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Multi-scale experimental deformation and damage initiation of clay-rich rocks: Coupling ultrasonic wave propagation and full field deformation measurements by digital image correlation (DIC)
No full textInternational audienceUnderstanding the damage processes in clay-bearing rocks is a decisive factor in geological engineering, and for instance considering nuclear waste deep geological repositories. But, more generally they may also contribute to localized deformation, and thus the rupture of fault gauges in seismic zones. However, owing to their complex mineralogy, multiscale microstructures and anisotropy, the mechanisms of clay-rich rock damage and their chronology are not yet well understood. Here we focus on the impact of micro-damage on ultrasonic wave propagation velocity, which is confronted with the corresponding full deformation fields calculated by digital image correlation (DIC). The aim is to associate the acoustic signature with the active deformation mechanisms identified by DIC. To this end, an integrated experimental approach is proposed to characterize localization and to identify the related deformation micro-mechanisms during uniaxial compression of natural clayey rock samples (Tournemire shales) with two simultaneous measurements: (i) the evolution of P-wave velocity within the sample by active acoustics, (ii) the development of the 2D mechanical full field by digital image correlation. Both experimental techniques are well known, but the innovation of our approach is to combine simultaneously both measurements. Deformation localization is a multiscale problem, which obviously occurs at the sample scale, but also at the fines scales of the microstructure. Therefore, we developed two different experimental setups. On the one hand, during uniaxial compression with a standard MTS loading frame the macro-scale localization patterns are characterized by optical observations, which image resolution is well suited to the cm sample scale (sample diameter: 3.6 cm and double in length). On the other hand, in order to characterize the initiation of micro-damage at the microstructure scale of the composite type of rock, the same loading protocol is reproduced (while keeping the acoustic diagnosis) on smaller scale mm-sized specimens (sample diameter : 8 mm, double in length), using a home-designed miniature loading frame fit for an environmental scanning electron microscope (ESEM). The latter analysis is carried out under controlled relative humidity of RH = 80%, hence preventing the samples to dry out due to the high vacuum Damage processes measured by digital image correlation (DIC) are usually represented over time as a series of deformation maps. This method is well suited to identify major damage events (macro-cracks). However, these highly localized events have little impacts on acoustic wave propagation velocity, unlike densely distributed micro-cracks. Therefore, we propose a method for analyzing the same data, to our knowledge original, revealing the temporal evolution of all deformation scales. A single map summarizes the entire temporal process with deformation classes as x-axis and time as y-axis. We call this representation the “Chronogram of Strain Distribution" (CSD). This projection has a number of advantages: (i) it removes micro-deformations from the ground noise of DIC; (ii) it explicitly links volumetric integration, inherent to Vp wave velocity measurement, to the full deformation field; (iii) it keeps the chronology of the different deformation process
Memory-Scalable and Simplified Functional Map Learning
No full textInternational audienceDeep functional maps have emerged in recent years as a prominent learning-based framework for non-rigid shape matching problems. While early methods in this domain only focused on learning in the functional domain, the latest techniques have demonstrated that by promoting consis-tency between functional and pointwise maps leads to significant improvements in accuracy. Unfortunately, existing approaches rely heavily on the computation of large dense matrices arising from soft pointwise maps, which compromises their efficiency and scalability. To address this limitation, we introduce a novel memory-scalable and efficient functional map learning pipeline. By leveraging the specific structure of functional maps, we offer the possibility to achieve identical results without ever storing the pointwise map in memory. Furthermore, based on the same approach,we present a differentiable map refinement layer adapted from an existing axiomatic refinement algorithm. Unlike many functional map learning methods, which use this algorithm at a post-processing step, ours can be easily used at train time, enabling to enforce consistency between therefined and initial versions of the map. Our resulting approach is both simpler, more efficient and more numerically stable, by avoiding differentiation through a linear system, while achieving close to state-of-the-art results in challenging scenarios
Intergranular stress and plastic strain formation during laser scanning of an additively manufactured stainless steel: an experimentally-driven thermomechanical simulation study
No full textInternational audienceA novel experiment-driven modelling and simulation approach is proposed to study the formation of intergranular stresses and plastic strains during laser scanning of additively manufactured 316L stainless steel. Recently, laser scanning experiments were performed using a unique coupling between a continuous-wave laser and a scanning electron microscope. These experiments form the basis for the development of a thermo-elasto-viscoplastic polycrystal model endowed with the minimum necessary physics of the problem in order to facilitate simulating sufficiently large domains to obtain reasonable stress magnitudes. The case of a single laser line scan performed in vacuum is analyzed in detail. Polar dislocation density magnitudes and distribution predicted from the simulation are compared with those measured experimentally. Results reveal that for 93% of the grains in the lasered zone, a statistical measure of the predicted polar dislocation density (Nye’s) tensor lies within a factor of 2 (higher or lower) of the experimental one; this result sets a benchmark for future experiment-simulation comparisons. Subsequent investigation studies the origin of the local residual stresses and plastic strains. On the lasered surface, the grain surface-averaged normal residual stress component and plastic strain component along the scan direction reaches a value and 0.04, respectively. The contribution of elastic anisotropy, plastic heterogeneity and strengthening due to microstructure refinement after laser scanning on the formation of stresses and plastic strains is studied in detail
Compression with Exact Error Distribution for Federated Learning
No full textInternational audienceCompression schemes have been extensively used in Federated Learning (FL) to reduce the communication cost of distributed learning. While most approaches rely on a bounded variance assumption of the noise produced by the compressor, this paper investigates the use of compression and aggregation schemes that produce a specific error distribution, e.g., Gaussian or Laplace, on the aggregated data. We present and analyze different aggregation schemes based on layered quantizers achieving exact error distribution. We provide different methods to leverage the proposed compression schemes to obtain compression-for-free in differential privacy applications. Our general compression methods can recover and improve standard FL schemes with Gaussian perturbations such as Langevin dynamics and randomized smoothing
Martian CO profiles from the solar occultation experiment of NOMAD on board TGO
No full textInternational audienceCarbon monoxide is one of the important minor species in the Martian atmosphere due to its role in the photochemical stability of the \ce{CO2} atmosphere and can also be used as a dynamical tracer. The SO spectrometer onboard the Trace Gas Orbiter (TGO) scans the Martian limb in the infrared provides transmittances with fine vertical sampling (km). In the spectral region, the sounding of \ce{CO} is found to be reliable due to its strong and well separated absorption lines. Here, we present the retrieval scheme for \ce{CO} from the solar occultation observation. Our scheme obtains density profiles up to 100 km with a vertical resolution better than km and errors below . The observations for the last two seasons of Mars Year 34 (MY34, April 2018 - March 2019) are analyzed here. We found important results such as a strong depletion of CO density during the global dust storm (GDS) and a clear dynamical influence of global Hadley circulation on the CO distribution
Learning to Mitigate Externalities: the Coase Theorem with Hindsight Rationality
No full textInternational audienceIn economic theory, the concept of externality refers to any indirect effect resulting from an interaction between players that affects the social welfare. Most of the models within which externality has been studied assume that agents have perfect knowledge of their environment and preferences. This is a major hindrance to the practical implementation of many proposed solutions. To address this issue, we consider a two-player bandit setting where the actions of one of the players affect the other player and we extend the Coase theorem [Coase, 1960]. This result shows that the optimal approach for maximizing the social welfare in the presence of externality is to establish property rights, i.e., enable transfers and bargaining between the players. Our work removes the classical assumption that bargainers possess perfect knowledge of the underlying game. We first demonstrate that in the absence of property rights, the social welfare breaks down. We then design a policy for the players which allows them to learn a bargaining strategy which maximizes the total welfare, recovering the Coase theorem under uncertainty
"Animal souffrant, animal soignant dans la peinture du Siècle d'Or (Espagne, Pays-Bas)"
No full textInternational audienceSuffering animal, animal as healer in the Painting of the Golden Age (Spain, Netherlands) During the XVIIth century, in Spain and the Netherlands (both Southern and Northern), the artists played a significant role in representing suffering animals and those providing care or healing. In numerous paintings, images of suffering, wounded, dead or dying animals as well as those of animals with healing powers -often linked to Christian Symbolism- can be associated with concepts of care serving human health.Au cours du XVIIe siècle, en Espagne et aux Pays-Bas (Méridionaux et du Nord), la peinture a joué un rôle important dans la représentation de l’animal souffrant et de l’animal soignant ou guérisseur. Dans de nombreux tableaux, les images d’animaux souffrants, blessés, morts ou agonisants, tout comme celles d’animaux guérisseurs aux propriétés curatives -images souvent liées à la symbolique chrétienne-, peuvent être associées à des notions de soin au service de la santé des hommes
Short Paper - Quadratic minimization: from conjugate gradient to an adaptive Polyak’s momentum method with Polyak step-sizes
No full textInternational audienceIn this work, we propose an adaptive variation on the classical Heavy-ball method for convex quadratic minimization. The adaptivity crucially relies on so-called “Polyak step-sizes”, which consists of using the knowledge of the optimal value of the optimization problem at hand instead of problem parameters such as a few eigenvalues of the Hessian of the problem. This method happens to also be equivalent to a variation of the classical conjugate gradient method, and thereby inherits many of its attractive features, including its finite-time convergence, instance optimality, and its worst-case convergence rates.The classical gradient method with Polyak step-sizes is known to behave very well in situations in which it can be used, and the question of whether incorporating momentum in this method is possible and can improve the method itself appeared to be open. We provide a definitive answer to this question for minimizing convex quadratic functions, an arguably necessary first step for developing such methods in more general setups
Advanced Methods for Analyzing in-Situ Observations of Magnetic Reconnection
No full textInternational audienceThere is ample evidence for magnetic reconnection in the solar system, but it is a nontrivial task to visualize, to determine the proper approaches and frames to study, and in turn to elucidate the physical processes at work in reconnection regions from in-situ measurements of plasma particles and electromagnetic fields. Here an overview is given of a variety of single- and multi-spacecraft data analysis techniques that are key to revealing the context of in-situ observations of magnetic reconnection in space and for detecting and analyzing the diffusion regions where ions and/or electrons are demagnetized. We focus on recent advances in the era of the Magnetospheric Multiscale mission, which has made electron-scale, multi-point measurements of magnetic reconnection in and around Earth's magnetosphere
Preparation and Ground-State Electronic Structure of Heterobimetallic Yb–Pt IV -Alkyl Complexes
No full textInternational audienceThis article focuses on the synthesis of heterobimetallic complexes of lanthanide and platinum. It describes the synthesis of the Cp*Yb(bipym)PtMe2 complex and its characterization, followed by its reactivity with oxidants, giving access to various Pt +IV compounds of trismethyl (PtMe3) and tetramethyl (PtMe4) fragments. Characterization of the electronic properties of the complexes by magnetic measurements demonstrated that the tetramethyl complex possesses a singlet ground state. The trismethyl fragments, on the other hand, have a ground state that evolves as a function of the ligand saturating the coordination sphere: singlet for triflate and pyridine and triplet for iodine, demonstrating the capacity for simple tuning of the electronic structure of these complexes. While the addition of B(C6F5)3 to the platinum +II bis methyl complex leads to FLP-like reactivity triggering THF opening, reactivity with [Ph3C] + [BPh4] -leads to oxidation of the bipym ligand. Furthermore, the light reactivity of the tetramethyl complex indicated the possible transfer of a methyl group, leading to the functionalization of the bridging bipym ligand. Associated ContentSupporting Information. Details on the solution NMR and X-ray crystallography (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. The crystal structures of 2 to 6, 5 + (py) OTf - and 2 + BPh4 -have been deposited in the CCDC with #2369155-2369161</div