9,668 research outputs found

    A random fiber bundle with many discontinuities in the threshold distribution

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    We study the breakdown of a random fiber bundle model (RFBM) with nn-discontinuities in the threshold distribution using the global load sharing scheme. In other words, n+1n+1 different classes of fibers identified on the basis of their threshold strengths are mixed such that the strengths of the fibers in the i−thi-th class are uniformly distributed between the values σ2i−2\sigma_{2i-2} and σ2i−1\sigma_{2i-1} where 1≤i≤n+11 \leq i \leq n+1. Moreover, there is a gap in the threshold distribution between i−thi-th and i+1−thi+1-th class. We show that although the critical stress depends on the parameter values of the system, the critical exponents are identical to that obtained in the recursive dynamics of a RFBM with a uniform distribution and global load sharing. The avalanche size distribution (ASD), on the other hand, shows a non-universal, non-power law behavior for smaller values of avalanche sizes which becomes prominent only when a critical distribution is approached. We establish that the behavior of the avalanche size distribution for an arbitrary nn is qualitatively similar to a RFBM with a single discontinuity in the threshold distribution (n=1n=1), especially when the density and the range of threshold values of fibers belonging to strongest (n+1n+1)-th class is kept identical in all the cases.Comment: 6 pages, 4 figures, Accepted in Phys. Rev.

    Shear-driven size segregation of granular materials: modeling and experiment

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    Granular materials segregate by size under shear, and the ability to quantitatively predict the time required to achieve complete segregation is a key test of our understanding of the segregation process. In this paper, we apply the Gray-Thornton model of segregation (developed for linear shear profiles) to a granular flow with an exponential profile, and evaluate its ability to describe the observed segregation dynamics. Our experiment is conducted in an annular Couette cell with a moving lower boundary. The granular material is initially prepared in an unstable configuration with a layer of small particles above a layer of large particles. Under shear, the sample mixes and then re-segregates so that the large particles are located in the top half of the system in the final state. During this segregation process, we measure the velocity profile and use the resulting exponential fit as input parameters to the model. To make a direct comparison between the continuum model and the observed segregation dynamics, we locally map the measured height of the experimental sample (which indicates the degree of segregation) to the local packing density. We observe that the model successfully captures the presence of a fast mixing process and relatively slower re-segregation process, but the model predicts a finite re-segregation time, while in the experiment re-segregation occurs only exponentially in time

    Crossover Behavior in Burst Avalanches of Fiber Bundles: Signature of Imminent Failure

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    Bundles of many fibers, with statistically distributed thresholds for breakdown of individual fibers and where the load carried by a bursting fiber is equally distributed among the surviving members, are considered. During the breakdown process, avalanches consisting of simultaneous rupture of several fibers occur, with a distribution D(Delta) of the magnitude Delta of such avalanches. We show that there is, for certain threshold distributions, a crossover behavior of D(Delta) between two power laws D(Delta) proportional to Delta^(-xi), with xi=3/2 or xi=5/2. The latter is known to be the generic behavior, and we give the condition for which the D(Delta) proportional to Delta^(-3/2) behavior is seen. This crossover is a signal of imminent catastrophic failure in the fiber bundle. We find the same crossover behavior in the fuse model.Comment: 4 pages, 4 figure

    Segregation by thermal diffusion in granular shear flows

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    Segregation by thermal diffusion of an intruder immersed in a sheared granular gas is analyzed from the (inelastic) Boltzmann equation. Segregation is induced by the presence of a temperature gradient orthogonal to the shear flow plane and parallel to gravity. We show that, like in analogous systems without shear, the segregation criterion yields a transition between upwards segregation and downwards segregation. The form of the phase diagrams is illustrated in detail showing that they depend sensitively on the value of gravity relative to the thermal gradient. Two specific situations are considered: i) absence of gravity, and ii) homogeneous temperature. We find that both mechanisms (upwards and downwards segregation) are stronger and more clearly separated when compared with segregation criteria in systems without shear.Comment: 8 figures. To appear in J. Stat. Mec

    Dynamic model of fiber bundles

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    A realistic continuous-time dynamics for fiber bundles is introduced and studied both analytically and numerically. The equation of motion reproduces known stationary-state results in the deterministic limit while the system under non-vanishing stress always breaks down in the presence of noise. Revealed in particular is the characteristic time evolution that the system tends to resist the stress for considerable time, followed by sudden complete rupture. The critical stress beyond which the complete rupture emerges is also obtained

    Semiflexible polymers: Dependence on ensemble and boundary orientations

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    We show that the mechanical properties of a worm-like-chain (WLC) polymer, of contour length LL and persistence length \l such that t=L/\l\sim{\cal O}(1), depend both on the ensemble and the constraint on end-orientations. In the Helmholtz ensemble, multiple minima in free energy near t=4t=4 persists for all kinds of orientational boundary conditions. The qualitative features of projected probability distribution of end to end vector depend crucially on the embedding dimensions. A mapping of the WLC model, to a quantum particle moving on the surface of an unit sphere, is used to obtain the statistical and mechanical properties of the polymer under various boundary conditions and ensembles. The results show excellent agreement with Monte-Carlo simulations.Comment: 15 pages, 15 figures; version accepted for publication in Phys. Rev. E; one new figure and discussions adde

    Spin-Correlation Coefficients and Phase-Shift Analysis for p+3^3He Elastic Scattering

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    Angular Distributions for the target spin-dependent observables A0y_{0y}, Axx_{xx}, and Ayy_{yy} have been measured using polarized proton beams at several energies between 2 and 6 MeV and a spin-exchange optical pumping polarized 3^3He target. These measurements have been included in a global phase-shift analysis following that of George and Knutson, who reported two best-fit phase-shift solutions to the previous global p+3^3He elastic scattering database below 12 MeV. These new measurements, along with measurements of cross-section and beam-analyzing power made over a similar energy range by Fisher \textit{et al.}, allowed a single, unique solution to be obtained. The new measurements and phase-shifts are compared with theoretical calculations using realistic nucleon-nucleon potential models.Comment: Submitted to Phys. Rev.

    Herb-drug interaction: Effect of aqueous extract of Bridelia ferruginea leaves on the pharmacokinetics of metformin

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    The concurrent use of herbal medicines and orthodox drugs, especially in the treatment and management of chronic ailments, may result in clinically significant herb-drug interactions. Bridelia ferruginea Benth (Euphorbiaceae) is a common medicinal plant with known anti-diabetic properties and has been reported to be taken alongside the orthodox medicine, metformin. The aim of this study was to investigate the effect of aqueous extract of B. ferruginea leaves on the pharmacokinetics of metformin in female Sprague-Dawley rats. Reconstituted freeze dried extract of B. ferruginea leaves (30 mg/kg), and metformin (7 mg/kg) were administered concurrently as a single dose to female Sprague-Dawley rats. Whole blood samples (1 ml) were aseptically withdrawn by tail bleeding at 1, 2, 4, 8, and 24 h after administration of the single dose for pharmacokinetics analyses. Concurrent administration of metformin and B. ferruginea significantly affected (P < 0.05) all the pharmacokinetics parameters of metformin except for the time to attain the maximum concentration (Tmax), which increased but insignificantly. Whereas the area under the curve, maximum whole blood concentration (Cmax) and half-life (T½) of metformin decreased significantly in the presence of B. ferruginea, the elimination rate constant (Kel), clearance (Cl), absorption rate constant (Ka), and volume of distribution (Vd) of metformin increased significantly in the presence of B. ferruginea. Therefore, in clinical practice, patients should be advised on the implication of concurrent administration of metformin and B. ferrruginea

    Cloaked Facebook pages: Exploring fake Islamist propaganda in social media

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    This research analyses cloaked Facebook pages that are created to spread political propaganda by cloaking a user profile and imitating the identity of a political opponent in order to spark hateful and aggressive reactions. This inquiry is pursued through a multi-sited online ethnographic case study of Danish Facebook pages disguised as radical Islamist pages, which provoked racist and anti-Muslim reactions as well as negative sentiments towards refugees and immigrants in Denmark in general. Drawing on Jessie Daniels’ critical insights into cloaked websites, this research furthermore analyses the epistemological, methodological and conceptual challenges of online propaganda. It enhances our understanding of disinformation and propaganda in an increasingly interactive social media environment and contributes to a critical inquiry into social media and subversive politics
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