Instrumented Projectile Penetration Testing of Granular Materials

The results of a series of penetration experiments with cylindrical projectiles (diameter: 30 mm, length: 158 mm) of different tip geometries (hemispherical, conic and flat) impacting with approx. 380 m/s on cylindrical sand targets (diameter: 250 mm, length: 1 m) are presented. The projectiles are instrumented with an on-board data recorder system with acceleration sensor (G-Rec), allowing for in-situ measurement of the decelerations experienced during the impact and during the subsequent penetration process. During and after the acceleration stage inside the gun barrel, the velocities derived from the G-Rec data show a very good agreement with an independent reference velocity measurement obtained by a light barrier system. Immediately after the initial impact on the granular material (compacted sand for this study), the sensor signals indicate very strong amplitude oscillating decelerations in the range of 50,000 to 80,000 g. The measured penetration depth varied widely for identical conditions of projectile impact, using identical preparation procedures for the sand sample. Variables considered for the experimental program included projectile tip shape, initial sand compaction state, and projectile velocity. A data reduction procedure to obtain velocity and position information of the projectile while penetrating the sand assembly is presented, taking into account a constant value (offset) observed in the raw signals of G-Rec at the end of penetration process. The formation of a false tip of agglomerated quartz powder in front of the projectile was observed in all cases. The observed shape of the false tip (penetrator tip with comminuted sand) was found to be identical for all projectile tip shapes (flat, hemispherical, and conical) considered in this study. Initial examination results of the false tip using X-ray computed tomography are presented.