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

We present some measurements of non-Newtonian behavior which clearly show the progression from Newtonian to power-law shear-thinning response and then to a shear stress limited by cohesive failure. Traction curvescalculated from these measurements are compared with EHL traction measurements at very low slide/roll ratio. It is shown that the shape or character of the traction curve is unchanged by shear-thinning but that the traction curves are shifted to higher shear rates. This rate shift is nearly indistinguishable from a reduction in viscosity with increasing pressure and may have been misinterpreted as Roelands behavior or even time dependent viscosity.

Ordinary shear-thinning behavior in liquids and its effect upon EHL traction

The property of fragility in glass forming liquids is introduced to elastohydrodynamic lubrication (EHL). Using viscosity measurements for two liquids that have been the subject of traction studies, the fragility of liquids is shown to be important to EHL traction and the property that most influences the representative (Eyring) stress. The derivative Stickel analysis is then carried out for lubricants. Using viscosity measurements, the dynamic crossover is detected for the first time in lubricants. The viscosity at the crossover is either constant or varies slowly with temperature and pressure and the crossover pressure varies with temperature in a manner similar to the glass transition pressure. The free volume model fails to predict the occurrence of the dynamic crossover.Copyright © 2007 by ASME

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Fragility and the Dynamic Crossover in Lubricants

Various types of rheometric apparatus have been developed to study and measure the shear stress behaviour of liquid lubricants. Elastohydrodynamic (EHD) lubrication is better studied with a high-pressure rheometer which allows independent control of temperature, pressure, and shear strain. With this, the authors propose an improved relation for the variation of viscosity through the glass transition, and present refined relations for shear modulus and limiting shear stress.

Lubricant rheological properties at high pressure

Tribological characteristics of sputtered thin films in rolling hertzian contacts

The Newtonian mixing rules for several binary systems have been experimentally investigated. Some systems show non-ideal mixing response and for some systems the non-ideal response is pressure-dependent, yielding an opportunity for manipulation of the pressure-viscosity behavior to advantage. The mixing of differing molecular weight straight cuts can produce very different pressure-viscosity response. This behavior underscores the difficulty in predicting the pressure-viscosity coefficient based upon chemical structure and ambient viscosity since the molecular weight distribution is also important, but it also provides another opportunity to control the high-pressure response by blending. The first experimental observation of double shear-thinning within a single flow curve is reported. Blending then provides the capability of adjusting not only the Newtonian viscosity but also the non-Newtonian shear-thinning response as well.

Scott Bair

Papers

Ordinary shear-thinning behavior in liquids and its effect upon EHL traction

Fragility and the Dynamic Crossover in Lubricants

Lubricant rheological properties at high pressure

Tribological characteristics of sputtered thin films in rolling hertzian contacts

The High Pressure Rheology of Mixtures