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

Twenty-five years of ultrafine-grained materials: achieving exceptional properties through grain refinement

A model for the axisymmetric growth and coalescence of small internal voids in elastoplastic solids is proposed and assessed using void cell computations. Two contributions existing in the literature have been integrated into the enhanced model. The first is the model of Gologanu-Leblond-Devaux, extending the Gurson model to void shape effects. The second is the approach of Thomason for the onset of void coalescence. Each of these has been extended heuristically to account for strain hardening. In addition, a micromechanically-based simple constitutive model for the void coalescence stage is proposed to supplement the criterion for the onset of coalescence. The fully enhanced Gurson model depends on the flow properties of the material and the dimensional ratios of the void-cell representative volume element. Phenomenological parameters such as critical porosities are not employed in the enhanced model. It incorporates the effect of void shape, relative void spacing, strain hardening, and porosity. The effect of the relative void spacing on void coalescence, which has not yet been carefully addressed in the literature. has received special attention. Using cell model computations, accurate predictions through final fracture have been obtained for a wide range of porosity, void spacing, initial void shape, strain hardening, and stress triaxiality. These predictions have been used to assess the enhanced model. (C) 2000 Elsevier Science Ltd. All rights reserved.

An extended model for void growth and coalescence - application to anisotropic ductile fracture

Recent developments in high-strength Mg-RE-based alloys: Focusing on Mg-Gd and Mg-Y systems

In the last two decades or so, spinning and flow forming have gradually matured as metal forming processes for the production of engineering components in small to medium batch quantities. Combined spinning and flow forming techniques are being utilised increasingly due to the great flexibility provided for producing complicated parts nearer to net shape, enabling customers to optimise designs and reduce weight and cost, all of which are vital, especially in automotive industries. In this paper, process details of spinning and flow forming are introduced. The state of the art is described and developments in terms of research and industrial applications are reviewed. Also, the direction of research and development for future industrial applications are indicated.

A review of spinning, shear forming and flow forming processes

Flow softening and microstructural evolution of TC11 titanium alloy during hot deformation

3D rigid-plastic FEM numerical simulation on tube spinning

Micro-electrical discharge machining (EDM) and micro-electrochemical machining (ECM) combined milling for 3D micro-structure is investigated in this paper. These processes that consist of micro-EDM shaping and micro-ECM finishing are carried out in sequence on the same machine tool with the same electrode but different dielectric medium. The processing conditions are investigated experimentally by the cavity milling. The electrode which was used both in micro-EDM and micro-ECM processes is online fabricated by using an anti-copying block. The EDMed surface roughness of 0.707 μm Ra is lowered to 0.143 μm Ra by applying micro-ECM finishing. Meanwhile, the size and shape of the workpiece by combined milling is controlled precisely, which is much better than that machined merely by micro-ECM. As the large machining parameter values, the machining efficiency is also improved. In order to verify the combined machining performance, some 3D micro-structures were fabricated. The results show that the machining precision and shape accuracy is much better than that machined merely by micro-ECM milling, which can be exactly controlled. Since the EDMed recast layer and surface defects are removed completely, the surface quality and mechanical property of the workpiece is improved, which is better than that machined merely by micro-EDM. It proves that this combined milling method is possible and useful in the field of 3D metallic micro-structure milling.

A study of micro-EDM and micro-ECM combined milling for 3D metallic micro-structures

When the feature dimension of metal forming parts is scaled down to micro-scale, size effects occur and the understanding of micro-forming becomes complex. In this paper, the size effect on deformation behavior of brass foil was investigated via micro-blanking test. The results show that the ultimate shearing strength increases with the decrease of the foil thickness and the fracture mechanism of micro-blanking significantly changes from fracture mode with ductile dimples. There exists a minimum peak value during micro-blanking of brass foil over the considered relative blanking clearance range. However, the deformation behavior of micro-blanking process not only relates with the blanking clearance, but also relates with the grain size of brass foil. The grain size has a significant influence on micro-blanking and the curves of coarse-grained foil specimen show a strong variation not only in the curve profile but also in blanking edge distribution. To explain the size effect mechanism of micro-blanking, a size effect model of micro-blanking was established by considering the blanking clearance and grain size and the results indicate that the ratio of blanking clearance to grain size is one of the main factors to affect micro deformation behavior in micro-blanking. The ultimate shearing strength reaches an extreme value when the blanking clearance to grain size ratio is equal to 1.

Debin Shan

Papers

Flow softening and microstructural evolution of TC11 titanium alloy during hot deformation

3D rigid-plastic FEM numerical simulation on tube spinning

A study of micro-EDM and micro-ECM combined milling for 3D metallic micro-structures

Blanking clearance and grain size effects on micro deformation behavior and fracture in micro-blanking of brass foil

Size effects of the cavity dimension on the microforming ability during coining process