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.

Direct correlation between mechanical failure and metallurgical reaction in flip chip solder joints

Abstract: We tested flip chip solder bonded Si samples under tensile and shear loading as a function of annealing time at 200 °C. The solder bump was eutectic SnPb and the underbump thin film metallization was Cu/Cr deposited on oxidized Si. We found that the failure mode is interfacial fracture and the fracture strength decreases rapidly with annealing time. From scanning electric microscope observations, the fracture occurs at the Cu–Sn/Cr interface. We conclude that it is the metallurgical reaction that has brought the solder into direct contact with the Cr surface. The weak joint is due to the spalling of Cu–Sn compound grains from the Cr surface, especially near the edges and corners of the joint.

Observation of 1- to 5-second microtremors and their application to earthquake engineering. Part III. A two-dimensional study of site effects in the San Fernando Valley

Abstract: A two-dimensional study of the influence of deep sediment on seismic ground motions was conducted by recording and analyzing long-period microtremors in the San Fernando Valley, California. The recordings were made at 50 regularly distributed sites in the valley. Three reference sites were employed at the baserock outcrop around the valley so as to observe the time-dependent characteristics of the microtremors . 1. Amplitudes in a period range of several seconds correlate with the thicknesses of the sedimentary layers . 2. A site amplification effect, which was evaluated in terms of sediment-to-rock spectral ratios, is qualitatively consistent with available geological and strong motion data . 3. The practical field observation procedure designed for this study, the two-dimensional approach, can be easily used in other surveys for estimating the spatial characteristics of earthquake ground motions in the period range of several seconds .

A Volume Integral Equation Technique for Multiple Inclusion and Crack Interaction Problems

Abstract: A volume integral equation method is introduced for the solution of elastostatic problems in heterogeneous solids containing interacting multiple inclusions, voids, and cracks. The method is applied to two-dimensional problems involving long parallel cylindrical inclusions and cracks. The influence of interface layers on the interfacial stress field is investigated. The stress intensity factors for microcracks in the presence of interacting inclusions or voids are also calculated for a variety of model geometries. The accuracy and efficiency of the method are examined through comparison with results obtained from analytical and boundary integral 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.