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  1. A method for detecting low-velocity impact damage in carbon fiber reinforced polymer (CFRP) is presented. It involves the use of the Impulse Excitation Technique (IET) and hysteresis loops to calculate the damping parameter of T700/NCT304-1 carbon/epoxy samples subjected to various low-velocity impact energies. The value of the coefficient of restitution (COR) is determined for each impact, ranging between 0.62 for the lowest impact energy to 0.48 for the highest one. The results reveal that a three-step increase in the damping parameter exists in all cases as the impact energy on the specimen increases. An abrupt jump in the damping parameter value is observed for impact energies exceeding ∼0.9 of the material's maximum capacity. Overall, at the highest impact energy equal to 3.65 J, the damping parameter increased by 43.3% compared to the pristine specimen. Additionally, two cases of cyclic tension-tension loading were applied to the specimens, with maximum stresses set at 150 MPa and 200 MPa. The measured values of plastic and elastic strain energy were used to determine the damping ratios. For both cases, the damping of the specimen subjected to the highest impact energy was ∼1.2 times greater than that of an intact specimen, with an increase pattern similar to the findings of the IET method. Optical microscope images of the specimens are provided to illustrate various damage modes observed in the composite materials.

     
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    Free, publicly-accessible full text available February 1, 2025
  2. Abstract

     We present a stochastic field line mapping model where the interplanetary magnetic field lines are described by a density distribution function satisfying a Fokker–Planck equation that is solved numerically. Due to the spiral geometry of the nominal Parker field and to the evolving nature of solar wind turbulence, the heliospheric diffusion of the magnetic field lines is both heterogeneous and anisotropic, including a radial component. The longitudinal distributions of the magnetic field lines are shown to be close to circular Gaussian distributions, although they develop a noticeable skewness. The magnetic field lines emanating from the Sun are found to differ, on average, from the spirals predicted by Parker. Although the spirals remain close to Archimedean, they are here underwound, on average. Our model predicts a spiral angle that is smaller by ∼5° than the Parker spiral angle at Earth’s orbit for the same solar wind speed ofVsw= 400 km s−1. It also predicts an angular position on the solar disk of the best magnetically connected footpoint to an observer at 1 au that is shifted westward by ∼10° with respect to the Parker’s field model. This significantly changes the angle of the most probable magnetic connection between possible sources on the Sun and observers in the inner heliosphere. The results have direct implications for the heliospheric transport of “scatter-free” electrons accelerated in the aftermath of solar eruptions.

     
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    Free, publicly-accessible full text available February 1, 2025
  3. Abstract

    A steady-state, semi-analytical model of energetic particle acceleration in radio-jet shear flows due to cosmic-ray viscosity obtained by Webb et al. is generalized to take into account more general cosmic-ray boundary spectra. This involves solving a mixed Dirichlet–Von Neumann boundary value problem at the edge of the jet. The energetic particle distribution functionf0(r,p) at cylindrical radiusrfrom the jet axis (assumed to lie along thez-axis) is given by convolving the particle momentum spectrumf0(,p)with the Green’s functionG(r,p;p), which describes the monoenergetic spectrum solution in whichf0δ(pp)asr→ ∞ . Previous work by Webb et al. studied only the Green’s function solution forG(r,p;p). In this paper, we explore for the first time, solutions for more general and realistic forms forf0(,p). The flow velocityu=u(r)ezis along the axis of the jet (thez-axis).uis independent ofz, andu(r) is a monotonic decreasing function ofr. The scattering timeτ(r,p)=τ0(p/p0)αin the shear flow region 0 <r<r2, andτ(r,p)=τ0(p/p0)α(r/r2)s, wheres> 0 in the regionr>r2is outside the jet. Other original aspects of the analysis are (i) the use of cosmic ray flow lines in (r,p) space to clarify the particle spatial transport and momentum changes and (ii) the determination of the probability distributionψp(r,p;p)that particles observed at (r,p) originated fromr→ ∞ with momentump. The acceleration of ultrahigh-energy cosmic rays in active galactic nuclei jet sources is discussed. Leaky box models for electron acceleration are described.

     
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    Free, publicly-accessible full text available November 22, 2024
  4. In response to COVID-19, many countries have mandated social distancing and banned large group gatherings in order to slow down the spread of SARS-CoV-2. These social interventions along with vaccines remain the best way forward to reduce the spread of SARS CoV-2. In order to increase vaccine accessibility, states such as Virginia have deployed mobile vaccination centers to distribute vaccines across the state. When choosing where to place these sites, there are two important factors to take into account: accessibility and equity. We formulate a combinatorial problem that captures these factors and then develop efficient algorithms with theoretical guarantees on both of these aspects. Furthermore, we study the inherent hardness of the problem, and demonstrate strong impossibility results. Finally, we run computational experiments on real-world data to show the efficacy of our methods. 
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  5. Efficient contact tracing and isolation is an effective strategy to control epidemics, as seen in the Ebola epidemic and COVID-19 pandemic. An important consideration in contact tracing is the budget on the number of individuals asked to quarantine—the budget is limited for socioeconomic reasons (e.g., having a limited number of contact tracers). Here, we present a Markov Decision Process (MDP) framework to formulate the problem of using contact tracing to reduce the size of an outbreak while limiting the number of people quarantined. We formulate each step of the MDP as a combinatorial problem, MinExposed, which we demonstrate is NP-Hard. Next, we develop two approximation algorithms, one based on rounding the solutions of a linear program and another (greedy algorithm) based on choosing nodes with a high (weighted) degree. A key feature of the greedy algorithm is that it does not need complete information of the underlying social contact network, making it implementable in practice. Using simulations over realistic networks, we show how the algorithms can help in bending the epidemic curve with a limited number of isolated individuals. 
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  6. Abstract An Earth-analog orbiting within the habitable zone of α Centauri B was shown to undergo large variations in its obliquity, or axial tilt, which affects the planetary climate by altering the radiative flux for a given latitude. We examine the potential implications of these obliquity variations for climate through Milankovitch cycles using an energy balance model with ice growth and retreat. Similar to previous studies, the largest amplitude obliquity variations from spin-orbit resonances induce snowball states within the habitable zone, while moderate variations can allow for persistent ice caps or an ice belt. Particular outcomes for the global ice distribution can depend on the planetary orbit, obliquity, spin precession, binary orbit, and which star the Earth-analog orbits. An Earth-analog with an inclined orbit relative to the binary orbital plane can periodically transition through several global ice distribution states and risk runaway glaciation when ice appears at both poles and the equator. When determining the potential habitability for planets in general stellar binaries, more care must be taken due to the orbital and spin dynamics. For Earth-analogs within the habitable zone of α Centauri B can experience a much greater range of climate states, which is in contrast to Earth-analogs in the habitable zone of α Centauri A. 
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