Oxidation induced changes in viscoelastic properties of a thermostable epoxy matrix

The thermal ageing of a neat epoxy matrix has been studied at 200°C in air by three complementary analytical techniques: optical microscopy, mechanical spectrometry and nano-indentation. Thermal oxidation is restricted in a superficial layer of about 195 µm of maximal thickness. It consists in a predominant chain scission process involving, in particular, chemical groups whose β motions have the highest degree of cooperativity and thus, are responsible for the high temperature side of β dissipation band. As a result, chain scissions decrease catastrophically the glass transition temperature, but also increase significantly the storage modulus at glassy plateau between Tβ and Tα. This phenomenon is called “internal antiplasticization”. Starting from these observations, the Di Marzio and Gilbert’s theories have been used in order to establish relationships between the glass transition temperature and number of chain scissions, and between the storage modulus and β transition activity respectively. The challenge is now to establish a relationship between the transition activity and the concentration of the corresponding chemical group

Dissolution features of gold particles in a lateritic profile at Dondo Mobi, Gabon

Cet article décrit l'évolution morphologique de particules d'or dans un profil d'altération latéritique sous forêt humide (Dondo Mobi, Gabon). Héritées d'une veine de quartz associée à des amphibolites, les particules d'or sont reconnues dans la saprolite à partir du front d'altération et dans les horizons meubles de surface dans la partie centrale du halo de dispersion de l'or supergène. Les particules d'or montrent des transformations dans les premiers dix mètre

Translation symmetry restoration under random unitary dynamics

The finite parts of a large, locally interacting many-body system prepared out-of-equilibrium eventually equilibrate. Characterising the underlying mechanisms of this process and its timescales, however, is particularly hard as it requires to decouple universal features from observable-specific ones. Recently, new insight came by studying how certain symmetries of the dynamics that are broken by the initial state are restored at the level of the reduced state of a given subsystem. This provides a high level, observable-independent probe. Until now this idea has been applied to the restoration of internal symmetries, e.g. U(1) symmetries related to charge conservation. Here we show that that the same logic can be applied to the restoration of space-time symmetries, and hence can be used to characterise the relaxation of fully generic systems. We illustrate this idea by considering the paradigmatic example of "generic" many-body dynamics, i.e. a local random unitary circuit. We show that, surprisingly, the restoration of translation symmetry in these systems only happens on time-scales proportional to the subsystem's volume. In fact, for large enough subsystems the time of symmetry restoration becomes initial-state independent (as long as the latter breaks the symmetry at time zero) and coincides with the thermalisation time. For intermediate subsystems, however, one can observe the so-called "quantum Mpemba effect", where the state of the system restores a symmetry faster if it is initially more asymmetric

Translation symmetry restoration under random unitary dynamics

The finite parts of a large, locally interacting many-body system prepared out-of-equilibrium eventually equilibrate. Characterising the underlying mechanisms of this process and its timescales, however, is particularly hard as it requires to decouple universal features from observable-specific ones. Recently, new insight came by studying how certain symmetries of the dynamics that are broken by the initial state are restored at the level of the reduced state of a given subsystem. This provides a high level, observable-independent probe. Until now this idea has been applied to the restoration of internal symmetries, e.g. U(1) symmetries related to charge conservation. Here we show that that the same logic can be applied to the restoration of space-time symmetries, and hence can be used to characterise the relaxation of fully generic systems. We illustrate this idea by considering the paradigmatic example of "generic" many-body dynamics, i.e. a local random unitary circuit. We show that, surprisingly, the restoration of translation symmetry in these systems only happens on time-scales proportional to the subsystem's volume. In fact, for large enough subsystems the time of symmetry restoration becomes initial-state independent (as long as the latter breaks the symmetry at time zero) and coincides with the thermalisation time. For intermediate subsystems, however, one can observe the so-called "quantum Mpemba effect", where the state of the system restores a symmetry faster if it is initially more asymmetric

Dynamics Of Internode And Stem Elongation In Three Cultivars Of Maize

The kinetics of elongation of individual internodes, the peduncle and panicle of maize were studied in field experiments in Gatton (Australia) using two tropical cultivars, and compared to similar data previously collected in Grignon (France) on a temperate cultivar. Data for phytomer initiation and organ extension and appearance were related to thermal time calculated from the temperature in the growing zone. Extension of internodes was analysed using a four stage framework:- an initial exponential stage, transition to rapid extension, rapid (linear) extension and transition to final length. The kinetics of internode extension were similar in Gatton and Grignon, though the rates of processes differed. Transition from stage 1 to stage 2 coincided with collar emergence. The commencement of rapid extension of the peduncle coincided with a transient reduction in the rates of extension of vegetative internodes. Further work is needed to assess whether they are effects of genotype only, or genotype and environment

Fatigue vibratoire des processus stochastiques, générés par les machines tournantes et modèle de fiabilité probabiliste associé

Les processus aléatoires de contrainte relevés sur les structures mécaniques des machines tournantes sont à structure non gaussienne. En effet, ces derniers sont logiquement composés d'un processus périodique déterministe, centré sur les harmoniques de la fréquence de rotation du rotor, et sur lequel vient en général se superposer un processus aléatoire à moyenne nulle de nature gaussienne. Ces processus aléatoires sont alors couramment appelés processus aléatoires Composites Sinus plus Bruit (C_S+B) ou Multi-Sinus plus Bruit. L'objectif de l'article est donc de présenter dans un premier temps les fondements mathématiques des modèles de fatigue vibratoire, adaptés ou non à cette nature spécifique de processus et couramment utilisés dans l'industrie. Puis dans un deuxième temps, les modèles probabilistes proposés seront comparés entre eux, afin d'en étudier leur degré de similitude et de conservatisme. Sur la base de la théorie de S.O Rice, associée au cas d'un processus aléatoire composite Sinus plus Bruit à bande étroite, un modèle analytique exact de fatigue vibratoire composite Sinus plus Bruit est alors proposé et discuté

Intérêt de la Méthode des Blocs Disjoints (MBD) de caractérisation des processus aléatoires non gaussiens et non stationnaires

Dans le domaine des véhicules Terrestres militaires, les processus aléatoires de vibrations générés par les véhicules à roues, en situation Tout-Terrain, sortent du schéma classique des processus stochastiques, à caractère stationnaire et gaussien. Le caractère non-gaussien des processus s'exprime notamment par des niveaux d'aplatissement très importants, pouvant remettre en cause le dimensionnement aux contraintes extremes et à la fatigue des structures mécaniques, acquis classiquement par les démarches spectrales. De ces considérations techniques, il convenait de faire évoluer les techniques de caractérisation des processus aléatoires d'excitation, générés par ce type de situation de roulage, en proposant des méthodes de caractérisation innovantes, non plus basées sur des démarches spectrales et/ou temporelles déterministes, mais sur des démarches temporelles, de nature stochastique. En effet, pour caractériser les valeurs extrêmes et l'endommagement par fatigue produit par les processus aléatoires non stationnaires et non gaussiens, l'auteur montre qu'il est désormais nécessaire de mixer les techniques de détection d'extrema et de comptage temporel utilisées dans le domaine de la fatigue vibratoire avec celles de la statistique de l'échantillonnage, utilisées dans la théorie de l'estimation. Cette approche permet en effet de pouvoir extrapoler favorablement dans le temps, les valeurs extremes et le niveau d'endommagement des structures, sous l'angle statistique, alors que cette phase d'extrapolation est dans la pratique réalisée de façon déterministe. Cette technique dénommée « MBD » pour Méthode des Blocs Disjoints a été éprouvée avec succès dans le cadre des techniques de spécification d'organes, sous l'angle fiabilité depuis 2010, et viennent d'intégrer récemment les normes AFNOR. L'avantage de cette méthode MBD couplée aux notions de Spectres de Réponse (SRX-SFX) sera mise en exergue, en s'appuyant sur le cas d'une mesure de roulage en tout terrain sur véhicules à roues, et ceci comparativement aux approches spectrales, basées sur l'hypothèse de gaussiannité des processus

Azimuthal dependence of the Garton-Tomkins orbit in crossed magnetic and electric fields

Work on classical closed orbits in the diamagnetic Kepler problem is predominately focused on the chaos observed in the polar launch angle as opposed to the azimuthal launch angle. This is due to atomic systems, along with widely studied external-field geometries (parallel magnetic and electric fields or pure magnetic field), being uniform in azimuthal angle, rendering the azimuthal angle unimportant. In the case of crossed magnetic and electric fields, this is no longer the case, and closed orbits do present an azimuthal launch angle dependence. In atomic systems, due to their spherical symmetry, the electric-field orientation in the plane perpendicular to the magnetic field does not affect the spectrum of orbits. However, in shallow n-type donors in anisotropic semiconductors such as silicon, the orientation of the external fields with respect to conduction-band valleys will be important. In this work we examine the Garton-Tomkins orbit in crossed magnetic and electric fields, and analyze how it and its harmonics\u27 azimuthal dependencies behave through variation of the scaled field or scaled energy. At low scaled fields, harmonics have either twofold or fourfold azimuthal dependencies determined by the rotational symmetry of the individual harmonics. As the scaled field or scaled energy is increased, several harmonics undergo significant bifurcations, resulting in large azimuthal angular regions of essentially closed orbits, which will lead to strong resonances in experimental work