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

Shear-banding is a ubiquitous plastic-deformation mode in materials. In metallic glasses, shear bands are particularly important as they play the decisive role in controlling plasticity and failure at room temperature. While there have been several reviews on the general mechanical properties of metallic glasses, a pressing need remains for an overview focused exclusively on shear bands, which have received tremendous attention in the past several years. This article attempts to provide a comprehensive and up-to-date review on the rapid progress achieved very recently on this subject. We describe the shear bands from the inside out, and treat key materials-science issues of general interest, including the initiation of shear localization starting from shear transformations, the temperature and velocity reached in the propagating or sliding band, the structural evolution inside the shear-band material, and the parameters that strongly influence shear-banding. Several new discoveries and concepts, such as stick-slip cold shear-banding and strength/plasticity enhancement at sub-micrometer sample sizes, will also be highlighted. The understanding built-up from these accounts will be used to explain the successful control of shear bands achieved so far in the laboratory. The review also identifies a number of key remaining questions to be answered, and presents an outlook for the field.

Shear bands in metallic glasses

The Communication program emphasizes theory, research, and application to examine the ways humans communicate, verbally and non-verbally, across a variety of levels and contexts. This is particularly important as communication shapes our ideas and values, gives rise to our politics, consumption and socialization, and helps to define our identities and realities. Its power and potential is inestimable. From the briefest of text messages to the grandest of public declarations, we indeed live within communication and invite you to join us in appreciating its increasing importance in contemporary society. From Twitter and reality television to family relationships and workplace dynamics, communication is about understanding ourselves, our media, our relationships, our culture and how these things connect.

의사 간호사 communication

High-pressure torsion (HPT) method currently receives much attention as a severe plastic deformation (SPD) technique mainly because of the reports of Prof. Ruslan Z. Valiev and his co-workers in 1988. They reported the efficiency of the method in creating ultrafine-grained (UFG) structures with predominantly high-angle grain boundaries, which started the new age of nanoSPD materials with novel properties. The HPT method was first introduced by Prof. Percy W. Bridgman in 1935. Bridgman pioneered application of high torsional shearing stress combined with high hydrostatic pressure to many different kinds of materials such as pure elements, metallic materials, glasses, geological materials (rocks and minerals), biological materials, polymers and many different kinds of organic and inorganic compounds. This paper reviews the findings of Bridgman and his successors from 1935 to 1988 using the HPT method and summarizes their historical importance in recent advancement of materials, properties, phase transformations and HPT machine designs.

A review on high-pressure torsion (HPT) from 1935 to 1988

Ionic liquids are an emerging class of materials with a diverse and extraordinary set of properties. Understanding the origins of these properties and how they can be controlled by design to serve valuable practical applications presents a wide array of challenges and opportunities to the chemical physics and physical chemistry community. We highlight here some of the significant progress already made and future research directions in this exciting area.

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Spotlight on ionic liquids

A conclusive demonstration has been provided that the nature of the shear-thinning, that affects both film thickness and traction in EHL contacts, follows the ordinary power-law rule that has been described by many empirical models of which Carreau is but one example. This was accomplished by accurate measurements in viscometers of the shear response of a PAO that possesses a very low critical stress for shear-thinning and accurate measurements in-contact of film thickness and traction under conditions which accentuate the shear-thinning effect. The in-contact central film thickness and traction were entirely predictable from the rheological properties obtained from viscometers using simple calculations. These data should be invaluable to researchers endeavoring to accurately simulate Hertz zone behavior since the shear-thinning rheology is extensively characterized and accurate in-contact data are available to test. In addition, a new model has been introduced that may be useful for the rheological characterization of mixtures.

A unified shear-thinning treatment of both film thickness and traction in EHD

The variation in viscosity with pressure and shear rate is required for modelling of film thickness and traction in elastohydrodynamic contacts. Two examples are presented with supporting data for lubricants. Then, five well-characterized simple, low molecular weight hydrocarbon liquids are investigated in detail. The effect of structure on the pressure-viscosity response and non-Newtonian response is discussed. A favourable comparison can be made with non-equilibrium molecular dynamics simulations on squalane.

The high pressure rheology of some simple model hydrocarbons

A film thickness correction for shear-thinning in elastohydrodynamic contacts has been available that requires parameters obtained from flow curves generated for the specific liquid. For instances for which reduced accuracy is acceptable, a new rough film thickness correction has been formulated for pure rolling and a moderate amount of sliding based on Van Krevelen's graphical technique of estimating shear-thinning from the molecular weight. It has been shown to be useful for high-molecular-weight base oils, when lubricant specific shear-thinning data is absent, although it should most often overestimate the film thinning.

A Rough Shear-Thinning Correction for EHD Film Thickness

A low-viscosity ionic liquid demonstrating superior lubricating performance from mixed to boundary lubrication

A new falling body viscometer was developed to extend the measurement of the limiting low shear viscosity to 1.4 GPa in pressure and 105 Pas in viscosity for temperatures above ambient. Rather complete characterizations of the temperature-pressure-viscosity behavior of three synthetic base oils were performed. Free volume models can reproduce and provide a physical explanation for the inflection observed in pressure viscosity isotherms and provide a relationship between viscosity and state behavior. Examples of traction calculations indicate that used alone, traction curves cannot be used to deduce flow curves. The interpretation of a traction curve in terms of shear rheology is ambiguous without knowledge of at least the pressure-viscosity behavior up to the maximum Hertz pressure. A pressure-viscosity correlation capable of providing accurate values for α at these pressures may be very different from a correlation which has proven useful for film thickness analysis. A need exists to extend the flow curv...

Scott Bair

Papers

A unified shear-thinning treatment of both film thickness and traction in EHD

The high pressure rheology of some simple model hydrocarbons

A Rough Shear-Thinning Correction for EHD Film Thickness

A low-viscosity ionic liquid demonstrating superior lubricating performance from mixed to boundary lubrication

Pressure-Viscosity Behavior of Lubricants to 1.4 GPa and Its Relation to EHD Traction