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

In an effort to unify the various phase-field models that have been used to study solidification, we have developed a class of phase-field models for crystallization of a pure substance from its melt. These models are based on an entropy functional, as in the treatment of Penrose and Fife, and are therefore thermodynamically consistent inasmuch as they guarantee spatially local positive entropy production. General conditions are developed to ensure that the phase field takes on constant values in the bulk phases. Specific forms of a phase-field function are chosen to produce two models that bear strong resemblances to the models proposed by Langer and Kobayashi. Our models contain additional nonlinear functions of the phase field that are necessary to guarantee thermodynamic consistency.

Thermodynamically-consistent phase-field models for solidification

Diffusion, coalescence, and reconstruction of vacancy defects in graphene layers are investigated by tight-binding molecular dynamics (TBMD) simulations and by first principles total energy calculations. It is observed in the TBMD simulations that two single vacancies coalesce into a 5-8-5 double vacancy at the temperature of 3000 K, and it is further reconstructed into a new defect structure, the 555-777 defect, by the Stone-Wales type transformation at higher temperatures. First principles calculations confirm that the 555-777 defect is energetically much more stable than two separated single vacancies, and the energy of the 555-777 defect is also slightly lower than that of the 5-8-5 double vacancy. In TBMD simulation, it is also found that the four single vacancies reconstruct into two collective 555-777 defects which is the unit for the hexagonal haeckelite structure proposed by Terrones et al. [Phys. Rev. Lett. 84, 1716 (2000)].

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Diffusion, Coalescence, and Reconstruction of Vacancy Defects in Graphene Layers

Electromagnetic shielding performance of carbon foams

Review: A review of active structural control: challenges for engineering informatics

Crashworthiness parameters of aluminum hexagonal honeycomb structures under impact loads are investigated by using finite element methods and conducting experiments. To validate the finite element models, numerical results are compared with experimental measurements and theoretical results reported in literature. In numerical simulations of honeycomb structures, out-of-plane loads are considered while the aluminum foil thickness, cell side size, cell expanding angle, impact velocity and mass are varying, and dynamic behavior and crashworthiness parameters are examined. It is observed that there are good agreements between numerical, experimental and theoretical results. Numerical simulations predict that crashworthiness parameters depend on cell specification and foil thickness of the honeycomb structure and are independent of impact mass and velocity.

Numerical and experimental study of crashworthiness parameters of honeycomb structures

Computational modeling of micro-cellular carbon foams

Nonlinear fracture analysis of single-layer graphene sheets

In this study, crashworthiness assessment and suggestions for the modification of a railroad passenger car are presented. To assess the crashworthiness, collision of the railroad passenger car onto a rigid wall is simulated by using finite element (FE) methods. A full-length, detailed passenger car model is used in FE analyses. In order to validate the FE model, simulation results obtained for different types of static loading conditions in compliance with various scenarios defined in UIC CODE OR 577 are compared with experimental measurements before running collision analyses of the railroad passenger car. The good agreement between static tests and FE analyses results indicates that the FE model accurately represents the real structure. Following the FE model validation, analysis of the collision behaviour of the railroad passenger car consists of two stages. In the first stage, the crashworthiness of the initial concept design of the railroad passenger car is analysed. It was observed that local buckli...

Railroad passenger car collision analysis and modifications for improved crashworthiness

Effect of attachment types and number of implants supporting mandibular overdentures on stress distribution: a computed tomography-based 3D finite element analysis.

Ata Mugan

Papers

Numerical and experimental study of crashworthiness parameters of honeycomb structures

Computational modeling of micro-cellular carbon foams

Nonlinear fracture analysis of single-layer graphene sheets

Railroad passenger car collision analysis and modifications for improved crashworthiness

Effect of attachment types and number of implants supporting mandibular overdentures on stress distribution: a computed tomography-based 3D finite element analysis.