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Ajit Mal

Bio: Ajit Mal is an academic researcher from University of California, Los Angeles. The author has contributed to research in topics: Lamb waves & Composite laminates. The author has an hindex of 38, co-authored 205 publications receiving 7217 citations. Previous affiliations of Ajit Mal include University of Southern California & California Institute of Technology.


Papers
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24 Mar 1998
TL;DR: In this article, the issue of aging is addressed in composite materials, where composites are being used at a significant level of usage for flaw critical structures and they are taking a growing percentage of the makeup of aircraft and spacecraft.
Abstract: Composite materials are being used at a significant level of usage for flaw critical structures and they are taking a growing percentage of the makeup of aircraft and spacecraft. Composite structues are now reaching service duration, for which the issue of aging is requiring adquate attention.

4 citations

Journal ArticleDOI
TL;DR: In this article, a broadband, hybrid system based on laser generation and air-coupled detection of ultrasound is employed for primary wave (P-wave) characterization of carbon foam.
Abstract: The nondestructive characterization of the material properties and the detection of hidden discontinuities in a highly porous, open cell, carbon foam using ultrasonics are presented. The propagation of ultrasonic waves in carbon foam specimens and their relationship with the elastic properties of the material are studied through theoretical modeling and laboratory experiments. The geometric and the overall elastic properties of the material are determined from microscopic data and wave propagation experiments performed under dry and fluid coupling conditions. A broadband, hybrid system based on laser generation and air-coupled detection of ultrasound is first employed for primary wave (P-wave) characterization. Significant discontinuities within the specimen are then detected using a narrowband, air-coupled ultrasonic setup as well as an automated fluid-coupled real-time intelligent ultrasonic setup. The use of the discrete wavelet transform is investigated as an effective tool for signal denoising in air-coupled ultrasonic probing of the foams. A simple, one-dimensional model, based on a periodic spring-mass system and a previously developed homogenization theory, is used to predict the wave speed.

4 citations

Proceedings ArticleDOI
TL;DR: In this paper, an experimental and numeri-cal studies are conducted, in an effort to understand the interaction of guided ultrasonic waves with simple models of the stiffener, in order to improve the reliability of ultrasonic damage detection in stiffened plate structures.
Abstract: Stiffeners are important structural components in modern composite and honey-comb structures. The safe operation of such composite structures, which are com-monly used in aeronautical applications, requires careful monitoring as hidden de-fects may compromise the structural safety. In order to improve the reliability of ultrasonic damage detection in stiffened plate structures, experimental and numeri-cal studies are conducted, in this paper, in an effort to understand the interaction of guided ultrasonic waves with simple models of the stiffener. Through a series of measurements in different positions, the amplitudes of scattered waves for various configurations of the stiffener are determined. Moreover, the group velocity of the waves in the stiffened structure is analyzed. The experimental findings are compared with results from numerical simulations. doi: 10.12783/SHM2015/248

4 citations

Proceedings ArticleDOI
11 Jun 2002
TL;DR: In this article, a finite element-based program called PZflex is used to calculate the mechanical and thermal effects caused by high-frequency ultrasound in different material systems, and the pressure distributions generated by plane and focused ultrasound beams are presented.
Abstract: This work is motivated by possible medical applications of focused ultrasound in minimally invasive treatment of a variety of disorders. The mechanical and thermal effects caused by high-frequency ultrasound in different material systems are calculated using a finite element based program called PZflex. The pressure distributions generated by plane as well as focused ultrasound beams are presented. For the focused beam, the temperature distribution in the focal zone is also calculated. The results indicate that the heating efficiency of the ultrasound energy in the focal region depends on the exciting frequency and the geometry of the focal zone depends on the material being tested. At higher excitation energies, cavitation and nonlinear effects need to be included in the simulations. These effects are under current investigation.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

4 citations

Journal ArticleDOI
TL;DR: In this article, an expression for the dispersion equation of the waves when the upper layer is of varying thickness was derived for the Love wave propagation over the spherical surface of a layered earth model with special emphasis on dispersion produced in the layer The velocity of the wave with large wave-length increases appreciably as compared to the case of plane layer.
Abstract: Propagation of Love waves over the spherical surface of a layered earth model has been discussed with special emphasis on the dispersion produced in the layer The velocity of the waves with large wave-length increases appreciably as compared to the case of plane layer The analysis has been extended to deduce an expression for the dispersion equation of the waves when the upper layer is of varying thickness The modifications imparted to the dispersion equation depends on the amplitude only and not the shape of the corrugations provided we neglect small quantities of the second order The effect is a substantial decrease in the phase velocity and becomes more pronounced if the amplitude of the corrugations increases

4 citations


Cited by
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01 May 1993
TL;DR: 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.
Abstract: 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.

29,323 citations

Journal ArticleDOI
TL;DR: Technical challenges that must be addressed if SHM is to gain wider application are discussed in a general manner and the historical overview and summarizing the SPR paradigm are provided.
Abstract: 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.

2,152 citations

Journal ArticleDOI
TL;DR: 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 akin to human skin.
Abstract: 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.

1,950 citations

Journal ArticleDOI
18 Nov 2011-Science
TL;DR: In this paper, the authors designed epoxy networks that can rearrange their topology by exchange reactions without depolymerization, and showed that they are insoluble and processable.
Abstract: 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

1,901 citations

Journal ArticleDOI
TL;DR: In this article, a route for the controlled synthesis of mesoporous polymer nanospheres, which can be further converted into carbon nanosphere through carbonization, is presented.
Abstract: 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.

1,542 citations