Asteroseismology of solar-type stars with K2

We present the first detections by the NASA K2 Mission of oscillations in solar-type stars, using short-cadence data collected during K2 Campaign\,1 (C1). We understand the asteroseismic detection thresholds for C1-like levels of photometric performance, and we can detect oscillations in subgiants having dominant oscillation frequencies around $1000\,\rm \mu Hz$. Changes to the operation of the fine-guidance sensors are expected to give significant improvements in the high-frequency performance from C3 onwards. A reduction in the excess high-frequency noise by a factor of two-and-a-half in amplitude would bring main-sequence stars with dominant oscillation frequencies as high as ${\simeq 2500}\,\rm \mu Hz$ into play as potential asteroseismic targets for K2.Comment: Accepted for publication in PASP; 16 pages, 2 figure

Influence of geometrical defects on the mechanical behaviour of hollow-sphere structures

AbstractHollow-sphere structures could represent an alternative to classical cellular materials, such as metal foams or honeycombs, for various structural applications; such stainless steel random structures are already on the market. One advantage of hollow-sphere structures unlike metal foams ensues from the possibility to stack the spheres regularly, even if in the literature there are only examples of limited size regular stackings for the moment. Higher mechanical properties than those of random cellular structures are expected for such regular structures according to modelling studies. Nevertheless, because of the difficulty in processing perfect regular stackings, it seems to be critical to study the influence of architectural defects on the overall mechanical behaviour of these cellular structures. Emphasis is on geometrical defects by introducing some dispersions on the sphere thickness and the meniscus size. Influences of both the magnitude of dispersion and the distribution of the defects on the mechanical behaviour of hollow-sphere structure are investigated. Especially, collapse mechanisms resulting from plasticity and their inhomogeneous localisation in the structure are studied in details. The case of periodic defects is addressed too in order to compare the mechanical response of infinite stackings to that of finite ones. This work highlights the significant influence of the defects on the effective mechanical behaviour of hollow-sphere structures. Most of the time, geometrical dispersion and defects are detrimental for the stacking behaviour, especially when understructures made of the defective hollow spheres or menisci are observed

Micromechanical modeling of packing and size effects in particulate composites

AbstractThis paper is devoted to the introduction of packing and size effects in micromechanical predictions of the overall elastic moduli of particulate composite materials. Whereas micromechanical models derived from the classical ‘point approach’ are known to be unable to model such effects, it is shown that the so-called ‘morphologically representative pattern-based approach’ (MRP-based approach) offers new means of taking some geometrical parameters into account such as the mean distance between nearest-neighbor particles or their size, so as to predict the dependence of the overall moduli on these parameters, at least in a relative way. Moreover, when internal lengths, such as the thickness of interphase shells of coated particles, are introduced, absolute size effects can be predicted as well. Illustrative applications are reported in view of comparisons between such new treatments and the predictions of some classical models which are shown to coincide with the ones derived from MRP-based models in definite limiting cases only