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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 article, the influence of the depth and material properties of a sedimentary deposit on earthquake ground motion is examie through theoretical modelling, where the strong stratification of a typical deposit is represented by a number of homogene layers of dissipative viscoelastic material.
Abstract: The influence of the depth and material properties of a sedimentary deposit on earthquake ground motion is examie through theoretical modelling. The strong stratification of a typical deposit is represented by a number of homogene layers of dissipative viscoelastic material. The motion is assumed to be produced by a line source. The calculations carried out by means of a recently developed theoretical technique. The near surface low velocity layers are found to has a very strong amplification effect on the ground motion at some frequencies. The widely used one dimensional approae for site effect evaluation is shown to be valid for smaller epicentral distances only. The deamplification effect of mate dissipation is shown to be extremely important in most cases.
03 Dec 2019
TL;DR: In this paper, an improved NDE method for detecting disbonds at the top and bottom interfaces is proposed based on experimental results, and the results show that the optimized damage index greatly improves the resolution and flexibility of NDE on HSP.
Abstract: Honeycomb sandwich panels (HSP) are widely used in the aerospace industry due to their high strength to stiffness ratio. Conducting non-destructive evaluation (NDE) of HSP is a topic of great current interest. However, the geometric complexity of honeycomb core makes NDE of HSP extremely difficult. Guided ultrasonic waves are ideal for large scale testing due to its large range and high sensitivity to defects in their path. Previous research has been successful in detecting disbonds at the core-skin interface using guided waves, but few of them have focused on the detection of disbonds at the bottom interface. An improved NDE method for detecting disbonds at the top and bottom interfaces is proposed based on experimental results. By applying excitation signals for different frequencies, the responses at the top and bottom skins are compared and analyzed. The response in a specific frequency range is further studied by introducing disbond at the bottom interface. It is shown that some components of the recorded signal in specific frequency range are more sensitive to the disbond and can be related to the size of the disbond. Finally, an improvement of the conventional damage index based on propagation velocity of guided waves is provided. The results show that the optimized damage index greatly improves the resolution and flexibility of NDE on HSP.
Proceedings ArticleDOI
TL;DR: In this article, the effect of the number of selected Lamb modes, mesh density and the size of the modeled plate segment is investigated for single and double cantilever configurations on a plate in the case of plain strain.
Abstract: Damping in miniature resonators is a consequence of many factors, one of which is due to interaction with the substrate to which the resonator is mounted. It is common practice to create a model of the resonator that includes a small segment of the substrate plate with a finite element (FE) software in conjunction with absorbing boundary elements. As an alternative to implementing absorbing boundary elements, semi-analytical methods have been developed in which such elements are replaced by analytical expressions for Lamb waves. This approach requires the specification of a harmonic load and the determination of the subsequent harmonic response at a point on the resonator. The modal frequency and damping can then be estimated from the computation of the frequency response function on a frequency grid. In this paper, the approach is demonstrated for single and double cantilever configurations on a plate in the case of plain strain. The influence of the number of selected Lamb modes, mesh density and the size of the modeled plate segment is investigated through parametric studies. Moreover, it is shown that the semi-analytical results are in good agreement with those from conventional transient finite element simulations.
01 Apr 1994
TL;DR: In this article, a device was developed to manipulate a pair of transmitting and receiving transducers at different angles of wave incidence and propagation with the fiber orientation to obtain reflected ultrasonic wave induced by oblique insonification of composite materials.
Abstract: The analysis of reflected ultrasonic waves induced by oblique insonification of composite materials is a powerful tool for providing informations about defects and material properties. A device was developed to manipulate a pair of transmitting and receiving transducers at vrious angles of wave incidence and propagation with the fiber orientation.
Proceedings ArticleDOI
11 Apr 2007
TL;DR: The conceptual structural health monitoring (SHM) system presented here involves a low level computational effort, has high reliability, and is able to treat the acquired data in real-time to identify the presence of existing as well as emerging damage in the structure.
Abstract: This paper is concerned with the detection and characterization of impact damage in stiffened composite structures using high frequency Lamb waves and low frequency modal vibrations. The geometric and material complexities of the structure present practical difficulties in the direct analysis of both wave propagation and modal vibration data using theoretical constructs. An improved test setup, consisting of high fidelity sensor arrays, laser scanning vibrometer, data acquisition boards, signal conditioning and dedicated software has been implemented. The conceptual structural health monitoring (SHM) system presented here involves a low level computational effort, has high reliability, and is able to treat the acquired data in real-time to identify the presence of existing as well as emerging damage in the structure. A statistical damage index algorithm is developed and automated by utilizing a diagnostic imaging tool to identify a defect right from its appearance, with high degree of confidence. The main advantage of the method is that it is relatively insensitive to environmental noise and structural complexities as it is based on the comparison between two adjacent dynamical states of the structure and the baseline for comparison is continuously updated to the previous state. The feasibility of developing a practical Intelligent Structural Health Monitoring (ISHM) System, based on the concept of "a structure requesting service when needed," is discussed.

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