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.

Fast parallel algorithms for short-range molecular dynamics

다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

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Using Multivariate Statistics

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

Phd by thesis

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.

https://science.sciencemag.org/content/sci/287/5453/637.full.pdf

Strength and breaking mechanism of multiwalled carbon nanotubes under tensile load

Low‐pressure plane impact experiments performed on Fe‐32 wt. % Ni‐0.035 wt. % C alloy revealed, after recovery, markings which are attributed to shock‐induced inhomogeneities. Shear of the material does not occur homogeneously, but in preferential planar regions. These regions are made visible by a martensitic transformation [fcc (austenite)→bcc (martensite)] produced by the tensile pulses generated by the reflection of the compressive shock wave at a free surface. The bands with higher plastic deformation served as preferential nucleation sites for martensitic transformation. The formation of these bands is attributed to inhomogeneous yielding due to work softening of the material during tensile loading.

Inhomogeneities of shock‐wave deformation in Fe‐32 wt. % Ni‐0.035 wt. % C alloy

Mineralized biological tissues offer insights into how nature has evolved these components to optimize multifunctional purposes. These mineral constituents are weak by themselves, but interact with the organic matrix to produce materials with unexpected mechanical properties. The hierarchical structure of these materials is at the crux of this enhancement. Microstructural features such as organized, layered organic/inorganic structure, and the presence of porous and fibrous elements are common in many biological components. The organic and inorganic portions interact at the molecular and micro-levels synergistically to enhance the mechanical function. In this paper, we report on recent progress on studies of the abalone shell, crab exoskeletons, antlers, tusks, teeth.

http://www.meyersgroup.ucsd.edu/papers/journals/Meyers%20304.pdf

Structure, function and mechanical properties of selected biological materials

Shock compression in nanocrystalline nickel is simulated over a range of pressures (10–80 GPa) and compared with experimental results. Laser compression carried out at Omega and Janus yields new information on the deformation mechanisms of nanocrystalline Ni. Although conventional deformation does not produce hardening, the extreme regime imparted by laser compression generates an increase in hardness, attributed to the residual dislocations observed in the structure by TEM. An analytical model is applied to predict the critical pressure for the onset of twinning in nanocrystalline nickel. The slip‐twinning transition pressure is shifted from 20 GPa, for polycrystalline Ni, to 80 GPa, for Ni with g. s. of 10 nm. Contributions to the net strain from the different mechanisms of plastic deformation (partials, perfect dislocations, twinning, and grain boundary shear) were quantified in the nanocrystalline samples through MD calculations. The effect of release, a phenomenon often neglected in MD simulations, o...

/pdf/laser-compression-of-nanocrystalline-metals-hu8lnm7fbg.pdf

Laser compression of nanocrystalline metals

Corrigendum to “Grain-size dependent mechanical behavior of nanocrystalline metals” [Mater. Sci. Eng. A 646 (2012) 101–134]

The evolution of multiple adiabatic shear bands was investigated in stainless steel (different grain sizes: 30 and 140 μm), titanium, and Ti‐6Al‐4V alloy through the radial collapse of a thick‐walled cylinder under high‐strain‐rate deformation (∼104 s−1) and different global strains(up to 0.9). Ti and Ti‐6Al‐4V displayed drastically different patterns of shear bands. The shear‐band spacing is compared with one‐dimensional theoretical predictions based on perturbation (Ockendon‐Wright and Molinari) and momentum diffusion (Grady‐Kipp) concepts. The experimentally observed spacing reveals a two‐dimensional character of self‐organization, not incorporated into the existing theories. A novel analytical description is proposed, in which embryos(potential initiation sites) are activated as a function of strain (greater than a threshold) according to a Weibull‐type distribution. The model incorporates embryo disactivation by stress shielding as well as selective growth of shear bands. The imposed strain rate, emb...

Marc A. Meyers

Papers

Inhomogeneities of shock‐wave deformation in Fe‐32 wt. % Ni‐0.035 wt. % C alloy

Structure, function and mechanical properties of selected biological materials

Laser compression of nanocrystalline metals

Corrigendum to “Grain-size dependent mechanical behavior of nanocrystalline metals” [Mater. Sci. Eng. A 646 (2012) 101–134]

Evolution in the Patterning of Adiabatic Shear Bands