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Développements expérimentaux pour l’analyse et l’optimisation de l’assistance CO2 supercritique en usinage
No full textWith the advent of increasingly difficult-to-machine materials and the requirements of REACH regulations to move toward more environmentally friendly lubrication solutions, this study focused on machining with supercritical CO₂ (scCO₂).The first chapter presents wear tests in milling, showing tool life improvements with scCO₂ assistance compared to emulsion-based cooling. In the second chapter, various lubricants (ionic liquids, vegetable oils, water, mineral oils) were combined with scCO₂ to evaluate, through tribological tests, the potential of new scCO₂-based mixtures. The greatest reductions in friction coefficients were observed for scCO₂ + ionic liquid mixtures.In the third chapter, to understand scCO₂ cooling, an optical method called Background Oriented Schlieren (BOS) was used to obtain temperature maps in the jet. This study evaluated the influence of upstream fluid parameters on cooling efficiency at the nozzle outlet. The experimental results contributed to developing an empirical model to predict centerline temperature, based on nozzle diameter, upstream pressure, and fluid temperature.Finally, the last chapter of the thesis presents a test rig designed to study heat transfer coefficients between the jet and a solid. This rig is intended to analyze the influence of nozzle geometry, jet-to-solid distance, and jet orientation relative to the solid on cooling efficiency.All these studies contribute to a better understanding, for an optimized use of the scCO₂ assistance in machining.Avec l’apparition de matériaux de plus en plus difficiles à usiner et la volonté des réglementations REACH d’aller vers des solutions de lubrifications moins nocives pour l’environnement, cette étude s’est intéressée à l’usinage avec CO2 supercritique (scCO2).Le premier chapitre présente des essais d’usure en fraisage montrant des gains en durée de vie avec l’assistance scCO2, en comparaison à une lubrification avec émulsion conventionnelle. Ensuite dans le deuxième chapitre, divers lubrifiants (liquides ioniques, huiles végétales, eau, huiles minérales) ont été combinés avec le scCO2, afin d’évaluer par des essais tribologiques, le potentiel de nouveaux mélanges à base de scCO2. Les plus grandes réductions de coefficients de frottement ont été observées pour les mélanges scCO2 + liquides ioniques.Dans le troisième chapitre, pour comprendre le refroidissement avec le scCO2, une méthode optique nommée « Background Oriented Schlieren (BOS) », a été utilisée pour obtenir les cartographies de températures dans le jet. Cette étude a permis d’évaluer l’influence des paramètres physiques du fluide en amont, sur le refroidissement en sortie de buse. L’ensemble des essais réalisés a contribué à la construction d’un modèle empirique permettant de prédire la température le long de l’axe du jet en fonction du diamètre de la buse, de la pression et de la température du fluide en amont.Enfin, le dernier chapitre de la thèse présente un banc d’essai conçu pour étudier les coefficients d’échanges thermiques entre le jet et un solide. Ce banc est destiné à analyser l’influence des géométries de buse, de la distance jet-solide, et de l’orientation du jet par rapport au solide, sur l’efficacité du refroidissement.Toutes ces études ont pour but de poser les bases nécessaires pour une compréhension approfondie, en vue d’une utilisation optimisée de l’assistance scCO2 en usinage
Fundamental mechanisms of discontinuous deformation in metals for cryogenic-environment applications
No full textInternational audienceMetallic materials are known to exhibit low-temperature discontinuous deformation (i.e., low-temperature serrated deformation, LTSD) at cryogenic temperatures, which can lead to sudden failures or catastrophic accidents. Therefore, understanding LTSD is crucial for ensuring material stability and reliability in a cryogenic environment. Thus far, the widely accepted explanations for the origins of LTSD can be categorized into two mechanisms: (i) dislocation-based mechanical instability and (ii) thermomechanical instability. However, interpreting LTSD using each theory independently has limitations in clearly elucidating the LTSD mechanism. Therefore, the current understanding of LTSD remains insufficient and is still subject to debate because it is challenging to prove experimentally. To address this issue, we suggest a novel LTSD mechanism, namely a thermally induced dislocation dynamics model, based on the experimental evidence that considers both the dislocation dynamics and thermomechanical characteristics at cryogenic temperatures. Furthermore, we present a modified deformation-mechanism map of a SS316L that incorporates the newly proposed LTSD mechanisms. The origin of LTSD is considered in the unique framework of dislocation behavior under severely limited thermal-vibration energy at cryogenic temperatures, leading to the dislocation avalanches and development of hierarchical dislocation networks, including multiple lattice defects. Therewith, the localized heating generated from dislocation avalanches induces multiple types of LTSD and gives rise to transitions from the heterogeneous to homogeneous deformation. Our findings highlight the rate-dependent nature of LTSD and negative strain-rate sensitivity in the strength-elongation relationship and include the first observation of changes in small stress fluctuations and their relationship to the changes in larger serrations
Exploring Dynamic Brain Oscillations in Motor Imagery and Low-frequency Sound
No full textInternational audienceAlthough both motor imagery (MI) and low-frequency sound listening have independently been shown to modulate brain activity, the potential synergistic effects that may arise from their combined application remains unexplored. Any further modulation derived from this combination may be relevant for motor learning and rehabilitation. We probed neurophysiological activity during these two processes, measuring alpha and beta band power amplitude by means of EEG recordings. Twenty healthy volunteers were instructed to (i) explicitly imagine right finger flexion/extension movements in a kinaesthetic modality, (ii) listen to low-frequency sounds, (iii) imagine right finger movements while listening to low-frequency sounds, or (iv) stay at rest. We observed a bimodal distribution of alpha-band reactivity to the conditions, suggesting the presence of variability in brain activity across participants during both MI and low-frequency sound listening. One group of participants (12 individuals) displayed increased alpha power within contralateral sensorimotor and ipsilateral medial parieto-occipital regions during MI. Another group (eight individuals) exhibited a decrease in alpha and beta band power within sensorimotor areas. Interestingly, low-frequency sound listening elicited a similar pattern of brain activity within both groups. The combination of MI and sound listening did not result in additional changes in alpha and beta power amplitudes, regardless of group (groups based on individual alpha-band reactivity). Altogether, these findings shed significant insight into the brain activity and its variability generated during MI and low-frequency sound listening. The simultaneous engagement of MI and low-frequency sound listening did not further modulate alpha power amplitude, possibly due to concurrent cortical activation. It remains possible that sequential performance of these tasks could elicit additional modulation
Scale‐by‐scale budget of turbulence kinetic energy in the convective atmospheric boundary layer: Analysis of structure functions
No full textInternational audienceWe investigate the scale‐by‐scale budget of turbulence kinetic energy in convective atmospheric boundary layers using airborne measurements performed in the shallow trade‐wind regime over the ocean by the ATR 42 aircraft during the Elucidating the Role of Cloud–Circulation Coupling in Climate (EUREC4A) measurement campaign. The simple and repeatable flight pattern sampled four altitude levels: near‐surface, the middle and top of the subcloud layer, and the cloud base. This approach provides acceptable statistical convergence, including mixed velocity–temperature and third‐order velocity structure functions. The scale‐by‐scale budget obtained from the data is approximately closed up to length‐scales of about 200 m at all four levels. At the near‐surface and mid‐subcloud levels, the buoyancy forcing supplies energy to the largest scales and becomes negligible at smaller scales. As a result, Kolmogorov equilibrium is observed over a wide range of scales, from a few to about 100 m. On the other hand, at the top‐subcloud and cloud‐base levels, the budget at scales above 10 m is far from Kolmogorov equilibrium, with a significant contribution of buoyancy forcing and turbulent transport. The contribution to the buoyancy forcing related to humidity variations is substantial and strictly positive at all four levels, even at the cloud base, where it balances the negative contribution related to temperature only
Formulation and 3D Printing of PVDF-Containing Photocurable Resins for Digital Light Processing
No full textInternational audienceAdditive manufacturing of electroactive polymers offers transformative potential for flexible electronics and smart devices, yet preserving the microstructure responsible for the electroactive property during processing remains a challenge. Here, we report a digital light processing (DLP) approach formulated without any volatile organic solvent to prepare poly(vinylidene fluoride) (PVDF)-based composites under ambient conditions, employing 1,6-hexanediol dimethacrylate (HDDMA) as a polymerizable matrix and phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (BAPO) as an efficient visible-light photoinitiator. Unlike conventional solvent-based methods relying on PVDF dissolution, this formulation enables direct dispersion of PVDF particles in the photocurable resin without the use of organic solvents that are typically used in the processing of PVDF. Formulation optimization enabled stable suspensions of PVDF up to 35 wt %, with rheological and optical properties leading to high-fidelity DLP printed samples. Atomic force microscopy (AFM) of cross sections of the 3D printed sample revealed uniform dispersion of PVDF-rich domains. Comprehensive characterization of the 3D printed sample using differential scanning calorimetry (DSC), infrared spectroscopy (IR), and X-ray diffraction confirmed the retention of the pristine PVDF's semicrystalline phases postprocessing. Preprinting modification of the PVDF and postprinting modifications of the 3D printed composite were conducted to confirm this observation. For instance, solvent-precipitated PVDF with enhanced β phase fraction, which is often associated with electroactivity, was used in the formulation without phase degradation during photopolymerization and postprint annealing of the 3D printed composite provided additional phase tuning, underscoring the versatility of this approach. This work establishes DLP as a robust platform for the additive manufacturing PVDF-based composites, allowing for precise control and retention over crystalline phase content and complex architectures, potentially relevant for electroactive applications in next-generation flexible electronics
Effects of Shot‐Blasting and Induction Hardening on the Fatigue Strength of Hot‐Forged Wheel Bearing Hubs
No full textInternational audienceHot-forged wheel bearing hubs are manufactured using a complicated fabrication process chain to obtain the desired microstructure and mechanical properties in different zones that experience varying local stress states. The different process steps modify the surface roughness and the residual stresses distribution and therefore affect the fatigue behavior of the component. This study focuses on the effects of the shot-blasting and the induction hardening stages on the fatigue failure mechanisms and aims to propose an appropriate fatigue design method. Fatigue tests are performed on industrial components and on conventional material specimens to evaluate the influence of the surface roughness and the residual stresses on the fatigue strength. The relaxation of residual stresses, caused by the heat treatment and the cyclic loading during fatigue testing, is shown to annihilate the positive effect of shot-blasting, and the fatigue strength decreases because of the detrimental surface roughness, created by shot-blasting. Different multiaxial fatigue criteria are tested to take into account the manufacturing process operations
Integration of piezoelectric transducers in hydrofoils made of composite materials
No full textInternational audienceBoat appendices, known as hydrofoils, can be subjected to significant dynamic stress during navigation at high Reynolds (Re > 10^5). The turbulent nature of the flow as well as vortex shedding at the trailing edge generate structural vibrations which couples with the resonant modes of the foil. These phenomena, turbulence induced vibration (TIV) and vortex induced vibrations (VIV), leads to undesirable structural fatigue and radiated noise. This study describes the integration of piezoelectric transducers in hydrofoils manufactured in composite material made of carbon fiber and epoxy resin. Macro Fiber Composite (MFC) patches are used to sense and possibly control bending and torsional vibrations induced by the flow. Two hydrofoils with the same external dimensions but obtained with dierent manufacturing processes are considered. The external shape corresponds to a NACA 006 truncated at 80% with a 100 mm chord and 191 mm wingspan. The internal geometries and the arrangement of the composite plies are described for both manufactured hydrofoils. In addition, their respective finite element (FE) models are built according to the manufacturing specifications. Particular attention is paid to the modeling of the piezoelectric transducers and the associated electromechanical coupling. Indeed, the FE models have been created to perform modal analyses of the hydrofoils in air and in water and then optimize coupling factors between the transducer and the structures. To do so, two types of configurations are tested: one with the MFC transducer disabled (short-circuit) and a second with the MFC transducer activated (open-circuit) in order to compute the natural frequencies of the structure in both configurations. The coupling factor, directly related to the dierence between these two natural frequencies, then depends on the location of the MFC on the foil span. The next step in this study is to experimentally validate the coupling factors computed by the FE model. This work is part of the HYDRAVIB project, which aims to develop a hydrofoil vibration mitigation systems based on the integration of piezoelectric transducers
Toward an incremental Anderson-Darling algorithm for drift detection on cable-driven robots
No full textInternational audienceThis article introduces a drift detection method based on an incremental algorithm for computing the Anderson-Darling hypothesis test statistic. This test makes few assumptions about the monitored data stream and the nature of the occurring drift, while remaining computationally efficient. The detection algorithm is applied to a use case in cabledriven robotics for detecting drift in measurements from tension sensors. Numerical experiments demonstrate the effectiveness of this algorithm in detecting several types of drift
Multi-Scale Study of the Influence of Bonding Temperature on the Diffusion Bonding of Equiatomic CoCrFeMnNi High Entropy Alloy and 316L Stainless Steel
No full textInternational audienceThe microstructural evolution and mechanical characteristics of CoCrFeMnNi high entropy alloy (HEA) joined to 316 L stainless steel (SS316L) were investigated by performing diffusion bonding at temperatures ranging from 975 °C to 1050 °C under a compressive pressure of 5 MPa. The diffusion-bonded samples were examined microstructurally with a Scanning Electron Microscope (SEM) equipped with an Energy Dispersive Spectrometer (EDS), Electron Back Scattered Diffraction (EBSD) and X-ray Diffraction (XRD) analyses, and the mechanical responses of the joints were assessed based on the fracture energy calculation from the shear test results. The CoCrFeMnNi-HEA was successfully joined to SS316L preserving a single face centred cubic (FCC) phase solid solution within the diffusion zone. Increasing the bonding temperature led to widening the diffusion zones and microstructural changes such as grain growth, which became more prominent, particularly for temperatures equal to 1025 °C and beyond. The optimum bonding temperature was observed to be 1000 °C, corresponding to a total diffusion width of 10.5 μm. These diffusion joints were characterised by the moderate presence of interfacial pores and the highest fracture energy
Modélisation musculo-squelettique du tronc et des membres supérieurs : application à l’escrime-fauteuil
No full textThe prevention of musculoskeletal disorders, particularly in the upper limbs, is a crucial health issue for workers, but also for people with disabilities and the elderly. Prevention requires an understanding of the causes. These are multiple and sometimes complex, but the mechanical load at the joints remains one of the first elements to be determined. To this end, musculoskeletal models capable of taking into account the kinematic, dynamic and muscular complexity of the parts concerned (upper limbs and trunk in the context of this work) have been developed and are being continuously improved. From a kinematics point of view, the work developed in this thesis therefore aims to :i) Aggregate the existing models into a single model under a single numerical environmentii) Propose customisation parameters and the associated protocol to determine themiii) Evaluate the personalised models according to their different components, i.e. geometric and kinematic parts.Finally, an application for the prevention of musculoskeletal disorders in para-sportsmen practising para-fencing will be carried out through the study of the dynamic phases of attaque.La prévention des troubles musculosquelettiques, notamment au niveau des membres supérieurs, est un enjeu crucial de santé pour les travailleurs, mais également pour les personnes en situation de handicap, ou encore les personnes âgées. Prévenir nécessite de comprendre les causes. Or celles-ci sont multiples et parfois complexes mais le chargement mécanique supporté par l’articulation reste un des premiers éléments à déterminer. Pour ce faire, des modèles musculosquelettiques capables de prendre en compte la complexité cinématique, dynamique et musculaire des parties concernées (membres supérieurs et tronc dans le cadre de ce travail) ont été développés et sont en continuelle amélioration. En abordant un point de vue cinématique, les travaux développés dans cette thèse s'attachent donc à :i) Agréger les modèles existants dans un même modèle sous un seul environnement numérique) Proposer des paramètres de personnalisation et le protocole associé pour les déterminer) Évaluer les modèles personnalisés suivant les différentes composantes de ceux-ci, i.e. parties géométrique et cinématique. Enfin, une application à visée de prévention des troubles musculosquelettiques chez les para-sportifs pratiquant l’escrime fauteuil sera réalisée à travers l’étude cinématique de l’attaque en coup droit