Raman thermometry, a Stokes/anti-Stokes Raman spectroscopy technique, has been fielded on plate impact and explosively-driven shock (EDS) experiments for measuring shock temperature in a plastic bonded explosive (PBX). Plate impact experiments were conducted on a single-stage light gas gun and the EDS shock experiments were accomplished in a walk-in firing vessel. Temperature measurements on PBX 9501 from gas gun shots are compared to temperatures at similar pressures from EDS and all experimental temperatures show good agreement with temperatures predicted from AWSD hydrocode calculations. We demonstrate the feasibility of coupling Raman thermometry to gas gun experiments for collecting routine temperature measurements needed to validate high explosive models for chemical kinetics and thermomechanics.

Predicting and controlling the failure of brittle materials against impacts have important applications in defense, mining, and medicine. To that end, the key is understanding at the mesoscale the events of crack initiation, propagation, branching, multiple crack interactions, and coalescence. Therefore, we developed an x-ray phase contrast imaging-based technique to directly visualize and quantify the cracking process. We chose to test our technique on single-crystal quartz because it has well-defined material and mechanical properties which computational models can use to accurately simulate the cracking process. Also, quartz serves as an ideal model material to developing experimental techniques/analysis and high-fidelity damage models for energetic materials and heterogenous geomaterials. To achieve the micron and nanosecond resolution required to resolve and track cracks in real-time, we use the high brilliance, spatially coherent synchrotron source at the Dynamic Compression Sector (Advanced Photon Source, Argonne National Laboratory) and the 8-frame LANL/DCS detector system coupled to a 150-μm thick single crystal LYSO scintillator. Quartz samples are uniaxially compressed at 103–104 s−1 strain rates with a custom-built Kolsky bar and stress-strain histories are measured using PDV probes. To characterize the evolving crack morphology, a physics-based inverse model is developed that converts the phase contrast-enhanced image intensity of the cracks into crack volume orientation distributions inside the sample. Using this model, we study how sample surface finish affects the dynamic behavior of cracks.

The catastrophic failure response of brittle solids is governed by the mechanics of crack nucleation and growth, which have been observed to be rate- and orientation-dependent. One property that is characteristic to this process is the failure strength. At low to intermediate strain rates, the failure strength has been observed to be nearly constant and equal to the strength observed under quasi-static conditions; however, at high-enough strain rates, the failure strength has been observed to become rate-dependent. The main objective of the present work is to interrogate the effects of loading rate and orientation on the failure strength of uniaxially compressed α-quartz at very high strain rates to ascertain the transition into rate sensitivity. For doing this, a miniature Kolsky bar is used to perform dynamic compression experiments on α-quartz at strain rates in the order of 103–104/s, and X-ray phase contrast imaging (XPCI) is used to directly visualize and quantify the cracking process. Experiments are carried out on nominally 1 mm and 2 mm rectangular α-quartz specimens compressed on the {-1,-1,2,0} and {-2,2,0,3} family of planes, resulting in strain rates of approximately 2000 - 5000/s to 20,000/s at the time of failure. The results show no appreciable orientation effects, suggesting that the loading configuration rather than the crystal orientation relative to the loading direction controls the orientation of crack propagation. However, the stress history and XPCI reveal that the failure strength is appreciably rate-sensitive within the present loading rate regimes. The stress history for both configurations exhibits an increasing average failure strength from around 2 GPa to 3 GPa as loading rates increase from 2000 - 5000/s to 20,000/s. The stress history and XPCI data are expected to provide crucial insight into the rate-dependence of the damage mechanisms occurring in this material.

Single-shot two-dimensional (2D) phase retrieval (PR) can recover the phase shift distribution within an object from a single 2D x-ray phase contrast image (XPCI). Two competing XPCI imaging modalities often used for single-shot 2D PR to recover material properties critical for predictive performance capabilities are: speckle-based (SP-XPCI) and propagation-based (PB-XPCI) XPCI imaging. However, PR from SP-XPCI and PB-XPCI images are, respectively, limited to reconstructing accurately slowly and rapidly varying features due to noise and differences in their contrast mechanisms. Herein, we consider a combined speckle- and propagation-based XPCI (SPB-XPCI) image by introducing a mask to generate a reference pattern and imaging in the near-to-holographic regime to induce intensity modulations in the image. We develop a single-shot 2D PR method for SPB-XPCI images of pure phase objects without imposing restrictions such as object support constraints. It is compared against PR methods inspired by those developed for SP-XPCI and PB-XPCI on simulated and experimental images of a thin glass shell before and during shockwave compression. Reconstructed phase maps show improvements in quantitative scores of root-mean-square error and structural similarity index measure using our proposed method.

Split-Hopkinson Pressure Bars (SHPB) or “Kolsky” bars are often employed for determining the high-rate compressive failure strength of high-strength brittle materials. However, experiments generating very high strain-ratesHigh strain-rate demand miniaturization of the setup for appropriately measuring decreasingly short loading events. Miniature aluminum and steel bars are often sufficient for this. However, for high enough strain-rates, miniaturization of these bars may require prohibitively small test specimens that can be inappropriate for inferring representative properties of materials with large grain size relative to the specimen size. The low Poisson’s ratio of beryllium relative to aluminum and steel is expected to minimize the effect of elastic wave dispersionDispersion on the measurable strain-rates in Kolsky barKolsky Bar experiments. For these reasons, we have developed a Be Kolsky barKolsky Bar apparatus, and, in this paper, we experimentally determine the dispersionDispersion characteristics of these bars and compare the results with those of similarly size setups made from aluminum and steel. The results show no appreciable dispersionDispersion in the data from the beryllium Kolsky barKolsky Bar setup, demonstrating its advantage over aluminum and steel.

Oblique shock wave–interface interactions of gases and liquids have been extensively studied in shock tubes using optical methods to measure equation-of-state (EOS) parameters. However, this is difficult with solids due to their opaqueness to visible light. X ray phase contrast imaging (XPCI) has the penetrative strength to probe solids while still being sensitive to mass density and enhancing the visibility of material boundaries. We investigate the accuracy and repeatability of measuring the mean value of the average mass density (areal density divided by thickness) over region S (BS) and flow deflection angle (θ) from XPCI images of a sample. To that end, a Hough transform-based method for measuring θ is developed. To measure BS, the XPCI image intensity probability density function (PDF) is modeled accounting for the spatial distribution of x ray energy, scintillator response, and pulse-to-pulse variation in the x ray intensity. In addition, a Monte Carlo-based algorithm for computing the BS PDF is developed. Both methods are validated on an impact-generated oblique shock wave interacting at a solid polymer-to-polymer interface. This is accomplished through a comparison to hydrodynamic simulations using well-established EOS. Under the modeling framework for the XPCI image intensity, BS is computed with an accuracy of <0.1% and precision of 3%–5%, while θ has an uncertainty of 0.2°, respectively. This shows that the XPCI-based model that is developed here could be an invaluable tool for high-fidelity testing of hydrodynamic models in shock polar configurations.

The dependence of the components of the elastic stiffness tensors (or elastic constants) of the organic explosives PETN, RDX, CL-20, DAAF, FOX-7, and HMX on hydrostatic pressure up to 10 GPa have been computed using dispersion-corrected density functional theory. We report the evolution of lattice parameters and the non-zero stiffnesses for the tetragonal, orthorhombic, and monoclinic crystal symmetries. Linear and quadratic dependencies of the components of the elastic stiffness tensors on volumetric compression and hydrostatic pressure are tabulated for use in single crystal plasticity models.

Conventional Split Hopkinson Pressure Bars (SHPB) or "Kolsky" bars are often used for determining the high-rate compressive yield and failure strength of materials. However, for experiments generating very high strain-rates (>10(3)/s) miniaturization of the setup is often required for minimizing the effects of elastic wave dispersion in order to enable the inference of decreasingly short loading events from the data. Miniature aluminum and steel bars are often sufficient for meeting these requirements. However, for high enough strain-rates, miniaturization of steel or aluminum Kolsky bars may require prohibitively small diameter bars and test specimens that could become inappropriate for inferring representative properties of materials with large grain size relative to the test specimen size. The use of a beryllium Kolsky bar setup is expected to enable high rates to be accessible with larger diameter bars/specimen combinations due to the inherent physical properties of beryllium, which are expected to minimize the effects of elastic wave dispersion. For this reason, a series of beryllium Kolsky bars have been developed, and, in this paper, the dispersion characteristics of these bars are measured and compare the data with those of similarly sized 7075-T6 aluminum and C350 maraging steel. The results, which agree well with the theory, show no appreciable frequency dependence of the elastic wavespeed in the data from the beryllium bars, demonstrating its advantage over aluminum and steel in application to Kolsky bars.

A new finite strain thermomechanical model for the high-rate deformation of the beta-polymorph of cyclotetramethylene tetranitramine (beta-HMX) has been developed and applied to simulations of plate impact experiments. The crystal plasticity model is based on a model developed previously for RDX (Luscher et al. 2017), which is extended to incorporate deformation twinning. Twinning during normal plate impacts is simulated with a phasefield twin model. First, material parameters governing the kinetics of dislocation slip are calibrated on the subset of simulations which had negative Schmid factors for the twin system. Second, a parametric study of the twin material parameters was performed to find suitable values. The results of the simulations with the phase-field twinning model are reported for impacts on several crystal orientations. We find that the twin growth decreases with increasing distance from the impact surface because of dissipation of the shock front via dislocation-mediated plasticity, and that the simulated interface velocity with and without phase-field twinning do not show appreciable differences. These modeling results suggest that the significance of twinning in beta-HMX cannot be determined with traditional loading configurations and diagnostics.

Mesoscale imperfections, such as pores and voids, can strongly modify the properties and the mechanical response of materials under extreme conditions. Tracking the material response and microstructure evolution during void collapse is crucial for understanding its performance. In particular, imperfections in the ablator materials, such as voids, can limit the efficiency of the fusion reaction and ultimately hinder ignition. To characterize how voids influence the response of materials during dynamic loading and seed hydrodynamic instabilities, we have developed a tailored fabrication procedure for designer targets with voids at specific locations. Our procedure uses SU-8 as a proxy for the ablator materials and hollow silica microspheres as a proxy for voids and pores. By using photolithography to design the targets' geometry, we demonstrate precise and highly reproducible placement of a single void within the sample, which is key for a detailed understanding of its behavior under shock compression. This fabrication technique will benefit high-repetition rate experiments at x-ray and laser facilities. Insight from shock compression experiments will provide benchmarks for the next generation of microphysics modeling.

Dispersion-corrected density functional theory calculations of the generalised stacking fault energy surface of the (101) plane in monoclinic beta-cyclotetramethylene tetranitramine (beta-HMX) indicate that a stacking fault with an energy of approximately 130 mi/m(2) at the translation vector t = 1/2[11 (1) over bar] is metastable. Using the energy of the metastable stacking fault and anisotropic linear elasticity theory, we have evaluated whether dislocations with total Burgers vectors b = [010], [10 (1) over bar] and [11 (1) over bar] will split into partial dislocations on (101). Our results suggest that it is not favourable for the [010) screw dislocation to split into 1/2 < 11 (1) over bar > partials but that the edge dislocation will split into partials separated by a narrow, 19.8 angstrom, stacking fault. Both screw and edge [11 (1) over bar] dislocations are predicted to split into two partials with splitting distances of about 28.7 and 43.4 angstrom, respectively. The screw and edge [10 (1) over bar] dislocation are also predicted to split into partials, with equilibrium distances of 16.3 and 43.2 angstrom, respectively. These results are consistent with recent analyses of Knoop indentation experiments in which the (101)[010) slip system is inactive whereas [10 (1) over bar] dislocations are glissile. Since twinning is an important deformation mechanism in beta-HMX, dispersion-corrected density functional theory calculations have also been used to compute the energy of the (101)[10 (1) over bar) twin boundary, for which we report a value of 163 mJ/m(2).

Inertial confinement fusion (ICF) holds increasing promise as a potential source of abundant, clean energy, but has been impeded by defects such as micro-voids in the ablator layer of the fuel capsules. It is critical to understand how these micro-voids interact with the laser-driven shock waves that compress the fuel pellet. At the Matter in Extreme Conditions (MEC) instrument at the Linac Coherent Light Source (LCLS), we utilized an x-ray pulse train with ns separation, an x-ray microscope, and an ultrafast x-ray imaging (UXI) detector to image shock wave interactions with micro-voids. To minimize the high- and low-frequency variations of the captured images, we incorporated principal component analysis (PCA) and image alignment for flat-field correction. After applying these techniques we generated phase and attenuation maps from a 2D hydrodynamic radiation code (xRAGE), which were used to simulate XPCI images that we qualitatively compare with experimental images, providing a one-to-one comparison for benchmarking material performance. Moreover, we implement a transport-of-intensity (TIE) based method to obtain the average projected mass density (areal density) of our experimental images, yielding insight into how defect-bearing ablator materials alter microstructural feature evolution, material compression, and shock wave propagation on ICF-relevant time scales.

Many questions regarding dynamic materials could be answered by using time-resolved ultra-fast imaging techniques to characterize the physical and chemical behavior of materials in extreme conditions and their evolution on the nanosecond scale. In this work, we perform multi-frame phase-contrast imaging (PCI) of micro-voids in low density polymers under laser-driven shock compression. At the Matter in Extreme Conditions (MEC) Instrument at the Linac Coherent Light Source (LCLS), we used a train of four x-ray free electron laser (XFEL) pulses to probe the evolution of the samples. To visualize the void and shock wave interaction, we deployed the Icarus V2 detector to record up to four XFEL pulses, separated by 1-3 nanoseconds. In this work, we image elastic waves interacting with the micro-voids at a pressure of several GPa. Monitoring how the material’s heterogeneities, like micro-voids, dictate its response to a compressive wave is important for benchmarking the performances of inertial confinement fusion energy materials. For the first time in a single sample, we have combined an ultrafast x-ray framing camera and four XFEL pulse train to create an ultrafast movie of micro-void evolution under laser-driven shock compression. Eventually, we hope this technique will resolve the material density as it evolves dynamically under laser shock compression.