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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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Book ChapterDOI
01 Jan 2011
TL;DR: In this paper, a single-component remendable polymer suitable for integration into aerospace composite structures is described. But, the authors focus on the development of a single component remendability polymer, which can be used as a replacement matrix material in fiber-reinforced composites and allows for in-situ site-specific healing of delamination and matrix cracking when combined with a small volume fraction of heat assisting materials such as magnetic particles.
Abstract: The work described in this research focuses on the development of a single-component remendable polymer suitable for integration into aerospace composite structures. The structural polymer can be used as a replacement matrix material in fiberreinforced composites and allows for in-situ site-specific healing of delamination and matrix cracking when combined with a small volume fraction of heat-assisting materials such as magnetic particles. Whereas previous studies focused on the optimal volume fraction and composition of magnetic particles, and the healing of cracks in carbon-fiber composite coupons, current studies focus on the barriers to adopt this material in a aircraft manufacturing environment. These barriers include the (1) the extensive labor involved in producing a limited quantity of Mendomer (1-3 grams), (2) the evolution and entrapment of voids for all but exquisitely controlled environments, and (3) the low melting temperature (ca. 125°C) of the material when compared against high-temperature matrix systems. Proprietary steps have been formulated to increase raw material yield, reduce viscosity, and increase the glass transition temperature. Interests have also been motivated by reducing costs and adhering to conventional composite fabrication techniques. Research has also involved the investigation of automated damage detection to locate the site of damage for further healing, followed by automated healing since the ultimate goal of this research is to develop an autonomously healing composite system. However, a remendable composite is in itself valuable where its successful incorporation will reduce the need for part replacement and maintenance as well as increase the longevity and reliability of the structure.

2 citations

Book ChapterDOI
01 Jan 1992
TL;DR: In this paper, the use of ultrasonic techniques involving guided waves and contact type transducers can provide powerful characterization methods for composites, however, the wave phenomena associated with these methods are, in general, far more complex and less well understood than those associated with conventional techniques, based on longitudinal waves.
Abstract: With the increasing use of advanced composites in a variety of modern applications, it has become necessary to employ reliable and effective nondestructive evaluation (NDE) methods to determine the integrity and serviceability of structural composites. The use of ultrasonic techniques involving guided waves and contact type transducers can provide powerful characterization methods for composites. However, the wave phenomena associated with these methods are, in general, far more complex and less well understood than those associated with conventional techniques, based on longitudinal waves. Realization of the full potential of these newer techniques will require a better understanding of the quantitative features of the wave phenomena than is available at present.

2 citations

Proceedings ArticleDOI
06 May 2020
TL;DR: In this article, a global-local model is used for Lamb wave modeling with and without defects in fiber-reinforced polymer matrix composites to identify useful signal features for damage detection.
Abstract: Fiber-reinforced polymer matrix composites have excellent in-plane stiffness and strength properties, and are therefore ideal for usage in panels of aircraft wings or fuselage as well as launch vehicle case segments. Those thin plates or shell structures are often stiffened with many locally increased thickness regions, or beams of various cross-sectional shapes such as flat or T-shaped. Small defects in any of those stiffened regions would greatly reduce the structural performance as a whole. Locating such defects is time consuming because of the large extend of the panels as well as the number of stiffeners. Guided ultrasonic wave-based techniques could be applied for damage detection in large areas. However, the scattering characteristics of stiffeners are complex. In particular, when multiple stiffeners are present, the incident Lamb wave signal is altered with the passing of each stiffener. Thus, the goal of this work is to efficiently model Lamb wave propagation when multiple stiffeners are present, with and without defects, in an effort to identify useful signal features for damage detection. To this end, the so-called global-local method is used for Lamb wave modeling. The global functions are used to represent the nominal composite region – parameters are obtained by means of waveguide finite element (WFE) method – and the stiffened region is represented by finite element discretization. With a recently developed coupling technique, a source problem, representing a surface-mounted transducer is coupled with multiple stiffener-scattering models to examine the transmission characteristics. The global-local model is validated by laboratory waveform measurements on a stiffened composite plate. The results from global-local method can then be used to efficiently determine the maximum number of stiffeners before the transmitted Lamb waves become too weak to identify defects.

2 citations

Y. Bar-Cohen1, Ajit Mal1, F. Feng, M. Kabo, J. Wang 
17 Mar 2002
TL;DR: In this article, focused ultrasound was used in minimally invasive treatment of a variety of disorders including those associated with soft tissue or disk element disruption in the vicinity of the spine causing impingement on the spinal cord.
Abstract: This work is motivated by the possible medical application of focused ultrasound in minimally invasive treatment of a variety of disorders including those associated with soft tissue or disk element disruption in the vicinity of the spine causing impingement on the spinal cord.

2 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