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.

Elastic Waves from a Distributed Surface Source in a Unidirectional Composite

Abstract: The response of a unidirectional composite plate of infirritc lateral dimensions to localized dynamic surface sourws is investigated through theoretical modeling and laboratory tests. In the theoretical sitnulations, the material ofthc plate is assumed to be dissipative and transversely isotropic with its symmetry axis parallel to the fibers. The source is assumed to hsrve an arbitrary spatial and time dependence. The associated clastodynamic boundary value problem is solved by means of an integral transform technique followed by numerical evaluation of the inversion intc~als. The laboratory tests are carried out on unidirectional graphite epoxy plates of thicknesses ranging from 1-25 mm and large Iateml dimensions (> 30 cm? excited by means of broadband transducers attached to its surface. The calculated surface response of the plate at different distances and directions from the source is shown to agree very well with the recorded response in the ultrasonic range. INTRODUCTION It is well known that Iaminatcd tibcr reinforced composites often suffer significant internal damage when they arc subjected to Iocalizcd dynamic surface loads. The damage may involve frbcr breakage and debonding as well as delamination bctwccn the individual laminae. Such damage has been observed to occur even at relatively low impact speeds resulting in a severe loss in the load carrying capacity of the larninac. Although the damage is clearly caused by the stresses which develop within the material, the prccisc nature of these stresses and their relationship to the degree and mode ofthc damage are not clearly understood at present. This is particularly true in the dynamic case where the stresses are caused by waves whose propagation characteristics are strongly influenced by the inherent an isotropy and heterogeneity of the composite material. Dynamic response of plates has been studied theoretically by many authors through past decades. The linear elastic solutions of the isotropic or anisotropic plates have been investigated by, c. While nurncrous analytical investigation of wave propagation in plate have been executed, concurrent experimental studies of such wave processes in the laboratory have been far Icss prevalent. Most clXort in this direction were devoted to a cmnpwiwn of pred Icted phase and group velocities of surface waves.

Rapid Calculation of Lamb Waves in Plates Due to Localized Sources

Abstract: Waveform based acoustic emission (AE) analysis is an emerging technique for nondestructive monitoring of the integrity of structures in service. In contrast to conventional AE studies, a detailed understanding of wave propagation characteristics is required in order to correlate the information from detected AE signals to their sources representing various failure modes. AE source characterization can be carried out by using the sensor output waveform, identifying unknown signals and evaluating their significance, and correlating identified signals to the failure modes. This requires efficient evaluation of the surface response due to each elementary microfracture source (represented by moment tensor components) in the plate.

Low Frequency Guided Plate Wave Propagation in Fiber Reinforced Composites

Abstract: The use of composite materials has increased steadily during the past two decades, particularly for aerospace, underwater and automotive structures. This is largely because many composite materials exhibit high strength-to-weight and stifihess-to-weight ratios, which make them ideally suited for use in weight-sensitive structures. The elastic properties of composite materials may be significantly different in specimens manufactured under the same general specifications and the bulk material properties may be different from those of the lamina. The elastic properties degrade as a result of aging, environmental and other effects (e.g., matrix cracking) resulting in overstress and eventual failure of the material. The elastic properties determine the performance of the material and it is necessary to assure the conformance of these properties with design requirements. Conventional destructive techniques for determining the elastic stiffness constants can be costly and often inaccurate. This is particularly true for the through-the-thickness properties. Nondestructive determination of these properties offers a better alternative for material characterization and for assuring structural performance.

Impact Damage Monitoring in Composite Plates

Abstract: This paper is concerned with the real-time detection of internal damage in composite structural components during impact using the far-field surface motion generated by these events. Impact tests are carried out on graphite epoxy composite plates using an instrumented impact testing system. Contact force and surface motion are measured at several locations on the plate surface. The far-field surface motions, both flexural and extensional waves in the composite plate, are modeled using both approximate and exact solution methods. Postimpact test were performed to determine the extent of internal damage caused by the impact load. Further research on the detection method can lead to the development of a viable impact monitoring system for composite aerospace structures using distributed sensors.Copyright © 2004 by ASME

Pressure and temperature distribution in biological tissues by focused ultrasound

Abstract: The interaction between ultrasound and biological tissues has been the subject of a number of investigators for nearly half a century and the number of applications of high intensity, focused ultrasound for therapeutic purposes continues to grow. This paper is motivated by possible medical applications of focused ultrasound in minimally invasive treatment of a variety of musculoskeletal disorders that are responsive to thermal treatment. The mechanical and thermal effects in a subject’s body induced by high-frequency ultrasound are simulated using PZFlex, a finite element based program. The FEM model described in this report is of a transverse section of the body at the level of the second lumbar vertebra (L2) extracted from a CT image. In order to protect the nerves inside the spinal canal as well as to obtain an effective heating result at the focal region within the intervertebral disk, a suitable orientation of axis of the focused ultrasound lens have to be determined in advance. The pressure, energy loss distribution and temperature distribution are investigated in this paper with the different orientations of the axis and different transverse diameter of the spherical ultrasound lens. Since nonlinear effects are expected to be important in the therapeutic application in some literatures, this paper also demonstrates the effects of nonlinearities on the pressure and temperature distribution induced by focused ultrasound in a two dimensional model. Finally, a comparison of the results between linear and nonlinear cases is reported.

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.