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

An overview of the self-organizing map algorithm, on which the papers in this issue are based, is presented in this article.

The Self-Organizing Map

Guided Lamb waves for identification of damage in composite structures: A review

Composites science and technology

Structural Health Monitoring (SHM) can be understood as the integration of sensing and intelligence to enable the structure loading and damage-provoking conditions to be recorded, analyzed, localized, and predicted in such a way that nondestructive testing becomes an integral part of them. In addition, SHM systems can include actuation devices to take proper reaction or correction actions. SHM sensing requirements are very well suited for the application of optical fiber sensors (OFS), in particular, to provide integrated, quasi-distributed or fully distributed technologies. In this tutorial, after a brief introduction of the basic SHM concepts, the main fiber optic techniques available for this application are reviewed, emphasizing the four most successful ones. Then, several examples of the use of OFS in real structures are also addressed, including those from the renewable energy, transportation, civil engineering and the oil and gas industry sectors. Finally, the most relevant current technical challenges and the key sector markets are identified. This paper provides a tutorial introduction, a comprehensive background on this subject and also a forecast of the future of OFS for SHM. In addition, some of the challenges to be faced in the near future are addressed.

Fiber Optic Sensors in Structural Health Monitoring

A 3D shear-lag model considering micro-damage and statistical strength prediction of unidirectional fiber-reinforced composites

The influence of the polymer matrix density, chemical cross-links in the interface, and geometrical defect in the carbon nanotubes (CNTs) on the CNT pull-out from polymer has been analyzed by the molecular dynamics simulation. The interfacial shear strength (ISS) has also been estimated with the change of total potential energy. In the simulation, the crystalline polyethylene matrix is set up in a hexagonal array with the polymer chains parallel to the CNT axis. First, we investigate the effect of the polymer matrix density on the ISS by changing the distance between the chains. Simulated results show that the ISS increases with the increase of matrix density. Next, we examine the cross-link effect on the ISS by adding polyethylene cross-links in the interface. Here, an energy based switching criterion addressing cross-links traveling on the CNT has been proposed. It is found that the presence of cross-links and the cross-link positions affect the ISS. Finally, pentagon–heptagon defect, which reduces the tensile strength of the CNT and has been experimentally observed by Hashimoto et al. [Hashimoto A, Suenaga K, Gloter A, Urita K, Iijima S. Direct evidence for atomic defects in graphene layers. Nature 2004;430:870–3], has been addressed as a geometrical defect in the CNT. When cross-links are present between the CNT and the polymer, this defect reduces the ISS due to the improper connections of cross-links around this defective region.

Computer simulation of carbon nanotube pull-out from polymer by the molecular dynamics method

The present paper addresses the correlation between mechanical damage and the change in electrical resistance of CFRP under tensile loading. A linear relation between the strain and the electrical resistance of single carbon fibers was obtained experimentally, and the electrical behavior of CFRP under tensile loading was investigated. At stresses approaching the failure stress, the composite resistance rises non-linearly, which is attributed to damage in the form of broken fibers. These experiments lead to the concept of electrical ineffective length over which a broken fiber does not carry electric current, in analogy to the well-established mechanical ineffective length over which a broken fiber carries reduced stress. Based on this concept, a DC circuit model consisting of a serial array of discrete parallel cells of length equal to the electrical ineffective length is proposed to explain the resistance evolution in the composite. An analytical model for fiber damage evolution within the electrical ineffective length is constructed using the Global Load Sharing model and the Weibull fiber strength distribution, The model successfully explains the experimental results on the resistance change of CFRP under tensile loading with an electrical ineffective length of 5 mm. (C) 2001 Elsevier Science Ltd. All rights reserved.

Tomonaga Okabe

Papers

A 3D shear-lag model considering micro-damage and statistical strength prediction of unidirectional fiber-reinforced composites

Computer simulation of carbon nanotube pull-out from polymer by the molecular dynamics method

Electromechanical modeling of unidirectional CFRP composites under tensile loading condition

Size effect on tensile strength of unidirectional CFRP composites— experiment and simulation

Micromechanical modeling of the microbond test to quantify the interfacial properties of fiber-reinforced composites