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 the response of traditional ballistic fabrics impregnated with fluids, the response of pure STFs to penetration has received limited attention. In the present study, the ballistic response of pure STFs is investigated and the effect of fluid density and particle strength on ballistic performance is isolated. The loss of ballistic resistance of STFs at higher impact velocities is governed by particle strength, indicating the range of velocities over which they may provide effective armor solutions.Comment: 4 pages, 4 figure

A study of the failure mechanism of detonations in homogeneous and heterogeneous explosives /

The present study measured the critical diameter and critical thickness of a variety of explosives. The explosives tested included two "unstable" homogeneous explosives (nitromethane and a nitromethane/nitroethane blend); a model heterogeneous explosive consisting of a packed bed of glass beads (Φ ~ 80 μm) saturated with the homogeneous nitromethane/nitroethane blend; and a commercial heterogeneous explosive, Apex Elite(TM). The comparison of the critical diameter and thickness of an explosive is used to identify the dominant propagation and failure mechanisms of the various explosives. The ratio of critical diameter to critical thickness for nitromethane, the nitromethane/nitroethane blend, the beaded heterogeneous explosive, and Apex Elite(TM) were found to be 3.2 +/- 0.6, 3.6 +/- 0.4, 2.3 +/- 0.1, and 3.5 +/- 1.2 respectively. According to accepted detonation failure theories, the energy losses associated with detonation front curvature are responsible for detonation failure. The curvature model, which is elaborated upon in the present work, leads to a predicted critical diameter to critical thickness ratio of exactly 2. The present study has shown that the only explosive which follows the behaviour predicted by curvature failure models is the beaded heterogeneous explosive, which exhibits fine scale heterogeneities. This seems to indicate that unstable liquid explosives and heterogeneous explosives with large scale heterogeneities do not fail simply due to the wave front curvature, but rather by a local mechanism of failure and reinitiation which dominates the detonation propagation

Spall strength measurements in EPON 828 epoxy and an epoxy/carbon nanotube composite

Polymer nanocomposites are seeing more frequent use in armor applications. The role of the microstructure on the performance of these materials under dynamic tensile loading is of particular interest. In the present study, plate impact experiments were conducted in order to observe the dynamic response of a neat epoxy (EPON 828) and an epoxy/carbon nanotube composite. The objective was to examine the effect of nano-scale particle inclusions on the spall strength of the composite. The material response was resolved with the combined use of shock pins and a multi-channel photonic Doppler velocimeter. The addition of raw carbon nanotubes (CNT) to epoxy resulted in a composite material with lower spall strengths compared to strengths measured for neat epoxy at elevated shock stresses. Tensile strain rate was found to have the greatest effect on spall strength. Recovered composite fragments were imaged with a scanning electron microscope. Instances of nanotube pull-out were identified on internal fracture surfaces. The low spall strengths of the epoxy/CNT composite were attributed to an increase in the density of potential nucleation sites for spallation caused by presence of the nanotubes in the matrix.Peer reviewed: YesNRC publication: Ye

Spall strength measurements in EPON 828 epoxy and an epoxy/carbon nanotube composite

Polymer nanocomposites are seeing more frequent use in armor applications. The role of the microstructure on the performance of these materials under dynamic tensile loading is of particular interest. In the present study, plate impact experiments were conducted in order to observe the dynamic response of a neat epoxy (EPON 828) and an epoxy/carbon nanotube composite. The objective was to examine the effect of nano-scale particle inclusions on the spall strength of the composite. The material response was resolved with the combined use of shock pins and a multi-channel photonic Doppler velocimeter. The addition of raw carbon nanotubes (CNT) to epoxy resulted in a composite material with lower spall strengths compared to strengths measured for neat epoxy at elevated shock stresses. Tensile strain rate was found to have the greatest effect on spall strength. Recovered composite fragments were imaged with a scanning electron microscope. Instances of nanotube pull-out were identified on internal fracture surfaces. The low spall strengths of the epoxy/CNT composite were attributed to an increase in the density of potential nucleation sites for spallation caused by presence of the nanotubes in the matrix

The effect of curing agent on the dynamic tensile failure of an epoxy subjected to plate impact

Dynamic tensile failure of epoxy resin cured with two different curing agents was studied in terms of spall strength, fracture toughness, and shock behaviour. Plate impact experiments were conducted to examine how the epoxy responds to one-dimensional, high-strain rate loading. Velocity measurements of the back surface of the targets were taken during impact with a photonic-Doppler velocimeter (PDV). The velocity profiles that resulted were analyzed to gain insight on the material interface/stress wave interactions that manifested within the samples. Spall strength measurements ranged from 404 to 585 MPa in EPON 828 cured with EPIKURE 3223, and from 339 to 462 MPa in EPON 828 cured with EPIKURE 3233. Evidence for the existence of a quantifiable relationship between the curing agent used to cure the resin and the dynamic tensile strength of the resulting epoxy is provided. The discrepancies in the measured spall strengths between the two epoxy systems were attributed to a difference in the electrostatic forces between adjacent polymer chains within the crosslinked epoxy network. Strength measurements in both epoxies demonstrated significant strain-rate dependency. Spall strength measurements presented in this study were noticeably higher than those listed in the literature for similar thermosetting polymers, likely the result of the choice of curing agent. Finally, shock and particle velocity measurements were shown to be consistent with previously published results, within experimental uncertainty

Spall Characterization of EPON 828 Epoxy with Embedded Carbon Nanotubes

The increasing use of polymer nanocomposites in armor applications requires an understanding of how these materials behave at strain-rates relevant to ballistic impacts. Of particular interest is the role of the microstructure on the failure of these materials under dynamic tensile loading. In the present study, plate impact experiments were conducted in order to measure the spall strength of a neat epoxy (EPON 828) and an epoxy–carbon nanotube composite. The addition of pristine carbon nanotubes (CNTs) to the epoxy resulted in a composite material with a lower spall strength than the neat epoxy matrix material. Recovered composite fragments were imaged with a scanning electron microscope. Instances of nanotube pull-out were identified on internal fracture surfaces. The lower spall threshold of the epoxy–CNT composite was attributed to the comparatively weak epoxy–CNT interface, which provides potential sites from which spall failure can favorably nucleate