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OBJECTIVE Transforming growth factor β (TGFβ) signaling plays a complex tissue-specific and nonlinear role in osteoarthritis (OA). This study was conducted to determine the osteocytic contributions of TGFβ signaling to OA. METHODS To identify the role of osteocytic TGFβ signaling in joint homeostasis, we used 16-week-old male mice (n = 9-11 per group) and female mice (n = 7-11 per group) with an osteocyte-intrinsic ablation of TGFβ receptor type II (TβRIIocy-/- mice) and assessed defects in cartilage degeneration, subchondral bone plate (SBP) thickness, and SBP sclerostin expression. To further investigate these mechanisms in 16-week-old male mice, we perturbed joint homeostasis by subjecting 8-week-old mice to medial meniscal/ligamentous injury (MLI), which preferentially disrupts the mechanical environment of the medial joint to induce OA. RESULTS In all contexts, independent of sex, genotype, or medial or lateral joint compartment, increased SBP thickness and SBP sclerostin expression were spatially associated with cartilage degeneration. Male TβRIIocy-/- mice, but not female TβRIIocy-/- mice, had increased cartilage degeneration, increased SBP thickness, and higher levels of SBP sclerostin compared with control mice (all P < 0.05), demonstrating that the role of osteocytic TGFβ signaling on joint homeostasis is sexually dimorphic. With changes in joint mechanics following injury, control mice had increased SBP thickness, subchondral bone volume, and SBP sclerostin expression (all P < 0.05). TβRIIocy-/- mice, however, were insensitive to subchondral bone changes with injury, suggesting that mechanosensation at the SBP requires osteocytic TGFβ signaling. CONCLUSION Our results provide new evidence that osteocytic TGFβ signaling is required for a mechanosensitive response to injury, and that osteocytes control SBP homeostasis to maintain cartilage health, identifying osteocytic TGFβ signaling as a novel therapeutic target for OA.

Bacteria have remarkably robust cell shape control mechanisms. For example, cell diameter only varies by a few percent across a given population. The bacterial actin homolog, MreB, is necessary for establishment and maintenance of rod shape although the detailed properties of MreB that are important for shape control remained unknown. In this study, we perturb MreB in two ways: by treating cells with the polymerization-inhibiting drug A22 and by creating point mutants in mreB. These perturbations modify the steady-state diameter of cells over a wide range, from 790 ± 30 nm to 1700 ± 20 nm. To determine which properties of MreB are important for diameter control, we correlated structural characteristics of fluorescently tagged MreB polymers with cell diameter by simultaneously analyzing three-dimensional images of MreB and cell shape. Our results indicate that the helical pitch angle of MreB inversely correlates with the cell diameter of Escherichia coli. Other correlations between MreB and cell diameter are not found to be significant. These results demonstrate that the physical properties of MreB filaments are important for shape control and support a model in which MreB organizes the cell wall growth machinery to produce a chiral cell wall structure and dictate cell diameter.

Species that can undergo changes in electronic configuration as a result of an external stimulus such as pH or solvent polarity can play an important role in sensors, conducting polymers, and molecular switches. One way to achieve such structures is to couple two redox-active fragments, where the redox activity of one of them is strongly dependent upon environment. We report on two new verdazyls, one subsituted with a di-tert-butyl phenol group and the other with a dimethylaminophenyl group, that have the potential for such behavior upon oxidation. Oxidation of both verdazyls with copper(II) triflate in acetonitrile gives diamagnetic verdazylium ions characterized by NMR and UV-vis spectroscopies. Deprotonation of the phenol-verdazylium results in electron transfer and a switch from a singlet state to a paramagnetic triplet diradical identified by electron spin resonance. The dimethylaminoverdazylium 9 has a diamagnetic ground state, in line with predictions from simple empirical methods and supported by density functional theory calculations. These results indicate that verdazyls may complement nitroxides as spin carriers in the design of organic molecular electronics.

The rod shape of most bacteria requires the actin homolog, MreB. Whereas MreB was initially thought to statically define rod shape, recent studies found that MreB dynamically rotates around the cell circumference dependent on cell wall synthesis. However, the mechanism by which cytoplasmic MreB is linked to extracytoplasmic cell wall synthesis and the function of this linkage for morphogenesis has remained unclear. Here we demonstrate that the transmembrane protein RodZ mediates MreB rotation by directly or indirectly coupling MreB to cell wall synthesis enzymes. Furthermore, we map the RodZ domains that link MreB to cell wall synthesis and identify mreB mutants that suppress the shape defect of ΔrodZ without restoring rotation, uncoupling rotation from rod-like growth. Surprisingly, MreB rotation is dispensable for rod-like shape determination under standard laboratory conditions but is required for the robustness of rod shape and growth under conditions of cell wall stress.

Pathogenic Vibrio cholerae remains a major human health concern. V. cholerae has a characteristic curved rod morphology, with a longer outer face and a shorter inner face. Previously, the mechanism and function of this curvature were unknown. Here we identify and characterize CrvA, the first curvature determinant in V. cholerae. CrvA self-assembles into filaments at the inner face of cell curvature. Unlike traditional cytoskeletons, CrvA localizes to the periplasm, and thus could be considered a periskeletal element. To quantify how curvature forms, we developed QuASAR (Quantitative Analysis of Sacculus Architecture Remodeling), which measures subcellular peptidoglycan dynamics. QuASAR reveals that CrvA asymmetrically patterns peptidoglycan insertion rather than removal, causing more material insertion into the outer face than the inner face. Furthermore, crvA is quorum regulated and CrvA-dependent curvature increases at high cell density. Finally, we demonstrate that CrvA promotes motility in hydrogels and confers an advantage in host colonization and pathogenesis.

Multi-million atom non-equilibrium molecular dynamics (MD) simulations give significant insight into the transient processes that occur under shock compression. Picosecond X-ray diffraction enables the probing of materials on a timescale fast enough to test such effects. In order to simulate diffraction patterns, Fourier methods are required to gain a picture of reciprocal lattice space. We present here results of fast Fourier transforms of atomic coordinates of shocked crystals simulated by MD, and comment on the computing power required as a function of problem size. The relationship between reciprocal space and particular experimental geometries is discussed. © 2007 American Institute of Physics.

In situ picosecond X-ray diffraction has proved to be a useful tool in furthering our understanding of the response of shocked crystals at the lattice level. To date the vast majority of this work has used single crystals as the shocked samples, owing to their diffraction efficiency, although the study of the response of polycrystalline samples is clearly of interest for many applications. We present here the results of experiments to develop sub-nanosecond powder/polycrystalline diffraction using a cylindrical pinhole camera. By allowing the incident X-ray beam to impinge on the sample at non-normal angles, the response of grains making a variety of angles to the shock propagation direction can potentially be interrogated. © 2007 American Institute of Physics.

Over the past few years we have developed an image-processing framework that allows us to extract precise 3D shapes of bacterial cells from fluorescence microscopy data. This active mesh approach minimizes the difference between an observed Z-stack and model shapes that have been convolved with the experimental point spread function. From these xyz coordinates, we calculate geometric parameters such as local curvatures, surface areas, and the relative enrichment of fluorescent signals. Our fluorescent signal is the concentration of the bacterial actin homolog MreB. We have integrated a fluorescent sandwich fusion of MreB with msfGFP at the native locus. This construct has unperturbed mass-doubling times and a proper rod-like shape.We have previously shown in both two and three dimensions that MreB is enriched at negative curvatures and away from positive curvatures [Ursell et al. PNAS 2014]. Here we show in the straight rod Escherichia coli and the curved rod Caulobacter crescentus that MreB senses local surface curvature and, as a result, helps ensure rod-like growth of the cell. The curvature sensed by Escherchia coli MreB is the Gaussian curvature of the surface, the product of the two principal curvatures. Using MreB point mutants that have altered curvature sensitivities, we are testing the hypothesis that cells straighten deformations by patterning growth at the proper geometry. To stably generate their comma shaped geometry, Caulobacter crescentus have developed ways to overcome this straightening mechanism. The enrichment profile of CcMreB shows a plateau in enrichment at small, positive Gaussian curvature values that depends on the protein crescentin. In addition to its structural role in bending the cell, we propose a mechanism whereby crescentin alters the curvature preference of MreB and allows the stabile propagation of the Caulobacter shape.

The ability to acquire large-scale recordings of neuronal activity in awake and unrestrained animals is needed to provide new insights into how populations of neurons generate animal behavior. We present an instrument capable of recording intracellular calcium transients from the majority of neurons in the head of a freely behaving Caenorhabditis elegans with cellular resolution while simultaneously recording the animal’s position, posture, and locomotion. This instrument provides whole-brain imaging with cellular resolution in an unrestrained and behaving animal. We use spinning-disk confocal microscopy to capture 3D volumetric fluorescent images of neurons expressing the calcium indicator GCaMP6s at 6 head-volumes/s. A suite of three cameras monitor neuronal fluorescence and the animal’s position and orientation. Custom software tracks the 3D position of the animal’s head in real time and two feedback loops adjust a motorized stage and objective to keep the animal’s head within the field of view as the animal roams freely. We observe calcium transients from up to 77 neurons for over 4 min and correlate this activity with the animal’s behavior. We characterize noise in the system due to animal motion and show that, across worms, multiple neurons show significant correlations with modes of behavior corresponding to forward, backward, and turning locomotion.

Cells typically maintain characteristic shapes, but the mechanisms of self-organization for robust morphological maintenance remain unclear in most systems. Precise regulation of rod-like shape in Escherichia coli cells requires the MreB actin-like cytoskeleton, but the mechanism by which MreB maintains rod-like shape is unknown. Here, we use time-lapse and 3D imaging coupled with computational analysis to map the growth, geometry, and cytoskeletal organization of single bacterial cells at subcellular resolution. Our results demonstrate that feedback between cell geometry and MreB localization maintains rod-like cell shape by targeting cell wall growth to regions of negative cell wall curvature. Pulse-chase labeling indicates that growth is heterogeneous and correlates spatially and temporally with MreB localization, whereas MreB inhibition results in more homogeneous growth, including growth in polar regions previously thought to be inert. Biophysical simulations establish that curvature feedback on the localization of cell wall growth is an effective mechanism for cell straightening and suggest that surface deformations caused by cell wall insertion could direct circumferential motion of MreB. Our work shows that MreB orchestrates persistent, heterogeneous growth at the subcellular scale, enabling robust, uniform growth at the cellular scale without requiring global organization.

We performed molecular dynamics simulations of shock wave propagation in diamond in the [110] crystallographic direction and observed an anomalous response of the material. This regime is characterized by absence of plastic deformation in the intermediate interval of shock wave intensities between shear‐deformation and overdriven rehybridization shock wave regimes.

We explore various relations for the detonation energy and velocity as they relate to the inverse radius of the cylinder. The effective detonation rate‐inverse slope relation seen in reactive flow models can be used to derive the familiar Eyring equation. Generalized inverse radii can be shown to fit large quantities of cylinder results. A rough relation between detonation energy and detonation velocity is found from collected JWL values. Cylinder test data for ammonium nitrate mixes down to 6.35 mm radii are presented, and a size energy effect is shown to exist in the Cylinder test data. The relation that detonation energy is roughly proportional to the square of the detonation velocity is shown by data and calculation.

A strong electric field can be generated when the shock‐induced depoling current from a normally poled PZT 95/5 sample is passed through a large resistive load. The portion of total depoling current that is retained on the sample electrodes to account for capacitance is governed by the dynamic dielectric properties of both unshocked and shocked material. Early studies used measured load currents from single samples to assess models for dielectric response. In more recent studies, we used shock‐driven circuits in which multiple PZT 95/5 elements were displaced both parallel and perpendicular to the shock motion. This allowed both load and charging currents to be measured for individual elements that are subjected to shock compression and release at different times. In the present study, these techniques have been utilized to examine dielectric properties in PZT 95/5 samples at initial temperatures from −56 to 74 °C. Significant changes in permittivity with temperature are observed in both unshocked and sho...

We are studying high‐pressure laser produced shock waves in silicon (100). To examine the material dynamics, we are performing pump‐probe style experiments utilizing 600 ps and 40 fs laser pulses from a Ti:sapphire laser. Two‐dimensional interferometry reveals information about the shock breakout, while third harmonic light generated at the rear surface is used to infer the crystalline state of the material as a function of time. Sustained third harmonic generation (THG) during a ∼100 kbar shock breakout indicate that the rear surface remains crystalline for at least 3 ns. However, a decrease in THG during a ∼300 kbar shock breakout suggests a different behavior, which could include a change in crystalline structure.

Work carried out on the development of a single‐stage implosion‐driven hypervelocity launcher is presented. Explosives surrounding a thin‐walled tube filled with helium works similar to the pump tube of a conventional light gas gun. Implosion of the tube drives a strong shock that reflects back and forth between the projectile and the implosion pinch, generating very high temperatures and pressures. Experiments to evaluate the implosion dynamics and performance of the pump tube were carried out, with attention given to the helium fill pressure, diameter of the pump tube, thickness of the explosive layer, and the presence of a tamper. Simple analytic models were used to approximate the performance of the launcher; their advantages and limitations are discussed. Experiments with an implosion‐driven launcher demonstrated muzzle velocities of 4 km/s with 5‐mm‐diameter aluminum projectiles, agreement with analytical models of performance is discussed. Projectile integrity was verified by high‐speed photography and other diagnostics.

We present analytic solutions to two test problems that can be used to check the hydrodynamic implementation in computer codes designed to calculate the propagation of shocks in spherically convergent geometry. Our analysis is restricted to fluid materials with constant bulk modulus. In the first problem we present the exact initial acceleration and pressure gradient at the outer surface of a sphere subjected to an exponentially decaying pressure of the form P(t) = P{sub 0}e{sup -at}. We show that finely-zoned hydro-code simulations are in good agreement with our analytic solution. In the second problem we discuss the implosions of incompressible spherical fluid shells and we present the radial pressure profile across the shell thickness. We also discuss a semi-analytic solution to the time-evolution of a nearly spherical shell with arbitrary but small initial 3-dimensional (3-D) perturbations on its inner and outer surfaces.

We have been developing the physically based QinetiQ Porter-Gould (P-G) model for the mechanical response of PBXs over a number of years and applying it to the solution of real scenarios involving impact and blast. The main difficulty with these models is predicting the intermediate strain rate regime where the relaxation time for the polymer is of the same order as the duration of the loading (e.g. as in a Hopkinson bar test). The other main issue is the ability of the model to predict the stress/strain data as a function of temperature up to and through the glass transition temperature. The paper presents predictions from the QinetiQ P-G model compared to quasi-static compression and Hopkinson bar compression test data and discusses the results in terms of requirements for future developments of the model. © 2007 American Institute of Physics.

Dynamic loading experiments were performed on inert Teflon (Polytetrafluoroethylene) samples, initially heated to the temperature of 200 °C, to test its behavior under these conditions for its use in other heated experiments. Tests were performed in the 100 mm diameter bore propellant driven gas gun with piezo‐resistive manganin pressure gauges imbedded into the samples to measure loading pressures. Experimental data provided new information on the shock velocity—particle velocity relationship for the heated material and showed no adverse effect of temperature on the insulating properties of the material.

Attenuating wave profiles from shock experiments on tungsten carbide powder are compared to calculations from the continuum P‐λ model and a 2‐D mesoscale model to gain insight into the suitability of the two models. When calibrated, both models accurately capture the Hugoniot response of the powder and the arrival times of unattenuated steady waves. Their amplitudes are more accurately given by the mesoscale model since its reshock states are above the Hugoniot as seen experimentally; the P‐λ model, in contrast, reshocks along the Hugoniot. When the attenuating wave is in the range of the Hugoniot data, the models predict attenuation correctly. However, when attenuation falls below the Hugoniot data both models are somewhat inaccurate, and the material response seems to lie between the two models. The final aspect considered is the wave rise time, which is qualitatively correct for the mesoscale model but completely inaccurate for the P‐λ model.

Dynamic fracture experiments are conducted for circular plates, vessel endplates and conical frustums of A2017‐7075 aluminum alloys and 304 stainless steel, using a testing apparatus developed applying wire‐row explosion technique to initiation, where tensile stress waves are generated producing spall in the specimens by the direct incidence of plane detonation waves of the explosive PETN. A VISAR system is adopted to observe the free‐surface velocity histories of the specimens. The signals for basic circular plate specimens indicate the characteristics of the failure for tested materials, effects of explosive thickness variations and the configuration of specimens. Hydro codes are satisfactorily applied to simulate the experimental signal data and observed damage phenomena of recovered specimens. Next, an explosive‐filled cylindrical vessel with an endplate at the one end is initiated at the other end surface and expanded by axially propagating explosive detonation to fracture. Both the VISAR signals and...

Isentropic compression of a solid to 100's of GPa by a ramped, planar compression wave allows measurement of material properties at high strain and at modest temperature. Introduction of a measurement plane disturbs the flow, requiring special analysis techniques. If the measurement interface is windowed, the unsteady nature of the wave in the window requires special treatment. When the flow is hyperbolic the equations of motion can be integrated backward in space in the sample to a region undisturbed by the interface interactions, fully accounting for the untoward interactions. For more complex materials like hysteretic elastic/plastic solids or phase changing material, hybrid analysis techniques are required.

Extended X-ray absorption fine structure (EXAFS) measurements on shocked polycrystalline iron have provided further evidence for the shock induced alpha - epsilon phase transition in iron. However, recent molecular dynamics investigation of this system has suggested the presence of fcc material in the shocked region. In this paper we will investigate the difficulties in simulating EXAFS signals from molecular dynamics data. We will aim to show that in the case of the shock induced a - E transition EXAFS is insensitive to the type of close packing of the product phase.

We assess the accuracy of a recently developed nonlocal density functional in treating van der Waals molecular systems. This new functional contains a non‐empirical treatment of dispersion interactions and exhibits the appropriate behavior in the high‐density limit. We present results for three model noncovalently bonded systems: the methane‐benzene complex, a dimethylnitramine dimer, and a dimer of triaminotrinitrobenzene in its crystalline symmetry. Predicted interaction energies are greatly improved with this new approach as compared to conventional density functionals. Intermolecular distances are also improved, though the degree of improvement varies with the character of the intermolecular interactions, with dispersion dominated systems showing the greatest improvement.

Previously established Ignition and Growth reactive flow models for the detonating triaminotrinitrobenzene (TATB) based plastic bonded explosives LX‐17 and PBX 9502 are applied to recent experimental detonation propagation/failure experiments using unconfined cones, confined arcs, and unconfined arcs. The conical experiments are initially overdriven by the convergent geometry and then fail to detonate at smaller diameters than do unconfined cylindrical charges when the radial rarefaction wave lowers the shock pressure and temperature and thus decreases the chemical energy release rate. Unconfined TATB arcs detonate more slowly than cylindrical charges on the inner surface and exhibit large phase velocities on the outer surface. Confinement reduces but does not eliminate these effects. The Ignition and Growth model calculations based on parameters normalized to a great deal of one‐, two‐ and three‐dimensional detonation propagation data reproduce these features and agree closely with experimental detonation velocity and arrival time data.

We report herein a series of ab initio simulations of water under both static and shocked conditions. We have calculated the coherent x-ray scattering intensity of several phases of water under high pressure, using ab initio Density Functional Theory (DFT). We provide new atomic scattering form factors for water at extreme conditions, which take into account frequently neglected changes in ionic charge and electron delocalization. We have also simulated liquid water undergoing shock loading of velocities from 5-11 km/s using the Multi-Scale Shock Technique (MSST). We show that Density Functional Theory (DFT) molecular dynamics results compare extremely well to experiments on the water shock Hugoniot.

We report on large-scale non-equilibrium atomistic simulations of shock-induced transformations in poly crystalline iron samples. These simulations show that, depending on the crystallographic orientation of the body-centered-cubic (bcc) parent phase grains with respect to the shock direction, a significant fraction of the product phase can be face-centered-cubic (fcc) instead of the expected hexagonal-close-packed (hcp) structure. This observation is explained by the existence of different transformation mechanisms for shocks along different crystallographic directions. We conclude that the observation of different product phases can be explained by simple geometric considerations of the involved transformation mechanisms between the parent bcc structure and the product hcp and fcc structures. Ultrafast high-energy laser-based experiments are underway to further investigate this subject. © 2007 American Institute of Physics.

Currently there are three platforms that offer quasi‐isentropic compression or ramp‐wave compression (RWC): light‐gas gun, magnetic flux (Z‐pinch), and laser. We focus here on the light‐gas gun technique and on some current theoretical insights from experimental data. An impedance gradient through the length of the impactor provides the pressure pulse upon impact to the subject material. Applications and results are given concerning high‐pressure strength and the liquid‐to‐solid, phase transition of water giving its first associated phase fraction history. We also introduce the Korteweg‐deVries‐Burgers equation as a means to understand the evolution of these RWC waves as they propagate through the thickness of the subject material. This model equation has the necessary competition between non‐linear, dispersion, and dissipation processes, which is shown through observed structures that are manifested in the experimental particle velocity histories. Such methodology points towards a possibility of quantifying dissipation, through which RWC experiments may be analyzed.

We present results of direct numerical simulations of impact of hypervelocity particle stream with a target. The stream of interest consists of submillimeter (30–300 micron) brittle ceramic particles. Current supercomputer capabilities make it possible to simulate a realistic size of streams (up to 20 mm in diameter and 500 mm in length) while resolving each particle individually. Such simulations make possible to study the damage of the target from synergistic effects of individual impacts. In our research we fixed the velocity distribution along the axis of the stream (1–4 km/s) and volume fraction of the solid material (1–10%) and study effects of particle size variation, particle and target material properties and surrounding air properties. We ran 3D calibration simulations with up to 10 million individual particles and conducted sensitivity studies with 2D cylindrically symmetric simulations. We used an Eulerian Godunov hydrocode with adaptive mesh refinement. The particles, target material and air ...

We conducted laser‐induced spall experiments aimed at studying how a material's microstructure affects the tensile fracture characteristics at high strain rates (>106 s−1). We used the Z‐Beamlet Laser at Sandia National Laboratory to drive shocks and to measure the spall strength of aluminum targets with various microstructures. The targets were recrystallized, high‐purity aluminum (Al‐HP RX), recrystallized aluminum + 3 wt.% magnesium (Al‐3Mg RX), and cold‐worked aluminum +3 wt.% magnesium (Al‐3Mg CW). The Al‐3Mg RX and Al‐3Mg CW are used to explore the roles that solid‐solution alloying and cold‐work strengthening play in the spall process. Using a line‐VISAR (Velocity Interferometer System for Any Reflector) and analysis of recovered samples, we were able to measure spall strength and determine failure morphology in these targets. We find that the spall strength is highest for Al‐HP RX. Analysis reveals that material grain size plays a vital role in the fracture morphology and spall strength results.

Since the carbon, diamond phase has such a high yield strength, dynamic simulations must account for strength even for strong shock waves ({approx} 3 Mbar). We have determined an initial parametrization of two strength models: Steinberg-Guinan (SG) and a modified or improved SG, that captures the high pressure dependence of the calculated shear modulus up to 10 Mbar. The models are based upon available experimental data and on calculated elastic moduli using robust density functional theory. Additionally, we have evaluated these models using hydrodynamic simulations of planar shocks experiments.

We developed a velocimeter system several years ago that uses the heterodyne method [1]. This system is assembled from commercially available components that were developed for the telecommunications industry. There are several advantages of this system over the traditional VISAR method that has made it increasingly popular. This system is compact, portable, and relatively inexpensive. The maximum velocity of this system is determined by the electrical bandwidth of the electronics and the digitizer sample rate. The maximum velocity for the system described here is over 5 km/s.

High strength fabrics provide lightweight impact protection and are employed in a wide range of applications. Examples include body armor for law enforcement and military personnel and orbital debris shielding for the International Space Station. Numerical simulation of impact effects on fabric protection systems is difficult, due to the complex woven structure of the fabric layers and the typical application of fabrics in a multilayer configuration. Recent research has applied a new particle‐element method to the simulation of impact effects on multilayer fabrics, applicable over a wide range of impact velocities, for use in body armor and orbital debris shielding design applications.

Recent investigations of the dynamic compressive strength of cold isostatically pressed composites of polytetrafluoroethylene (PTFE), tungsten (W) and aluminum (Al) powders show significant differences depending on the size of metallic particles. The addition of W increases the density and overall strength of the sample. To investigate relatively large deformations multi-material Eulerian and arbitrary Lagrangian-Eulerian methods, which have the ability to efficiently handle the formation of free surfaces, were used. The calculations indicate that the increased strength of the sample with fine metallic particles is due to the formation of force chains under dynamic loading. This phenomenon occurs even at larger porosity of the PTFE matrix in comparison with samples with larger particle size of W and higher density of the PTFE matrix., 5 pages, 6 figures

The Isentropic Compression Experiment (ICE) technique has proven to be a valuable complement to the well‐established method of shock compression of condensed matter. However, whereas the high‐pressure compression response of interferometer window materials has been studied extensively under shock loading, similar knowledge of these materials under quasi‐isentropic loading is limited. We present recent experimental results on the quasi‐isentropic compression of the high‐pressure window LiF on the Veloce pulsed power generator. While it is a frequently used window material in quasi‐isentropic loading and unloading experiments, the unloading response of LiF is usually neglected. It will be shown how the strength of LiF may influence the wave profile analysis and thus the inferred compressive strength of the material being studied.

At Sandia National Laboratories, we have coined the term “microenergetics” to describe sub‐millimeter energetic material studies aimed at gaining knowledge of combustion and detonation behavior at the mesoscale. Our approach is to apply technologies developed by the microelectronics industry to fabricate test samples with well‐defined geometries. Substrates have been fabricated from materials such as silicon and ceramics, with channels to contain the energetic material. Energetic materials have been loaded into the channels, either as powders, femtosecond laser‐micromachined pellets, or as vapor‐deposited films. Ignition of the samples has been achieved by simple hotwires, integrated semiconductor bridges, and also by lasers. Additionally, grain‐scale patterning has been performed on explosive films using both oxygen plasma etching and femtosecond laser micromachining. We have demonstrated simple work functions in microenergetic devices, such as piston motion, which is also a relevant diagnostic to examine combustion properties. Detonation has been achieved in deposited explosive films, recorded by high‐speed photography.

We report the first experimental observation of highly nonlinear signals propagating in a two dimensional system composed of granular chains. In this system one of the chains contacts two others to allow splitting and redirecting the solitary-like signal formed in the first chain. The system consists of a double Y-shaped guide in which high- and low-modulus chains of spheres are arranged in various geometries. We observed fast splitting of the initial pulse, rapid chaotization of the signal and sharp bending of the propagating acoustic information. Pulse and energy trapping was also observed in composite systems assembled from hard- and soft-particles in the branches.

Enhanced heating from shock compression of a porous material can potentially suppress or delay cracking of the material on subsequent expansion. In this paper we quantify the expected enhanced heating in an experiment in which a sector of a thin cylindrical shell is driven from the inside surface by SEMTEX high explosive ({approx}1 {micro}s FWHM pressure pulse with peak pressure {approx}21.5 GPa). We first derive an analytical equation of state (EOS) for porous metals, then discuss the coupling of this EOS with material elastic-plastic response in a 2D hydrocode, and then discuss the modeling of the HE experiment with both fully dense and 10% porous Ta and a Bi/Ta composite. Finally, we compare our modeling with some recent experimental data.

Material heterogeneity appears to give rise to variability in the yield behavior of ceramics and metals under shock loading conditions. The line‐imaging VISAR provides a way to measure this variability, which may then be quantified by Weibull statistics or other methods. Weibull methods assign a 2‐parameter representation of failure phenomena and variability. We have conducted experiments with tantalum (25 and 40 μm grains) and silicon carbide (SiC‐N with 5 μm grains). The tantalum HEL variability did not depend systematically on peak stress, grain size or sample thickness, although the previously observed precursor attenuation was present. SiC‐N HEL variability within a single shot was approximately half that of single‐point variability in a large family of shots; these results are more consistent with sample‐to‐sample variation than with variability due to changing shot parameters.

Alumina‐filled epoxies are composites having constituents with highly dissimilar mechanical properties, resulting in complex behavior during shock compression and release. Two distinguishing characteristics are amplitude‐dependent wave structures and high release wave velocities. Recent studies examined the effects of various compositional changes on these shock properties. As expected, the strongest effects were observed when the total alumina volume fraction was reduced in steps from a nominal 43% to 0%. In the present study, compositions prepared over the same range of alumina loadings were examined at initial temperatures that were nominally −55 °C or 70 °C. Experimental configurations were identical to previous room‐temperature experiments. Laser interferometry and wave timing were used to obtain transmitted wave profiles, Hugoniot states, and release wave velocities. Initial densities were determined from thermal expansion coefficients measured for each composition. Although initial density changes ...

Al 6061‐T6 powder particles with a partially nanocrystalline graded microstructure in three distinct morphologies are compacted at an impact velocity of 650 m/s. Recovered samples are characterized to determine degree of compaction, deformation characteristics, and mechanical properties. Compacts range from 96–98% of theoretical density, exhibiting relatively low elastic moduli. Nano‐indentation yields relatively consistent hardness values of ∼1.4 GPa, indicating hardness of starting powders is preserved after compaction. Micro‐indentation indicates varying degrees of compaction through specimen cross‐section, which is supported by EBSD and optical microscopy.

This work predicts the critical conditions required for the onset of reaction runaway in a narrow high‐explosive slot intended to simulate a crack. We review ongoing experiments where flames propagated through such slots at velocities up to 10 km/s, reaching pressures in excess of 1 kbar. A model is developed where slot pressurization is attributed to gas‐dynamic choking at the slot exit. The combination of choking and a pressure‐dependent reaction rate is shown to be capable of runaway reaction for a range of slot dimensions and pressures. This model agrees with experimental pressure measurements of reaction runaway in slots and provides a mechanism for the erratic burning observed with some explosives under high pressure.

MD simulations in monocrystalline and bicrystalline copper were carried out with LAMMPS to reveal void growth mechanisms. The specimens were subjected to both tensile uniaxial and hydrostatic strains; the results confirm that the emission of (shear) loops is the primary mechanism of void growth. However, these shear loops develop along two slip planes (and not one, as previously thought), in a heretofore unidentified mechanism of cooperative growth. The emission of dislocations from voids is the first stage, and their reaction and interaction is the second stage. These loops, forming initially on different {111} planes, join at the intersection, the Burgers vector of the dislocations being parallel to the intersection of two {111} planes: a 〈110〉 direction. Thus, the two dislocations cancel at the intersection and a biplanar shear loop is formed. The expansion of the loops and their cross slip leads to the severely work hardened layer surrounding a growing void. Calculations were carried out on voids with different sizes, and a size dependence of the stress response to emitted dislocations was observed, in disagreement with the Gurson model[1] which is scale independent. Calculations were also carried out for a void at the interface between two grains.

Hypervelocity impact tests were carried out at 4.8 km/s using the Open University's All Axis Light Gas Gun (AALGG) in the Planetary and Space Sciences Research Institute (PSSRI)'s Hypervelocity Impact Laboratory. A first estimate of the peak loading pressures was made using preliminary hydrocode simulations, supported by calculations. Following a review of existing published quartz and sandstone data, our previously published plate impact data were combined with high pressure quartz data to produce a synthetic Hugoniot. This will form the basis of future hydrocode modelling, as a linear Us‐Up relationship does not adequately represent the behaviour of sandstone over the pressure range of interest, as indicated by experimental data on Coconino sandstone. This work is a precursor to investigating the biological effects of shock on microorganisms in sandstone targets. This paper also contains the first presentation of results of ultra high speed imaging of hypervelocity impact at the Open University.

Shock initiation experiments on the LLM‐105 based explosive RX‐55‐AA (95% LLM‐105, 5% Viton by weight) were performed at 25 °C and 150 °C to obtain in‐situ pressure gauge data, run‐distance‐to‐detonation thresholds, and Ignition and Growth modeling parameters. A 101 mm diameter propellant driven gas gun was utilized to initiate the explosive sample with manganin piezoresistive pressure gauge packages placed between sample slices. The run‐distance‐to‐detonation points on the Pop‐plot for these experiments showed agreement at 25 °C with previously published data on a similar LLM‐105 based formulation RX‐55‐AB as well as a slight sensitivity increase at elevated temperature (150 °C) as expected. Ignition and Growth modeling parameters were obtained with a reasonable fit to the experimental data.

Shock initiation experiments on the explosive PBX 9501 (95% HMX, 2.5% estane, and 2.5% nitroplasticizer by weight) were performed at pressures below 3 GPa to obtain in‐situ pressure gauge data, run‐distance‐to‐detonation thresholds, and Ignition and Growth modeling parameters. Propellant driven gas guns (101 mm and 155 mm) were utilized to initiate the PBX 9501 explosive with manganin piezoresistive pressure gauge packages placed between sample slices. The run‐distance‐to‐detonation points on the Pop‐plot for these experiments showed agreement with previously published data and Ignition and Growth modeling parameters were obtained with a good fit to the experimental data. This parameter set will allow accurate code predictions to be calculated for safety scenarios in the low‐pressure regime (below 3 GPa) involving PBX 9501 explosive.

Stress‐density (σ‐ρ) loading paths of both Ta and W under ramped compression were measured up to 300 GPa. For similar ramp loading conditions, σ(ρ) for Ta lies close to the cold curve and significantly below the Hugoniot, while σ(ρ) for W lies close to the Hugoniot and significantly above the cold curve. The elastic yield limit is reported for W and Ta with thicknesses 400–700 μm.