An Experimental Study of Particle Crushing in Granular Material and Projectile Penetration

Laboratory projectile penetration tests have shown extensive particle crushing along the path of the projectile. Careful extraction of projectile tip has shown formation of a false tip, composed of comminuted sand particles, that travels along with the projectile during the penetration process. Particle size analysis of the comminuted fines from the tip and path have shown orders of magnitude in size reduction. Current penetration depth prediction methods (empirical and analytical) do not explicitly account for energy dissipation in the vicinity of the projectile. In the far field, there is very little movement of the bulk mass which can be modelled using a continuum approach. Modeling the penetration process therefore requires a multi-scale approach, able to simulate the far field, the grain-projectile and grain-grain interactions, and granular fracture. A suite of tests was performed on sands with different grain morphology, Ottawa sand (sub-rounded), Q-Rok (angular), and Euroquarz Siligran (sub-angular) to determine the role of particle shape on continuum and micro scale properties. Triaxial tests were performed to investigate the role of particle shape on strength volume relationship. Additionally, digital image correlation technique was employed to better understand the initiation and propagation of localized deformations associated with frictional end triaxial tests. Role of particle shape and size in comminution was explored by performing single grain crushing tests encompassing particles of different shapes and sizes. The tensile strength of single grains decreased with angularity. It was quantitatively shown, using acoustic emission measurements, that crushing smaller particles requires considerably greater energy than crushing larger particles. In high strain rate tests, particle crushing decreased with moisture content and no significant differences in particle crushing were observed for different specimen densities. The role of projectile and target characteristics in the response of projectiles impacting granular material was explored with the help of full flight time histories from onboard data acquisition system. Penetration depth increased with impact velocity. Both projectile and target characteristics produced visible differences in the recorded projectile response. Projectiles with softer tip material went further in the target than those with stiffer tips.