Experimental and Numerical Investigation on Failure Characteristics of High-Temperature Granite Subjected to Impact Loads

The exploration of hot dry rock is hindered by the high temperature, considerable strength, and strong abrasiveness of rocks. Field tests have demonstrated that percussive drilling is an efficient rock breaking method. However, failure mechanisms of high-temperature rocks under dynamic loads remain unclear, thus limiting the further optimization of drilling parameters. In addition, previous studies primarily focus on the failure of rocks at the well bottom while neglecting the damage to the wellbore. In this research, a series of experiments and simulations are performed to study rock failure characteristics. The variations in stress waves and 3D topography of penetration pits are monitored. Furthermore, the evolution of compressive and tensile damage is calculated, thus revealing rock failure mechanisms under different conditions. Based on the results, penetration pits with relatively regular shapes are observed at low temperatures (< 200 degrees C) and small impact velocities (< 5 m/s). At these conditions, rock failure predominantly occurs at the well bottom. With the further increase in rock temperature and impact velocity, cracks initiate from penetration pits and propagate towards the rock boundary, indicating the failure of the wellbore. This could decrease the fracturing pressure and improve the permeability of the rock near the wellbore. Meanwhile, increasing the forward rake angle similarly affects rock failure characteristics. Beyond 20 degrees, the induced horizontal force aggravates rock damage and facilitates the propagation of cracks. Simulation results demonstrate that tensile damage plays a primary role in the generation of cracks, while compression damage spreads around penetration pits, leading to the existence of dense core and larger volume of penetration pits. The results of this study provide an in-depth understanding of rock failure mechanisms and offer insights for optimizing impact parameters.