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

Hidden damage caused by foreign object impact in a composite structure, if left undetected, can grow and lead to a catastrophic failure of the structure. Detection of impact events and characterization of the degree of damage caused by them, preferably in real time, would be extremely helpful in safe continued operation of composite structures. In this paper, low velocity impact experiments are carried out on AS4/3501-6 [0/90]8S cross-ply graphite epoxy composite plates. An instrumented impact testing system is used to record the contact force and the surface motion at locations away from the impact point. The response of the plate due to localized sources is calculated using a modified laminate theory providing detailed information on the relationship between the impact load and the signals generated by the load. For thin plates, the far-field response is dominated by plate guided Lamb waves. It is shown that the occurrence of an impact loading can be easily detected from the recorded signals. Delamination damage, if any, can also be determined through careful analysis of the recorded waveforms. Practical applications of the technique in structural health monitoring will require careful investigation and elimination of environmental noise.

Acoustic emission waveforms in composite laminates under low velocity impact

A theory is presented for the calculation of the acoustic material signature of a multilayered elastic half‐space overlain by a fluid The solid layers are composed of homogeneous isotropic linearly elastic materials and are firmly bonded at the interfaces The calculation procedure is valid at an arbitrarily high frequency of excitation Results are presented for a uniform, a single layered and a four layered model of the half‐space at two frequencies of excitation; one moderate (35 MHz) and the other relatively high (370 MHz) Several new features of the material signatures and their possible use in the material characterization of layered specimen are indicated

Calculation of the acoustic material signature of a layered solid

The integrity of safety-critical structural composites can be enhanced by the use of innovative ultrasonic nondestructive evaluation techniques. Among the various existing techniques, guided wave methods provide a good promise in terms of sensitivity to a variety of damage types or defects and the extent of the area that can be monitored, given the ability of these waves to travel relatively long distances within the structure under investigation. In comparison with isotropic metallic structures, wave propagation in composite structures presents additional complexity for effective damage identification. The material inhomogeneity, anisotropy, and the multilayered construction of composite materials lead to significant dependence of wave modes on laminate layup configurations, direction of propagation, frequency, and interface conditions. In this article, a specific structure will be analyzed with different levels of complexities in an effort to determine the propagation characteristics of the waves. The i...

Guided waves in a stiffened composite laminate with a delamination

Lamb wave propagation in a plate with step discontinuities

This paper is concerned with the development of acoustic emission (AE) waveform analysis in advanced structural composites. The relationship between the surface response and microfracture modes in composite laminates is studied to establish the theoretical background for waveform analysis of AE signals. Lamb waves produced by arbitrary internal sources in unidirectional and cross-ply composite laminates are investigated. Laboratory experiments are performed to validate the theoretical models. The results of this research should be useful in developing practical nondestructive testing tools to monitor damage initiation and evolution in composite structures.

Ajit Mal

Papers

Acoustic emission waveforms in composite laminates under low velocity impact

Calculation of the acoustic material signature of a layered solid

Guided waves in a stiffened composite laminate with a delamination

Lamb wave propagation in a plate with step discontinuities

Wave theory of acoustic emission in composite laminates