Energy transfer efficiency and rock damage characteristics of a hydraulic impact hammer with different tool shapes

The energy transfer efficiency and damage characteristics are the theoretical basis for the selection and optimization of hydraulic impact hammer tools. In this paper, quarter three-dimensional models of the piston-toolrock system are established based on the continuum and discontinuum element method. The incident and reflected stress waves and rock damage characteristics obtained from the simulation model are in good agreement with experimental and theoretical results. The effects of piston impact velocity, tool shape, rock strength, confining pressure and repeated impacts on rock damage zone and energy transfer efficiency between tool and rock are analyzed. The force-penetration curves and penetration coefficients under different impact velocities, rock strength, and confining pressure conditions are obtained. The results illustrate that the energy transfer efficiency and penetration coefficient increase with the increase of rock strength, confining pressure and tip area of tool, but decrease with the increase of impact velocity. Based on the simulation results, a prediction formula for penetration force with consideration of the shape of the tool tip is proposed and the calculation accuracy is verified. Finally, the proportion of energy actually used for rock fracturing is analyzed.