Measurement and analysis of critical crack tip processes during fatigue crack growth

The mechanics of fatigue crack growth under constant-amplitudes and variable-amplitude loading were examined. Critical loading histories involving relatively simple overload and overload/underload cycles were studied to provide a basic understanding of the underlying physical processes controlling crack growth. The material used for this study was 7091-T7E69, a powder metallurgy aluminum alloy. Local crack-tip parameters were measured at various times before, during, and after the overloads, these include crack-tip opening loads and displacements, and crack-tip strain fields. The latter were useed, in combination with the materials cyclic and monotonic stress-strain properties, to compute crack-tip residual stresses. The experimental results are also compared with analytical predictions obtained using the FAST-2 computer code. The sensitivity of the analytical model to constant-amplitude fatigue crack growth rate properties and to through-thickness constrain are studied

Radiative Models of Sagittarius A* and M87 from Relativistic MHD Simulations

Ongoing millimeter VLBI observations with the Event Horizon Telescope allow unprecedented study of the innermost portion of black hole accretion flows. Interpreting the observations requires relativistic, time-dependent physical modeling. We discuss the comparison of radiative transfer calculations from general relativistic MHD simulations of Sagittarius A* and M87 with current and future mm-VLBI observations. This comparison allows estimates of the viewing geometry and physical conditions of the Sgr A* accretion flow. The viewing geometry for M87 is already constrained from observations of its large-scale jet, but, unlike Sgr A*, there is no consensus for its millimeter emission geometry or electron population. Despite this uncertainty, as long as the emission region is compact, robust predictions for the size of its jet launching region can be made. For both sources, the black hole shadow may be detected with future observations including ALMA and/or the LMT, which would constitute the first direct evidence for a black hole event horizon.Comment: 8 pages, 2 figures, submitted to the proceedings of AHAR 2011: The Central Kiloparse

A Sustainable Water Resources Management Assessment Framework (SWRM-AF) for Arid and Semi-Arid Regions:Part 1: Developing the Conceptual Framework

The evaluation of water resources management practices is essential for water usage decisions in regions with limited water resources. The literature provides numerous assessment frameworks, but many ignore the unique characteristics and conditions of some special arid and semi-arid regions, such as the Gulf Cooperation Council (GCC) countries, which lack any permanent rivers or lakes. Thus, this study, the first in a two-part series, seeks to develop a conceptual Sustainable Water Resources Management Assessment Framework (SWRM-AF). General and particular criteria explain how components and indicators were identified. The conceptual SWRM-AF provided here has four components (environment, economy, society, and infrastructure) and 24 indicators. Almost every indicator has been selected from the literature and is briefly explained and justified. This research presents, possibly for the first time, clear and straightforward directions for evaluating each indicator in colour-coded tables. To create a more holistic framework for arid and semi-arid regions, social indicators like “intervention acceptability” and environmental indicators for assessing the impacts of desalination treatment plants have been added to form a unique framework applicable to such regions. Therefore, the components and indicators of conceptual SWRM-AF could work collectively to aid the process of decision-making. The next phase is validating this framework using a participatory approach

Resonant Energy Exchange between Atoms in Dispersing and Absorbing Surroundings

Within the framework of quantization of the macroscopic electromagnetic field, a master equation describing both the resonant dipole-dipole interaction (RDDI) and the resonant atom-field interaction (RAFI) in the presence of dispersing and absorbing macroscopic bodies is derived, with the relevant couplings being expressed in terms of the surroundings-assisted Green tensor. It is shown that under certain conditions the RDDI can be regarded as being governed by an effective Hamiltonian. The theory, which applies to both weak and strong atom-field coupling, is used to study the resonant energy exchange between two (two-level) atoms sharing initially a single excitation. In particular, it is shown that in the regime of weak atom-field coupling there is a time window, where the energy transfer follows a transfer-rate law of the type obtained by ordinary second-order perturbation theory. Finally, the spectrum of the light emitted during the energy transfer is studied and the line splittings are discussed.Comment: 9 pages, 5 figs, Proceedings of ICQO'2002, Raubichi, to appear in Optics and Spectroscop

Spontaneous decay of an excited atom in an absorbing dielectric

Starting from the quantized version of Maxwell's equations for the electromagnetic field in an arbitrary linear Kramers-Kronig dielectric, spontaneous decay of the excited state of a two-level atom embedded in a dispersive and absorbing medium is studied and the decay rate is calculated. The calculations are performed for both the (Clausius-Mosotti) virtual cavity model and the (Glauber-Lewenstein) real cavity model. It is shown that owing to nonradiative decay associated with absorption the rate of spontaneous decay sensitively depends on the cavity radius when the atomic transition frequency approaches an absorption band of the medium. Only when the effect of absorption is fully disregarded, then the familiar local-field correction factors are recovered.Comment: 28 pages, 6 figures, typeset using RevTe

Dissecting X-ray-emitting Gas around the Center of our Galaxy

Most supermassive black holes (SMBHs) are accreting at very low levels and are difficult to distinguish from the galaxy centers where they reside. Our own Galaxy's SMBH provides a uniquely instructive exception, and we present a close-up view of its quiescent X-ray emission based on 3 mega-second of Chandra observations. Although the X-ray emission is elongated and aligns well with a surrounding disk of massive stars, we can rule out a concentration of low-mass coronally active stars as the origin of the emission based on the lack of predicted Fe Kalpha emission. The extremely weak H-like Fe Kalpha line further suggests the presence of an outflow from the accretion flow onto the SMBH. These results provide important constraints for models of the prevalent radiatively inefficient accretion state.Comment: 18 pages, 5 PDF figures, pdflatex format; Final version, published in Scienc

Quasi-degenerate self-trapping in one-dimensional charge transfer exciton

The self-trapping by the nondiagonal particle-phonon interaction between two quasi-degenerate energy levels of excitonic system, is studied. We propose this is realized in charge transfer exciton, where the directions of the polarization give the quasi-degeneracy. It is shown that this mechanism, unlike the conventional diagonal one, allows a coexistence and resonance of the free and self-trapped states even in one-dimensional systems and a quantitative theory for the optical properties (light absorption and time-resolved luminescence) of the resonating states is presented. This theory gives a consistent resolution for the long-standing puzzles in quasi-one-dimensional compound A-PMDA.Comment: accepted to Phys. Rev. Letter