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

This introduction begins with a brief history of SHM technology development. Recent research has begun to recognise that a productive approach to the Structural Health Monitoring (SHM) problem is to regard it as one of statistical pattern recognition (SPR); a paradigm addressing the problem in such a way is described in detail herein as it forms the basis for the organisation of this book. In the process of providing the historical overview and summarising the SPR paradigm, the subsequent chapters in this book are cited in an effort to show how they fit into this overview of SHM. In the conclusions are stated a number of technical challenges that the authors believe must be addressed if SHM is to gain wider acceptance.

An introduction to structural health monitoring

Human skin is a remarkable organ. It consists of an integrated, stretchable network of sensors that relay information about tactile and thermal stimuli to the brain, allowing us to maneuver within our environment safely and effectively. Interest in large-area networks of electronic devices inspired by human skin is motivated by the promise of creating autonomous intelligent robots and biomimetic prosthetics, among other applications. The development of electronic networks comprised of flexible, stretchable, and robust devices that are compatible with large-area implementation and integrated with multiple functionalities is a testament to the progress in developing an electronic skin (e-skin) akin to human skin. E-skins are already capable of providing augmented performance over their organic counterpart, both in superior spatial resolution and thermal sensitivity. They could be further improved through the incorporation of additional functionalities (e.g., chemical and biological sensing) and desired properties (e.g., biodegradability and self-powering). Continued rapid progress in this area is promising for the development of a fully integrated e-skin in the near future.

25th Anniversary Article: The Evolution of Electronic Skin (E-Skin): A Brief History, Design Considerations, and Recent Progress

Permanently cross-linked materials have outstanding mechanical properties and solvent resistance, but they cannot be processed and reshaped once synthesized Non–cross-linked polymers and those with reversible cross-links are processable, but they are soluble We designed epoxy networks that can rearrange their topology by exchange reactions without depolymerization and showed that they are insoluble and processable Unlike organic compounds and polymers whose viscosity varies abruptly near the glass transition, these networks show Arrhenius-like gradual viscosity variations like those of vitreous silica Like silica, the materials can be wrought and welded to make complex objects by local heating without the use of molds The concept of a glass made by reversible topology freezing in epoxy networks can be readily scaled up for applications and generalized to other chemistries

Silica-Like Malleable Materials from Permanent Organic Networks

The controlled synthesis of monodisperse nanospheres faces a number of difficulties, such as extensive crosslinking during hydrothermal processes. Here, the authors show a route for the controlled synthesis of mesoporous polymer nanospheres, which can be further converted into carbon nanospheres through carbonization.

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A facile soft-template synthesis of mesoporous polymeric and carbonaceous nanospheres

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Diffraction of elastic waves by a penny-shaped crack

The time histories and spectral characteristics of acoustic pulses reflected from fiberreinforced composite laminates immersed in water are recorded in the laboratory and analyzed through a generalized ray theory and an exact theory. Calculated results for an unidirectional laminate and two angle-ply laminates are compared with measured data. It is shown that for a unidirectional laminate a finite number of mode-converted waves contribute to the overall signal reflected from a thick specimen resulting in certain irregular behavior of the reflected pulses. A phenomenological model of wave attenuation is introduced in the theoretical simulation and the damping parameters are shown to have a strong influence on the amplitude of the reflected pulses. The phase velocity of the guided waves in the laminates are shown to be nearly independent of water loading and material dissipation in a broad frequency range. Agreement between measured and calculated results is found to be excellent to very good in all cases.

Analysis of Acoustic Pulses Reflected From Fiber-Reinforced Composite Laminates

Fatigue life prediction of fiber-reinforced titanium matrix composites

The problems of guided waves propagating across a lap joint as well as reflected by the free edge of a semi-infinite plate with defects are investigated both theoretically and experimentally. The theoretical analysis is accomplished by using a hybrid method called the global local finite element method. A bounded region enclosing the lap joint, or the free edge, is described and analyzed by the finite element method, and the Lamb wave modal expansion is used to represent the wave field outside this region. The experiments are performed by using the fracture wave detector. The theoretical and experimental results are compared and show good agreement.

Ajit Mal

Papers

Diffraction of elastic waves by a penny-shaped crack

Analysis of Acoustic Pulses Reflected From Fiber-Reinforced Composite Laminates

Fatigue life prediction of fiber-reinforced titanium matrix composites

Lap-joint inspection using plate waves

A differential equation for surface waves in layers with varying thickness