Crater Formation and Penetration of Laterally Confined Granite Under High-Speed Projectile Impact: Effects of Cone Angle and Impact Velocity

High-speed projectile impact is a promising approach for assisting hard-rock fragmentation; however, the coupled effects of projectile geometry and impact velocity on crater formation and penetration behavior under lateral confinement remain insufficiently understood. In this study, seven granite penetration tests were conducted using a biomass-fueled combustion-driven gas gun system coupled with a biaxial lateral confinement device, including one unconfined case and six cases under 2 MPa lateral confinement. Equal-mass YG20 cemented carbide projectiles with cone angles of 60°, 90°, and 120° were tested at nominal velocities of approximately 220 and 400 m/s. The results show that the two velocity levels produced different crater morphologies, and cone angle regulated the relative development of axial penetration and transverse crater expansion. At the lower velocity level, surface crushing dominated, with penetration depth decreasing from 13.5 to 10.5 mm and equivalent crater diameter decreasing from 74.5 to 44.4 mm as cone angle increased. At the higher velocity level, funnel-shaped craters were formed, with penetration depth decreasing from 45.0 to 30.0 mm, while equivalent crater diameter exhibited a non-monotonic variation. Dynamic strain measurements revealed coupled radial compression and circumferential tensile responses, with more complex transient fluctuations observed for the 120° projectile at high velocity. Based on the Young/Sandia framework, a condition-specific penetration-depth correlation was established. The full seven-test calibration gave an in-sample RMSE of 1.87 mm and R2 = 0.977, while leave-one-condition-out evaluation of the five conditions not used to define the paired-test boundary-state reduction ratio K2 yielded an RMSE of 3.40 mm and R2 = 0.937. The held-out analysis is interpreted as an internal stability check rather than independent external validation. These findings provide insights into the coupled influence of projectile cone angle and impact velocity on granite crater formation under controlled lateral confinement.