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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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Journal ArticleDOI
TL;DR: In this paper, the capabilities of existing ultrasonic NDE methods for the strength-related properties of bonds in structural components are reviewed and the severe limitations of conventional NDE method in yielding quantitative results are indicated.
Abstract: The capabilities of existing ultrasonic nondestructive evaluation (NDE) methods for the strength-related properties of bonds in structural components is reviewed. The severe limitations of conventional NDE methods in yielding quantitative results are indicated. Some recent results of a joint theoretical and experimental program of research using leaky Lamb waves (LLW) in laboratory specimens are presented. The LLW technique is shown to have several advantages over conventional techniques. Potential applications of the technique to determine non-destructively the quality of bonds in a variety of models are discussed.

24 citations

Journal ArticleDOI
TL;DR: In this paper, a theoretical analysis is carried out in an effort to understand certain unusual properties of transient guided waves produced in a thin unidirectional graphite/epoxy composite plate by a localized dynamic surface load.
Abstract: A theoretical analysis is carried out in an effort to understand certain unusual properties of transient guided waves produced in a thin unidirectional graphite/epoxy composite plate by a localized dynamic surface load. The surface motion is calculated using an approximate plate theory, called the shear deformation plate theory (SDPT), as well as a recently developed finite element analysis (FEA), for their mutual verification. The results obtained by the two methods are shown to have excellent agreement. An interesting, nearly periodic "phase reversal" of the signal with propagation distance is observed for each propagation direction relative to the fiber direction. For clarification, a closed form analytical expression for the vertical surface displacement in an aluminum plate to an impulsive point force is obtained using the steepest descent method. It is found that the strong dispersion of the first antisymmetric waves at low frequencies is the main reason behind the phase reversal. This is verified further by measuring the surface response of a relatively thick aluminum plate to a pencil lead break source. The understanding developed in the paper is expected to

24 citations

Journal ArticleDOI
TL;DR: In this paper, the propagation of coherent elastic shear waves in a medium containing a dilute and uniform distribution of cylindrical inclusions is discussed, where the waves are assumed to be polarized parallel to the axes of the cylinders while the cylinders are thin compared to the wavelength.
Abstract: The propagation of coherent elastic shear waves in a medium containing a dilute and uniform distribution of cylindrical inclusions is discussed. The waves are assumed to be polarized parallel to the axes of the cylinders while the cylinders are assumed to be thin compared to the wavelength. The complex wavenumber is calculated to a second order of approximation, the real part of which gives the propagation speed and the imaginary part gives the damping due to scattering. Numerical results are obtained for the case of identical inclusions with the properties of the inclusions varying between hollow (void) and rigid.

24 citations

Journal ArticleDOI
01 Nov 2007
TL;DR: In this article, the authors used a combined higher-order time-frequency (CHOTF) method to locate and characterize an impact load in thick plates, which is based on the analysis of the acoustic waveforms measured by a sensor array located on the plate surface in combination with the theoretical Green's function for the plate.
Abstract: The current paper presents a novel approach to precisely locate and characterize an impact load in thick plates The approach is based on the analysis of the acoustic waveforms measured by a sensor array located on the plate surface in combination with the theoretical Green's function for the plate The Green's functions are derived based on either the exact elastodynamic theory or an approximate shear deformation plate theory For accurate estimation of the location of the impact source, the time differences in the arrival times of the waves at the sensors and their propagation velocities are determined first This is accomplished through the use of a combined higher-order time-frequency (CHOTF) method, which is capable of detecting signals with lower signal to noise ratio compared with other available methods Since most of the energy in the wave is carried by the flexural waves (A0 mode), the group velocity of this mode is extracted using the CHOTF technique for estimating the impact source loc

23 citations

Journal ArticleDOI
TL;DR: In this article, a volume integral equation method (VIEM) is used to calculate the plane elastostatic field in an unbounded isotropic elastic medium containing multiple orthotropic cylindrical inclusions subject to remote loading.

22 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