Oren E. Petel

Particle jet formation during explosive dispersal of solid particles

Previous experimental studies have shown that when a layer of solid particles is explosively dispersed, the particles often develop a non-uniform spatial distribution. The instabilities within the particle bed and at the particle layer interface likely form on the timescale of the shock propagation through the particles. The mesoscale perturbations are manifested at later times in experiments by the formation of coherent clusters of particles or jet-like particle structures, which are aerodynamically stable. A number of different mechanisms likely contribute to the jet formation including shock fracturing of the particle bed and particle-particle interactions in the early stages of the dense gas-particle flow. Aerodynamic wake effects at later times contribute to maintaining the stability of the jets. The experiments shown in this fluid dynamics video were carried out in either spherical or cylindrical geometry and illustrate the formation of particle jets during the explosive dispersal process. The number of jet-like structures that are generated during the dispersal of a dry powder bed is compared with the number formed during the dispersal of the same volume of water. The liquid dispersal generates a larger number of jets, but they fragment and dissipate sooner. When the particle bed is saturated with water and explosively dispersed, the number of particle jets formed is larger than both the dry powder and pure water charges. More extensive experiments that explore the effect of particle size, density and the mass ratio of explosive to particles on the susceptibility for jet formation are reported in Frost et al. (Proc. of 23rd ICDERS, Irvine, CA, 2011).

The effect of particle strength on the ballistic resistance of shear thickening fluids

The response of shear thickening fluids (STFs) under ballistic impact has received considerable attention due to its field-responsive nature. While efforts have primarily focused on traditional ballistic fabrics impregnated with these fluids, the response of pure STFs to penetration has received limited attention. In the present study, the ballistic response of particle-based STFs is investigated and the effects of fluid density and particle strength on ballistic performance are isolated. It is shown that the loss of ballistic resistance in the STFs at higher impact velocities is governed by the material strength of the particles in suspension. The results illustrate the range of velocities over which these STFs may provide effective armor solutions.

Shock wave propagation in dense particle suspensions

Shock wave propagation in a multiphase suspension is studied experimentally. Particle suspensions are used as a means of obtaining a system in which there is limited initial interparticle contacts with a large degree of parametric variability. Suspensions were created in ethylene glycol at several volume fractions (41%, 48%, and 54%) of silicon carbide particles. Plate impact experiments are conducted to obtain the shock Hugoniots of the various suspensions at particle velocities in the range of 200–900 m/s. Transitions are shown to exist in the Us-up Hugoniots of the suspensions. In situ longitudinal and lateral stress measurements are made in the 48% suspension at two different impact velocities demonstrating a deviatoric stress component to the stress state in the suspension. The results are discussed in terms of the development of extensive interparticle contacts in a mechanism analogous to classical shear thickening in dense suspensions.

Shock Hugoniot measurements in foam

The present study outlines a new approach to collecting shock Hugoniot data in foams using photonic Doppler velocimetry to perform mid-plane measurements of the foam deformation. Plate impact experiments were carried out to investigate wave propagation in a closed-cell polymeric foam and an open-cell aluminum foam. Dual-wave structures were observed in both materials with the leading precursor wave determined to be an elastic wave. The discussion of the results focuses on the nature of foam compression under high-rate loading, particularly the difference between the strain history in a foam undergoing uniform stress compaction and uniaxial strain compression. These results are discussed in reference to the current interpretations of Taylor-Hopkinson bar experiments on similar metallic foams. The importance of gas-filtration driven flows in the wave dynamics of open-cell foams is discussed in relation to the nature of the precursor waves.

Lateral stress measurements in dense suspensions

The present study investigates the deviatoric response of dense particle suspensions consisting of silicon carbide suspended in ethylene glycol using piezoresistive stress gages as a means of experimentally measuring the dynamic strength of materials. The validity of the method is first shown in a pure liquid environment, recovering the hydrodynamic behavior of the liquid. The deviatoric response of the dense suspension is indicative of a shock-induced stiffening within the mixture, whereby the stress state is largely influenced by the suspended particles. A mesh-free numerical approach (Smoothed Particle Hydrodynamics) is used to investigate the shock-induced mesostructural deformation of the suspension. The measured deviatoric response of the suspensions is discussed in terms of the formation of inter-particle contact networks, force chains, which result from the shock-induced mesostructural changes in the suspensions.

Shock-induced deformation in wetted particle beds

The high-strain-rate response of granular media has received considerable attention due to increasing interest in granular penetration. In the present study, we investigate the response of wetted packed particle beds under varying flyer plate-induced shock loadings. We investigate the critical conditions for the onset of particle deformation in systems of spherical macroscopic glass beads. Resulting particle deformations from the shock compression are characterized using microscopy as well as particle size analysis, and the effects of shock strength are compared. A fracturing response with a bimodal particle distribution is observed, with an increasing shift to the lower particle size range as shock loading is initially increased. As the transmitted shock pressure exceeds 1 GPa, a significant decrease in the mean particle size is observed.

High Rate Characterization of Polymeric Closed-Cell Foams: Challenges Related to Size Effects

Polymeric foams including expanded polystyrene and low-density polyethylene have been used extensively in the design of military protective systems to help mitigate threats that can range from low velocity impacts to explosive events. Polymeric foams are significantly rate dependent and have very low wave speeds, which can complicate their response in specific conditions. In the present study, two polymeric foams were characterized in compression at quasi-static and high strain rates. Rates from 1 s−1 were obtained with a standard hydraulic test machine. Acrylic Hopkinson bars were used to generate compression rates on the order of 103 s−1. The two closed-cell polymeric foams investigated in this study were of similar density but with a significantly different macro-structure. Low and high strain rate testing on a relatively consistent cell-size material (low density polyethylene) demonstrated expected trends and results, while the effect of strain rate was masked for a material with high structural variability (expanded polystyrene).

Comparison of Failure Thickness and Critical Diameter of Nitromethane

The critical diameter and failure thickness of both neat liquid nitromethane and a 65% nitromethane/35% nitroethane blend confined by aluminum are determined experimentally. A comparison of these two parameters provides insight into the failure mechanism of detonation in these explosives. If the failure of detonation in a critical charge diameter (or thickness) experiment is due to reaction quenching resulting from expansion losses (wave curvature), then it is expected that the failure thickness should be half the value of the critical diameter. The critical diameter and failure thickness of neat nitromethane confined in aluminum are found to be 2.5 mm and 0.75 mm respectively for a temperature range of 26 ± 1°C. The critical diameter and failure thickness of the 65NM/35NE blend confined in aluminum are found to be 6.2 mm and 1.7 mm respectively for a temperature range of 28 ± 1°C. The ratio of critical diameter to failure thickness for these experiments is found to lie between 3:1 and 4:1 rather than 2:1...

Shock-induced formation of a disordered solid from a dense particle suspension

Shock wave propagation in multiphase media is dominated by the relative compressibility of the mixture components, which can result in shock-induced variations in the volume fraction of the suspension. As the post-shock volume fraction of a dense suspension increases with increasing shock strength, a loosely disordered solid can take form which causes an increase in the stiffness of the suspension. The present study investigates the formation of disordered structures within dense suspensions as well as the effect that such structures have on wave propagation through shock Hugoniot data for dense suspensions of silicon carbide in ethylene glycol. An analytical model will be used to illustrate the shock-induced variations of the mesostructure within the suspensions.

Development of instabilities in explosively dispersed particles

Models for explosives containing metal particles must consider the complex gas-particle flow that is generated following detonation of the explosive. Previous experimental studies have shown that when a layer of solid particles is explosively dispersed, the particles often develop a non-uniform spatial distribution. The instabilities within the particles and at the particle layer interface likely form on the timescale of the detonation propagation through the particles. The mesoscale perturbations are manifested at later times in experiments by the formation of clusters of particles or coherent jet-like particle structures which persist for some distance during the dispersal process. These particle jets influence the particle-gas mixing and hence particle reaction rates at the macro scale. The particle instabilities that occur in explosively dispersed particles are investigated with a mesh-free computational method (smoothed-particle hydrodynamics). The simulations are compared with experimental results f...