Marc A. Meyers

Bio: Marc A. Meyers is an academic researcher from University of California, San Diego. The author has contributed to research in topics: Deformation (engineering) & Dislocation. The author has an hindex of 85, co-authored 487 publications receiving 36646 citations. Previous affiliations of Marc A. Meyers include University of California & Instituto Militar de Engenharia.

Shock compression of monocrystalline copper: Experiments, characterization, and analysis

Abstract: Monocrystalline copper samples with [0 0 1] and [2 2 1] orientations were subjected to shock/recovery experiments at 30 and 57 GPa and 90 K. The slip system activity and the microstructural evolution were investigated. Different defect structures, including dislocations, stacking faults, twins, microbands, and recrystallized grains were observed in the specimens. The residual microstructures were dependent on crystalline orientation and pressure. The differences with crystalline orientations are most likely due to different resolved shear stresses on specific crystalline planes. The geometric relationships between the shock propagation direction and crystalline orientation are presented under uniaxial strain. It is shown that the [2 2 1] orientation, by virtue of having fewer highly activated slip systems, exhibits greater concentration of deformation with more intense shear on the primary system. This, in turn leads to greater local temperature rise and full recrystallization, in spite of the thermodynamic residual temperature of ∼500 K and rapid cooling (within 20 s) to ambient temperature. The profuse observation of microbands is interpreted in terms of the mechanism proposed by Huang and Gray [J.C. Huang, G.T. Gray III, Acta Metallurgica 37 (1989) 3335–3347].

Multiple film plane diagnostic for shocked lattice measurements (invited)

Abstract: Laser-based shock experiments have been conducted in thin Si and Cu crystals at pressures above the Hugoniot elastic limit. In these experiments, static film and x-ray streak cameras recorded x rays diffracted from lattice planes both parallel and perpendicular to the shock direction. These data showed uniaxial compression of Si(100) along the shock direction and three-dimensional compression of Cu(100). In the case of the Si diffraction, there was a multiple wave structure observed, which may be due to a one-dimensional phase transition or a time variation in the shock pressure. A new film-based detector has been developed for these in situ dynamic diffraction experiments. This large-angle detector consists of three film cassettes that are positioned to record x rays diffracted from a shocked crystal anywhere within a full π steradian. It records x rays that are diffracted from multiple lattice planes both parallel and at oblique angles with respect to the shock direction. It is a time-integrating measurement, but time-resolved data may be recorded using a short duration laser pulse to create the diffraction source x rays. This new instrument has been fielded at the OMEGA and Janus lasers to study single-crystal materials shock compressed by direct laser irradiation. In these experiments, a multiple wave structure was observed on many different lattice planes in Si. These data provide information on the structure under compression.

Bioinspired composite segmented armour: Numerical simulations

Abstract: Nature has evolved ingenious armour designs, like the flexible carapaces of armadillo and boxfish consisting of hexagonal segments connected by collagen fibres, that serve as bioinspiration for modern ballistic armours. Here, Finite element modelling (FEM) used to analyze the effect of scale geometry and other impact parameters on the ballistic protection provided by a bioinspired segmented ceramic armour. For this purpose, the impact of cylindrical fragment simulating projectiles (FSPs) onto alumina-epoxy non-overlapping scaled plates was simulated. Scale geometrical parameters (size, thickness and shape) and impact conditions (FSP diameter, speed, location) are varied and the amount of damage produced in the ceramic tiles and the final residual velocity of the FSP after the impact are evaluated. It is found that segmentation drastically reduces the size of the damaged area without significantly reducing the ballistic protection in centred impact, provided the tile size is kept over a critical value. Such critical tile size (∼20 mm, inscribed diameter, for impacts at 650 m/s) is independent of the scale thickness, but decreases with projectile speed, although never below the diameter of the projectile. Off-centred impacts reduce the ballistic protection and increase the damaged area, but this can be minimized with an appropriate tile shape. In this sense and in agreement with the natural hexagonal tiles of the boxfish and armadillo, hexagonal scales are found to be optimal, exhibiting a variation of ballistic protection—measured as reduction of projectile speed—with impact location under 12%. Design guidelines for the fabrication of segmented protection systems are proposed in the light of these numerical results.

Synthesis of nanocrystalline titanium carbide by spark erosion

Abstract: Interest in nanocrystaJline ceramic powders is due to their attractive processing and mechanical properties. The disadvantages of conventional ceramics, such as large internal flaws, negligible ductility, hightemperature processing, can possibly be eliminated or minimized by starting with finegrained powders. A previous study on the sinterability of nanocrystalline ceramic powders showed that high densities can be achieved at lower temperatures, both with and without the’ application of pressure [ 11. In addition, it is remarkable that superplastic behavior has been reported in nanocrystalline ceramics at room temperature [2]. One of the novel methods for nrenaring nanocrvstalline ceramic powders is reactive spark erosion [3-51. An important feature of this method is the-capability to synthesize new materials by reacting the eroded particles with the dielectric liguid. This technique utilizes two metal or conducting electrodes sparked in a suitable dielectric liquid that reacts with the pa&les to produce the desired ceramic powders. The high temperature provided by the spark results in vaporization of the electrodes and the surrounding dielectric liquid; the reaction products formed between the various vapor species are quenched in the liquid and produce spherical particles. The aim of the present note is to report on the titanium carbide particle formation in a carbonic dielectric liquid (pentane) with titanium electrodes and charges. The nanocrystalline titanium carbide was produced in a wide range of particle size distribution and in the form of single phase spheres.

Shock-Wave Consolidation of Rapidly Solidified Superalloy Powders

Abstract: The feasibility of consolidating rapidly solidified MAR M-200 powders by explosive means is demonstrated. MAR M-200 powders produced by the rapid solidification rate (RSR) technique and exhibiting a microdendritic structure were consolidated in an axisymmetric set-up, consisting of a steel pipe (in which the powder was placed) surrounded by explosives. The converging shock waves produced full densification, after optimization of the process parameters. The microindentation hardness showed a dramatic increase: 357 HV in the as-received condition, to 700 HV in the shock-consolidated condition. The substructure was analyzed by TEM, and two distinct regions were identified. The center of the particles exhibited planar arrays of dislocations, stacking-faults, and twin faults characteristic of shock-loaded superalloys, while the melted and re-solidified interfacial layers consisted of a microcrystalline structure with homogeneous composition.

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Using Multivariate Statistics

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Phd by thesis

Abstract: Deposits of clastic carbonate-dominated (calciclastic) sedimentary slope systems in the rock record have been identified mostly as linearly-consistent carbonate apron deposits, even though most ancient clastic carbonate slope deposits fit the submarine fan systems better. Calciclastic submarine fans are consequently rarely described and are poorly understood. Subsequently, very little is known especially in mud-dominated calciclastic submarine fan systems. Presented in this study are a sedimentological core and petrographic characterisation of samples from eleven boreholes from the Lower Carboniferous of Bowland Basin (Northwest England) that reveals a >250 m thick calciturbidite complex deposited in a calciclastic submarine fan setting. Seven facies are recognised from core and thin section characterisation and are grouped into three carbonate turbidite sequences. They include: 1) Calciturbidites, comprising mostly of highto low-density, wavy-laminated bioclast-rich facies; 2) low-density densite mudstones which are characterised by planar laminated and unlaminated muddominated facies; and 3) Calcidebrites which are muddy or hyper-concentrated debrisflow deposits occurring as poorly-sorted, chaotic, mud-supported floatstones. These

Strength and breaking mechanism of multiwalled carbon nanotubes under tensile load

Abstract: The tensile strengths of individual multiwalled carbon nanotubes (MWCNTs) were measured with a “nanostressing stage” located within a scanning electron microscope. The tensile-loading experiment was prepared and observed entirely within the microscope and was recorded on video. The MWCNTs broke in the outermost layer (“sword-in-sheath” failure), and the tensile strength of this layer ranged from 11 to 63 gigapascals for the set of 19 MWCNTs that were loaded. Analysis of the stress-strain curves for individual MWCNTs indicated that the Young's modulus E of the outermost layer varied from 270 to 950 gigapascals. Transmission electron microscopic examination of the broken nanotube fragments revealed a variety of structures, such as a nanotube ribbon, a wave pattern, and partial radial collapse.