Jerry W. Forbes

Isentropic compression of cyclotetramethylene tetranitramine (HMX) single crystals to 50GPa

Single crystals of cyclotetramethylene tetranitramine (HMX) were isentropically compressed perpendicular to (010) and (011) faces at the Sandia Z-Machine. A 50GPa ramped magnetic pressure load of about 200ns rise time loaded four specimens of each orientation. HMX specimens were from 300–600μm thick. Velocity histories at the rear of each crystal were measured by Doppler velocimetry. Although a phase change in HMX at 27GPa has been proposed based upon isothermal data, no evidence of this change is seen in our analyses between 5 and 50GPa along the isentrope. Previous isentropic loading experiments on HMX had not shown evidence of a phase change either, but those experiments were complicated by the use of NaCl interferometer windows that have a phase change near the pressure of interest. The experiments described in this paper employed LiF interferometer windows that are known to be absent phase changes in the regime of application. Accurate determination of isentropic compressibility for HMX was not possi...

Shock Sensitivity of LX-04 Containing Delta Phase HMX at Elevated Temperatures

LX‐04 is a widely used HMX‐based plastic bonded explosive, which contains 85 weight % HMX and 15 weight % Viton binder. The sensitivity of LX‐04 to a single stimulus such as heat, impact, and shock has been previously studied. However, hazard scenarios can involve multiple stimuli, such as heating to temperatures close to thermal explosion conditions followed by fragment impact, producing a shock in the hot explosive. The sensitivity of HMX at elevated temperatures is further complicated by the beta to delta solid‐state phase transition, which occurs at approximately 165°C. This paper presents the results of shock initiation experiments conducted with LX‐04 preheated to 190°C, as well as density measurements and small scale safety test results of the δ phase HMX at room temperature. This work shows that LX‐04 at 190°C is more shock sensitive than LX‐04 at 150°C or 170°C due to the volume increase during the β to δ solid phase transition, which creates more hot spots, and the faster growth of reaction duri...

Shock sensitivity of LX-04 at elevated temperatures

Hazard scenarios can involve multiple stimuli, such as heating followed by fragment impact (shock). The shock response of LX-04 (85 weight % HMX and 15 weight % Viton binder) preheated to temperatures near 170C is studied in a 10.2 cm bore diameter gas gun using embedded manganin pressure gauges. The pressure histories at various depths in the LX-04 targets and the run distances to detonation at several input shock pressures are measured and compared to those obtained in ambient temperature LX-04. The hot LX-04 is significantly more shock sensitive than ambient LX-04. Ignition and Growth reactive flow models are developed for ambient and hot LX-04 to allow predictions of impact scenarios that can not be tested directly.

Investigation of Steven Impact Test Using a Transportation Hook Projectile with Gauged Experiments and 3D Modeling

The Steven Impact Test and associated modeling offer valuable practical predictions for evaluating numerous safety scenarios involving low velocity impact of energetic materials by different projectile geometries. One such scenario is the impact of energetic material by a transportation hook during shipping, which offers complexity because of the irregular hook projectile shape. Experiments were performed using gauged Steven Test targets with PBX9404 impacted by a transportation hook projectile to compliment previous non‐gauged experiments that established an impact threshold of approximately 69 m/s. Modeling of these experiments was performed with LS‐DYNA code using an Ignition and Growth reaction criteria with a friction term. Comparison of the experiment to the model shows reasonable agreement with some details requiring more attention. The experimental results (including carbon resistor gauge records), model calculations, and a discussion of the dominant reaction mechanisms in light of comparisons bet...

Isentropic compression loading of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) and the pressure-induced phase transition at 27GPa

The 27GPa pressure-induced epsilon–phi phase transition in octahydro-1,3,5,7-tetranitro-l,3,5,7-tetrazocine (HMX) is explored using the isentropic compression experiment (ICE) technique at the Sandia National Laboratories Z-machine facility. Our data indicate that this phase transition is sluggish and if it does occur to any extent under the time scales (200–500ns) and strain rates (5×105) typical of ICE loading conditions, the amount of conversion is small.

Measurement of Low Level Explosives Reaction in Gauged Multi-Dimensional Steven Impact Tests

The Steven Test was developed to determine relative impact sensitivity of metal encased solid high explosives and also be amenable to two‐dimensional modeling. Low level reaction thresholds occur at impact velocities below those required for shock initiation. To assist in understanding this test, multi‐dimensional gauge techniques utilizing carbon foil and carbon resistor gauges were used to measure pressure and event times. Carbon resistor gauges indicated late time low level reactions 200–540 μs after projectile impact, creating 0.39–2.00 kb peak shocks centered in PBX 9501 explosives discs and a 0.60 kb peak shock in a LX‐04 disk. Steven Test modeling results, based on ignition and growth criteria, are presented for two PBX 9501 scenarios: one with projectile impact velocity just under threshold (51 m/s) and one with projectile impact velocity just over threshold (55 m/s). Modeling results are presented and compared to experimental data.

Nonequilibrium Zeldovich-von Neumann-Doring theory and reactive flow modeling of detonation

This paper discusses the Nonequilibrium Zeldovich-von Neumann-Doring (NEZND) theory of self-sustaining detonation waves and the Ignition and Growth reactive flow model of shock initiation and detonation wave propagation in solid explosives. The NEZND theory identified the nonequilibrium excitation processes that precede and follow the exothermic decomposition of a large high explosive molecule into several small reaction product molecules. The thermal energy deposited by the leading shock wave must be distributed to the vibrational modes of the explosive molecule before chemical reactions can occur. The induction time for the onset of the initial endothermic reactions can be calculated using high pressure-high temperature transition state theory. Since the chemical energy is released well behind the leading shock front of a detonation wave, a physical mechanism is required for this chemical energy to reinforce the leading shock front and maintain its overall constant velocity. This mechanism is the amplification of pressure wavelets in the reaction zone by the process of de-excitation of the initially highly vibrationally excited reaction product molecules. This process leads to the development of the three-dimensional structure of detonation waves observed for all explosives. For practical predictions of shock initiation and detonation in hydrodynamic codes, phenomenological reactive flow models have been developed. The Ignition and Growth reactive flow model of shock initiation and detonation in solid explosives has been very successful in describing the overall flow measured by embedded gauges and laser interferometry. This reactive flow model uses pressure and compression dependent reaction rates, because time-resolved experimental temperature data is not yet available. Since all chemical reaction rates are ultimately controlled by temperature, the next generation of reactive flow models will use temperature dependent reaction rates. Progress on a statistical hot spot ignition and growth reactive flow model with multistep Arrhenius chemical reaction pathways is discussed.

Manganin Gauge and Reactive Flow Modeling Study of the Shock Initiation of PBX 9501

A series of 101mm diameter gas gun experiments was fired using manganin pressure gauges embedded in the HMX‐based explosive PBX 9501 at initial temperatures of 20°C and 50°C. Flyer plate impact velocities were chosen to produce impact pressure levels in PBX 9501 at which the growth of explosive reaction preceding detonation was measured on most of the gauges and detonation pressure profiles were recorded on some of the gauges placed deepest into the explosive targets. All measured pressure histories for initial temperatures of 25°C and 50°C were essentially identical. Measured run distances to detonation at three input shock pressures agreed with previous results. An existing Ignition and Growth reactive flow computer model for shock initiation and detonation of PBX 9501, which was developed based on LANL embedded particle velocity gauge data, was tested on these pressure gauge results. The agreement was excellent, indicating that the embedded pressure and particle velocity gauge techniques yielded consistent results.

The Isentrope of Unreacted LX‐04 to 170 kbar

We present new data on the unreacted approximate isentrope of the HMX-based explosive LX-04, measured to 170 kbar, using newly developed long pulse isentropic compression techniques at the Sandia National Laboratories Z Machine facility. This study extends in pressure by 70% the previous state of the art on unreacted LX-04 using this technique. This isentrope will give the unreacted Hugoniot from thermodynamic relations using a Gruneisen equation of state model. The unreacted Hugoniot of LX-04 is important in understanding the structure of the reaction front in the detonating explosive. We find that a Hugoniot given by U{sub s}= 2950 m/s + 1.69 u{sub p} yields for an isentrope a curve which fits our LX-04 ICE data well.

Pressure Wave Measurements Resulting from Thermal Cook‐Off of the HMX Based High Explosive LX‐04

Experiments that investigate thermal and nearby explosion scenarios are needed to provide essential data to models for accurate predictions. A porous LX‐04 (85/15 wt% HMX/Viton) sample was heated in a heavily confined donor charge until it thermally exploded. The reaction accelerated a steel cover plate across a 10 cm gap into a preheated gauged acceptor cylinder (near its theoretical maximum density) of LX‐04. The carbon resistor gauges in the acceptor measured the resulting multi‐dimensional ramp wave as it propagated through the pre‐heated LX‐04. Detonation of the LX‐04 acceptor does not occur. Results are compared to similar experiments with acceptors at room temperature.