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.

Leaky Lamb Wave Phenomena in Composites Using Pulses

Abstract: The leaky Lamb wave (LLW) phenomenon in composite laminates has been studied using pulses rather than tone burst or continuous waveforms. For unidirectional laminates, the leaky wave pulses propagating obliquely to the fibers are shown to be strongly influenced by the presence of flaws within the laminate. The differences in the pulse characteristics have been employed to detect and image ply-gaps, delaminations, and variations in the resin/fiber ratio in laboratory specimens. Moreover, the depth distribution of porosity, simulated by microballoons, has been clearly shown with a C-scan system. Theoretical simulations of pulse propagation have been conducted for multilayered, multiorientation laminates, and the calculated reflected pulses are in close agreement with those obtained from the LLW experiments for a variety of specimens.

Uncertainty characterization of guided ultrasonic wave properties in composite materials

Abstract: Guided ultrasonic wave-based methods are promising for structural health monitoring of isotropic and composite materials and structures. The technology has seen a lot of attention in the research community over the past decades, and many analytical and numerical methods have been developed to describe different aspects of guided wave propagation and scattering phenomena as well as damage detection. However, very little research was geared towards the influence of the uncertainty in the material properties for the calculation of the dispersion curves. The lack of knowledge of the exact material properties together with manufacturing tolerances could lead to erroneous conclusions. Hence, in this study, an uncertainty analysis for the material properties of fiber-reinforced composites is conducted to quantify the effect of uncertain material constants on the dispersion curves. A fuzzy arithmetical approach based on the Transformation Method is used to generate the dispersion curves with uncertain parameters in conjunction with a root-finding algorithm. The uncertain parameters are modeled as linear fuzzy numbers. Using triangular membership functions, both the nominal value and the worst-case interval are adequately combined into one fuzzy number. Furthermore, it is shown that the measure of influence for the uncertain material parameters on the group velocities of the considered Lamb waves is not equally weighted. These findings might allow for the development of efficient, nondestructive material characterization techniques in the future.

Toward Quantitative Imaging with the Acoustic Microscope

Abstract: Since the first disclosure of the acoustic material signature1, the acoustic microscope has been explored to perform both imaging and metrology functions2 on a variety of specimens in industry and in the scientific laboratory. The acoustic material signature (AMS) has been refined3 to yield increasingly quantitative information of acoustic velocities in homogeneous and layered structures. However, the imaging mode, though it led to the visualization of subsurface structures5 and associated defects6, has to date been used largely as a qualitative tool in nondestructive evaluation.

Perturbation solution for the interaction of Lamb waves with localized surface defects

Abstract: We present an analytical solution of the transmitted and reflected wave fields due to the interaction of Lamb waves with localized, small-scale defects on late surfaces. The surface defect is represented by a distribution of surface sources, and the overall solution is obtained as a superposition of the incident wave field plus the 'small' wave field associated with the surface sources. The reflection and transmission coefficients of various Lamb modes are also determined. It is shown that the scattered field consists of a superposition of various possible modes at the frequency of the incident mode. For the fundamental modes, it is found that the reflection coefficients are periodic functions of the defect width with the period and magnitude being dependent on the surface defect profile. The transmission coefficient for the converted mode is also found to be a periodic function of the defect width while that corresponding to the incident mode does not deviate significantly from unity. A parametric study has also shown that to this approximation the numerical values of the reflection and transmission coefficients are not sensitive to the maximum slope of the defect profile.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Effects of Homogenization and Quasi-Isotropy Assumptions on Guided Wave-Based Nondestructive Testing Methods

Abstract: Past research on guided ultrasonic wave-based methods for nondestructive testing of composite materials often assumed quasi-isotropy of composite laminates. Addition- ally, the quasi-isotropic laminate is homogenized and effective material properties for the laminate, modeled as a single layer material, are employed in subsequent analytical, numerical or experimental studies. However, deviations from the intended layup cause a directional dependency of the propagation velocities, thus requiring the modeling of the entire laminate. This not only alters wave fronts but it may also lead to significant errors in time-of-flight analyses for longer propagation ranges. Hence, in this paper, a comprehensive numerical-experimental approach is taken to investigate the quasi-isotropy of composite laminates in the context of guided ultrasonic wave propagation. A combination of semi-analytical and analytical tools are used to determine the dispersion curves and the propagation characteristics for several composite plate structures. The methods are applied for quasi-isotropic laminate that can be homogenized as well as a general composite laminate. The results are validated through dispersion experiments, and implications for damage detection are highlighted.

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.