Gary R. Parker

Frictional heating and ignition of energetic materials

For many years, powder friction tests have been an integral part of explosives sensitivity and safety testing. More recently, oblique impact tests have been used in the hazard assessment of monolithic charges. However, these tests are simply threshold tests for reaction, and relatively little work has been done to try to examine the processes that lead to frictional heating and ignition of energetic materials. We report the results from a series of experiments in which energetic materials are subjected to frictional heating under closely‐controlled conditions (normal load, sliding speed, grit quantity and composition, substrate). The response of the energetic material and grit, if present, is observed by optical and infrared high‐speed photography to determine the nature of the interactions between the test material, grit and substrate, and the mechanisms by which the energetic material may be heated to ignition.

Interplay of explosive thermal reaction dynamics and structural confinement

Explosives play a significant role in human affairs; however, their behavior in circumstances other than intentional detonation is poorly understood. Accidents may have catastrophic consequences, especially if additional hazardous materials are involved. Abnormal ignition stimuli, such as impact, spark, friction, and heat may lead to a very violent outcome, potentially including detonation. An important factor influencing the behavior subsequent to abnormal ignition is the strength and inertia of the vessel confining the explosive, i.e., the near-field structural/mechanical environment, also known as confinement (inertial or mechanical). However, a comprehensive and quantified understanding of how confinement affects reaction violence does not yet exist. In the research discussed here, we have investigated a wide range of confinement conditions and related the explosive response to the fundamentals of the combustion process in the explosive. In our experiments, a charge of an octahydrotetranitrotetrazine-...

Dynamic Measurement of the Permeability of an Explosive Undergoing Thermal Damage

We have constructed a gas permeameter to measure the permeability of explosives that have been thermally damaged. We report on the calibration of the instrument, as well as measurements of the permeability of PBX 9501, a plastic bonded explosive. The limitations of this technique are shown, as well as the effects that crystalline phase transition, volumetric expansion, and chemical reaction have on the permeability of this material.

Note: A technique to capture and compose streak images of explosive events with unpredictable timing

The authors describe a method to capture optical data and construct digitized streak images for analysis of high-speed phenomena with unpredictable timing by using a high-speed video camera and software routines. Advances in high-speed video camera technology have led to development of cameras with frame rates (1 x 10(6) frames per second) and spatial resolution (1280 x 800 pixels) suitable to capture fast phenomena, such as detonation in high explosives (< or = 10 km s(-1)), on small enough scales to be convenient for laboratory experiments. Further, relatively long-duration recordings (> or = 1 s) are maintained in a rolling buffer in volatile memory allowing the entire frame sequence to be recorded pretrigger, thus obviating the need for precisely located diagnostic triggers. The method described was used to capture the progression of luminous reaction during the deflagration-to-detonation transition of the HMX-based (octahydro-1, 3, 5, 7-tetranitro-1, 3, 5, 7-tetrazocine) plastic bonded explosive (PBX) formulation during cookoff.

Effect of Thermal Damage on the Permeability of PBX 9501

It has been known for many years that thermally‐damaged explosive has significantly different mechanical properties when compared to its pristine state. These differences can have profound effects on the response to external stimuli. We present the results of a study to examine the effects of dynamic thermal damage on the permeability of PBX 9501 and compare these results with a nonenergetic simulant.

Quantification of reaction violence and combustion enthalpy of plastic bonded explosive 9501 under strong confinement

The confinement experienced by an explosive during thermal self-initiation can substantially affect performance in terms of deflagration–to–detonation characteristics and explosion/detonation violence. To this end, we have developed an experiment to quantitatively observe enthalpy change and reaction violence in thermally initiated plastic bonded explosive (PBX) 9501. Traditionally, researchers attempt to quantify violence using terminal observations of fragment size, fragment velocity, and through subjective observations. In the work presented here, the explosive was loaded into a heated gun assembly where we subjected a 300 mg charge to a cook-off schedule and a range of static and inertial confinements. Static confinement was controlled using rupture disks calibrated at 34.5 and 138 MPa. The use of 3.15 and 6.3 g projectile masses provided a variation in inertial confinement. This was a regime of strong confinement; a significant fraction of the explosive energy was required to rupture the disk, and th...

Morphology Changes during Thermal Decomposition of PBX9501

Our goal is to be able to predict the morphology of PBX9501 as a function of time at temperature. This is necessary in order to be able to predict the behavior of material that has been subjected to a known temperature trajectory. We have begun by studying the mechanism of the initial solid state phase transition between the beta and delta phases of HMX. This leads to a volume expansion and a large degree of mechanical damage in the material. On continued heating above the solid‐solid phase transition, a solid to gas transformation occurs. We have monitored the solid to gas transition as a function of isothermal temperature and particle size in order to address mechanistic questions concerning chemical production of gas phase species by surface regression or internal pore formation. We have also performed experiments to characterize the morphology changes in the material caused by the loss of solid mass.

Violent cookoff reactions in HMX-based explosives in DDT tubes: Tracking luminous waves with streak imaging

Recent implementation of modern high-speed video cameras has permitted the experimental flexibility needed to revisit classic deflagration-to-detonation (DDT) tube experiments and capture novel and valuable results displaying the progression of luminous reaction from a cookoff event. The authors present select data from a series of experiments where the HMX-based high explosives PBX 9501 and LX-07 were heated above 180°C for various durations to impose damage (i.e. phase transitions and void generation) before being driven to cook off. These two explosives have different polymeric binders, HMX mass fractions and cook off responses and a comparison between the two offers mechanistic insights on how thermal explosions evolve. From this series, results will be displayed indicating a wide range of violence from somewhat mild pressure bursts, to intermediate power compressive burns, to high-violence DDT. Image data from high temperature DDT tube experiments, where the explosive was ignited on one end, were als...

NON‐RANDOM CRACK OPENING IN PARTIALLY CONFINED, THERMALLY DAMAGED PBX 9501 AND OBSERVATIONSON ITS EFFECTS ON COMBUSTION

In this work, we present evidence for how strong radial confinement can result in aligned macro‐scale crack opening. We damage cylinders in a tight‐fitting quartz sleeve, open on both ends, and observe the occurrence of aligned cracks opening normal to the longitudinal axis. This geometry and confinement is common in experimental arrangements such as strand burners and DDT tubes. Further, we observe, with high‐speed photography, how this non‐random crack opening affects combustion, and propose mechanisms, garnered from time‐lapse photography and elastic stress analysis, for how it occurs.

Understanding the Mechanisms Leading to Gas Permeation in Thermally Damaged PBX 9501

We present data that indicate that thermally damaged PBX 9501 is substantially more permeable than the pristine material and that this may have a significant effect on the pre‐ignition slow cook‐off process, as well as the post‐ignition flame spread process. Experiments indicate that the mechanism responsible for the formation of interconnected matrix porosity is likely dominated by nitroplasticizer decomposition in the early stages of the permeability evolution followed by secondary, slower HMX decomposition.