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Characterization of plastic limit surface and bifurcation domain of geomaterials
No full textInternational audienceInstabilities and failure in ductile non associated materials have been widely investigated during last decades especially in the case of geomaterials. It has been shown experimentally that collapse of some specimens can occur strictly within the ultimate plasticity limit, which is experimentally characterized by the maximum shear stress in a drained triaxial test. From a theoretical point of view such instability problems are well described using the so called second order work criterion derived from the Hill's stability analysis (Hill [1958]). Hence a question arises as to the experimental characterization of the ultimate plasticity limit with respect to the choice of stress paths. After a few reminders on Hill's theory, we prove in a general framework that the drained triaxial paths allow to determine with certainty this ultimate plasticity limit without any risk of preliminary bifurcation whatever the elasto-plastic material considered. We conclude that the plasticity limit is only slightly sensitive to variation of the internal state of the material, which can be described by different micromechanical quantities such as the void ratio and the fabric tensor. Furthermore, we define the limit of the bifurcation domain as the surface drawn in the 6-dimensional stress space that delimits the unconditionally stable space from the one where instabilities and failures can occur within the plasticity limit. However, we show that this latter limit is itself very sensitive to the evolution of the internal state of the soil sample
Butterfly factorization with error guarantees
No full textInternational audienceIn this paper, we investigate the butterfly factorization problem, i.e., the problem of approximating a matrix by a product of sparse and structured factors. We propose a new formal mathematical description of such factors, that encompasses many different variations of butterfly factorization with different choices of the prescribed sparsity patterns. Among these supports, we identify those that ensure that the factorization problem admits an optimum, thanks to a new property called ``chainability". For those supports we propose a new butterfly algorithm that yields an approximate solution to the butterfly factorization problem and that is supported by stronger theoretical guarantees than existing factorization methods. Specifically, we show that the ratio of the approximation error by the minimum value is bounded by a constant, independent of the target matrix
Linking Fluorescence Spectroscopy and 31P MRS in NADH Phantoms
No full textGlioblastoma exhibits significant metabolic alterations, making tracking energy metabolism important for its characterization. This could be crucial for glioblastoma resection in neurosurgery. We link two NADH monitoring methods, showing linear dependence on phantom concentrations
Losses Analysis of a Three-Phase Bidirectional Active Split Source Inverter for Traction Applications
No full textInternational audienceIn automotive applications, industrials use DC-DC boosting stages interfaced upstream of the Voltage Source Inverter (VSI) to increase the DC bus voltage. These topologies enable the motors to be driven with higher DC bus voltage than that delivered by the battery. Apparently, the use of extra DC-DC converter decreases the efficiency of the whole system. Recently a single-stage topology called the Split Source Inverter (SSI) was derived. This topology enables the boosting function. Unfortunatly, it exhibits very poor utilization of the DC bus voltage, thus preventing its use for traction applications. In previous work, the authors introduced a new single-stage topology called the Bidirectional Active Split Source Inverter (B-ASSI). It enables the boosting function with bidirectional power flow capability and a large utilization of the DC bus voltage. However, its efficiency was not thoroughly studied and comparison with conventionnal two-stage solution not conducted. This paper proposes an in-depth losses analysis of the B-ASSI. Analytical expression of the currents are proposed and validated by simulation. An extensive experimental validations are carry out on a permanent magnet synchronous machine (PMSM) test-bench. The derived expressions and proposed approadh enables a theoretical comparison of the B-ASSI to two-stage solutions and to the SSI but also giving insights about the design of the converter. This theoretical comparison is performed on a normalized driving cycle. It demonstrates that for the given design constraints the B-ASSI exhibits lower losses than the classical two-stage solution and the SSI for low-speed operating conditions corresponding to urban drive profiles
Hardware Fixed-Point 2D and 3D norms
No full textInternational audienceThis article studies the hardware implementation of the Euclidean norm in 2 and 3 dimensions with fixed-point inputs and outputs. It compares the CORDIC shift-and-add algorithm to a "naive" architecture combining squarers, adders and square root, with a common specification: faithful accuracy. This specification is used in both cases to determine bounds on architectural parameters such as the number of CORDIC iterations and the bit-width of internal data-paths. Several architectural variants of the "naive" architecture are investigated. Their relevance domains are discussed based on synthesis results on FPGAs. For two dimensions, CORDIC has lower area but longer latency than a well-researched naive version. 3D variants of CORDIC, however, are worse than the naive architecture both in area and delay
Méthode de formation de nanoparticules à la surface de substrats
No full textLa présente invention se rapporte à un procédé de fonctionnalisation de surface permettant de générer des nanoparticules à la surface d'un matériau. En particulier, ces nanoparticules ont une composition chimique différente de celle du matériau traité, et elles présentent un ancrage mécanique dans le matériau de base. Le traitement obtenu est localisé tant en termes de profondeur du matériau concerné qu'en position sur la surface traitée.</div
Density-valued symplectic forms from a multisymplectic viewpoint
No full textWe give and intrinsic characterization of multisymplectic manifolds that have the linear type of density-valued symplectic forms in each tangent space, prove Darboux-type theorems for these forms and investigate their symmetries
A numerical model of coupled phloem-xylem flows for dynamic long-distance transport in trees
No full textInternational audienceIn trees, the vascular system is dual in structure and function. Sap flows upwards in the xylem to hydrate tissues and refill reserves from transpiration loss, in the leaves, some of the water recirculates into the phloem and sap, loaded with photoassimilates, flows downwards to supply tissues with carbohydrates. Flow and counter-flow occur in physically separated but hydraulically connected pathways. Water exchanges take place all along them. Understanding the entire system and its subprocesses is essential to precisely quantify the carbon-water fluxes at the soil-atmosphere interface. That understanding is also key to predict the functional limits of vascular transport and if dysfunction occurs, how it will impact the vitality of plant communities in response to drought events and foliar pathogen outbreaks. Here we present an integrated, spatially explicit model of phloem-xylem transport. The model implements Münch’s osmo-regulated pressure flow hypothesis for phloem transport and cohesion-tension for xylem transport. The evolution of phloem pressure, carbohydrate concentration and xylem pressure are governed by three coupled nonlinear partial differential equations. Sap flow velocity, volume flow and the amount of shrinkage and swelling are calculated as derived variables. The model uses a special-purpose numerical scheme and can simulate response to dynamic forcing such as the diurnal patterns of phloem loading and transpiration. Unlike in other models, transport equations are solved for a surface and account for tangential movement of water and carbohydrates. Phloem and xylem are treated as elasto-porous media with distributed hydraulic and mechanical properties. We also present a semi-automated image processing method to calculate the theoretical hydraulic conductivity of phloem and xylem tissues based on their anatomy. Key hydraulic characteristics are given for the phloem of juvenile Pinus radiata D. Don
Human cortical bone intrinsic permeability distribution based on 3D canalicular morphology
No full textInternational audienceBone permeability is a key parameter that drives osteocyte-based mechanobiological modelling and remodelling. While previous experimental and numerical studies have estimated bone permeability based on the morphology of the lacuno-canalicular network, these studies often relied on simplified geometries. In the current study, bone permeability was characterized using more realistic canalicular geometry for the morphological data. Bone samples harvested from 27 human femoral bones were investigated using synchrotron radiation-based nanocomputed tomography with a voxel size of 100 nm. After segmenting the canaliculi and lacunae, each canaliculus was investigated individually by applying a distance map and watershed algorithms. Bone permeability based on canalicular morphology was then assessed using the Kozeny relation, which defines the permeability of a porous medium with capillary-like pores. An averaged intrinsic permeability value of 8.8 10 -18 m 2 was obtained. It should be noted that this study considered an empty canalicular network, however in vivo, both cellular and pericellular matrices decrease space for interstitial fluid flow and thus permeability. Furthermore, a voxel size of 100 nm does not allow for the detection of smaller canaliculi, which may also modify average permeability. With the current data set and the analytic process applied, the results showed a heterogeneous permeability distribution within bone tissue, both when comparing osteonal and interstitial tissues and within an individual osteon. A difference was observed between male and female samples, and permeability appeared to significantly decrease with age. Finally, a significant correlation was found between permeability and canalicular length density, defined as canalicular length per unit bone volume. This study proposes a new form of the Kozeny law to express bone canalicular permeability as a proportional relationship with the canalicular length density. Importantly, this parameter can be directly quantified through confocal fluorescence microcopy, which is more convenient than synchrotron radiation-based nano-computed tomography. In conclusion, the current study confirms that confocal microscopy can be serve as a reliable tool to estimate bone permeability. However, the permeability values calculated here are solely based on canalicular morphology and do not consider cellular and peri-cellular intra-canalicular features