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

We consider SH wave propagation in a fiber-reinforced composite which consists of a homogeneous, isotropic matrix, containing long, parallel, randomly distributed circular fibers of identical properties. The scattering of waves in the elastically inhomogeneous medium results in a frequency dependent velocity and attenuation of the coherent wave.

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1256&context=qnde

Phase Velocity and Attenuation of SH Waves in a Fiber-Reinforced Composite

A volume integral equation method (VIEM) is used to study elastostatic problems in an unbounded elastic solid containing multiple elliptical inclusions of arbitrary orientation subject to uniform tensile stress at infinity. The inclusions are assumed to be long parallel elliptical cylinders composed of isotropic or anisotropic elastic materials and perfectly bonded to the isotropic matrix. The solid is assumed to be under plane strain on the plane normal to the cylinders. In contrast to previous studies cited in this paper where only one or a few specific types of inclusions were considered, a detailed analysis of the stress field at the matrix-inclusion interface for square and hexagonal packing arrays is carried out herein, taking into account different values for the number, aspect ratio, orientation angle and concentration of the elliptical inclusions. The accuracy and efficiency of the method are examined through comparison with results obtained from analytical and finite element methods.

Calculation of interfacial stresses in composites containing elliptical inclusions of various types

Lap joints are common elements of aircraft and other engineered structures. They are often subject to hidden defects which are caused by corrosion and fatigue, and are very difficult to detect. Development of accurate and efficient methods for the early detection of corrosion and fatigue cracks in lap joints is of considerable current interest. Ultrasonic techniques using guided waves offer the possibility of improving the technology of detecting and characterizing flaws within lap joints. It is well known that the characteristics of guided waves can be used to detect defects in plates [1]. However, the geometry of the lap joint makes it difficult to extend these techniques to lap joints. In this paper we consider the theoretical problem of the propagation of guided waves across a simple model of the lap joint in an effort to understand the interaction of the guided waves with the geometrical features of the lap joint. The geometry of the lap joint, including the vertical stress free boundaries, the rectangular corners, and the change in thickness, makes it impossible to derive a closed form solution to the problem of wave propagation across it. The problem can only be attacked by numerical methods. Conventional finite element methods fail when dealing with problems with infinite domains. In order to handle problems with local inhomogeneities or irregular shapes in an infinite domain, hybrid methods must be used.

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=3067&context=qnde

Lamb Wave Propagation Across a Lap Joint

This paper is concerned with the quantitative nondestructive evaluation of corrosion around and cracks emanating from rivet holes in typical aircraft structural components. Current techniques for detecting defects around rivet holes are time consuming and qualitative. A recently developed ultrasonic technique using plate guided Lamb waves as probes has been found to be successful in laboratory specimens and appear to have the potential to improve the speed and robustness of the inspection. The general features and future prospects of the method is described in the paper.

NDE of rivet holes in aging aircraft components using Lamb waves

In composite structures, damages are often invisible from the surface and can grow to reach a critical size, potentially causing catastrophic failure of the entire structure. Thus safe operation of these structures requires careful monitoring of the initiation and growth of such defects. Ultrasonic methods using guided waves offer a reliable and cost-effective method for structural health monitoring in advanced structures. Guided waves allow for long monitoring ranges and are very sensitive to defects within their propagation path. In this work, the relevant properties of guided Lamb waves for damage detection in composite structures are investigated. An efficient numerical approach is used to determine their dispersion characteristics, and these results are compared to those from laboratory experiments. The experiments are based on a pitch-catch method, in which a pair of movable transducers is placed on one surface of the structure to induce and detect guided Lamb waves. The specific cases considered include an aluminum plate and an aluminum honeycomb sandwich panel with woven composite face sheets. In addition, a disbond of the interface between one of the face sheets and the honeycomb core of the sandwich panel is also considered, and the dispersion characteristics of the two resultant waveguides are determined. Good agreement between numerical and experimental dispersion results is found, and suggestions on the applicability of the pitch-catch system for structural health monitoring are made.

Ajit Mal

Papers

Phase Velocity and Attenuation of SH Waves in a Fiber-Reinforced Composite

Calculation of interfacial stresses in composites containing elliptical inclusions of various types

Lamb Wave Propagation Across a Lap Joint

NDE of rivet holes in aging aircraft components using Lamb waves

Dispersion of guided waves in composite laminates and sandwich panels