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

There have been a number of review papers on layered silicate and carbon nanotube reinforced polymer nanocomposites, in which the fillers have high aspect ratios. Particulate–polymer nanocomposites containing fillers with small aspect ratios are also an important class of polymer composites. However, they have been apparently overlooked. Thus, in this paper, detailed discussions on the effects of particle size, particle/matrix interface adhesion and particle loading on the stiffness, strength and toughness of such particulate–polymer composites are reviewed. To develop high performance particulate composites, it is necessary to have some basic understanding of the stiffening, strengthening and toughening mechanisms of these composites. A critical evaluation of published experimental results in comparison with theoretical models is given.

/pdf/effects-of-particle-size-particle-matrix-interface-adhesion-yv055zq7xi.pdf

Effects of particle size, particle/matrix interface adhesion and particle loading on mechanical properties of particulate–polymer composites

Changes in portlandite morphology with solvent composition: Atomistic simulations and experiment

A new approach to the application of Mori-Tanaka's theory in composite materials

The purpose of the present survey is to review the analysis of composite materials from the applied mechanics and engineering science point of view. The subjects under consideration will be analysis of the following properties of various kinds of composite materials: elasticity, thermal expansion, moisture swelling, viscoelasticity, conductivity (which includes, by mathematical analogy, dielectrics, magnetics, and diffusion) static strength, and fatigue failure.

Analysis of Composite Materials—A Survey

Advanced reactors from the Gen IV program are required to deliver electricity and process heat with high efficiency. The electric power production may be through a high pressure steam generator (Rankine cycle) or a director indirect-cycle gas turbine (Brayton cycle). The process heat applications may include co-generation, coal-to-liquids conversion, and synthesis of chemical feedstocks. These applications of these advanced reactors are critically dependent upon an efficient intermediate heat exchanger (IHX), which is a key component transferring heat from the primary coolant to a secondary coolant which will then further transfer heat to a chemical process or a power conversion system. Several candidate designs for IHX are available, with the final decision still to be made. These designs include shell and tube, and compact heat exchangers: plate-and-fin, and micro-channel (printed circuit heat exchangers or PCHEs). The secondary heat transfer medium may be helium (power generation through Brayton cycle) or molten salts (process heat applications). Despite its significance, the safety implications of the nuclear reactor-IHX-chemical process coupling have not received much attention. The complexity of the system requires the development of advanced techniques to ensure proper control of the system. The goal of the proposed research is to model the behavior of the Advanced Reactor-IHX-Chemical Process system and develop advanced control techniques (based on genetic algorithms or neural networks) for off-normal conditions. The specific objectives are: 1.To develop mathematical models to describe the advanced nuclear reactor-IHX-Chemical Process coupling during normal and off-normal operations, and simulate the models using a multiphysics software such as Comsol; 2. To develop control strategies using genetic algorithm techniques and couple these techniques with the multiphysics software; and, 3. To validate the models experimentally using the existing facilities at The Ohio State University. The proposed research will provide information on the thermal designs and advanced control strategies for different IHX types for various combinations of coolants and operating temperatures and conditions. The data and information obtained through the research will be valuable in understanding the coupled response of the reactor-IHX system to various transients and in the development of intelligent control systems for the next generation nuclear reactor systems.

Advanced Reactors-Intermediate Heat Exchanger (IHX) Coupling: Theoretical Modeling and Experimental Validation

The problem of special interest is the nature of the mode of failure in uniaxial compression at the brittle limit. This problem is known by observation to undergo a splitting mode of failure. The present work gives a full theoretical treatment and proof for this mode of failure. The general failure theory of Christensen for isotropic materials provides the basis for the derivation. The solution demonstrates the depth of technical capability that is required from the failure theory to treat such a classically difficult problem.

Proof of the Splitting Mode of Compressive Brittle Failure and Integration With General Failure Theory

Failure Behavior of Closed Cell Foams

A nonlinear, two constant stress-deformation form is deduced for elastic materials. At very large stretch ratios of greater than about 3 or 4, the model exhibits the strain stiffening behavior common to many elastomers. The constitutive form is very simple since the two material constants enter it as multiplying constants times certain nonlinear deformation terms. The model is evaluated with respect to data upon natural rubber under both uniaxial and bi-axial stress conditions. The model is also used to evaluate data obtained from a nonlinear membrane inflation experiment. The latter experimental capability and corresponding data are new.

Evaluation of a simple nonlinear elastic constitutive form

\n The recently developed general materials failure theory is specialized to the two-dimensional state of plane stress. It takes a form that is virtually no more involved than that of the Mises criterion. Yet it remains applicable to the entire range of materials types and thus retains that generality. The Mises form has absolutely no capability for generality. This plane stress form of the new failure theory reveals the existence of three independent modes and mechanisms of failure, not two, not four, purely three. The Mises criterion has one mode of failure. These three modes of failure are fully examined. It is verified that these modes of failure under plane stress conditions are exactly the same as those operative in the three-dimensional case. The simple plane stress form of the failure theory has major appeal and likely use as a teaching tool to introduce failure and to help de-mystify the vitally important general subject of materials failure.

Richard M. Christensen

Papers

Advanced Reactors-Intermediate Heat Exchanger (IHX) Coupling: Theoretical Modeling and Experimental Validation

Proof of the Splitting Mode of Compressive Brittle Failure and Integration With General Failure Theory

Failure Behavior of Closed Cell Foams

Evaluation of a simple nonlinear elastic constitutive form

The Three Controlling Modes of Failure in Homogeneous and Isotropic Materials With Proof Thereof Through Critical Plane Stress Conditions