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.

Feasibility Analysis of Various Sensing Methods for Nondestructive Testing of Composites

Abstract: Guided ultrasonic wave-based methods are promising for detecting defects over long ranges in isotropic and composite materials. 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 wave propagation and scattering phenomena. However, very little research was geared towards the physical implementation in practical applications. Hence, in this study, the detectability of defects in composite materials (delaminations and core-skin disbonds) with different sensor technologies is investigated. To induce and record scattered guided ultrasonic waves, broadband contact transducers, air-coupled transducers and a laser Doppler vibrometer are used. It is shown that the thickness of typical sandwich panels used in aerospace industry and the non-homogeneous nature of the honeycomb core material make precise localization and identification of defects challenging, in particular in non-contact scenarios.

Composite Materials Stiffness Determination and Defects Characterization Using Leaky Lamb Wave (LLW) Dispersion Data

Abstract: The Leaky Lamb wave (LLW) technique is approaching a maturity level that is making it an attractive quantitative NDE tool for composites and bonded joints.

Inverse fuzzy arithmetic for material characterization of composites using guided waves

Abstract: In this study, a technique based on guided ultrasonic waves coupled with an uncertainty analysis is developed to quantify the deviation from the assumed nominal value of the material constants of quasi-isotropic fiber-reinforced composites. It is shown that the measured group velocities vary depending on the location within the laminate, opening the possibility of questioning whether the assumed nominal values of the material properties could accurately represent the entire material system at any region. Furthermore, after the identified material parameters are defuzzyfied, a new set of nominal values for the material properties is determined. These preliminary findings might allow for the development of other efficient, nondestructive material characterization techniques in the future.

Ultrasonic Characterization of Defects in Lap Joints

Abstract: This paper is concerned with the detection and characterization of material loss within lap joints. One of the most commonly used nondestructive methods to inspect lap joints is the eddy current method [1]. In this technique, the percentage of material loss is determined by comparing the signal from the defective joint with a reference signal. Ultrasonic tests based on pulse-echo technique can also be used to detect thickness reduction in lap joints. However both of these methods require that the sensor be directly above the defects, thus making their practical implementation extremely time consuming. Improving the efficiency of lap joint inspection is one of the most critical tasks currently facing the NDE community. In this paper we discuss the feasibility of using an ultrasonic technique based on guided waves launched across the lap joint. It is well known that the characteristics of guided waves can be used to detect defects in plates [2]. The geometry of the lap joint makes it much more difficult to extend the guided wave based method to lap joints. There is no closed form solution to the problem of wave propagation across lap joints. Due to the change of thickness and the existence of vertical stress free boundaries, nonpropagating modes may play an important role inside and near the overlapped region. With material loss inside the lap joint, the geometry becomes even more complicated. Thus the problem of wave propagation in the lap joint can only be solved by numerical and experimental methods.

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.