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

This work describes the cross-cutting technology for using remendable polymers for Ballistic Missile Defense System (BMDS) Interceptors to heal damaged missile structures Matrix cracking and associated delamination of carbon-fiber ply skins on interceptor structures is addressed using a Mendomer-based polymer composite developed by the University of California, Santa Barbara (UCSB), the University of California, Los Angeles (UCLA), NextGen Aeronautics, US Army Aviation and Missile Research Development and Engineering Center (AMRDEC), and Missile Defense Agency (MDA) scientists and engineers When used in place of conventional composite matrix materials, this material system enables in-situ healing of damaged carbon-fiber components Both neat and composite samples were fabricated and examined for their remendable and shape memory properties The neat polymer samples and polymer/Ni particle composites contained a significant number of voids The carbon-fiber-polymer composite samples exhibit similar gas entrapment as well as inadequate carbonfiber wet-out during fabrication Despite the undesirable void content and inadequate carbon-fiber wet-out, the coupons exhibit appreciable remendable and shape memory properties Further improvement and optimization of the composite layup process are needed to produce remendable carbon-fiber composites with adequate mechanical and structural properties We also present preliminary molecular dynamics (MD) and finite element analysis (FEA) simulations to capture the behavior of these remendable material systems

Ballistic Missile Defense System (BMDS) Solutions Using Remendable Polymers

The fracture and failure of fiber-reinforced composites are known to be strongly influenced by the properties of the fiber-matrix interface zone. Since these properties may be altered during processing, or may suffer degradation during service, the quality control of interface zones through nondestructive evaluation techniques is highly desirable. The overall properties of fiber-reinforced composites are related to their microstructure. In particular, the velocity of the average waves that can be transmitted in these materials depends on the elastic moduli and the densities of the constituents as well as the properties of the fiber-matrix interface. Thus, the interface conditions can, in principle, be determined from the measurement and analysis of wavespeeds in these materials. However, in practice, this is not a straightforward exercise due to the fact that the relationship between the overall and microstructural properties of a composite are in general nonlinear and thus may lead to an amplification of measurement errors. It is nevertheless extremely important to develop the capability to monitor the integrity of the fiber-matrix interface, especially in high temperature applications where the composites must operate in harsh environments.

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=2549&context=qnde

Characterization of Fiber-Matrix Interface Degradation in a Metal Matrix Composite

Non-destructive testing of critical structural components is time consuming, while necessary for maintaining safe
operation. Large aerospace structures, such as the vertical stabilizers of aircraft undergo inspection at regular intervals
for damage diagnostics. However, conventional techniques for damage detection and identification before repair can be
scheduled are conducted off-line and therefore can take weeks. The use of guided ultrasound waves is being investigated
to expedite damage detection in composites. We measure the frequency dependent loss of ultrasonic guided waves for a
structure comprising a boron-nitride composite skin sandwiching an aluminum honeycomb. A wide range of ultrasound
frequencies propagate as measured using PZTs, with the lowest attenuation observed about 200-250 kHz. These
measurements are confirmed using optical fiber Bragg grating arrays used as ultrasound transducers.

Propogation loss with frequency of ultrasound guided waves in a composite metal-honeycomb structure

The behavior of the wave field produced in a thin unidirectional graphite/epoxy composite plate by a dynamic point load is studied using an approximate shear deformation plate theory (S.D.P.T) and a finite element analysis (F.E.A). Comparisons are made for propagation at 0 o , 45 , and 90 directions relative to the fibers showing excellent agreement between the two model approaches. The approximate method is then used to calculate the response of a composite plate as well as of an aluminum plate to a uniform dynamic surface load distributed in a circular region. A periodic reversal in the phase of the signal with propagation distance is observed. It is found that this is caused by the strong dispersion of the first antisymmetric waves at low frequencies. For clarification, the steepest descent method is applied to obtain a closed form analytical expression for the far field response in the aluminum plate for a Dirac delta source. It is shown that the waveform carries a singularity that reverses its phase at regular intervals. The present work should be helpful in understanding the nature of waveform signals produced by impact loads and in the detection and characterization of impact damage in composite structures. Keywords: Thin plate, unidirectional graphite/epoxy composite, periodic phase reversal, low frequency, first antisymmetric mode, the steepest descent method

Ultrasonic lamb waves in a thin plate

A method of heating a tissue using pulsed focused ultrasonic waves, and an apparatus for carrying out the method. The method comprises depositing an amount of ultrasound energy sufficient to heat the selected site without adversely damaging other parts of the tissue. The apparatus comprises an ultrasonic source for producing pulsed ultrasonic waves, a lens for focusing the ultrasonic waves, and a controller for controlling wave frequency and transmission parameters of the ultrasonic waves. The method and apparatus can be used to treat spine or other joint disorders associated with lax ligamentous tissues, i.e., soft tissue disruption.

Ajit Mal

Papers

Ballistic Missile Defense System (BMDS) Solutions Using Remendable Polymers

Characterization of Fiber-Matrix Interface Degradation in a Metal Matrix Composite

Propogation loss with frequency of ultrasound guided waves in a composite metal-honeycomb structure

Ultrasonic lamb waves in a thin plate

Transcutaneous spine trauma and disorders treatment using ultrasonically induced confined heat zone