Christopher Neel

Equation of state and high pressure properties of a fluorinated terpolymer: THV 500

We present the results of an investigation of the static compressive and dynamic (shock) responses of a fluorinated terpolymer of tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride (Dyneon THV 500), in an effort to further understand its behavior under static and dynamic high pressures, and elucidate its equation of state properties. Fluorinated polymers, and, in particular, their copolymers, have garnered increasing attention by the static high pressure and shock wave communities, due to their widespread use in engineering applications, and formulation into energetic materials as binders. Shock wave compression experiments performed at two laboratories showed good consistency, and provide the first Hugoniot data for this fluorinated terpolymer. The Hugoniot of THV 500 is in general agreement with that of the related fluoropolymers polytetrafluoroethylene and poly(chlorotrifluoroethylene-co-vinylidene fluoride), reported previously. The static compressive data, combined with measurement of the am...

Shock compression response of alumina-THV composites

The shock compression response of several THV-alumina particle composites (where THV is a terpolymer of tetrafluoroethylene, hexafluoropropylene, and vinyldiene fluoride) was investigated based on experiments performed in the pressure range of 2–6 GPa. The composites, composed of 25% by volume of either 1, 10, or 100 μm Al2O3, had varying degrees of porosity. The high level of porosity, particularly in the 1 μm Al2O3 composite, obscured determination of any particle size effects. In general, the composites displayed a stiffer shock response than expected, based on the known response of the constituent materials, with the 10 μm Al2O3 composite being slightly stiffer than the 100 μm composite. It is argued that a possible way to describe the stiffer shock compression response is by assigning a higher value for the “effective” Gruneisen parameter γ to the composites. The higher value is consistent with arguments made in the literature that γ for polymers is much higher (by as much as an order of magnitude) t...

Shock and release wave speed of an alumina epoxy composite

A filled epoxy with 43% by volume of disk-shaped Al2O3 particles is used to investigate the effect of high-aspect ratio (4:1) particle shape on composite shock and release response. Stress profiles are recorded during a symmetric impact experiment performed at an impact velocity of 569 m/s. The shock and release velocities are respectively observed to be 3511 and 5685 m/s, which agrees with data previously reported, indicating that particle shape has no effect on the Hugoniot or the isentrope. However, the high-aspect ratio particles are found to increase the acoustic wave speeds, especially in shear, and also to increase shock wave dispersion, as indicated by the measured rise time of the stress profile.

Dynamic characterization of eglin steel by symmetric impact experimentation

Well-controlled impact studies have been conducted on heat treated ES-1 (i.e. Eglin steel) to determine their dynamic material properties. In particular gas-gun and time-resolved laser interferometry was used to measure the fine structure in the particle velocity profile resulting from symmetric impact. Nominal impact pressures range from 8 to 20 GPa at corresponding impact velocities of 0.400 km/s and 1.00 km/s, respectively. These experiments have allowed us to estimate the dynamic yield and spall strengths and phase transition kinetics of the material.

SIMULTANEOUS MEASUREMENTS OF SHOCK WAVE PROFILES FROM VISAR VELOCITY GAUGES AND PVDF STRESS GAUGES

Parallel plate impact tests were performed at ∼3 GPa on two well characterized polymers, PTFE and PMMA, as well as on OFHC copper. Simultaneous time‐resolved measurements at the interface between the sample and fused silica backer were performed using PVDF stress gauges and VISAR velocity interferometry probes. The gauges were shielded to eliminate the effect of stress‐induced polarization on the PVDF gauges. Differences in the loading traces for the two measurement methods are discussed.

Laser interferometry and emission spectroscopy measurements of cold-sprayed copper thermite shocked to 35 GPa

Plate impact experiments were conducted on a cold-sprayed Al-CuO thermite at peak stresses between 5-35 GPa to determine the Hugoniot curve and characterize any shock induced energetic reaction. Photon Doppler Velocimetry (PDV) measurements were used to obtain particle velocity histories and shock speed information for both the shock loading and unloading behavior of the material. A jump in shock velocity was observed in the Hugoniot curve when the material was shocked beyond 20 GPa, suggesting a volume-increasing reaction occurs in this shocked Al-CuO thermite near 20 GPa. To better characterize any shock-induced thermite reactions, emission spectroscopy measurements were obtained at stresses above 20 GPa. The best time-resolved spectra obtained thus far, at 25 GPa, does not support the fast thermite reaction hypothesis.

A review of reshock data for PMMA above the phase transition and the implied Grüneisen coefficient

PMMA (poly methyl methacrylate) is an important material to characterize, both as a model glassy polymer and as a window for interferometry techniques. Recently, PMMA reshock experimental results have been reported which implied a large thermal pressure component for PMMA reshocked from about 45 GPa. This work calls into question the high pressure, primary Hugoniot data the original conclusions were based on and presents an alternative explanation, namely, that the average Gruneisen coefficient, as indicated by the Mie-Gruneisen EOS, is too small to be inferred by the experimental data.

Shock Propagation and Deformation of Additively-Manufactured Polymer Foams with Engineered Porosity

The propagation of shocks through additively-manufactured (AM) polymeric structures containing multiple length scales of engineered porosity is studied both experimentally and computationally. In this study, a single-stage light gas gun is used to impact cube-shaped specimens, 40 mm on a side, instrumented with photon Doppler velocimeter (PDV) light probes to capture free surface velocities and side-looking high-speed video to capture deformation history. A combined Eulerian-Lagrangian finite element (FE) model has been developed which reproduces the majority of the observed experimental trends, based on an independently-measured shock Hugoniot for the bulk AM polymer. After initial calibration, the FE model has been used to suggest candidate geometries for experimental investigation, based on the desired shock response. Geometries for structurally-efficient shock mitigation have been investigated. In a separate set of experiments, miniature (6 mm × 6 mm) square specimens have been impacted at the Dynamic Compression Sector at the Advanced Photon Source (APS), and imaged using x-ray Phase Contrast Imaging (PCI). This technique gives strong evidence for the propagation of discrete shocks within the engineered foam structures, in agreement with our models.

A high-purity alumina for studies of shock loaded samples

Planar impact experiments have been performed on a potential new “standard” material, Coorstek 99.9% Plasmapure-UC™ polycrystalline alumina, for use as a replacement for the now unavailable Coors Vistal™ alumina in studies of shocked materials. The shock response of the Plasmapure-UC alumina (i.e. particle velocities, shock velocities, the Hugoniot elastic limit, and release wave speeds) was characterized up to 14 GPa for use in impedance matching calculations. These measurements were compared with other types of high purity alumina to identify differences in the shock response related to material pedigree. As a result of this work, the Hugoniot elastic limit for Plasmapure-UC alumina was found to be at ∼5.6 GPa, below that of other previously studied polycrystalline alumina. However, the impedance change at 5.6 GPa is minor, with a larger change occurring at 7.5 GPa. Beyond 7.5 GPa, the material exhibits an elastic-plastic response slightly different from that observed in similar high purity polycrystalline alumina. The slight impedance change and elastic-plastic response is discussed.Planar impact experiments have been performed on a potential new “standard” material, Coorstek 99.9% Plasmapure-UC™ polycrystalline alumina, for use as a replacement for the now unavailable Coors Vistal™ alumina in studies of shocked materials. The shock response of the Plasmapure-UC alumina (i.e. particle velocities, shock velocities, the Hugoniot elastic limit, and release wave speeds) was characterized up to 14 GPa for use in impedance matching calculations. These measurements were compared with other types of high purity alumina to identify differences in the shock response related to material pedigree. As a result of this work, the Hugoniot elastic limit for Plasmapure-UC alumina was found to be at ∼5.6 GPa, below that of other previously studied polycrystalline alumina. However, the impedance change at 5.6 GPa is minor, with a larger change occurring at 7.5 GPa. Beyond 7.5 GPa, the material exhibits an elastic-plastic response slightly different from that observed in similar high purity polycrystall...

Using mid-Infrared External Reflectance Spectroscopy to Distinguish Between Different Commercially Produced Poly[Methyl MethAcrylate] (PMMA) Samples

We report (null) results of experiments testing the hypothesis that mid-infrared (mid-IR) spectroscopy can be used to distinguish samples of poly[methyl methacrylate] (PMMA) obtained from different commercial suppliers. This work was motivated by the desire for a simple non-destructive and non-invasive test for pre-sorting PMMA samples prior to use in shock and high-strain-rate experiments, where PMMA is commonly used as a standard material. We discuss the reasoning behind our choice of mid-IR external reflectance spectroscopy. We describe our approach to recording reflectance spectra at near-normal (θi = 0 ± 5°) incidence, and for processing the raw reflectance data via a Kramers-Kronig analysis to obtain the wavelength-dependent complex index of refraction; the imaginary component of which corresponds to the wavelength-weighted absorption spectrum. We employ extensive signal averaging to improve the signal:noise ratio of our data, which necessitates adopting a special experimental protocol to mitigate the effects of instrumental drift. Finally, we report spectra of three PMMA samples with different commercial pedigrees, and show that they are virtually identical (± 1 % error, 95 % confidence); obviating the use of mid-IR reflectance spectroscopy to tell the samples apart.We report (null) results of experiments testing the hypothesis that mid-infrared (mid-IR) spectroscopy can be used to distinguish samples of poly[methyl methacrylate] (PMMA) obtained from different commercial suppliers. This work was motivated by the desire for a simple non-destructive and non-invasive test for pre-sorting PMMA samples prior to use in shock and high-strain-rate experiments, where PMMA is commonly used as a standard material. We discuss the reasoning behind our choice of mid-IR external reflectance spectroscopy. We describe our approach to recording reflectance spectra at near-normal (θi = 0 ± 5°) incidence, and for processing the raw reflectance data via a Kramers-Kronig analysis to obtain the wavelength-dependent complex index of refraction; the imaginary component of which corresponds to the wavelength-weighted absorption spectrum. We employ extensive signal averaging to improve the signal:noise ratio of our data, which necessitates adopting a special experimental protocol to mitigate t...