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

Micromechanical modeling of packing and size effects in particulate composites

International audienc

FE modelling of cellular materials under compressive load

Structures made of tubes, stacked in square or hexagonal patterns, have been considered here as model cellular materials. Compression tests have shown large deformations, transformations and also many contact points. Two Finite Element codes have been used to investigate the influence of the elements type (quadrangular or triangular, linear or quadratic) and the numerical scheme (implicit or explicit) on the structural numerical responses. A contact algorithm based on the Pinball method has been implemented in the explicit code. A very good agreement has been found between the predictions of both codes. The numerical responses are close for a given meshes order, whatever the elements type. Whereas the linear meshes cannot be considered as converged, the quadratic meshes predict very well the experimental responses of the structures, especially for the square stacking. The overestimation of the numerical response of the hexagonal stacking might be explained because, experimentally, this stacking exhibits a more scattered and irregular behaviour due to defects (missing brazes, tube misalignment) which were not modelled.JRC.E.4-Safety and Security of Building

About the use of micro-indentation to determine the constitutive material’s mechanical behaviour of cellular structures

Architectured cellular materials seem to be very promising materials since they can combine interesting specific mechanical, acoustical and thermal properties. Nevertheless the process routes used to elaborate such materials, especially the thermal treatments and the added compounds, may alter their constitutive materials for which the mechanical behaviour becomes unknown. In order to determine locally the mechanical behaviour of the constitutive materials of cellular structures we propose the use of microindentation. The model structures considered here are brazed tube stackings. Macroscopic tensile tests and micro-indentation measurements have been performed on isolated tubes having been submitted to different thermal treatment durations. The mechanical properties estimated thanks to both methods show a good agreement. These results also put in evidence the strong influence of the thermal treatment on the behaviour of the constitutive material of the tubes. Micro-indentation measurements performed in the braze joints gave access to their mechanical properties which are strongly heterogeneous. A comparison between the experimental mechanical response of the tube stackings under compression and the one predicted by the finite-element method is proposed too

About the use of micro-indentation to determine the constitutive material’s mechanical behaviour of cellular structures

Architectured cellular materials seem to be very promising materials since they can combine interesting specific mechanical, acoustical and thermal properties. Nevertheless the process routes used to elaborate such materials, especially the thermal treatments and the added compounds, may alter their constitutive materials for which the mechanical behaviour becomes unknown. In order to determine locally the mechanical behaviour of the constitutive materials of cellular structures we propose the use of microindentation. The model structures considered here are brazed tube stackings. Macroscopic tensile tests and micro-indentation measurements have been performed on isolated tubes having been submitted to different thermal treatment durations. The mechanical properties estimated thanks to both methods show a good agreement. These results also put in evidence the strong influence of the thermal treatment on the behaviour of the constitutive material of the tubes. Micro-indentation measurements performed in the braze joints gave access to their mechanical properties which are strongly heterogeneous. A comparison between the experimental mechanical response of the tube stackings under compression and the one predicted by the finite-element method is proposed too

Modélisation par élément finis de la compression de matériaux cellulaires

International audienceStructures made of tubes, stacked in square or hexagonal patterns, have been considered here as model cellular materials. Compression tests have shown large deformations, transformations and also many contact points. Two Finite Element codes have been used to investigate the influence of the elements type (quadrangular or triangular, linear or quadratic) and the numerical scheme (implicit or explicit) on the structural numerical responses. A contact algorithm based on the Pinball method has been implemented in the explicit code. A very good agreement has been found between the predictions of both codes. The numerical responses are close for a given meshes order, whatever the elements type. Whereas the linear meshes cannot be considered as converged, the quadratic meshes predict very well the experimental responses of the structures, especially for the square stacking. The overestimation of the numerical response of the hexagonal stacking might be explained because, experimentally, this stacking exhibits a more scattered and irregular behaviour due to defects (missing brazes, tube misalignment) which were not modelled

Experimental and Finite Element Analysis of cellular materials under large compaction levels

This work aims at investigating the experimental characterisation and the modelling of the mechanical behaviour of cellular sandwich structures for large compaction levels, especially focusing on the collapse mechanisms of their constitutive cells and the role of the contacts created between neighbour cells. For that purpose, brazed cellular sandwich structures made of tube stackings have been considered as model architectures. The experimental characterisation of stackings consisting of either a square pattern or a hexagonal one has highlighted that the collapse mechanism was very reproducible in the case of the square stacking. On the contrary, the one observed for the hexagonal stacking showed an important sensitivity to the architectural defects such as missing braze joints or tube misalignment. Internal self-contacts created plaid also an important role regarding the densification plateau. In parallel, these compression tests have been simulated through the finite-element method; two different codes have been considered, one implicit (Z-set) and one explicit (Europlexus). The predictions of both codes have been compared to investigate their differences depending on finite strain and contact formulations. The comparison of their predictions with the experimental results has highlighted that quadratic meshes were necessary, involving the implementation of a second-order pinball method for the modelling of contacts in Europlexus. Both codes have also shown very close predictions whatever the mesh order and the finite strain formulation.JRC.E.4-Safety and Security of Building