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

Composite structures require a rigorous program of nondestructive inspection and maintenance to detect and characterize
hidden defects at an early stage of their occurrence so that preventive measures can be taken before the structure loses its load
carrying capacity or suffers from catastrophic failure. Current methods for defects detection in large aircraft and aerospace
structures are slow, labor intensive and costly. This is especially true for composite structures where conventional
techniques are often ineffective. Ultrasonic guided waves offer an attractive complementary tool for improving inspection
techniques in relatively large plate-like structural components due to their large propagation range and sensitivity to defects in
their propagation path. Since the waves are affected by the geometrical structural features (e.g. stringers) as well as harmful
defects (e.g. delaminations), the application of guided waves in the NDE or SHM of real structures requires a good
understanding of these interaction effects. This will help identify the defects from their distinguishing features in the signal in
structural components with complex geometry. In this paper a detailed study of the interaction of guided waves with defects
in an aluminum plate and a honeycomb composite sandwich structure is carried out using numerical simulations and
laboratory experiments. The simpler aluminum plate is used for model validation and understanding the basic characteristics
of the interaction phenomena. The agreement between the simulated waveforms and those measured from the experiments
are found to be excellent in both cases indicating the possibility of applying guided wave based techniques to more realistic
structures.

NDE of composite structures using ultrasonic guided waves

A model of atherosclerotic calcification and plaque vulnerability

This paper is concerned with the detection and characterization of hidden defects in advanced structures before they grow to a critical size. A methodology for automatic damage identification and localization is developed using a combination of vibration and wave propagation data. The structure is assumed to be instrumented with an array of actuators and sensors to excite and record its dynamic response, including vibration and wave propagation effects. A damage index, calculated from the measured dynamical response of the structure in a reference state and the current state, is introduced as a determinant of structural damage. The indices are used to identify low velocity impact damages in composite as well as aluminium plates for different arrangements of the source and the receivers. The potential applications of the approach in developing health monitoring systems in defects-critical structures is discussed.

Automated structural health monitoring system using acoustic emission and modal data

Frequency conversion is an essential tool in modern communication devices. Traditionally, frequency conversion is achieved through parametric coupling via nonlinear inductors or capacitors whose reactance is modulated by a carrier wave. In this study, nonlinear acoustic Lamb wave devices are explored for simultaneous signal filtering and frequency conversion. Three sets of interdigitated transducers are fabricated on a piezoelectric aluminum nitride (AlN) thin film to provide a carrier wave, a low-power signal wave, and to receive a frequency converted mixed wave. Two devices are fabricated and tested to demonstrate frequency upconversion and downconversion by utilizing mechanical nonlinearity of AlN, and the results are compared to a nonlinear circuit model. The nonlinear circuit model is used to link experimentally observed phenomenon to the acoustic material's intrinsic nonlinearity. The nonlinearity of AlN reaches a maximum of 2.8% with a carrier wave power at 28 dBm. An analytical model is used to predict device performance along with physical dimensions. These analytical results show that nonlinear acoustic mixers and filters can approach sub-millimeter sizes, which is orders-of-magnitude smaller than conventional structures using nonlinear inductors and capacitors.

Frequency conversion through nonlinear mixing in acoustic waves

A rigid circular disc embedded in an infinite, isotropic elastic solid is assumed to be excited by a normally-incident plane, harmonic compressional wave. The disc is assumed to be either in perfectly welded or perfectly smooth contact with the surrounding solid. In each case, the problem is reduced to the solution of Fredholm integral equations of the second kind. For the case of the welded contact, exact numeral solutions valid at any given frequency are given. Approximate solutions valid at low and high frequencies are obtained for both cases and are compared with the exact solution. The displacement and compliance of the disc as well as other quantities of interest are presented graphically as functions of frequency.

Ajit Mal

Papers

NDE of composite structures using ultrasonic guided waves

A model of atherosclerotic calcification and plaque vulnerability

Automated structural health monitoring system using acoustic emission and modal data

Frequency conversion through nonlinear mixing in acoustic waves

Motion of a rigid disc in an elastic solid