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 & Ultrasonic sensor. 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.

A conceptual structural health monitoring system based on vibration and wave propagation

Abstract: Development of efficient methodologies to determine the presence, location, and severity of hidden damage in critical structural components is an important task in the design and construction of structural health monitoring systems in aging as well as new structures. In this article, a methodology for automatic damage identification and localization is presented. The structure is assumed to be instrumented with an array of actuators and sensors to excite and record its dynamic response, including vibration and wave propagation effects. In the vibrational approach, the data consist of the modal response of the structure produced by the actuators while in the wave propagation approach, they are the broadband signals due to ultrasonic waves propagating in the structures. Both types of signals are affected by the presence of defects. The approximate location and severity of an unknown defect is determined using a damage correlation index calculated from the frequency response function (FRF) of the structure. ...

Multiple scattering of elastic waves in a fiber-reinforced composite

Abstract: Propagation of elastic waves in a composite containing randomly distributed parallel fibers is studied in this paper. Both anti-plane (SH waves) and in-plane (P and SV waves) cases are considered. A multiple scattering theory and a statistical averaging procedure are implemented by means of the so-called Generalized Self Consistent Model (GSCM). Multiple scattering in the inhomogeneous medium results in a frequency dependent velocity and attenuation of the waves. The effective phase velocity and attenuation of the coherent waves are calculated for a wide range of frequencies and concentrations. The degree of interaction between the fibers is determined and the average strain calculated in a given inclusion by direct analysis and approximate homogenization is compared. The proposed method recovers three well-known static effective moduli of fiber-reinforced composites in the Rayleigh limit and the results at higher frequencies are physically reasonable.

Dynamic elastic moduli of a suspension of imperfectly bonded spheres

Abstract: An isotropic elastic material containing a random distribution of identical spherical particles of another elastic material is considered. The bonding between the spheres and the matrix is imperfect, so that slip may occur at interfaces when stress is applied to the medium. The shear stresses at the interface is assumed to be proportional to the amount of slip. The velocity and attenuation of the average harmonic elastic waves propagating through such a medium are calculated. The results are valid to the lowest order in frequency for wave lengths long compared with the radius of the sphere. The dynamic elastic moduli are obtained from these results and are compared with available results for welded contact. The variations in the P and S wave velocities for propagation across earthquake faults is discussed.

Response of an Elastic Plate to Localized Transient Sources

Abstract: The surface response of an infinite, homogeneous elastic plate to an internal dislocation across an infinitestimal area is investigated. As a companion problem, the normal displacement of the plate surface due to a time-dependent surface load is also calculated. The first problem is relevant for the detection of crack initiation in structural materials through the analysis of high-frequency elastic waves generated by the event. The solution to the second problem is needed for the calibration of test equipment used in the detection of the waves. The problems are formulated by means of a classical integral transform in the frequency domain and the spectral response of the plate is expressed in terms of the modal contributions due to the real, imaginary, and complex roots of the Rayleigh-Lamb equation. Time histories of the response are obtained through the inversion of the spectra by a Fast Fourier Transform (FFT) routine.

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.