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Structural characterization of iron oxide grown on 18% Ni-Co-Mo-Ti ferrous base alloy aged under superheated steam atmosphere
No full textInternational audience18% Ni-Co-Mo-Ti Ferrous base alloys are special materials, widely used in the industry of isotopic enrichment after specific annealing and aging thermal treatment. The desirable high mechanical properties can then be attained by adequate aging heat treatment, answering the structural materials specifications required by defense applications in aerospace and nuclear engineering. For instance, the isotopic enrichment, in rocket engine envelope application, when associated with high temperature and chemical residues like acidic solutions, can induce corrosion and hydrogen embrittlement in martensite structures. To limit these corrosion and hydrogen embrittlement phenomena, an adherent and protective layer of iron oxides can be grown on the material surface by using adequate atmosphere during the aging treatment. Due to its application in strategic areas, the characterization of these oxide layers in maraging steels is of importance as well as the understanding of their growth kinetics. For this purpose, several techniques, such as Optical Microscopy (OM), Scanning Electron Microscopy (SEM), Glow Discharge Optical Emission Spectroscopy (GDOES), Microabrasive wear testing, Hardness, Grazing Incidence X-ray Diffraction (GIXRD) and X-ray Photoelectron Spectroscopy (XPS), have been performed for chemical and structural characterization of the oxide films formed after vapor exposed thermal aging at 510°C . The oxide layer consists mostly in two sub-layers composed by magnetite (Fe3O4) and an external layer of hematite (Fe2O3). A thick interface between the oxide layer and the bulk is enriched in Ti and Mo, whereas the analyses of deep bulk material show an enriched area with Ni and Co
Quantification of mechanical fields inside invasive cancer cells: Description of a new method using Digital Volume Correlation and Confocal Microscopy
No full textCell invasion process, which appears in the progression of tumors, such as glioblastoma, is highly dependent on cellular mobility. Cellular movement results from the interaction of chemical, biological and mechanical factors both inside and outside the invasive cancer cell. To identify and understand the relationship between these factors, it is necessary to quantify and visualise the extra- and intracellular kinematic fields during cell movement. This study proposes a new methodology for the experimental measurement of full kinematic fields inside cancer cells and the use of a digital twin simulation of the cell to obtain the stress and force fields. This approach combines confocal microscopy, Digital Volume Correlation (DVC), and the Finite Element Method (FEM) to achieve precise and comprehensive measurements. To demonstrate the efficiency of this methodology, highly invasive cells from human glioblastomas have been used as a model, and their mechanical behaviour on a substrate was studied over time during their development
Embrittlement after high-temperature exposure to air of a near-γ TiAl alloy
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Concept of a simple conductive thermal transistor based on an oxide phase change material
No full textInternational audienceWe theoretically investigate the behaviors of the main features of a simple conductive thermal transistor (CTT) concept. The two operating modes, amplification and switching, of the CTT are achieved using a solid oxide phase change material (PCM) that shows a metal–insulator transition and examined in detail by individually changing various parameters. We model the CTT by exploiting the properties of one of the promising PCMs, which is VO2. We found that the ratio of the thermal conductances between the states of the PCM and the stiffness of the phase transition greatly affect the amplification. Modifying the stiffness of the phase transition also allows modifying the operating temperature range of the amplification mode to some extent. Besides, we observed that depending on the temperatures of the collector, the emitter and the phase transition temperature of the PCM, a bistability regime occurs and this allows the CTT to operate in the switch mode. Therefore, the CTT can operate in both amplification and switch modes depending on the temperatures of its three elements (collector, base and emitter). The CTT provides the highest amplification before it transitions to the switch mode. Conversely, its switching efficiency decreases to the lowest value before it transitions to the amplification mode. Moreover, we derived analytical expressions, which are generally applicable to a CTT similar to the one we investigate in this study and its base element consists of solid-oxide PCMs showing similar phase transition characteristics with VO2, to determine the base temperature resulting in the highest amplification factor and calculate the maximum amplification factor. We analytically demonstrate that the base temperature that induces the highest amplification factor is always less than the phase transition temperature, but lies in its vicinity
Secondary telephone cord buckle in thickness gradient and tapered delamination
No full textWe investigated the formation of telephone-cord buckling patterns in stressed nickel thin films deposited by ion beam sputtering on polycarbonate substrates with a controlled thickness gradient. Initially, the application of an external compressive stress induced straight-sided buckles in the nickel films. Upon releasing the applied stress, these buckles transformed into telephone-cord patterns oriented parallel to the thickness gradient, exhibiting stripe widths that varied accordingly. Experimental observations revealed that both the wavelength and width of the telephone-cord buckles decreased with decreasing film thickness. To further understand this behavior, finite element method simulations were performed to systematically study the effects of film thickness, stripe-width gradients and internal stress on the buckle wavelength. The numerical results are presented and compared with experimental data, showing good agreement and providing deeper insight into the mechanics of gradient-induced buckling in thin films
Non-Intrusive Potential Measurement in Charged Dielectric Surfaces with a Capacitive Traveling Sensor
No full textElectrostatic phenomena are among the most feared phenomena in the industry. Under the effect of mechanical, chemical and/or thermal mechanisms, electrical charges are generated and locally accumulated on the surface of materials, such as the dielectrics. The electric fields induced by these charges can reach the dielectric breakdown of the medium, leading to partial or full discharges which can cause an early aging and weaken of dielectrics. Quantifying, locating and/or mapping these charge accumulations remains a very important issue in the prevention of electrostatic hazard. In this context, a specific capacitive sensor was developed in order to measure a spatial charge distribution within the dielectric surface during a longitudinal traveling displacement. The main objective of this work is to define the feasibility and limitations of this experimental technique for a stair-shaped potential distribution at a solid dielectric surface. The study of the several parameters (traveling speed, electrode width and gap) was carried out and allowed to obtain the best compromise between the sensitivity and the resolution of the measurement
Hybrid Supervised Learning and Constrained Optimization for Task-Space Trajectory Generation in Tendon-Driven Continuum Robots
No full textInternational audienceTendon-driven continuum robots (TDCRs) offer exceptional dexterity and compliance, making them suitable for manipulation in constrained and complex environments. However, solving the inverse-kinematics (IK) problem with sufficient accuracy and physical feasibility remains challenging due to nonlinear curvature coupling and actuation uncertainties. This paper presents a hybrid inverse-kinematics framework that integrates supervised deep learning with constrained optimization to achieve precise and physically consistent task-space trajectory tracking. Neural networks provide fast initial pose-to-configuration estimates, which are subsequently refined through constraint-aware optimization, ensuring geometric and actuation feasibility. The training data were generated from optimal IK solutions obtained via constrained optimization across sixteen trajectory-constraint combinations, covering four representative trajectories circular, elliptical, helical, and butterfly, and four end-effector orientation modes. Extensive simulations on a two-segment tendon-driven continuum robot demonstrated statistically significant improvements (p ≤ 0.01) in both position and orientation accuracy compared with standalone neural or optimization-based approaches. The hybrid method achieves micrometerlevel positional accuracy and micro-degree-level orientation precision while maintaining computational efficiency suitable for real-time applications. The present work focuses on a quasi-static modeling and trajectory-generation framework. Future work will extend constraint handling to include dynamic obstacle avoidance and smoothness optimization, explore global solvers such as genetic algorithms and particle swarm optimization, and address sim-to-real transfer through adaptive learning for multi-segment robots in realistic environments
Morphological properties of random arrays of infinitely long overlapping cylinders for modeling statistically homogeneous and isotropic fibrous media
No full textInternational audienceIn this article, we examine random disordered arrays of infinitely long cylinders of revolution, all with the same diameter and allowed to overlap. These arrangements can be considered as simple representations of fibrous media, which are widely used in the field of high-performance thermal insulation. In a cylinder of revolution of infinite length, the expression of the cumulative distribution function of internal i-random chord lengths Fi(s) is given in terms of elliptic integrals, and some notable properties are highlighted. It is shown that there is a link between Fi(s) and the sphere / solid cylinder intersection area. A statistically homogeneous and isotropic arrangement of infinite cylinders is then studied. An algorithm for generating such a medium is reviewed and the resulting elementary properties are given. Finally, a study in the field of stochastic geometry makes it possible to determine an analytical expression for the autocorrelation function (two-point probability function) inside the solid phase and, more generally, for the number of overlaps probability function. This makes it possible to deduce, in the Laplace domain, an expression for the cumulative distribution function of i-random chord lengths Fi(Π, s) within the solid phase for any porosity Π. The autocorrelation function also allows one to define a Representative Elementary Volume (REV) related to local fluctuations in porosity
Conductive thermal diode based on vanadium dioxide under dynamical boundary conditions
No full textInternational audienceWe conduct a large-scale case study for a conductive thermal diode that is made of VO2 and SiO2 by considering the full characteristics of VO2, including the hysteresis zone and by applying dynamical boundary conditions. The study extends the analysis by examining the role of the capacitive effect, the influence of which is also analyzed separately in detail by changing various parameters. We find out that the phase transition of VO2 significantly affects the thermal rectification factor R of the conductive thermal diode. Moreover, the capacitive effect has a significant impact as there is even more pronounced variations on R when the heat capacity is temperature-dependent. We also see that as the frequency of the applied boundary conditions increases, R oscillates more and its maximum value increases