Simon Ouellet

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.

Investigation of Head Response to Blast Loading

BACKGROUND Head injury resulting from blast loading, specifically mild traumatic brain injury, has been identified as a possible and important blast-related injury for soldiers in modern conflict zones. A study was undertaken to evaluate head response to blast loading scenarios using an explicit finite element numerical model and to comment on the potential for head injury. METHODS The blast loading and simplified human body numerical models were validated using impulse, peak acceleration and the Head Injury Criterion from experimental blast test data. A study was then undertaken to evaluate head response at varying distances and orientations from the explosive. RESULTS The accelerations and injury metrics for the head increased with decreasing distance to the explosive, as expected, but were also significant at intermediate distances from the explosive for larger charge sizes and intermediate heights of burst. Varying lateral position with constant standoff did not have a significant effect on the head kinematic response. CONCLUSIONS The head injury criteria considered were exceeded in close proximity to the explosive (<35 charge radii) and depended on the height of burst for the range of charge sizes considered. The injury criteria were also exceeded at intermediate distances for larger charge sizes because of the influence of the mach stem. Although the injury criteria used in this study are typically applied to longer duration events, and may not be applicable for shorter duration blast loading, aggressive loading is predicted at small standoff distances and confirmed by the resulting head kinematics.

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.

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).

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 and validation of a biofidelic head form model to assess blast-induced traumatic brain injury

Traumatic Blast Injury (TBI) associated with the human head is caused by exposure to a blast loading, resulting in decreased level of consciousness, skull fracture, lesions, or death. This paper presents the simulation of blast loading of a human head form from a free-field blast with the end goal of providing insight into how TBI develops in the human head. The developed numerical model contains all the major components of the human head, the skull, and brain, including the tentorium, cerebral falx, and gray and white matter. A nonlinear finite element analysis was employed to perform the simulation using the Arbitrary Lagrangian–Eulerian finite element method. The simulation captures the propagation of the blast wave through the air, its interaction with the skull, and its transition into the brain matter. The model quantifies the pressure histories of the blast wave from the explosive source to the overpressure on the skull and the intracranial pressure. This paper discusses the technical approach used to model the head, the outcome from the analysis, and the implication of the results on brain injury.

Analysis of the ballistic impact response of a composite material using FAST Infrared Imagery

The level of protection offered by a given ballistic material is typically evaluated in terms of a set of projectiles and their associated velocity at which a certain percentage of the projectiles are expected to perforate. (i.e. FSP 17gr : V50 = 500m/s, 9mm FMJ; V0=500m/s). These metrics give little information about the physical phenomena by which energy is dispersed, spread or absorbed in a specific target material. Aside from post-test inspection of the impacted material, additional information on the target response is traditionally obtained during a test from the use of high speed imaging, whether it is from a single camera aimed at the impact surface or the backface, or from a set of camera allowing full 3-D reconstruction of a deformed surface. Again, this kind of data may be difficult to interpret if the interest is in the way energy is managed in the target in real time. Recent technological progress in scientific grade high-speed infrared (IR) camera demonstrated that these phenomena can straightforwardly be measured using IR thermal imaging. This paper presents promising results obtained from Telops FAST-IR 1500 infrared camera on an aramid-based ballistic composite during an impact from a small caliber fragment simulating projectile (FSP).

Dynamic strength, particle deformation, and fracture within fluids with impact-activated microstructures

The evolution of material strength within several dense particle suspensions impacted by a projectile is investigated and shown to be strongly dependent on the particle material in suspension. For stronger particles, such as silicon carbide, the shear strength of the fluid is shown to increase with the ballistic impact strength. For weaker particles, such as silica, the shear strength of the suspension is found to be independent of impact strength in this dynamic range of tests. A soft-capture technique is employed to collect ejecta samples of a silica-based shear thickening fluid, following a ballistic impact and penetration event. Ejecta samples that were collected from impacts at three different velocities are observed and compared to the benchmark particles using a Scanning Electron Microscope. The images show evidence of fractured and deformed silica particles recovered among the nominally 1 μm diameter monodisperse spheres. There is also evidence of particle fragments that appear to be the result of...