications and research results.