Session L2: ERM: Advanced and additive manufacturing

Diameter effects on the directional anisotropic detonation behavior of strand structured additively manufactured explosives.

UV-curing direct ink write (DIW) techniques have recently been used to introduce ordered linear porosity that demonstrate directional anisotropic detonation behavior in structured explosives. This exerted control over the detonation behavior of high explosives (HE) through structure by additive manufacturing relies on the unimpeded passage of a fast precursor wave escaping the detonation front, causing desensitization of the upstream HE material. We have shown that for a single strand diameter, \textasciitilde 600 \textmu m, exceeding a critical value for dimension of the interstitial space between these printed HE strands results in detonation failure. Here we will present a parametric study of the dependence of this critical interstitial spacing value on the diameter of the printed HE strands, and the resulting control over the detonation propagation and failure gained through this additional parameter. Overall critical dimensions at which detonation failure occurs will also be discussed. Experiments will be compared with calibrated simulations using the Scaled Uniform Reactive Front model. LA-UR-19-21617   

Investigating Typical Additive Manufacturing Defect Geometries using Physical Vapor Deposition Explosives as a Model System

Additive Manufacturing (AM) techniques are increasingly being utilized for energetic material processes and research. The downside to utilizing current AM techniques is that energetic samples fabricated using these techniques often develop artifacts or defects during the manufacturing process. In this work, we use Physical Vapor Deposition (PVD) explosive samples as a model system to investigate the effects of these typical AM artifacts or defects on explosive samples created through AM techniques. PVD techniques allow for precise control of geometry to simulate typical AM artifacts or defects embedded into explosive samples. This experiment specifically investigates triangular and diamond-shaped artifacts that often result during direct-ink-writing (robocasting). Samples were prepared with different sizes of voids embedded into the films. An ultra-high-speed framing camera and streak camera were used to view the samples under dynamic shock loading. It was determined that both geometry and size of the defects have a significant impact on the detonation front.   

A Simple 3D Printed Plane Wave Explosive Lens Based on Fritz Parameters

The development of additive manufacturing (3D printing) has opened up avenues previously unexplored due to prohibitive cost and/or complexity. Printing of inert parts for use in shock property characterization has reached a new level by allowing high resolution (10's of micron) wave shapers to be designed and employed at varying dimensions; the ability to save time on HE machining, casting, and cost of HE is undeniable. Herein, we report the design of a polyjet-printed wave shaper paired with a cast-cure HE formulation to generate a planar output shock; guided by CTH simulations, the design was iterated to increase planarity. Lens fabrication followed guidelines by J. Fritz, using PMMA Hugoniot data as a substitute for the chemically similar 3D printed acrylates. Front curvature characterization of these minimal explosive mass, small diameter (2.54 cm) charges showed reliable planarity below 100 ns and optimized to \textasciitilde 28 ns. Following this characterization, the plane wave generators were used to launch flyers at varying materials to investigate shock and particle velocities and chemical reactions. In this fashion, Us-up curves were created and aided follow-on gas-gun experiments. LA-UR-21802   

Controlling thermite reactivity with engineered porosity and architecture

Thermites are two-component mixtures containing a metal fuel and a metal oxide as an oxidizer. The reactivity of these materials can vary from slow burns to rapid deflagrations depending on a variety of parameters such as the particle size, composition, and configuration. This shift has been attributed to a change in the mode of energy transport from a conductive mode to a convective mode. Additive manufacturing (AM) gives us unique control of the architecture, which can mean composition or the addition of microstructural features. With this tool, we can design and test thermite samples to control both the reaction rate and heat transfer modes in order to build a better understanding of the reaction mechanisms. In this work, we mix and Al and CuO ink on-the-fly during an extrusion process to create thermite samples. The porosity and filament size were varied in a systematic way to produce lattices of different surface area and density. The resultant reactivity was measured two ways; the velocity of the sweeping luminous front as well as the total time of luminous emission. In general, we observe that at least some porosity is needed in order to achieve rapid energy release and that intermediate gas trapping is important for accelerating the reaction. A ``materials design plot'' which plots the range of energy release rate as a function of volumetric energy density was constructed. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-768219   

Shock Interactions in Multilayer Explosive Films

Mixing two energetic materials with different properties can be an effective method for controlling performance. However, reactions at material interfaces are poorly understood and performance may be highly dependent on the degree of mixing. In this work, we use vapor-deposited explosive multilayers as a model system to investigate shock interactions between different explosive materials with precisely controlled spacings. Samples consisted of alternating pentaerythritol tetranitrate (PETN) and hexanitrostilbene (HNS) layers, materials that have substantial differences in detonation velocity, with individual layer thicknesses in the vicinity of the critical thickness for detonation propagation of each material (100 -- 200 microns). Hydrocode simulations were employed to simulate detonation performance, using an Arrhenius reactive burn model that was parameterized from detonation velocity and failure data from each constituent material. The shape of the detonation front was determined experimentally by streak camera imaging of the breakout surface and compared with hydrocode simulation results. Differences between experimental and simulated results will be discussed in the context of the mechanisms dictating performance at these length scales.   

Detonation Wave Manipulation via Inert 3D Printed Open Cell Siloxane Lattices Backfilled with a Liquid Explosive

Additive manufacturing (AM) has generated recent interest in the field of energetic materials and shock physics by generating structural methods to manipulate the dynamic behavior of materials. X-ray phase contrast imaging studies of the shock response of Direct Ink Write AM siloxane lattices has shown that by slightly varying structure, while maintaining other geometric constraints (extrudate size, bulk density, etc\textellipsis ), the material response to a transiting shock can be manipulated from jet-like extrusion to a sinusoidal crush-up response, in comparison to the planar wave associated with stochastic foams. Previous studies of AM using high explosive (HE) feedstocks has explored the anisotropic sensitivity of strand structured explosives by using internal structure as guides for weak precursor shocks that can travel at rates (\textasciitilde 12 km/s), much faster than the bulk HE detonation velocity (\textasciitilde 8 km/s) leading to pre-shock desensitization. In this work, we investigate simple cubic and face centered tetragonal siloxane lattices backfilled with the liquid explosives nitromethane or trimethylolethane trinitrate using back-lit high speed imaging techniques. Using this technique, density gradients are easily programmed into inert printed structures, thus altering the local detonation wave velocity in the liquid explosives.