Introduction

Introduction by David Guillet and Douglas Uzzel

Dynamic cratering of graphite : experimental results and simulations

The cratering process in brittle materials under hypervelocity impact (HVI) is of major relevance for debris shielding in spacecraft or high-power laser applications. Amongst other materials, carbon is of particular interest since it is widely used as elementary component in composite materials. In this paper we study a porous polycrystalline graphite under HVI and laser impact, both leading to strong debris ejection and cratering. First, we report new experimental data for normal impacts at 4100 and 4200 m s-1 of a 500-μm-diameter steel sphere on a thick sample of graphite. In a second step, dynamic loadings have been performed with a high-power nanosecond laser facility. High-resolution X-ray tomographies and observations with a scanning electron microscope have been performed in order to visualize the crater shape and the subsurface cracks. These two post-mortem diagnostics also provide evidence that, in the case of HVI tests, the fragmented steel sphere was buried into the graphite target below the crater surface. The current study aims to propose an interpretation of the results, including projectile trapping. In spite of their efficiency to capture overall trends in crater size and shape, semi-empirical scaling laws do not usually predict these phenomena. Hence, to offer better insight into the processes leading to this observation, the need for a computational damage model is argued. After discussing energy partitioning in order to identify the dominant physical mechanisms occurring in our experiments, we propose a simple damage model for porous and brittle materials. Compaction and fracture phenomena are included in the model. A failure criterion relying on Weibull theory is used to relate material tensile strength to deformation rate and damage. These constitutive relations have been implemented in an Eulerian hydrocode in order to compute numerical simulations and confront them with experiments. In this paper, we propose a simple fitting procedure of the unknown Weibull parameters based on HVI results. Good agreement is found with experimental observations of crater shapes and dimensions, as well as debris velocity. The projectile inclusion below the crater is also reproduced by the model and a mechanism is proposed for the trapping process. At least two sets of Weibull parameters can be used to match the results. Finally, we show that laser experiment simulations may discriminate in favor of one set of parameters

Predicting persistence or withdrawal in female handballers with Social Exchange theory

International audienceTwo complementary studies were conducted to explain the dropout phenomenon with French female handball players, utilizing the tenets of social exchange theory (Thibaut & Kelley, 1959; Rusbult, 1980). In the first study, the aim was to more fully explore the sources of enjoyment by emphasizing the key variables in the costs/benefits analysis. We postulate that the costs/benefits analysis consists in each athlete estimating the probability of reaching the most desired consequences for him or her (e.g., learn and improve skill, affiliation with others, be better than the others). The subjects were 488 French women aged from 15-19 years. Structural equation modeling (SEM) revealed that enjoyment in handball was predicted by a latent variable (named perceived benefits) subjacent to perceptions of competence, autonomy, relatedness, progress, coach's support, and time of play. In the second study, we tested a sport commitment model based on the social exchange postulates, using SEM analyses and a prospective design over 8 months. In view of the results of study 1, we replaced the construct of enjoyment with perceived benefit. This analysis provides a more comprehensive understanding of the enjoyment construct and is more consistent with the notion of cost and benefits outlined in social exchange theory (Thibaut and Kelley, 1959). Participants were 253 French handball players between the ages of 14 and 16 years. A first set of analyses, focused on the differences, showed that dropout players perceived themselves as significantly less competent, less autonomous, less related to their team, lower in progress and less supported by their coach than persistent players. The second set of analyses with SEM revealed that the commitment level was positively associated with perceived benefits and negatively with social constraints and alternatives opportunities. Finally, a lack of commitment led to dropping out of the sport 8-months later.Deux études complémentaires destinée à mieux comprendre le phénomène de l'abandon chez des handballeuses françaises à partir des postulats de la théorie de l'échange social (Thibaut & Kelley, 1959; Rusbult, 1980), ont été conduites. La première étude s'est intéressée aux sources du plaisir lié à la pratique sportive. Nous avons postulé que l'analyse coûts/bénéfices correspondait à une estimation d'atteindre les motifs les plus importants de pratique (e.g., apprendre et progresser, avoir des amis, être meilleur que les autres). Les sujets étaient 488 handballeuses françaises âgées de 15 à 19 ans. Les résultats d'analyses par équations structurelles (AES) ont révélé que le plaisir en handball était prédit par une variable latente (dénommée “bénéfices perçus” dans l'activité) sous jacente aux perceptions de compétence, d'autonomie, d'affiliation, de progrès, du soutien de l'entraîneur, et du temps de jeu moyen en match. Dans la deuxième étude, nous avons testé un modèle de l'engagement sportif basé sur les postulats de la théorie de l'échange social, en utilisant des AES et un suivi longitudinal de 8 mois. Au regard, des résultats de la première étude, nous avons remplacé le construit de plaisir par celui des bénéfices perçus. Cette analyse permet une compréhension plus complète des notions de coûts et de bénéfices développées dans la théorie de l'échange social (Thibaut et Kelley, 1959). Les participants étaient 253 joueuses âgées de 14 à 16 ans. Les résultats ont montré que les joueuses qui avaient abandonné se percevaient moins compétentes, moins autonomes, moins liées à leur équipe, moins en progrès et moins encouragées par leur entraîneur que les joueuses persistantes. De plus, l'AES a révélé que le niveau d'engagement était associé positivement aux bénéfices perçus, et négativement aux contraintes sociales et aux activités alternatives. En retour, un faible niveau d'engagement prédisait un taux élevé d'abandon 8 mois plus tard

The MIT Media Laboratory videodisc : the process and interactive model

Thesis (M.S.V.S.)--Massachusetts Institute of Technology, Dept. of Architecture, 1986.MICROFICHE COPY AVAILABLE IN ARCHIVES AND ROTCH.Bibliography: leaves 56-57.by Guy Guillet.M.S.V.S

High quality factor nitride-based optical cavities: microdisks with embedded GaN/Al(Ga)N quantum dots

We compare the quality factor values of the whispery gallery modes of microdisks incorporating GaN quantum dots (QDs) grown on AlN and AlGaN barriers by performing room temperature photoluminescence (PL) spectroscopy. The PL measurements show a large number of high Q factor (Q) resonant modes on the whole spectrum which allows us to identify the different radial mode families and to compare them with simulations. We report a considerable improvement of the Q factor which reflect the etching quality and the relatively low cavity loss by inserting QDs into the cavity. GaN/AlN QDs based microdisks show very high Q values (Q > 7000) whereas the Q factor is only up to 2000 in microdisks embedding QDs grown on AlGaN barrier layer. We attribute this difference to the lower absorption below bandgap for AlN barrier layers at the energies of our experimental investigation

Retos y respuestas en la gestión europea de las aguas: los sistemas de riego por aspersión en el Noroeste de España

Los sistemas eficientes de distribución de aguas están siendo cada vez más reconocidos por su potencial para incrementar la agricultura sostenible en regiones áridas y semiáridas al sur de Europa. En España, los sistemas de riego por aspersión han sido adoptados ampliamente desde la década de los sesenta. La cuenca del río Duero es una de las regiones donde su uso ha sido más acentuado. Al sur del Duero, en donde predominan las grandes propiedades de tierra, los sistemas de pivote central y movimiento lineal permanentes han sido las técnicas más utilizadas. Al norte del Duero, donde las propiedades son más reducidas, las familias de agricultores prefieren los sistemas de riego manuales o fijos. Este artículo trata de la adopción y el uso de sistemas de riego por aspersión por los agricultores en el norte del Duero, en el área del río Órbigo, en la provincia de León. Los datos recogidos provienen de una encuesta que se llevó a cabo en 1997 con 356 agricultores provenientes de dieciséis localidades. Se comprobó que las diferencias agroecológicas en cada lugar son el factor más determinante en la adopción del sistema de riego por aspersión. Distintas variables en cada hogar explican las diferencias individuales en el uso de éste. La sección que concluye este artículo evalúa el papel que juegan los sistemas de riego por aspersión de pequeña escala utilizados por las familias agricultores para la gestión sostenible de las aguas en el sur de Europ

Tuning, Impedance Matching, and Temperature Regulation during High-Temperature Microwave Sintering of Ceramics

International audienceOver the years, microwave radiation has emerged as an efficient source of energy for material processing. This technology provides a rapid and a volumetric heating of material. However, the main issues that prevent microwave technology from being widespread in material processing are temperature control regulation and heating distribution within the sample. Most of the experimental works are usually manually monitored, and their reproducibility is rarely evaluated and discussed. In this work, an originally designed 915MHz microwave single-mode applicator for high-temperature processing is presented. The overall microwave system is described in terms of an equivalent electrical circuit. This circuit has allowed to point out the different parameters which need to be adjusted to get a fully controlled heating process. The basic principle of regulation is then depicted in terms of a block function diagram. From it, the process has been developed and tested to sinter zirconia-and spinel-based ceramics. It is clearly shown that the process can be successfully used to program multistep temperature cycles up to similar to 1550 degrees C, improving significantly the reproducibility and the ease of use of this emerging high-temperature process technology

The Coyote Universe I: Precision Determination of the Nonlinear Matter Power Spectrum

Near-future cosmological observations targeted at investigations of dark energy pose stringent requirements on the accuracy of theoretical predictions for the clustering of matter. Currently, N-body simulations comprise the only viable approach to this problem. In this paper we demonstrate that N-body simulations can indeed be sufficiently controlled to fulfill these requirements for the needs of ongoing and near-future weak lensing surveys. By performing a large suite of cosmological simulation comparison and convergence tests we show that results for the nonlinear matter power spectrum can be obtained at 1% accuracy out to k~1 h/Mpc. The key components of these high accuracy simulations are: precise initial conditions, very large simulation volumes, sufficient mass resolution, and accurate time stepping. This paper is the first in a series of three, with the final aim to provide a high-accuracy prediction scheme for the nonlinear matter power spectrum.Comment: 18 pages, 22 figures, minor changes to address referee repor

Photon Physics in Heavy Ion Collisions at the LHC

Various pion and photon production mechanisms in high-energy nuclear collisions at RHIC and LHC are discussed. Comparison with RHIC data is done whenever possible. The prospect of using electromagnetic probes to characterize quark-gluon plasma formation is assessed.Comment: Writeup of the working group "Photon Physics" for the CERN Yellow Report on "Hard Probes in Heavy Ion Collisions at the LHC", 134 pages. One figure added in chapter 5 (comparison with PHENIX data). Some figures and correponding text corrected in chapter 6 (off-chemical equilibrium thermal photon rates). Some figures modified in chapter 7 (off-chemical equilibrium photon rates) and comparison with PHENIX data adde

Nosocomial Malaria and Saline Flush

An investigation of malaria in a US patient without recent travel established Plasmodium falciparum molecular genotype identity in 2 patients who shared a hospital room. P. falciparum can be transmitted in a hospital environment from patient to patient by blood inoculum if standard precautions are breached