Search

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.

A bacteria's shape plays a large role in determining its mechanism of motility, energy requirements, and ability to avoid predation. Although it is a major factor in cell fitness, little is known about how cell shape is determined or maintained. These problems are made worse by a lack of accurate methods to measure cell shape in vivo, as current methods do not account for blurring artifacts introduced by the microscope. Here, we introduce a method using 2D active surfaces and forward convolution with a measured point spread function to measure the 3D shape of different strains of E. coli from fluorescent images. Using this technique, we are also able to measure the distribution of fluorescent molecules, such as polymers, on the cell surface. This quantification of the surface geometry and fluorescence distribution allow for a more precise measure of 3D cell shape and is a useful tool for measuring protein localization and the mechanisms of bacterial shape control.

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.

Lead is a material of fundamental scientific interest because of its relatively low melting point and ductility. Alloys of lead are used for practical reasons which include the desire to have a slightly harder form of the metal to be able to handle without damage. A practical question is how lead and alloys with small amounts of alloying element respond to dynamic loading. Here we report the results of fundamental shock wave compression experiments on an alloy of lead that contains approximately 3.5 wt.% antimony. Good results for several dynamic properties have been obtained, and are reported here.

In the present paper we address the phenomenon by which embedded manganin gauges between soda‐lime plates always fail to record the full stress history when the shock level is above 5.5 GPa. Our explanation for this early failure is related to the fracture wave generated in the backing glass plate which affects the gauge or its leads, in such a way as to disconnect them. 1D numerical simulations and several plate impact experiments were made in order to support our assumption.

To further understand the deformation mechanisms associated with the dynamic yield point and shock initiation of single crystals of explosives, shock wave loading experiments were performed on oriented crystals of RDX at impact stresses of approximately 1 GPa. Defect content in the crystals and the surface polishing technique were varied and the resulting materials were characterized by optical microscopy. In previous experiments on (100) oriented crystals, multiple loading inflections in the proximity of the dynamic yield point were thought to be due to cracks introduced during experiment assembly. Experiments were performed to try to reproduce these features on other crystal orientations by deliberately inducing cracks and/or choosing crystals with voids. Previously observed inflections in the proximity of the dynamic yield point can be interpreted as discrete yielding events potentially caused by one or more defects that eventually lead to a disperse elastic‐plastic transition as observed in randomly oriented powders or perhaps sufficiently defective single crystals. The intent of this effort is to understand the effects of orientation dependent crystal plasticity and large defects, such as surface finish, cracks, and voids, on dynamic deformation separately so that the efficiencies and importance of deformation mechanisms in the initiation process can begin to be delineated for energetic molecular single crystals.

The interaction of high‐power pulsed‐laser beam with metal targets in air from 1.06 μm, 5 ns, 3 J/pulse max, Nd:YAG pulsed laser is investigated together with hydrodynamic theories of laser‐supported detonation (LSD) wave and multi‐material reactive Euler equations. The high speed blast wave generated by the laser ablation of metal reaches maximum velocity of several thousand meters per second. The apparently similar flow conditions to those of reactive shock wave allow one to apply the equations of motion for energetic materials and to understand the explosive behavior of metal vaporization upon laser ablation. The characteristic time at which planar to spherical wave transition occurs is confirmed at low (20 mJ/pulse) to higher (200 mJ/pulse) beam intensities. The flow structure behind the leading shock wave during the early planar shock state is confirmed by the high‐resolution multi‐material hydrocode originally developed for shock compression of condensed matter.

Plate impact experiments have been performed to determine the dynamic behavior of conventional and high strength mortars. Longitudinal stresses have been directly measured by means of embedded polyvinylidene fiuoride (PVDF) gauges up to 1 GPa. A 200 mm‐cal. powder gun enable us to measure longitudinal stresses at several point from the impact surface, simultaneously. The longitudinal stress profile of conventional mortar, which static compression strength is 35 MPa, shows a two‐wave structure associated with the compaction of pores and the onset of damage around 200 MPa. Although the longitudinal stress profile of high strength mortar, which static compression strength is 120 MPa, does not show a two‐wave structure, longitudinal stress to phase velocity relationship indicates the onset of pore collapse around 500 MPa.

For condensed explosives containing metal particle additives, a characteristic parameter, δ, can be defined as the ratio of particle diameter to detonation reaction zone length. Particle heating and acceleration are studied between the interaction limits ranging from frozen shock to thin‐detonation‐front diffraction followed by an expanding products flow. The intermediate case where the particle diameter and reaction zone length scales are comparable is considered as a function of metal mass fraction and particle packing to determine momentum and heat transfer during the detonation interaction time. The present investigation employs 3D mesoscale simulations to further conduct parametric studies in the 0.1⩽δ⩽10 range by varying the particle diameter and reaction zone length. The results are quantified as velocity and temperature transmission factors, which indicate a strong dependence of particle acceleration and heating rate on δ for dilute particles and high metal mass fraction conditions.For condensed explosives containing metal particle additives, a characteristic parameter, δ, can be defined as the ratio of particle diameter to detonation reaction zone length. Particle heating and acceleration are studied between the interaction limits ranging from frozen shock to thin‐detonation‐front diffraction followed by an expanding products flow. The intermediate case where the particle diameter and reaction zone length scales are comparable is considered as a function of metal mass fraction and particle packing to determine momentum and heat transfer during the detonation interaction time. The present investigation employs 3D mesoscale simulations to further conduct parametric studies in the 0.1⩽δ⩽10 range by varying the particle diameter and reaction zone length. The results are quantified as velocity and temperature transmission factors, which indicate a strong dependence of particle acceleration and heating rate on δ for dilute particles and high metal mass fraction conditions.

Composite materials have been of significant interest due to widespread application of anisotropic materials in aerospace and civil engineering problems. For example, composite materials are one of the important types of materials in the construction of modern aircraft due to their mechanical properties. The strain rate dependent mechanical behaviour of composite materials is important for applications involving impact and dynamic loading. Therefore, we are interested in understanding the composite material mechanical properties and behaviour for loading rates between quasistatic and 1×108 s−1. This paper investigates modelling of shock wave propagation in orthotropic materials in general and a specific type of CFC composite material. The determination of the equation of state and its coupling with the rest of the constitutive model for these materials is presented and discussed along with validation from three dimensional impact tests.

A series of plate impact experiments was performed using the single stage light gas gun facility at the Cavendish Laboratory, Cambridge University to determine the dynamic tensile, or spall strength of a hypabyssal kimberlite and a siltstone. Two distinct impact configurations were employed, over a range of impact velocities up to 185 m/s. VISAR and manganin gauges were used to monitor the spalling of the material. Thicker targets were found to be less suited for spall measurements as the signal appeared to be degraded by the increased amount of material traversed before reaching the measuring plane. A comparison with a theoretical scheme relating the spall strength to the Hugoniot elastic limit is performed, and while the agreement is not perfect it is of the same order of magnitude.

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.

Recently, pressure tuning of the thermoelectric figure of merit has been reported for several materials Bi2Te3 based thermoelectric materials [2],[10],[12]. In order to investigate the bulk properties of Bi2Te3, Sb2Te3, and their solid solution in detail, we have performed structural studies up to 20 GPa. Our diffraction results show that all three compounds transform from the ambient pressure structure to a high pressure phase between 7 and 10 GPa. In addition, these diffraction results have been converted to Vinet and Holzapfel equations of state to test the claim of electronic topological transitions in these structures [3].

In conventional Split Hopkinson Pressure Bar (SHPB) experiments the striker bar is a single rod and the sample is loaded at one strain rate. In this study, we present results from a system that uses a striker bar formed from two rods of different materials. A two level loading pulse is generated, subjecting the sample to two different strain‐rates: either a low strain‐rate followed by a higher strain‐rate (“step up”) or vice versa (“step‐down”). The sample does not experience repeat loading or significant unloading between the two regimes. The “step‐down” loading sequence, of equal durations at 2300 s−1 and 400 s−1, has been applied to iron and copper samples. History independence is observed for iron, as expected from its BCC structure.

In order to study the SDT process in a granular non‐ideal explosive (NIE) an experimental technique has been developed that allows the granular explosive to be shock initiated at a well controlled “tap density”. The granular NIE was contained in a PMMA cone and a planar shock was delivered to the explosive through a buffer plate of varying material. A combination of piezoelectric probes, ionisation pins, PVDF stress gauges and a high speed framing camera were used to measure the input shock pressure and shock and detonation wave positions in the explosive. Four trials were performed to characterise the run to detonation distance versus input pressure relationship (Pop plot) of the granular NH4 NO3 based explosive. Input pressures ranged from close to the 4 GPa predicted CJ pressure of the granular explosive down to 1.4 GPa, giving run distances up to ∼12 mm for the lowest pressure. The data indicates a steady acceleration of the input shock to detonation velocity, implying significant reaction growth at t...

A gas gun has been used to investigate the impact initiation of rods of a mixture of 72 wt% PTFE (polytetrafluoroethylene) and 28 wt% aluminum powders. The rods were sabot‐launched in vacuum into steel anvils at impact velocities ranging from 468 to 969 m/s. A framing camera was used to observe the time sequence of events following impact. At the lowest impact stress of 25 kbar no light was observed. Above the initiation threshold, the initiation time dropped from 74 μs just above threshold to 14 μs at 64 kbar. These results are compared with earlier rod impact experiments for a similar material in which the only major difference is a smaller aluminum particle size.

Shock recovery experiment on sillimanite was conducted using a two‐stage light‐gas gun to clarify origin of the peculiar nano‐frgmentation observed in mullite. Recovered sillimanite was examined by X‐ray powder diffraction (XRD) and transmission electron microscopy (TEM). Shock recovered sillimanite showed amorphization by XRD measurements as seen in mullite. However, microtexture observed by TEM showed characteristic planar deformation structures and following amorphization, which are considerably different from those observed in mullite. Importance of oxygen vacancies in crystal structure of mullite for the nano‐fragmentation was inferred from comparison to the present result on sillimanite.

Improved relationships were developed in this study for the thermodynamic properties of solid and liquid silicon and silicon dioxide for use with JAGUAR thermo‐chemical equation of state routines. Analyses of experimental melting temperature curves for silicon and silicon dioxide indicated complex phase behavior and that improved coefficients were required for solid and liquid thermodynamic properties. Advanced optimization routines were utilized in conjunction with the experimental melting point data to establish volumetric coefficients for these substances. The new property libraries resulted in agreement with available experimental values, including Hugoniot data at elevated pressures.

A reactive force field (ReaxFF) is used with molecular dynamics to probe the chemistry induced by intense heating (‘accelerated cook‐off’) of 1,3,5‐triamino‐2,4,6‐trinitrobenzene (TATB). Large‐system simulations are desired for TATB because of the high degree of carbon clustering expected in this material. Using small, 32‐molecule simulations, we calculate the reaction rate as a function of temperature and compare the Arrhenius‐predicted activation energy with experiment. Decomposition product evolution (mainly N2, H2O, CO2 and graphitic carbon clusters) is followed using a 576‐molecule larger simulation, which also illustrates the effect of system size on both carbon clustering and reaction rate.

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.

MnAs1−xSbx material is one of the ferromagnets which can contribute to a magnetic refrigeration system that is useable at room temperature. The consolidation experiments of MnAs1−xSbx (x = 0.068, 0.073) powder are carried out by a shock compaction method using underwater shock wave with shock pressure of 16 GPa. The magnetic entropy change, ΔSmag, of annealed sample (x = 0.073) after press caused by a magnetic field of 1 T is 27.8 J/Kkg at the maximum value, and the sample annealed after press undergoes a first‐order ferromagnetic to paramagnetic transition at 304.05 K.

The evolution and decay of aluminum and aluminum monoxide emission signatures following a laser ablation event were used to compare the relative reaction rates of three aluminum based materials. Time‐resolved emission results of oxide‐free, C13F27CO2H‐passivated materials were compared with uncoated, oxide passivated aluminum nanoparticles and those coated with the same acid used in for passivation C13F27CO2H. Excited state Al and AlO emission is reduced in time for the oxide free material when compared to coated, 50 nm, oxide passivated particles mixed on an equal active Al: C13F27CO2H ratio. This is interpreted as an increase in the reaction rate afforded by the elimination of the oxide coating and proximity of oxidizing species in the SAM‐based nanocomposite.

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 extend the Walsh and Christian (1955) analysis in a manner which allows a reasonable estimation of the expected deviation from the mirror image approximation due to solid liquid phase transition. This estimate is limited to a phase transition that occurs at shock unloading. This analysis provides a good estimate to metals with low melting pressure such as Tin and Lead.

The precursor decay anomaly in single‐crystal lithium fluoride has been evaluated for estimated dislocation densities at the leading edges of the followers on the Sano's decay curve, which is much lower than the Asay's decay curve. The equation for the density derived reveals that the density depends only on the slopes of the decay curves for particle velocity and stress, irrespective of the form of the follower and the slope at the leading edge. The density distribution estimated has a maximum at the beginning of the decay process, and the maximum density is about 1/22 times as large as that estimated on the Asay's decay curve. Thus, the anomaly is reduced significantly. By estimating the densities at the rears of the near‐steady precursors on the Sano's decay curve, it is also shown that a large number of dislocations are multiplied in the vicinity of the leading edges of the followers. The multiplication of dislocations results from initial dislocations near the impact surface and at the subgrain boundaries as well as those in the bulk.

Direct free energy calculations for liquid metals from ab initio potential surfaces are very computationally intensive. Here we investigate Monte Carlo methods that involve sampling on the surface defined by a reference system. This allows for large gains in efficiency because the random walk is carried out on the (much faster) reference potential, and the ab initio potential is only evaluated on a small subset of uncorrelated configurations. We investigate the feasibility of these methods, by first applying them to liquid copper using an embedded atom potential, with a 1/r12 pair potential as the reference system. We find that the free energy perturbation series converges well and is numerically tractable. Second and third order perturbation theory give results that are weakly dependent on the reference system parameters. A preliminary application to an ab initio potential surface for liquid magnesium shows that we can obtain free energies accurate at the meV/atom level with only ∼100 evaluations of the ...

Results of shock‐wave experiments with free and pre‐stressed samples of K8 crown glass, K14 crown glass and fused quartz are presented. Controlled confinement pressure on the specimen in the range of 200 MPa to 300 MPa was provided by installing a shrink‐fit metal sleeve on the lateral surface of the sample disk. The shock compression pulses of approximately triangular profile were created using thin aluminum impactors and PMMA base plates. The peak shock stresses in the range of 5.5 GPa to 8.5 GPa were sufficient to initiate the failure waves, whereas following unloading stopped the cracking. Results of measurements of the free surface velocity histories show that pre‐stressing results in earlier braking of the failure wave. Thus, the compressive transversal stress increases the failure threshold that is in agreement with existing criteria of compressive fracture.

New advanced investigations of aluminum properties at extreme conditions include experiments and theoretical calculations. Experimental researches on shock and isentropic compression of aluminum at high pressures up to 500 GPa have demonstrated smooth monotonic dependencies, resulting in more “stiff” compression curve and shock adiabat for aluminum that it was figured in previously. Adiabatic‐expansion measurements of shocked aluminum have brought new accurate data for the region of hot expanded liquid. Theoretical “ab initio” QMD calculations in the region in the vicinity of the critical point established the reference domain of thermodynamic data. This complex of data serves as a fundament for a new multi‐phase EOS for aluminum.

Previous research at room temperature and quasi‐static loading rates has shown that above the glass transition conditions of their binders, PBX Brazilian disc specimens fail by the localisation of strain in binder rich areas, followed by debonding of the binder from the largest energetic crystals. Crystal fracture is rare and the process is predominantly intergranular. In contrast we show, for a UK PBX, that when the temperature is reduced to below the glass transition condition of the binder system, failure becomes predominantly transgranular. Such data are required for the development and validation of PBX micromechanical models. This paper outlines the current state of research and details the important observations to date.

The incidental effects of gaps in detonating explosives have been observed for over two centuries, yet the root cause of peripheral damage due to these features has been a partial mystery. To evaluate such damage for PBX 9501, a test series has been performed that examines single and multiply‐directed detonations crossing gaps of varying widths and angles relative to the detonation wave fronts. Damage is evaluated with steel witness plates and quantified through trench profiling and photographic analysis. In addition, streak camera traces are used to track detonations along explosive material surfaces and across gaps. Such traces allow the quantification of timing delays due to the gap reinitiation process. For some reinitiation tests, a second detonation wave is directed to interfere at varying times with the post‐gap initiation process of the first wave, thus allowing complex wave‐wave interactions to be investigated in detail. With these cumulative observations, further insight into the mechanism of ex...

A modification of REBO potential has been proposed for the molecular dynamics simulation of a TATB‐like condensed explosive whose molecule initially consists of four different atoms. TATB‐like means bulk properties of initial state and parameters at CJ point similar to those of real TATB. Parameters of the potential are subdivided into two groups that are responsible for CJ parameters and reaction zone width. The possibility of formation of intermediate detonation products allows variation of reaction zone characteristics without changing CJ parameters. Provided are a number of test MD calculations on the thermodynamic properties of both the original explosive and detonation products, parameters at CJ point, reactions rates and reaction zone width as dependent upon the potential parameters as well as the evaluation of critical diameter. Mechanism of the detonation initiation proper to heterogeneous explosives has been investigated.

Experiments performed on the Z machine at Sandia Labs used magnetically generated planar ramp waves to quasi‐isentropically compress pre‐heated solid tin across the equilibrium β‐γ phase boundary. Velocity history measurements at a tin/window interface exhibited features that could be consistently related, through simulations, to the β‐γ structural transformation occurring in the bulk tin. The simulations used a homogeneous phase‐mixture model with a γ‐phase energy offset that was adjusted to match the measured velocity feature. This determined the phase‐boundary pressure from experiment and the phase‐boundary temperature from the β‐phase equation of state. Due to wave interactions, measurements using sapphire windows were more difficult to interpret than those using LiF windows and thus led to results with larger uncertainty. Results did not display the hysteretic overshoot seen in static and shock experiments on other phase‐transforming materials.

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.

The molecular dynamics simulation is applied to study the influence of nanoprecipitates on the microscopic mechanisms of the spall fracture initiation and corresponding plastic deformation. The systems under study are single crystals doped with nanoclusters of another metal. The calculation results for the Cu‐Al system is presented. Three models are considered: the triaxial uniform expansion, the shock wave propagation and the reflection of release wave in the impactor‐target model and the model of an edge dislocation interaction with a nanocluster. Different characteristic consequences of the nanoprecipitate presence on material response are analyzed.

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.

In order to get simultaneous high speed streak and framing images of exploding metal wires in a water environment, we have employed a CW green laser as a back light source of shadow photography and a laser beam splitter as a device separating images of exploding wires. By filtering the light emitted from the exploding wires with the help of a laser line band pass filter, the images could become much finer than those pictures taken without the filter. The stability of the cylindrical plasma column could be monitored by the framing pictures, and the evolution of the shock wave and plasma expansion could be measured precisely by the streak pictures. The measured data were applied to understand dense plasma characteristics, e.g. electrical conductivity or thermodynamic properties of the dense metal plasma.