Defence Research Agency

About: Defence Research Agency is a based out in . It is known for research contribution in the topics: Synthetic aperture radar & Radar. The organization has 1211 authors who have published 1109 publications receiving 31542 citations.

The dual boundary element formulation for elastoplastic fracture mechanics

Abstract: In this paper the extension of the dual boundary element method (DBEM) to the analysis of elastoplastic fracture mechanics (EPFM) problems is presented. The dual equations of the method are the displacement and the traction boundary integral equations. When the displacement equation is applied on one of the crack surfaces and the traction equation on the other, general mixed-mode crack problems can be solved with a single-region formulation. In order to avoid collocation at crack tips, crack kinks and crack-edge corners, both crack surfaces are discretized with discontinuous quadratic boundary elements. The elastoplastic behaviour is modelled through the use of an approximation for the plastic component of the strain tensor on the region expected to yield. This region is discretized with internal quadratic, quadrilateral and/or triangular cells. This formulation was implemented for two-dimensional domains only, although there is no theoretical or numerical limitation to its application to three-dimensional ones. A centre-cracked plate and a slant edge-cracked plate subjected to tensile load are analysed and the results are compared with others available in the literature. J-type integrals are calculated.

Recent results on metalorganic vapor phase epitaxially grown HgCdTe heterostructure devices

Abstract: The ability to grow complex multilayer structures in Hg1-xCdxTe by epitaxial techniques has made it possible to produce a range of new devices such as infrared LEDs, lasers, and two-color infrared detector arrays. The devices described here, however, are designed to operate at temperatures above 145K and include both infrared sources and detectors. Three layer ppn structures, where the underlined symbols mean wider gap, have close to Auger limited RoAs at temperatures above 145K. Under reverse bias, the devices exhibit Auger suppression leading to useful detectivities at room temperature. The diodes exhibit forward biased electroluminescence at room temperature although the efficiency of this emission is found to fall rapidly as the peak wavelength is increased toward 9 μm due to increased Auger recombination rates. By reverse biasing them, however, the devices show negative luminescence as a result of reducing the electron and hole densities below their thermal equilibrium value. The diode emitters have a higher quantum efficiency when used in this mode due to Auger suppression of the dark current.

The treatment of state in optimistic systems

Abstract: Optimistic computation methods typically save copies of objects' state information, so that they can recover from erroneous “over-optimistic” computations. Such state saving is generally time and space consuming, and can be rather complicated both to implement and to use.I show how the data structure community's theory of persistence can be used not only to analyse and explain the treatment of state in optimistic systems, but also as a simple yet general mechanism for performing the necessary state saving with minimal impact on application code.Preliminary results based on a benchmark application and an existing optimistic simulator are presented, showing that providing support for fully general object states is a realistic and practical option. In addition, I show how some existing state saving techniques—including support for shared state—can be derived, and discuss a number of ways in which the model might be extended.

Monte Carlo simulation of impact ionization and current multiplication in short GaAs diodes

Abstract: We have modelled carrier transport and impact ionization in bulk GaAs and GaAs diodes using two Monte Carlo models, one using analytical band structure, the other employing more realistic pseudopotential band structure. Despite the relative lack of sophistication of the analytical model and the poor representation of band structure at higher energies, the analytical model reproduced accurate drift velocities, mean energies and impact ionization rates in good agreement with experiment and the more sophisticated model. Both models accurately simulated the diodes, agreeing well with experimental results and the two models also agreed with each other with respect to the microscopic aspects of the carrier transport in these devices.