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.

/pdf/a-facile-soft-template-synthesis-of-mesoporous-polymeric-and-8vdw1lf6wb.pdf

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

Determination of geometric role and damage assessment in hybrid fiber metal laminate (FML) joints based on acoustic emission

NDE methods are needed to determine the structure integrity, stiffness and durability (residual life) of structures and they can be extremely useful in assuring the performance of structures using smaller safety factors. While the integrity and stiffness can be extracted directly from NDE measurements, strength and durability can not be associated with physical parameters and therefore, cannot be measured by NDE methods. Specifically, NDE methods are developed to detect and characterize flaws and to determine the material properties of test specimens. For many years, composites as multi-layered anisotropic media, have posed a challenge to the NDE research community. Pulse-echo and through-transmission are the leading methods that are used in practice to evaluate the quality of composites. However, these methods provide limited and mostly qualitative information about the material properties and many defects. Following the discovery of the LLW and the Polar Backscattering phenomena in composites [1,2], numerous experimental and analytical studies have taken place using obliquely insonified ultrasonic waves [3–5]. These studies led to the development of effective quantitative NDE capabilities to determine the elastic properties, to accurately characterize defects and even to evaluate the quality of adhesively bonded joints [6, 7]. In spite of the progress that was made both theoretically and experimentally, oblique insonification techniques are still academic tools and have not yet become standard industrial test methods for NDE of composite materials. The authors investigated the issues that are hampering the transition of these methods to the practical world of NDE and are involved with extensive studies to address these issues. This paper covers the progress that was made by the investigators in tackling the theoretical and experimental issues to solidify the foundation of the techniques and their transition to practical NDE tools.

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=3743&context=qnde

Rapid Characterization of the Degradation of Composites Using Plate Waves Dispersion Data

Based on the metric of "healing window", the ionomer composite containing 2-3% nano iron oxide particles and heated at induction frequencies between 250 and 300 kHz yields the most favorable results. The metric of healing window is defined as the time difference between initiation of healing (melting) and onset of deformation. At this aforementioned ideal volume fraction, composition and induction frequency range, healing of a damaged thick film of ionomeric polymer occurs between 5 and 10 seconds. In this study, both compliant and structural remendable polymers are combined with a small amount of heating material (<5% volume fraction) to facilitate site-specific healing and facilitate future active self-healing. The compliant remenable polymer is an ionomer. The structural remendable polymer is a highly cross-linked polymers called "Mendomer." The heating materials include nanoscale and microscale magnetic particles used with induction heating and carbon fibers used with resistive heating. The inductive heating approach compared to the resistive heating approach has the advantage of being non-contact. Current approaches describe in-situ healing where external power is delivered to generate heating. Active self-healing will be made possible by coupling the healing mechanism with a damage event. Remending polymers can be layered or combined with conventional materials to yield a multifunctional solution offering both a conventional function plus the added remendable feature. One application combines ionomeric remendable polymers layered with polyimide and polytetrafluoroethylene to form remendable wire insulation multilayer composites. Another application uses the Mendomer as the matrix for conventional carbon-fiber composite fabrication to mitigate damage from matrix cracking and subsequent delamination. Preliminary and quantitative results are given and discussed for ionomers, ionomers combined with magnetic particles, ionomers combined with carbon fiber, Mendomers, and Mendomers combined with magnetic particles. For the case of magnetic particles used with induction heating, optimal volume fractions are between 2 and 3%.

Smart Self-Healing Material Systems Using Inductive and Resistive Heating

Diffraction of SH Waves by a Near Surface Crack

Composite materials are being used increasingly in advanced aircraft and aerospace structures. Despite their many advantages including high strength to weight ratio, formability and low coefficient of thermal expansion, composites are often susceptible to hidden damage that may occur during their manufacturing and/or service of the structure. Safe operation of composite structures requires careful monitoring of the initiation and growth of such defects before they grow to a critical size resulting in possible catastrophic failure of the structure. Ultrasonic methods using guided waves offer a reliable and cost effective method for defects monitoring in advanced structures due to their long propagation range and their sensitivity to defects in their propagation path. In this paper some of the useful properties of guided Lamb type waves are investigated in an effort to provide the knowledge base required for the development of viable defects monitoring systems in composite structures. Some of our recent research in this area is presented in this paper. The research includes laboratory experiments using a pitch catch method in which a pair of moveable transducers are placed on the outside surface of the structure for generating and recording the wave signals. The recorded signals are analyzed to construct the dispersion and other relevant properties of the guided waves. Theoretical simulations using analytical and numerical methods are carried out and compared with the experimental results. The specific cases considered include an aluminum plate, a woven quasi-isotropic composite panel and an aluminum honeycomb panel with woven composite face sheets. The agreement between the experimental and theoretical results are shown to be excellent in certain frequency ranges, but not for others, providing a guidance for the design of effective inspection systems.

Ajit Mal

Papers

Determination of geometric role and damage assessment in hybrid fiber metal laminate (FML) joints based on acoustic emission

Rapid Characterization of the Degradation of Composites Using Plate Waves Dispersion Data

Smart Self-Healing Material Systems Using Inductive and Resistive Heating

Diffraction of SH Waves by a Near Surface Crack

Lamb Waves in a Honeycomb Composite Sandwich Plate