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Heterogeneous fatigue damage in a nickel-based single-crystal superalloy unraveled using correlative 3D X-ray technology
No full textInternational audienceNickel-based single-crystal (Ni-SX) superalloys under cyclic stress are susceptible to cracking at stress-concentration sites, eventually leading to low-cycle fatigue (LCF) failure. LCF cracks typically originate from intrinsic defects (e.g., voids and carbides) within solidified dendrites. However, systematic quantitative experimental analyses of defect-mediated local damage remain limited. To thoroughly understand the microscopic origins and evolution of LCF damage, correlated 3D mapping of dendrites across various regions is essential. Here, macroscale micro-computed tomography (μ-CT) was initially used to capture internal interdendritic secondary cracks within bulk DD413 superalloy after LCF testing at 760 °C. Subsequently, a multimodal methodology combining synchrotron 3D microdiffraction (3D-μXRD), high-resolution μ-CT, and electron microscopy was established. This approach allowed precise localization of internal damage zones near interdendritic secondary cracks and detailed mapping of the 3D correlated distributions of dendrites, defects, and residual stress/strain fields within these zones at submicron spatial resolution. Finally, the same approach was applied to specimens subjected to interrupted loading at approximately 40 % of the fatigue life to uncover the early damage states of dendrites. The dendrite cores (DCs) and interdendritic regions (IDs) exhibit microscale heterogeneous mechanical responses: nearly defect-free DCs accumulate local irreversible slip along specific slip systems to generate slip bands, while the IDs containing various defects accommodate local microplasticity through the activation of multiple slip systems around these defects. The local tensile stress near defects in the IDs exceeds that in the DC slip band regions by more than threefold, leading to the generation of local damage zones within the IDs. Chain-like defect distributions facilitate the interconnection of these local zones into a continuous damage region, further elevating the overall tensile stress in the IDs. Additionally, geometrically necessary dislocations alone are insufficient as indicators of LCF damage; both the internal stress state and its magnitude must be considered. These experimental results provide critical data and insights for the development of multi-physics fatigue models
Methodology for the Design and Control of a Soft Finger Based on the Mullins Effect and Material Training
No full textThe advancement of soft multi-digital grippers with both adaptive grasping and in-hand manipulation capabilities remains a challenging issue for the development of human-like dexterous manipulation. Despite four decades of research, the most advanced grippers remain encumbered by excessive complexity and a lack of robustness. The field of soft robotics presents a promising avenue for reducing the level of complexity and enhancing the safety of grasping and interaction with the environment. This work presents a methodology for the design and control of a soft finger, with the objective of ultimately developing a highly dexterous gripper. To address these challenges, it is essential tomaster the design, fabrication process, and behavior of the finger’s soft material. The iterative design and fabrication process requires a comprehensive understanding of the theoretical and experimental aspects, as detailed in this paper. Given that the finger is constructed from silicone, the proposed methodology and outcomes demonstrate the importance of accounting for the Mullins effect and conducting finger training prior to controlling the pneumatic soft finger. The proposed hard real-time control architecture guarantees the robustness of the analysis and control of the finger’s behavior, while also offering perspectives for coordinated multi-fingered manipulation
Dynamic behavior of the nucleus pulposus within the intervertebral disc loading: a systematic review and meta-analysis exploring the concept of dynamic disc model
No full textInternational audienceIntroduction: The dynamic disc model (DDM) is a theoretical framework in spine mechanics that theorizes the behavior of the nucleus pulposus within the intervertebral disc under various loads. The model predicts displacement of the nucleus pulposus away from the bending loads, for example backward displacement of the nucleus pulposus with a flexion load. These predictions are regularly used as a theoretical basis for explaining certain disc pathologies, such as disc herniation.Methods: We screened seven databases (CENTRAL, Embase, MEDLINE, CINAHL, ScienceDirect, Google Scholar, and HAL) up to July 2024, identifying studies through a PRISMA-guided approach that detailed the mechanical transformation (displacement and deformation) of the nucleus pulposus under bending load on the intervertebral disc. We conducted a double-blind data extraction and quality assessment of the body of evidence. Finally, we performed a meta-analysis of proportions.Results: From the 9,269 articles screened, 14 studies were included in the systematic review and meta-analysis. Magnetic Resonance Imaging (MRI) was employed in 92.8% of the studies, revealing four strategies for assessing nucleus pulposus transformation. The meta-analysis of asymptomatic subjects’ data demonstrated that the nucleus pulposus behavior aligned with dynamic disc model predictions in 85.4% (95% CI = [79.4–91.4]) across spinal regions and bending directions. However, significant heterogeneity and low study quality were noted. Only one study used discography to assess the DDM in a discogenic pain population, identifying discrepancies in nucleus pulposus transformation and contrast agent leakage.Conclusion: Evidence for the dynamic disc model for intact discs is of low strength, whereas very limited evidence challenges the dynamic disc model for fissured discs. New multiparametric MRI studies may help guiding future clinical assessment protocols.Systematic Review Registration: CRD42022331774
Model identification of a cable-based mechanical transmission for robotics using a Best Linear Approximation approach
No full textMechanical properties of hydrogen-passivated silicon and silicon carbide nanoparticles
No full textInternational audienceUnlike the perfect models often used in numerical simulations, real nanoparticles (NPs) are usually characterized by an oxidized or passivated surface, whose the effect on mechanical properties is not well known. In the present work we perform first principles molecular dynamics calculations to simulate the flat punch compression of small hydrogen passivated silicon and silicon carbide NPs. They reveal that the NPs yield at high strains and preferentially by amorphization. Small rotations are often observed before yielding. Our investigations suggest that these rotations are favored by the presence of the hydrogen passivated layer. Another consequence is a notable reduction of stiffness, due to the lower bending strength of Si/C–H bonds compared to the compression strength of the Si/C lattice. At last it is found that the amorphization of silicon carbide is facilitated by the presence of the hydrogen passivated layer
A Soft Variable Stiffness Actuator with a Chain Mail Structure as a Particle Jamming Interface
No full textInternational audienceVariable stiffness actuators (VSAs) have attracted considerable attention in wearable robotics and soft exoskeletons due to their ability to adapt to various load conditions. This study presents a modular design for VSAs that incorporates a chain mail structure with various link topologies, allowing for a reconfiguration of stiffness. The proposed VSA consists of three main parts: the vacuum chamber, the VSA actuator, and the chain mail structure. The VSA fabrication process was carried out in five stages: (1) mold fabrication by 3D FDM printing, incorporating a film of oil to facilitate easy demolding; (2) mold preparation using silicone, with a precise ratio of 1:1 weight-based mixture to optimize material utilization; (3) silicone pouring into molds while applying vibration to eliminate air bubbles; (4) curing for four hours to achieve optimal mechanical properties; and (5) careful demolding to prevent damage. Experimental tests were conducted to characterize the stiffness of actuators with different chain mail fabric configurations, using an experimental setup designed to securely fix the actuator and accurately measure the pneumatic pressure and the angle of deformation after applying weights at its end. The European 6-in-1 and rounded square configurations were shown to be the most effective, increasing stiffness up to 382% compared to the chain mail-free configuration, highlighting the positive impact of these structural designs
Linear dynamics of over-expanded annular supersonic jets
No full textInternational audienceThis article delves into the dynamics of inviscid annular supersonic jets, akin to those exiting converging–diverging nozzles in over-expanded regimes. It focuses on the first azimuthal Fourier mode of flow fluctuations and examines their behaviour with varying mixing layer parameters and expansion regimes. The study reveals that two unstable Kelvin–Helmholtz waves exist in all cases, with the outer-layer wave being more unstable due to differences in the velocity gradient. The inner-layer wave is more sensitive to changes in base flow and extends beyond the jet, potentially contributing to nozzle resonances. The article also investigates upstream propagating guided-jet modes, which are found to be robust and not highly sensitive to changes in base flow, which makes them essential for understanding jet dynamics. A simplified model is used to obtain ideal base flows but with realistic shape in order to study the effects of varying nozzle pressure ratios on the dynamics of the waves supported by the jet
Water hammer waves simulations using a homogeneous equilibrium model
No full textInternational audienceWater hammer problems involving shock waves and cavitation are investigated numerically. The Homogeneous Equilibrium Model (HEM) of gas-liquid two-phase flow is considered for modeling the maximum and minimum resulting pressure waves. This is carried out using a set of conservation laws based on the continuity and momentum equations. The discretization of the model is carried out using Godunov-type finite volume technique based upon the wave propagation algorithm that mainly addresses second-order accuracy in water hammer problems. The capabilities of this model have been assessed on different cases of water hammer in air-water and CO2-water flows including the prediction of pressure, phase velocities and void fraction. Further, the wide applicability and advantageous properties of Godunov methods are demonstrated through such cases together with proper boundary conditions. Simulation results are compared with those provided by other solution techniques showing the superior accuracy as well as efficiency relations used by Godunov methods over other numerical methods. Additionally, the results are compared with experimental data available in the literature and found to be in a very good agreement. The current range of numerical simulations is discussed and illustrated including highlighting both their limitations and their advantages. Overall, this work will provide a systematic representation of past and current developments and directions for future research related to water hammer modeling nuclear power plants
Mechanical behavior characterization of glioblastoma cell using Scanning Ion Conductance Microscope (SICM)
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Analysis of the traction direction during a simulated forceps delivery
No full textInternational audienceThe use of forceps in deliveries has decreased significantly in favor of vacuum extraction (Leray & Lelong, 2021). However, when forceps are necessary, less experienced obstetricians may unintentionally cause serious and preventable perineal or foetal injuries (Coste Mazeau et al., 2020). Despite the importance of training, there is a lack of clear recommendations on the specific techniques and postures to adopt during forceps deliveries. Our previous study revealed that obstetricians used four different postures (“standing without trunk flexion”, “standing with trunk flexion”, “chevalier servant”, “squatting”) adopted when crossing the first plane and that very different pulling techniques were applied throughout delivery. Some recommendations suggest that traction should follow the umbilico-coxygeal axis, which points down- wards after crossing the first plane (Schaal, 2012). This raises questions about the compatibility of certain postures, such as “standing without trunk flexion,” with the recommended downward pull technique since the hands and forearms are not in this direction during this posture. To address these concerns, the objective of this study was then to qualify, by means of a three-dimensional kinematic analysis, the direction of the force produced during foetal delivery in relation to the obstetricians’posture