Asymmetric friction: Modelling and experiments

International audienceThe effect of asymmetry with respect to sliding direction has been recently proved to be important in the analysis of static contact interactions of some automotive mechanical structures. In the present paper, we consider a nonclassical friction law, based on a dissymmetry of friction in a given sliding direction. A simple analytical micro-model has been adopted to derive the macroscopic model via averaging. Not only does it allow the law macroscopic characteristics to be determined but it also theoretically proves the existence of such dissymmetry of friction. The thermodynamic admissibility of the law is then shown from the derivation of a generalized standard model including friction. To complete the analysis, the existence of asymmetric friction conditions is confirmed from an experimental point of view. We then compare theoretical computations resulting from the analytical model with experimental results

Searching for Outbursts in the Ground-Based Photometry of 67P/Churyumov-Gerasimenko

67P/Churyumov-Gerasimenko is a Jupiter-family comet that was the target of the Rosetta mission, the first mission to successfully orbit and land a probe on a comet. This mission was accompanied by a large ground-based observing campaign. We have developed a pipeline to calibrate and measure photometry of comet 67P during its 2016 perihelion passage, making use of all visible wavelength broadband imaging collected across a wide range of facilities. The pipeline calibrates the brightness of the comet to a common photometric system (Pan-STARRS 1) using background stars within the field allowing for compilation and comparison of multiple data sets. Results follow the predictions based on previous apparitions: 67P shows no obvious change in activity levels from orbit-to-orbit and coma colours remain constant throughout the apparition. We detected an outburst on 2015 August 22 of ∼0.14 mag. The brightness and estimated mass of this outburst puts it in line with the outbursts directly observed on the nucleus by Rosetta. An in situ outburst was observed at the same time as the one seen from the ground, however linking these two events directly remains challenging

Carbon-rich dust in comet 67P/Churyumov-Gerasimenko measured by COSIMA/Rosetta

Cometary ices are rich in CO2, CO and organic volatile compounds, but the carbon content of cometary dust was only measured for the Oort Cloud comet 1P/Halley, during its flyby in 1986. The COmetary Secondary Ion Mass Analyzer (COSIMA)/Rosetta mass spectrometer analysed dust particles with sizes ranging from 50 to 1000 μm, collected over 2 yr, from 67P/Churyumov-Gerasimenko (67P), a Jupiter family comet. Here, we report 67P dust composition focusing on the elements C and O. It has a high carbon content (atomic |${\rm{C}}/{\rm{Si}} = 5.5{\rm{\ }}_{ - 1.2}^{ + 1.4}\ \ {\rm{on\ average}}$ |⁠) close to the solar value and comparable to the 1P/Halley data. From COSIMA measurements, we conclude that 67P particles are made of nearly 50 per cent organic matter in mass, mixed with mineral phases that are mostly anhydrous. The whole composition, rich in carbon and non-hydrated minerals, points to a primitive matter that likely preserved its initial characteristics since the comet accretion in the outer regions of the protoplanetary disc.</p

Study of icy bodies in the solar system through two main targets of spatial exploration : the comet 67P/C-G and the satellite Europa

La majeure partie de mon travail de thèse a porté sur l’étude de la composition chimique de la surface d’Europe. Afin d’apporter une plus-value par rapport aux résultats de l’instrument NIMS à bord de la sonde Galileo qui orbita dans le système jovien de 1995 à 2003, une campagne d’observations depuis le sol a été menée avec le spectromètre imageur infra-rouge SINFONI au VLT. Cinq observations en optique adaptative possédant une résolution spatiale d’environ 160 km et une résolution spectrale R=1500 dans le proche infra-rouge furent acquises. Une procédure complexe de réduction des données a été mise en place afin de construire un cube hyperspectral global en réflectance de la surface. La modélisation linéaire de chacun des spectres du cube a permis d’aboutir aux premières cartes d’abondances absolues jamais obtenues pour la surface d’Europe. Ces cartes confirment la présence des deux espèces majoritaires, la glace d’eau et l’acide sulfurique hydraté. La distribution de l’acide sulfurique est centrée sur l’hémisphère orbital arrière qui est préférentiellement impacté par un flux d’ions de soufre originaires du tore de plasma produit par l’activité volcanique d’Io. Cependant, deux résultats inattendus ont été obtenus. Le premier concerne la glace d’eau, dont la forme cristalline est près de deux fois plus abondante que la forme amorphe selon les résultats de la modélisation. Ce résultat, surprenant compte tenu du taux d’irradiation très élevé auquel la surface est soumise, pourrait s’expliquer par l’existence d’un fort gradient de cristallinité au sein de la couche de glace, mais aussi par l’existence d’une activité endogène relativement soutenue qui se traduit visuellement par une surface peu cratérisée, donc jeune. La corrélation entre la distribution des grains cristallins et la géomorphologie semble accréditer la seconde hypothèse. Le second résultat concerne la détection de sels chlorés à partir de la modélisation des spectres hautement résolus de SINFONI, et non de sulfates, remettant en cause les détections marginales annoncées par les observations de l’instrument NIMS/Galileo. La distribution des sels chlorés, tout comme celle de la glace d’eau cristalline, est corrélée à la géomorphologie, ce qui confirme le rôle important des apports endogènes. Des processus tectoniques et cryovolcaniques mis en évidence récemment pourraient être à l’origine de cette distribution. L’autre versant de ma thèse a été consacré aux caractéristiques physiques des grains cométaires de 67P/C-G. L’instrument COSIMA embarqué sur l’orbiter de la sonde Rosetta a permis la collecte, l'imagerie et l'analyse chimique élémentaire des grains présents dans l’environnement proche de 67P/C-G. Une approche automatisée de la détection des grains à partir des images prises par la caméra « COSISCOPE » a été mise en place et a permis de détecter environ 35000 grains ayant une surface de plus de 100 µm2 entre août 2014 et mai 2016. La résolution de 13.7 µm/pixel a rendu possible la caractérisation en détail de la forme et de la structure des grains, et le nombre important de détections a permis d’obtenir des statistiques robustes concernant la distribution en taille et l’évolution de celle-ci au cours du temps. Deux grandes familles de grains ont été identifiées : les grains compacts, qui ne représentent qu’une faible minorité des grains et qui ont été majoritairement collectés en début de mission, et les agrégats, qui ont une structure très poreuse similaire à celle des IDPs et des micrométéorites collectées en Antarctique. La distribution en taille obtenue suit une loi de puissance intégrale en r-2.66. La comparaison avec des lois obtenues à des échelles différentes par d’autres instruments met en évidence des différences qui peuvent être interprétées par des mécanismes d’éjection dépendant de la taille associés à un biais du processus de collecte en orbite.The major part of my work focused on the study of the chemical composition of Europa’s surface. In order to provide additional insights in comparison to the results of the NIMS instrument onboard the Galileo spacecraft that orbited in the Jovian system from 1995 to 2003, a ground-based observations campaign was conducted with the infrared imaging spectrometer SINFONI on the VLT. Several observations using adaptive optics with a spatial resolution of about 160 km and a spectral resolution R = 1500 in the near-infrared were acquired and then combined. A specific data reduction pipeline was developed to build a global hyperspectral cube in surface reflectance. The linear modeling of each spectra of this cube leads to the first global abundance maps ever obtained for the surface of Europa. These maps confirm the presence of the two major species, namely water ice and hydrated sulfuric acid. The distribution of the hydrated sulfuric acid is centered on the trailing orbital hemisphere preferentially affected by a sulfur ion flux coming from the plasma torus produced by Io volcanic activity. Two surprising results were obtained. The first one concerns water ice, which crystalline form is about twice more abundant than the amorphous form according to the modeling results. This result, unexpected given the very high radiation rate on the surface, could be explained by a strong crystallinity gradient through the ice slab. However, it could also point out an endogenous activity possibly strong as first suggested by its poorly well-known cratered surface. The correlation between the crystalline grains distribution and the geomorphology seems in favor to the second hypothesis. The second result is related to the detection of chlorinated salts from the modeling of highly resolved spectra from SINFONI. Sulfates, first reported by several analyses of NIMS observations are marginal in the modeled composition, hence challenging their presence on the surface of Europa. The chloride distribution, as well as the one of the crystalline water ice, is correlated to geomorphology, potentially confirming significant endogenous contributions as the result of tectonic and cryovolcanic processes recently highlighted. The second part of my PhD was dedicated to the physical characteristics of the 67P/C-G’s cometary grains. The COSIMA instrument onboard the Rosetta orbiter allowed the collection, imaging and elemental chemical analysis of grains present in the immediate environment of 67P/C-G. An automated approach of the grains detection based on images taken by the camera "COSISCOPE" was set up and able to detect about 35.000 grains having an area of more than 100 µm² between August 2014 and May 2016. The resolution of 13.7 µm/pixel allowed to perform a detailed characterization of the shape and the structure of the grains, and the large number of detections permitted to obtain significant statistics on the size distribution and its evolution over the time. Two large families of grains have been identified: the compact grains, which represent only a small minority of grains mostly collected at the beginning of the mission, and aggregates, which have a very porous structure similar to those of IDPs and micrometeorites collected in Antarctica. The size distribution obtained follows an integrated power law in r-2.66. The comparison with the laws obtained at different scales by other instruments highlights differences that can be interpreted by ejection mechanisms depending on the size associated with a selection bias during the collection process in orbit

Etude des corps glacés du système solaire à travers deux cibles majeures de l'exploration spatiale : la comète 67P/C-G et le satellite Europe

The major part of my work focused on the study of the chemical composition of Europa’s surface. In order to provide additional insights in comparison to the results of the NIMS instrument onboard the Galileo spacecraft that orbited in the Jovian system from 1995 to 2003, a ground-based observations campaign was conducted with the infrared imaging spectrometer SINFONI on the VLT. Several observations using adaptive optics with a spatial resolution of about 160 km and a spectral resolution R = 1500 in the near-infrared were acquired and then combined. A specific data reduction pipeline was developed to build a global hyperspectral cube in surface reflectance. The linear modeling of each spectra of this cube leads to the first global abundance maps ever obtained for the surface of Europa. These maps confirm the presence of the two major species, namely water ice and hydrated sulfuric acid. The distribution of the hydrated sulfuric acid is centered on the trailing orbital hemisphere preferentially affected by a sulfur ion flux coming from the plasma torus produced by Io volcanic activity. Two surprising results were obtained. The first one concerns water ice, which crystalline form is about twice more abundant than the amorphous form according to the modeling results. This result, unexpected given the very high radiation rate on the surface, could be explained by a strong crystallinity gradient through the ice slab. However, it could also point out an endogenous activity possibly strong as first suggested by its poorly well-known cratered surface. The correlation between the crystalline grains distribution and the geomorphology seems in favor to the second hypothesis. The second result is related to the detection of chlorinated salts from the modeling of highly resolved spectra from SINFONI. Sulfates, first reported by several analyses of NIMS observations are marginal in the modeled composition, hence challenging their presence on the surface of Europa. The chloride distribution, as well as the one of the crystalline water ice, is correlated to geomorphology, potentially confirming significant endogenous contributions as the result of tectonic and cryovolcanic processes recently highlighted. The second part of my PhD was dedicated to the physical characteristics of the 67P/C-G’s cometary grains. The COSIMA instrument onboard the Rosetta orbiter allowed the collection, imaging and elemental chemical analysis of grains present in the immediate environment of 67P/C-G. An automated approach of the grains detection based on images taken by the camera "COSISCOPE" was set up and able to detect about 35.000 grains having an area of more than 100 µm² between August 2014 and May 2016. The resolution of 13.7 µm/pixel allowed to perform a detailed characterization of the shape and the structure of the grains, and the large number of detections permitted to obtain significant statistics on the size distribution and its evolution over the time. Two large families of grains have been identified: the compact grains, which represent only a small minority of grains mostly collected at the beginning of the mission, and aggregates, which have a very porous structure similar to those of IDPs and micrometeorites collected in Antarctica. The size distribution obtained follows an integrated power law in r-2.66. The comparison with the laws obtained at different scales by other instruments highlights differences that can be interpreted by ejection mechanisms depending on the size associated with a selection bias during the collection process in orbit.La majeure partie de mon travail de thèse a porté sur l’étude de la composition chimique de la surface d’Europe. Afin d’apporter une plus-value par rapport aux résultats de l’instrument NIMS à bord de la sonde Galileo qui orbita dans le système jovien de 1995 à 2003, une campagne d’observations depuis le sol a été menée avec le spectromètre imageur infra-rouge SINFONI au VLT. Cinq observations en optique adaptative possédant une résolution spatiale d’environ 160 km et une résolution spectrale R=1500 dans le proche infra-rouge furent acquises. Une procédure complexe de réduction des données a été mise en place afin de construire un cube hyperspectral global en réflectance de la surface. La modélisation linéaire de chacun des spectres du cube a permis d’aboutir aux premières cartes d’abondances absolues jamais obtenues pour la surface d’Europe. Ces cartes confirment la présence des deux espèces majoritaires, la glace d’eau et l’acide sulfurique hydraté. La distribution de l’acide sulfurique est centrée sur l’hémisphère orbital arrière qui est préférentiellement impacté par un flux d’ions de soufre originaires du tore de plasma produit par l’activité volcanique d’Io. Cependant, deux résultats inattendus ont été obtenus. Le premier concerne la glace d’eau, dont la forme cristalline est près de deux fois plus abondante que la forme amorphe selon les résultats de la modélisation. Ce résultat, surprenant compte tenu du taux d’irradiation très élevé auquel la surface est soumise, pourrait s’expliquer par l’existence d’un fort gradient de cristallinité au sein de la couche de glace, mais aussi par l’existence d’une activité endogène relativement soutenue qui se traduit visuellement par une surface peu cratérisée, donc jeune. La corrélation entre la distribution des grains cristallins et la géomorphologie semble accréditer la seconde hypothèse. Le second résultat concerne la détection de sels chlorés à partir de la modélisation des spectres hautement résolus de SINFONI, et non de sulfates, remettant en cause les détections marginales annoncées par les observations de l’instrument NIMS/Galileo. La distribution des sels chlorés, tout comme celle de la glace d’eau cristalline, est corrélée à la géomorphologie, ce qui confirme le rôle important des apports endogènes. Des processus tectoniques et cryovolcaniques mis en évidence récemment pourraient être à l’origine de cette distribution. L’autre versant de ma thèse a été consacré aux caractéristiques physiques des grains cométaires de 67P/C-G. L’instrument COSIMA embarqué sur l’orbiter de la sonde Rosetta a permis la collecte, l'imagerie et l'analyse chimique élémentaire des grains présents dans l’environnement proche de 67P/C-G. Une approche automatisée de la détection des grains à partir des images prises par la caméra « COSISCOPE » a été mise en place et a permis de détecter environ 35000 grains ayant une surface de plus de 100 µm2 entre août 2014 et mai 2016. La résolution de 13.7 µm/pixel a rendu possible la caractérisation en détail de la forme et de la structure des grains, et le nombre important de détections a permis d’obtenir des statistiques robustes concernant la distribution en taille et l’évolution de celle-ci au cours du temps. Deux grandes familles de grains ont été identifiées : les grains compacts, qui ne représentent qu’une faible minorité des grains et qui ont été majoritairement collectés en début de mission, et les agrégats, qui ont une structure très poreuse similaire à celle des IDPs et des micrométéorites collectées en Antarctique. La distribution en taille obtenue suit une loi de puissance intégrale en r-2.66. La comparaison avec des lois obtenues à des échelles différentes par d’autres instruments met en évidence des différences qui peuvent être interprétées par des mécanismes d’éjection dépendant de la taille associés à un biais du processus de collecte en orbite

Cumulative microslip at the interface of mechanical assemblies under cyclic loading

International audienc

On the cumulative microslip phenomenon

The cumulative microslip phenomenon is the accumulation of relative slips in a tangential direction on the contact interface of two solids under cyclic loadings. This lead to significant global relative displacement between components and can account for the failure of some assembly parts in mechanical structures. Practical examples from the automotive industry are presented in this paper to describe the cumulative microslip in real situations. The phenomenon is then discussed from a theoretical point of view as an asymptotic behaviour by analogy with the ratchetting effects in elastoplasticity. Accommodation and slip shakedown are introduced in the same spirit. A slip shakedown theorem is proposed and leads to the definition of a safety coefficient with respect to slips when a standard law of friction is assumed. The safety coefficient can be computed from two static and kinematic approches in min-max duality. A simple example is given

Birth experience during COVID-19 confinement (CONFINE): protocol for a multicentre prospective study

International audienceProtection des Personnes) SUD OUEST ET OUTRE-MER IV on 16th of April 2020 with reference number CPP2020-04-040. The results of this study will be published in a peer-reviewed journal and will be presented at relevant conferences. Trial registration number NCT04348929

Dust particle flux and size distribution in the coma of 67P/Churyumov-Gerasimenko measured in situ by the COSIMA instrument on board Rosetta

Context. The COmetary Secondary Ion Mass Analyzer (COSIMA) on board Rosetta is dedicated to the collection and compositional analysis of the dust particles in the coma of 67P/Churyumov-Gerasimenko (67P). Aims. Investigation of the physical properties of the dust particles collected along the comet trajectory around the Sun starting at a heliocentric distance of 3.5 AU. Methods. The flux, size distribution, and morphology of the dust particles collected in the vicinity of the nucleus of comet 67P were measured with a daily to weekly time resolution. Results. The particles collected by COSIMA can be classified according to their morphology into two main types: compact particles and porous aggregates. In low-resolution images, the porous material appears similar to the chondritic-porous interplanetary dust particles collected in Earth’s stratosphere in terms of texture. We show that this porous material represents 75% in volume and 50% in number of the large dust particles collected by COSIMA. Compact particles have typical sizes from a few tens of microns to a few hundreds of microns, while porous aggregates can be as large as a millimeter. The particles are not collected as a continuous flow but appear in bursts. This could be due to limited time resolution and/or fragmentation either in the collection funnel or few meters away from the spacecraft. The average collection rate of dust particles as a function of nucleo-centric distance shows that, at high phase angle, the dust flux follows a 1/d2comet law, excluding fragmentation of the dust particles along their journey to the spacecraft. At low phase angle, the dust flux is much more dispersed compared to the 1/d2comet law but cannot be explained by fragmentation of the particles along their trajectory since their velocity, indirectly deduced from the COSIMA data, does not support such a phenomenon. The cumulative size distribution of particles larger than 150 μm follows a power law close to r− 0.8 ± 0.1, confirming measurements made by another Rosetta dust instrument Grain Impact Analyser and Dust Accumulator (GIADA). The cumulative size distribution of particles between 30 μm and 150 μm has a power index of −1.9 ± 0.3. The excess of dust in the 10–100 μm  range in comparison to the 100 μm–1 mm range together with no evidence for fragmentation in the inner coma, implies that these particles could have been released or fragmented at the nucleus right after lift-off of larger particles. Below 30 μm, particles exhibit a flat size distribution. We interprete this knee in the size distribution at small sizes as the consequence of strong binding forces between the sub-constitutents. For aggregates smaller than 30 μm, forces stronger than Van-der-Waals forces would be needed to break them apart