Session G00: Poster Session (5:30pm-7:30pm PDT)

An Engineering Approach to Increasing Proton Radiography Sensitivity to Material Shock Front Detection

The proton radiography facility at the Los Alamos Neutron Science Center (pRad) is used to probe shocked materials from ~1 g cm-2 to 50 g cm-2 with temporal resolution down to 100 ns. Its sensitivity is adjusted by installing a collimator specific to the areal density range under interrogation. This presents a problem because collimators change often, and such changes are labor intensive and time consuming. This work has seen the development of a push-button actuated collimator capable of remotely installing available collimator settings within seconds. The suite of collimators imposes an angular acceptance of 5, 7.5, 10 and 20 mrad, and inverse (dark field) collimators, which block the beam not scattered, with an angular acceptance of 2, 3, 4 and 5 mrad. The addition of the dark field collimation settings, combined with an upstream pre-collimator, allows for the investigation and implementation of the Dark Field Proton Radiographic technique, which has been demonstrated to increase sensitivity to areal density changes by a factor of two, and visualize thinner materials. This system will be implemented at the LANSCE proton radiography facility in 2022 to validate the forward model of the dark field concept and visualize shock fronts and the turbulent mixing regions in noble gases.   

Optical return study for common surface preparations used in Photon Doppler Velocimetry

We present detailed measurements of the bidirectional reflectance distribution function (BRDF) for common surface preparations and materials used in Photon Doppler Velocimetry (PDV), i.e. four different surface preparations (Scotch-Brite hand and drill roughened, ball-rolled, and grit blasted), and four different materials (copper, aluminum, stainless steel, and tantalum). These measurements employ a conventional PDV probe and obtain diffraction limited angular resolution with 10 pW accuracy and 2.6% repeatability. In addition to scattering data, we employ scattering theory and simulations to accurately emulate the measured data. We also present a straightforward method to derive the average scattering distribution from surface profilometry and note several qualitative aspects of the scattering data which may help to optimize PDV signals.   

Phase investigation in Sn with plate impact and isentropic compression experiments with preheating devices.

For nearly two decades, the CEA Gramat operates several gas guns for shock loading and high pulsed power (HPP) drivers dedicated to Isentropic Compression Experiments (ICE) up to several GPa. These experimental devices associated with diagnostics (velocimetry and temperature measurements) help to collect new experimental data to study kinetics under dynamic transition in a more rigorous manner verified on various compression paths and contribute to constrain equation of state (EOS) models. Loading metallic materials from various non ambient initial temperatures can significantly extend the range of our studies into previously unexplored thermodynamic paths.

Several results on shock loading at various initial temperatures and on isentropic compression on the new HPP drivers ICE16 contribute to explore the phase diagram of Tin and the influence of the structure with the use of single crystals and polycrystalline Tin samples.   

Uncertainty analysis of data collected using embedded magnetic particle velocity gauges

The uncertainty in particle velocity data collected using embedded magnetic gauges is presented, as is a discussion on the uncertainties in the position and rise time of each element within the gauge package. When embedded within an explosive gas gun target, magnetic particle velocity gauges can be used to record the shock propagation through the explosive and the point of turnover to detonation, as well as the shock particle velocity and subsequent reactive growth history at discrete points throughout the target. An attempt was made at an analysis of the uncertainty for each of these data types using experimental data for embedded gauges within PCTFE and CompB targets. The significant sources of uncertainty affecting the rise time, positional error and particle velocity are identified and initial thoughts on how to consider these uncertainties when analysing data are presented.

UK Ministry of Defence © Crown Owned Copyright 2022/AWE   

Optical Response and Hugoniot State of Shock-Compressed Heavy Liquids

The shock response of heavy liquids was tested in a gas gun to determine index of refraction and Hugoniot equation of state data. Perfluorooctane (PFO), as well as deuterium oxide (D₂O) solutions of sodium polytungstate (SPT) and lithium heteropolytungstate (LST), were confined in a cell with a fused-silica window and subjected to one-dimensional compression from a copper impactor and driver. Photonic Doppler Velocimetry (PDV) was used to measure shock and particle velocity through the liquid cell, and verify optical transparency at pressures between 6.1 and 10.6 GPa. SPT and LST are adjustable density solutions up to approximately 3 g/cm³ and have low toxicity, which allows them to be used as impedance-matched window materials for equation of state experiments. Furthermore, these heavy liquids are good candidates for tamped Richtmyer–Meshkov Instability (RMI) experiments to interrogate dynamic interface instabilities at non-zero pressure. We will present initial radiography measurements and PDV results from RMI experiments in which PFO is used as a tamper.   

Development of a Two-frame Holographic Imager for Shock Physics Research

Here we introduce a novel multi-frame off-axis digital holographic velocimetry diagnostic for measuring shock front topography, reflectivity, and velocity all spatially resolved in dimensions transverse to the probe beam. The probe laser duration sets the time resolution of reflectivity and topography data, while that of the velocity is fixed by a choice of etalon. In this poster, we will describe the optical layout of the diagnostic, processing techniques, and design considerations for application to shock physics research. We also demonstrate that numerical parametric lenses can remedy wavefront errors in the optical system and that images can be numerically refocused post-shot, vastly improving the depth of field. Finally, we describe a proposed experiment relying on the diagnostic.   

Transverse Velocity for Angled Probes; Estimation, Uncertainty Quantification, and Sensitivity Analysis

Consider a surface being measured by Photon Doppler Velocimetry (PDV) and Broadband Laser Ranging (BLR) using a multiplexed PDV+BLR probe. The velocity measurements from PDV and the displacement measurements from BLR can be naïvely compared through differentiation or integration, although only under strict geometric and physical circumstances are these comparisons expected to agree (all surface motion colinear with the probe axis). When the probe axis is not colinear with surface motion, transverse velocity (motion perpendicular to the probe axis) becomes non-zero. We present a dynamic surface reconstruction approach to estimation of transverse velocity (velocity parallel to the plane of the initial coupon) made possible with axis-symmetric assumptions and geometric considerations of the probe-axis/surface interaction. Distinction between transverse velocity and velocity components perpendicular to initial coupon surface is relevant because of the use of diagonal (non-orthogonal) probes. We show that diagonal probes lower uncertainty estimates in the transverse velocity by at least an order of magnitude compared to orthogonal probes. This work was done by Mission Support and Test Services, LLC, under Contract No. DE-NA0003624 with the U.S. Department of Energy.   

On calculating shock velocities by the overtake method behind splitting waves

Overtake method is commonly used in impact experiments to determine and calibrate the equation of state outside the principle Hugoniot. The method is based on measuring the time of arrival of either consecutive shock waves or rarefaction waves from an impactor at targets of different thickness. In some cases, the leading shock wave that created at the impact splits into two waves (e.g. elastic precursor or due to phase transition), so the speed of the second wave cannot be determined by the overtake method. In this work we investigate the difficulties that arise from such wave splitting, and specifically show that additional target widths do not add any new information that can be used when calculating wave velocities. We look at special cases where the speed of the second wave may nevertheless be approximated, albeit with lesser precision, and identify the main error sources. 1-dimensional simulations were done to support our calculations to good agreement. The present work may act as a basis for additional calculations that will expand our knowledge of equations of state in uncharted areas.

    

Comparison between x-ray free electron laser phase contrast imaging and hydrodynamical simulations of void collapse in PETN single crystals

Much is still unknown about how detonation in high explosives initiates from insults such as shock, especially when the energy from the shock is lower than the energy barrier of the chemical reaction. The current hypothesis is that void collapse or other heterogeneity interactions with the shock form hot spots that initiate the chemical reaction needed to sustain shock wave propagation. Single-shot X-ray Free Electron Laser (XFEL) phase contrast imaging (PCI) was performed at the Matter in Extreme Conditions (MEC) Instrument at the Linac Coherent Light Source (LCLS). Pentaerythritol tetranitrate (PETN) high explosive single crystals with laser milled 10 micron voids were shocked with 5 GPa and probed with 40 fs, 5.822 keV X-ray pulse from the LCLS. Here, we compare hydrocode simulations of the experiment to PCI detector images. The hydrocode simulation density is converted to index of refraction and then summed along the optical axis into a 2D exit surface wave, and computationally propagated to the detector. The results of this proof of principle experiment represent the highest resolution imaging of shocked high explosives and will improve our understanding of hot spot formation in high explosives. 

    

Development of a confinement system for single-stage gun experiments

Over the years, there has been significant effort in coupling impact system to light sources that include the Los Alamos Neutron Science Center (LANSCE) and the Advanced Photon Source (APS) to use diagnostics such as proton radiography or X-ray imaging and diffraction to study matter at extremes.  The ability to confine or contain the impact event would allow the study of hazardous materials including explosives, radiological, and pyrophoric materials, for example.  The current work highlights the development of a confinement system designed specifically for use on single stage guns for impact experiments.  A description of this capability, developed for small bore guns (~12 mm bore), will be presented along with recent experimental results.  Further work to scale this to larger bore or higher performance gun systems will also be discussed.    

Development and modeling for a small-scale, rapid heated explosives experiment

Most small-scale tests of explosive sensitivity, the drop-weight test in particular, convolute so many complex phenomena that it has been extremely challenging to decipher how an explosive ignites and propagates reactions.  For instance, an impact generates heat through a range of dissipation mechanisms, which can in turn, depending on the reaction rates of the explosive, lead to chemical decomposition.  In order to deconvolute the various contributions to the sub-shock initiation and propagation of explosive reactions, we will discuss the development and modeling of the High Explosives Initiation Time (HEIT) test - a new, small-scale, high throughput experiment designed to rapidly heat small quantities of energetic materials within small diameter steel needles. Specifically, we have designed and modeled a 250 Joule pulsed power system capable of rapidly delivering electrical current to the needles, resulting in rapid heat delivery to the sample. The energy rate deposition into the needle is controlled by different transmission line topologies. Modeling in COMSOL is performed to understand the energy required to heat up the explosive sample, and the electrical current is modeled as a decaying sinusoid.

    

Extending imaging VISARs into 3D with digital holography and phase retrieval

Velocity Interferometer Systems for Any Reflector (VISAR) systems can operate with fiber-coupled probes (non-imaging) or with spatially resolving image relays (imaging).  Imaging VISARs can resolve the motion temporally in one spatial dimension (as in line-imaging VISAR on a streak camera) or spatially resolved in two dimensions and time gated (as in two-dimensional VISAR).  Because the probe light is coherent it is possible to extend the imaging modality into the third spatial dimension using digital holographic techniques.  A holographic recording provides both amplitude and phase of the reflected beam, which can then be propagated numerically and interrogated for detailed analysis.  By combining a two-dimensional VISAR recording with a digital hologram we can resolve the velocity and spatial structure of reflecting systems throughout a three-dimensional volume down to the resolution limit of the imager.  We list several possible applications of this capability: (1) simultaneous determination of both the amplitude (surface topography) and velocity (motion of the topographic structure) of a corrugated reflecting shock front; (2) a similar determination of the corrugation structure and motion of a reflecting perturbed interface between two media after passage of a shock (Richtmyer-Meshkov); and, (3) simultaneous measurement of the surface topography and velocity of the free-surface of a material upon release.  In this talk, we will describe an extension to the two-dimensional VISAR diagnostic that provides the digital holographic capability and show initial benchtop experimental results. LLNL-ABS-832654   

Symmetrization Techniques for High Explosive Test Data

A significant fraction of high-explosive experiments involve the viewing of a nominally-axially-symmetric detonation wave breaking out of a charge surface. In such cases the emerging wave virtually always exhibits a degree of tilt. One generally wants to remove this while also quantifying its direction and magnitude. In some cases, such as front-curvature rate sticks and Onionskin (OS)-type tests, the diagnostic is a single-slit streak camera. In those instances, the tilt correction is 1D. In other cases, such as a Plane-Wave Lens characterization test or a Furball test, multiple slits or fibers provide data over parts of a surface. In those instances, the tilt correction is 2D. Our 2D technique represents the breakout time as the sum of a symmetric component and an asymmetric component (a tilted plane). The two tilt angle components are found that minimize the data scatter associated with the symmetric component. The most compelling example is the Furball test, an OS- variant for which the breakout time over the hemispherical observation surface is measured at many points using optical fibers. Here we are able to construct detonation breakout trace vs polar angle in the direction of maximum tilt, even though there are generally no fibers at that direction. This provides a distinct advantage over the traditional OS-test, for which the streak camera slit is randomly oriented with respect to the direction of maximum tilt (such that the probability of observing the worst-case tilt is very small).   

From 2D to 3D: Resolving Flow Fields Around Sand Penetrators Using Flash X-ray Imaging

The dynamic penetration of projectiles into brittle granular materials is important in a broad range of fields including planetary science and defense.  As a projectile impacts a granular material, various material mechanisms are activated, including grain fracture, plasticity, fragmentation, granular flow, and pore collapse.  Experimental visualization of these mechanisms is extremely difficult without disrupting the mechanisms at work; however, quantifying of the role of each mechanism in projectile behavior is essential for validation of constitutive laws.  In this poster we discuss a novel approach to visualizing the 3D flow fields inside a sand sample during penetration by using lead tracer particles that are embedded in matrix of sand.  The 3D positions of the lead particles are known prior to impact from X-ray computed tomography.  During impact, two orthogonally placed flash x-ray sources capture the displacement of the lead particles at a single instance in time.  By using a known position of particles both before and during penetration, a full 3D flow field can be obtained.  The 3D flow fields are investigated for both dry and fully-saturated Ottawa sand samples at different times during penetration of samples at 1.5 km/s.   

Speckle-based x-ray phase contrast imaging at free-electron lasers

X-ray free electron laser (XFEL)-based x-ray phase contrast imaging (XPCI) harnesses the highly-coherent, brilliant x-rays to probe into dynamic processes with femtosecond and submicron resolution. Optical elements, such as compound refractive lenses, are critically important for optimizing the imaging conditions specific to the experiment; however, introduced artefacts in the image cannot easily be eliminated with a simple flat field correction due to the stochastic nature and sample-induced distortion of the x-ray pulse. This makes both interpreting the image and retrieving the phase to extract quantitative information challenging. To address these challenges, we treated the image artefacts as a reference pattern and implemented a speckle-based approach to retrieve the phase. We demonstrate our approach on in-situ XPCI images of laser shock-induced void collapse in a polymer recorded at the Matter in Extreme Conditions (MEC) Instrument at the Linac Coherent Light Source (LCLS). We calculate the areal density from the recovered object phase, which informs our understanding and development of constitutive models of materials at extreme conditions.   

Multiscale Temperature Measurements in Shocked High Explosives Using Raman Thermometry

Raman thermometry (RT) provides an attractive route for measuring the dynamic temperatures of shocked materials given its high time resolution, optical coupling, and its exclusion of material-dependent parameters. In this work, we plan to couple two shock drive sources- laser driven flyer plates and gas gun-driven impactors- to RT for measuring in-situ shock temperatures in single crystalline materials. Initial proof-of-principle experiments with RT and laser driven flyer plates were conducted on single crystal quartz samples and will be followed by experiments on single crystals of 1,3,5,7-Tetranitro-1,3,5,7-tetrazocane (HMX). Implementing RT with laser driven flyer plates will enable benchtop temperature measurements on many micron-scale samples under multiple impact conditions with nanosecond time resolution. We will compare shocked temperatures from benchtop experiments to gas gun experiments which will extend our temperature measurements into longer time and length scales. Ultimately, we aim for these data to provide the vital, yet elusive, experimental temperatures needed to validate current reactive burn models and aid in creating next-generation models.   

Predicting Velocimetry Curves from Photonic Doppler Velocimetry (PDV) Signals using Neural Networks

Photonic Doppler Velocimetry (PDV) is a popular diagnostic technique for generating particle (or free surface) velocity profiles during shock compression experiments. However, the signal gathered by the probe requires post-processing using software that involves either human intervention for different settings (SIRHEN) or brute force averaging (HiFiPDV) to produce the final velocity profile. For experiments with multiple PDV probes, it becomes difficult to perform a consistent and rigorous analysis to identify and isolate the effects of material microstructure, especially for heterogeneous materials.

The goal of this work is to establish velocimetry curve consistency between multiple PDV probes through use of a Neural Network (NN) model, thereby reducing the uncertainties and errors associated with the current processing methods. The raw PDV signal is pre-processed and fed into the NN model with hidden layers, which outputs the predicted velocimetry curve. The work specifically utilizes LSTM (Long Short-Term Memory) Recurrent Neural Networks (RNNs). The method is expected to predict optimum outputs for different materials if it is fed a wide range of training data, because it learns from past inferences. The RNN model description and results obtained to date will be presented.   

Jointly Embedded Machine Learning Approach for Polymer Combustion Properties

High energy density polymeric binders are a class of polymer materials that can be used in lieu of inert binders in high energy density mixtures. By using higher energy binders, the overall internal energy of the mixture can be designed intentionally and proactively. In this presentation, we will showcase our recent efforts to develop a machine learning approach to learn, predict, and design novel energetic polymers. The scarcity of data available for energetic polymers is a particular challenge that we overcome through transfer learning techniques. Generally-speaking, transfer learning is a class of machine learning algorithm that assists the learning of general trends within one dataset using alternate datasets. In our approach, we use a feature transfer learning approach based on low-level physiochemical data that may be obtained for any molecule. We first train the model to learn the form of repeat unit structures using an open synthetic dataset containing 1 million polymer repeat units. Then, the model is trained on a dataset containing <170 polymer repeat units for which thermochemical properties are known. The model is then developed to perform generation functions and new polymers may be proposed with desirable attributes. The resulting machine-learned molecule property estimates are then compared with theoretical thermochemical models. Through the transfer learning approach, we will also address the importance of synthesizability and discuss how the proposed techniques can be used to increase the likelihood of developing synthesizable candidate polymer molecules.   

Learning continuum strength models for meso-scale simulations of HMX from molecular dynamics using deep neural networks

The fidelity of meso-scale void-collapse computations for predicting detonation initiation in energetic materials (EMs) hinges critically on the constitutive models employed in simulations. Isotropic rate-dependent plasticity models are popular for meso-scale simulations of void-collapse in EMs under shocks. However, for these models to be physically correct, they must capture molecular-scale physics. In this work, we develop an isotropic rate-dependent Johnson-Cook (JC) plasticity model that is informed by molecular dynamics (MD) simulations. The overall goal is to capture the shear band and hot-spot dynamics observed in MD. To learn the model parameters, the power of deep learning (DL) is leveraged to create a predictive environment for an HMX sample with a void experiencing shock loading. This environment is trained by an ensemble of void collapse simulations; these use an isotropic rate-dependent JC model with varying JC constants. The rapid predictive nature of DL is then used iteratively to identify the JC constants such that meso-scale computations replicate the dynamics of MD void-collapse. This procedure will result in a strength model that incorporates molecular physics and is robust for performing mesoscale computations using isotropic plasticity models.   

Effects of Microstructure and Binder on the Shock-to-Detonation Behavior of Hexanitrostilbene (HNS)

We developed a mesoscale shock-to-detonation model for HNS that predicts the effects of microstructure and binder on the ignition and growth processes. In this model, shock-wave energy is localized by voids and defects in the HNS, creating hotspots that release energy to support shock-wave growth to steady detonation. An Arrhenius reaction rate, based on the local temperature in the HNS, was calibrated using experimental threshold flyer-velocity data from exploding foil initiator (EFI) tests. We simulated flyers of various thicknesses impacting the HNS and obtained reasonably good agreement between the predicted and experimental threshold velocities. Moreover, we compared simulations of both fine-grained and coarse-grained HNS where the only difference between the simulations was the initial microstructure. For very thin flyers, fine-grained HNS was more sensitive than coarse-grained HNS. For thicker flyers, a cross-over in sensitivity was predicted, making coarse-grained HNS more sensitive. We also performed three-phase simulations of the same coarse-grained HNS with a small amount of binder added to the HNS and pore interfaces. These simulations enabled detailed predictions of the relative change in initiation sensitivity and threshold flyer-velocity due to the binder content.   

Connecting details of heterogeneous explosives reactive wave profiles to the hydrodynamic reaction rate

The future of energetic materials development, especially high-performance high explosives, demands new efficient and cost-effective experimental methods to understand the performance of new materials when only limited, development-scale quantities are available. We also need efficient methods to understand the effects on explosive performance that result from manufacturing variability, service environment, and aging effects. Here, we explore the quantitative relationship between characteristics of a reactive wave profile and the hydrodynamic reaction rate for use in model-based assessments. The wave profiles were recorded from a relatively efficient so-called ‘cut-back’ experimental method, using laser velocimetry for the diagnostic. The initial pressurization rate, or particle acceleration, behind the initial shock wave directly and quantitatively reflects the reaction rate for a given shock velocity-pressure. With this knowledge, and equations-of-state information, we can use relatively sparse data to determine the reaction rate over a pressure range useful to simulate and assess the shock initiation regime. We deem the approach useful for efficient evaluation of new explosive formulations and variations in existing formulations.   

Dependence of the blast on the shape of a high explosive charge

The high number of high explosive with very different compositions implies the need to compare them. That is the reason of the use of TNT equivalency. Even if this notion seems simple, it may lead to misuses, due to the physical differences between all compositions. The equivalent should be different for each effect, external blast, internal blast, ballistic capability, indentation, etc. For example, HMX is superior to TNT for external blast but inferior in internal blast.

Blast TNT equivalents are generally determined with overpressure gauges around an explosive charge. Even if a spherical shape ignited in its center is the best configuration to perform accurate measurements, the difficulty to machine a sphere lead often to use cylindrical charges. This charge is put on the ground surrounded by gauges at different distances on support at same height, leading to see the charge with different incidence. This setup is generally associated with the conclusion that TNT equivalent is a poor approach.

To assess the influence of the shape of the charge and of the incidence, 2D calculations have been performed with Ouranos hydrocode. Twenty numerical gauges were considered on each of 9 different directions, around cylinders of same mass and ratio length over diameter 1, 4, 10, 1/4 and 1/10 and compared to the pressure obtained for a sphere. The calculations show large differences on the time of arrival, the overpressure and the impulse. The difference for overpressure can be greater than a factor of 4. The influence of the ratio length over diameter, the incidence and the distance will be discussed.   

Designing a High-Throughput Drop-Weight-Impact Instrument for Real-Time Reaction Imaging of Energetic Materials

Characterizing the handling safety and sensitivity of explosives has been a challenging area of study for over 60 years. Historically one of the most accessible and widely utilized experiments has been the drop-weight impact test, which involves dropping a weight on a small sample sandwiched between two anvils. Because this experiment generally only utilizes sound thresholds to determine whether or not a sample reacted, the physical parameters governing sensitivity remain convolved. Better understanding of chemical and material characteristics is needed to give the chemistry and engineering communities a predictive tool to determine the handling sensitivity of explosives prior to pursuing expensive and potentially hazardous synthesis and formulation operations. We are developing a high-throughput drop tower instrument capable of imaging energetic material deformation during impact and the resulting thermal ignition and propagation events. Herein, we present key design features including a drop tower module and diagnostic impact chamber enabling high-speed visible and thermal imaging of explosive initiation by sub-shock impacts. Combined with thermal testing results, this instrument will provide data needed to untangle the physical parameters that govern explosive sensitivity.   

Laser Etched Fiducial Marks for Detonation Front Curvature Measurements

Front-curvature measurements provide information on the detonation wave shape, which is an essential experimental data component used in high explosive model calibration. We typically perform these measurements on rate-sticks and other test geometries using a mirror destruct technique. Typically, the mirror is a layer of vapor deposited aluminum, 0.45 micrometers thick, applied to a clear plastic window placed in intimate contact with the breakout surface of the charge. During the experiment, light is reflected from this mirror surface to a streak camera until the detonation wave arrives at the aluminum coating, destroying the mirror. An important feature of our front-curvature windows is the placement of fiducial marks beyond the edges of the high explosive charge. These fiducial marks are small defects in the mirror where light is not reflected, and are placed at a known distance apart from each other, allowing for accurate spatial scaling of the streak camera record. The marks are visible in the still images taken prior to firing the shot, and also produce lines on the streak camera dynamic record since they interrupt the reflected light. Previously, we've created these fiducial marks by center point milling, which requires a very sharp tool cutter and using the knee of the milling machine to bring the window into contact with the cutter. To keep the mark small and precise, the tool should only cut about 75 microns deep. Producing fiducial marks of consistent size and shape requires a skilled machinist, and is a time consuming process. Here, we describe a process to generate front curvature fiducial window marks using a laser etching technique. We show that the consistency of the fiducial mark size and shape as well as positioning accuracy are improved relative to our prior technique. In addition, front curvature results for both methods are shown and compared.   

Meshless Front Tracking with Acceleration Effect for Detonation Propagation in Multiple Explosives

We have shown previously that an acceleration effect is necessary in explaining the behavior of a concave detonation with detonation-shock-dynamics (DSD). The acceleration effect can also be used to derive the structure of the initiation-zone attached to explosive-interface as shown in the case of dual initiation. Because the propagation of a detonation between multiple explosives inevitably creates concave detonation front, we conclude that using a D_n-kappa relation in DSD is not adequate and acceleration effects must be included in the front evolution equation of a detonation. To the best of our knowledge, existing DSD implementations do not capture acceleration effects. To obtain reasonable solutions for detonation tracking of concave fronts, we implement acceleration effects in LLNL’s meshless detonation tracker, SDOT.  Results using evolution equations calibrated for practical explosives show that the SDOT solutions closely match direct numerical simulations (DNS). Because front tracking is orders-of-magnitude faster than DNS, we expect significant efficiency gains using SDOT. This work will enables the design of complex multi-material charges.    

    

Evaluation of Explosively-Driven Flash Coatings

The application of flash coatings to detonation tests for light enhancement with high-speed imaging is a well-established technique within the community. From quickly assembled tests that use cigarette paper or even sand to nano-size particles suspended in a working fluid that is subsequently airbrushed onto samples, each group has created their own standard for what works best. These standards have remained in the background of many research studies that simply note a flash coating was used with little technical specifications or details given. Though one or two studies exist on the introduction of a new coating, information on how the morphology, application, and material type affects explosively-driven light output has been practically nonexistent. A review of past applications has yielded little information as many researchers who use flash coatings will state, "it's more of an art than a science." This work seeks to review non-reactive flash coatings that have been stated to work with a focus on aluminum oxide to investigate the effect of coating thickness and particle size. Streak camera imaging will be used to measure flash coating performance. Additional simulations using CTH will investigate how the coating material EOS and particle morphology affect temperature rise time, and will be used for comparison with experimental results.   

Microstructural Investigation of PBX 9501: Comparing Wet Slurry and Resonant Acoustic Mixing Techniques

The microstructure of a high explosive (HE) material directly influences the reactive behavior under a variety of insults. Furthermore, the preparation method of polymer-bonded explosives (PBX), as well as the constituent material particle size distribution and consolidation parameters, will dominant the microstructural features that are observed. Here, we compare batches of PBX 9501 prepared by the typical wet slurry process and resonant acoustic mixing. The prills produced by the wet slurry process will lead to an inherently different microstructure than the coated powder produced by resonant acoustic mixing. We observed that the conditions required to consolidate (press) the material to the nominal PBX 9501 density of 1.835 g/cc varied depending on the prill size and preparation technique. SEM imaging, X-ray microcomputed tomography, and ultra small-angle x-ray scattering analysis reveal distinct differences in the void structure and void size distribution.   

Structural Transformation and Chemical Stability of a Shock Compressed Insensitive High Explosive Single Crystal: Time-Resolved Raman Spectroscopy

Despite the considerable interest in insensitive high explosives (IHEs), real-time measurements to directly probe the molecular-level response of shock compressed IHE single crystals are lacking. To address this need, plate impact experiments were conducted to measure time-resolved Raman spectra for 1,1-diamino-2,2-dinitroethene (FOX-7) single crystals shock compressed to 20 GPa. Vibrational frequencies from 800 cm^-1 to 1500 cm^-1 were examined with 15 nanosecond time resolution at several peak stresses. At 4 – 6 GPa, two new Raman peaks appeared, consistent with onset of the α′ – ε structural transformation reported previously in static compression work. Raman results to 20 GPa showed neither additional transformations nor any indication of chemical decomposition, in marked contrast to the decomposition observed at lower stresses in shock compressed conventional HE single crystals. Our Raman results support the previous suggestion that strengthening of intra- and inter-molecular bonds, due to the α′ – ε transformation, plays a significant role in the insensitivity of FOX-7 single crystals to shock initiation. The present work provides the first experimental insight into the molecular-level response of a shock compressed IHE single crystal.   

Porosity: The Key to Initiating Metallic Composite Particles under Shock Compression

Thermitic composites of metal and metal oxides can produce twice as much energy as conventional explosives but the diffusion-controlled reaction is too slow (microseconds) to produce detonations. If we can design metal-based composites that can react in tens of nanoseconds in strong shock waves, they can be used to boost the energy of conventional explosives or possibly be used as high explosives. One way to increase the reaction rate is to design composites with void structures that can produce hot spots. Recent work has shown that arrested reactive ball milling in fluid emulsions can produce micrometer-sized composites with a variety of void configurations. We can characterize the void structures by cross-sectioning the composite particles. We can then determine the efficiency of these void structures in producing fast energetic chemistries using a tabletop high-throughput system that uses km/s laser launched flyer plates to input detonation-strength shocks into the particles, whose shock reactivity can be characterized with nanosecond video and optical pyrometry   

High-pressure, high-temperature phase diagram and equations of state of RDX polymorphs

High-explosive crystals pass through a range of pressure-temperature conditions on the path to shock-induced detonation. Accurate identification of phase boundaries is essential to predicting the phases involved in initiation and at the leading edge of the detonation front. RDX (cyclotrimethylenetrinitramine) is a commonly-used explosive that is known to have a complex phase diagram to 10 GPa and 200 °C. At ambient temperatures, α-RDX transforms to γ-RDX around 4 GPa, further transforming to δ-RDX around 16-18 GPa. At high temperatures, both α-RDX and γ-RDX transform to the ε phase, but the location of the α-γ-ε triple point has been debated. Here, we explore the high-pressure, high-temperature phase diagram of RDX to 25 GPa and 150 °C in a diamond anvil cell with in situ synchrotron X-ray diffraction (XRD). We confirm the α-to-γ phase transition around 4 GPa at all temperatures explored, and the γ-to-δ phase transition around 16 GPa. We did not observe the ε phase of RDX up to 150 °C, more tightly constraining the α-γ-ε triple point for this material. However, we did observe some time-dependent decomposition at this temperature. These data were used to inform thermal equations of state of all three phases explored.   

Investigation of core/shell nanoparticles properties by classical molecular dynamic study

The synthesis of hexolite by Spray Flash Evaporation (SFE) recently led to the formation of new core/shell highly energetical nanoparticles [1]. The detonation of such compounds produce very small nanodiamonds of high interest for electronics and biomedical applications. It was revealed that such production is influenced by the core/shell ratio, their size and interfaces features (significant at nanometric level). To manage these factors, the determination of the core/shell thermophysical properties and the underlying mecanisms leading to their formation is crucial.

Due to the thermal instability of hexolite core/shell structures, experimental characterization of these systems is challenging therefore a theoretical investigation appeared as a relevant approach to elucidate the properties of these heteregenous materials under ambiant and extreme conditions.

This task is undertaken by molecular dynamic simulation, performed using the STAMP code developed at CEA/DAM [2].

In this work, we present a molecular dynamics study of the thermophysical properties of model core-shell RDX/TNT nanoparticles. We focus on the nature of the core-shell interface with comparison to regular planar interfaces. The SFE process is also tentatively simulated in order to unravel the first mechanisms involved in the formation of this peculiar structures.   

Radiolytic Degradation of Common Energetic Functional Groups

Understanding the radiolytic degradation of high explosives (HE) is of critical importance, but it is not possible to know which HEs or even which energetic functional groups (FGs) will be used in future applications. It is therefore necessary to understand how each energetic functional group degrades when exposed to ionizing radiation. This talk will discuss results pertaining to the radiolytic degradation of various energetic FGs including: azide (-N₃), nitro (-NO₂), and nitrate ester (-ONO₂). Materials containing energetic FGs were irradiated with γ-rays and the irradiated materials were compared to the unirradiated materials using proton nuclear magnetic resonance (¹H-NMR) and gas chromatography (GC) to assess trace chemical changes. Comparisons between the different energetic FGs will be discussed as well as their connections to photolytic calculations. Lastly, the effect of ionizing radiation on aromatic versus aliphatic backbones will be discussed.   

Measurement of the Coefficient of Thermal Expansion for PBX 9502

Detonation performance modeling of PBX 9502 at elevated initial temperatures requires knowledge of the change in charge size with increasing temperature. Previous measurements of the coefficient of thermal expansion (CTE) of PBX 9502 have shown large variations across different studies, as well as anisotropic growth in directions parallel and perpendicular to the pressing direction [Skidmore et al., The Elusive Coefficients of Thermal Expansion in PBX 9502, LA-14003, 2003]. In this study, we examine the thermally induced change in the radial and axial dimensions of isostatically-pressed cylindrical pellets of PBX 9502 during a single heating event from 20 °C to 75 °C, with intermediate measurements taken at 40 °C and 60 °C. Two virgin lots [BAE20F755-002 and HOL88H891–008] and one recycled lot [HOL88B891–007] are examined. For each lot, two pellets were measured. We did not observe anisotropic growth for any of the pellets between 20 °C and 75 °C, with comparable thermal expansion rates observed in both radial and axial directions. Our new growth rates for PBX 9502 are analyzed and compared with prior measurements. We show that the length scaling ratio versus temperature is slightly lower than that obtained by Skidmore et al. [2003].   

Microstructure of Pre-Pressed PBX Prills Explored by X-ray Computed Tomography

PBX (plastic bonded explosive) prills are spongy nuggets a few millimeters in diameter, comprising a nonuniform distribution of HE crystals and binder. When pressed—especially at elevated temperatures at which the binder has softened or melted—one hopes that it will flow to uniformly coat the HE crystals. In reality it does so imperfectly, such that X-ray tomographic scans often look like a collection of prills mashed together. Lore is that the larger the prill, the more heterogeneous the binder distribution. Even if that were not so, the larger the prill the farther binder must flow in order to homogenize. Thus, the degree to which it homogenizes depends in part on prill size. The degree to which binder flows during pressing in turn affects the void distribution within pressed charges, which one suspects will affect shock sensitivity and material strength. In this paper we will use X-ray CT to measure and compare the pre-pressed prill structure for three PBX 9502 formulation batches using the same TATB powder lot, and three PBX 9501 lots.   

Study of decomposition properties of PETN.

Pentaerythritol tetranitrate (PETN) powder is a secondary high explosive widely used in military applications, namely as a booster in detonators.  PETN powder has a melting temperature of 141° C that is close to the onset of decomposition. The fact that these two main characteristic temperatures so close in value makes the separation of these two processes challenging. We have observed a large hysteresis in melt after bringing a PETN to 145° C through melt and then recrystallizing and remelting. The second melt is broader and at lower temperature than the pristine melt.  In this presentation we investigated decomposition of PETN heated to temperatures below melt and held for various lengths of time before taking the sample to 141° C. We use thermal analytical techniques (TG and DSC) to look for any effects of low temperature decomposition on PETN melt.

    

Shock Hugoniot and a broad-range multiphase EOS of Ti-6Al-4V to more than 1 TPa.

Titanium and its alloys are a fascinating family of metals with excellent mechanical and biocompatibility properties with applications in aerospace, defense, biomedical and more. In this work we focus our attention on Ti-6Al-4V, commonly referred to at Ti64. We use ab initio molecular dynamics (AIMD) simulations, which we validate with experimental shock data to over 1 TPa on Sandia's Z machine, in order to develop a high-fidelity, multiphase Equation of State for (EOS) Ti64, spanning a broad range of temperature and pressures. The resulting EOS is suitable for use in hydrodynamic simulations involving shock compressions of solid materials.   

Broad-range multiphase EOS of a strong oxide to more than 1 TPa.

We present ongoing work on the design of a broad-range multiphase equation of state (EOS) of gallium

oxide. Ga2O3 is an important functional material with rich physicochemical properties and emerging

candidate for applications in different fields like high power electronics, optoelectronics and more. Many

of the standard EOS models currently in use are geared to simple elemental metals. Ga2O3 pushes the

modeling beyond the basic physics assumptions. The complex nature of the crystalline structures lends to

a directional dependency. These dependencies make various quantities like thermal expansion and the

compression response change according to crystalline orientation. We are currently modelling the EOS

for four of the solid phases and the liquid.

    

Multiphase Equation of State Optimisation Utilising Legendre Polynomial Fits for the Cost Function

Multiphase equation of state (EoS) optimisation requires a cost function defined as a sum of the square residuals between the EoS output and the measured data divided by the measurement error. This cost function assumes that the data are independent measurements, however this is unlikely to be true, therefore, a new cost function must be developed. Legendre polynomials are fit through both the measured data and the EoS output, at the location of the measured data, in order to consider measurement correlations and to reduce the complexity of the cost function. A new cost function is defined by the comparison of the coefficients of these Legendre polynomial fits. An application of this methodology to the measurements on tin are presented. UK Ministry of Defence © Crown Owned Copyright 2022/AWE   

Equations of State and Phase Transformations in Rocky Materials to TPa Pressures

Recent advances in theory and technology have enabled constraining the equation of state (EOS) and phase transformation of materials to TPa pressures—conditions relevant to early Earth and large planets. This presentation summarizes our latest findings for a series of planetary materials listed below based on first-principles simulations and compared to laser-driven experiments when available:

(1) MgSiO₃. Hugoniots of enstatite and bridgmanite calculated to 1.4 TPa, in addition to the Grüneisen parameter as a function of temperature and density.¹

(2) SiO₂. Mechanism of the bonded-to-atomic transition in liquid silica clarified and a new transition boundary rendered based on calculations and extensive analysis of thermodynamic, structural, and electronic properties.²

(3) MgO. The B1–B2 phase boundary benchmarked by quantum Monte Carlo calculations and accurate free-energy data (via thermodynamic integration) at T = 0 K up to the triple point.³

(4) Al₂O₃. Melting curves of the post-perovskite and U₂S₃ phases constrained to 1 TPa.⁴

The results necessitate reconsideration of predictions by planetary models that are based on empirical EOS models.   

Sound velocity, second shock velocity and off-Hugoniot measurements of lead compressed up to 83 GPa.

Two series of shock wave experiments to investigate the sound velocity and second shock velocity of lead were carried out using a powder gun facility. An overtaking method [1] has been utilized in both types of experiments. A Photon Doppler velocimeter (PDV) was used to monitor the sample/window (either PMMA or LiF) interface velocity. The waveforms recorded in sound speed and re-shock experiments exhibit a shock wave front followed by a plateau and eventually by a release eave or a second shock wave front, respectively. Values of sound velocity obtained in present study are in good agreement with literature data for lead. Off-Hugoniot state of lead behind the release wave emanating from the sample/window interface were estimated as well. Mie-Gruneisen equation of state was calibrated to the data presented in this work.

[1] R. G. McQueen, J. W. Hopson, J. N. Fritz, Rev. Scient. Instr. 53, 245 (1982)     

High-temperature, high-pressure behavior of common windows and pressure calibrants: LiF and MgO

Lithium fluoride (LiF) and periclase (MgO) are used extensively as optical windows in dynamic compression experiments and also used as pressure media and thermal insulation layers in laser-heated diamond-anvil cell (LHDAC) experiments.  To better understand these important high-pressure standards and ubiquitous window materials, we present their solid high-temperature equations of state as determined in a LHDAC.  Transparent samples of LiF and MgO foils were coated with either Mo, Ag or Au in order to couple with the near-infrared laser for heating between ~1000-2500 K and pressures up to ~80 GPa. Finite element analysis was used to determine temperatures of the insulation layers based on the measured temperature of the metal coatings.  These measurements complement already measured low-T isotherms performed with resistively-heated DACs. 

 

    

Characterizing Structure in Simulated Silicon Under Shock Compression

Silicon is an important material used in many industries and is well studied. Here we use it as a baseline for shock simulations of materials with covalent bonding. The shock structure of silicon is investigated through classical molecular dynamics simulations in the range of 10-20 GPa using the modified Tersoff interatomic potential. Simulations in the literature have shown presence of amorphous regions at the areas with high dislocation density; however, the disordered structure outside of those amorphous regions was not fully identified. The structure is characterized here through the use of radial distribution and angular distribution functions comparing pristine silicon structures with those within the shocked silicon bulk. Additionally, measurements are made of the flow stress and energy stored by the phase change within the shocked regions.   

Anisotropic properties of 3D-printed rock-like materials under dynamic Brazilian disc tests

Rock materials are commonly endowed with anisotropic structures, like beddings, flaws and fillers, which casues difficulty when trying to predict strength, deformation and fracture properties under dynamic loading. Considering the structural complexity and randomness in rock materials, it is difficult to get samples for experimental studies. 3D printing (3DP) is emerging as a promising method to replicate the complicated meso- and micro- structures found in natural geological materials, but which is able to repetitively fabricate  identical specimens. In this study, 3D printed rock-like specimens are fabricated into cylinders for Brazilian disc testing (diameter: 50 mm and thickness: 25 mm) using extrudable geomaterials (extrusion width 1 mm and layer thickness 0.5 mm). By combining a split Hopkinson pressure bar and a high-speed camera, Brazilian disc tests were conducted on the 3DP specimens with bedding angle α (i.e. angle between impact direction and bedding orientation) taking a variety of values (0˚, 30˚, 45˚, 60˚, and 90˚) to investigate anisotropic mechanical and fracturing properties. Results show that the dynamic peak strength gradually increases as the bedding angle α increases from 0˚ to 90˚. Failure patterns of 3DP specimens exhibits  tensile dominated failure in both bedding angles 0˚ and 90˚, and mixed tensile-shear failure in other bedding angles (30˚, 45˚ and 60˚). The experimental results are consistent with a previous study using natural rock materials, indicating that the 3D printing technology has promise to allow more controlled studies of rock materials under dynamic loadings, avoiding the structural inconsistencies in natural rock specimens.   

Using shockless ramp compression to investigate melting in the Earth's mantle.

A major feature of the Earth’s interior structure is the mantle transition zone (MTZ), which lies between 410-660 km depths (12-20 GPa), defined by discontinuous increases in seismic velocity associated with mineral phase transitions. Geophysical observations from seismic tomography show slower than average seismic velocities just above and below the MTZ. It is proposed that due to the convective flow of the mantle, these regions of low velocities indicate the presence of partially molten rock near the upper and lower boundaries of the MTZ. Testing this hypothesis requires experimental determination of velocity for silicate melts at high pressures and elevated temperatures. Static studies of silicate melts at one-atmosphere and silicate glasses at high-pressure exhibit little compositional or density dependence for velocity. Whether this observation holds at MTZ conditions has not been demonstrated. To investigate the composition-velocity relationship at elevated pressure and temperature, we conduct shockless dynamic compression (ramp) of glasses along the SiO₂-MgSiO₃ compositional join to >20 GPa on the pulsed-power machine Thor at Sandia National Laboratories. These experiments probe an isentropic pressure-temperature path, similar to that of the mantle geotherm, and will illuminate the seismic properties of partial melts, if they exist in the MTZ.   

The Z to Planets Project: Exploring giant impacts and rocky exoplanet interiors at the Sandia Z Machine

The Z to Planets project in the Z Fundamental Science Program has focused on measuring the physical properties of the major building blocks of Earth-like and Super-Earth planets. The program has built a body of equation of state data on major silicate minerals, building upon previous work on the end-member compositions in the MgO-SiO₂-Fe system. Through multi-sample planar shock-and-release experiments at 100’s GPa, our project has measured the principal Hugoniot states (PVT), partial release states, and states on the liquid branch of the vapor curve for forsterite (Mg₂SiO₄), olivine ((Mg,Fe)₂SiO₄), enstatite (MgSiO₃), bronzite ((Mg,Fe)SiO₃), and a synthetic glass with a bulk composition similar to the silicate portion of the Earth. These data are being used to improve equation of state models for calculations of planetary collisions and interior structures. Here we present the experimental design and highlight the dataset and applications to giant impacts.

    

Water in silicates: A combined shock and spectroscopy study

Water is abundant in planetary building blocks. Two fundamentally important questions for understanding impact events and interpreting planetary evolution are 1) how does the incorporation of water into silicate materials influence physical properties during shock and 2) where do volatiles like water go during impact events? To this end, we present results from shock experiments on the compressed gas-driven gun housed at the Dynamic Integrated Compression Experimental (DICE) facility at Sandia National Laboratories for two samples: SiO2 glasses with <1ppm OH and 1000ppm OH. Raman and FTIR data from recovered sample materials will be used to determine the degree of devolatilization that has occurred in these shock experiments. These combined datasets will permit us to evaluate both the effect of low levels of hydration on physical properties of silicates, as well as determine what happens to volatile species on impact.    

Hot Spot Initiation and Growth in Shock-Induced RDX through MD-informed Void Collapse Simulations

Continuum-scale shock-induced void collapse resulting in hot spot formation is crucial to understanding the initiation of detonation inof energetic materials. Hotspots, i.e., localized areas of high temperature, can lead to chemical reactions in the material causing a shock to detonation wave transition. In this work, circular void collapse computations were conducted using 1,3,5-Trinitro-1,3,5-triazinane (RDX) as the energetic material. These simulations were conducted with an Eulerian hydrocode, SCIMITAR3D, which uses a sharp interface, and level set based methods for modeling material dynamics. A circular void is embedded into a block of RDX which then undergoes a reverse ballistic shock, where . A varying a range of shock velocities and void sizes are explored. The calculations employ MD-derived material models such as a polynomial equation of state, temperature-dependent specific heat function, pressure-dependent melting temperatures and chemical reaction rates. The resulting void collapse and hot spot characteristics can be extracted and compared with coarse grain and atomistic model simulations. The overall goal of this work is to develop a rate-dependent Johnson-Cook plasticity model for RDX, and additionally eventually to establish an understanding of hot spot formation in the material.

    

Theoretical Calculation of Proton Radius

A single proton is almost non-radiating. It can be considered that a proton is a black hole in the microscopic world, and the densities of macroscopic black holes and microscopic black holes should be equal. My theory calculates that the density of black holes is constant, and the density of black holes is proportional to the cube of the speed of light. According to the theory that stars evolve into black holes, it can be deduced that the proportionality constant is: the density of the sun is: K=1.76×10^-7. Let's calculate the density of the black hole again, and substitute ρ=KC³==1.76×10^-7×(3×10⁸)³=4.752×10¹⁸(kg/m³), so the density of any black hole is 4.752×10¹⁸(kg /m³). The density of protons should be: 4.752 × 10¹⁸ (kg/m³). We know that the mass of the proton is 1.67×10^-27kg, we think that the proton is a sphere, ρ=M/V=M/(4/3)πR³, R³=M/ρ(4/3)π=1.67 ×10^-27/4.752×10¹⁸(4/3) π=0.84×10^-46(m), so R=0.44×10^-15(m), that is, the radius of the proton is theoretically estimated to be 0.44×10^-15(m) , This calculation is quite close to the experimental value of scientists.

    

Dynamic Tensile Extrusion Behavior of Fine-Grained OFHC Cu Fabricated by Powder Injection Molding and Equal Channel Angular Pressing

The dynamic tensile extrusion (DTE) behavior and microstructural evolution of fine-grained (FG, ~1 μm < d < ~10 μm) Cu fabricated by powder injection molding (PIM) and equal-channel angular pressing (ECAP) were investigated. The FGM Cu was fabricated by PIM with commercial micro-sized Cu powder sintering at 850 °C for 2 h, while the FGH Cu was developed by the hot isostatic pressing of the FGM at 780 °C for 2 h under a pressure of 1000 bar. In order to compare the DTE behavior of the FG Cu depending on the manufacturing process, the ultrafine-grained-B (UFG-B, d < ~1 μm) Cu was developed by performing 16 passes of ECAP with route B_c, and the FG-150 and FG-200 Cu were fabricated by annealing the UFG-B Cu bar at 150 °C and 200 °C for 1 h, respectively. The DTE tests were performed with identical flyer velocities using an all-vacuum gas gun. The fragments and remnants were carefully recovered after the DTE tests and examined by EBSD measurement and a micro-Vickers hardness test. In order to compare microstructure evolution during the DTE deformation, an analysis of misorientation distribution and grain orientation spread was carried out from EBSD results. In addition, fracture characterization was accompanied by an investigation of the DTE fragments’ morphology.   

The Effects of Prior Cold Work on the Shock Response of Silver

The shock response of simple metals and alloys are influenced by a number of factors, including the unit cell, prior deformation history and alloying. In this presentation, we discuss the effects of prior cold work on silver. Silver has been studied in two states; an as received hot forged recrystallised condition and a cold rolled state to a reduction in thickness of around 21%. This was chosen so as to mimic the state of commercially available half-hard copper. The main difference is that silver has a significantly lower stacking fault energy than copper, with a corresponding increase in the spacing of partial dislocations, which in turn will affect dislocation motion and generation. In particular, changes in the Hugoniot Elastic Limit (HEL) and spall strength are discussed.   

Hydrodynamic simulations of rippled shock wave driven by laser ablation and initial pressure discontinuity

When lasers irradiate a target with a rippled surface, the ablation pressure launches a rippled shock induced by the initial target surface. We study the evolution of a rippled shock driven by an ablation surface and the associated flow field using inviscid hydrodynamic simulations in FLASH. For strong shocks (O(100 GPa)), the shock separates from the ablation surface at very long times. Hence, the reverberations of pressure waves in the region between the shock front and the ablation surface can interact and modify the rippled shock evolution. We compare the evolution of the decaying amplitude of a rippled shock driven by: 1) laser-driven ablation and 2) initial pressure discontinuity across an interface separated by two media. First, we match the zeroth order flow profiles in the shock-compressed region in both the cases by driving a steady planar shock of equal strength. Next, we conduct simulations on sinusoidally-perturbed target surface (interface) of multiple wavelengths. Finally, we analyze the effect of including dynamic viscosity and compare our results with previous theoretical models of Miller et al. (1991) and Ishizaki et al. (1996).   

Microstructural features informing continuum scale dynamic behavior of an additively manufactured titanium alloy

The connection between process-structure-properties-performance (PSPP) is an overarching goal in materials design. Increasingly complex and integrated physics models attempt to predict the entire path of a material from processing all the way through performance. The advent of additive manufacturing (AM) has increased the range but also uncertainty in nearly all aspects of PSPP which can be viewed as both a challenge and an opportunity. We will present an effort to incorporate processing, structure, and properties effects on the dynamic performance of a dual-phase titanium alloy produced through AM. Specifically we will focus on equation-of-state, strength, and damage models to understand and incorporate microstructure information to predict HEL, Hugoniot, and spall response of our titanium alloy. Added wrinkles to this include porosity from AM processing, phase fraction, interfacial strength, and effect of heat affected zones, to name a few. Attempts to incorporate all of these microstructural characteristics into modeling of dynamic performance within established continuum frameworks will be discussed and where this approach might break down. Time permitting, we discuss efforts to model the dynamic performance explicitly from the grain scale up.

    

High strain-rate strength response of single crystal Tantalum through in-situ hole closure imaging experiments

The properties of crystalline materials often depend on directionality and operating conditions. Specifically, the strength of materials can depend anisotropically on crystal direction and the loading condition. To probe these effects, a preliminary series of high strain-rate (>10⁵/s) strength plate-impact hole closure experiments were performed on high purity single crystal Tantalum cubes. The impact/loading condition and orientation of the single crystals with respect to loading were varied to provide data to inform crystal plasticity modeling efforts. The experiments consist of in-situ high-resolution X-ray radiographic imaging of the hole collapse under dynamic compression conditions to infer the material strength via its resistance to closure at ever increasing levels of plastic strain. The experiments are compared against hydrocode simulation predictions. The samples are recovered and characterized with EBSD to evaluate the deformation structure that developed during the extreme loading.

    

Effect of PBX Prill Size on the Compressive Properties of PBX 9502

PBX prills are agglomerates a few millimeters in diameter composed of High Explosive crystals non-uniformly distributed in and surrounded by a polymer binder. When pressed at elevated temperatures at or above the binder melt temperature the polymer flows to uniformly coat the HE crystals to create a well-consolidated compaction. In reality it does so imperfectly, such that x-ray tomographic scans often look like a collection of prills mashed together. It is speculated that the larger the prill, the more non-uniform the binder distribution but this has never been proven. Even if that were not so, the larger the prill the farther binder must flow toward the middle in order to homogenize. Thus, the degree of homogenization depends in part on prill size. The degree to which binder flows during pressing in turn affects the void distribution within pressed charges, which one suspects will affect shock sensitivity and material strength. In this paper we explore, for ~30 PBX 9502 formulation batches using the same TATB powder lot, how prill size effects the compressive mechanical properties.

    

Local dynamic effects on impact welding process window variability

Joining metallic materials with dynamic welding techniques (e.g. impact welding) provides a means to metallurgically bond material via mechanical mixing, extreme deformation rates, and local material instability. Process variables of interest include the selected metals, dimension, impact velocity, and impact angle. Even with limited process variables and relatively simple boundary conditions, local dynamic effects arise from the complexity of the wave propagation events, which can lead to material state evolution which modifies process windows. This talk presents a computational analysis of local wave dynamics present in vaporizing foil actuator welding (VFAW), comparing hydrocode simulation results with experimental observations of impact weld properties. Local conditions leading to instability and weld thickness will be analyzed and compared with experiments as a function of material selected, impact velocity, and impact angle. Process window variability is discussed in this context and presented for similar and dissimilar materials.   

On the numerical simulation of DTE test of OFHC Copper: Influence of constitutive modeling and numerical parameters

The dynamic tensile extrusion (DTE) allows to investigate the material behavior under extreme conditions, very large plastic deformations, and high temperatures and strain rates. It is therefore an essential tool for constitutive modeling validation. At the same time, the numerical simulation of the DTE test is particularly challenging since several aspects need to be evaluated before model verification. In this work, the influence of material modelling and of numerical parameters for the simulation of the DTE test on OFHC Copper was investigated and the results were compared to experimental data. The performances of three different constitutive models, phenomenological (Modified Johnson and Cook, MJC), physically based (Mechanical Threshold Stress, MTS) and hybrid (Zerilli-Armstrong, ZA) were evaluated. For each model, the material parameters were identified using uniaxial stress-strain data at different temperatures and strain rates. Independently from constitutive modeling, a sensitivity analysis was carried out in order to evaluate the role of computational parameters as well.   

Elastic anisotropy of 1,3,5-Triamino-2,4,6-Trinitrobenzene (TATB) as a function of temperature and pressure: A Molecular Dynamics study

The equation of state of the triclinic compound 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) as well as its second-order isothermal elastic tensor are computed through classical molecular dynamics simulations under various temperature and pressure conditions. Hydrostatic pressures similar to previous diamond anvil cell experiments are imposed within the range [0, 60] GPa and temperatures chosen between 100 and 900 K in conjunction with the most recent version of an all-atom fully-flexible molecule force field.

The isothermal elastic constants are computed using the generalized Hooke’s law by fitting Cauchy stress vs. linear strain curves. Along isobaric pathways, TATB single crystal stiffness is found to undergo linear softening, less pronounced at high pressure, while maintaining its elastic anisotropy. On the other hand, along isothermal pathways, a non-linear increase is observed in the elastic constants with a significant decrease in anisotropy.

Towards a precise mesoscopic modeling of TATB single crystal mechanical behavior, we provide “ready to plug-in” analytical formulations of the P,V,T equation of state and pressure-temperature dependent non-linear elasticity.   

Simulating Plasma Ignition of HMX via Microwave Radiation at Substandard Bulk Temperatures

Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) is an energetic material used in a range of applications from plastic bonded explosives (PBX 9501, LX-14, etc.), solid rocket propellants, to military applications. Over the past decade, HMX (neat, doped and PBX) has been studied in microwave fields (MWFs). Although the direct interaction of neat HMX with MWFs is minor, it is hypothesized that the gas-phase intermediates generated by thermal decay strongly couple to the MWFs resulting in ignition at substandard bulk temperatures. In this work, simulations will provide insight to experiments that observed the plasma ignition of gaseous intermediates. We couple the Fridman-Macheret α-Model (plasma), reversible chemical kinetics, MWF and heat transfer within COMSOL Multiphysics software suite. These simulations will provide support for our hypothesis that the intense optical signal observed during the microwave pulse is the result of the ignition of HMX intermediates that couple to the MWFs.   

Influence of solid constituent properties on thermal response of porous materials under shock loading

The heat output is determined using high speed pyrometry for various powders under shock loading in the present work. The pyrometry data for copper and silica powders, obtained in earlier work, are complemented with data for an iron powder. Comparison of the data for the three powders shows an unusually high heat output for the iron powder. However, the two-phase constitutive CTH modelling reveals a link of this higher output with particle size. Calculations for the powders with similar particle sizes demonstrate that the heat output for the iron powder lies between that for the copper and silica powders. This agrees with the inter-phase heat fluxes substantiated by the thermal properties of the condensed constituents.   

Shock compaction response of Al2O3 powders over range of initial density and high-pressure states

Recent studies into the shock response of granular Al₂O₃ have produced only a portion of the distended Hugoniot. The existing data shows distinct compaction curves for each initially distended state under shock compression, of which only initial states of 50% and 58% of theoretical maximum density (TMD) have been investigated. Additionally, these datasets contain a substantial gap in the peak pressure states, as granular Al₂O₃ has only been investigated up to 2.5 GPa.  In this study, the effect of initial distention on the densification trends observed under shock compression is analyzed, including discussion on expected trends in the powder compaction curves of varying initial distended density states and extrapolating those trends to higher initial density states. Using this framework, a series of planar shock-compression experiments on Al₂O₃ powders over a range of initial density states, with peak pressure states in the 2-15 GPa regime, is proposed. These experiments, involving multi-probe PDV measurements of shock wave profiles, will be used to generate high-precision datasets for Al2O3 and will be correlated with other material systems to investigate the role of initial density and material strength on shock densification.   

Phase transition and sound velocity study of shock loaded CaF2

Fluorite, CaF₂, is a model ionic solid and prototypical structure type in condensed matter physics, undergoing pressure-induced phase transitions to highly coordinated AX₂ structure. In this study, the shock-induced phase transitions in CaF₂ were investigated by plate impact experiments carried out with the gas guns at Soreq NRC and NRC Negev and powder gun at Soreq NRC. Single-crystal samples in (111) and (100) orientations were shock loaded to pressures between 7 GPa to 45 GPa. The particle velocities were probed by VISAR and PDV. The wave structure evolution measured in samples of different thicknesses allowed us to extract the shock velocity and sound velocity in the shock compressed material. We observe evidence of a transition to a high-pressure phase, where the phase transition seems to be sensitive to pressure and strain-rate.   

High Pressure Response of Additively Manufactured AlSi10Mg

Additive manufacturing (AM) is a rapidly emerging technology with the potential to expedite current engineering design processes. However, the AM process does not always lend itself to traditional engineering alloys. Often, unique alloy compositions are used to improve the AM process. A common example is AlSi10Mg, which uses high Si content to lower its melting point, improve fluidity, and reduce thermal expansion. In this work, we present a synergistic study using shockless compression on Sandia National Laboratories' Z machine and structural x-ray diffraction (XRD) measurements at high-pressures in a diamond anvil cell (DAC) at the High Pressure Collaborative Access Team (HPCAT) on AM AlSi10Mg to quantify its high pressure thermodynamic response. We find that the pressure-induced phase transitions present in Si alter the bulk response of the AM AlSi10Mg alloy.   

Fracture of PC and PMMA cylinders in the Taylor tests

The Taylor cylinder impact test reveal different failure patterns for flat-ended cylindrical rod for two polymers, polymethylmethacrylate (PMMA) and polycarbonate (PC) at different impact velocities against a steel anvil. Numerical modelling, employing an updated two-phase model with phase transitions and a time accumulated damage, shows the importance of phase transition and loading modes on the difference in the failure patterns. In experiments, the loading modes were varied with a range impact face conditions to control failure in the rod and resolve compressed regions within the recovered polymer cylinders. The combination of macroscopic high-speed photography and three-dimensional X-ray imaging with new constitutive modeling has identified and described the development of failure of the rods made from these polymers.

    

Shock compression behavior of additively manufactured highly solids loaded polymer composites

Additively manufactured highly solids loading polymer composites (HiSoLPCs) were impacted via plate-on-plate impact experiments in order to study the influence of heterogeneities on the shock response. HiSoLPCs have heterogeneities ranging from mesoscale features such as particles, interfaces, and voids to macroscale features like a hierarchical structure. Previous work has shown the dispersive and dissipative effect of particles and voids on shock waves propagating through polymer composites, but do not explore the effect of multiple particles with different material properties. Additionally, the influence of a hierarchical AM microstructure in polymer composites has not been studied under shock compression previously. Plate impact experiments with varying impact velocities, sample orientations and number of AM layers (thicknesses) are explored. EOS data (particle and shock velocities) and shock profile data (rise times and wave profile shape) are collected, in addition to novel optomechanical sensors to quantify the presence of multiple pressures in the same sample. CTH simulations utilizing real microstructures with mesoscale resolution are run to explore the in-material behavior. The experimental and CTH results are discussed in how the influence of particles; voids; multiple, heterogeneous particles; and the hierarchical structure affect the shock compression response with comparisons to other relevant composite material systems from the literature.   

Shock and Rarefaction Wave Induced Activation in Mechanophores

Mechanophores are a novel class of material in which mechanical activation leads to a premeditated reaction within the polymer that leads to an alteration of some property. Numerous experimental and theoretical studies have explored and characterized mechanoluminescence, mechanocatalytic, and mechanochromic responses under strain loading. However, mechanical loading in condensed matter, especially shock compression, results in more complex, many-body (MB) deformations. Mounting theoretical work has shown that these MB deformations can result in complex accelerations and alterations to chemical reactions. Here, we perform shock compression in a PMMA + Spiropyran system and analyze how isomerization reactions occur during both compression and high strain release due to rarefaction waves and spall. We find that, for strong shocks, both compression and expansion waves can induce significant activation on the timescale of the loading. The many-body deformations of the loaded mechanophores are assessed to evaluate the mechanochemical influence on activation.   

A shock wave through a polymer/TATB interface : a molecular dynamics study

The interface between a polymer and a solid is instrumental for the mechanical behavior of many filled polymers and polymer bonded powders such as those used in car equipment, sport gear and polymer-bonded explosives. This poster presents an on-going work focused on the behavior of an interface between a polymer and TATB at equilibrium and under shock loading. The study uses molecular dynamics to describe the interface. This tool enables us to compute thermodynamic quantities such as cohesion energy density [1], surface free energy [2] and surface stress [3], but also mechanical quantities such as the full stress tensor. This tool is also well suited to describe the stress relaxation of the polymer, up to 100 ns, ensuing the arrival of the shock wave [4].

[1] Chen, F. and Ren, Y. and He, L. and An, C. and Wen, S. and Shen, F. Molecular dynamics simulation of the interface interaction and mechanical properties of PYX and polymer binder AIP Advances (2022) 12:025307.

[2] Brahmbhatt, J. and Bidault, X. and Chaudhuri, S. Quantifying surface free energy of molecular crystal-HMX using non-equilibrium molecular dynamics methods J. Appl. Phys. (2021) 130:135105.

[3] Frolov, T. and Mishin, Y. Thermodynamics of coherent interfaces under mechanical stresses. I. Theory Phys. Rev. B (2012) 85:224106.

[4] Lecoutre, G. and Lemarchand, C. A. and Soulard, L. and Pineau, N. Hugoniostat and Direct Shock Simulations in cis-1,4-Polybutadiene Melts Macromolecular Theory and Simulations (2021) 30:2000068.

    

Comparison of two models for the simulation of hypervelocity impacts on porous graphite

Hypervelocity impacts (HVI) on porous brittle materials is an important mechanism for various applications such as planetary defense, human-made spacecraft protection, and high power laser facility safety. In either case, the target fragmentation upon impact can lead to significant cratering and debris ejection. This paper presents a numerical investigation of HVI experiments on porous graphite. Two porosity models are compared. In the first one, pore collapse is assumed to be an irreversible process during compaction, whereas the second one considers that pore collapse is mainly due to elastic, i.e. reversible, deformation. Various experimental data are analyzed and compared to the simulations with both models. Three experimental data are in favor of the second model: i) volume recovering upon release after quasistatic oedometric tests; ii) crater shapes resulting from HVI experiments; iii) ejecta velocity distribution observed under HVI experiments. The detailed analysis of these results suggests that they are related to the modelling of release waves velocity, which is significantly different between both models.

    

Understanding microstructural effects of spallation in additively manufactured Ti-5Al-5V-5Mo-3Cr towards the development of a two-phase microstructurally aware model.

Additive manufactured Ti-5Al-5V-5Mo-3Cr (Ti-5553) is being considered as an AM insertion material for engineering applications because of its superior strength properties compared to other titanium alloys. Here we describe the constitutive behavior as a function of strain rate for as-built and annealed Ti-5553. Spallation experiments were conducted at three impact velocities and materials characterization including computed tomography, electron backscatter diffraction, and scanning electron microscopy was conducted on the samples pre and post shot. Inconsistencies in the Hugoniot elastic limit were observed for as-built Ti-5553 indicating a phase change during compression. Preliminary CTH modeling of the dynamic experiments is described. Finally, we discuss a novel approach to enhance existing peridynamics research code Emu and SPPARKS Monte Carlo (MC) capabilities to include multiphases and demonstrate a preliminary simulation of Ti-5553 microstructure with the integration of alpha phase.

    

Simulations of jetting behavior from a shocked micro-groove surface

Shock compression of metal surfaces with machined grooves can create matter jets at high temperatures and pressures with conditions that mimic those of astrophysical and subterranean geophysical processes. Recent work using the OMEGA facility has measured properties of such jets at low and high pressure drives where the jet material is solid and liquid on release, respectively. However, less is known about the behavior of the jets under a wide range of drive strengths and sample dimensions. In this work, we use hydrodynamic simulations to elucidate salient physics of shock-driven jetting behavior in tin surfaces. In addition to observing a near linear scaling in total mass ejection with increasing sample thickness, we also observe a power law in the density distribution of the jet as it moves downstream. We characterize the power law exponent for different groove angles and drive strengths. Understanding such physics in a single jet will prove useful for the design of more complicated interactions involving jet collisions.

    

Comparisons of Explosive Dispersal in Static and Supersonic Conditions

Explosive dispersal is a rich area of research in the multiphase flow community. When considering environments such as supersonic regimes for multiphase flow interesting questions can arise. We simulate an explosive dispersal in static, Mach 3 and 6 ambient conditions using an Eulerian-Lagrangian finite volume code. The explosive dispersal is simulated in an axisymmetric barrel with an exit into ambient conditions. The supersonic cases allow a bow shock to form over the barrel before the release of the explosive. The explosive's detonation is initialized with a reactive burn model prior to the start of the explosive dispersal simulations. Cases with the particle bed have it inserted flush with the barrel exit and the explosive detonation. Three different locations downstream of the barrel exit contain virtual probe plates to capture flow metrics. Incipient flow and particle properties at various times throughout the simulation are captured.   

Determining burn model parameters of ZPP using small scale tests

Zirconium potassium perchlorate (ZPP) is a primary explosive which possesses fast burning rates, desirable ignition characteristics, and high energy densities. Despite being widely used as the igniting propellant in an explosive train, its burning mechanism is not widely reported. In this work, the combustion model for ZPP is assumed to follow Vielle’s law. The parameters are calibrated and optimized based on measured combustion pressures from specially designed small scale tests. Two different diagnostics are explored (a) direct measurement in a confined fixture using pressure transducers and (b) indirect measurementusing polyvinylidene fluoride (PVDF) gauges.

While transducers directly measure the combustion pressure, the voltage signals obtained from the transducers were found to plateau, possibly due to the fixture design preventing deformation of the diaphragm in the transducer. Subsequently, PVDF gauges were considered due to its simplicity. Isochoric combustion pressures were determined from thermochemical equilibrium calculations and compared with test measurements. The burn model of ZPP was then calibrated and further optimized as more PVDF test data are attained. The calibrated burn model parameters act as inputs to the hydrodynamics model of an explosive train comprising of the ZPP stimulus, donor and acceptor explosives. With the calibrated burn model of ZPP, the response to different amounts of stimulus, acceptor and donor could be studied. This methodology can be applied to study the burn models of energetic materials with little reported reaction mechanisms.   

Model Testing and Diagnostic Simulation of Implosion Experiments on the National Ignition Facility

Implosion experiments reach the most extreme high energy density states in the laboratory though the convergent amplification of shock waves and the stagnation of material. These systems are critical to a variety of applications including inertial confinement fusion and serve as the basis of fundamental physics platforms focusing on studying the behavior of matter at conditions relevant to the deep interior of celestial bodies. Diagnosing these experiments is challenging due to the lack of diagnostic access in spherical geometry leading to the primary measurements being self-emission of x-ray and particle radiation that is emitted at around peak convergence. These quantities encode information about the underlying mechanisms that lead to their generation but require sophisticated techniques to extract this information. This work describes how the code PYthon Radiation IMaging and Detection Simulation (PYRIMADS) can be used to test various proposed models for the behavior of an implosion experiment by fully simulating the diagnostic signatures that would be measured to compare against real data.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. IM release number LLNL-ABS-835827.