Daniel W. Greenwood

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...

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...

Threshold Studies of Heated HMX-Based Energetic Material Targets Using the Steven Impact Test

Impact tests performed at low velocity on heated energetic material samples are of interest when considering the situation of energetic materials involved in a fire. To determine heated reaction thresholds, Steven Test targets containing PBX 9404 or LX‐04 samples heated to the range of 150–170°C were impacted at velocities up to 150 m/s by two different projectile head geometries. Comparing these measured thresholds to ambient temperature thresholds revealed that the heated LX‐04 thresholds were considerably higher than ambient, whereas the heated PBX 9404 thresholds were only slightly higher than the ambient temperature thresholds. The violence of reaction level of the PBX 9404 was considerably higher than that of the LX‐04 as measured with four overpressure gauges. The varying results in these samples with different HMX/binder configurations indicate that friction plays a dominant role in reaction ignition during impact. This work outlines the experimental details, compares the thresholds and violence l...

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.

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.

Carbon Resistor Pressure Gauge Calibration at Low Stresses

The 470 Ohm carbon resistor gauge has been used in the stress range up to 4–5 GPa for highly heterogeneous materials and/or divergent flow experiments. The attractiveness of the gauge is its rugged nature, simple construction, low cost, reproducibility, and survivability in dynamic events. Gauge drawbacks are the long time response to pressure equilibration and gauge resistance hysteresis. In the regime below 0.4 GPa, gauge calibration has been extrapolated. Because of the need for calibration data within this low stress regime, calibration experiments were performed using a split‐Hopkinson bar, drop tower apparatus, and gas pressure chamber. Since the performance of the gauge at elevated temperatures is a concern, the change in resistance due to heating at atmospheric pressure was also investigated. Details of the various calibration arrangements and the results are discussed and compared to a calibration curve fit to previously published calibration data.

TIME-SEQUENCED X-RAY OBSERVATION OF A THERMAL EXPLOSION

The evolution of a thermally‐initiated explosion is studied using a multiple‐image x‐ray system. HMX‐based PBX 9501 is used in this work, enabling direct comparison to recently‐published data obtained with proton radiography [1]. Multiple x‐ray images of the explosion are obtained with image spacing of ten microseconds or more. The explosion is simultaneously characterized with a high‐speed camera using an interframe spacing of 11 μs. X‐ray and camera images were both initiated passively by signals from an embedded thermocouple array, as opposed to being actively triggered by a laser pulse or other external source. X‐ray images show an accelerating reacting front within the explosive, and also show unreacted explosive at the time the containment vessel bursts. High‐speed camera images show debris ejected from the vessel expanding at 800–2100 m/s in the first tens of μs after the container wall failure. The effective center of the initiation volume is about 6 mm from the geometric center of the explosive.

Thermal Cook-off of an HMX Based Explosive: Pressure Gauge Experiments and Modeling

Safety issues related to thermal cook-off are important for handling and storing explosive devices. Violence of an event as a function of confinement is important for prediction of collateral events. There are major issues which require an understanding of the following events: (1) transit to detonation of a pressure wave from a cook-off event, (2) sensitivity of changes in HMX-based explosives with thermally induced phase transitions, and (3) the potential danger of neighboring explosive devices being affected by a cook-off reaction. Results of cook-off events of known size, confinement, and thermal history allow for development and/or calibration of computer models for calculating events that are difficult to measure experimentally.

THERMAL COOK-OFF EXPERIMENTS OF THE HMX BASED HIGH EXPLOSIVE LX-04 TO CHARACTERIZE VIOLENCE WITH VARYING CONFINEMENT

Thermal cook‐off experiments were carried out using LX‐04 explosive (85% HMX and 15% Viton by weight) with different levels of confinement to characterize the effect of confinement on the reaction violence. These experiments involved heating a porous LX‐04 sample in a stainless steel container with varying container end plate thickness and assembly bolt diameter to control overall confinement. As expected, detonation did not occur and reducing the overall confinement lowered the reaction violence. This is consistent with modeling results that predict that a lower confinement will act to lower the cook‐off pressure and thus the overall burn rate which lowers the overall violence. These results suggest that controlling the overall system confinement can modify the relative safety in a given scenario.

Pressure Wave Measurements from Thermal Cook‐Off of an HMX Based High Explosive PBX 9501

A better understanding of thermal cook‐off is important for safe handling and storing explosive devices. A number of safety issues exist about what occurs when a cased explosive thermally cooks off. For example, violence of the events as a function of confinement is important for predictions of collateral damage. This paper demonstrates how adjacent materials can be gauged to measure the resulting pressure wave and how this wave propagates in this adjacent material. The output pulse from the thermal cook‐off explosive containing fixture is of obvious interest for assessing many scenarios.