A multi-scale approach to a greater understanding of the behavior of heterogeneous materials under dynamic loading

Abstract

The penetration of granular materials is of interest to a variety of different fields, and is an active area of research. The objective of this project is to gain understanding of the dynamics of a projectile penetrating into a granular material. To do this, experiments were run and a numerical model was created.

A dart gun was used to accelerate an aluminum dart to velocities around 100 m/s, which then impacted a target tank filled with Ottawa sand. The dart flew along a view window, which allowed for a recording of the penetration event using a high speed camera. Pressure gauges inserted into the target tank measured the timing and magnitude of the compaction wave created by the dart. In these penetration events a two wave structure was discovered; a compaction wave and a fracture wave. The fracture wave is characterized by a white cone around the nose of the dart, which is created by increased reflectance from the newly created fracture surfaces in the grains of sand.

An experiment was conducted in which single grain of sand was crushed. From this experiment it was discovered that the phenomenon that creates increased reflectivity is the creation of fractures faces in the sand, and is not triboluminescence. Stress-strain data for the sand was also gathered, to be used in the numerical simulation. An ultrasonic pulser/receiver was used to gather data on the longitudinal and shear wave sound speeds through "as poured" Ottawa sand; 263 m/s and 209 m/s respectively. It was determined that the compaction and damage wave speeds were not related to either the longitudinal or shear wave speeds.

A numerical model was created using an EMU Peridynamic code. This code utilizes integral rather than differential equations, which allows for the modeling of crack propagation and fracture. The numerical simulations run were two-dimensional and on a smaller scale than the penetration experiments. The numerical simulation showed evidence of a compaction wave, force chain creation, and grain fracture, all of which were also observed in the penetration experiments.