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.

Modeling Ultrasonic Elastic Waves in Fiber-Metal Laminate Structures in Presence of Sources and Defects

Abstract: This paper presents a modeling and simulation method for studying ultrasonic guided wave propagation in hybrid metal-composites, also known as fiber-metal laminates. The objective is to develop an efficient and versatile modeling tool to aid in the design of cost-effective nondestructive evaluation technologies. The global–local method, which combines finite element discretization and Lamb wave modal expansion is used. An extension to the traditional global–local method is made to couple the source problem with the scattering problem to deal with a surface source generating Lamb waves that interact with defects in multilayered structures. This framework is used to study the sensitivity of different excitation frequencies to ply gap defects of various sizes. The coupled model considers the transducer contact conditions and the ultrasonic system response in the Lamb wave excitation, along with the scattering phenomenon caused by the defects. This combined result is used to define the optimal excitation frequency for the strongest transmission or reflection for a given defect size that can be observed in a physical experiment. Such results can be applied to the design of a damage detection scheme in realistic aerospace structures.

Love and Rayleigh waves in the San Fernando Valley

Abstract: By means of a recently developed finite element technique, the propagation of Love and Rayleigh waves along an approximately north-south section of the San Fernando Valley is studied. At periods of 1 sec or less, the surface amplitudes of both Love and Rayleigh waves increase considerably with distance south into the valley as the alluvium thickens. Dipping structures give rise to higher Love and Rayleigh modes which modulate the surface displacement caused by the fundamental Love and Rayleigh modes. At periods of 1 sec or less, the fundamental Love and Rayleigh modes travel almost entirely in the uppermost 9000 ft (2743 m) of the Earth9s crust. Conversion of the first higher Love mode to the fundamental mode can contribute significantly to the Love wave surface displacement. Finally, at periods of 1 sec or less, the phase velocities of the fundamental Love and Rayleigh modes depend on the displacements of these modes near the surface of the Earth. These velocities may be considerably different from the arithmetic mean of the phase velocities of the Love and Rayleigh modes at the ends of the model.

Elastic Waves in Layered Media with Interface Features

Abstract: A simple and efficient matrix method is applied to analyze the reflected waves from a layered plate with certain interface features. It is shown that a thin, adhesive layer can be modeled as an equivalent elastic interface with zero thickness characterized by two adjustable parameters. The accuracy of the model is examined through comparison with calculations from the exact case in which the adhesive layer is explicitly modeled. The influence of the imperfect interface on the velocity of layer guided waves is investigated.

Acoustic emission waveform simulation in multilayered composites

Abstract: In this paper, an efficient and accurate semi-analytical method is developed to calculate the elastodynamic field produced by localized dynamic loads in a relatively thick composite plate. Two types of loads are considered: a pencil lead break source located on the surface and a localized shear delamination within the interior of the plate. In the case of the pencil lead break source, the calculated results for the surface motion are compared with those obtained in laboratory experiments on a 4.4mm thick 32 layered cross-ply graphite/ epoxy using high-fidelity broadband transducers. The waveforms consist of both flexural and extensional modes; the amplitude variations of these modes are found to be strongly dependent on their propagation direction. For the delamination source, the results from the exact calculation are compared with those from an approximate laminate theory with a shear correction factor and 'moment tensor' representation of the source. The results obtained by the two methods are shown to have excellent agreement in the low-frequency ranges. Although the motion due to the delamination is dominated by flexural waves of lower frequency in both thin and thick plates, the presence of extensional waves are observed in thicker laminates.

Fast parallel algorithms for short-range molecular dynamics

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.

An introduction to structural health monitoring

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.

25th Anniversary Article: The Evolution of Electronic Skin (E-Skin): A Brief History, Design Considerations, and Recent Progress

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.

Silica-Like Malleable Materials from Permanent Organic Networks

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

A facile soft-template synthesis of mesoporous polymeric and carbonaceous nanospheres

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.