David L. Robbins

Laser-driven MiniFlyer induced gold spall

A laser-launched miniature flyer system (MiniFlyer) is being used to study the dynamic properties of materials. A 3-mm diameter and 0.05-mm thick flyer plate is accelerated by a laser-pulse-induced plasma contained between a clear window substrate and the flyer plate. The substrate is coated with carbon, aluminum oxide, and aluminum to enhance the plasma formation process. The flyer impacts a gold target plate of 0.10 or 0.26 mm thickness, producing a shock. The shock pulse interacts with the free surface and reflects as a rarefaction wave, producing tension in the foil. Dynamic measurements of the free surface particle velocity were made using VISAR (Velocity Interferometer System for Any Reflector). Cross-sections of the gold targets exhibit spall planes at the expected locations.

EQUATION OF STATE OF AMMONIUM NITRATE

Ammonium nitrate (AN) is a widely used fertilizer and mining explosive. AN is commonly used in ammonium nitrate‐fuel oil (ANFO), which is a mixture of explosive‐grade AN prills and fuel oil in a 94:6 ratio by weight. ANFO is a non‐ideal explosive with measured detonation velocities around 4 km/s. The equation of state properties and known initiation behavior of neat AN are limited. We present the results of a series of gas gun‐driven plate impact experiments on pressed neat ammonium nitrate at 1.72 g/cm3. No evidence of initiation was observed under shock loading to 22 GPa. High pressure x‐ray diffraction experiments in diamond anvil cells provided insight into the high pressure phase behavior over the same pressure range (to 25 GPa), as well as a static isotherm at ambient temperature. From the isotherm and thermodynamic properties at ambient conditions, a preliminary unreacted equation of state (EOS) has been developed based on the Murnaghan isotherm and Helmholtz formalism [1], which compares favorably...

High Pressure Hugoniot and Reaction Rate Measurements in PBX9501

Single‐stage and two‐stage gas gun experiments have been completed to measure the unreacted Hugoniot of PBX9501 high explosive (HE). Two types of experiments were done: 1) PBX9501 was impacted with a higher impedance projectile and the interface particle velocity history was measured using a magnetic gauge glued to the HE front; 2) a PBX9501 disc was mounted in the front of a projectile that impacted a LiF window and velocity interferometers (VISAR) were used to measure the impact interface particle velocity history. Inputs to the PBX9501 ranged from 3 to 15 GPa in these experiments. Particle velocity waveforms show an induction time followed by a particle velocity change (the nature of the change depends on the type of experiment) corresponding to shock‐induced reaction in the PBX9501. The induction part of the waveform provided unreacted Hugoniot information so several new high‐pressure Hugoniot points were generated. These data do not indicate a softening in the unreacted Hugoniot at high pressures; more experiments will be necessary to determine this. By using an estimate for the reaction product EOS, it was possible to estimate the average PBX9501 initial reaction rate for each experiment. The induction time decreases with pressure and the reaction rate increases with pressure.Single‐stage and two‐stage gas gun experiments have been completed to measure the unreacted Hugoniot of PBX9501 high explosive (HE). Two types of experiments were done: 1) PBX9501 was impacted with a higher impedance projectile and the interface particle velocity history was measured using a magnetic gauge glued to the HE front; 2) a PBX9501 disc was mounted in the front of a projectile that impacted a LiF window and velocity interferometers (VISAR) were used to measure the impact interface particle velocity history. Inputs to the PBX9501 ranged from 3 to 15 GPa in these experiments. Particle velocity waveforms show an induction time followed by a particle velocity change (the nature of the change depends on the type of experiment) corresponding to shock‐induced reaction in the PBX9501. The induction part of the waveform provided unreacted Hugoniot information so several new high‐pressure Hugoniot points were generated. These data do not indicate a softening in the unreacted Hugoniot at high pressures; mo...

Magnetic Particle Velocity Measurements of Shocked Teflon

A series of shock compression experiments have been undertaken on Teflon using single‐ and two‐stage gas‐guns. Peak pressures in these experiments range from a few kbars to over 10 kbars, as well as one shot completed at 117 kbar. Multiple particle velocity wave profiles, at a number of Langrangian positions, are obtained for each experiment using in‐situ magnetic gauges. Shock velocity is calculated from arrival times at both the particle velocity gauges and at embedded shock trackers. These direct measurements of particle and shock velocity are compared to previous shock compression results on Teflon. Particular attention is focused in the region below 10 kbar where evidence of a shock induced phase transition has been reported, based upon a cusp in the Hugoniot. The volume change for this transition is only ∼ 2.2 % making its observation difficult. A two‐wave structure on the shock front would be strong evidence of the shock‐induced transition, but has not been observed in these initial low‐pressure ex...

FLYER VELOCITY CHARACTERISTICS OF THE LASER-DRIVEN MINIFLYER SYSTEM

The laser-driven MiniFlyer system is used to launch a small, thin flyer plate for impact on a target. Consequently, it is an indirect drive technique that de-couples the shock from the laser beam profile. The flyer velocity can be controlled by adjustment of the laser energy. The upper limits on the flyer velocity involve the ability of the substrate window to transmit the laser light without absorbing, reflecting, etc.; i.e., a maximum amount of laser energy is directly converted into kinetic energy of the flyer plate. We have investigated the use of sapphire, quartz, and BK-7 glass as substrate windows. In the past, a particular type of sapphire has been used for nearly all MiniFlyer experiments. Results of this study in terms of the performance of these window materials, based on flyer velocity are discussed.

Shock‐Induced Chemical Reaction in Organic and Silicon Based Liquids

Shock‐induced chemical reactions remain an area in shock physics that needs further investigation, particularly for determining the influence of pressure, temperature, and chemical structure on reactivity. Several studies have been done in the past that indicate dimerization, polymerization, and decomposition take place in different shock‐produced pressure and temperature regimes depending on chemical functionality. We present results obtained from single‐shock experiments in which liquids were studied using embedded multiple magnetic gauges to make in‐situ measurements of the particle velocity profiles at up to ten Lagrangian positions in the liquid. One of the liquids was organic (tert‐butylacetylene) and the other was a closely related silicon‐based material (ethynyltrimethylsilane). Multiple wave structures were measured in each liquid when the input pressure was above a certain threshold. Here, the reactivity of these materials are compared.

Stereo camera system for three-dimensional reconstruction of a flyer plate in flight

A stereo camera system has been developed for use with the laser-driven MiniFlyer apparatus. The objective of the stereo camera is to determine the three-dimensional reconstruction of the surface of a flyer plate in flight. The resolution of the system is designed to be 10 μm, based on the maximum blur expected due to the flyer plate motion during the exposure. Illumination of the flyer plate in flight is accomplished with a Q-switched Nd:YAG laser. A grid is projected onto the flyer plate surface to provide reference points for the reconstruction. The software algorithm used for the reconstruction utilizes a bilinear interpolation to fill in the data between the grid lines. Data for copper and titanium flyer plates are discussed.

Hugoniot and properties of diesel fuel used in ANFO

One of the more common ammonium nitrate(AN)-based explosives is ANFO, which is a mixture of AN prills and diesel fuel oil (FO) in a 94:6 ratio by weight. Since there are no available shock data on FO, a series of shock compression experiments have been completed using a two-stage light gas gun with a sealed liquid target cell. The FO studied was diesel #2 which has been used in a number of ANFO explosive shots. Density and sound speed data were measured and used to predict and compare the data to a universal liquid Hugoniot (ULH). In-situ magnetic gauges in the target cell were used to measure the particle velocity, shock velocity, and shock wave profiles. Projectile impact velocities ranged from 1.5 to 3.2 km/s, generating input pressures to the FO between 3 and 17 GPa (depending on the impactor material being used). No shock-induced reaction was observed in the FO in any of the experiments. Since the ULH was found to slightly under predict the FO Hugoniot states, a linear Hugoniot was fit to the data - ...

Quasi‐Static and Shock Compressive Response of Fluorinated Polymers: Kel‐F 800

Fluoropolymers are widely used in a variety of applications due to their favorable properties that include chemical inertness, low coefficient of friction, and ability to withstand high‐temperature operating conditions. Additionally, their high densities also make them desirable materials for use as high explosive binders in plastic bonded explosives. Here, we present the first investigation of the dynamic (shock) and static compression behavior of the fluorinated binder poly(chlorotrifluoroethylene‐co‐vinylidene fluoride) (Kel‐F 800). Kel‐F 800 samples prepared by both compression molding and melt‐processing methods were studied. We observed little difference in the dynamic behavior of the two materials, despite a difference in crystallinity of 10–15%. A linear Rankine‐Hugoniot fit to the Hugoniot loci in the Us‐up plane gives Us = 1.838 + 1.824 up. Langrangian sound velocities at pressure were also measured at 3.0 (cL = 4.7 mm/μs) and 4.9 (cL = 5.8 mm/μs) GPa.

Temperature Controller System for Gas Gun Targets

A temperature controller system capable of heating and cooling gas gun targets over the range −75°C to +120°C was designed and tested. The system uses cold nitrogen gas from a liquid nitrogen Dewar for cooling and compressed air for heating. Two gas flow heaters control the gas temperature for both heating and cooling. One heater controls the temperature of the target mounting plate and the other the temperature of a copper tubing coil surrounding the target. Each heater is separately adjustable, so the target material will achieve a uniform temperature throughout its volume. A magnetic gauge membrane with integrated thermocouples was developed to measure the internal temperature of the target. Using this system, multiple magnetic gauge shock experiments, including equation‐of‐state measurements and shock initiation of high explosives, can be performed over a range of initial temperatures. Successful heating and cooling tests were completed on Teflon samples.