John P. Borg

Aspects of simulating the dynamic compaction of a granular ceramic

Mesoscale hydrodynamic calculations have been conducted in order to gain further insight into the dynamic compaction characteristics of granular ceramics. With a mesoscale approach each individual grain, as well as the porosity, is modeled explicitly; the bulk behavior of the porous material can be resolved as a result. From these calculations bulk material characteristics such as shock speed, stress and density have been obtained and compared with experimental results. A parametric study has been conducted in order to explore the variation and sensitivity of the computationally derived dynamic response characteristics to micro-scale material properties such as Poissons ratio, dynamic yield and tensile failure strength; macro-scale parameters such as volume fraction, particle morphology and size distribution were explored as well. The results indicate that the baseline bulk Hugoniot response under-predicts the experimentally measured response. These results are sensitive to the volume fraction, dynamic yield strength and particle arrangement, somewhat sensitive to failure strength and insensitive to the micro-scale Hugoniot and grain morphology. A discussion as to the shortcomings in the mesoscale modeling technique, as well as future considerations, is included.

Dynamic compaction of porous silica powder

The dynamic compaction characteristics of a porous silicon dioxide (SiO2) powder are reported. The initial specific volumes of the samples were either V00=1.30, 4.0, or 10.0cm3∕g whereas the silicon dioxide has a matrix specific volume of V0=0.455cm3∕g. The impact velocity ranges from 0.25to1.0km∕s and the shock incident pressure on the silica ranges from 0.77to2.25GPa. The shock velocity–particle velocity exhibited a linear relationship within this range. Although these tests represent the low end of dynamic compaction, the dynamic tests compare favorably to extrapolated data available in the open literature. Theoretical pressure–particle velocity and shock velocity–particle velocity curves were generated using a P-α compaction curve. The P-α compaction curve accurately represented the pressure–particle velocity and shock velocity–particle velocity Hugoniot curves for the low specific volume powder, specifically V00=1.30cm3∕g. However, the P-α compaction curve did not accurately represent the pressure–pa...

THE DYNAMIC COMPACTION OF SAND AND RELATED POROUS SYSTEMS

Porous and granular materials are widely found in a number of environments. One of the most important groups both geographically and in the construction industry are the sands. A review of the response of sand (42% porous) over a wide range of strain rates is presented. Factors such as water content and density variation are addressed. In addition a very low‐density silica dust (95% porous) is also discussed in relation to its contrasting behaviour.

Instability and fragmentation of expanding liquid systems

This analysis pursues the underlying physics governing the expansion, dispersal and breakup of a thin walled steel right circular cylinder filled with liquid after being impacted by a high velocity aluminum sphere. The impact generates a radially expanding coherent thin shell of liquid which stays together to at least a diameter 8 times that of the original cylinder. An instability criterion is proposed and developed based on the opposing forces of stabilizing inertial pressures and destabilizing viscous resistance. This criterion is compared to test data where possible in order to ascertain its ability to predict liquid breakup of the shell. The breakup theory developed here predicts the extensive expansion of the unthickened liquid prior to fragmentation as observed in the experiments. This result lends some credence to the underlying physics pursued here and its ability to predict the onset of liquid fragmentation.

Dynamic Compaction Modeling of Porous Silica Powder

A computational analysis of the dynamic compaction of porous silica is presented and compared with experimental measurements. The experiments were conducted at Cambridge University’s one‐dimensional flyer plate facility. The experiments shock loaded samples of silica dust of various initial porous densities up to a pressure of 2.25 GPa. The computational simulations utilized a linear Us‐Up Hugoniot. The compaction events were modeled with CTH, a 3D Eulerian hydrocode developed at Sandia National Laboratory. Simulated pressures at two test locations are presented and compared with measurements.

Dynamic response of dry and water-saturated sand systems

The effect of grain size and moisture content on the dynamic macroscopic response of granular geological materials was explored by performing uniaxial planar impact experiments on high purity, Oklahoma #1, sand samples composed of either fine (75–150 μm) or coarse (425–500 μm) grain sizes in either dry or fully water-saturated conditions. Oklahoma #1 sand was chosen for its smooth, quasi-spherical grain shapes, narrow grain size distributions, and nearly pure SiO2 composition (99.8 wt. %). The water-saturated samples were completely saturated ensuring a two-phase mixture with roughly 65% sand and 35% water. Sand samples were dynamically loaded to pressures between 1 and 11 GPa. Three-dimensional meso-scale simulations using an Eulerian hydrocode, CTH, were created to model the response of each sand sample. Multi-phase equations of state were used for both silicon dioxide, which comprised individual sand grains, and water, which surrounded individual grains. Particle velocity profiles measured from the rea...

Mesoscale Simulations of Dry Sand

There is an interest in producing accurate and reliable computer simulations to predict the dynamic behavior of heterogeneous materials and to use these simulations to gain further insight into experimental results. In so doing, a more complete understanding of the multiple-length scale involved in heterogeneous material compaction can be obtained. In this work, planar shock impact experiments were simulated using two different hydrocode formulations: iSALE and CTH. The simulations, which were based on a Georgia Tech experimental setup, consisted of a flyer of varying thickness impacting dry sand over a range of impact. Particle velocity traces obtained from the computer simulations were compared to VISAR and PDV measurements obtained from experiments. The mesoscale simulations compare well with the dynamic behavior of dry sand. Improvements on these simulations with the inclusion of these mesoscale phenomena are presented and discussed.

Ballistic penetration of sand with small caliber projectiles

In this work a series of experiments were carried out in which right-circular cylinders were launched into sand targets at velocities ranging from 70 to 150 m/s. The projectiles were launched along a view window in order to record the penetration event with high-speed photography. Stress measurements of the transmitted wave forms were simultaneously collected from a piezoelectric load cells buried in the sand. A particle image velocimetry (PIV) technique, which extracted information from the photographic images, was used to resolve transmitted wave profiles. A two wave structure was observed. The first wave, a compaction wave, moves at the bulk sound speed of the sand. The second is an attached fracture wave which is stationary relative to the projectile. Together these experiments further our understanding of high-speed granular penetration events.

A REVIEW OF MESOSCALE SIMULATIONS OF GRANULAR MATERIALS

With the advent of increased computing power, mesoscale simulations have been used to explore grain level phenomenology of dynamic compaction events of various heterogenous systems including foams, reactive materials and porous granular materials. This paper presents an overview of several mesoscale studies on a variety of materials including tungsten carbide, wet and dry sand, and an inert mixture of Al‐MnO2‐Epoxy. This paper focuses on relating bulk and compaction wave phenomenology from the mesoscale modeling to experimental results and exploring the nature of the compaction wave. In addition, lessons learned during these explorations, modeling techniques, strengths and weaknesses of hydrodynamic mesoscale simulations are also discussed.

MESOSCALE AND CONTINUUM CALCULATIONS OF WAVE PROFILES FOR SHOCK‐LOADED GRANULAR CERAMICS

Attenuating wave profiles from shock experiments on tungsten carbide powder are compared to calculations from the continuum P‐λ model and a 2‐D mesoscale model to gain insight into the suitability of the two models. When calibrated, both models accurately capture the Hugoniot response of the powder and the arrival times of unattenuated steady waves. Their amplitudes are more accurately given by the mesoscale model since its reshock states are above the Hugoniot as seen experimentally; the P‐λ model, in contrast, reshocks along the Hugoniot. When the attenuating wave is in the range of the Hugoniot data, the models predict attenuation correctly. However, when attenuation falls below the Hugoniot data both models are somewhat inaccurate, and the material response seems to lie between the two models. The final aspect considered is the wave rise time, which is qualitatively correct for the mesoscale model but completely inaccurate for the P‐λ model.