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Understanding the deformation mechanisms of hexagonal close-packed (HCP) polycrystals at the grain scale is crucial for developing both macro and micro scale predictive models. Slip and twinning are the two main deformation mechanisms of HCP polycrystals at room temperature. In this paper, the development of grain-level stress tensors during nucleation and growth of twins is investigated. A pure zirconium specimen with HCP crystals is deformed in-situ while the centre-of-mass, orientation, elastic strain, and stress of individual grains are measured by three-dimensional synchrotron X-ray diffraction (3D-XRD). The observed microstructure is subsequently imported into a crystal plasticity finite element (CPFE) model to simulate the deformation of the polycrystal. The evolution of stress in twin-parent pairs at the early stages of plasticity, further into plasticity zone, and unload is studied. It is shown that twins do not relax very much at the nucleation step, but the difference between the measured stress in the twin and parent increases further into plastic zone where twins relax. While at the early stages of plasticity all six twin variants are active, a slightly better estimation of active variants is obtained using the measured grain-resolved stress tensors. Discovery Grant from the Natural Sciences and Engineering Research Council of Canada (NSERC).

International audience; We describe a process for transferring a 200 nm thick, 200 mm wide monocrystalline silicon (mono c-Si) thin film from a silicon-on-insulator onto a flexible polymer substrate. The result is a stretchable and flexible ultra-thin semiconductor film that can be subjected to tensile stress experiments. The process uses off-the-shelf 200 mm wafers and standard polymer temporary bonding techniques. The backside substrate and buried oxide are removed using grinding and wet etching processes. No cracks or wrinkles are observed on the film prior to the tensile stress experiments. The stretching of the flexible structure results in up to 1.5% uniaxial tensile elastic strain on the thin mono c-Si film.

Polyamides belong to the major class of engineering polymers with very attractive properties but fail in a brittle manner under stress concentrators. PA6/ABS blends are well-established commercial materials due to their stiffness/toughness balance even when stress concentrators are present. The pair PA6/ABS is immiscible and exhibits poor mechanical properties due to unfavorable interactions, thus, an efficient compatibilization is mandatory for practical applications. Deep understanding of the processing-structure-properties interplays requires multiple characterization techniques to provide insights on the impact of the processing/compatibilization on morphology and phases' distribution. In this work an advanced co-localized AFM-Raman strategy was used to study the compatibilization of PA6/ABS blends with SAN-g-Maleic Anhydride. AFM and TEM allowed nanomechanical analysis and characterization of the blends' morphologies. Co-localized AFM-Raman provided new fundamental insights on the compatibilizer's effect on the PA6's γ polymorphic crystallization. The amount and distribution of this γ phase were found to depend on the blending protocol.

Proton-Exchange Membrane Fuel Cells is a promising emission free energy technology. New generation of hybrid membrane for hydrogen fuel cells were produced from commercial sPEEK membranes, chemically and mechanically stabilized with a Sol-Gel (SG) phase. To study the process-structure-properties relationship of our alternative membranes we coupled Atomic Force Microscopy and Raman microspectroscopy on membranes prepared by cryo-ultramicrotomy without epoxy embedding. This paper demonstrates the powerfulness of co-localized AFM-Raman analysis, revealing the inner structure of hybrid membranes. We obtained quantitative data on the diffusion/condensation of SG precursors inside the sPEEK membrane. Co-localized analyses revealed the formation of skin layers with lower SG concentration on both sides of the membrane. The diffusion of species from the SG phase at the cross-section surface was observed at the first step of fabrication (insufficient SG condensation) and disappeared after hydrothermal treatments (improved SG condensation). The nano-mechanical data revealed a densification of the SG phase through the fabrication process.

The optimization of manufacturing processes and the investigation of polymer materials during their life cycle imply an extended knowledge of the physical and chemical properties of the material. T...

Operando Raman spectroscopy and synchrotron X-ray diffraction were combined to probe the evolution of strain in Li-ion battery anodes made of crystalline silicon nanoparticles. The internal structure of the nanoparticles during two discharge/charge cycles was evaluated by analyzing the intensity and position of Si diffraction peaks and Raman TO-LO phonons. Lithiation/delithiation of the silicon under limited capacity conditions triggers the formation of "crystalline core-amorphous shell" particles, which we evidenced as a stepwise decrease in core size, as well as sequences of compressive/tensile strain due to the stress applied by the shell. In particular, we showed that different sequences occur in the first and the second cycle, due to different lithiation processes. We further evidenced critical experimental conditions for accurate operando Raman spectroscopy measurements due to the different heat conductivity of lithiated and delithiated Si. Values of the stress extracted from both operando XRD and Raman are in excellent agreement. Long-term ex situ measurements confirmed the continuous increase of the internal compressive strain, unfavorable to the Si lithiation and contributing to the capacity fading. Finally, a simple mechanical model was used to estimate the sub-nanometer thickness of the interfacial shell applying the stress on the crystalline core. Our complete operando diagnosis of the strain and stress in SiNPs provides both a detailed scenario of the mechanical consequences of lithiation/delithiation in SiNP and also experimental values that are much needed for the benchmarking of theoretical models and for the further rational design of SiNP-based electrodes.

Via SAXS, herein, we studied how a sPEEK microstructure evolves when it is immersed in water at a wide range of temperatures (20-100 °C) and time scales (from a few hours to dozens of days). In particular, we scrutinized the behavior of sPEEK at the temperature and time associated with the appearance of a well-defined nanosegregated morphology. At 80 °C, we observed nanoscale swelling along with smoothing of the water/polymer interface over a long period of time (several days). Herein, two of the main membrane properties, i.e., water uptake and proton conductivity, were studied for different immersion times and temperatures. It was found that the abovementioned properties were remarkably correlated with the evolution of the membrane microstructure, which was partly conserved after drying. The present findings helped us to understand that the thermally activated evolutions observed at both the nanoscale and macroscale were associated with the sPEEK β-relaxation crossover. Therefore, the very different swelling behaviors of sPEEK and Nafion are correlated to the much higher β-relaxation of sPEEK vs. Nafion (75 °C vs. -20 °C - dry state). From a practical viewpoint, this study emphasizes, for membranes alternative to Nafion, the importance and impact of the membrane pretreatment on their functional properties.

In this study, the influence of geometrical defects on the High Cycle Fatigue (HCF) resistance of aluminium is investigated, with emphasis on the impact of local microstructure on fatigue crack initiation. In order to meet this objective, an experimental approach, using a commercial purity polycrystalline aluminium alloy, is proposed. First, different thermomechanical treatments are applied to the aluminium alloy to obtain two homogeneous microstructures with respective mean grain sizes of 100 and 1000 µm. Then, fatigue specimens with an artificial hemispherical surface defect of diameter 1000 µm are subjected to fully reversed stress-controlled cyclic loading conditions. In-situ observations are carried out to monitor the crack length during fatigue tests. It is noted that, for a higher grain size, the number of cycles needed for the initiation of a 100 µm-long surface crack is lower. A study of the influence of the defect size relative to the grain size is also conducted. Two sizes of defects are used, and the influence of characteristic sizes seems to be explained by the role of cyclic plasticity in the crack initiation process.

National audience

International audience; Obstacle avoidance systems for autonomous vehicles combine multiple sensing technologies (i.e. LiDAR, Radar, Ultrasound and Visual) to detect different types of obstacles across the full range of lighting and weather conditions. Sensor data are fused with vehicle orientation (obtained for instance from an Inertial Measurement Unit and/or compass) and navigation subsystems. Power hungry, they require powerful computational capability, which limits their use to high-end vehicles and robots. 2 INSPEX ambition The H2020 INSPEX project plans to make obstacle detection capabilities available as a personal portable multi-sensors, miniaturised, low power device. This device will detect, locate and warn of obstacles under different environmental conditions, in indoor/outdoor environments, with static and mobile obstacles. Potential applications range from safer human navigation in reduced visibility conditions (e.g. for first responders and fire brigades), small robot/drone obstacle avoidance systems to navigation for the visually and mobility impaired people. As primary demonstrator (Fig.1), we will plug the INSPEX device on a white cane (see Fig. 1) for Visually Impaired and Blind (VIB) people to detect obstacle over the whole person height, provide audio feedback about harmful obstacles, improve their mobility confidence and reduce injuries, especially at waist and head levels [1]. The device will offer a "safety cocoon" to its user.

In this work, a hyperelastic constitutive model is developed to describe the thermo-mechanical behavior of the Ti17 titanium alloy. The grain shape and the crystallographic orientation are explicitly taken into account. The behavior of both the α and β phases is modelled with a crystal plasticity formulation coupled to a CDM (Continuum Damage Model). The constitutive model is implemented in the ABAQUS/Explicit finite element solver with a user-defined subroutine. The model parameters are identified from experimental tests. According to the cutting simulation results, both strain localization and chip segmentation are strongly impacted by the crystallographic orientation. Agglomération Angers Loire Métropole

WOS:000450307600018

Additive manufacturing processes allow developing complex optimized geometries, hence making them very attractive for aeronautical applications. In recent years, the possibility of fabricating secondary structural parts with additive manufacturing techniques has been explored by Airbus. Because of the thermal gradients caused by local heating, these techniques also have some drawbacks such as residual stresses and distortions. To solve these issues, post treatments, like stress relieving and Hot Isostatic Pressing (HIP) treatments, are usually applied. The present work deals with the development of a numerical tool for the prediction of the final state of Ti-6Al-4V industrial components produced by Selective Laser Melting (SLM). This study focuses on the simulation of the stress relaxation occurring during heat treatments. The objective is to analyse the creep phenomenon of the Ti-6Al-4V alloy at high temperatures (between 500°C and 900°C). Creep tests are performed under a constant stress (tensile and compression), using a Gleeble© machine. Specimens initially exhibit a fully martensitic microstructure resulting from the SLM process. Observations show that the microstructure evolves according to the temperature and influences the creep strain rate. A constitutive equation, based on the Arrhenius law, is thus proposed to describe the material behaviour during stress relief heat treatment and HIP. The model parameters are fitted using a least squares method showing a good agreement with experimental data. This phenomenological model is implemented in ABAQUS with a user subroutine. Numerical simulations are performed on simple test cases inspired from literature. The simulation results correspond well to the experimental data. In a near future, the model will be used to simulate complex shaped parts.

In this work, the impact of the microstructure and the loading conditions on the mechanical behavior of a β-rich Ti17 titanium alloy is investigated. For this purpose, two different initial microstructures are considered : (i) a two-phase lamellar α + β microstructure and (ii) a single-phase equiaxed β-treated microstructure. First, compression tests are performed at different strain rates (from $$10^{-1}$$ to 10 s−1) and different temperatures (from 25 to $$900\,^\circ $$C) for both microstructures. Then, optical microscopy, scanning electron microscopy, EBSD and X-ray diffraction analyses of deformed specimens are carried out. Whatever the loading conditions are, the flow stress of the as-received α + β Ti17 is higher than that of the β-treated Ti17. Also, because of a higher strain-rate sensitivity, the β-treated Ti17 is less prone to shear banding. At low temperatures (i.e., $$T \le 450\,^\circ $$C), the deformation behavior of both the as-received α + β and the β-treated Ti17 is controlled by strain hardening. For the β-treated Ti17 alloy, martensitic transformation is systematically detected in this temperature range. The softening behavior of the as-received α + β Ti17 observed at high temperatures is due to the joint effect of dynamic recrystallization, dynamic transformation, adiabatic heating and morphological texture evolution. For the β-treated Ti17 alloy, when the temperature exceeds $$700\,^\circ $$C, stress–strain curves display a yield drop phenomenon, which is explained by dynamic recrystallization.

The present work focuses on the simulation of the heat treatment applied after printing of Ti-6Al-4V parts. The numerical tool aims at predicting the influence of heat treatment conditions (e.g. holding time, temperature) on the residual stress field and the distortions for SLM produced parts. The numerical model relies on a thermo-viscoplastic constitutive model. To determine the corresponding material parameters, different creep tests have been performed at temperatures ranging from 723 K to 1173 K. According to the results, the stationary creep strain rate is independent of the hydrostatic pressure, which indicates that the high temperature behavior is not impacted by the initial porosity. Also, the material parameters are observed to change significantly from 873 K, which is due to the progressive transformation of the initial martensitic α ′ microstructure into the α + β lamellar microstructure. To validate the proposed approach, some numerical simulations have been performed for two different parts, for which distortions have been measured. The numerical and experimental distortions have then been compared to each other. For both parts, the agreement between experimental and numerical data is correct.

The mechanical response of polycrystalline materials to an externally applied load and their in-service performance depend on the local load partitioning among the constituent crystals. In hexagonal close-packed polycrystals such load partitioning is significantly affected by deformation twinning. Here we report in-situ compression-tension experiments conducted on magnesium specimens to measure the evolution of grain resolved tensorial stresses and formation and annihilation of twins. More than 13000 grains and 1300 twin-parent pairs are studied individually using three-dimensional synchrotron X-ray diffraction. It is shown that at the early stages of plasticity, the axial stress in twins is higher than that of parents, yet twins relax with further loading. While a sign reversal is observed for the resolved shear stress (RSS) acting on the twin habit plane in the parent, the sign of RSS within the majority of twins stays unchanged until twin annihilation during the load reversal. The variations of measured average stresses across parents and twins are also investigated. Load partitioning to grains during deformation of a polycrystalline material is affected by the presence of twins. Here, synchrotron diffraction is used to study 1300 twin-parent grains, revealing that stress is higher in twins than in the parent grain during the early stages of plastic deformation.

Gold nanoparticles, capped by 3-mercapto propane sulfonate (Au-3MPS), were synthesized inside a swollen sulfonated poly(ether ether ketone) membrane (SPEEK). The formation of the Au-3MPS nanoparticles in the swollen sPEEK membrane was observed by spectroscopic and microscopic techniques. The nanocomposite containing the gold nanoparticles grown in the sPEEK membrane, showed the plasmon resonance λmax at about 520 nm, which remained stable over a testing period of three months. The size distribution of the nanoparticles was assessed, and the sPEEK membrane roughness, both before and after the synthesis of nanoparticles, was studied by AFM. The XPS measurements confirm Au-3MPS formation in the sPEEK membrane. Moreover, AFM experiments recorded in fluid allowed the production of images of the Au-3MPSatsPEEK composite in water at different pH levels, achieving a better understanding of the membrane behavior in a water environment; the dynamic hydration process of the Au-3MPSatsPEEK membrane was investigated. These preliminary results suggest that the newly developed nanocomposite membranes could be promising materials for fuel cell applications.

In this work, a crystal plasticity constitutive model is proposed to describe the mechanical behavior of metallic materials for which twinning plays a significant role in the deformation process. Constitutive relations are obtained from a micromechanical approach that explicitly considers the interactions between twinned and untwinned domains. Then, based on a thermodynamical analysis of the problem, a new expression for the driving force for the expansion of twinned domains is proposed. Finally, to account for the polycrystalline nature of metallic materials, the constitutive model is implemented in a FFT spectral solver. In the second part of this paper, the model is used to study the mechanical behavior of a AZ31 magnesium alloy under compression, for which a significant amount of experimental data is available in the literature. The comparison between numerical and experimental data allows for discussion of the influence of the different deformation modes on the development of both crystallographic texture and lattice strains. The evolution of lattice strains is found to be largely influenced by the internal stress redistribution process associated with the expansion of twinned domains. Also, the polycrystalline plasticity model provides a correct description of how the morphological texture is strongly altered during the deformation process due to the important activity of twinning systems. The authors wish to thank Bjørn Clausen from LANSCE-LC for providing the experimental dataset for magnesium. This work was financed by Nu-Tech Precision metals, NSERC, COG, and OPG under the Industrial Research Chair program in Nuclear Materials at Queen's University.

International audience; Because of their improved magnetic properties, Fe-Si alloys are widely used for new electric motor generations. The use of punching process to obtain these components specially affects their mechanical behavior and fatigue strength. This work aims at studying the influence of punching operations on the fatigue behavior of a Fe-Si alloy. High cycle fatigue tests are performed on different smooth specimen configurations with either punched or polished edges. Results show a significant decrease of the fatigue strength for punched specimens compared to polished ones. To understand the origin of the fatigue failure on punched specimens, SEM observations of the fracture surfaces are carried out. They reveal that crack initiation always occurs on a punch defect. Additional experimental techniques are combined to characterize how the edges are altered by punching. The impact of punching operations on residual stresses and hardening is then investigated. Residual stresses are quantified on punched edges using X-ray diffraction techniques. Important tensile residual stresses exist in the loading direction as a result of punching operations. Also, according to XRD analyses and micro-hardness measurements, teh hardened zone depht is about 200µm. To dissociate teh respective influences of strain hardening, residual stresses and geometrical defects, a heat tratment is applied to both punched and polished specimens in order to quantify the contribution of each parameter to the high cycle fatigue resistance. Results show that the geometry of defects is one of teh most influent parameters. Consequantly, a finite element model is developed to simulate teh influence of edge defects on the fatigue strength of punched components. A non-local high cycle fatigue criterion is finally used as a post-processing of FEA to consider the effect of defets and teh associated stress-strain gradients in the HCF strength assessment.

Stress-controlled fatigue tests were run at different R ratios (= sigma min / sigma max) up to at most 3 million cycles on a 2.5%Cr–1%Mo steel (ASTM A182 F22) used in riser tubes connectors for offshore oil drilling. The fatigue lives, as well as the slope of the S – N curves were found to decrease with increasing R and the endurance limit to follow Gerber’s parabola. Surface crack initiation without any defect involved, was most often observed for R=-1, 0.5 and 0, while an R ratio of 0.25 triggered crack initiation from either surface or internal pores or chemically inhomogeneous areas, leading, in the latter case, to fish-eye patterns for relatively low numbers of cycles. A further increase in R ratio to 0.5 promoted only defect-initiated surface cracks, while no fatigue fracture was observed within 10 million cycles above R=0.6. These transitions in crack initiation mode are discussed based on X-ray diffraction analyses of residual stresses, elastic–plastic F.E. computations on a unit cell model containing a pore and some fracture mechanics analyses, with a particular attention to environmental effects. The authors acknowledge the assistance of Dr. E. Heripre for FIB sectioning and observations with a dual beam microscope bought within the Equipex Matmeca funding program.

Because of its excellent properties, such as good corrosion resistance, high specific strength and important ductility, the AA5383 aluminum alloy is largely employed for naval applications. In this work, the mechanical behavior of the AA5383 alloy at elevated temperatures, which is an important aspect for the control of forming operations, is investigated. For this purpose, an experimental campaign, including uniaxial tension, biaxial tension, and shear tests, is performed to cover an important range of temperatures (623∼723 K) and strain rates (10−4∼10-1 s-1). A constitutive model for the description of the high temperature behavior of the AA5383 alloy is then proposed. For the deformation behavior, this model combines a viscoplastic flow rule with the BBC2003 anisotropic yield criterion. Also, the prediction of ductile fracture, which is an important aspect for formability, relies on an extended version of the modified Mohr-Coulomb criterion. The extension allows including the impact of temperature and strain rate on ductile fracture as well as a cut-off value for stress triaxiality. Finally, numerical simulations of the experimental tests are performed to identify the flow rule, yield criterion and fracture criterion parameters by combining different optimization methods. The numerical and experimental results of the different tests are in good agreement, which indicates that the proposed constitutive model is well suited for investigating the impact of process conditions on the formability of the AA5383 alloy at high temperatures.

The model of Johnson and Cook, which includes a viscoplastic flow rule and a damage criterion, is widely used to describe the mechanical behaviour of metallic materials subjected to severe loading conditions, such as those encountered during fabrication operations or impact. This model has been built on empirical, rather than physical, grounds. The present paper therefore aims at revisiting the model of Johnson and Cook from the view point of thermodynamics with internal variables. The interest of this approach is twofold. First, it provides a guide for the construction of a complete thermomechanical constitutive model, with some constitutive relations not only for the stress tensor but also specific internal energy, specific entropy and heat flux vector. Second, it allows highlighting some possible limitations of the original model of Johnson and Cook. Such limitations can be circumvented with an alternative model, which is described in the present work. For illustration purpose, some applications of both the original and alternative models are presented in the final section.

Inspired by the abilities of contemporary autonomous vehicles to navigate with a high degree of effectiveness, the INSPEX Project aims to create a minaturised smart obstacle detection system, which could find use in a wide variety of leading edge smart applications. The primary use case focused on in the project is producing an advanced prototype for a device which can be attached to a visually impaired or blind (VIB) person’s white cane, and which, through the integration of a variety of minaturised sensors, and of the processing of their data via sophisticated algorithms, can offer the VIB user greater precision of information about their environment. The increasing complexity of such systems creates increasing challenges to assure their correct operation, inviting the introduction of formal techniques to aid in maximising system dependability. However, the major challenge to building such systems resides at the hardware end of the development. This impedes the routine application of top-down formal methods approaches. Some ingenuity must be brought to bear, in order that normally mutually hostile formal and mainstream approaches can contribute positively towards system dependability, rather than conflicting unproductively. This aspect is illustrated using two strands of the INSPEX Project.

International audience

International audience; Polycrystalline elasto-plasticity models provide a general framework for investigating the effect of microstructural heterogeneities (e.g. grains, inclusions, pores) on the high cycle fatigue behavior of metallic materials. In this work, continuum damage mechanics is used to construct a set of constitutive relations to describe the progressive degradation of certain mechanical properties at the grain scale. The damage is considered to be coupled with the elastic behavior of the material. Special care is taken to include the anisotropic aspect of fatigue damage and the effect of intragranular internal stresses. The constitutive relations are then implemented within a self-consistent model to evaluate intergranular interactions. Finally, the model is used to investigate the high cycle fatigue behavior of polycrystalline copper. It is shown that the influence of certain loading conditions on the high cycle behavior is correctly reproduced. Specifically, the application of a mean shear stress does not result in an increase in damage; however, a mean normal stress is damaging. That is, a decrease in the fatigue resistance is predicted when the mean normal stress is increased.

International audience; For metallic materials, fabrication processes (e.g. machining and forging) may involve important strain rates and high temperatures. For such severe loading conditions, the development of damage is often associated with the formation of Adiabatic Shear Bands (ASB). In this work, the impact of loading conditions (strain rate, temperature) on the formation of ASB in a beta rich titanium alloy (Ti17) is investigated. In this perspective, uniaxial compression tests have been conducted on cylindrical samples with a Gleeble-3500 thermo-mechanical simulator at temperatures ranging from 25◦C to 800◦C and strain rates ranging from 0.1 to 50 s−1 with axial strains of approximately 50 %. According to the experimental results, the flow curves exhibit hardening from25◦C to 550◦C and softening from 600◦C to 800◦C. When looking at the evolution of flow stress, the strain rate sensitivity is found to increase significantly with increasing temperatures. Also, adiabatic shear bands are preferably observed for high strainrates and low temperatures. The formation of ASB thus seems to be quite dependent on the evolution of the strain rate sensitivity of Ti17. Finally, metallographic observations have been carried out to better understand the process leading to the formation of ASB. Such observations demonstrate that the average width of ASB increases with increasing temperatures and decreasing strain rates. However, such observations do not allow for identifying whether some specific microstructural transformations (e.g. recrystallization or phase transformation) could explain the formation of ASB or not.

International audience; Geometrical defects are known to have a detrimental influence on the high cycle fatigue resistance of metallic alloys, smaller defects being less harmful. In this experimental work, the influence of the defect size on the high cycle fatigue behavior of polycrystalline aluminium with different grain sizes is investigated, to better understand the role of internal length scales. Firstly, different thermomechanical treatments are applied to obtain aluminium samples with either small (100 μm) or large (1000 μm) grains. The samples are used for preparing fatigue specimens, with either small (100 μm) or large (1000 μm) hemispherical defects. Fully reversed stress-controlled fatigue tests are then carried out. According to fatigue test results, surface crack initiation is delayed when the grain size is reduced, while an approximation of the fatigue limit shows that it is not much influenced by the average grain size. The influence of grain size seems to be explained by the role of cyclic plasticity in the crack initiation process. Finally, Electron BackScattered Diffraction (EBSD) maps are collected for specimens with large grains and small defects. Based on this experimental dataset, fatigue crack initiation from a defect is found to be strongly impacted by the crystallographic orientation of the surrounding grain, crack initiation preferably occurring in crystals being favorably oriented for plastic slip.

In this work, the influence of the geometrical defect size on the high cycle fatigue behavior of polycrystalline aluminium with different grain sizes is investigated, to better understand the role of internal length scales. Two sizes of grains and defect are used: 100 μm and 1000 μm, the grain size being controlled with thermomechanical treatments. Fully reversed stress-controlled fatigue tests are then carried out. According to fatigue test results, surface crack initiation is delayed when the grain size is reduced, while an approximation of the fatigue limit shows that it is not much influenced by the average grain size. The relative defect diameter (compared to the grain size) seems to be the leading parameter influencing fatigue crack initiation from a defect. Finally, Electron BackScattered Diffraction (EBSD) maps are collected for specimens with large grains and small defects. Fatigue crack initiation from a defect is found to be strongly impacted by the crystallographic orientation of the surrounding grain, crack initiation preferably occurring in crystals being favorably oriented for plastic slip.

The INSPEX project aims to miniaturize state-of-the-art obstacle detection technology comprising heterogeneous sensors and advanced processing, so that it can be used for wearable devices. The project focuses on enhancing the white cane used by some visually impaired and blind people. Due to high demand for reliability and performance, the project is a good candidate for the use of formal methods. In this paper, we report lessons we have learned from formal modelling exercises related to the pre-processing of sensor information in INSPEX.

International audience; Since a decrease of the fatigue strength may result from punching operations, this study proposes a methodology for designing punched parts against high cycle fatigue crack initiation. To reach this goal, high cycle fatigue tests are performed on different specimens configurations with either punched or polished edges. Due to punching effects, the fatigue strength of punched specimens is significantly decreased. Fracture surfaces observations reveal that crack initiation occurs always on a punch defect. Additional investigations are combined to characterize how the edges are altered by the punching operations. High tensile residual stress levels along the loading direction are quantified using X-Ray diffraction techniques. Furthermore, micro-hardness measurements and X-Ray diffraction results reveals a strong hardness gradient due to punching operation. For a better understanding of crack initiation mechanisms, the edge geometries have been scanned with 3D optical microscopy, allowing us to identify the most critical defect (and its real geometry) by comparing the edges before and after fatigue failure. Finally, FEA are performed on identified defects. A non-local high cycle multiaxial fatigue strength criterion has been used as post-processing of FEA to take into account the effect on the HCF strength of defects and the strong stress-strain gradients around them.

International audience

The macroscopic behavior of polycrystalline materials is largely influenced by the shape, the arrangement and the orientation of crystallites. Different methods have thus been developed to determine the effective behavior of such materials as a function of their microstructural features. In this work, which focuses on polycrystalline materials with an elastic–viscoplastic behavior, the self-consistent, finite element and spectral methods are compared. These common methods are used to determine the effective behavior of different 316L polycrystalline aggregates subjected to various loading conditions. Though no major difference is observed at the macroscopic scale, the hardening rate is found to be slightly overestimated with the finite element method. Indeed, spatial convergence cannot be guaranteed for finite element calculations, even when fine mesh resolutions, for which the computational cost is important, are used. Also, as the self-consistent method does not explicitly account for neighborhood effects, important discrepancies between the self-consistent method and the other methods exist regarding the mechanical response of a specific grain. The self-consistent method nevertheless provides a reasonable description of the average response obtained for a group of grains with identical features (e.g. shape, orientation).

Resume L’utilite de l’hypnose dans la prise en charge de la douleur est maintenant largement reconnue. Cet outil trouve donc tout naturellement sa place en kinesitherapie ou, d’une part, les patients sont adresses pour la prise en charge de douleurs aigues ou chroniques, et, d’autre part, la reeducation en elle-meme peut parfois etre douloureuse. Differentes techniques sont disponibles : de l’hypnose conversationnelle accessible a tous les soignants a l’hypnose conventionnelle requerant une formation specifique. Toutes ces techniques facilitent l’alliance therapeutique avec le patient. Les enfants sont plus « competents » en hypnose que les adultes car ils sont plus creatifs et entrent plus facilement dans leur monde imaginaire. Cela facilite grandement leur reeducation et la rend plus ludique. Un autre interet de l’hypnose est de rendre le patient autonome en lui apprenant l’auto-hypnose. Le travail en collaboration medecin-kinesitherapeute, formes tous les deux a l’hypnose, ameliore d’autant plus la prise en charge du patient. L’utilisation de l’hypnose en reeducation pediatrique est illustree de cas cliniques commentes. Niveau de preuve NA.

6th Fatigue Design conference, Fatigue Design 2015 Rotors of electric machines are built from stacks of thin steel sheets. The fabrication process of these components usually involves punching operations that generate defects on the steel sheet edges. In this study, high cycle fatigue tests are performed on punched and polished edges specimens to investigate the effect of the punching process on the fatigue behaviour of these thin sheets. Results show a significant decrease of the fatigue strength for punched specimens. SEM observations of fracture surfaces reveal that crack initiation always occurs on a punching defect. Residual stresses on punched edges are analysed using X-Ray diffraction techniques. High tensile residual stresses along the loading direction are found. Some specimens edges were scanned using 3D topography prior to the fatigue tests. This allows for identifying the real geometry of the most critical defect. Murakami criterion was then evaluated in order to take into account the effect of defects. The best trend of the experimental results is given when residual stresses are taken into account. Local elastic stresses for 3 defects geometries have been calculated using FEA. Crossland fatigue criterion has been evaluated to try accounting for the local stress state around defects. Results show that the assessed fatigue strength is overestimated using this criterion.