Session R03: Shock to Detonation Transition (SDT) and Initiation Phenomonology I

Overview of Recent Research Efforts in Shock Initiaiton of Energetic Materials

Shock initiation experiments using energetic materials with in-situ gauges to track the process of the initiation front is relatively established and the data obtained facilitates the calibration of various models. In the recent decade, advancements have extended the technique into the areas of studying increased porosity for a variety of energetic materials as well as utilizing multiple or thin-pulse shocks to study kinetic effects in Insensitive High Explosives (IHE's). The ability of various models to reproduce the results has also shown advancement. This work will provide a brief history of using the in-situ gauge technique followed by highlights of recent work with porous energetics or utilizing multiple/thin-pulse shocks. A short discussion of modeling advancements with suggestions for future work will also be included. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

The Effect of PETN Particle Size and Choice of EFI Technology on Detonator Performance Thresholds

Exploding Foil Initiators (EFIs) can be one of two types: a flexible foil or a chip. As the name suggests flexible foils are not rigid but comprise alternating layers of a metal conductor and an insulator. Whereas a chip EFI has the conducting metal layers and insulating flyer deposited onto a hard, ceramic substrate. It has commonly been suggested that chip EFIs are more efficient than an equivalent flexible foil EFI, however there appears to have been limited direct comparisons published. The relationship between surface area or particle size on and EBW detonator performance has been widely published but the effect on EFI performance less so. Therefore, a four-way threshold test has been performed to quantify the effects of both technology type and PETN particle. The results (threshold voltages and standard deviations) are reported for high and low surface area, detonator-grade PETNs with both flexible foils and chips. Powder compaction mechanisms are also discussed.

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Hayesian Inference: Exploring the Relationship between Supercritical Hot Spot Number Density & the Speed of Deflagration Waves they Trigger

On Jan 2, 2023, the SCCM community lost one of its most senior and vital members: Dennis B. Hayes, formerly of Sandia and (in his retirement) Los Alamos National Laboratories. Although Dennis was primarily known as a shock-wave theorist, he made forays into x-rays, detonation, and many other topics. He independently derived the Statistical Hot Spot (SHS) model in 1981, and was the first to apply it to HE reaction zones. (The first derivation was by Soviet mathematician Andrey Kolmogorov, of turbulence fame, in 1937, his interest being heterogeneous phase transition in metals.) Dennis realized that the SHS result provides a relationship between the number density of supercritical hot spots, eta, and the speed of deflagration waves, V, that they trigger. He applied this relationship to experiments on HNS explosive. Based on the powder granularity he estimated eta, and hence (via SHS) V-values between about 5 and 50 m/s. These estimates are fairly consistent with more recent measurements and estimates of V. Here we refine his method—which we call Hayesian Inference—to explore the same relationship for two PBXs. The results show that eta is exceedingly large, 10⁹/mm³ being a conservative number in many cases. This tells us that, although detonation reaction zones are very thin, the continuum approximation does conveniently hold. It also tells us that most efforts we make to seed homogenous explosives with artificial heterogeneities have eta-values orders of magnitude smaller than do PBXs.   

Varied Thin Pulse Duration Experiments Performed on Insensitive High Explosives

Experiments were performed to measure the thin pulse shock initiation of insensitive high explosive (IHE) formulations at varied pulse durations to build on previous experiments. The goal of these thin pulse experiments is to bridge the gap between shorter duration E-gun tests and longer duration standard shock initiation testing. The thin pulses were generated using stainless steel flyers of varied thicknesses backed by low-density foam on the front of 100 mm diameter projectiles. These flyers were shot at IHE targets at speeds which induced pressures necessary to cause a transition to detonation at ambient temperature. Manganin gauges embedded in the IHE target were used to determine the initial pressure and run-distance to detonation. Data obtained is used to validate and/or calibrate models in development. Test parameters, comparisons to prior testing and simulation results, and future experiments will be discussed. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

Detonation Sensitivity of Thermally Damaged PBX 9501

Polymer bonded explosives (PBXs) are a combination of high explosives (HE) and an energetic and/or inert binder. In the case of PBX 9501, the HE is 95% octogen (HMX) and the binder is 2.5% BDNPA-F and 2.5% estane. To study how damage may affect the detonation sensitivity of this HE we exploited the beta-to-delta transition that PBX 9501 goes through above 150 ^oC. This crystalline transition is accompanied by a volume expansion that remains when the HE is cooled to room temperature. We thermally damaged multiple HE samples to various temperatures and then performed (5) multi-slug gas gun experiments to study how the initiation sensitivity changed. Computed tomography was also collected before and after thermal damage to observe the effect on the morphology of the samples.

 

LA-UR-23-20555