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Continuum hot spot models for explosives now incorporate information on pore size distribution. However, there is a limited understanding of the participation of different sized pores in shock initiation scenarios. The objective of this simulation- based study is to determine the criticality of single hot spots in HMX as a function of circular pore diameter and shock pressure. Simulations are performed with the multi-physics hydrocode, ALE3D. A coupled thermochemical code provides the equation-of-states and the chemical kinetics. We also employ a strain, strain-rate, and pressure-dependent strength model. We fit the results of our pore collapse simulations to an ignition criticality equation: Rig(P) = (RmaxRmin)(P/P0)−a + Rmin. For the 2D plane strain case, Rmax = 2.5 µm, Rmin=0.25 µm, P0=5.0 GPa, and a=3.5. For the 2D axisymmetric case, Rmax=0.5 µm, Rmin=0.04 µm, P0=5.0 GPa, and a = 1. Non-reactive pore collapse simulations are also used to determine the ignition threshold but extracting definitive hot spot information from these simulations is complicated by their complex geometries and non-uniform temperature distributions. We found that using the mass of a hot spot with T > 2Tbulk appears to be a fair discriminator for ignition in complex hot spots. These studies are important because they enable upscaling grain-scale information to the continuum-scale as part of developing morphology-aware models. [ABSTRACT FROM AUTHOR]

We present computational studies of experiments that measure the impact shock response in energetic materials on nanosecond-micrometer scales using a multi-material, arbitrary Lagrangian-Eulerian code, ALE3D. In the experiments, a laser launches an aluminum flyer-plate that impacts the energetic material. After impacting the energetic material, the output shock is detected by the motion imparted to a glass window. Experiments are also performed without energetic materials to focus on the flyer-plate drive conditions. Numerical results demonstrate agreement with experimental data for the laser-driven acceleration of the aluminum flyer and the first ∼5 ns after the flyer impacts the glass window. Our simulations indicate that we have to account for back-surface curvature in the flyer produced during the launch to give agreement with the experiment. Preliminary results are presented with an energetic liquid, nitromethane, using an equation-of-state and chemical kinetics model generated from the thermo-chemical code, Cheetah. [ABSTRACT FROM AUTHOR]

Shocked Ce metal in contact with a reactive gas such as H2 or D2 produces a distribution of ejecta particles that react with the gas to form Ce hydrides or deuterides. We present an average particle reaction diffusion model to calculate particle and gas temperatures and reaction fractions. We compare model results with recent HE driven Ce experiments into reactive D2 and non-reactive He gases for a variety of initial gas pressures from 2–8 atmospheres at initial temperatures of 300 K. We find consistent agreement with radiance temperature measurements as a function of time using particle distributions from Mie scattering data resulting in Ce deuteride mass conversion fractions in D2 gas of order 10 – 20 %. [ABSTRACT FROM AUTHOR]

In this work, the shock-material interface behavior at very oblique angles is examined using shock polar analysis as well as full hydrodynamic simulations for two cases : (1) polymethylpentene driving polymethylmethacrylate and (2) polytetraflu-oroethylene driving polymethylmethacrylate while using Mie-Grüneisen equations of state. The hydrodynamic simulations were performed using a ghost fluid methodology which utilizes nominally fifth-order spatial and third-order temporal discretizations. The semi-analytical, steady-state, shock polar analysis and full-time dependent hydrodynamic simulations are compared with experimental images obtained using X-ray phase contrast imaging. These comparisons show great qualitative agreement with slight discrepancies present in the streamline flow deflection angles further away from the shock-material interface point. [ABSTRACT FROM AUTHOR]

A full multiphase description of the behavior of tin is proposed. It is based on a multiphase equation of state that feeds multiphase strength and damage modellings. A good agreement with a series of plate impact experiments is shown. [ABSTRACT FROM AUTHOR]

An analytical model describing void growth in a ductile viscoplastic material is proposed. It accounts for inertial effects and compressibility. Its good accuracy is studied by comparing its results to various hydrodynamic simulations. [ABSTRACT FROM AUTHOR]

We know that microstructure and density affect the shock initiation of an insensitive high explosive (IHE), such as PBX 9502 (95% TATB). We suspect that those factors also affect some performance and propagation metrics, especially curvature effects and the explosive's ability to turn corners. A recently developed hydrodynamic burn model, πSURF, provides insight and predictive capability for the initiation regime of shock response based partly on a statistical characterization of the microstructure. Here we examine the ability of the πSURF model to predict the effects of microstructural features on the propagation regime characteristics of curvature and corner turning of the IHE PBX 9502. Specifically, we explore the hypothesis that informing the model about the presence or absence of intra-granular porosity (the 'nano-scopic' voids, as opposed to the larger, inter-granular 'micro-scopic' voids), as revealed by a void size distribution, will predict the corner turning behavior observed in the Enhanced COrner Turning (ECOT) experiments for these two classes of PBX 9502. Indeed, we learned that the model does show the expected behavior. We also learned that the model smoothly predicts, without adjustment, both the initiation and propagation regimes of shock response (other models require mathematical or code switching between the regimes). [ABSTRACT FROM AUTHOR]

In order to determine the dominant mechanism underlying the effect of preshock desensitization in heterogeneous explosives, two-dimensional, meso-resolved simulations are performed to capture the dynamics of double-shock initiation in mixtures of liquid nitromethane with air-filled cavities. Without invoking any phenomenological reaction models to account for the meso-scale effects, temperature-dependent Arrhenius chemical kinetics and a statistically significant amount of heterogeneities are explicitly considered in these simulations. The desensitizing effect is captured in the simulations considering double-shock scenarios with various delay times between the two shocks. Single-shock scenarios with a post-shock state that is isentropic or isothermal to the double-shocked state are also simulated. For the selected set of parameters, the results suggest that a smaller increase in temperature behind double shocks is likely the dominant mechanism underlying the effect of preshock desensitization. [ABSTRACT FROM AUTHOR]

Heterogeneous polycrystalline microstructure in plastic bonded explosives (PBX) leads to thermal localization under dynamic impact. We describe a method to characterize the heterogeneity of these temperature fields as a function of the grain size. We performed a systematic study of how grain size affects temperature in model PBXs using finite element simulations and correlation techniques. The modeling framework combines a dislocation-based, anisotropic, single crystal plasticity model for RDX with a visco-elastic constitutive model for the estane binder. Our simulations used constant binder volume while the grain size was varied in the range of 25 to 200 microns. Smaller grains gave rise to lower average temperatures, with lower spatial correlation amplitudes. The microstructure of the preceding grains hit by the shock wave are observed to affect the later shock wave evolution itself and thereby the temperature of the ensuing grains. Fast Fourier Transform analysis of the spatial correlation revealed this effect in each of the grain sizes tested. High thermal localization in all grain sizes revealed a grain size length-scale effect. The spatial correlation between temperatures in the shock direction decreased, while the temperatures in the transverse direction retained their strong correlations. [ABSTRACT FROM AUTHOR]

The localization of shear strain into narrow bands is a common failure mechanism under dynamic loading, and we present an approach to capture this behavior in a numerical framework. The approach employs a mixture theory for embedding the band material, and thus avoids the daunting task of computational discretization of the band's geometric features that are often on the scale of micrometers. An iterative solver for traction balance is used in the mixture theory, and this strategy for treating material interfaces has advantages across a wider class of problems. The traction balance methodology solves the appropriate compatibility and stress equilibrium conditions between the shear band and the bulk material within a given computational zone. The propensity for strain localization thus affects the macroscopic behavior of the zone without the need to fully discretize the shear band. For general loading scenarios, the code must be able to detect elements in which shear bands might form and the orientation of such bands. A novel shear band insertion approach has been developed for this task. Example applications of the traction balance methodology coupled with shear band insertion are shown. [ABSTRACT FROM AUTHOR]

The aquarium test provides optical data on important aspects of detonation performance, namely the detonation velocity, shock wave shape in the surrounding water, and expansion rate of the condensed phase explosive products. It is commonly used for calibrating reaction rate laws and products equations of state (EOS). An important aspect, with regards to the analysis, is an adequate representation of the confining material to avoid inaccuracies. We conduct a series of two-dimensional axisymmetric reactive flow simulations for an ANFO-PMMA-water system using the Ghost Fluid Method. The goal is to evaluate the results obtained using three different EOSs for water: Tait, Murnaghan, and Tillotson. The numerical calculations are compared to large-scale aquarium experiments where the HE-water interface and the water shock front locations are directly measured from time-resolved image data. [ABSTRACT FROM AUTHOR]

When confining metal is accelerated by a grazing detonation it launches outwards at an angle relative to its initial orientation. Consequently, the metal velocity vector contains both lateral and longitudinal components. Since photonic Doppler velocimetry (PDV) can only resolve the component of velocity aligned with the collimated beam, recorded velocities must be tilt-corrected to obtain true wall velocity and casing shape. Correction is important for fitting detonation product equations of state. For standardized configurations like the cylinder test, the tilt angle is small (less than 15°). The error involved in approximate corrections for these configurations is thus negligible. However, if very thin flyers are launched, the tilt angles can approach 50°, making the correction large. In the present study we consider tilt-correction of PDV histories from explosively driven flyer experiments where large tilt angles were observed. We adapted the vector decomposition from Taylor's tubular bomb model and used a Galilean transformation to account for translation of the steady, detonation-attached wall motion past a stationary, laterally observing PDV probe. This approximate correction was compared to a more exact correction using two misaligned PDV probes to determine wall shape without any assumed relationship between wall velocity and wall angle. The Taylor approximation resulted in significant errors at large tilt angles. However, this error can be nearly eliminated by inclining the single probe. [ABSTRACT FROM AUTHOR]

Using a Split Hopkinson Tension Bar (SHTB) to test ductile materials with high strain to failure (of the order of ∼50%), poses some challenges. 1) Interpretation of SHTB tests is not straight forward due to neck formation, which causes non-uniform stress and strain distributions along the specimen. 2) Neck location varies with specimen geometry and loading conditions, and it is not clear if this may influence the strain to failure. 3) To cause failure of a long specimen requires a long loading pulse, and this may be practically limited by the maximum striker's length possible for a given system. We address the latter problem by using a technique that practically doubles the duration of the loading pulse without changing the striker's length. We address the latter problem by using a technique that practically doubles the duration of the loading pulse without changing the striker's length. We address problems 1 and 2 by using full numerical simulations (including the striker, the bars and the specimen) to predict the test results. In this way we are able to calibrate the strength model, taking into account necking, neck location and plastic heating. [ABSTRACT FROM AUTHOR]

We describe a hydrodynamic model of the flow field created by pyrotechnic explosions. It is based on a 3-phase version of our AMR code. It contains the following elements: (i) a gas-dynamic model of the expansion and mixing in the fireball, (ii) a Discrete Lagrangian Particles model of the burning projectiles, and (iii) a heterogeneous continuum model of the particle wakes. The 3 sets of conservation laws are coupled through drag and heat transfer with the gas; also, the projectiles loose mass, creating a mass source for the wakes. Adaptive Mesh Refinement: AMR (Bell et al., 1996) is used to capture turbulent mixing and combustion on the grid. The grid was initialized with similarity solutions for: (i) the detonation products (Kuhl 2015) and (ii) the particles (Stanyukovich 1960)—thereby defining the particular pyrotechnic charge configuration to be studied. The 3 sets of hyperbolic conservation laws were integrated with our high-order Godunov schemes (Bell et al. 1989). Results of the numerical simulations are compared with data from pyrotechnic explosion tests. Most striking is the bright turbulent combustion structures in the fireball and the bright streamers formed by combustion of the DLP-wake systems. [ABSTRACT FROM AUTHOR]

A methodology was developed to predict pressure and temperature regimes achieved during detonation of RDX/TNT compositions with inert granular inclusions. The predicted pressures and temperatures are used as inputs for thermochemical simulations to design detonation synthesis experiments that utilize shock-induced chemical reactions to produce ceramic nanomaterials. This study computationally assessed the effects of inert spherical sand inclusions and porosity produced by inert additives on the sensitivity of the explosive composition during the shock-to-detonation transition using a limited scope approach through Lee-Tarver ignition and growth modeling. On the continuum scale, the effects of inert additives on pressure generation behind the detonation wave and within the reaction zone were parameterized through numerical modeling using Jones-Wilkins-Lee equations of state coupled with programmed burn and ignition and growth burn models. The developed model predicted convergent shock loading within the sand inclusions producing localized pressures as great as 135 GPa with associated superheating to temperatures greater than 5000 Kelvin. These results imply that phase formation in an inert inclusion will depend on the location of the material relative to the point of shock convergence where pressures and temperatures can far exceed the Chapman-Jouguet values for the explosive matrix used in the tests. [ABSTRACT FROM AUTHOR]

Polymethyl methacrylate (PMMA) is a transparent thermoplastic often used in shock compression science. Previous proceedings (AIP Conf. Proc. 845, 131 (2006)) described plate impact experiments in which PMMA was reshocked to pressures of up to ∼130 GPa, well above that at which it decomposes on its principal Hugoniot. Because some of the conclusions of the original analysis were surprising, the results were reanalyzed in a later proceeding (ibid. 1426, 771 (2012)). In addition to our own reanalysis of the data, we performed hydrodynamic simulations of the experiments based on a new equation of state for PMMA shocked above its threshold for decomposition. [ABSTRACT FROM AUTHOR]

We investigate the problem of diffusion effects near discontinuities in TNT explosions. The model consists of gas phase conservation laws (i.e., the compressible Navier-Stokes equations), coupled with a heterogeneous continuum model for the carbon particle phase of the detonation products. The problem is assumed to be point symmetric, so 1D spherical coordinates are used. The hyperbolic terms are integrated with a 2nd-order Godunov scheme, while the viscous terms are advanced by a 2nd-order Runge-Kutta method. Adaptive Mesh Refinement is used to resolve steep gradients in the flow. A tabular EOS, based on equilibrium thermodynamics, is used for computational efficiency. Three converged solutions were found: inviscid, viscous and two-phase. The blast wave solution was self-similar (i.e., it scales with the cube root of the charge mass), however, species concentrations near the DP-Air interface and peak temperatures were smeared by molecular diffusion effects, and the shock front was smeared due to viscous effects. Similarity solutions for the latter show that diffusion effects scale with the appropriately-defined similarity functions that are related to the Peclet and Reynolds numbers of this problem. [ABSTRACT FROM AUTHOR]

Advances in computational power present an opportunity to further optimize the design of an engineered energetic system. This work presents the application of a proposed optimization scheme which combines the shock-physics hydrocode CTH with the DAKOTA optimization package to automate shaped-charge jet design. The formation of an explosively driven hypervelocity jet can be simulated using CTH. The jet can be characterized by selecting the slope of the jet's kinetic energy profile. Then using parametric and genetic optimization algorithms, the objective function can be optimized. By parameterizing the initial liner shape and thickness, many iterations can be executed to establish the surface of the basis function. Once an optimization algorithm is selected, DAKOTA automatically iterates on the parameters controlling the charge's liner to find any local extrema of this surface. A converged solution is presented herein. [ABSTRACT FROM AUTHOR]

The shock initiation of heterogeneous solid explosives, such as binderized PETN-based explosives, is governed by mesoscopic processes. However, there is a scarcity of direct measurements probing these phenomenon so we employ modeling to improve our understanding of them. Here we perform grain-scale simulations in the multi-physics code, ALE3D, to investigate the shock response of explicitly resolved energetic grains and pores following planar impact. Non-reactive simulations are used to calculate an unreacted equation of state (EOS) for the heterogeneous explosive and constituent ingredients. These calculated EOSs are compared to historic models. The effects of pore size and distribution on the non-uniform temperature and pressure fields within the shocked material is investigated. These studies are important because they provide insight on mesoscopic processes and form the basis for new morphology aware explosive models. [ABSTRACT FROM AUTHOR]

It is widely accepted that the shock initiation of explosive materials is largely dependent upon the localization of energy at material defects commonly referred to as "hot-spots". Many studies have investigated the effect of size and distribution of the shock-induced collapse of voids within explosive materials. However, there is limited understanding of the effect of void shape on the collapse process and subsequent energy localization. In this work, complex Fourier descriptors were used to characterize a series of 2-dimensional void structures imaged via scanning electron microscopy of pressed HMX material. The shape information of the void structures was modified by a series of decreasing low-pass filters to yield increasingly "smoothed" void structures. The shock-induced collapse of these modified void structures was then simulated to investigate the effect of shape resolution. This methodology allows for the evaluation of spatial resolution needed to "adequately" characterize a void under shock loading. [ABSTRACT FROM AUTHOR]

Shock induced Richtmyer-Meshkov instabilities at perturbed metal vacuum/gas interfaces result in metal material ejecta. For strong shock waves, the material ejected is initially in the form of fluid sheets when the machining grooves are two-dimensional. These sheets ultimately break up to form a distribution of droplets. Large-scale non-equilibrium molecular dynamics simulations of Richtmyer-Meshkov instabilities allow the investigation of the dynamics of the breakup process but are limited in length and time scales by rarefaction wave reflections at the boundaries, which put the material behind the perturbed interface under tension eventually leading to spall. A time-dependent boundary condition based on the self-similar character of the release wave is shown to mitigate boundary reflections and reduce unwanted tensile waves behind the perturbed interface zone and thereby can be used to increase the wavelength and time scale for breakup simulations. We discuss the details of this method and results of NEMD simulations of a shocked Cu interface with a single mode perturbation characterized by a wavelength of λ = 13 nm and a wavenumber amplitude product kh0 = 1/2. [ABSTRACT FROM AUTHOR]

Designing materials with microstructural features like multiphase interfaces have shown significant promise for usage in the next generation defense and nuclear applications. The dynamic response of these interfaces in extreme environments is observed to vary with the deformability of individual phases, which could subsequently alter the favored damage nucleation sites related to spall failure. This study investigates the role of spacing of interfaces in a nanocrystalline layered Aluminum-Silicon system on the shock wave propagation behavior, and microstructural and defect evolution using classical molecular dynamics simulations. The simulations are carried out with different hypothetical Al/Si microstructures including variations in the distribution of Si grains as layers in a nanocrystalline Al matrix. The molecular dynamics simulations suggest that spall failure is preferentially initiated at Al/Si interfaces. In addition, for the same system volume and same concentration of Si, a higher number of layers is marked by an increase in shock wave velocity and reduced resistance to spall failure. [ABSTRACT FROM AUTHOR]

Although the high-pressure phase diagram of carbon at extreme temperatures and pressures is in focus of theoretical and experimental dynamic compression studies, there still exist outstanding problems including disagreement between theoretical predictions and experiments. Using first-principles molecular dynamics simulations at high temperatures and pressures and employing large unit cells, we construct an accurate phase diagram of carbon using two-phase and Z-methods. In accord with previous simulations, a large positive slope of the melting line is observed for pressures from 0 to 200 GPa, whereas at pressures above 500 GPa a very small negative slope exists, which is in contrast to most of previous simulations and experiment. Our accurate results demonstrate the necessity for future dynamic compression experiments to clarify behavior of carbon at extreme conditions including its melting line. [ABSTRACT FROM AUTHOR]

State of the art computer-aided synthesis planning models are naturally biased toward commonly reported chemical reactions, thus reducing the usefulness of those models for the unusual chemistry relevant to shock physics. To address this problem, a neural network was trained to recognize reaction template applicability for small organic molecules to supplement the rare reaction examples of relevance to energetic materials. The training data for the neural network was generated by brute force determination of template subgraph matching for product molecules from a database of reactions in U.S. patent literature. This data generation strategy successfully augmented the information about template applicability for rare reaction mechanisms in the reaction database. The increased ability to recognize rare reaction templates was demonstrated for reaction templates of interest for energetic material synthesis such as heterocycle ring formation. [ABSTRACT FROM AUTHOR]

Material response at front face of target rear plate in Pressure-Shear Plate Impact (PSPI) experiments has been determined directly from measured velocity-time profiles at traction-free rear face of the target plate. Conceptual advance is the recognition that the usual forward problem for a mixed initial and boundary value problem can be reformulated as an initial value problem by a change of independent variables. For this reformulation the governing system of first-order, hyperbolic partial differential equations have been solved by a second order accurate characteristics method. While applications have been made to PSPI experiments, the approach applies equally well to the more common case of normal impact. The new methodology requires an accurate constitutive model for the rear plate of the target assembly. With such a model, the inverse problem approach provides a convenient means for extending PSPI experiments into higher impact regimes where the rear plate is no longer the hard, elastic material that was envisioned in earlier PSPI experiments. Results are presented for cases where the sandwiching plates are made of tungsten carbide. [ABSTRACT FROM AUTHOR]

We describe construction of a new multiphase equation of state (EOS) for beryllium calibrated almost entirely to density functional theory calculations based on the AM05 exchange-correlation functional. The EOS is in tabular form (as SESAME 92025) and includes hcp, bcc, and liquid/plasma phases. We find hcp→bcc transitions at ∼325 GPa at zero and room temperatures, and at ∼150 GPa on the Hugoniot. Shock melting from the bcc phase proceeds at 205-230 GPa. We also present experimental results for the isentropic bulk modulus as a function of temperature, based on resonant ultrasound spectroscopy measurements. [ABSTRACT FROM AUTHOR]

We present a new Eulerian multi-material hydrodynamics code to simulate the mechanics of impact crater formation. It is based on the open source astrophysical radiation magnetohydrodynamics with adaptive mesh refinement code ATHENA++. The hydrodynamics method is a directionally unsplit, high-resolution shock capturing Godunov scheme. We have developed and implemented a new multi-material capability into ATHENA++, where the evolution of several materials (gas and/or solids) is modeled in a fluid approximation with a set of conservative equations coupled to the basic hydrodynamic equations. Each material is governed by a separate equation of state (EOS) where we assume pressure equilibrium closure for mixed cells. The current implementation includes analytic EOS, but an extension to tabular form is straightforward. In this paper, we describe the numerical method and apply it to early-time models of asteroid impacts into layers of water and granite. Each layer is modeled using a Mie-Gruneisen EOS based on experimental shock data. We focus the study on oblique shocks and compare simulation results with analytic solutions from shock polar analysis. We discuss the coupling to strength and fracture models with this code. [ABSTRACT FROM AUTHOR]

The delay times prior to the thermal runaway of the explosives erythritol tetranitrate (ETN), erythritol tetraazide (ETA), pentaerythritol tetranitrate (PETN), cyclotrimethylene trinitramine (RDX), and cyclotetramethylene tetranitramine (HMX) have been obtained using molecular dynamics simulations with a semi-empirical model for the interatomic forces. We find that the mean delay times for each of the explosives follow Arrhenius chemical kinetics despite the complexity of the underlying chemical reactions. The temperatures required for these explosives to transition to thermal runaway appears to be correlated with their impact sensitivities, in accord with observations of Wenograd in the early 1960s. [ABSTRACT FROM AUTHOR]

Predictive tools are often employed to determine detonation parameters of explosives prior to development. However, thermodynamic-hydrodynamic based theoretical codes seldom take into account particle size as a basis for the computational analysis as they primarily focus on the equation of state of the detonation products, heat of formation and density of the explosive composition. The microstructure effects on the detonation velocity of RDX containing compositions have previously been reported. It was demonstrated that compositions containing smaller average particle sizes of RDX generated higher detonation velocities. The study utilized regression analysis to model the relationship between the microstructure characteristics and detonation velocity of the experimental compositions of 73 wt. % solids and 27 wt. % polyurethane binder. The principal characteristics examined were the average particle size, impurity within the explosive particles, method of manufacture, and compositional density. Statistical analysis demonstrated the relevancy of the microstructure influence on the detonation velocity of the experimental compositions. The developed model underscored the significance of the relationship between the average particle size and detonation velocity. In this study, we have continued the investigation to include other nitramines, such as HMX. [ABSTRACT FROM AUTHOR]

Many experiments of interest, particularly those meant to probe flow strength response under dynamic conditions, involve reversals in the loading condition. Transients in the material response during these reversals can significantly influence experimental observables. In materials for which deformation is mediated by the motion of dislocations, aspects of the transients are thought to be associated with motion of dislocations during the transient; and the strains so produced are sometimes described as being anelastic. Appropriate formulations for anelastic response can be distinct from standard models used to capture plasticity response over larger monotonic strains. We present results from a new anelasticity formulation and its numerical implementation. In this formulation, the anelastic strain production is influenced by the characteristics of the dislocation network, with these characteristics evolving over the course of material deformation. [ABSTRACT FROM AUTHOR]

Solid materials behavior under ultra-high strain rate loading such as shock compression may involve various processes including plastic deformation, structural phase transformations, fracture, melting, etc., whose kinetics and coupling are complex functions of strain rate, initial conditions and microstructure. Here, we present molecular dynamics simulations of the dynamic response of single-crystal iron to either ramp or shock compression, at strain rates on the order of 109 s−1, where we focus on the coupling between plastic deformation and the bcc-hcp phase transition. Defect-free crystal at 50 K initial temperature was found to yield via twinning when compressed along the [001] direction. Then, the onset of the bcc-hcp transformation was shown to be tightly dependent on the plasticity history through a shear-stiffening effect, which in some conditions inhibits the nucleation of the hcp phase [N. Amadou et al., Phys. Rev. B 98, 024104, 2018]. Yet, changing the direction of load application versus crystal orientation, or introducing lattice defects lead to very different behavior, including dislocation-mediated plasticity. [ABSTRACT FROM AUTHOR]

Detonation wave propagation in TATB-based explosives such as LX-17 is important to measure to understand explosives' performance, but the details of shock interactions with surfaces and interfaces are often inherently three dimensional. They are therefore challenging to observe with traditional diagnostics which rely on point measurements (e.g., with photonic Doppler velocimetry) or two-dimensional imaging. Flash X-rays are uniquely well-suited to observation of detonation phenomena due to short (∼25 ns) exposure times and the fact that X-ray contrast is correlated with spatially localized explosive density. Here we report results obtained with a few-view X-ray computed tomography (CT) approach to observe an asymmetrically-initiated LX-17 cylinder. 3D characterization of shock wave evolution is enabled by advanced reconstruction algorithms and a fifteen source multi-energy 3D flash X-ray imaging system. [ABSTRACT FROM AUTHOR]

Voids in high explosives (HE) are the most effective way of producing sensitizing "hot spots", areas of shock-induced energy localization involved in detonation. Thus it can be important to know whether a charge is close to or at Theoretical Maximum Density (TMD). For example, PBX 9502 has a nominal density of 1.890 g/cc with a void fraction of 1.8%. When the sample density is increased to 1.907 g/cc, the void fraction decreases to 0.9%, producing a measurable effect on the sensitivity and detonation properties. Immersion density measurements have shown that density gradients of this amount or more (from tenths to a few percent) can appear across pressed HE parts. Immersion methods can measure bulk density with very high accuracy. However, to measure density variation in a part requires it to be cut into pieces (≥ 13 mm or 0.5"), which is time consuming and destructive. Thus the method can't be used to correlate observed gradients to performance, or applied to every part being made. A non-destructive imaging method such as x-ray computed tomography (CT) would be ideal to evaluate the density variation in intact parts with better spatial resolution. However, getting a quantitative measurement of such small variations in density is challenging for x-ray imaging. Artifacts due to beam attenuation and scattering can often obscure the desired features of interest in thick parts. In this work, we use x-ray CT to study a variety of pressed HE parts. We show that obtaining radiographs of materials of density bracketing that of the HE can be used to gain a quantitative measure of density. 1% changes in density between pressed HE parts can be resolved. We also show the successful use of a beam hardening algorithm to help correct one of the major artifacts, "cupping", that can obscure the radial density gradients within an HE part. During our scans of pressed HE parts we also observed macroscopic features such as areas of relatively higher density that may be inclusions of other material or undissolved material from the manufacturing process. CT methods such as dual energy to further improve the sensitivity of measuring density gradients and help identify these inclusions are presented. [ABSTRACT FROM AUTHOR]

Extrudable high explosive XTX-8004 was hand-loaded into custom-cast polydimethylsiloxane (PDMS) witness blocks in a cylindrical charge geometry. In the PDMS casting process a thin volume of polymer containing sparsely distributed tracer particles was added to allow direct observation of radial-drive velocities using ultra-high-speed explosive particle image velocimetry (ExPIV). The shock wave image framing technique (SWIFT), a laser-backlit derivative of focused shadowgraphy, simultaneously captured both detonation-front position along the HE charge length, as well as the temporal evolution of leading shock fronts propagating radially outward from the charge interface. The visualized axisymmetric and self-similar shock-front geometries were used to initialize a three-dimensional ray-tracing scheme that estimates the density-gradient-based optical distortion realized when observing the object plane through the curved shock fronts in PDMS. After applying a corrective mapping the particle positions can be compared across images, which provided a field measurement of particle velocity throughout the shocked PDMS. Novel aspects of data analysis will be discussed, along with preliminary results quantifying XTX-8004 drive into PDMS witness media. [ABSTRACT FROM AUTHOR]

The discrete nature of granular materials such as sands results in complex mechanical interactions such as non- affine deformations including slippage at grain contacts, force chain buckling and shear banding. Even until now, geomechanics community largely relies on triaxial testing as the basis for constitutive behavior at continuum scale. However, the continued evidence with the lack of suitable predictive models with reasonable number of parameters to capture phenomenological effects makes us believe that this problem is too complicated for any continuum-based approach. The pressing need is to incorporate mesoscale effects which are discrete and non-repeating in the numerical modeling, and hence FEM and X-ray CT imaging are explored concurrently. The conventional triaxial loading condition with actual morphology of particles, geometric contacts and arrangement of solid, void phases are modelled in this work. The comparison of results from simulations and experiments is demonstrated in addition to microstructural insights from strain localization. [ABSTRACT FROM AUTHOR]

The temperature, water vapor concentration, and pressure within a kilogram-scale high-explosive fireball are probed using a custom, tunable diode laser absorption spectroscopy setup housed in a ruggedized gauge. Explosive fireballs are generated by the detonation of 2.2 kg spherical charges of C-4 high explosive at one end of a partially enclosed concrete tunnel structure. The 0.3 m fixed path-length absorption gauge is placed at varying stand-off distances from the charge at 6.4 m, 3.9 m, and 2.4 m, over several tests, to show survivability, measurement quality, and a repeatability. Changing the explosive composition to a 2.2 kg aluminized charge resulted in an explosive fireball that caused heavy beam attenuation at a 2.4 m stand-off distance due to the presence of added condensed-phase material. Reliable temperature measurements in the aluminized charge fireball were not possible due to the low signal-to-noise ratio and distortion of the background signal. Numerical simulations of the explosion in the hallway structure are performed using the CRAFT computational fluid dynamics code. While the simulations demonstrate general agreement with the hydrostatic pressure features measured in the experiment, the model predicts temperatures significantly higher than the temperature sensitivity limit of the probed spectral band feature and exhibits fluctuations of temperature on the order of several hundred kelvin due to turbulence. [ABSTRACT FROM AUTHOR]

The temperature of shock-compressed z-cut quartz was determined at 99 and 93 GPa using a calibrated streaked spectral pyrometry system for use in high-pressure gas-gun experiments. The temperatures obtained are found to agree with the majority of previous literature values within 5%. Sensitivities to various error sources, including geometric corrections, response-measurement errors, and wavelength-assignment errors are discussed. The combined uncertainties due to these errors are estimated to be 0.7-1.1%, a substantial improvement over other methods which eliminates errors due to lower temperature calibration, and reinforces previous results at intermediate shock pressures. [ABSTRACT FROM AUTHOR]

We report on the results of experiments to constrain the size distributions of particles ejected from shocked cerium wafer surfaces into vacuum and various pressures of helium and deuterium by measuring the angular distribution of laser light scattered by the particles. These data are analyzed using the Mie solution to Maxwell's equations and assuming that the particle size distribution function is log normal thereby yielding a continuous record of the size distribution with time, or alternatively with ejected particle velocity, throughout the duration of each experiment. [ABSTRACT FROM AUTHOR]

The flow stress in a metal is dependent on a variety of factors such as strain, strain rate, microstructure, and temperature. Experiments (i.e. quasi-static tensile testing or Kolsky bar testing) with well characterized stress states have been used to determine the relation between flow stress and these many factors. However, for higher strain rates (>105/s) there is a dearth of high-fidelity data at high plastic strains. Here, we present results of a recent in-situ gas-gun experimental technique that can probe strength effects at strain rates of >105/s. By measuring the diameter of a long cylindrical hole using x-ray imaging in conjunction with back surface velocimetry while the sample is subjected to controlled dynamic loading, the factors affecting flow stress can be inferred. Materials with higher dynamic flow stresses tend to exhibit less diameter reduction than materials with lower flow stresses all else being equal. Typically, the hole size is measured through imaging, but if the hole is blocked, it becomes necessary to infer the amount of closure from another diagnostic like velocimetry. We present preliminary analysis and results on the velocimetry traces from a suite of hole closure experiments that indicates hole size information may be embedded in velocimetry traces. [ABSTRACT FROM AUTHOR]

Broadband Laser Ranging (BLR) is a recently developed instrument for measuring the position of rapidly moving surfaces. Two gas gun experiments were performed to characterize a 16-channel Broadband Laser Ranging system. The precision of the measured position data exceeded expectations and has revealed systematic errors of up to 12 microns. These errors appear to be caused by the interaction of the data analysis routines with the recording hardware's frequency-dependent signal response. [ABSTRACT FROM AUTHOR]

Simultaneous ultra-high-speed framing and electronic streak photography are applied to the working surface of cylindrical, detonator-scale explosives to quantitatively assess detonation-wave breakout kinematics. The framing camera captures the evolution of two-dimensional cross-sections of the detonation wave exiting a detonator output pellet, and the streak camera records one-dimensional, space-time breakout curvature with sub-nanosecond resolution. Both data sets are analyzed to assess the geometrical characteristics of the detonation wave. An existing and idealized model equation for hyperbolic breakout of cylindrical high-explosive charges has been parameterized for nominal detonator-explosive performance and used to initially inform the geometry of the detonation wave within a functioning detonator pellet. Accurate extraction of output pellet detonation velocity is attempted using the idealized model equation, and uniqueness results are discussed. [ABSTRACT FROM AUTHOR]

Charged particle radiography with 800 MeV protons has been used for decades at LANL and developed around the world to study dynamic material properties. Recently, charged particle radiography has been demonstrated with the use of high-energy electrons. Because of the difference in the mass of the electron compared to the mass of the proton, the radiographic processes are substantially different and well suited to the study of fast dynamic processes in relatively thin systems. This presentation will show the layout required for such measurements, along with data collected from this recent demonstration performed with 14 GeV electrons generated at the SLAC National Accelerator Laboratory. The radiographic performance for flash measurements will be presented, along with the limitations of this measurement technique. [ABSTRACT FROM AUTHOR]

Vibrational spectroscopy allows identification of molecules with very high specificity. It is therefore often applied for the measurement of species under shock compression, especially when chemical reactions are likely such as in energetic materials. There are unique complications for these vibrational spectroscopic methods in the applications to shock-compressed materials, which are the subjects of this paper. The complications illustrated and discussed here include band-broadening mechanisms, thin film interference effects, signal integration along a path through shocked and unshocked layers, convolutions of signal path integration with reaction rates, and the non-resonant background in CARS. [ABSTRACT FROM AUTHOR]

Microwave interferometry (MI) provides several advantages over more traditional shock and deflagration front diagnostics. Most importantly, it directly interrogates these fronts, instead of measuring the evolution of containment surfaces or light from detonation breakout. The copper cylinders commonly employed in cylinder test experiments (CYLEX) act as microwave cavities or waveguide structures. Experimental geometries with large dimensions (relative to the ∼cm-scale MI wavelength) result in artifacts in the MI signal due to higher-order modes propagating in the explosive/metal system. We have developed a microwave coupling design to suppress higher order modes present in 1" diameter cylinder tests of high explosives. We demonstrate the effectiveness of this structure and show sub-microsecond scale microwave tracking of detonation front velocity in cylinder tests with a variety of explosives diameters, materials, and MI frequencies. These results illustrate the importance of selecting appropriate microwave frequencies and coupling for specific experimental geometries. [ABSTRACT FROM AUTHOR]

Anisotropy of single crystals plays an integral role in meso-scale behavior of materials. In energetic materials, the anisotropy of thermal conductivity and elasticity plays a key role in hot spot generation during dynamic loading, potentially leading to deflagration or detonation. Precise measurements are needed to validate predictive models of these materials during accident scenarios. Toward these goals, we are investigating single crystal orthorhombic 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) using impulsive stimulated thermal scattering (ISTS). Here we present results of the experimentally determined thermal diffusivity and its comparison with the anisotropic values predicted by atomistic simulations, as well as experimentally measured anisotropic acoustic velocities. [ABSTRACT FROM AUTHOR]

The ability to probe the time-dependent hot spot formation processes in explosives under shock compression is critical to understanding the initiation of explosives used in munitions and nuclear weapons. We have developed a tabletop apparatus that allows us to impart shocks on a sample to study the relationship between hot spot formation and shock initiation. By using laser-driven impactors, we impart shocks lasting several ns and compress sample targets up to 10s of GPa. Using multi-frame fast photography with ns temporal resolution we visually track thermal emission and its evolution over time. Additionally, we measure the time dependent temperatures using a multichannel optical pyrometer coupled to a microscope objective that can resolve features down to a few microns. The sample targets in this study consist of miniature arrays of explosive crystals embedded in various polymers. To date we have produced time-resolved images and simultaneous temperature measurements of shock-induced hot spots in a model plastic-bonded explosive. [ABSTRACT FROM AUTHOR]

Transverse surface velocity measurements are fundamental to performing pressure-shear plate impact experiments. Recent works have utilized slight variations of traditional velocimetry techniques that measure apparent velocity from a combination of fiber optic probes aligned at non-zero angles relative to a surface normal. Various methods involving active and/or passive angled probes are proposed. This work compares the uncertainty in each approach using a multi-component velocity test case and explores the influence of parameters such as probe angle relative to the surface normal, impact angle, and measurement uncertainty of the velocimetry system. Recommendations for optimal configuration are presented. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525. [ABSTRACT FROM AUTHOR]

Photon Doppler velocimetry has been used increasingly to measure transverse velocity in dynamic experiments. This work presents results of an oblique impact experiment which was performed using a slotted-barrel gas gun to generate normal and shear stress waves resulting in longitudinal and transverse velocity. Multiple transverse PDV diagnostic configurations were fielded on a single test enabling the results to be directly compared. Results indicate variability between the velocimetry results due to varying levels of measurement uncertainty, and illustrate best practices aiming to further advance transverse velocimetry measurements. [ABSTRACT FROM AUTHOR]

Linear shaped charges (LSC) are typically manufactured in continuous lengths and formed into an inverted "V". They use explosive force to cut through a target with a straight blade, usually in the demolition industry, but there is also significant interest in cutting a circle with the LSC formation for military and breaching applications. While some curved LSCs do exist, there are limitations for the curve due to the manufacturing process; additionally depth of penetration is reduced as the blade is formed at an angle due to varying inside and outside dimensions of the LSC. The run-up/run-down effect that is prevalent with the use of LSCs also poses as an obstacle towards cutting a full circle, as the optimal penetration depth is not reached in the run-up/run-down areas. Additive manufacturing allows for geometric complexity not possible in other manufacturing techniques. In this work, selective laser melting with a Renishaw 250 system was utilized. Using additive manufacturing, two separate design challenges were addressed; reducing the amount of run-up that occurs, and producing curved penetration with the LSC blade. LSC performance was evaluated by the depth of penetration, reduction of the amount of run-up that occurs, and curvature in the cut compared to traditional liners. The aim of this work is to show the potential for reducing the amount of run-up and curving the blade of a LSC through additive manufacturing of LSC liners. [ABSTRACT FROM AUTHOR]

Concurrent temperature measurement in a Pressure-Shear Plate Impact (PSPI) experiment has been developed by collecting the infrared radiation emitted from the sample/rear-plate interface. In order to convert the measured emittance to sample temperature accurately, the dynamic emissivity of the sample/rear-plate interface is required. The initial emissivity is calibrated statically by comparing the collected emittance from the sample surface with that of a surface with known emissivity. In a PSPI experiment the effective emissivity may change due to pressure-shear loading. At thermodynamic equilibrium, the dynamic change of emissivity could be indicated through the change of reflectivity. Pilot experiments are conducted to gain a qualitative understanding of, if and by how much, the emissivity changes during a PSPI experiment. A CO2 laser is used to reflect an incident beam off the sample/rear-plate interface. The intensity of this reflected beam is monitored by a fast HgCdTe detector through a pair of 90° off-axis parabolic reflectors. This measured change of intensity during a PSPI experiment provides an indication of the required change of emissivity. [ABSTRACT FROM AUTHOR]