Search

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]

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]

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]

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]

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]

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]

Mesoscale simulations of a LiF impactor colliding with a PMMA capsule containing aluminum powder (ρ00 = 1.5 g/cc) have been performed to investigate shock-induced melting in porous metals. Impact velocities of 1-2.5 km/s are chosen to coincide with in situ X-ray diffraction experiments, which provide direct evidence of shock-induced melting in aluminum powders. Mesoscale simulations show shock heating within the powder is highly nonuniform and melting remains incomplete over hundreds of nanoseconds behind the shock front despite equilibrium pressure-temperature states from continuum simulations lying above the experimental melt line. Such incomplete melting behavior is consistent with X-ray diffraction data obtained in experiment. For an impact velocity of ∼1 km/s, mesoscale simulations predict re- solidification behind the shock front as high-temperature regions are cooled below the melt line. Reducing the grain size of the powder by a factor of two leads to a reduction in the time required to reach complete melt such that total melting of the powder may be observed experimentally for an impact velocity of 2.42 km/s. [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]

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]

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]

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]

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]

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]

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]

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]

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]

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]