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

The effect of thickening of the crustal layer in mountainous region on the dispersion curve of Love waves has been studied Perturbation method has been applied to obtain the modified frequency equation for Love waves through the surface of separation between a semi-infinite material and a layer the thickness of which abruptly increases throughout a certain length of the path The effect is to decrease the phase velocity of the waves particularly in the low period range It has been pointed out that by proper study, the amount of thickening may be obtained

On the frequency equation for love waves due to abrupt thickening of the crustal layer

Development of efficient tools to successfully localize and characterize hidden damage in critical structural components is an important task in the design and construction of structural health monitoring systems in aging as well as new structures. In this paper two methodologies for damage identification and localization will be presented. The first is an automatic numerical scheme using a state space system identification approach and the second is based on certain damage correlation indices associated with changes in the frequency response of the structure in presence of flaws. In each case, the structure is to be instrumented with an array of sensors to record its dynamic response including vibration and wave propagation effects. To determine the type and location of an unknown defect, the sensor data detected is used to identify a new system, which then is compared to a database of state-space models to find the nearest match. The second method deals with the definition of a set of damage correlation indices obtained from the frequency response analysis of the structure. Two types of indices have been considered. The first uses the correlation between the responses of the defect free and damaged structure at the same point, and the second uses correlation at two different points. The potential application of the general approach in developing health monitoring systems in defects-critical structures is discussed.

Damage detection in structures from vibration and wave propagation data

The analysis of reflected ultrasonic waves induced by oblique insonification of composite materials is increasingly recognized as a powerful tool in providing information about defects and material properties. The theoretical modeling of the wave behavior for tone-burst and pulses has been very successful in accurately corroborating the experimental results. The data repeatability and accuracy make oblique insonification methods easy to standardize for practical applications. Recent theoretical and experimental results and the progress in using frequency- and time-domain data are discussed with an emphasis on the application to graphite/epoxy composites.

NDE of composite materials using ultrasonic oblique insonification

This study is concerned with the detection and characterization of impact damage in a stiffened woven composite structure using high frequency Lamb waves and low frequency modal vibrations. The geometric and material complexities of the structure present practical difficulties in the direct analysis of both wave propagation and modal vibration data using theoretical constructs. Improved ultrasonic and vibration test setups consisting of distributed, high fidelity, and surface mounted sensor arrays are used here to determine changes in the dynamical properties of the composite structural components in the presence of damage. The sensors are assumed to provide both the low frequency global response (i.e., modal frequencies and mode shapes) of the structure to external loads and the (local) high frequency signals due to wave propagation effects in either passive or active mode of the ultrasonic array. A damage index, comparing the measured dynamical response of two successive states of the structure is introduced as a determinant of structural damage. The method relies on the fact that the dynamical properties of a structure change with the initiation or growth of damage. A diagnostic imaging tool is used for the interpretation and graphical representation of the indices to enable automated monitoring of the changes in the indices at a given instant of time. The value of the index at a given sensor increases with the proximity of the damage to the sensor. A sensitivity analysis is carried out in an effort to determine a threshold value of the index below which no reliable information about the state of health of the structure can be estimated. It is shown that the automated procedure is able to identify a defect right from its appearance, with some degree of confidence. The feasibility of developing a practical intelligent structural health monitoring system (ISHMS), based on the concept of 'a structure requesting service when needed,' is discussed.

Ajit Mal

Papers

On the frequency equation for love waves due to abrupt thickening of the crustal layer

Damage detection in structures from vibration and wave propagation data

NDE of composite materials using ultrasonic oblique insonification

Autonomous Impact Damage Monitoring in a Stiffened Composite Panel

Elastic waves from localized sources in composite laminates