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.

Impact damage detection in composite structures using Lamb waves

Abstract: This paper is concerned with the detection of low velocity impact and the associated internal damage in composite structures using Lamb waves. Impact tests are carried out on a cross ply graphite epoxy plate using an instrumented impact testing system. The contact force and the surface motion caused by the impact load are recorded at several points on the plate surface away from the impact location and are analyzed based on theoretical simulations. The Lamb waves generated by the impact load and internal damage to the plate caused by it are shown to be highly effective tools for damage detection in laboratory specimens. Ultrasonic and impact tests are also conducted on a stiffened, woven composite panel in an effort to examine the propagation characteristics of ultrasonic waves in realistic composite structural components. Preliminary analysis of the recorded waveforms indicates that Lamb waves can be used to interrogate relatively large composite structures.

Determination of the Elastic Constants of Composites Through the Inversion of Leaky Lamb Wave Data

Abstract: Analysis and prediction of the response of composite laminates to external loads are essential for the design of composite structures This in turn requires a precise knowledge of their mechanical properties including their constitutive behavior It is reasonable to assume that, in the bulk, the overall behavior of unidirectional graphite/epoxy composites is the same as that of a homogeneous, transversely isotropic material with its symmetry axis along the fiber direction Then the linear elastic response of the material can be described by means of five elastic constants If the values of these constants can be determined, then the stress analysis of a laminate with a given number and stacking order of the laminae can, in principle, be carried out However, the measurement of the elastic constants by conventional, destructive techniques is difficult and often, inaccurate Thus, the availability of alternative, preferably nondestructive methods, for the determination of the elastic costants of the material would be extremely helpful

Leaky lamb waves for the ultrasonic nondestructive evaluation of adhesive bonds

Abstract: The critical role played by interface zones in the fracture and failure of composites and other bonded materials is well known. However, the existing nondestructive evaluation methods are not capable of yielding useful quantitative information on either elastic or strength related properties of the interface. The authors are investigating the feasibility of applying an ultrasonic method to determine some of the interface properties nondestructively. The method uses guided waves and is based on the fact that the dispersive properties of these waves are strongly affected by the elastic properties of the interface. A coordinated theoretical and experimental program of research has revealed that the correlation between the interfacial properties and the phase velocity of the guided waves is quite strong and is identifiable at least in laboratory specimens. Some recent results of this research are reported in this paper.

Rayleigh wave diffraction from surface‐breaking discontinuities

Abstract: This letter presents theoretical and experimental results treating the interaction between Rayleigh waves and surface‐breaking discontinuities, such as cracks and grooves, in the regime where crack depth is large compared to the Rayleigh wavelength. Particular emphasis is placed on the mode conversion to bulk waves at the crack tip. Radiation patterns are presented for both the longitudinal and shear wave contributions. The experimental data agree well with theory and show that the generation of shear waves polarized perpendicular to the crack face is particularly efficient.

Composite material defect characterization using leaky Lamb wave dispersion data

Abstract: Leaky Lamb wave (LLW) propagation in composite materials has been studied extensively since it was first observed in 1982. The wave is induced using a pitch-catch arrangement and the plate wave modes are detected by identifying minima in the reflected spectra to obtain the dispersion data. The wave behavior in multi-orientation laminates was well documented and corroborated experimentally with a very high accuracy. The sensitivity of the wave to the elastic constants of the material and to its boundary condition led to several studies where the elastic properties were inverted and the characteristics of bonded joint were evaluated. Recently, the authors modified their experimental setup to allow measuring dispersion curves at a significantly higher speed than ever recorded. A set of 20 angles of incidence along a single polar angle of a composite laminate are acquired in about 45 seconds. The reflection spectra are acquired in real time while filtering the high frequency noise providing reliable data at amplitude levels that are significantly lower that were acquired in prior studies. This new method makes the LLW a practical quantitative tool for both inversion of the elastic properties and characterization of flaws. The emphasis of the current study is on the detection and characterization of flaws. The composite is modeled as transversely isotropic and dissipative medium and the effect of flaws is analyzed and compared to the experimental data using a C-scan mounted LLW scanner.

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.